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Introduction
Following the advent of antiretroviral therapy (ART), people living with HIV-1 can suppress plasma viremia to undetectable levels and live relatively normal and healthy lives. However, reservoirs of persistent HIV-1 infection endure and are thought to represent a major barrier to a cure.1-9 Typically, the HIV-1 reservoir is defined as latently infected resting CD4+ T cells, which are impervious to ART.1,2,4 This latent HIV-1 reservoir is formed early during viral infection likely via one of two (or both) mechanisms: (1) some HIV-1 infected activated CD4+ T cells survive and subsequently return to a memory phenotype2,3,5,7,10-12; and/or (2) HIV-1 directly infects resting memory CD4+ T cells, particularly in the presence of the CCR7 ligands CCL19 or CCL21.13 These chemokines, which are constitutively expressed in lymphoid organs, do not increase expression of T cell activation or proliferation markers but increase the permissiveness of these cells to HIV-1 infection.13 The latent HIV-1 reservoir in resting CD4+ T cells is maintained due to the long half-lives of these cells and via homeostatic proliferation.1,5,6 While CD4+ T cells represent a major reservoir of HIV-1 infection, other cellular and anatomical reservoirs exist, including macrophages, myeloid dendritic cells and astrocytes, as well as the central nervous system, gastrointestinal tract, lymphatics and urinary tract, respectively.5,8,9,14 Reservoirs in these privileged or complex sites need to be acknowledged in potential cure strategies as many clinical studies are focused only on the circulating immune cells in the peripheral blood.

Maintenance of the HIV-1 reservoirs during suppressive ART may also occur as a result of suboptimal ART levels in some anatomical sites, allowing for cryptic viral replication and therefore new infections, although this hypothesis is controversial.6,8,15,16 Cell-to-cell transfer of HIV-1, or trans infection, has also been implicated in the establishment and maintenance of the latent viral reservoir.6,7,17-20 Compared to direct, cis infection, trans infection can be 10- to 1000-fold more efficient and may lead to multiple infections per cell likely due to a high multiplicity of infection (MOI) at the site of cell-to-cell contact.17,18,20-22 Cell-to-cell transmission of HIV-1 utilizes the natural interactions between immune cells to allow for HIV-1 to be transferred into uninfected cells leading to virus infection.6,20 Of note, HIV-1 trans infection is insensitive to antiviral drugs and neutralizing antibodies.6,7,17,21,23 Cell-to-cell HIV-1 transmission has been shown between infected and uninfected CD4+ T cells as well as being mediated by mast cells and basophils.19,24,25 Infection of CD4+ T cells can also be mediated by professional antigen presenting cells (APC), i.e., dendritic cells (DC), B lymphocytes, and monocytes/macrophages, and is termed APC-mediated trans infection.26-29 In this review, we focus on DC-, B cell-, and macrophage- mediated trans infection of CD4+ T cells and consider the impact that these events have on the establishment and maintenance of the viral reservoirs. Additionally, we speculate as to how APC-mediated trans infection may be involved in disease progression, emphasizing the importance of addressing this mechanism of HIV-1 pathogenesis when investigating potential cure strategies.

DC-mediated HIV-1 trans infection
Myeloid DC express the HIV-1 receptor CD4 and the co-receptors CCR5 and CXCR4, and therefore can be productively infected by both R5- and X4- tropic HIV-1 strains.28,30-32 Due to higher levels of CCR5 expression, R5-tropic HIV-1 strains are more efficient at infecting myeloid DC.30 While DC can be productively infected, HIV-1 replication in these cells is much lower, as is the frequency of HIV-1 infected DCs in vivo compared to CD4+ T cells.30,31,33 However, myeloid DC can bind HIV-1 gp120 independent of CD4 and CCR5/CXCR4 and subsequently trans infect CD4+ T cell targets.28,30,32,34-38 C-type lectin receptors on DC, including the DC-specific intercellular adhesion molecule-3 grabbing nonintegrin (DC-SIGN), Langerin, DC-Immunoreceptor (DCIR), and mannose receptor (MR), can facilitate this process.28,30-32,38-41 DC-SIGN, which has been studied extensively for its role in DC-mediated trans infection, binds and internalizes HIV-1 via endocytosis into early endosomes before it is transferred to target CD4+ T cells.6,18,28,30-32,38,39,41-43 With high levels of DC- SIGN expression on their surface, monocyte- derived immature DC (iDC) can bind HIV-1 through gp120, which has a higher affinity for DC-SIGN compared to the CD4 receptor.39,44,45 Of note, iDC derived from CD14+ monocytes cultured with granulocyte-macrophage colony-stimulating factor (GM-CSF) and recombinant human interleukin-4 (IL-4) can trans infect CD4+ T cells following incubation with a sub-infectious dose of HIV-1BaL (CCR5-tropic HIV-1 strain), i.e., a multiplicity of infection (MOI) that is not able to productively infect CD4+ T cells in cis (MOI 10-3). However, these iDC, while able to trans infect total CD4+ T cells, do so at a significantly lower efficiency than B cells. Additionally, while they can trans infect central memory CD4+ T cells (TCM), they are inefficient at trans infecting naïve CD4+ T cells (TN), unlike B cells, which we discuss in more detail below.27

Mature DC (mDC) have also been implicated in trans infection where they store HIV-1 in endosomal compartments or invaginations of the plasma membrane before transfer to CD4+ T cell targets.43,46,47 Similar to iDC, CD40L-matured DC are inefficient at mediating HIV-1 trans infection of TN cells.27 While DC-SIGN expression is reduced upon DC maturation22,43,48,49, HIV-1 capture and transfer efficiency is increased.37,49 Unlike HIV-1 capture by iDC via DC-SIGN, capture by mDC does not require viral envelope glycoproteins.43,50,51 Instead, the sialic-acid binding immunoglobulin-like lectin 1 (Siglec-1) receptor is implicated in mDC- mediated trans infection. Siglec-1, which is highly expressed on mDC, recognizes sialyllactose- containing membrane gangliosides on HIV-1, specifically GM3, a virus particle-associated ligand. Siglec-1 expression correlates with HIV-1 capture and trans infection of CD4+ T cells while anti-Siglec-1 antibodies inhibit HIV-1 capture.7,18,22,32,36,37,49,52-55 Furthermore, it was found that Siglec-1 is upregulated on myeloid DCs matured by lipopolysaccharide (LPS) or type I interferon, and using infectious HIV-1, LPS-matured mDC capture significantly more virus than iDC.37,43,49

iDC have long been implicated in the formation of the viral reservoir during transmission events as they can capture HIV-1 via DC-SIGN at the site of infection, i.e., rectal or vaginal mucosa after sexual exposure, and subsequently travel to secondary lymphoid tissues to mount an immune response where they have many interactions with target CD4+ T cells.6,22,37-39,41-43,45 Similarly, mDC in lymphoid tissues have numerous interactions with uninfected CD4+ T cells which densely populate these areas. Lymph nodes (LN) removed from an HIV-1-infected individual before and after initiation of ART showed Siglec-1 positive cells in the perivascular, sub-capsular, and perifollicular areas with a similar pattern before and after ART.36 Knowing that cell-to-cell HIV-1 transmission can occur even during ART and that there may be lower ART concentrations in lymphoid tissues suggests that mDC-mediated trans infection via Siglec-1 may occur in vivo. Importantly, DC may be able to trans infect multiple CD4+ T cells during exploratory contacts as they search for their cognate interaction.49 Additionally, DC-mediated trans infection that occurs during a cognate interaction with antigen-specific CD4+ T cells is highly efficient.56 Considering lymphoid tissues are a major site of HIV-1 replication and that DC migrate to these tissues where they repeatedly establish interactions with CD4+ T cells implies the role DC- mediated HIV-1 trans infection can play in both the establishment and maintenance of the viral reservoir.

