Introduction: Back to basics: mucosal immunity and novel HIV vaccine concepts

The 5th Axel Key symposium on 'Mucosal Immunity and Novel HIV-vaccine Concepts' was held in Stockholm, Sweden. This review summarizes some of the key messages and suggests areas of future scientific research priorities on HIV transmission and pathogenesis. The consensus of this meeting was that additional basic science research on HIV cell entry and mucosal immunology is likely to contribute significantly to insights that can be capitalized upon for the development of an efficacious HIV vaccine. In the current volume of the Journal of Internal Medicine, several of the symposium participants have provided comprehensive reviews of their specific fields of research on mucosal immunology. Most HIV-1 transmissions result from virus exposure at mucosal surfaces and the time-interval between initial viral exposures and detection of viral RNA in the blood is approximately 7–21 days. The cellular and molecular events that occur during the first critical hours or days after viral exposure and the establishment of a productive infection remain to be elucidated. Whilst mucosal tissue inflammation and a high viral inoculum may result in transmission of heterogeneous virus populations, a largely intact mucosal barrier and other unknown host and viral factors may only allow for more restricted viral sequences to passage the epithelial layer. Dr Hunter (Emory University) presented data on the identification of the transmitted virus and sequences evolving from it by single genome analysis that show that virus populations in the acute phase of infection were generally genetically highly homogenous. Thus, although multiple variants may be transmitted, more often single or small populations of virus are transmitted and/or there is selection of a more homogeneous viral population in the postinfection period [1] (Fig. 1). HIV-1 invasion in the genital mucosa. Viral invasion occurs through the nonkeratinized squamous epithelium of the vagina and ectocervix, as well as through the single-layer columnar epithelium of the endocervix. The endocervical mucus plug normally provides a barrier by trapping virions. Innate immune factors and the alkalinization by semen can decrease the barrier function. Virus can migrate between the stratified epithelial cells or enter through breaches. Invading HIV-1 particles may be captured and internalized into endocytic compartments by Langerhans cells (LCs) that reside within the epithelium. Interaction with the Langerin may result in degradation in the Birbeck granules, whilst binding to CCR5/CD4 may cause viral replication. Virus interaction with the epithelium induces massive MIP3-α release and subsequent accumulation of myeloid dendritic cells (mDCs). Such stromal DCs that express both DC-SIGN and CCR5 have been implicated in HIV-1 infection. The former receptor is involved in trans-infectivity whilst CCR5/CD4 interaction may lead to direct viral replication in the DCs. Most likely entrapment, antimicrobial peptide activity and C-type lectin receptor-mediated degradation dramatically reduce the pool of invasive HIV-1 quasispecies into very restricted numbers that actually reach the lamina propria. Here establishment of infection is a consequence of HIV-1 targeting CD4+ T cells that generates massive viral replication. The majority of these cells are CD45R0+ memory T cells that express higher CCR5 than those circulating in the blood. The fight back is a complex battle. HIV-1 binding to TLR7 and TLR9 result in IFN-α release in plasmacytoid DCs (pDCs) and activation of intracellular ABOPEC and other endogenous antiviral proteins. mDC derived chemokine production form a two edge sword by blocking HIV-1-CCR5 receptor utilization but also increase infectability by generating accumulation of new potential target cells. Viral induced MHC-class I down-regulation plus IFN-α and Th1 cytokines cause NK-cell activation and subsequent T cell killing of infected cells. They also produce antiviral factors that suppress replication. Formation of HIV-1-neutralizing antibodies are limited by the massive viral mutagenicity and carbohydrate-mediated shields of the critical HIV-1-envelope epitopes. These events cause inflammation including IDO-release that blocks proliferation of T cells, which can limit virus infection, but can also impair antiviral T-cell responses. A consequence of the NFκB activation is enhanced viral replication. Regulatory T cells (Treg) are accumulated that restrict the immune activation and the HIV-1 specific antiviral responses including the Th17-mediated control of mucosal microbial permeability. Establishment of the infection occurs if the infected cells are allowed to migrate to lymph nodes and lymphatic tissue of the gut. Dr Haase (University of Minnesota) described the sequential events that occur in the SIV model during the first few weeks following inoculation of virus in the female genital tract mucosa. An intact mucosal barrier indeed limits the infection of cervico-vaginal tissues and only allows a small initial founder population of infected cells. These populations must expand