Current Immunology Reviews (Discontinued) - Volume 15, Issue 1, 2019

Most new HIV infections, over 80%, occur through sexual transmission. During sexual transmission, the virus must bypass specific female and male reproductive tract anatomical barriers to encounter viable target cells. Understanding the generally efficient ability of these barriers to exclude HIV and the precise mechanisms of HIV translocation beyond these genital barriers is essential for vaccine and novel therapeutic development. In this review, we explore the mucosal, barriers of cervico-vaginal and penile tissues that comprise the female and male reproductive tracts. The unique cellular assemblies of the squamous and columnar epithelium are illustrated highlighting their structure and function. Each anatomical tissue offers a unique barrier to virus entry in healthy individuals. Unfortunately barrier dysfunction can lead to HIV transmission. How these diverse mucosal barriers have the potential to fail is considered, highlighting those anatomical areas that are postulated to offer a weaker barrier and are; therefore, more susceptible to viral ingress. Risk factors, such as sexually transmitted infections, microbiome dysbiosis, and high progestin environments are also associated with increased acquisition of HIV. How these states may affect the integrity of mucosal barriers leading to HIV acquisition are discussed suggesting mechanisms of transmission and revealing potential targets for intervention.

The transmission of HIV is generally inefficient. Despite the development of a diverse viral quasispecies in a chronically infected individual, a severe genetic bottleneck is observed during transmission, leading to only one or a few genetic variants establishing infection. This genetic bottleneck is the result of both stochastic events and selection pressures, such that viruses with specific traits are favored during transmission. This chapter discusses current models of HIV mucosal transmission, evidence for selection of specific viral traits during this process, and the biological characterization of transmitted founder viruses based on monkey models and human cohorts. The impact of transmitted viral phenotypes on disease progression is also described. Understanding in greater depth the key viral features required for transmission will be essential to the development of effective interventions for HIV prevention.

The mucosal surfaces of the genital and gastrointestinal tracts are the routes by which HIV-1 is acquired, excluding persons infected parenterally. Identification of the mucosal target cells and the receptors by which HIV-1 enters these cells is fundamental to elucidating the biology of HIV-1 transmission. The mucosal target cells include epithelial cells, dendritic cells, Langerhans cells, CD4+ T-cells, macrophages and even mast cells, but the contribution of each cell type is highly dependent on the mucosal surface - genital versus gastrointestinal. Importantly, mucosal target cells may also play key roles in the immunobiology and latency of HIV-1 infection. Given the pivotal role of mucosal cells in HIV-1 transmission and pathogenesis, an effective vaccine to bring the HIV-1 pandemic under control must be effective at the level of the key target cells in both the genital and gastrointestinal mucosae.

Because HIV is sexually transmitted, there is considerable interest in defining the nature of anti-HIV immunity in the female reproductive tract (FRT) and in developing ways to elicit antiviral immunity in the FRT through vaccination. Although it is assumed that the mucosal immune system of the FRT is of central importance for protection against sexually transmitted diseases, including HIV, this arm of the immune system has only recently been studied. Here, we provide a brief review of the role of T cells in the FRT in blocking and facilitating HIV transmission.

The mucosal immune systems of the genital and intestinal tracts are considered as the most frequent sites of HIV-1 entry, displaying remarkable immunological differences in the systemic immune compartment which must be considered in the evaluation of humoral and cellular immune responses to HIV- 1. Marked differences in the fluids from the genital and intestinal tracts and in plasma with respect to the Ig isotypes, their levels, molecular forms and distinct effector functions must be taken into consideration in the evaluation and interpretation of humoral immune responses. Because of the low levels and highly pronounced variation in Ig content, HIV-1-specific antibody concentrations should always be related to the levels of total Ig of a given isotype. This practice will avoid inevitable differences due to the small volumes of collected fluids and sample dilution during the collection and processing of samples from external secretions. Furthermore, appropriate controls and immunochemical assays should be used to complement and confirm results generated by ELISA, which is prone to false positivity. In the evaluation of antibodymediated virus neutralization in external secretions, precautions and rigorous controls must be used to exclude the effect of innate humoral factors. The evaluation of cell-mediated immune responses in mucosal tissues is difficult due to the low yields of cells obtained from tissue biopsies or cytobrush scrapings. Furthermore, tissue biopsies of, for example, rectal mucosa, provide information pertaining exclusively to this local site, which due to the differences in the distribution of cells of different phenotypes, does not provide generalized information to the entire intestinal tract. Importantly, studies concerning the kinetics of cellular responses are difficult to perform due to the limited availability of samples or the inability of obtaining frequently repeated tissue biopsies. For sampling the female genital tract, parallel collection of menstrual and peripheral blood yields high numbers of cells that permit their detailed phenotypic and functional analyses. In contrast to tissue biopsies, this non-traumatic collection procedure results in high cell yields and repeated monthly sampling permits extensive and parallel functional studies of kinetics and unique characteristics of HIV-1-specific cellular responses in the female genital tract and peripheral blood.

