Pattern Recognition Receptors in HIV Transmission

Strategies to overcome HIV drug resistance-current and future perspectives

The availability of combined antiretroviral therapy (cART) has revolutionized the course of HIV infection, suppressing HIV viremia, restoring the immune system, and improving the quality of life of HIV infected patients. However, the emergence of drug resistant and multidrug resistant strains remains an important contributor to cART failure, associated with a higher risk of HIV-disease progression and mortality. According to the latest WHO HIV Drug Resistance Report, the prevalence of acquired and transmitted HIV drug resistance in ART naive individuals has exponentially increased in the recent years, being an important obstacle in ending HIV-1 epidemic as a public health threat by 2030. The prevalence of three and four-class resistance is estimated to range from 5 to 10% in Europe and less than 3% in North America. The new drug development strategies are focused on improved safety and resistance profile within the existing antiretroviral classes, discovery of drugs with novel mechanisms of action (e.g., attachment/post-attachment inhibitors, capsid inhibitors, maturation inhibitors, nucleoside reverse transcriptase translocation inhibitors), combination therapies with improved adherence, and treatment simplification with infrequent dosing. This review highlight the current progress in the management of salvage therapy for patients with multidrug-resistant HIV-1 infection, discussing the recently approved and under development antiretroviral agents, as well as the new drug targets that are providing a new avenue for the development of therapeutic interventions in HIV infection.

Mucosal Dendritic Cell Subsets Control HIV-1's Viral Fitness

Dendritic cell (DC) subsets are abundantly present in genital and intestinal mucosal tissue and are among the first innate immune cells that encounter human immunodeficiency virus type 1 (HIV-1) after sexual contact. Although DCs have specific characteristics that greatly enhance HIV-1 transmission, it is becoming evident that most DC subsets also have virus restriction mechanisms that exert selective pressure on the viruses during sexual transmission. In this review we discuss the current concepts of the immediate events following viral exposure at genital mucosal sites that lead to selection of specific HIV-1 variants called transmitted founder (TF) viruses. We highlight the importance of the TF HIV-1 phenotype and the role of different DC subsets in establishing infection. Understanding the biology of HIV-1 transmission will contribute to the design of novel treatment strategies preventing HIV-1 dissemination.

Myeloid C-Type Lectin Receptors in Tuberculosis and HIV Immunity: Insights Into Co-infection?

C-type lectin receptors (CLRs) are carbohydrate binding pattern recognition receptors (PRRs) which play a central role in host recognition of pathogenic microorganisms. Signaling through CLRs displayed on antigen presenting cells dictates important innate and adaptive immune responses. Several pathogens have evolved mechanisms to exploit the receptors or signaling pathways of the CLR system to gain entry or propagate in host cells. CLR responses to high priority pathogens such as Mycobacterium tuberculosis (Mtb), HIV, Ebola, and others are described and considered potential avenues for therapeutic intervention. Mtb and HIV are the leading causes of death due to infectious disease and have a synergistic relationship that further promotes aggressive disease in co-infected persons. Immune recognition through CLRs and other PRRs are important determinants of disease outcomes for both TB and HIV. Investigations of CLR responses to Mtb and HIV, to date, have primarily focused on single infection outcomes and do not account for the potential effects of co-infection. This review will focus on CLRs recognition of Mtb and HIV motifs. We will describe their respective roles in protective immunity and immune evasion or exploitation, as well as their potential as genetic determinants of disease susceptibility, and as avenues for development of therapeutic interventions. The potential convergence of CLR-driven responses of the innate and adaptive immune systems in the setting of Mtb and HIV co-infection will further be discussed relevant to disease pathogenesis and development of clinical interventions.

Microbicides for topical immunoprevention of HIV infection

Microbicides are antiseptic topical drugs that help directly or indirectly inhibit the penetration of an infectious agent into the human body, thereby preventing the sexual transmission of HIV-infection and other sexually transmitted diseases. Microbicides have an antiviral mechanism of action in the sexual transmission of HIV and affect the components of mucosal immunity in the vagina. In this article, the pharmaceutical and biomedical aspects of microbicide application are examined and diverse classifications of microbicides are presented. For each group of chemicals, the most important representatives and their mechanisms of action are described. This article also presents the structure and function of mucosal immunity, and shows the importance of the mucosal immune response in the sexual transmission of HIV. This work also exhibits the experimental models for testing of candidate microbicides. For each compound described, a review of preclinical research and clinical trials is provided, covering its development as a microbicide. This paper gives an overview of microbicides, a new class of chemically diverse immunobiological medications reducing the risk of sexual transmission of HIV. The use of microbicides is believed to curb the HIV/AIDS epidemic in the nearest future.

