Polyanionic microbicides modify Toll-like receptor-mediated cervicovaginal immune responses

Gardnerella vaginalis alters cervicovaginal epithelial cell function through microbe-specific immune responses

BACKGROUND

The cervicovaginal (CV) microbiome is highly associated with vaginal health and disease in both pregnant and nonpregnant individuals. An overabundance of Gardnerella vaginalis (G. vaginalis) in the CV space is commonly associated with adverse reproductive outcomes including bacterial vaginosis (BV), sexually transmitted diseases, and preterm birth, while the presence of Lactobacillus spp. is often associated with reproductive health. While host-microbial interactions are hypothesized to contribute to CV health and disease, the mechanisms by which these interactions regulate CV epithelial function remain largely unknown.

RESULTS

Using an in vitro co-culture model, we assessed the effects of Lactobacillus crispatus (L. crispatus) and G. vaginalis on the CV epithelial barrier, the immune mediators that could be contributing to decreased barrier integrity and the immune signaling pathways regulating the immune response. G. vaginalis, but not L. crispatus, significantly increased epithelial cell death and decreased epithelial barrier integrity in an epithelial cell-specific manner. A G. vaginalis-mediated epithelial immune response including NF-κB activation and proinflammatory cytokine release was initiated partially through TLR2-dependent signaling pathways. Additionally, investigation of the cytokine immune profile in human CV fluid showed distinctive clustering of cytokines by Gardnerella spp. abundance and birth outcome.

CONCLUSIONS

The results of this study show microbe-specific effects on CV epithelial function. Altered epithelial barrier function through cell death and immune-mediated mechanisms by G. vaginalis, but not L. crispatus, indicates that host epithelial cells respond to bacteria-associated signals, resulting in altered epithelial function and ultimately CV disease. Additionally, distinct immune signatures associated with Gardnerella spp. or birth outcome provide further evidence that host-microbial interactions may contribute significantly to the biological mechanisms regulating reproductive outcomes. Video Abstract.

Protozoan-Viral-Bacterial Co-Infections Alter Galectin Levels and Associated Immunity Mediators in the Female Genital Tract

Co-infections with sexually transmittable pathogens are common and more likely in women with disturbed vaginal bacteriome. Among those pathogens, the protozoan parasite Trichomonas vaginalis (TV) is most common after accounting for the highly persistent DNA viruses human papillomavirus (HPV) and genital herpes. The parasitic infection often concurs with the dysbiotic syndrome diagnosed as bacterial vaginosis (BV) and both are associated with risks of superimposed viral infections. Yet, the mechanisms of microbial synergisms in evading host immunity remain elusive. We present clinical and experimental evidence for a new role of galectins, glycan-sensing family of proteins, in mixed infections. We assessed participants of the HIV Epidemiology Research Study (HERS) at each of their incident TV visits (223 case visits) matched to controls who remained TV-negative throughout the study. Matching criteria included age, race, BV (by Nugent score), HIV status, hysterectomy, and contraceptive use. Non-matched variables included BV status at 6 months before the matched visit, and variables examined at baseline, within 6 months of and/or at the matched visit e.g. HSV-2, HPV, and relevant laboratory and socio-demographic parameters. Conditional logistic regression models using generalized estimating equations calculated odds ratios (OR) for incident TV occurrence with each log₁₀ unit higher cervicovaginal concentration of galectins and cytokines. Incident TV was associated with higher levels of galectin-1, galectin-9, IL-1β and chemokines (ORs 1.53 to 2.91, p <0.001). Galectin-9, IL-1β and chemokines were up and galectin-3 down in TV cases with BV or intermediate Nugent versus normal Nugent scores (p <0.001). Galectin-9, IL-1β and chemokines were up in TV-HIV and down in TV-HPV co-infections. In-vitro, TV synergized with its endosymbiont Trichomonasvirus (TVV) and BV bacteria to upregulate galectin-1, galectin-9, and inflammatory cytokines. The BV-bacterium Prevotella bivia alone and together with TV downregulated galectin-3 and synergistically upregulated galectin-1, galectin-9 and IL-1β, mirroring the clinical findings of mixed TV–BV infections. P. bivia also downregulated TVV+TV-induced anti-viral response e.g. IP-10 and RANTES, providing a mechanism for conducing viral persistence in TV-BV co-infections. Collectively, the experimental and clinical data suggest that galectin-mediated immunity may be dysregulated and exploited by viral–protozoan–bacterial synergisms exacerbating inflammatory complications from dysbiosis and sexually transmitted infections.

The impact of aging on innate and adaptive immunity in the human female genital tract

Mucosal tissues in the human female reproductive tract (FRT) are primary sites for both gynecological cancers and infections by a spectrum of sexually transmitted pathogens, including human immunodeficiency virus (HIV), that compromise women's health. While the regulation of innate and adaptive immune protection in the FRT by hormonal cyclic changes across the menstrual cycle and pregnancy are being intensely studied, little to nothing is known about the alterations in mucosal immune protection that occur throughout the FRT as women age following menopause. The immune system in the FRT has two key functions: defense against pathogens and reproduction. After menopause, natural reproductive function ends, and therefore, two overlapping processes contribute to alterations in immune protection in aging women: menopause and immunosenescence. The goal of this review is to summarize the multiple immune changes that occur in the FRT with aging, including the impact on the function of epithelial cells, immune cells, and stromal fibroblasts. These studies indicate that major aspects of innate and adaptive immunity in the FRT are compromised in a site‐specific manner in the FRT as women age. Further, at some FRT sites, immunological compensation occurs. Overall, alterations in mucosal immune protection contribute to the increased risk of sexually transmitted infections (STI), urogenital infections, and gynecological cancers. Further studies are essential to provide a foundation for the development of novel therapeutic interventions to restore immune protection and reverse conditions that threaten women's lives as they age.