in order to overcome local innate immune defences that otherwise would prevent dissemination to distal sites and systemic infection. However, Dr Haase stressed that the window of vulnerability for the virus, in which there is an opportunity for preventive interventions, before its spread to the lymphatic tissues, is a matter of only a few days. Thus, the virus specific CD8+ T cell immune response may arrive on the scene too late to contain and/or eliminate the small founder populations, and perhaps only after HIV-1 has already established a latent reservoir. It suggests an extant mucosal immune response, or very rapid recall response might be able to prevent systemic infection. Dr Haase also presented data that suggested the concept that inhibiting the NFκB regulated innate and inflammatory response, which fuels the local expansion necessary to establish systemic infection, could prevent acute SIV infection. Dr Hope (Northwestern University) described ex vivo analyses of the early interactions of HIV-1 with human foreskin and cervical tissue, and of SIV in the rhesus macaque female genital tract. By labelling virions with fluorescent proteins [2] he identified SIV that had productively entered the target cells within the first few hours of virus exposure. Importantly, the virions were shown to diffuse interstitially in the intact multi-stratified epithelium of the genital tract. This was a pH-dependent process since the particles were immobilized at physiological conditions (pH = 4). In contrast, an increase in pH caused by seminal fluid permitted migration of the virions. Intraepithelial target cells such as Langerhans cells could easily establish contact with the infectious particles within the seemingly intact epithelium (Fig. 1). Breaches in the epithelium, due to sexually transmitted diseases (STDs) or physical abrasions, are thus not prerequisite for infection although such disruptions allow a more direct access of the virions to the submucosa and an extended spectrum of target cells. Furthermore, Dr Hope provided evidence that relaxation of the tight junction in inflamed tissue may facilitate the transepithelial migration of HIV-1. The generally low HIV-1 transmission rate during heterosexual vaginal intercourse is estimated to 1/200–1/2000 per exposure. Thus, the genital tissues form a protective barrier that HIV-1 has to circumvent, in addition to inhibiting factors in cervico-vaginal secretions, hindrance by the mucus layer covering the ectocervix and the endocervical mucus plug. The next barrier in the lower female genital tract is the multi-stratified epithelium of the vagina and ectocervix. Langerhans cells are amongst the first cells to encounter HIV-1 at this site. Dr Geijtenbeek (VU University Medical Center, Amsterdam) and Dr McElrath (University of Washington) have used different models for studying the role of Langerhans cells in HIV-1 transmission and they provided their insights into how this cell type may impact on the initial events in genital infections. Clearly, the function of these cells in HIV-1 transmission is complicated. HIV-1 can bind to Langerhans cells through different receptor structures including the CD4/CCR5 pathway that may result in viral replication or via initial attachment to the C-type lectin Langerin (Fig. 1). Additionally, Dr Geijtenbeek presented data showing that Langerin associated with either endosomal uptake and inactivation or spread of virions. Thus in a viral dose-dependent fashion Langerhans cells may either contain HIV-1 or efficiently transfer the virus to T cells and thereby promote subsequent systemic dissemination. Preventive strategies against HIV-1 primary infection must therefore be carefully designed in order to maintain the potential beneficial role of Langerin-HIV-1 interactions. Dr Cunningham (Westmead Millenium Institute) described the interplay between different subsets of dendritic cells (Langerhans cells as well as plasmacytoid and myeloid dendritic cells) with herpes simplex and varicella-zoster viruses. Although HIV-1 and herpes viruses interact with dendritic cells during initial infection of the genital tract, the degree of productive infection and the effects on cell maturation, IFN-release, apoptosis and subsequent cross-presentation were quite different. This reflects differences in biology and pathogenesis and has implications for understanding co-infections and the increased risk of HIV-acquisition upon concomitant herpes simplex virus infection. Indeed, CCR5 expression, which is an important infection rate limiting event for HIV-1, seems to be under the influence of local Th1 versus Th2 type of cytokine responses. The role of concomitant STDs in relation to HIV-1-acquisition in the female genital tract was further elucidated by Dr Hirbod (Karolinska Institutet). Genital tract biopsies from healthy individuals as well as from women at high-risk of acquiring HIV-1 infection were studied for identification and distribution of HIV-1 target cells. Langerhans cells were localized intra-epithelially whereas dendritic cells that express DC-SIGN and mannose-receptor (MR) were restricted to the submucosal layer