HIV-1 border patrols: Langerhans cells control antiviral responses and viral transmission

Langerhans cells (LCs) reside in the mucosal epithelia and are refractory to HIV-1 infection; HIV-1 capture by C-type lectin receptor langerin and subsequent targeting to Birbeck granules prevents infection. Furthermore, LCs restrict transmission of CXCR4-using HIV-1 variants, which underscores the role of immature LCs as gatekeepers in the selection of HIV-1 variants. Interaction of langerin on LCs with hyaluronic acid on dendritic cells facilitates cross-presentation of HIV-1 to CD8+ T cells. Activation of LCs upon inflammation bypasses the langerin-dependent barrier, which favors cross-presentation and increases susceptibility of LCs to HIV-1 infection. These recent developments not only highlight the plasticity of LCs but also define an important role for LC-dendritic cell crosstalk at the periphery in directing adaptive immune responses to viruses.

Which Antibody Functions are Important for an HIV Vaccine?

HIV antibody (Ab) functions capable of preventing mucosal cell-free or cell-to-cell HIV transmission are critical for the development of effective prophylactic and therapeutic vaccines. In addition to CD4(+) T cells, other potential HIV-target cell types including antigen-presenting cells (APCs) (dendritic cells, macrophages) residing at mucosal sites are infected. Moreover, the interactions between APCs and HIV lead to HIV cell-to-cell transmission. Recently discovered broadly neutralizing antibodies (NAbs) are able to neutralize a broad spectrum of HIV strains, inhibit cell-to-cell transfer, and efficiently protect from infection in the experimentally challenged macaque model. However, the 31% protection observed in the RV144 vaccine trial in the absence of detectable NAbs in blood samples pointed to the possible role of additional Ab inhibitory functions. Increasing evidence suggests that IgG Fcγ receptor (FcγR)-mediated inhibition of Abs present at the mucosal site may play a role in protection against HIV mucosal transmission. Moreover, mucosal IgA Abs may be determinant in protection against HIV sexual transmission. Therefore, defining Ab inhibitory functions that could lead to protection is critical for further HIV vaccine design. Here, we review different inhibitory properties of HIV-specific Abs and discuss their potential role in protection against HIV sexual transmission.

DC-SIGN plays a stronger role than DCIR in mediating HIV-1 capture and transfer

The C-type lectin receptors (CLRs) expressed on dendritic cells (DCs), in particular DC-SIGN and DCIR, likely play an important role in HIV-1 early infection. Here, we systematically compared the capture and transfer capability of DC-SIGN and DCIR using a wide range of HIV-1 isolates. Our results indicated that DC-SIGN plays a stronger role than DCIR in DC-mediated HIV-1 capture and transfer. This was further strengthened by the data from transient and stable transfectants, showing that DC-SIGN had better capability, compared with DCIR in HIV-1 capture and transfer. Following constructing and analyzing a series of soluble DC-SIGN and DCIR truncates and chimeras, we demonstrated that the neck domain, but not the CRD, renders DC-SIGN higher binding affinity to gp120 likely via the formation of tetramerization. Our findings provide insights into CLR-mediated HIV-1 capture and transfer, highlighting potential targets for intervention strategies against gp120-CLR interactions.

Genetic characterisation of Langerin gene in human immunodeficiency virus-1-infected women from Bahia, Brazil

Studies on human genetic variations are a useful source of knowledge about human immunodeficiency virus (HIV)-1 infection. The Langerin protein, found at the surface of Langerhans cells, has an important protective role in HIV-1 infection. Differences in Langerin function due to host genetic factors could influence susceptibility to HIV-1 infection. To verify the frequency of mutations in the Langerin gene, 118 samples from HIV-1-infected women and 99 samples from HIV-1-uninfected individuals were selected for sequencing of the promoter and carbohydrate recognition domain (CRD)-encoding regions of the Langerin gene. Langerin promoter analysis revealed two single nucleotide polymorphisms (SNPs) and one mutation in both studied groups, which created new binding sites for certain transcription factors, such as NFAT5, HOXB9.01 and STAT6.01, according to MatInspector software analysis. Three SNPs were observed in the CRD-encoding region in HIV-1-infected and uninfected individuals: p.K313I, c.941C>T and c.983C>T. This study shows that mutations in the Langerin gene are present in the analysed populations at different genotypic and allelic frequencies. Further studies should be conducted to verify the role of these mutations in HIV-1 susceptibility.