Differential Response of Gestational Tissues to TLR3 Viral Priming Prior to Exposure to Bacterial TLR2 and TLR2/6 Agonists

Background: Infection/inflammation is an important causal factor in spontaneous preterm birth (sPTB). Most mechanistic studies have concentrated on the role of bacteria, with limited focus on the role of viruses in sPTB. Murine studies support a potential multi-pathogen aetiology in which a double or sequential hit of both viral and bacterial pathogens leads to a higher risk preterm labour. This study aimed to determine the effect of viral priming on bacterial induced inflammation in human in vitro models of ascending and haematogenous infection.

Methods: Vaginal epithelial cells, and primary amnion epithelial cells and myocytes were used to represent cell targets of ascending infection while interactions between peripheral blood mononuclear cells (PBMCs) and placental explants were used to model systemic infection. To model the effect of viral priming upon the subsequent response to bacterial stimuli, each cell type was stimulated first with a TLR3 viral agonist, and then with either a TLR2 or TLR2/6 agonist, and responses compared to those of each agonist alone. Immunoblotting was used to detect cellular NF-κB, AP-1, and IRF-3 activation. Cellular TLR3, TLR2, and TLR6 mRNA was quantified by RT-qPCR. Immunoassays were used to measure supernatant cytokine, chemokine and PGE2 concentrations.

Results: TLR3 (“viral”) priming prior to TLR2/6 agonist (“bacterial”) exposure augmented the pro-inflammatory, pro-labour response in VECs, AECs, myocytes and PBMCs when compared to the effects of agonists alone. In contrast, enhanced anti-inflammatory cytokine production (IL-10) was observed in placental explants. Culturing placental explants in conditioned media derived from PBMCs primed with a TLR3 agonist enhanced TLR2/6 agonist stimulated production of IL-6 and IL-8, suggesting a differential response by the placenta to systemic inflammation compared to direct infection as a result of haematogenous spread. TLR3 agonism generally caused increased mRNA expression of TLR3 and TLR2 but not TLR6.

Conclusion: This study provides human in vitro evidence that viral infection may increase the susceptibility of women to bacterial-induced sPTB. Improved understanding of interactions between viral and bacterial components of the maternal microbiome and host immune response may offer new therapeutic options, such as antivirals for the prevention of PTB.

Highly-Exposed HIV-1 seronegative Female Commercial Sex Workers sustain in their genital mucosa increased frequencies of tolerogenic myeloid and regulatory T-cells

We and others have shown that HIV-1 highly-exposed seronegative (HESN) female commercial sex workers (CSWs) maintain low genital inflammatory conditions to prevent HIV infection. HIV-1 interacts with toll-like receptors (TLR)-7/8 to induce interferon (IFN)-α, an important antiviral and immunomodulatory cytokine, which act together with interleukin (IL)-10, human leukocyte antigen (HLA)-G and immunoglobulin-like transcript (ILT)-4 to initiate a "tolerogenic/regulatory" anti-inflammatory loop. In view of further unravelling elements associated with natural immunity to HIV-1, we have characterised TLR-7, IFN-α, IL-10, HLA-G and ILT-4 expression profiles in the genital tract of female CSWs and HIV-1-uninfected non-CSWs from Benin. Endocervical myeloid HLA-DR⁺ cells from HESN CSWs expressed higher levels of IFN-α, TLR-7, IL-10 and HLA-G than those from both HIV-1-infected CSWs and HIV-1-uninfected non-CSWs. Further characterization of the endocervical myeloid HLA-DR⁺ cells in HESN CSWs revealed a population of "tolerogenic" CD103⁺ CD14⁺ CD11c⁺ myeloid cells expressing high levels of IFN-α and IL-10. Concomitantly, HESN CSWs had higher frequencies of endocervical regulatory CD4⁺ T-cells when compared to those from the two other groups of women. These novel findings provide strong evidence to support the implication of tolerogenic myeloid cells expressing high levels of antiviral molecules in shaping the genital mucosal immune response to prevent HIV infection.