of the ectocervix in both study groups. Although the total numbers of dendritic cells were similar between the groups, the high-risk women had significantly higher expression of the HIV-1 binding receptors Langerin, MR and DC-SIGN (Fig. 1). Variations in expression and localization of potential HIV-1 receptors should be considered when developing topical prophylactic measures, i.e. microbicides. In related studies, Dr Geijtenbeek described the susceptibility of Langerhans cells to HIV-1-infection during various inflammatory conditions. It was clear that genital co-infections directly activated Langerhans cells by pathogenic and inflammatory factors, thereby increasing risk of HIV-1-acquisition. The innate defenses against HIV-1 must be very efficient, since only a small minority of sexual exposures to HIV-1 results in established infection. Indeed, the mucosal fluid and the thick mucus layer that covers the cervical and vaginal epithelia in the female genital tract contain a mixture of molecules including secreted antibodies, cytokines and other antimicrobial molecules. These innate immune factors derive from cervical vestibular glands, endometrial and oviductal fluids as well as plasma transudates. The genital epithelia and underlying leukocytes also actively secrete molecules that act as effectors of the innate host defense. Suggested antiviral molecules include lysozyme, lactoferrin, secretory leucocyte protease inhibitor (SLPI) and defensins [3] (Fig.1). By collecting cervico-vaginal fluid samples from healthy individuals, Dr Cole (University of Central Florida) demonstrated that these innate molecules synergize in complex ways thereby mediating potential anti-HIV-1 activity at physiological concentrations. An altered commensal bacterial flora, variations in pH and hormonal changes may greatly influence the concentrations of many of these factors which in turn affects the susceptibility to HIV-1 acquisition. Interestingly, retrocyclin, an ancestral human theta-defensin, was demonstrated by Dr Cole to be induced by aminoglycosides and to have potential for development as a topical microbicide to prevent HIV-1 transmission. Dr Hirbod and Dr Kaul presented data from studies of women at high-risk of acquiring HIV-1-infection. The expression of antimicrobial peptides and proteins in the genital tract of these individuals varied with regard to the presence of STDs. Cervico-vaginal secretions from some of the high-risk women contained in vitro HIV-1-neutralizing activity possibly mediated by antimicrobial peptides (Fig. 1). This contributed to the total HIV-1-inhibitory activity, which also consisted of HIV-1-neutralizing IgA antibodies [4]. Since the incidence of STDs is high in these study groups it will be essential to characterize baseline levels of these naturally occurring innate immune molecules. The composition of innate immune molecules in different risk groups, depending on genetic, hormonal and behavioural factors, must be taken into account when evaluating microbicide products. Vaccine-induced immune responses at mucosal sites are likely to be influenced by the natural variations in innate immune factors in target cell populations. Induced levels of mucosal innate immune parameters may have both beneficial and detrimental effects in the in vivo situation. Whereas some of the innate immune factors block viral entry and subsequent infection, Dr Haase presented data that they also recruit immune cells, which are potential target cells for HIV infection. A better understanding of the critical balance of having these potent molecules 'at the right place at the right time' is crucial for future prophylactic human trials, especially those involving topically applied substances, as well as for development of vaccine immunogens. The epithelial barriers in the female and male ano-genital tract are thus protected by innate immune factors in the mucus covering the epithelium. Some of these factors are also present within and beneath the multi-stratified epithelial layer thereby constituting a 'second line of defense' before HIV-1 may reach other target cells including dendritic cells, macrophages and lymphocytes. Dr Geijtenbeek pointed out the role of inflammatory conditions with pro-inflammatory cytokine and chemokine expression in the recruitment of HIV-1- target cells. Moreover, pathogen interactions with intact epithelium as well as micro-abrasions as a result of sexual intercourse can induce inflammatory cytokine production, such as TNF-α. These can in turn enhance HIV-1 replication in Langerhans cells and increase transmission to T-lymphocytes [5, 6]. In addition to the soluble innate immune molecules, innate cellular immune responses have a direct antiviral potency. However, very little is known about the local effect of NK-cells, NKT-cells and γδ-T-cells at the mucosal level and its role in preventing primary HIV-1 infection (Fig. 1). Dr Altfeld (Harvard Medical School) has studied NK-cells during established infection and showed that NK-cell-mediated immunity clearly has an impact on viral evolution through immune pressure. Dr Altfeld emphasized the importance of performing