B cell-mediated HIV-1 trans infection
Unlike DC and macrophages, B cells cannot be productively infected with HIV-1. While they express high levels of CXCR4, they express only low levels of CD4 and lack CCR5 expression.57,58 B cells were first implicated in trans infection of CD4+ T cells through binding of HIV-1 immune complexes (HIV-1 IC) via CD21 (Complement Receptor-2). Through HIV-1 IC and CD21 binding, virus remained on the cell surface but was infectious for target CD4+ T cells with trans infection being significantly reduced when anti-CD21 antibodies were used before HIV-1 IC incubation.59-63 As previously described, DC can bind HIV-1 via DC-SIGN and subsequently trans infect CD4+ T cells. Interestingly, a subset of B cells from the blood and tonsils of uninfected individuals express DC-SIGN and expression is increased after stimulation with CD40L and IL-4. These activated B cells can also bind and internalize both CCR5- and CXCR4-tropic HIV-1 via DC-SIGN and mediate trans infection of CD4+ T cells in a similar manner to DC. When DC-SIGN is blocked prior to HIV-1 incubation, trans infection is significantly inhibited.58 This work provides an HIV- 1 IC-independent mechanism for trans infection of CD4+ T cells by B cells.

TN cells were previously thought to contribute minimally to the viral reservoir due to their significantly lower frequency of infection, as measured by HIV-1 DNA copies, compared to the memory CD4+ T cell subsets.1,64 However, Zerbato et al recently showed, both in vitro and ex vivo, that despite harboring less HIV-1 DNA than TCM cells, TN cells produce as much if not more virus after reactivation with latency reversing agents.65,66 A possible explanation for this finding is that the contribution of intact HIV-1 proviruses in TN cells is greater than in TCM cells.67,68 Additionally, the proviral sequences in TN cells are unique, with the number of clones increasing as cells differentiate from TN to effector memory, in which they were mainly clonal, suggesting TN cells repopulate the effector memory viral reservoir.68,69 These findings suggest that TN cells contribute more to the reservoir than initially thought. Furthermore, although TN cells are extremely resistant to cis infection with CCR5- tropic HIV-1 in vitro due to minimal CCR5 expression, HIV-1 DNA is detectable in TN cells of viremic and virus-suppressed individuals and CCR5- tropic virus has been recovered from them.65,67,68,70,71 Since CCR5-tropic HIV-1 is responsible for the vast majority of transmission and early stages of infection, the mechanism by which CCR5-tropic HIV-1 can infect TN cells is of importance, especially considering the impact infection of this CD4+ T cell subset has on the viral reservoir.72,73

To explain this phenomenon, it was hypothesized that B cells can mediate trans infection of TN cells with CCR5-tropic HIV-1, bypassing the need for CCR5-co-receptor expression. While both B cells and iDC can mediate HIV-1BaL trans infection of TCM and bulk CD4+ T cells, only B cells, compared to iDC and mDC, can efficiently trans infect TN cells in vitro.27 Again, it is important to note that in this model APC are incubated with a sub-infectious dose of HIV-1BaL, i.e., an MOI that is not able to productively infect CD4+ T cells in cis (MOI 10-3), emphasizing the efficiency of trans infection compared to cis infection. Additionally, this finding was consistent whether the T cell targets were previously activated with phytohemagglutinin (PHA) and IL-2 or exposed to CCL19 which does not induce global T cell activation.13 Furthermore, co- culture of B cells with TN cells does not affect the TN phenotype, with the TN cell subset remaining predominately CCR5-.27

B cells and CD4+ T cells interact regularly in vivo in and around B cell follicles in secondary lymphoid organs (SLO). These interactions are important for B cell and CD4+ T follicular helper cell (TFH) differentiation and the formation and maintenance of antibody responses.74-77 Importantly, TFH cells have also been implicated as a major reservoir of HIV-1 infection in humans and in simian immunodeficiency virus (SIV) infection in macaques, harboring high levels of viral DNA and RNA.78-83 Additionally, TFH cell populations, along with germinal center (GC) B cells, increase during HIV- 1 infection compared to uninfected persons, and these increases are not normalized by ART, providing a long-term reservoir for HIV-1 infection.81,83 Regarding CCR5 expression, macaque and human TFH cells predominately lack CCR5 expression, but do include a precursor subset termed pre-TFH. These cells are present during the proliferation and differentiation phase following initial activation of a TN cell by a DC, but before interaction with their cognate B cell, which is categorized by some CCR5 expression. Like TN cells, despite there being only minimal CCR5 expression on TFH cells, they are infected in vivo with CCR5- tropic virus, thus their mode of infection is also of importance.78,80,81 Similar to B cell-mediated HIV-1 trans infection of TN cells, we hypothesize that B cells can also mediate trans infection of TFH cells in SLO, aiding in the establishment and maintenance of the viral reservoir.

Macrophage-mediated HIV-1 trans infection
Macrophages express CD4, CCR5 and CXCR4, and therefore can be productively infected with R5 and X4-tropic HIV-1.26,84-86 Due to higher CCR5 expression they are more efficiently infected by CCR5-tropic HIV-1. However, how monocytes are differentiated into macrophages also impacts their co-receptor expression.84 Differentiation of monocytes with macrophage colony stimulating factor (M-CSF) causes an increase in both CCR5 and CXCR4 expression, while differentiation with GM- CSF causes an upregulation in CCR5 expression, but a down regulation in CXCR4 expression.57 In addition to cis infection, monocyte-derived macrophages (MDM) have also been implicated as mediators of trans infection of CD4+ T cells through binding HIV-1 via DC-SIGN, Siglec-1, and MR.26,84,87-92 Moreover, DC-SIGN is upregulated on MDM exposed to IL-13.91,92 These MDM can trans infect CD4+ T cells with HIV-1BaL and trans infection efficiency is associated with DC-SIGN expression. Additionally, blocking DC-SIGN on MDM prior to virus exposure reduces trans infection by 87% by day 12 post co-culture.26 Another study found that in those MDM that lack DC-SIGN expression, 60% of HIV-1 binding is MR-mediated and that blocking MR reduces trans infection of CD4+ T cells up to 80%.89 Activating MDM with IFN-α causes an upregulation in Siglec-1 expression, although to a lesser extent than on DC. However, MDM can still mediate trans infection of CD4+ T cells via Siglec-1 binding of HIV-1 and when Siglec-1 is blocked, it largely inhibits trans infection.36 Like DC, macrophages are one of the first cells to come in contact with HIV-1 during sexual transmission in mucosal tissue.87-89 As APC, macrophages have many transient adhesive contacts with CD4+ T cells in lymphoid organs where HIV-1 can also be transferred, implicating these cells in both the formation and maintenance of the viral reservoir.88

APC-mediated HIV-1 trans infections impact on disease progression
While most HIV-1-infected individuals will progress to AIDS in the absence of ART (progressors; PR), there is a small percentage of individuals who naturally control disease progression in the absence of ART and are broadly termed non-progressors (NP). These NP can be further classified into elite controllers (EC), viremic controllers (VC), and long- term NP (LTNP). The criteria for these subgroups are as follows: EC= undetectable viral load >7 years post infection, VC= at least two viral load measures below 2,000 copies HIV-1 RNA/mL, and LTNP= CD4+ T cell counts >500 cells/mm3 over 7 years post infection.27,93 Understanding how NP naturally control HIV-1 infection may provide insight for new therapeutics that could mimic this control in PR.