Can one size fit all? Approach to bacterial vaginosis in sub-Saharan Africa

Bacterial vaginosis (BV) is the most common vaginal disorder affecting women of reproductive age and is associated with increased risk of sexually transmitted infections such as human immunodeficiency syndrome (HIV-1). Sub-Saharan Africa has the highest BV and HIV-1 burden and yet very few studies have focused on understanding the aetiology of BV and its association with HIV in this region. It has been suggested that we need to accurately diagnose and treat BV to lower the risk of HIV infection globally. However, effective diagnosis requires knowledge of what constitutes a "healthy" cervicovaginal microbiome and current studies indicate that Lactobacillus crispatus might not be the only commensal protective against BV: healthy women from different countries and ethnicities harbour alternative commensals. Microbiotas associated with BV have also shown global variation, further complicating effective diagnosis via culture-based assays as some species are difficult to grow. Antibiotics and probiotics have been suggested to be key in controlling BV infection, but the efficacy of this treatment might rely on reconstituting endogenous commensals while targeting a specific species of BV-associated bacteria (BVAB). Alternatively, therapy could inhibit essential BV bacterial growth factors e.g. sialidases or provide anti-microbial compounds e.g. lactic acid associated with a healthy cervicovaginal microbiome. But without global investigation into the mechanism of BV pathogenesis and its association with HIV, selection of such compounds could be limited to Caucasian women from certain regions. To confirm this suggestion and guide future therapy we require standardised diagnostic assays and research methodologies. This review will focus on research papers that describe the global variation of BV aetiology and how this influences the identification of determinants of BV pathogenesis and potential probiotic and antimicrobial therapy.

Trichomonas vaginalis Lipophosphoglycan Exploits Binding to Galectin-1 and -3 to Modulate Epithelial Immunity

Trichomoniasis is the most common non-viral sexually transmitted infection caused by the vaginotropic extracellular protozoan parasite Trichomonas vaginalis. The infection is recurrent, with no lasting immunity, often asymptomatic, and linked to pregnancy complications and risk of viral infection. The molecular mechanisms of immune evasion by the parasite are poorly understood. We demonstrate that galectin-1 and -3 are expressed by the human cervical and vaginal epithelial cells and act as pathogen-recognition receptors for the ceramide phosphoinositol glycan core (CPI-GC) of the dominant surface protozoan lipophosphoglycan (LPG). We used an in vitro model with siRNA galectin knockdown epithelial clones, recombinant galectins, clinical Trichomonas isolates, and mutant protozoan derivatives to dissect the function of galectin-1 and -3 in the context of Trichomonas infection. Galectin-1 suppressed chemokines that facilitate recruitment of phagocytes, which can eliminate extracellular protozoa (IL-8) or bridge innate to adaptive immunity (MIP-3α and RANTES (regulated on activation normal T cell expressed and secreted)). Silencing galectin-1 increased and adding exogenous galectin-1 suppressed chemokine responses to Trichomonas or CPI-GC/LPG. In contrast, silencing galectin-3 reduced IL-8 response to LPG. Live Trichomonas depleted the extracellular levels of galectin-3. Clinical isolates and mutant Trichomonas CPI-GC that had reduced affinity to galectin-3 but maintained affinity to galectin-1 suppressed chemokine expression. Thus via CPI-GC binding, Trichomonas is capable of regulating galectin bioavailability and function to the benefit of its parasitic survival. These findings suggest novel approaches to control trichomoniasis and warrant further studies of galectin-binding diversity among clinical isolates as a possible source for symptom disparity in parasitic infections.

Recent advances on anti-HIV vaginal delivery systems development

A review of the recent outcomes regarding technologies to prevent vaginal transmission of HIV, mainly by using antiretroviral (ARV) drugs formulated as microbicides. An introduction about the HIV transmission mechanisms by the vaginal route is included, together with the recent challenges faced for development of successful microbicide products. The outcomes of clinical evaluations are mentioned, and the different formulation strategies studied to-date, with the requirements, advantages, disadvantages and limitations of each dosage-form type, are presented. Finally, the recent attempts to apply various types of nanotechnologies in order to develop advanced microbicide-products and overcome existing limitations, are discussed.

Enhancing Interferon Regulatory Factor 7 Mediated Antiviral Responses and Decreasing Nuclear Factor Kappa B Expression Limit HIV-1 Replication in Cervical Tissues

Establishment of a productive HIV-1 infection in the female reproductive tract likely depends on the balance between anti-viral and pro-inflammatory responses leading to activation and proliferation of HIV target cells. Immune modulators that boost anti-viral and depress pro-inflammatory immune responses may decrease HIV-1 infection or replication. Polyinosinic:polycytidylic [Poly (I:C)] has been reported to down-regulate HIV-1 replication in immune cell subsets and lymphoid tissues, yet the scope and mechanisms of poly (I:C) regulation of HIV-1 replication in the cervicovaginal mucosa, the main portal of viral entry in women remain unknown. Using a relevant, underexplored ex vivo cervical tissue model, we demonstrated that poly (I:C) enhanced Interferon Regulatory Factor (IRF)7 mediated antiviral responses and decreased tissue Nuclear Factor Kappa B (NFκB) RNA expression. This pattern of cellular transcription factor expression correlated with decreased HIV-1 transcription and viral release. Reducing IRF7 expression up-regulated HIV-1 and NFκB transcription, providing proof of concept for the critical involvement of IRF7 in cervical tissues. By combining poly (I:C) with a suboptimal concentration of tenofovir, the leading anti-HIV prophylactic microbicide candidate, we demonstrated an earlier and greater decrease in HIV replication in poly (I:C)/tenofovir treated tissues compared with tissues treated with tenofovir alone, indicating overall improved efficacy. Poly (I:C) decreases HIV-1 replication by stimulating IRF7 mediated antiviral responses while reducing NFκB expression. Early during the infection, poly (I:C) improved the anti-HIV-1 activity of suboptimal concentrations of tenofovir likely to be present during periods of poor adherence i.e. inconsistent or inadequate drug use. Understanding interactions between anti-viral and pro-inflammatory immune responses in the genital mucosa will provide crucial insights for the identification of targets that can be harnessed to develop preventative combination strategies to improve the efficacy of topical or systemic antiviral prophylactic agents and protect women from HIV-1 and other sexually transmitted infections.

