Research priorities for an HIV cure: International AIDS Society Global Scientific Strategy 2021

Despite the success of antiretroviral therapy (ART) for people living with HIV, lifelong treatment is required and there is no cure. HIV can integrate in the host genome and persist for the life span of the infected cell. These latently infected cells are not recognized as foreign because they are largely transcriptionally silent, but contain replication-competent virus that drives resurgence of the infection once ART is stopped. With a combination of immune activators, neutralizing antibodies, and therapeutic vaccines, some nonhuman primate models have been cured, providing optimism for these approaches now being evaluated in human clinical trials. In vivo delivery of gene-editing tools to either target the virus, boost immunity or protect cells from infection, also holds promise for future HIV cure strategies. In this Review, we discuss advances related to HIV cure in the last 5 years, highlight remaining knowledge gaps and identify priority areas for research for the next 5 years.

Langerhans Cell-Dendritic Cell Cross-Talk via Langerin and Hyaluronic Acid Mediates Antigen Transfer and Cross-Presentation of HIV-1

Human epidermal and mucosal Langerhans cells (LCs) express the C-type lectin receptor langerin that functions as a pattern recognition receptor. LCs are among the first immune cells to interact with HIV-1 during sexual transmission. In this study, we demonstrate that langerin not only functions as a pattern recognition receptor but also as an adhesion receptor mediating clustering between LCs and dendritic cells (DCs). Langerin recognized hyaluronic acid on DCs and removal of these carbohydrate structures partially abrogated LC-DC clustering. Because LCs did not cross-present HIV-1-derived Ags to CD8(+) T cells in a cross-presentation model, we investigated whether LCs were able to transfer Ags to DCs. LC-DC clustering led to maturation of DCs and facilitated Ag transfer of HIV-1 to DCs, which subsequently induced activation of CD8(+) cells. The rapid transfer of Ags to DCs, in contrast to productive infection of LCs, suggests that this might be an important mechanism for induction of anti-HIV-1 CD8(+) T cells. Induction of the enzyme hyaluronidase-2 by DC maturation allowed degradation of hyaluronic acid and abrogated LC-DC interactions. Thus, we have identified an important function of langerin in mediating LC-DC clustering, which allows Ag transfer to induce CTL responses to HIV-1. Furthermore, we showed this interaction is mediated by hyaluronidase-2 upregulation after DC maturation. These data underscore the importance of LCs and DCs in orchestrating adaptive immunity to HIV-1. Novel strategies might be developed to harness this mechanism for vaccination.

Human Langerhans cells capture measles virus through Langerin and present viral antigens to CD4⁺ T cells but are incapable of cross-presentation

Langerhans cells (LCs) are a subset of DCs that reside in the upper respiratory tract and are ideally suited to sense respiratory virus infections. Measles virus (MV) is a highly infectious lymphotropic and myelotropic virus that enters the host via the respiratory tract. Here, we show that human primary LCs are capable of capturing MV through the C-type lectin Langerin. Both immature and mature LCs presented MV-derived antigens in the context of HLA class II to MV-specific CD4(+) T cells. Immature LCs were not susceptible to productive infection by MV and did not present endogenous viral antigens in the context of HLA class I. In contrast, mature LCs could be infected by MV and presented de novo synthesized viral antigens to MV-specific CD8(+) T cells. Notably, neither immature nor mature LCs were able to cross-present exogenous UV-inactivated MV or MV-infected apoptotic cells. The lack of direct infection of immature LCs, and the inability of both immature and mature LCs to cross-present MV antigens, suggest that human LCs may not be directly involved in priming MV-specific CD8(+) T cells. Immune activation of LCs seems a prerequisite for MV infection of LCs and subsequent CD8(+) T-cell priming via the endogenous antigen presentation pathway.

Burn injury suppresses human dermal dendritic cell and Langerhans cell function

Human skin contains epidermal Langerhans cells (LCs) and dermal dendritic cells (DCs) that are key players in induction of adaptive immunity upon infection. After major burn injury, suppressed adaptive immunity has been observed in patients. Here we demonstrate that burn injury affects adaptive immunity by altering both epidermal LC and dermal DC functions. We developed a human ex vivo burn injury model to study the function of DCs in thermally injured skin. No differences were observed in the capacity of both LCs and dermal DCs to migrate out of burned skin compared to unburned skin. Similarly, expression levels of co-stimulatory molecules were unaltered. Notably, we observed a strong reduction of T cell activation induced by antigen presenting cell (APC) subsets that migrated from burned skin through soluble burn factors. Further analyses demonstrated that both epidermal LCs and dermal DCs have a decreased T cell stimulatory capacity after burn injury. Restoring the T cell stimulatory capacity of DC subsets might improve tissue regeneration in patients with burn wounds.

