Hiv targets what type of cells

An Erratum to this article was published on 08 February Current antiretroviral therapies have improved the duration and quality of life of people living with HIV However, viral reservoirs impede complete eradication of the virus. Although there are many strategies to eliminate infectious virus, the most actively pursued are latency reversing agents in conjunction with immune modulation.

This is in part because of the dearth of conclusive evidence about the existence of non-T cell reservoirs. Studies of non-T cell reservoirs have been difficult to interpret because of technical and biological issues that have hampered a better understanding. This review considers the current knowledge of non-T cell reservoirs, the challenges encountered in a better understanding of these populations, and their implications for HIV-1 cure research.

However, whereas ART is remarkably effective at preventing new cells from becoming infected, it does not eliminate long-lived cells that are already infected prior to ART initiation. Latent reservoirs have thwarted attempts to eliminate all replication competent forms of the virus from infected individuals [ 1 , 2 , 3 , 4 , 5 , 6 ]. There is reason for balanced optimism in the HIV-1 cure field.

Central to each case of a potential cure or ART-free remission has been a reduction in the size of the HIV-1 reservoir. Therefore, it is critical for cure strategies to target all potential reservoirs. Many cells are susceptible to HIV-1 in vitro, but not all potential reservoirs have been studied in vivo during ART with the same rigor. For cells to constitute an HIV-1 reservoir, they have to harbor replication competent forms of the virus that persist for years despite long-term ART suppression of viremia [ 14 ].

Against the standard of the T cell reservoir, in this review we consider evidence suggesting the possible long-term persistence of non-T cell reservoirs in individuals on ART, and the current challenges involved in their identification. Viral latency is defined as a reversible nonproductive state of infection in individual cells [ 15 ]. Reservoirs are cells that harbor replicative forms of HIV-1 following long periods of ART-suppressed viremia [ 14 , 16 ]. The phases of plasma HIV-1 RNA decline on ART have been attributed to infection of different cell types that are infected by the virus, and much has been inferred about the identities of those cells without clear evidence Fig.

Here, we enumerate several candidate cell types that could potentially serve as HIV-1 reservoirs Table 1. The multiphasic decay in plasma viremia following initiation of ART has been attributed to the varying half-life of infected cells. The slower second phase during which viremia becomes undetectable is contributed to by cells with a half-life in the order of weeks.

The cells contributing to the second phase have not been conclusively identified. Found primarily in tissues, macrophages are mononuclear leukocytes that are key components of innate immunity. For decades, the origin of tissue resident macrophages TRM was explained by the concept of the mononuclear-phagocyte system: monocytes were thought to continually replenish TRM that died in tissues [ 34 , 35 ].

Consistent with this early concept, the death of HIV-1 infected macrophages was thought to be responsible for the second phase of HIV-1 viral kinetic decline during ART. However, recent findings based on murine models suggest that the principal origin of TRM in steady state is from embryonic haematopoietic precursors, while monocytes only contribute in the setting of inflammation and injury [ 36 ].

Similarly, detection of TRM even in individuals with monocytopenia suggests monocyte-independent maintenance, a long half-life of embryonically derived macrophages, or likely a combination of both [ 37 ]. Studies in patients who received lung transplantation have also shown long-term persistence of donor alveolar macrophages [ 32 ].

In parallel, the rapid second phase decline of HIV-1 was found not to be attributable to macrophages [ 38 ]. Taken together, these findings have led to a marked revision in our understanding of the maintenance and longevity of TRM. It is well established in animal models and in vitro that macrophages can be productively infected by lab strains of HIV-1 [ 39 , 40 ], although there may be anatomical variation in their susceptibility to HIV-1 infection.

Vaginal macrophages have been shown to support HIV-1 replication better than intestinal macrophages, which may be explained by differential expression of entry co-receptors [ 43 ]. Comparative in situ fluorescence also suggests higher HIV-1 susceptibility of rectal macrophages compared to colonic macrophages [ 44 ].

Cai et al. Our lab has extended earlier studies of liver macrophages Kupffer cells , the largest population of TRM in the body, to show that these cells can harbor virus from individuals on ART for as long as 11 years, although their functional significance is still unclear [ 25 ]. Other tissue macrophages that have also been implicated as harboring HIV-1 include those in the seminal vesicle, duodenum, urethra, adipose tissue, and liver [ 25 , 46 , 52 , 53 , 54 , 55 ].

The study of HIV-1 infection of macrophages is not without controversy. Recent in vivo data from an SIV macaque model has demonstrated the presence of both proviral DNA and T cell receptors TCR in myeloid cells: the authors concluded that the presence of viral DNA in macrophages was due to phagocytosis of infected dying cell rather than de novo infection of myeloid cells [ 56 ]. However, a subsequent report by Baxter et al.