As APC-mediated trans infection is an extremely efficient means of viral dissemination and is potentially important for HIV-1 reservoir seeding and maintenance, the efficiency of B cell- and DC- mediated trans infection in seronegative (SN), PR, and NP samples from the Multicenter AIDS Cohort Study and the Women’s Interagency HIV Study (MWCCS) was examined to determine if there was a link between trans infection efficiency and disease progression. Interestingly, both DC and B cells from NP cannot mediate trans infection of CD4+ T cells, unlike PR and SN APC.93 Of note, CD4+ T cells from NP are efficiently HIV-1 cis-infected in vitro. Additionally, B cells and DC from NP and SN revealed no difference in DC-SIGN expression or their ability to bind HIV-1. Cholesterol is essential for DC- and macrophage-mediated trans infection as when DC are treated with peroxisome proliferator-activated receptor γ (PPARγ) and liver X receptor (LXR), HIV-1 trans infection is inhibited by an increase in cholesterol efflux through ATP- binding cassette transporter A1 (ABCA1) activity.94,95 Thus, it was hypothesized that increased cholesterol efflux is related to the inability of APC from NP to trans infect CD4+ T cells. Indeed, this lack of trans infection is linked to lower cholesterol levels and an increase in ABCA1 levels in APC from NP. Additionally, NP APC’s ability to trans infect is restored by cholesterol reconstitution and siRNA knockdown of ABCA1.93 It is also important to note that this trait is present in APC from NP before HIV-1 infection, indicating it is hereditary.93 This work was followed by an analysis of macrophage-mediated trans infection by SN, PR, and NP.26 Similarly, MDM from NP cannot trans infect CD4+ T cells, while MDM from SN and PR are efficient mediators of trans infection. However, in contrast to B cells and DC, trans infection efficiency is linked to the number of DC-SIGN+ MDM with significantly fewer NP MDM expressing DC-SIGN. Additionally, cholesterol levels, lipid rafting, and virus internalization into early endosomes is significantly lower in NP MDM than PR MDM.26

As previously stated, B cells can mediate CCR5- tropic HIV-1 trans infection of predominately CCR5- TN cells.27 If this mechanism of TN cell infection is a major mechanism by which infection occurs, then NP may have a reduced or absent level of HIV-1 DNA in this CD4+ T cell subset. HIV-1 DNA levels were quantified in TN and total CD4+ T cells from 7 NP not under ART and 7 PR on ART. Interestingly, HIV- 1 DNA was not detected in TN cells from the EC tested, while a low number of HIV-1 DNA copies were detected in the other NP (LTNP and VC). Importantly, TN cells from NP harbor significantly lower copies of HIV-1 DNA than TN cells from PR.27 Consistent with these data, Pinzone et al performed proviral sequencing on CD4+ T cell subsets from 5 EC not on ART and 5 PR on ART and found that TN cell infection was barely detectable in EC and was significantly lower than in TN cells from PR.69 Importantly, these EC had predominately CCR5- tropic HIV-1 proviruses. Additionally, EC had overall less HIV-1 infection compared to PR, as seen by significantly lower levels of HIV-1 DNA in total CD4+ T cells, as well as in TN and memory subsets, consistent with other findings of smaller reservoirs in CD4+ T cells of EC compared to PR.69,96-98 This work supports the role of APC-mediated trans infection in disease progression, specifically in its potential impact on the viral reservoir size and TN cell infection.

Mechanisms of APC-mediated HIV-1 trans infection
Various mechanisms for HIV-1 transfer from APC to CD4+ T cells have been proposed with one being exosomes.35,99-103 HIV-1 infected monocyte derived iDCs generate fibronectin and galactin-3 containing exosomes which are 4-fold more infective than cell free HIV-1. Additionally, anti-fibronectin antibodies and β-lactose, a galectin-3 antagonist, significantly block DC exosome-mediated HIV-1 trans infection of CD4+ T cells.35 Similarly, mDC have also been implicated in trans infecting CD4+ T cells via exosomes in an envelope glycoprotein-independent manner.99,104 This mDC-mediated trans infection via exosomes also allows for efficient trans infection of resting CD4+ T cells.100 Another mechanism for viral transfer is filopodia, which are actin-rich membrane protrusions.7,18,19,105 Filopodia are induced in iDC after DC-SIGN-mediated HIV-1 binding which then allows for trans infection of CD4+ T cell targets.18,19,105-107 Other membrane protrusions, namely tunneling nanotubes (TNT), have also been implicated as a mode of viral transfer in APC- mediated trans infection. Macrophages produce TNT after HIV-1 infection through involvement of Nef, with HIV-1 trafficking through endocytic compartments in TNT.7,18,19,108-111 Additionally, DC matured by type 1 immunity mediators produce TNT-like structures in response to CD40L with these structures able to traffic HIV-1 like particles from donor to recipient cells.112 In regard to reservoir formation and maintenance, TNT are of importance as they allow for protection of HIV-1 from immune surveillance and a mechanism of efficient spreading.7,18,111

The virological synapse is frequently used to define viral cell-to-cell transmission and is similar to the immunological synapse formed between APC and T cells during antigen presentation.18 There can be recruitment of cell adhesion molecules, such as the lymphocyte function associated antigen-1 (LFA-1) and its ligand, intercellular adhesion molecule- 1(ICAM-1), and of receptors including CD4, CCR5, and CXCR4, as well as actin rearrangement.18,113,114 However, the virological synapse is commonly described for CD4+ T cell-to- CD4+ T cell transfer, and modes of cell-to-cell HIV- 1  transmission  not  requiring  HIV-1  receptors/co- receptors and adhesion molecules have also been shown.18,27,113-115 Virological, or infectious, synapses have been implicated in DC-mediated trans infection.7,19,22,116 Interestingly, Akiyama et al found that not only can DC bind HIV-1 through Siglec-1, but that after virus is internalized, the cytoplasmic tail of Siglec-1 allows for HIV-1 trafficking and trans infection to CD4+ T cells. Additionally, this mechanism of trans infection was not susceptible to neutralizing antibodies.55 The varying hypotheses on requirements for viral transfer likely depend on the specific cells involved in trans infection and are not “one size fits all”. Immunological synapses between APC and CD4+ T cells could provide HIV- 1 with the ability to hijack these natural, frequent interactions allowing for highly efficient viral spread. Also, an increase in TNT or filopodia due to the presence of HIV-1 in turn can increase cell-to- cell contact, thus increasing the likelihood of trans infection.

Conclusions
While CD4+ T cells are the major reservoir for HIV- 1 infection, the processes by which they can be infected in anatomical reservoirs such as the brain/CNS, gastrointestinal tract, lymphatics, and urinary tract is of importance.5,8,9,14 Notably, APC- mediated HIV-1 trans infection is significantly more efficient than cis infection due to the transfer of multiple virus particles and the high MOI at the site of cell-to-cell contact, thus increasing the likelihood that virus escapes immune responses.17,18,20-22 Additionally, higher copies of HIV-1 DNA in CD4+ T cells isolated from tissue compared to peripheral blood indicates trans infection as a likely means of CD4+ T cell infection as it can lead to multiple infections per cell.17,18,20,117 At the site of sexually transmitted HIV-1 infections, macrophages and iDC are some of the first cells to interact with HIV-1. Macrophages and iDC in anal and vaginal mucosa can bind and internalize HIV-1 via DC-SIGN and Siglec-1. Subsequently they could trans infect activated CD4+ T cells present in the submucosa, establishing the HIV-1 reservoir very early in infection (Figure 1A).118 Alternatively, iDC that bind HIV-1 via DC-SIGN, can mature and travel to the draining LN via the CCL19/21 chemokine gradient through their CCR7 expression for antigen presentation to CD4+ T cells (Figure 1B, E).119-121 In the LN paracortex, mDC survey CD4+ T cells for their cognate TN cell. In addition to TN cells, there are TCM cells present that home to SLO due to CCR7 expression, as well as activated CD4+ T cells that transiently interact with mDC, potentially allowing for HIV-1 trans infection (Figure 1E).120,122 Additionally, iDC migrate through SLO and thus if they have bound HIV-1 through DC-SIGN, could also trans infect the CD4+ T cells in the paracortex of LN (Figure 1E).122 Pathogens are also transported to draining LNs through lymphatic vessels where they enter the subcapsular sinus (SCS). The SCS is lined with SCS macrophages that express Siglec-1 through which they can bind antigens.121,123,124 As the SCS also borders the T cell zone (paracortex), SCS macrophages that have bound HIV-1 through Siglec-1 may be able to trans infect CD4+ T cells they encounter (Figure 1C).