The protective function of mucosal HIV-1- or SIV-specific antibodies against viral infection has stimulated extensive studies of their Ig isotype association with differences in specificity and in effector functions. In contrast to many mucosally acquired microbial infections in which the humoral responses are dominated by induction of secretory IgA (S-IgA), HIV-1/SIV infections stimulate vigorous IgG responses in sera as well as in external secretions but low IgA virus-specific antibodies although the total levels of IgA in these fluids remain unaltered. The diminished or even absent IgA responses to HIV-1/SIV and to other mucosal antigens in external secretions and their replacement with IgG is likely to influence the functionality of mucosal barriers and eliminate antiinflammatory effector functions of IgA antibodies. Furthermore, the polymeric character of S-IgA with 4-8 antigen-binding sites, exquisite resistance to proteolysis and anti-inflammatory potential are of great advantage in mucosal protection. The markedly different effector functions of mucosal antibodies of IgG and IgA isotypes must be considered in the design of HIV-1 vaccines to stimulate S-IgA responses at sites of virus entry and IgG responses in the systemic compartment.

As our understanding of mucosal immunity increases, it is becoming clear that the host response to HIV-1 is more complex and nuanced than originally believed. The mucosal landscape is populated with a variety of specialized cell types whose functions include combating infectious agents while preserving commensal microbiota, maintaining barrier integrity, and ensuring immune homeostasis. Advances in multiparameter flow cytometry, gene expression analysis and bioinformatics have allowed more detailed characterization of these cell types and their roles in host defense than was previously possible. This review provides an overview of existing literature on immunity to HIV-1 and SIVmac in mucosal tissues of the female reproductive tract and the gastrointestinal tract, focusing on major effector cell populations and briefly summarizing new information on tissue-resident memory T cells, Treg, Th17, Th22 and innate lymphocytes (ILC), subsets that have been studied primarily in the gastrointestinal mucosa.

Among the most significant findings in the pathogenesis of HIV infection was the discovery that almost total depletion of intestinal CD4+ T cells occurs rapidly after SIV or HIV infection, regardless of the route of exposure, and long before CD4+ T cell losses occur in blood or lymph nodes. Since these seminal discoveries, we have learned much about mucosal and systemic CD4+ T cells, and found several key differences between the circulating and intestinal CD4+ T cell subsets, both in phenotype, relative proportions, and functional capabilities. Further, specific subsets of CD4+ T cells are selectively targeted and eliminated first, especially cells critically important for initiating primary immune responses, and for maintenance of mucosal integrity (Th1, Th17, and Th22 cells). This simultaneously results in loss of innate immune responses, and loss of mucosal integrity, resulting in mucosal, and systemic immune activation that drives proliferation and activation of new target cells throughout the course of infection. The propensity for the SIV/HIV to infect and efficiently replicate in specific cells also permits viral persistence, as the mucosal and systemic activation that ensues continues to damage mucosal barriers, resulting in continued influx of target cells to maintain viral replication. Finally, infection and elimination of recently activated and proliferating CD4+ T cells, and infection and dysregulation of Tfh and other key CD4+ T cell results in hyperactive, yet non-protective immune responses that support active viral replication and evolution, and thus persistence in host tissue reservoirs, all of which continue to challenge our efforts to design effective vaccine or cure strategies.