Langerhans cells in innate defense against pathogens

Langerhans cells (LCs) are at the frontline in defense against mucosal infections because they line the mucosal tissues and are ideally situated to intercept pathogens. Recent data suggest that LCs have an innate anti-HIV-1 function. LCs express the LC-specific C-type lectin Langerin that efficiently captures HIV-1, which prevents HIV-1 transmission. However, immune activation of LCs changes these protective cells into HIV-1-transmitting cells, which indicates that the antiviral function of LCs depends on several factors including co-infections. In this review, we discuss the dual role of LCs in innate defense against pathogens, with a focus on HIV-1 dissemination.

Herpes simplex virus type 2 enhances HIV-1 susceptibility by affecting Langerhans cell function

Genital herpes is the most prevalent viral sexually transmitted infection worldwide and is mainly caused by HSV type 2 (HSV-2). HSV-2 infection enhances HIV-1 susceptibility, even in the absence of clinical symptoms. In this study, we investigated the effect of HSV-2 on HIV-1 transmission by mucosal Langerhans cells (LCs). LCs are important in heterosexual transmission because they form a barrier against HIV-1 infection; LCs efficiently capture and degrade HIV-1 through the C-type lectin langerin, thereby preventing HIV-1 transmission. Notably, our data showed that HSV-2 enhanced HIV-1 infection of LCs and subsequent HIV-1 transmission to T cells. HSV-2 interfered with HIV-1 capture by langerin, which allowed efficient HIV-1 infection of LCs. HSV-2 inhibited the antiviral function of langerin at two levels; HSV-2 decreased langerin expression and competed with HIV-1 for langerin binding. HSV-2 replication was not required, because both UV-inactivated HSV-2 and TLR-3 agonist polyinosinic:polycytidylic acid similarly increased HIV-1 transmission by LCs. Therefore, we identified a mechanism by which HSV-2 enhances HIV-1 susceptibility, even in the absence of clinical symptoms. Our data demonstrated that viral coinfections, such as HSV-2, breach the protective function of LCs by abrogating langerin function, which increases HIV-1 susceptibility. These data reinforce the importance of preventing sexually transmitted infections, such as HSV-2, to reduce the transmission of HIV-1.

Isolation of immature primary Langerhans cells from human epidermal skin

Langerhans cells (LCs) are the main population of antigen-presenting cells lining the epidermis and stratified mucosal epithelia (1). Therefore, they play an important role in the first line of defense against invading pathogens. Upon capture of these pathogens, LCs subsequently migrate to the lymph nodes where they present pathogen-derived antigens to T cells to initiate an adaptive immune response. During this migration, LCs up-regulate cell surface marker HLA-DR and co-stimulatory molecules, while the LC-specific C-type lectin Langerin is down-regulated (reviewed in Refs. (2,3)). In the epidermis, LCs are the only cell population expressing CD1a and this marker is therefore extremely useful to isolate LCs from epidermis (4). Here we discuss a method to isolate human primary LCs from the epidermis in an as immature state as possible. The use of immature LCs is especially important in the investigation of the function of these cells, since few acceptable LC models are available. Immature LCs can be used to further elucidate the function of LCs in pathogen interactions and adaptive immunity.

Mutz-3-derived Langerhans cells are a model to study HIV-1 transmission and potential inhibitors

Sexual transmission is the primary route of HIV-1 infection, and DC subsets are thought to be involved in viral dissemination to T cells. In the genital mucosa, two main subsets of DCs are present: epithelial LCs capture and degrade HIV-1 through C-type lectin Langerin, whereas subepithelial DCs express DC-SIGN, which facilitates HIV-1 transmission to T cells. As there is currently no HIV-1 vaccine available, microbicides provide an alternative strategy to limit HIV-1 spread. However, research into the function of LCs is hampered by the low availability and donor differences. Here, we set out to investigate whether LCs derived from the Mutz-3 cell line (Mu-LCs) provide a valuable tool to investigate the role of LCs in HIV-1 transmission and identify suitable potential microbicides. We demonstrate that Mu-LCs phenotypically resemble human primary LCs; Mu-LCs do not transmit HIV-1 efficiently, and inhibition of Langerin enhances HIV-1 transmission to T cells. We show that carbohydrate structures blocking DC-SIGN but not Langerin are potential microbicides, as they prevent HIV-1 transmission by DCs but do not affect the antiviral function of LCs. Therefore, Mu-LCs are a suitable model to investigate the role of LCs in HIV-1 transmission and to screen potential microbicides.