A quantitative multiplex nuclease protection assay reveals immunotoxicity gene expression profiles in the rabbit model for vaginal drug safety evaluation

Any vaginal product that alters the mucosal environment and impairs the immune barrier increases the risk of sexually transmitted infections, especially HIV infection, which thrives on mucosal damage and inflammation. The FDA-recommended rabbit vaginal irritation (RVI) model serves as a first line selection tool for vaginal products; however, for decades it has been limited to histopathology scoring, insufficient to select safe anti-HIV microbicides. In this study we incorporate to the RVI model a novel quantitative nuclease protection assay (qNPA) to quantify mRNA levels of 25 genes representing leukocyte differentiation markers, toll-like receptors (TLR), cytokines, chemokines, epithelial repair, microbicidal and vascular markers, by designing two multiplex arrays. Tissue sections were obtained from 36 rabbits (6 per treatment arm) after 14 daily applications of a placebo gel, saline, 4% nonoxynol-9 (N-9), and three combinations of the anti-HIV microbicides tenofovir (TFV) and UC781 in escalating concentrations (highest: 10% TFV+2.5%UC781). Results showed that increased expression levels of toll-like receptor (TLR)-4, interleukin (IL)-1β, CXCL8, epithelial membrane protein (EMP)-1 (P<0.05), and decreased levels of TLR2 (P<0.05), TLR3 and bactericidal permeability increasing protein (BPI) (P<0.001) were associated with cervicovaginal mucosal alteration (histopathology). Seven markers showed a significant linear trend predicting epithelial damage (up with CD4, IL-1β, CXCL8, CCL2, CCL21, EMP1 and down with BPI). Despite the low tissue damage RVI scores, the high-dose microbicide combination gel caused activation of HIV host cells (SLC and CD4) while N-9 caused proinflammatory gene upregulation (IL-8 and TLR4) suggesting a potential for increasing risk of HIV via different mechanisms depending on the chemical nature of the test product.

Toward early safety alert endpoints: exploring biomarkers suggestive of microbicide failure

Several microbicides, including nonoxynol-9 (N-9) and cellulose sulfate (CS), looked promising during early trials but failed in efficacy trials. We aimed to identify Phase I mucosal safety endpoints that might explain that failure. In a blinded, randomized, parallel trial, 60 healthy premenopausal sexually abstinent women applied Universal HEC placebo, 6% CS or 4% N-9 gel twice daily for 13½ days. Endpoints included immune biomarkers in cervicovaginal lavage (CVL) and endocervical cytobrushes, inflammatory infiltrates in vaginal biopsies, epithelial integrity by naked eye, colposcopy, and histology, CVL anti-HIV activity, vaginal microflora, pH, and adverse events. Twenty women enrolled per group. Soluble/cellular markers were similar with CS and placebo, except secretory leukocyte protease inhibitor (SLPI) levels decreased in CVL, and CD3(+) and CD45(+) cells increased in biopsies after CS use. Increases in interleukin (IL)-8, IL-1, IL-1RA, and myeloperoxidase (MPO) and decreases in SLPI were significant with N-9. CVL anti-HIV activity was significantly higher during CS use compared to N-9 or placebo. CS users tended to have a higher prevalence of intermediate Nugent score, Escherichia coli, and Enterococcus and fewer gram-negative rods. Most Nugent scores diagnostic for bacterial vaginosis were in N-9 users. All cases of histological inflammation or deep epithelial disruption occurred in N-9 users. While the surfactant N-9 showed obvious biochemical and histological signs of inflammation, more subtle changes, including depression of SLPI, tissue influx of CD45(+) and CD3(+) cells, and subclinical microflora shifts were associated with CS use and may help to explain the clinical failure of nonsurfactant microbicides.

The villain team-up or how Trichomonas vaginalis and bacterial vaginosis alter innate immunity in concert

Objectives

Complex interactions of vaginal microorganisms with the genital tract epithelium shape mucosal innate immunity, which holds the key to sexual and reproductive health. Bacterial vaginosis (BV), a microbiome-disturbance syndrome prevalent in reproductive-age women, occurs commonly in concert with trichomoniasis, and both are associated with increased risk of adverse reproductive outcomes and viral infections, largely attributable to inflammation. To investigate the causative relationships among inflammation, BV and trichomoniasis, we established a model of human cervicovaginal epithelial cells colonised by vaginal Lactobacillus isolates, dominant in healthy women, and common BV species (Atopobium vaginae, Gardnerella vaginalis and Prevotella bivia).

Methods

Colonised epithelia were infected with Trichomonas vaginalis (TV) or exposed to purified TV virulence factors (membrane lipophosphoglycan (LPG), its ceramide-phosphoinositol-glycan core (CPI-GC) or the endosymbiont Trichomonas vaginalis virus (TVV)), followed by assessment of bacterial colony-forming units, the mucosal anti-inflammatory microbicide secretory leucocyte protease inhibitor (SLPI), and chemokines that drive pro-inflammatory, antigen-presenting and T cells.