Thus it is important to differentiate between phagocytosis and actual infection of macrophages following detection of nucleic acids in macrophages. Monocytes, closely related myeloid cells, were initially reported as being infected in vivo; however, it has now been shown that monocytes are not susceptible to HIV-1, and largely lack proviral HIV-1 DNA in both viremic and ART suppressed individuals [ 24 , 60 ].

Dendritic cells DCs are a heterogeneous group of antigen-presenting cells that play vital roles in orchestrating immune responses [ 61 ]. DCs can be broadly divided into those of myeloid or lymphoid origin [ 62 ], and further categorized as plasmacytoid pDCs , myeloid mDCs , Langerhans cells found in the epidermis , and interstitial [ 63 ]. Although DCs comprise a small proportion of cells in various anatomical sites [ 64 ], their role as immunologic sentries makes them among the first cells that encounter invading pathogens like HIV There have been several reports of productive HIV-1 infection of DCs in vitro for as long as 45 days [ 72 , 73 , 74 , 75 ], but limited data in vivo.

Langerhans cells have been considered as a potential reservoir, but largely based on data in the pre-ART era [ 76 , 77 ]. During trans infection, compartmentalized HIV-1 has been observed to emerge from DCs and fuse with the T cell membrane [ 81 ]. Envelope specific inhibitors maintain their potency against these compartmentalized virions [ 81 ]. These are tantalizing hypotheses that have been difficult to find evidence for in vivo.

Follicular dendritic cells FDCs that are found in B cell follicles in secondary lymphoid organs are not typical DCs, although they are similarly named: FDCs develop from perivascular precursors of stromal cell origin and are not known to present antigens using MHC-restricted pathways [ 26 , 64 ].

FDCs can potentially serve as viral reservoirs by maintaining a stable pool of HIV-1 on their surface without being infected [ 82 , 83 ]. In vitro studies have revealed that HIV-1 virions adhere on the surface of FDCs through interactions with complement receptors mediated via a C3-dependent mechanism [ 84 ].

HIV-1 has been known to persist on these cells even in the presence of neutralizing antibodies, with reports suggesting that FDCs can restore the infectivity of neutralized viruses [ 85 , 86 ]. FDCs transfer antigens in the B cell follicles of all secondary lymphoid tissues, and in the process may transfer HIV-1 to T follicular helper cells that are also present in the B cell follicles [ 21 ].

In mice, FDCs have been shown to trap HIV-1 following a single exposure, and these virions remained infectious for at least 9 months [ 85 ].

Mathematical models have suggested that FDCs are the major contributor to the low-level viremia detected during the third phase of viral decay, and have been estimated to have a half-life of 39 months [ 22 ].

There have been reports suggesting the possible infection and transmission of infection by epithelial cells even though they do not express CD4 and have undetectable or low expression of the co-receptors CCR5and CXCR4 [ 88 , 89 ]. Renal epithelial cells have been reported to be susceptible to HIV-1 in vitro [ 90 ].

Cultures of renal tubule epithelial cells were productively infected by HIV-1 following co-culture with infected T cells [ 90 ]. Transmission of infection was observed to occur by formation of virological synapses [ 91 ]. Phylogenetic analyses of sequences obtained from renal epithelial cells were found to cluster together within the radiation of sequences obtained from peripheral blood mononuclear cells [ 93 ]. These cells could play a role in persistence of HIV-1 infection in individuals on ART based on indirect evidence [ 94 , 95 ].

Mammary epithelial cells have been conjectured to harbor a separate compartment of HIV phlyogenetic analyses of HIV-1 DNA from paired breast-milk and peripheral blood samples from HIV-1 infected women have shown the existence of genetically distinct compartments [ 96 , 97 ].

Similar to DCs, oral keratinocytes have been shown to support transmission of virus to susceptible cells without supporting replication [ , ]. However there is no evidence that these cells serve as HIV-1 reservoirs, and there are no published data on the half-life of epithelial cells in vivo in this context. Kong et al. In addition, hepatic stellate cells have also been shown to release infectious virus following infection in vitro [ ].

However, the translation of this research to studies of in vivo reservoirs has been more challenging, and data are lacking. There have been isolated reports of other cells that can possibly be infected with HIV However, there are no data on whether fibrocytes are HIV-1 infected in vivo [ ].

Hematopoietic progenitor cells HPCs that were initially reported to harbor infectious virus are now not considered to fulfill the criteria to be a reservoir following development of enhanced techniques to purify HSCs from bone marrow [ , ]. Estimation of these numbers in ART-suppressed individuals requires isolation of millions of cells from large volume blood draws [ ]. Similar studies on cells from HIV-1 infected people that have low or absent numbers in circulation, or that are principally found in tissues, have been technically challenging or unethical [ 25 , 51 ].