In the B cell follicles of LN, naïve B cells survey SCS macrophages and follicular DC (fDC) for their cognate antigen. Extracellular HIV-1 accumulates on fDC and can survive for long periods of time, remaining infectious even in the presence of neutralizing antibodies, and SCS macrophages bind HIV-1 via Siglec-1.79 B cells survive in part through the TNF family cytokine BAFF, which is secreted by resident stromal cells, with production upregulated during HIV-1 infection. This B cell survival factor can also induce expression of C-type lectins, such as DC- SIGN on B cells.58,125,126 These circumstances allow B cells to bind and internalize HIV-1 via DC-SIGN as they survey for their cognate antigen. Once B cells encounter their cognate antigen they upregulate markers including CCR7, which allows them to migrate to the B:T cell border through the CCL19/21 chemokine gradient. Here B cells will form an antigen-specific interaction with pre-TFH cells that previously encountered the same antigen. Subsequently, B cells and now fully differentiated TFH cells can form GC where more antigen- dependent interactions occur that give rise to antibody class switching and affinity maturation.74-77 Additionally, as B cells survey for their cognate TFH cell, they engage in other CD4+ T cell interactions. We postulate that these interactions, along with the antigen specific B:TFH cell interactions at the B:T border and in GC, result in HIV-1 trans infection of CD4+ T cells (Figure 1D).

During HIV-1 infection, the majority of viral replication occurs in SLO, even during ART-treated infections, presenting an important source of viral reservoirs that need to be addressed.79,80,83,117 The enrichment of infected cells in these areas is likely mediated by several factors. B cell follicles provide a sanctuary for the virus to escape CD8+ T cell- mediated elimination, as the frequencies of these cells are lower in this area. Additionally, a chronic state of inflammation during HIV-1 infection can enhance viral production from infected cells, allowing replenishment of the reservoir.79,117 Viral persistence in SLO during ART despite viral suppression in peripheral blood has also been linked to lower drug concentrations in these tissues.8,15,17 While in their active form HIV-1 infected TFH cells may not be considered part of the latent reservoir, after the GC response has ended a large proportion of TFH cells survive as memory cells in SLO. Thus, this subset is a major reservoir of HIV-1 that largely does not express the CCR5 co- receptor needed for cis infection.81,127,128 The chemokine CCL19, which is constitutively expressed in SLO for DC and T cell trafficking, enhances the permissiveness of CD4+ T cells to HIV-1 infection and allows for efficient B cell-mediated trans infection of TN cells.13,27 In addition to TN cells being able to be trans infected, it is suggested that TN cells repopulate the effector memory CD4+ T cell reservoir.68,69 Finally, through DC-SIGN and Siglec- 1 expression, macrophages as well as iDC and mDC are efficient mediators of HIV-1 trans infection of CD4+ T cells. In sum, we hypothesize that HIV-1 hijacks the natural, frequent interactions between APC and CD4+ T cells. This allows for highly efficient trans infection to occur in vivo which provides a means for both establishment and maintenance of the HIV-1 reservoir in CD4+ T cells. To this end, these mechanisms need to be addressed in any potential cure strategies.

Funding
This work was supported by National Institutes of Health (NIH) grants R01-AI162615 and U01- HL146208.

Conflicts of interest
The authors have no conflicts of interest to declare.

References
1.    Chomont N, El-Far M, Ancuta P, et al. HIV reservoir size and persistence are driven by T cell survival and homeostatic proliferation. Nat Med. Aug 2009;15(8):893-900. doi:10.1038/nm.1972
2.    Finzi D, Blankson J, Siliciano JD, et al. Latent infection of CD4+ T cells provides a mechanism for lifelong persistence of HIV-1, even in patients on effective combination therapy. Nat Med. May 1999;5(5):512-7. doi:10.1038/8394
3.    Eisele E, Siliciano RF. Redefining the viral reservoirs that prevent HIV-1 eradication. Immunity. Sep    21    2012;37(3):377-88. doi:10.1016/j.immuni.2012.08.010
4.    Chun TW, Stuyver L, Mizell SB, et al. Presence of an inducible HIV-1 latent reservoir during highly active antiretroviral therapy. Proc Natl Acad Sci U S A. Nov 25 1997;94(24):13193- 7. doi:10.1073/pnas.94.24.13193
5.    Smith MZ, Wightman F, Lewin SR. HIV reservoirs and strategies for eradication. Curr HIV/AIDS Rep. Mar 2012;9(1):5-15. doi:10.1007/s11904-011-01082
6.    Sigal A, Baltimore D. As good as it gets? The    problem    of    HIV    persistence    despite antiretroviral drugs. Cell Host Microbe. Aug 16 2012;12(2):132-8. doi:10.1016/j.chom.2012.07.005
7.    Pedro KD, Henderson AJ, Agosto LM. Mechanisms of HIV-1 cell-to-cell transmission and the establishment of the latent reservoir. Virus Res. May    2019;265:115-121. doi:10.1016/j.virusres.2019.03.014
8.    Lorenzo-Redondo R, Fryer HR, Bedford T, et al. Persistent HIV-1 replication maintains the tissue reservoir during therapy. Nature. Feb 4 2016;530(7588):51-56. doi:10.1038/nature16933
9.    Henderson LJ, Reoma LB, Kovacs JA, Nath A. Advances toward Curing HIV-1 Infection in Tissue Reservoirs. J Virol. Jan 17 2020;94(3)doi:10.1128/JVI.00375-19
10.    Han Y, Wind-Rotolo M, Yang HC, Siliciano JD, Siliciano RF. Experimental approaches to the study of HIV-1 latency. Nat Rev Microbiol. Feb 2007;5(2):95-106. doi:10.1038/nrmicro1580
11.    Ward AR, Mota TM, Jones RB. Immunological approaches to HIV cure. Semin Immunol.    Jan    2021;51:101412. doi:10.1016/j.smim.2020.101412
12.    Fromentin R, Chomont N. HIV persistence in subsets of CD4+ T cells: 50 shades of reservoirs. Semin Immunol. Jan 2021;51:101438. doi:10.1016/j.smim.2020.101438
13.    Saleh S, Solomon A, Wightman F, Xhilaga M, Cameron PU, Lewin SR. CCR7 ligands CCL19 and CCL21 increase permissiveness of resting memory CD4+ T cells to HIV-1 infection: a novel model of HIV-1 latency. Blood. Dec 15 2007;110(13):4161-4. doi:10.1182/blood-2007-
06-097907
14.    Barton K, Winckelmann A, Palmer S. HIV-1 Reservoirs During Suppressive Therapy. Trends Microbiol. May 2016;24(5):345-355. doi:10.1016/j.tim.2016.01.006
15.    Fletcher CV, Staskus K, Wietgrefe SW, et al. Persistent HIV-1 replication is associated with lower        antiretroviral    drug    concentrations    in lymphatic tissues. Proc Natl Acad Sci U S A. Feb 11 2014;111(6):2307-12. doi:10.1073/pnas.1318249111
16.    Kearney MF, Wiegand A, Shao W, et al. Ongoing HIV Replication During ART Reconsidered. Open Forum Infect Dis. Summer 2017;4(3):ofx173. doi:10.1093/ofid/ofx173
17.    Sigal A, Kim JT, Balazs AB, et al. Cell-to- cell spread of HIV permits ongoing replication despite antiretroviral therapy. Nature. Aug 17 2011;477(7362):95-8. doi:10.1038/nature10347
18.    Bracq L, Xie M, Benichou S, Bouchet J. Mechanisms for Cell-to-Cell Transmission of HIV-1. Front Immunol. 2018;9:260. doi:10.3389/fimmu.2018.00260
19.    Dufloo J, Bruel T, Schwartz O. HIV-1 cell- to-cell transmission and broadly neutralizing antibodies. Retrovirology. Jul 28 2018;15(1):51. doi:10.1186/s12977-018-0434-1
20.    Do T, Murphy G, Earl LA, et al. Three- dimensional imaging of HIV-1 virological synapses reveals membrane architectures involved in virus transmission. J Virol. Sep 2014;88(18):10327-39. doi:10.1128/JVI.00788-14
21.    Zhong P, Agosto LM, Ilinskaya A, et al. Cell- to-cell transmission can overcome multiple donor and target cell barriers imposed on cell-free HIV. PLoS One. 2013;8(1):e53138. doi:10.1371/journal.pone.0053138
22.    Dutartre H, Claviere M, Journo C, Mahieux
R. Cell-Free versus Cell-to-Cell Infection by Human Immunodeficiency Virus Type 1 and Human T- Lymphotropic Virus Type 1: Exploring the Link among Viral Source, Viral Trafficking, and Viral Replication. J Virol. Sep 1 2016;90(17):7607-17. doi:10.1128/JVI.00407-16
23.    Rappocciolo G, Sluis-Cremer N, Rinaldo CR. Efficient HIV-1 Trans Infection of CD4(+) T Cells Occurs in the Presence of Antiretroviral Therapy. Open Forum Infect Dis. Jul 2019;6(7):ofz253. doi:10.1093/ofid/ofz253