Antigen-presenting cell candidates for HIV-1 transmission in human distal colonic mucosa defined by CD207 dendritic cells and CD209 macrophages

A common route for HIV-1 infection is sexual transmission across colorectal mucosa, which is thought to be 10-2,000 times more vulnerable to infection than that of the female genital tract. Mucosal surfaces are the first line of defense against many pathogens but the antigen-presenting cells (APCs), key regulators of innate immunity and determinants of adaptive immunity, are not well defined in these target tissues. Using immunohistochemistry, dendritic cells expressing Langerin (CD207(+)), a lectin known to bind and internalize HIV-1, were detected in the periphery of colonic glands and sparsely scattered in the submucosa similarly in colorectal mucosa. This cell type, well known in skin, has generally not been reported in colonic/rectal mucosa. Unexpectedly, the largest APC population observed was a macrophage-like population expressing the well-characterized tissue macrophage markers CD68 and CD163. Confocal microscopy of these cells revealed colocalization of CD209 (DC-SIGN), a presumed dendritic cell marker believed to facilitate HIV-1 transmission, but not other dendritic cell markers. These results show evidence of the unconfirmed presence of Langerhans cells in colorectal mucosa and a predominance of macrophage-like APCs that express CD209 (DC-SIGN). These findings define potential target cells in the pathogenesis of HIV-1 transmission, which may have key implications for the study of early transmission events in normal colorectal mucosa, as well as other infectious diseases and primary immune diseases involving the gut.

The Neonatal Fc receptor (FcRn) enhances human immunodeficiency virus type 1 (HIV-1) transcytosis across epithelial cells

The mechanisms by which human immunodeficiency virus type 1 (HIV-1) crosses mucosal surfaces to establish infection are unknown. Acidic genital secretions of HIV-1-infected women contain HIV-1 likely coated by antibody. We found that the combination of acidic pH and Env-specific IgG, including that from cervicovaginal and seminal fluids of HIV-1-infected individuals, augmented transcytosis across epithelial cells as much as 20-fold compared with Env-specific IgG at neutral pH or non-specific IgG at either pH. Enhanced transcytosis was observed with clinical HIV-1 isolates, including transmitted/founder strains, and was eliminated in Fc neonatal receptor (FcRn)-knockdown epithelial cells. Non-neutralizing antibodies allowed similar or less transcytosis than neutralizing antibodies. However, the ratio of total:infectious virus was higher for neutralizing antibodies, indicating that they allowed transcytosis while blocking infectivity of transcytosed virus. Immunocytochemistry revealed abundant FcRn expression in columnar epithelia lining the human endocervix and penile urethra. Acidity and Env-specific IgG enhance transcytosis of virus across epithelial cells via FcRn and could facilitate translocation of virus to susceptible target cells following sexual exposure.

HIV-1 causes a sexually transmitted disease. However, the mechanisms employed by the virus to cross genital tract tissue and establish infection are uncertain. Since cervicovaginal fluid is acidic and HIV-1 in cervicovaginal fluid is likely coated with antibodies, we explored the effect of low pH and HIV-1-specific antibodies on transcytosis, the movement of HIV-1 across tight-junctioned epithelial cells. We found that the combination of HIV-1-specific antibodies and low pH enhanced transcytosis as much as 20-fold. Virus that underwent transcytosis under these conditions was infectious, and infectivity was highly influenced by whether or not the antibody neutralized the virus. We observed enhanced transcytosis using antibody from cervicovaginal and seminal fluids and using transmitted/founder strains of HIV-1. We also found that the enhanced transcytosis was due to the Fc neonatal receptor (FcRn), which binds immune complexes at acidic pH and releases them at neutral pH. Finally, staining of human tissue revealed abundant FcRn expression on columnar epithelial cells of penile urethra and endocervix. Our findings reveal a novel mechanism wherein HIV-1 may facilitate its own transmission by usurping the antibody response directed against itself. These results have important implications for HIV vaccine development and for understanding the earliest events in HIV transmission.

HIV-1 Trans Infection of CD4(+) T Cells by Professional Antigen Presenting Cells

Since the 1990s we have known of the fascinating ability of a complex set of professional antigen presenting cells (APCs; dendritic cells, monocytes/macrophages, and B lymphocytes) to mediate HIV-1 trans infection of CD4(+) T cells. This results in a burst of virus replication in the T cells that is much greater than that resulting from direct, cis infection of either APC or T cells, or trans infection between T cells. Such APC-to-T cell trans infection first involves a complex set of virus subtype, attachment, entry, and replication patterns that have many similarities among APC, as well as distinct differences related to virus receptors, intracellular trafficking, and productive and nonproductive replication pathways. The end result is that HIV-1 can sequester within the APC for several days and be transmitted via membrane extensions intracellularly and extracellularly to T cells across the virologic synapse. Virus replication requires activated T cells that can develop concurrently with the events of virus transmission. Further research is essential to fill the many gaps in our understanding of these trans infection processes and their role in natural HIV-1 infection.