The potency of the QVOA is that it hinges on the recovery of infectious, replication competent HIV-1 that propagates exponentially, plausibly explaining the virological rebound seen in patients who discontinue ART. The QVOA is a highly consistent assay, but nonetheless poses a number of technical challenges, including that it is expensive, time-consuming, requires large amounts of starting materials, has a limited dynamic range, and underestimates the size of the latent reservoir [ , , ].

Several groups have employed PCR-based approaches as alternative tools [ 23 ]. Although easier, PCR-based approaches do not differentiate between replication competent and defective viruses, of which the latter constitute the majority of viral forms, and do not correlate well with the number of cells with replication competent virus [ 13 ].

PCR-based approaches typically yield infected cell frequencies that are — times higher than what is resulted from the QVOA [ 23 ].

This assay has a quick turnaround time and requires fewer than a million cells of starting material. However, the TILDA does not measure virus production and does not address whether measured RNAs derive from replication competent viruses [ , ]. Therefore, as of now the most accurate measurement of the replication competent viral reservoir requires the QVOA, limiting the quantification of HIV-1 reservoirs in tissues that are poorly accessible.

To address the challenges posed in isolating a large number of these cells to study latency, the field has resorted to the use of alternate models that complement each other—in vitro, animal, and mathematical models [ 22 , 58 , , ]. Although more feasible, these approaches have their drawbacks. In vitro models are used frequently because of their convenience, but do not fully mimic in vivo infections. Similarly, heterogeneous cell phenotypes can be observed in in vitro models, such as in monocyte-derived macrophages MDMs subpopulations [ , , ].

Fundamentally, HIV-1 susceptibility and longevity in vitro may be quite different than in the immunological context of natural infection. Hence, in vitro modeling can only be used to complement findings in vivo. Non-human primates NHP and humanized mice models have been invaluable for understanding HIV-1 pathogenesis [ 24 , 27 , 58 ]. Instead, they encode for vpx , which may be a critical difference: vpx degrades SAM and HD domain containing deoxynucleoside triphosphate triphosphohydrolase 1 SAMHD1 , a key retroviral restriction factor in macrophages and DCs [ , ].

Nevertheless, SIV infection of NHP remains a key experimental tool, especially for in vivo and ex vivo studies of tissues that are inaccessible in humans, such as the brain.

Recent advances in humanized mouse technology have facilitated their infection with HIV-1 [ , , ]. However, a major hurdle impeding more widespread use of humanized mice is that each experiment requires the surgical engraftment of human tissue, since this aspect cannot be bred [ ]. A promising and creative use of humanized mice is in the development of a murine viral outgrowth assay where HIV-1 latency is estimated by adoptive transfer of human cells into humanized mice [ ].

Whereas promising improvements to antiretroviral therapy have improved the quality of life of PLWH, they have not bridged the gap toward an HIV-1 cure [ ]. Although it has been debated whether resources for HIV-1 research should be focused on a cure when there are other challenges facing PLWH, we argue that latent reservoirs harbor the potential for high-level virologic rebound in each of the 37 million HIV-1 infected people worldwide, which bears both individual and public harm.

Dendritic cells DCs. DCs are large cells with dendritic cytoplasmic extensions. These cells present processed antigens to T lymphocytes in lymph nodes. These cells transport HIV from the site of infection to lymphoid tissue. The follicular DCs, found in lymphoid tissue, are also key antigen-presenting cells that trap and present antigens on their cell surfaces. In the lymph node follicles, DCs provide signals for the activation of B lymphocytes.

Natural killer NK cells. NK cells have lytic activity against cells that have diminished expression of m ajor h istocompatibility c omplex MHC I antigens. NK cells proliferate in response to type 1 interferon secreted by DCs. Cellular immune response to HIV. The cellular immune response is induced upon the entry of HIV into the target cells e. This results in declining viraemia after primary infection.

Humoral response to HIV. The humoral immune response occurs later in infection; therefore, the level of antibodies during the acute infection is very low. Non-neutralising antibodies to structural proteins i. P17 and P24 are first to appear and generally do not persist. Later neutralising antibodies specific to proteins, involved in the entry of the virus into the cells, will be generated.

These antibodies are specific to: 1 the variable region of gp V3 ; 2 CD4 binding sites and chemokine receptors i. There are various reasons which can contribute to the failure of the immune system to control HIV infection and prevent AIDS development. Antigenic mutation within the T-cell epitopes can affect the binding capacity of MHC molecules to the viral peptides, resulting in the inability of the TCRs to recognise the MHC-peptide complex.