24.    Jiang AP, Jiang JF, Wei JF, et al. Human Mucosal Mast Cells Capture HIV-1 and Mediate Viral trans-Infection of CD4+ T Cells. J Virol. Dec 30 2015;90(6):2928-37.cdoi:10.1128/JVI.03008-15
25.    Jiang AP, Jiang JF, Guo MG, Jin YM, Li YY, Wang JH. Human Blood-Circulating Basophils Capture HIV-1 and Mediate Viral trans-Infection of CD4+ T Cells. J Virol. Aug 2015;89(15):8050-62. doi:10.1128/JVI.01021-15
26.    DeLucia DC, Rinaldo CR, Rappocciolo G. Inefficient HIV-1 trans Infection of CD4(+) T Cells by Macrophages    from    HIV-1    Nonprogressors    Is Associated with Altered Membrane Cholesterol and DC-SIGN. J Virol. Jul 1 2018;92(13) doi:10.1128/JVI.00092-18
27.    Gerberick A, DeLucia DC, Piazza P, et al. B Lymphocytes, but Not Dendritic Cells, Efficiently HIV-1 Trans Infect Naive CD4(+) T Cells: Implications for the Viral Reservoir. mBio. Mar 9 2021;12(2)doi:10.1128/mBio.02998-20
28.    Ahmed Z, Kawamura T, Shimada S, Piguet
V. The role of human dendritic cells in HIV-1 infection. J Invest Dermatol. May 2015;135(5):1225-1233. doi:10.1038/jid.2014.490
29.    Rinaldo CR. HIV-1 Trans Infection of CD4(+) T Cells by Professional Antigen Presenting Cells. Scientifica (Cairo). 2013;2013:164203. doi:10.1155/2013/164203
30.    Wu L, KewalRamani VN. Dendritic-cell interactions with HIV: infection and viral dissemination. Nat Rev Immunol. Nov 2006;6(11):859-68. doi:10.1038/nri1960
31.    Manches O, Frleta D, Bhardwaj N. Dendritic cells in progression and pathology of HIV infection. Trends Immunol. Mar 2014;35(3):114-22. doi:10.1016/j.it.2013.10.003
32.    Kijewski SD, Gummuluru S. A mechanistic overview of dendritic cell-mediated HIV-1 trans infection: the story so far. Future Virol. Mar 2015;10(3):257-269. doi:10.2217/fvl.15.2
33.    McIlroy D, Autran B, Cheynier R, et al. Infection frequency of dendritic cells and CD4+ T lymphocytes in spleens of human immunodeficiency virus-positive patients. J Virol. Aug 1995;69(8):4737-45. doi:10.1128/JVI.69.8.4737-4745.1995
34.    Turville SG, Santos JJ, Frank I, et al. Immunodeficiency virus uptake, turnover, and 2- phase transfer in human dendritic cells. Blood. Mar 15 2004;103(6):2170-9. doi:10.1182/blood- 2003-09-3129
35.    Kulkarni R, Prasad A. Exosomes Derived from HIV-1 Infected DCs Mediate Viral trans- Infection via Fibronectin and Galectin-3. Sci Rep.cNov 1 2017;7(1):14787. doi:10.1038/s41598=017-14817-8
36.    Pino M, Erkizia I, Benet S, et al. HIV-1 immune activation induces Siglec-1 expression and enhances viral trans-infection in blood and tissue myeloid cells. Retrovirology. May 7 2015;12:37. doi:10.1186/s12977-015-0160-x
37.    Izquierdo-Useros N, Lorizate M, Puertas MC, et al. Siglec-1 is a novel dendritic cell receptor that    mediates    HIV-1    trans-infection    through recognition of viral membrane gangliosides. PLoS Biol. 2012;10(12):e1001448. doi:10.1371/journal.pbio.1001448
38.    Geijtenbeek TB, van Kooyk Y. DC-SIGN: a novel HIV receptor on DCs that mediates HIV-1 transmission. Curr Top Microbiol Immunol. 2003;276:31-54.          doi:10.1007/978-3-662- 06508-2_2
39.    Geijtenbeek TB, Kwon DS, Torensma R, et al. DC-SIGN, a dendritic cell-specific HIV-1-binding protein that enhances trans-infection of T cells. Cell. Mar 3 2000;100(5):587-97. doi:10.1016/s0092- 8674(00)80694-7
40.    Lambert AA, Gilbert C, Richard M, Beaulieu AD, Tremblay MJ. The C-type lectin surface receptor DCIR acts as a new attachment factor for HIV-1 in dendritic cells and contributes to trans- and cis-infection pathways. Blood. Aug 15 2008;112(4):1299-307. doi:10.1182/blood- 2008-01-136473
41.    Geijtenbeek TB, van Kooyk Y. Pathogens target DC-SIGN to influence their fate DC-SIGN functions as a pathogen receptor with broad specificity. APMIS. Jul-Aug 2003;111(7-8):698- 714.    doi:10.1034/j.1600- 0463.2003.11107803.x
42.    Geijtenbeek TB, van Vliet SJ, van Duijnhoven GC, Figdor CG, van Kooyk Y. DC-SIGN, a dentritic cell-specific HIV-1 receptor present in placenta that infects T cells in trans-a review. Placenta. Apr 2001;22 Suppl A:S19-23. doi:10.1053/plac.2001.0674
43.    Izquierdo-Useros N, Lorizate M, McLaren PJ, Telenti A, Krausslich HG, Martinez-Picado J. HIV- 1 capture and transmission by dendritic cells: the role of viral glycolipids and the cellular receptor Siglec-1. PLoS Pathog. Jul 2014;10(7):e1004146. doi:10.1371/journal.ppat.1004146
44.    Curtis BM, Scharnowske S, Watson AJ. Sequence and expression of a membrane- associated C-type lectin that exhibits CD4- independent binding of human immunodeficiency virus envelope glycoprotein gp120. Proc Natl Acad Sci U S A. Sep 1 1992;89(17):8356-60. doi:10.1073/pnas.89.17.8356