Results

TV reduced colonisation by Lactobacillus but not by BV species, which were found inside epithelial cells. TV increased interleukin (IL)-8 and suppressed SLPI, likely via LPG/CPI-GC, and upregulated IL-8 and RANTES, likely via TVV as suggested by use of purified pathogenic determinants. BV species A vaginae and G vaginalis induced IL-8 and RANTES, and also amplified the pro-inflammatory responses to both LPG/CPI-GC and TVV, whereas P bivia suppressed the TV/TVV-induced chemokines.

Conclusions

These molecular host–parasite–endosymbiont–bacteria interactions explain epidemiological associations and suggest a revised paradigm for restoring vaginal immunity and preventing BV/TV-attributable inflammatory sequelae in women.

Homeostatic properties of Lactobacillus jensenii engineered as a live vaginal anti-HIV microbicide

Background

Vaginal probiotics are investigated as a binary strategy for prevention of bacterial vaginosis and HIV. We applied an innovative experimental model using primary and immortalized human cervical and vaginal epithelial cells to assess the functional properties of Lactobacillus jensenii, a predominant constituent of the healthy vaginal microbiome, engineered to express the HIV-1 entry inhibitor modified cyanovirin-N (mCV-N). In this model bacteria colonize the epithelial cells over a period of 24-72 h. Staurosporine and the Toll-like receptor 2/6 ligand macrophage-activating lipopeptide-2 (MALP-2) serve as positive controls for apoptosis and proinflammatory activation, respectively. In 24-hour intervals, the colonized epithelium is assessed microscopically, supernatants are collected for measurement of soluble immunoinflammatory mediators and production of CV-N, and cells are lysed for assessment of: 1) apoptosis by cleaved versus total caspase-3 assay; 2) NF-κB activation by a luciferase reporter assay; or 3) epithelia-associated colony forming units (CFU) in Brucella agar.

Results

Wild type (WT) L. jensenii 1153 consistently colonized cervical and vaginal cells in the absence of epithelial damage and apoptosis. The bioengineered derivatives expressing mCV-N or control plasmids showed the same stable colonization pattern, which was reproducible between technologists and bacterial batches (CFU coefficient of variation <10% within and between experiments and epithelial cell types). MALP-2 activated NF-κB and caused fold-increased levels of proinflammatory mediators with clinically established significance in the cervicovaginal environment (IL-1α, IL-1β, IL-6, TNF-α, IL-8, RANTES, MIP-3α, and ICAM-1), measured by a multiplex electrochemiluminescence assay. At the same time levels of protective anti-inflammatory mediators interleukin 1 receptor antagonist (IL-1RA) and secretory leukocyte protease inhibitor (SLPI), both measured by ELISA, remained constant (IL-1RA) or moderately increased (SLPI). Similarly to MALP-2, colonization by L. jensenii WT activated NF-κB; however, unlike the synthetic TLR2/6 ligand, the live microorganisms did not induce significant changes in the secreted levels across all inflammation-associated proteins. The mCV-N production and function were confirmed by western blot and a HIV-1 gp120 binding assay, respectively. The bioengineered lactobacilli expressed mCV-N with anti-HIV activity preserved in the epithelial cell context and caused no significant immunoinflammatory changes as compared to the WT L. jensenii.

Conclusions

These results highlight the translational value of the colonization model and justify further clinical investigation of the homeostatic and anti-HIV effectiveness of the L. jensenii derivates.

Twice-daily application of HIV microbicides alter the vaginal microbiota

Vaginal HIV microbicides offer great promise in preventing HIV transmission, but failures of phase 3 clinical trials, in which microbicide-treated subjects had an increased risk of HIV transmission, raised concerns about endpoints used to evaluate microbicide safety. A possible explanation for the increased transmission risk is that the agents shifted the vaginal bacterial community, resulting in loss of natural protection and enhanced HIV transmission susceptibility. We characterized vaginal microbiota, using pyrosequencing of bar-coded 16S rRNA gene fragments, in samples from 35 healthy, sexually abstinent female volunteer subjects (ages 18 to 50 years) with regular menses in a repeat phase 1 study of twice-daily application over 13.5 days of 1 of 3 gel products: a hydroxyethylcellulose (HEC)-based “universal” placebo (10 subjects), 6% cellulose sulfate (CS; 13 subjects), and 4% nonoxynol-9 (N-9; 12 subjects). We used mixed effects models inferred using Bayesian Markov chain Monte Carlo methods, which showed that treatment with active agents shifted the microbiota toward a community type lacking significant numbers of Lactobacillus spp. and dominated by strict anaerobes. This state of the vaginal microbiota was associated with a low or intermediate Nugent score and was not identical to bacterial vaginosis, an HIV transmission risk factor. The placebo arm contained a higher proportion of communities dominated by Lactobacillus spp., particularly L. crispatus, throughout treatment. The data suggest that molecular evaluation of microbicide effects on vaginal microbiota may be a critical endpoint that should be incorporated in early clinical assessment of microbicide candidates.