Fifty-four participants were enrolled in the Nairobi-based studies. Of these 54 participants, HIV entry into cervical T-cell targets was assessed in 41 participants median age, 28 years; range, 19—46 years , and 13 participants were enrolled in studies of mucosal Ki67 expression 30 years; 19—39 years.

In blood, median virus entry was 4. Bottom rows in a and b show representative cytosolic HIV entry or blue vs. Graph shows median values, and statistical comparisons are done by Wilcoxon rank-pairs test. PowerPoint slide. Next we compared expression of Ki67, a marker of metabolic activity and cell proliferation, between blood and cervical compartments, as metabolically active cells are more likely to sustain productive HIV infection.

Ki67 expression was 1. Furthermore, median HIV entry dropped from Graphs show median values, and statistical comparisons are done by Wilcoxon rank-pairs test. Treatment with T20 Fuzeon , which prevents fusion of viral and host membranes, resulted in near-complete ablation of viral entry in PBMCs median 2.

However, the reverse transcriptase inhibitor lamivudine, which blocks the postviral entry step of reverse transcription of viral RNA into DNA, had no effect on HIV entry data not shown. Next, we assessed the association of integrin expression with cellular HIV entry. Graphs show median values and statistical comparisons are done by Wilcoxon rank-pairs test. In blood, HIV entry was 3.

First we confirmed proper folding of Qd2. In contrast, Qd2. Representative figure from three independent experiments is shown in a and b. Neither inhibitor had any impact on virus entry: median virus entry after Act-1 incubation was 3. Thus far, the data presented were obtained using BlaM-Vpr pseudotypes containing the Qd2.

HIV infection after sexual exposure is thought to be caused by the infection of a small number of highly susceptible mucosal target cells, 27 with subsequent slow local expansion leading to eventual systemic dissemination. However, rather than increased susceptibility of the latter subsets being due to any direct interaction of these integrins with the HIV envelope, our results suggest that increased virus entry was likely because of increased CCR5 expression in these subsets and potentially other as yet undefined factors.

However, our assessment of ex vivo HIV entry does have limitations. In addition, the complete inhibition of HIV entry by the fusion inhibitor T20 Fuzeon demonstrates that HIV entry was not because of nonspecific effects. Nevertheless, the detection of cellular HIV targets that appear to be CCR5 - by flow cytometry may in fact constitute an advantage of the BlaM-Vpr assay, as it can identify cells that might otherwise not be considered as HIV targets.

Nonetheless, Cavrois et al. Enhanced susceptibility of the latter subsets did not appear to be mediated by direct integrin—envelope interaction, and was related in part to increased CCR5 expression. Ethics statement. The described studies were conducted according to the principles expressed in the Declaration of Helsinki. Study participants. Female participants were recruited from two outpatient clinics in Nairobi, Kenya.

Recruited participants self-reported to be HIV negative, were not actively menstruating at the time of sample collection, and did not present with clinical signs and symptoms of genital inflammation. Sample collection and processing. PBMCs were isolated by Ficoll density separation and reconstituted at 10 7 cells per ml in complete media. As our primary goal was to develop an infection assay relevant for sexual transmission of HIV, the envelope used for pseudotyping the BlaM-Vpr virions was early transmitted, CCR5-tropic and Clade A, the most common clade in Kenya, 43 where we performed our mucosal studies.

As described elsewhere, the Qd2. For ex vivo infections, 10 6 PBMCs were incubated with BlaM-Vpr pseudovirus or phosphate-buffered saline negative control; mock infection. Processed cervical cell suspension from each participant was divided in two and similarly incubated with either virus or mock. For analysis of HIV entry within each individual, the same flow cytometry gating strategy used for blood was applied to cervix, with the exception of the gates denoting HIV entry, as these were not consistent between the two compartments.

Flow cytometry. Intracellular staining for Ki67 could not be performed on the same cervical samples used for the HIV entry assay because cell permeabilization, which is required for detection of Ki67, caused leakage of CCF2-AM.

All cellular events were recorded during flow cytometric analysis of cytobrush samples. Gp production and purification. Mammalian codon-optimized Qd2. Purified protein was analyzed by sodium dodecyl sulfate—polyacrylamide gel electrophoresis. Proper folding of the purified protein was assessed by immunoprecipitation for 1. Integrin-binding assay. Statistical analysis. Flow cytometry data were analyzed in FlowJo v.

Ashland, OR. Haase, A. Hladik, F. Setting the stage: host invasion by HIV. McKinnon, L. Passmore, J. Boston, MA: Levinson, P. Levels of innate immune factors in genital fluids: association of alpha defensins and LL with genital infections and increased HIV acquisition. AIDS 23 , — Kaul, R. AIDS 22 , — Nazli, A. Exposure to HIV-1 directly impairs mucosal epithelial barrier integrity allowing microbial translocation.