45.    Geijtenbeek TB, Engering A, Van Kooyk Y. DC-SIGN, a C-type lectin on dendritic cells that unveils many aspects of dendritic cell biology. J Leukoc Biol. Jun 2002;71(6):921-31.
46.    Yu HJ, Reuter MA, McDonald D. HIV traffics through        a    specialized,    surface-accessible intracellular compartment during trans-infection of T cells by mature dendritic cells. PLoS Pathog. Aug 22 2008;4(8):e1000134. doi:10.1371/journal.ppat.1000134
47.    Garcia E, Pion M, Pelchen-Matthews A, et al. HIV-1 trafficking to the dendritic cell-T-cell infectious synapse uses a pathway of tetraspanin sorting to the immunological synapse. Traffic. Jun 2005;6(6):488-501.    doi:10.1111/j.1600- 0854.2005.00293.x
48.    Relloso M, Puig-Kroger A, Pello OM, et al. DC-SIGN (CD209) expression is IL-4 dependent and is negatively regulated by IFN, TGF-beta, and anti-inflammatory agents. J Immunol. Mar 15 2002;168(6):2634-43. doi:10.4049/jimmunol.168.6.2634
49.    Puryear WB, Akiyama H, Geer SD, et al. Interferon-inducible mechanism of dendritic cell- mediated HIV-1 dissemination is dependent on Siglec-1/CD169. PLoS Pathog. 2013;9(4):e1003291. doi:10.1371/journal.ppat.1003291
50.    Hatch SC, Archer J, Gummuluru S. Glycosphingolipid composition of human immunodeficiency virus type 1 (HIV-1) particles is a crucial determinant for dendritic cell-mediated HIV-
1 trans-infection. J Virol. Apr 2009;83(8):3496- 506. doi:10.1128/JVI.02249-08
51.    Izquierdo-Useros N, Blanco J, Erkizia I, et al. Maturation of blood-derived dendritic cells enhances human immunodeficiency virus type 1 capture and transmission. J Virol. Jul 2007;81(14):7559-70. doi:10.1128/JVI.02572- 06
52.    Perez-Zsolt D, Raich-Regue D, Munoz- Basagoiti J, et al. HIV-1 trans-Infection Mediated by DCs: The Tip of the Iceberg of Cell-to-Cell Viral Transmission. Pathogens. Dec 31
2021;11(1)doi:10.3390/pathogens11010039
53.    Perez-Zsolt D, Cantero-Perez J, Erkizia I, et al. Dendritic Cells From the Cervical Mucosa Capture and Transfer HIV-1 via Siglec-1. Front Immunol.    2019;10:825. doi:10.3389/fimmu.2019.00825
54.    Sewald X, Ladinsky MS, Uchil PD, et al. Retroviruses use CD169-mediated trans-infection of permissive lymphocytes to establish infection. Science. Oct 30 2015;350(6260):563-567. doi:10.1126/science.aab2749
55.    Akiyama H, Ramirez NG, Gudheti MV, Gummuluru S. CD169-mediated trafficking of HIV to plasma membrane invaginations in dendritic cells attenuates    efficacy    of    anti-gp120    broadly neutralizing        antibodies.    PLoS    Pathog.    Mar 2015;11(3):e1004751. doi:10.1371/journal.ppat.1004751
56.    Lore K, Smed-Sorensen A, Vasudevan J, Mascola JR, Koup RA. Myeloid and plasmacytoid dendritic cells transfer HIV-1 preferentially to antigen-specific CD4+ T cells. J Exp Med. Jun 20 2005;201(12):2023-33. doi:10.1084/jem.20042413
57.    Lee B, Sharron M, Montaner LJ, Weissman D, Doms RW. Quantification of CD4, CCR5, and CXCR4 levels on lymphocyte subsets, dendritic cells, and differentially conditioned monocyte-derived macrophages. Proc Natl Acad Sci. 1999;96
58.    Rappocciolo G, Piazza P, Fuller CL, et al. DC-SIGN on B lymphocytes is required for transmission of HIV-1 to T lymphocytes. PLoS Pathog. Jul 2006;2(7):e70. doi:10.1371/journal.ppat.0020070
59.    Moir S, Malaspina A, Li Y, et al. B cells of HIV-1-infected patients bind virions through CD21- complement interactions and transmit infectious virus to activated T cells. J Exp Med. Sep 4 2000;192(5):637-46. doi:10.1084/jem.192.5.637
60.    Malaspina A, Moir S, Nickle DC, et al. Human immunodeficiency virus type 1 bound to B cells: relationship to virus replicating in CD4+ T cells and    circulating    in    plasma.    J    Virol.    Sep 2002;76(17):8855-63. doi:10.1128/jvi.76.17.8855-8863.2002
61.    Jakubik JJ, Saifuddin M, Takefman DM, Spear GT. Immune complexes containing human immunodeficiency virus type 1 primary isolates bind to lymphoid tissue B lymphocytes and are infectious for T lymphocytes. J Virol. Jan 2000;74(1):552-5. doi:10.1128/jvi.74.1.552-555.2000
62.    Dopper S, Wilflingseder D, Prodinger WM, et al. Mechanism(s) promoting HIV-1 infection of primary unstimulated T lymphocytes in autologous B cell/T    cell    co-cultures.    Eur    J    Immunol.    Aug 2003;33(8):2098-107. doi:10.1002/eji.200323932
63.    Jakubik JJ, Saifuddin M, Takefman DM, Spear GT. B lymphocytes in lymph nodes and peripheral blood are important for binding immune complexes containing HIV-1. Immunology. Apr 1999;96(4):612-9. doi:10.1046/j.1365-2567.1999.00304.x
64.    Schnittman SM, Lane HC, Greenhouse J, Justement JS, Baseler M, Fauci AS. Preferential infection of CD4+ memory T cells by human immunodeficiency virus type 1: evidence for a role in the selective T-cell functional defects observed in infected individuals. Proc Natl Acad Sci U S A. Aug 1990;87(16):6058-62. doi:10.1073/pnas.87.16.6058