Despite large prevention efforts, HIV transmission and acquisition rates remain unacceptably high. In developing countries, transmission mainly occurs through heterosexual intercourse, where women are significantly more vulnerable to infection than men. Vaginal microbicides are considered to be one of the most promising female-controlled products, in that women themselves insert the microbicides into the vagina to prevent HIV transmission during sexual intercourse. The failure of several microbicides in clinical trials has raised questions concerning the low in vivo efficacy of such anti-HIV molecules. This study was designed to gain insights into the failures of two microbicides by testing the hypothesis that the microbicides negatively affect a critical line of defense against HIV, the vaginal microbiota. The results suggest that in the early assessment of candidate microbicides, culture-independent evaluation of their effect on the vaginal microbiota should be considered and may constitute a critical endpoint.

Natural Immunity to HIV: a delicate balance between strength and control

Understanding how the mucosal immune system in the human female reproductive tract might prevent or facilitate HIV infection has important implications for the design of effective interventions. We and others have established cohorts of highly-exposed, HIV-seronegative individuals, such as HIV-uninfected commercial sex workers, who have remained HIV-negative after more than 5 years of active prostitution. Observations obtained in studies of such individuals, who represent a model of natural immunity to HIV, indicate that HIV resistance may be associated with the host's capacity to preserve systemic integrity by constraining immune activity and controlling inflammatory conditions at the mucosal point of entry. This likely necessitates the orchestration of balanced, first-line and adaptive immune responses.

Endobiont viruses sensed by the human host - beyond conventional antiparasitic therapy

Wide-spread protozoan parasites carry endosymbiotic dsRNA viruses with uncharted implications to the human host. Among them, Trichomonas vaginalis, a parasite adapted to the human genitourinary tract, infects globally ∼250 million each year rendering them more susceptible to devastating pregnancy complications (especially preterm birth), HIV infection and HPV-related cancer. While first-line antibiotic treatment (metronidazole) commonly kills the protozoan pathogen, it fails to improve reproductive outcome. We show that endosymbiotic Trichomonasvirus, highly prevalent in T. vaginalis clinical isolates, is sensed by the human epithelial cells via Toll-like receptor 3, triggering Interferon Regulating Factor -3, interferon type I and proinflammatory cascades previously implicated in preterm birth and HIV-1 susceptibility. Metronidazole treatment amplified these proinflammatory responses. Thus, a new paradigm targeting the protozoan viruses along with the protozoan host may prevent trichomoniasis-attributable inflammatory sequelae.

Factors Important to the Prioritization and Development of Successful Topical Microbicides for HIV-1

Significant advancements in topical microbicide development have occurred since the prevention strategy was first described as a means to inhibit the sexual transmission of HIV-1. The lack of clinical efficacy of the first generation microbicide products has focused development attention on specific antiretroviral agents, and these agents have proven partially successful in human clinical trials. With greater understanding of vaginal and rectal virus infection, replication, and dissemination, better microbicide products and delivery strategies should result in products with enhanced potency. However, a variety of development gaps exist which relate to product dosing, formulation and delivery, and pharmacokinetics and pharmacodynamics which must be better understood in order to prioritize microbicide products for clinical development. In vitro, ex vivo, and in vivo models must be optimized with regard to these development gaps in order to put the right product at the right place, at the right time, and at the right concentration for effective inhibition of virus transmission. As the microbicide field continues to evolve, we must harness the knowledge gained from unsuccessful and successful clinical trials and development programs to continuously enhance our preclinical development algorithms.

Microbial products alter the expression of membrane-associated mucin and antimicrobial peptides in a three-dimensional human endocervical epithelial cell model

Our understanding of the mechanisms that regulate tissue-specific mucosal defense can be limited by the lack of appropriate human in vitro models. The endocervix lies between the microbe-rich vaginal cavity and the relatively sterile endometrium and is a major portal of entry for Chlamydia trachomatis, Neisseria gonorrhoeae, Mycoplasma genitalium, human immunodeficiency virus (HIV), and herpes simplex virus (HSV) infection in women. The endocervix is lined with a simple epithelium, and these cells produce mucus, which plays a key role in immune defense and reproduction. Here we describe the development of a human three-dimensional endocervical epithelial cell model generated by rotating wall vessel bioreactor technology. The model is composed of cellular aggregates that recapitulate major structural and barrier properties essential for the function and protection of the endocervix, including junctional complexes, microvilli, innate immune receptors, antimicrobial peptides, and mucins, the major structural component of mucus. Using this model, we also report, for the first time, that the membrane-associated mucin genes MUC1, MUC4, and MUC16 are differentially regulated in these aggregates by different bacterial and viral products. Differential induction of antimicrobial peptides was also observed with these products. Together these data define unique and flexible innate endocervical immune signatures that follow exposure to microbial products and that likely play a critical role in the outcome of pathogen challenge at this site.

Targeting HIV-1 innate immune responses therapeutically

PURPOSE OF REVIEW

The early stage of HIV-1 infection is when the virus is most vulnerable, and should therefore offer the best opportunity for therapeutic interventions. This review addresses the recent progress in the understanding of innate immune responses against HIV-1 with focus on the potential targets for prevention of viral acquisition, replication and dissemination.