interventional pilot trials assessing the potential of boosting NK-cell activity in vivo on viral control. The importance of NK-cells is further implied by cellular-genetic epidemiological studies which have demonstrated that the combined expression of specific NK-cell receptors, killer cell Ig-like receptors (KIRs), in conjunction with their HLA-class I ligands, is protective in HIV-1 disease [7]. Understanding these mechanisms in mucosal tissue will be crucial to harness the potency of these cytotoxic effector cells for therapeutic interventions. Furthermore, the added potency of antibody-mediated cellular cytotoxicity, ADCC, at the mucosal level has been suggested in previous studies of the female genital tract [8]. Moreover, the HIV-1 Nef protein has been shown to have many different functions, one of which is immune evasion [9]. Killer cell Ig-like receptors are a diverse family of cell-surface glycoproteins that are expressed on NK cells and a subpopulation of CD8+ T cells with an activated or memory phenotype. KIR trigger inhibitory or activating signals upon binding to their MHC class I ligands, thereby regulating cell cytotoxicity and cytokine secretion. The KIRs show extensive genetic polymorphisms. Dr Thomas (National Cancer Institute) presented data on the identification of novel KIR3DL1 alleles and characterized their expression patterns on NK cells in different individuals [10]. The functional consequences of these genetic variants could affect susceptibility or resistance to HIV-1 infection. Further studies on the roles for HLA and KIR polymorphisms in NK cell repertoire selection and modulation of effector function are important to further elucidate the host genetic impact on susceptibility to HIV-1 infection and disease. There is a need to explore how HIV-1 might interfere with the innate immune system to alter adaptive immune responses. Dendritic cells bridge innate and adaptive immunity by interacting with microorganisms at the periphery. Through this interaction they produce effector cytokines and initiate or re-stimulate activation of T and B lymphocytes through antigen-presentation. Several soluble innate immune molecules also possess bridging properties by different mechanisms. One such example is the chemokine CCL20/MIP3α and the antimicrobial peptide human β-defensin-2 that are structurally similar and both act as ligands for the chemokine receptor CCR6. Both molecules are synthesized by epithelial cells and can independently chemoattract immature dendritic cells and lymphocytes to sites of infection (Fig. 1). The role of HIV-1 in modulating dendritic cell function affecting anti-viral immunity, especially TLR expression and cytokine and chemokine secretion, are subjects of future investigation. Plasmacytoid dendritic cells (pDCs) represent a unique cell type which produces a significant amount of IFN-α, a hallmark of the innate immune response (Fig. 1). The pDCs also trigger adaptive immunity by the ability to mature into potent antigen-presenting cells. Human pDCs express a specific pattern of TLRs, preferentially the nucleic acid-specific TLR7 and TLR9. Dr Guiducci (Dynavax Technologies Corporation) discussed the key signaling pathways leading to IFN-α production in human pDC. It is becoming clear that whereas IFN-α can have a key role in triggering the adaptive immunity it can also have detrimental effects leading to autoimmunity if chronically over-expressed. Therapeutic approaches targeting TLR7 and TLR9 on human pDCs are thus likely to open up novel prospects not yet applied on HIV infection [11]. Dr Andersson (Harvard Medical School) presented novel concepts from 'outside' the HIV field. A pathway including two molecules, TIM-3 and its ligand galectin-9 has been discovered that regulates both adaptive and innate immunity. Both genetic evidence and experimental data using human specimens demonstrate that this pathway may contribute to human inflammatory disease. It is rare, perhaps unprecedented, for a single molecule (TIM-3) to both promote an immune response (when engaged on APCs) and terminate an immune response (when engaged on T cells). The expression of these molecules now needs to be analysed in the genital mucosal tissue in addition to studies on peripheral blood cells. Inappropriate or undesired activation of this pathway could perpetuate a mucosal inflammatory response in the early stages of infection. Dr Shacklett (University of California Davis) showed that the HIV-1-specific CD8+ T cell responses in the gut were robust and complex, including of and with low to in the of had and CD8+ T cell responses in the gut as to blood This suggests an important role for these responses in control of HIV-1. studies evaluating T-cell responses in blood were thus by important mucosal many individuals with T-cell responses in blood may in and complex responses in of T-cell in the genital tract might a for understanding how at one mucosal induces at other mucosal The adaptive T cells in the genital tract are from gut mucosal T-cell populations that express the which is a identified receptor with binding to HIV-1 and to through gut Dr (University of