65.    Zerbato JM, Serrao E, Lenzi G, et al. Establishment and Reversal of HIV-1 Latency in Naive and Central Memory CD4+ T Cells In Vitro. J Virol. Sep 15 2016;90(18):8059-73. doi:10.1128/JVI.00553-16
66.    Zerbato JM, McMahon DK, Sobolewski MD, Mellors JW, Sluis-Cremer N. Naive CD4+ T Cells Harbor a Large Inducible Reservoir of Latent, Replication-competent    Human    Immunodeficiency Virus    Type    1.    Clin        Infect        Dis.    Nov    13 2019;69(11):1919-1925. doi:10.1093/cid/ciz108
67.    Venanzi Rullo E, Cannon L, Pinzone MR, Ceccarelli M, Nunnari G, O Doherty U. Genetic Evidence That Naive T Cells Can Contribute Significantly to the Human Immunodeficiency Virus Intact Reservoir: Time to Re-evaluate Their Role. Clin Infect Dis. Nov 27 2019;69(12):2236-2237. doi:10.1093/cid/ciz378
68.    Venanzi Rullo E, Pinzone MR, Cannon L, et al. Persistence of an intact HIV reservoir in phenotypically naive T cells. JCI Insight. Oct 15 2020;5(20)doi:10.1172/jci.insight.133157
69.    Pinzone MR, Weissman S, Pasternak AO, Zurakowski R, Migueles S, O Doherty U. Naive infection predicts reservoir diversity and is a formidable hurdle to HIV eradication. JCI Insight. Aug 23 2021;6(16)doi:10.1172/jci.insight.150794
70.    Ostrowski MA, Chun TW, Justement SJ, et al. Both memory and CD45RA+/CD62L+ naive CD4(+) T cells are infected in human immunodeficiency virus type 1-infected individuals. J Virol. Aug 1999;73(8):6430-5.
71.    Wightman F, Solomon A, Khoury G, et al. Both CD31(+) and CD31(-) naive CD4(+) T cells are persistent    HIV    type    1-infected    reservoirs    in individuals receiving antiretroviral therapy. J Infect Dis. Dec 1 2010;202(11):1738-48. doi:10.1086/656721
72.    Margolis L, Shattock R. Selective transmission of CCR5-utilizing HIV-1: the gatekeeper problem resolved? Nat Rev Microbiol. Apr 2006;4(4):312-7. doi:10.1038/nrmicro1387
73.    Grivel JC, Shattock RJ, Margolis LB. Selective transmission of R5 HIV-1 variants: where is the gatekeeper? J Transl Med. Jan 27 2011;9 Suppl 1:S6. doi:10.1186/1479-5876-9-S1-S6
74.    Crotty S. T Follicular Helper Cell Biology: A Decade of Discovery and Diseases. Immunity. May 21 2019;50(5):1132-1148. doi:10.1016/j.immuni.2019.04.011 
75.    Crotty S. T follicular helper cell differentiation, function, and roles in disease. Immunity. Oct 16 2014;41(4):529-42. doi:10.1016/j.immuni.2014.10.004
76.    Cyster JG. B cell follicles and antigen encounters of the third kind. Nat Immunol. Nov 2010;11(11):989-96. doi:10.1038/ni.1946
77.    Cyster JG, Allen CDC. B Cell Responses: Cell Interaction Dynamics and Decisions. Cell. Apr 18    2019;177(3):524-540. doi:10.1016/j.cell.2019.03.016
78.    Xu Y, Phetsouphanh C, Suzuki K, et al. HIV- 1 and SIV Predominantly Use CCR5 Expressed on a Precursor Population to Establish Infection in T Follicular Helper Cells. Front Immunol. 2017;8:376. doi:10.3389/fimmu.2017.00376
79.    Bronnimann MP, Skinner PJ, Connick E. The B-Cell Follicle in HIV Infection: Barrier to a Cure. Front Immunol. 2018;doi:10.3389/
80.    Rabezanahary H, Moukambi F, Palesch D, et al. Despite early antiretroviral therapy effector memory and follicular helper CD4 T cells are major reservoirs in visceral lymphoid tissues of SIV- infected macaques. Mucosal Immunol. Jan 2020;13(1):149-160. doi:10.1038/s41385-019- 0221-x
81.    Zaunders J, Xu Y, Kent SJ, Koelsch KK, Kelleher AD. Divergent Expression of CXCR5 and CCR5 on CD4(+) T Cells and the Paradoxical Accumulation of T Follicular Helper Cells during HIV Infection. Front Immunol. 2017;8:495. doi:10.3389/fimmu.2017.00495
82.    Moukambi F, Rodrigues V, Fortier Y, et al. CD4 T Follicular Helper Cells and HIV Infection: Friends or Enemies? Front Immunol. 2017;8:135. doi:10.3389/fimmu.2017.00135
83.    Perreau M, Savoye AL, De Crignis E, et al. Follicular helper T cells serve as the major CD4 T cell compartment for HIV-1 infection, replication, and    production.    J    Exp    Med.    Jan    14 2013;210(1):143-56. doi:10.1084/jem.20121932
84.    Cannon G, Yi Y, Ni H, et al. HIV envelope binding        by    macrophage-expressed    gp340 promotes HIV-1 infection. J Immunol. Aug 1 2008;181(3):2065-70. doi:10.4049/jimmunol.181.3.2065
85.    Shen R, Richter HE, Clements RH, et al. Macrophages in vaginal but not intestinal mucosa are monocyte-like and permissive to human immunodeficiency virus type 1 infection. J Virol. Apr 2009;83(7):3258-67. doi:10.1128/JVI.01796-08
86.    Duncan CJ, Sattentau QJ. Viral determinants of HIV-1 macrophage tropism. Viruses.    Nov    2011;3(11):2255-79. doi:10.3390/v3112255