MUC1 in human milk blocks transmission of human immunodeficiency virus from dendritic cells to T cells

Mother-to-child transmission of human immunodeficiency virus-1 (HIV-1) occurs frequently via breast-feeding. HIV-1 targets DC-SIGN+ dendritic cells (DCs) in mucosal areas that allow efficient transmission of the virus to T cells. Here, we demonstrate that the epithelial mucin MUC1, abundant in milk, efficiently bound to DC-SIGN on DC. The O-linked glycans within the mucin domain contained Lewis X structures, that were specifically recognized by the receptor. Interestingly, MUC1 prevented DC-SIGN-mediated transmission of HIV-1 from DCs to CD4+ T cells. We hypothesize that repetitive units of Lewis X, within the mucin domain, play an important role in inhibiting transmission of HIV-1 from mother to child.

TNF-alpha and TLR agonists increase susceptibility to HIV-1 transmission by human Langerhans cells ex vivo

Genital coinfections increase an individual's risk of becoming infected with HIV-1 by sexual contact. Several mechanisms have been proposed to explain this, such as the presence of ulceration and bleeding caused by the coinfecting pathogen. Here we demonstrate that Langerhans cells (LCs) are involved in the increased susceptibility to HIV-1 in the presence of genital coinfections. Although LCs are a target for HIV-1 infection in genital tissues, we found that immature LCs did not efficiently mediate HIV-1 transmission in an ex vivo human skin explant model. However, the inflammatory stimuli TNF-alpha and Pam3CysSerLys4 (Pam3CSK4), the ligand for the TLR1/TLR2 heterodimer, strongly increased HIV-1 transmission by LCs through distinct mechanisms. TNF-alpha enhanced transmission by increasing HIV-1 replication in LCs, whereas Pam3CSK4 acted by increasing LC capture of HIV-1 and subsequent trans-infection of T cells. Genital infections such as Candida albicans and Neisseria gonorrhea not only triggered TLRs but also induced TNF-alpha production in vaginal and skin explants. Thus, during coinfection, LCs could be directly activated by pathogenic structures and indirectly activated by inflammatory factors, thereby increasing the risk of acquiring HIV-1. Our data demonstrate a decisive role for LCs in HIV-1 transmission during genital coinfections and suggest antiinflammatory therapies as potential strategies to prevent HIV-1 transmission.

C-type lectin Mermaid inhibits dendritic cell mediated HIV-1 transmission to CD4+ T cells

Dendritic cells (DCs) are important in HIV-1 transmission; DCs capture invading HIV-1 through the interaction of the gp120 oligosaccharides with the C-type lectin DC-SIGN and migrate to the lymphoid tissues where HIV-1 is transmitted to T cells. Thus, the HIV-1 envelope glycoprotein gp120 is an attractive target to prevent interactions with DCs and subsequent viral transmission. Here, we have investigated whether the structural homologue of DC-SIGN, the nematode C-type lectin Mermaid can be used to prevent HIV-1 transmission by DCs. Our data demonstrate that Mermaid interacts with high mannose structures present on HIV-1 gp120 and thereby inhibits HIV-1 binding to DC-SIGN on DCs. Moreover, Mermaid inhibits DC-SIGN-mediated HIV-1 transmission from DC to T cells. We have identified Mermaid as a non-cytotoxic agent that shares the glycan specificity with DC-SIGN and inhibits DC-SIGN-gp120 interaction. The results are important for the anti-HIV-1 microbicide development directed at preventing DC-HIV-1 interactions.

Langerin is a natural barrier to HIV-1 transmission by Langerhans cells

Human immunodeficiency virus-1 (HIV-1) is primarily transmitted sexually. Dendritic cells (DCs) in the subepithelium transmit HIV-1 to T cells through the C-type lectin DC-specific intercellular adhesion molecule (ICAM)-3-grabbing nonintegrin (DC-SIGN). However, the epithelial Langerhans cells (LCs) are the first DC subset to encounter HIV-1. It has generally been assumed that LCs mediate the transmission of HIV-1 to T cells through the C-type lectin Langerin, similarly to transmission by DC-SIGN on dendritic cells (DCs). Here we show that in stark contrast to DC-SIGN, Langerin prevents HIV-1 transmission by LCs. HIV-1 captured by Langerin was internalized into Birbeck granules and degraded. Langerin inhibited LC infection and this mechanism kept LCs refractory to HIV-1 transmission; inhibition of Langerin allowed LC infection and subsequent HIV-1 transmission. Notably, LCs also inhibited T-cell infection by viral clearance through Langerin. Thus Langerin is a natural barrier to HIV-1 infection, and strategies to combat infection must enhance, preserve or, at the very least, not interfere with Langerin expression and function.