Minnesota) presented results on how the to memory T-cell in cells that are not present in tissues T cells were in the small epithelium and not pointed out that mucosal of may not be for the establishment of mucosal memory T-cell responses but that need to more about the of these responses at different mucosal Dr (University of California Davis) showed that from vaginal is mediated by CD8+ T cells in the vaginal of rhesus with an Interestingly, after vaginal SIV not the genital tract of and the only significant expansion of T cells in these in the CD4+ and CD8+ T cells are indeed in the lamina and epithelium of the vagina and of rhesus Thus, an mucosal memory T-cell response at the of entry may be able to limit viral replication in the genital tract and prevent the establishment of a systemic infection (Fig. 1). is known about the immunology in the female genital tract, in models in The role of B cells and antibodies in transmission and pathogenesis as well as the innate cellular immune responses including NK-cells and subsets of T cells have been a of studies in relation to HIV-1 infection at mucosal In addition to new insights into mucosal Dr University) data on the IgA responses in which place in both and and through both T-cell and T-cell This to production of mucosal and antibodies with potential antiviral activity Dr showed that the protein with the production of IgA antibodies in the of the HIV-1 IgA production which may be of a and of the very little about the of class events during HIV-1 infection. An local antibody-mediated immune response at both the gut and genital mucosal is clearly Genital HIV-1-neutralizing IgA has indeed been suggested to be associated with HIV in a of high-risk female [4]. However, antibodies can virus infection by many mechanisms and it is important to the between and IgA antibodies with HIV-1-inhibitory The may not for HIV-1 mechanisms for may include down-regulation or blocking of receptors or responses against HLA molecules. by HIV-1 is characterized by functional and activation of T cells, due to that are largely factors include caused by HIV-1 replication and presence of circulating factors of microbial of T-cell activation are not in natural of remain high viral replication in Dr on key events occurring upon contact between HIV-1 and pDCs and suggested a model (Fig. 1). Thus, activation of and subsequent production of IFN-α and contributed to T-cell activation and of T-cell during HIV-1 infection The may thus at a very early following primary infection. The early viral with innate immune cells, as well as with memory T-cell may in also immune responses. immune activation and to are after SIV infection in but not in better this Dr (University of California acute pathogenic in to infection in but not systemic immune of cells and of the balance between and cells in and mucosal tissue was in cells were to be of systemic and T cell data thus that of the to balance was related to has a importance for the release of antimicrobial peptides and thus regulates gut mucosal Thus, a new on that both and cells may a unique and possibly role in the of acute HIV-1 infection. regard to HIV-1 vaccine Dr pointed out that can not on the of a robust antiviral adaptive systemic immune it may be beneficial to responses that limit mucosal of cells after acute infection the balance that in Such a vaccine might result in a that of the is It may lead to a response that the level of immune thereby the of viral replication and Dr (University of has studied how naturally remain high virus replication results that the of activated CD4+ T cells, than immune control of SIV is the of viral during natural SIV infection of Furthermore, a of this model of infection is the of immune activation which is one of the of disease in HIV-1 infected A better understanding of the between CD4+ T cell activation and to replication as well as the of mechanisms at the pool of functional cells are Dr (National of described that a of to the and induce an intracellular that increase expression on CD4+ T cells This interaction in turn the of the and the transmission of the virus from one cell to It to be elucidated this interaction and subsequent signaling a direct role in the of CD4+ T cells or of other immune cells. HIV-1 thus many of the current in immunology. Dr (National of pointed out that there is an to better the cell interactions and how they immune mechanisms. The key that Dr were to binding is in unique by C-type lectin binding to HIV-1 also the mechanisms. One could also blocking with antibodies or of HIV-1 will or will not interfere with the immune mechanisms by immune individuals are by to HIV but remain HIV also but individuals, and HIV RNA by in blood. to with different exposure including for to sexual to HIV infected individuals and on risk degree of exposure and of host immune several and genetic have been associated with resistance against HIV-1-acquisition. the HIV-1 some of these individuals both systemic and mucosal immune responses against HIV-1 infection. the genetic factors the significant of described to include those to the

Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.