87.    Peressin M, Proust A, Schmidt S, et al. Efficient transfer of HIV-1 in trans and in cis from Langerhans dendritic cells and macrophages to autologous    T    lymphocytes.    AIDS.    Mar    13 2014;28(5):667-77. doi:10.1097/QAD.0000000000000193
88.    Groot F, Welsch S, Sattentau QJ. Efficient HIV-1 transmission from macrophages to T cells across transient virological synapses. Blood. May 1 2008;111(9):4660-3. doi:10.1182/blood-2007- 12-130070
89.    Nguyen DG, Hildreth JE. Involvement of macrophage mannose receptor in the binding and transmission of HIV by macrophages. Eur J Immunol. Feb 2003;33(2):483-93. doi:10.1002/immu.200310024
90.    Sharova N, Swingler C, Sharkey M, Stevenson M. Macrophages archive HIV-1 virions for dissemination    in    trans.    EMBO    J.    Jul    6 2005;24(13):2481-9. doi:10.1038/sj.emboj.7600707
91.    Soilleux EJ, Morris LS, Leslie G, et al. Constitutive and induced expression of DC-SIGN on dendritic cell and macrophage subpopulations in situ and in vitro. J Leukoc Biol. Mar 2002;71(3):445- 57.
92.    Mikulak J, Teichberg S, Arora S, et al. DC- specific ICAM-3-grabbing nonintegrin mediates internalization of HIV-1 into human podocytes. Am J Physiol Renal Physiol. Sep 2010;299(3):F664-73. doi:10.1152/ajprenal.00629.2009
93.    Rappocciolo G, Jais M, Piazza P, et al. Alterations in cholesterol metabolism restrict HIV-1 trans infection in nonprogressors. mBio. Apr 29 2014;5(3):e01031-13. doi:10.1128/mBio.01031- 13
94.    Hanley TM, Viglianti GA. Nuclear receptor signaling inhibits HIV-1 replication in macrophages through multiple trans-repression mechanisms. J Virol.    Oct    2011;85(20):10834-50. doi:10.1128/JVI.00789-11
95.    Hanley TM, Blay Puryear W, Gummuluru S, Viglianti GA. PPARgamma and LXR signaling inhibit dendritic cell-mediated HIV-1 capture and trans- infection. PLoS Pathog. Jul 1 2010;6(7):e1000981. doi:10.1371/journal.ppat.1000981
96.    Graf EH, Mexas AM, Yu JJ, et al. Elite suppressors harbor low levels of integrated HIV DNA and high levels of 2-LTR circular HIV DNA compared to HIV+ patients on and off HAART. PLoS Pathog.    Feb    2011;7(2):e1001300. doi:10.1371/journal.ppat.1001300
97.    Kwaa AK, Garliss CC, Ritter KD, Laird GM, Blankson JN. Elite suppressors have low frequencies of intact HIV-1 proviral DNA. AIDS. Mar 15 2020;34(4):641-643. doi:10.1097/QAD.0000000000002474
98.    Julg B, Pereyra F, Buzon MJ, et al. Infrequent recovery of HIV from but robust exogenous infection of activated CD4(+) T cells in HIV elite controllers. Clin Infect Dis. Jul 15 2010;51(2):233-8. doi:10.1086/653677
99.    Izquierdo-Useros N, Naranjo-Gomez M, Archer J, et al. Capture and transfer of HIV-1 particles by mature dendritic cells converges with the exosome-dissemination pathway. Blood. Mar 19 2009;113(12):2732-41. doi:10.1182/blood- 2008-05-158642
100.    Chiozzini C, Arenaccio C, Olivetta E, et al. Trans-dissemination of exosomes from HIV-1- infected cells fosters both HIV-1 trans-infection in resting CD4(+) T lymphocytes and reactivation of the HIV-1 reservoir. Arch Virol. Sep 2017;162(9):2565-2577.   doi:10.1007/s00705- 017-3391-4
101.    Chen J, Li C, Li R, Chen H, Chen D, Li W. Exosomes in HIV infection. Curr Opin HIV AIDS. Sep 1    2021;16(5):262-270.
doi:10.1097/COH.0000000000000694
102.    Wiley    RD,    Gummuluru    S.    Immature dendritic cell-derived exosomes can mediate HIV-1 trans infection. Proc Natl Acad Sci U S A. Jan 17 2006;103(3):738-43. doi:10.1073/pnas.0507995103
103.    Patters BJ, Kumar S. The role of exosomal transport of viral agents in persistent HIV pathogenesis. Retrovirology. Dec 22
2018;15(1):79. doi:10.1186/s12977-018-0462-x
104.    Izquierdo-Useros N, Naranjo-Gomez M, Erkizia I, et al. HIV and mature dendritic cells: Trojan exosomes riding the Trojan horse? PLoS Pathog. Mar 26    2010;6(3):e1000740. doi:10.1371/journal.ppat.1000740
105.    Menager MM, Littman DR. Actin Dynamics Regulates Dendritic Cell-Mediated Transfer of HIV- 1 to T Cells. Cell. Feb 11 2016;164(4):695-709. doi:10.1016/j.cell.2015.12.036
106.    Nikolic DS, Lehmann M, Felts R, et al. HIV-1 activates Cdc42 and induces membrane extensions in immature dendritic cells to facilitate cell-to-cell virus propagation. Blood. Nov 3 2011;118(18):4841-52. doi:10.1182/blood- 2010-09-305417
107.    Aggarwal A, Iemma TL, Shih I, et al. Mobilization of HIV spread by diaphanous    2 dependent filopodia in infected dendritic cells. PLoS Pathog.    2012;8(6):e1002762. doi:10.1371/journal.ppat.1002762
108.    Eugenin EA, Gaskill PJ, Berman JW. Tunneling nanotubes (TNT) are induced by HIV- infection of macrophages: a potential mechanism for intercellular HIV trafficking. Cell Immunol. 2009;254(2):142-8.
doi:10.1016/j.cellimm.2008.08.005
109.    Hashimoto M, Bhuyan F, Hiyoshi M, et al. Potential Role of the Formation of Tunneling Nanotubes in HIV-1 Spread in Macrophages. J Immunol. Feb 15 2016;196(4):1832-41. doi:10.4049/jimmunol.1500845
110.    Kadiu I, Gendelman HE. Human immunodeficiency virus type 1 endocytic trafficking through macrophage bridging conduits facilitates spread of infection. J Neuroimmune Pharmacol. Dec 2011;6(4):658-75. doi:10.1007/s11481-011- 9298-z
111.    Kadiu I, Gendelman HE. Macrophage bridging conduit trafficking of HIV-1 through the endoplasmic reticulum and Golgi network. J Proteome Res. Jul 1 2011;10(7):3225-38. doi:10.1021/pr200262q
112.    Zaccard CR, Watkins SC, Kalinski P, et al. CD40L        induces    functional    tunneling    nanotube networks exclusively in dendritic cells programmed by mediators of type 1 immunity. J Immunol. Feb 1 2015;194(3):1047-56. doi:10.4049/jimmunol.1401832
113.    Jolly C, Kashefi K, Hollinshead M, Sattentau QJ. HIV-1 cell to cell transfer across an Env-induced, actin-dependent synapse. J Exp Med. Jan 19 2004;199(2):283-93. doi:10.1084/jem.20030648
114.    Jolly C, Sattentau QJ. Retroviral spread by induction of virological synapses. Traffic. Sep 2004;5(9):643-50.    doi:10.1111/j.1600- 0854.2004.00209.x
115.    Puigdomenech I, Massanella M, Izquierdo- Useros N, et al. HIV transfer between CD4 T cells does not require LFA-1 binding to ICAM-1 and is governed by the interaction of HIV envelope glycoprotein with CD4. Retrovirology. Mar 31 2008;5:32. doi:10.1186/1742-4690-5-32
116.    McDonald D. Dendritic Cells and HIV-1 Trans-Infection. Viruses. Aug 2010;2(8):1704-17. doi:10.3390/v2081704
117.    Kulpa DA, Brehm JH, Fromentin R, et al. The immunological synapse: the gateway to the HIV reservoir. Immunol Rev. Jul 2013;254(1):305-25. doi:10.1111/imr.12080
118.    Gonzalez SM, Aguilar-Jimenez W, Su RC, Rugeles MT. Mucosa: Key Interactions Determining Sexual Transmission of the HIV Infection. Front Immunol.    2019;10:144. doi:10.3389/fimmu.2019.00144
119.    Mbongue JC, Nieves HA, Torrez TW, Langridge WH. The Role of Dendritic Cell Maturation in the Induction of Insulin-Dependent Diabetes Mellitus. Front Immunol. 2017;8:327. doi:10.3389/fimmu.2017.00327
120.    Hampton HR, Chtanova T. Lymphatic Migration of Immune Cells. Front Immunol. 2019;10:1168. doi:10.3389/fimmu.2019.01168
121.    Louie DAP, Liao S. Lymph Node Subcapsular Sinus Macrophages as the Frontline of Lymphatic Immune Defense. Front Immunol. 2019;10:347. doi:10.3389/fimmu.2019.00347
122.    de Winde CM, Munday C, Acton SE. Molecular mechanisms of dendritic cell migration in immunity and cancer. Med Microbiol Immunol. Aug 2020;209(4):515-529.       doi:10.1007/s00430- 020-00680-4
123.    Gray EE, Cyster JG. Lymph node macrophages. J Innate Immun. 2012;4(5-6):424- 36. doi:10.1159/000337007
124.    Camara A, Lavanant AC, Abe J, et al. CD169(+) macrophages in lymph node and spleen critically depend on dual RANK and LTbetaR signaling. Proc Natl Acad Sci U S A. Jan 18 2022;119(3)doi:10.1073/pnas.2108540119
125.    He B, Qiao X, Klasse PJ, et al. HIV-1 envelope triggers polyclonal Ig class switch recombination through a CD40-independent mechanism involving BAFF and C-type lectin receptors. J Immunol. Apr 1 2006;176(7):3931-41. doi:10.4049/jimmunol.176.7.3931
126.    Borhis G, Trovato M, Chaoul N, Ibrahim HM, Richard Y. B-Cell-Activating Factor and the B-Cell Compartment in HIV/SIV Infection. Front Immunol. 2017;8:1338. doi:10.3389/fimmu.2017.01338
127.    Banga R, Procopio FA, Noto A, et al. PD- 1(+) and follicular helper T cells are responsible for persistent HIV-1 transcription in treated aviremic individuals. Nat Med. Jul 2016;22(7):754-61. doi:10.1038/nm.4113
128.    Weber JP, Fuhrmann F, Hutloff A. T- follicular helper cells survive as long-term memory cells. Eur J Immunol. Aug 2012;42(8):1981-8. doi:10.1002/eji.201242540