RECENT FINDINGS

Research indicates that the host-derived factor trappin-2/elafin is protective against HIV, whereas semen-derived enhancer of viral infection and defensins 5 and 6 enhance viral transmission. Further, studies suggest that stimulation of TLR4 and inhibition of TLR7-9 pathways may be HIV suppressive. The regulation and function of viral restriction factors tetherin and APOBEC3G have been investigated and a molecule mimicking the premature uncoating achieved by TRIM5α, PF74, has been identified. Chloroquine has been shown to inhibit plasmacytoid dendritic cell activation and suppress negative modulators of T-cell responses. Blockade of HMBG1 has been found to restore natural-killer-cell-mediated killing of infected dendritic cells, normally suppressed by HIV-1. Interestingly, when used as adjuvants, EAT-2 and heat shock protein gp96 reportedly enhance innate immune responses.

SUMMARY

Several targets for innate immunity-mediated therapeutics have been identified. Nonetheless, more research is required to unveil their underlying mechanisms and interactions before testing these molecules in clinical trials.

HIV gp120 binds to mannose receptor on vaginal epithelial cells and induces production of matrix metalloproteinases

Background

During sexual transmission of HIV in women, the virus breaches the multi-layered CD4 negative stratified squamous epithelial barrier of the vagina, to infect the sub-epithelial CD4 positive immune cells. However the mechanisms by which HIV gains entry into the sub-epithelial zone is hitherto unknown. We have previously reported human mannose receptor (hMR) as a CD4 independent receptor playing a role in HIV transmission on human spermatozoa. The current study was undertaken to investigate the expression of hMR in vaginal epithelial cells, its HIV gp120 binding potential, affinity constants and the induction of matrix metalloproteinases (MMPs) downstream of HIV gp120 binding to hMR.

Principal Findings

Human vaginal epithelial cells and the immortalized vaginal epithelial cell line Vk2/E6E7 were used in this study. hMR mRNA and protein were expressed in vaginal epithelial cells and cell line, with a molecular weight of 155 kDa. HIV gp120 bound to vaginal proteins with high affinity, (Kd = 1.2±0.2 nM for vaginal cells, 1.4±0.2 nM for cell line) and the hMR antagonist mannan dose dependently inhibited this binding. Both HIV gp120 binding and hMR exhibited identical patterns of localization in the epithelial cells by immunofluorescence. HIV gp120 bound to immunopurified hMR and affinity constants were 2.9±0.4 nM and 3.2±0.6 nM for vaginal cells and Vk2/E6E7 cell line respectively. HIV gp120 induced an increase in MMP-9 mRNA expression and activity by zymography, which could be inhibited by an anti-hMR antibody.

Conclusion

hMR expressed by vaginal epithelial cells has high affinity for HIV gp120 and this binding induces production of MMPs. We propose that the induction of MMPs in response to HIV gp120 may lead to degradation of tight junction proteins and the extracellular matrix proteins in the vaginal epithelium and basement membrane, leading to weakening of the epithelial barrier; thereby facilitating transport of HIV across the vaginal epithelium.

Influence of lactic acid on endogenous and viral RNA-induced immune mediator production by vaginal epithelial cells

OBJECTIVE

To evaluate the influence of lactic acid on immune mediator release from vaginal epithelial cells.

METHODS

The human vaginal epithelial cell line, VK2/E6E7, was cultured in the presence or absence of physiological concentrations of lactic acid, and in the presence or absence of the viral Toll-like receptor 3 agonist, poly (inosinic acid:cytidylic acid). Supernatants were assayed by enzyme-linked immunosorbent assay (ELISA) for interleukin (IL)-1β, IL-6, IL-8, IL-23, transforming growth factor (TGF)-β and secretory leukocyte protease inhibitor.

RESULTS

Vaginal epithelial cells spontaneously released IL-1β (25.9 pg/mL), IL-8 (1.0 ng/mL), TGF-β (175 pg/mL), and secretory leukocyte protease inhibitor (33.8 ng/mL). Only TGF-β production was marginally enhanced (49%) by addition of lactic acid alone. Poly (inosinic acid:cytidylic acid) by itself stimulated the release of IL-6 (305 pg/mL) and enhanced IL-8 production (2.8 ng/mL). The combination of poly (inosinic acid:cytidylic acid) and lactic acid markedly increased IL-8 production (5.0 ng/mL) and induced the release of IL-1β (96.2 pg/mL). The poly (inosinic acid:cytidylic acid)-mediated lactic acid effect on IL-1β and IL-8 release was abrogated when the lactic acid was neutralized or if acetic acid was substituted for lactic acid.

CONCLUSION

Lactic acid enhances the release of selective mediators from vaginal epithelial cells and stimulates antiviral immune responses.

Primary human epithelial cell culture system for studying interactions between female upper genital tract and sexually transmitted viruses, HSV-2 and HIV-1

Evidence from clinical and epidemiological studies indicates that women are disproportionately susceptible to sexually transmitted viral infections. To understand the underlying biological basis for this increased susceptibility, more studies are needed to examine the acute events in the female reproductive tract following exposure to viruses during sexual transmission. The epithelial lining of the female reproductive tract is the primary barrier that sexually transmitted viruses, such as HIV-1 and HSV-2 need to infect or traverse, in order to initiate and establish productive infection. We have established an ex-vivo primary culture system to grow genital epithelial cells from upper reproductive tract tissues of women. Using these cultures, we have extensively examined the interactions between epithelial cells of the female genital tract and HSV-2 and HIV-1. In this review, we describe in detail the experimental protocol to grow these cultures, monitor their differentiation and inoculate with HSV-2 and HIV-1. Prospective use of these cultures to re-create the microenvironment in the reproductive tract is discussed.