Immune protection in the female reproductive tract (FRT) has evolved to meet the challenges of sexually transmitted bacterial and viral pathogens, allogeneic spermatozoa, and an immunologically distinct semi-allogeneic fetus. Throughout the FRT, the innate immune system is essential for the recognition and initial response to incoming pathogens. Key mediators of innate immune protection examined in this review include epithelial cells, stromal fibroblasts, macrophages, DC, and neutrophils from the Fallopian tubes, uterus, cervix and vagina. These innate immune cells respond to pathogens resulting in the secretion of cytokines, chemokines, antimicrobials, and production of intracellular proteins that protect, activate and recruit both innate and adaptive immune cells. Human immunodeficiency virus (HIV) infection can occur throughout the FRT, including the ovary, and is modulated by multiple factors including age of the individual, epithelial barrier integrity, composition of the vaginal microbiome, and hormonal status. Alterations in immune function due to hormonal changes that optimize conditions for successful fertilization create a hypothesized “window of vulnerability” that lasts from ovulation into the secretory stage of the menstrual cycle. The goal of this review is to summarize the multiple levels of protection against HIV infection in the FRT and thereby providing a foundation for the design of vaccines for protection against sexually-transmitted infections (STI) including HIV.

Optimal protective immunity to HIV will likely require that plasma cells, memory B cells and memory T cells be stationed in mucosal tissues at portals of viral entry. Mucosal vaccine administration is more effective than parenteral vaccine delivery for this purpose. The challenge has been to achieve efficient vaccine uptake at mucosal surfaces, and to identify safe and effective adjuvants, especially for mucosally administered HIV envelope protein immunogens. Here, we discuss strategies used to deliver potential HIV vaccine candidates in the intestine, respiratory tract, and male and female genital tract of humans and nonhuman primates. We also review mucosal adjuvants, including Toll-like receptor agonists, which may adjuvant both mucosal humoral and cellular immune responses to HIV protein immunogens.

Passive immunization has been used since the late 1800’s to prevent and treat human infectious diseases. Administration of animal immune sera and human immunoglobulin has given way to the use of monoclonal antibodies (mAbs) for passive immunization, and highly potent broadly neutralizing anti-HIV antibodies (bNAbs) are now being considered for HIV therapy and prophylaxis. Recent studies have shown that systemic and topical administration of bNAbs can effectively inhibit HIV/SHIV mucosal transmission in macaques and in humanized mice, and selected bNAbs are currently being tested in clinical trials for safety and efficacy in humans. In this review, we outline strategies for the selection, engineering and manufacture of human bNAbs to prevent the sexual transmission of HIV, describe the proof-of-concept animal studies that have demonstrated mAb-mediated protection against mucosal HIV transmission, and review clinical trials currently underway to test the safety and efficacy of mAb-based HIV prevention in humans.

Increased availability of antiretroviral therapy to pregnant and breastfeeding women in resource-limited areas has proven remarkably successful at reducing HIV vertical transmission rates over the past several decades. Yet, still, more than 170,000 children are infected annually due to failures in therapy implementation, monitoring, and adherence. Mother-to-child transmission (MTCT) of HIV-1 can occur at one of several distinct stages of infant development – intrauterine, intrapartum, and postpartum. The heterogeneity of the maternal-fetal interface at each of these modes of transmission poses a challenge for the implementation of immune interventions to prevent all modes of HIV MTCT. However, using mother-infant human cohorts and nonhuman primate models of infant simian immunodeficiency virus (SIV) acquisition, investigators have made an important observation about the biology of pediatric HIV infection and have identified unique protective immune factors for each mode of transmission. Knowledge of immune factors protective against HIV MTCT will be critical to the development of targeted immune therapies to prevent infant HIV acquisition and to bring an end to the pediatric AIDS epidemic.

HIV-1 is present in many secretions including oral, intestinal, genital, and breast milk. However, most people exposed to HIV-1 within these mucosal compartments do not become infected despite often frequent and repetitive exposure over prolonged periods of time. In this review, we discuss what is known about the levels of cell-free HIV RNA, cell-associated HIV DNA and cellassociated HIV RNA in external secretions. Levels of virus are usually lower than contemporaneously obtained blood, increased in settings of inflammation and infection, and decreased in response to antiretroviral therapy. Additionally, each mucosal compartment has unique innate and adaptive immune responses that affect the composition and presence of HIV-1 within each external secretion. We discuss the current state of knowledge about the types and amounts of virus present in the various excretions, touch on innate and adaptive immune responses as they affect viral levels, and highlight important areas for further study.