Nuclear receptor signaling inhibits HIV-1 replication in macrophages through multiple trans-repression mechanisms

Sexually transmitted pathogens activate HIV-1 replication and inflammatory gene expression in macrophages through engagement of Toll-like receptors (TLRs). Ligand-activated nuclear receptor (NR) transcription factors, including glucocorticoid receptor (GR), peroxisome proliferator-activated receptor gamma (PPARγ), and liver X receptor (LXR), are potent inhibitors of TLR-induced inflammatory gene expression. We therefore hypothesized that ligand-activated NRs repress both basal and pathogen-enhanced HIV-1 replication in macrophages by directly repressing HIV-1 transcription and by ameliorating the local proinflammatory response to pathogens. We show that the TLR2 ligand PAM3CSK4 activated virus transcription in macrophages and that NR signaling repressed both basal and TLR-induced HIV-1 transcription. NR ligand treatment repressed HIV-1 expression when added concurrently with TLR ligands and in the presence of cycloheximide, demonstrating that they act independently of new cellular gene expression. We found that treatment with NR ligands inhibited the association of AP-1 and NF-κB subunits, as well as the coactivator CBP, with the long terminal repeat (LTR). We show for the first time that the nuclear corepressor NCoR is bound to HIV-1 LTR in unstimulated macrophages and is released from the LTR after TLR engagement. Treatment with PPARγ and LXR ligands, but not GR ligands, prevented this TLR-induced clearance of NCoR from the LTR. Our data demonstrate that both classical and nonclassical trans-repression mechanisms account for NR-mediated HIV-1 repression. Finally, NR ligand treatment inhibited the potent proinflammatory response induced by PAM3CSK4 that would otherwise activate HIV-1 expression in infected cells. Our findings provide a rationale for studying ligand-activated NRs as modulators of basal and inflammation-induced HIV-1 replication.

Differential regulation of endogenous pro-inflammatory cytokine genes by medroxyprogesterone acetate and norethisterone acetate in cell lines of the female genital tract

BACKGROUND

Medroxyprogesterone acetate (MPA) and norethisterone (NET) and its derivatives are widely used in female reproductive therapy, but little is known about their mechanisms of action via steroid receptors in the female genital tract. MPA used as a contraceptive has been implicated in effects on local immune function. However, the relative effects of progesterone (Prog), MPA and norethisterone acetate (NET-A) on cytokine gene expression in the female genital tract are unknown.

STUDY DESIGN

Using two epithelial cell lines generated from normal human vaginal (Vk2/E6E7) and ectocervical (Ect1/E6E7) cells as in vitro cell culture model systems for mucosal immunity of the female cervicovaginal environment, we investigated steroid receptor expression and activity as well as regulation of cytokine/chemokine genes by MPA and NET-A, as compared to the endogenous hormone Prog.

RESULTS

We show that the Prog, androgen, glucocorticoid and estrogen receptors (PR, AR, GR and ER, respectively) are expressed in both the Vk2/E6E7 and Ect1/E6E7 cell lines, and that the GR and AR are transcriptionally active. This study is the first to show ligand-, promoter- and cell-specific regulation of IL-6, IL-8 and RANTES (regulated-upon-activation, normal T cell expressed and secreted) gene expression by Prog, MPA and NET-A in these cell lines. Moreover, we show that the repression of the TNF-α-induced RANTES gene by MPA in the Ect1/E6E7 cell line is mediated by the AR.

CONCLUSION

Collectively, these data demonstrate that cell lines from different anatomical sites of the female genital tract respond differently to Prog and the synthetic progestins, most likely due to differential actions via different steroid receptors. The results highlight the importance of choice of progestins for immune function in the cervicovaginal environment. They further suggest that choice of progestins in endocrine therapy may have implications for women's risk of susceptibility to infections due to differential actions on genes involved in inflammation and immune function.

Glucocorticoid-independent repression of tumor necrosis factor (TNF) alpha-stimulated interleukin (IL)-6 expression by the glucocorticoid receptor: a potential mechanism for protection against an excessive inflammatory response

TNFα signaling and cytokine levels play a crucial role in cervical immunity and the host response to infections. We investigated the role of liganded and unliganded glucocorticoid receptor (GR) in IL-6 and IL-8 gene regulation in response to TNFα in the End1/E6E7 immortalized human endocervical epithelial cell line. In the absence of glucocorticoids, both decreasing GR protein levels by an siRNA strategy and results with the GR antagonist RU486 suggest a role for the unliganded GR in reduction of TNFα-induced IL-6 and IL-8 mRNA levels in End1/E6E7 cells. Moreover, GR-dependent repression of endogenous IL-6 mRNA as well as a minimal IL-6 promoter-reporter gene is also demonstrated in COS-1 cells in the absence of GR ligand, suggesting a transcriptional mechanism that is not cell-specific. TNFα induced recruitment of both the unliganded GR and GR-interacting protein type 1 (GRIP-1) to the IL-6 promoter. This, together with GRIP-1 overexpression studies, suggests a function for GRIP-1 as a GR co-repressor in this context. TNFα was shown to induce phosphorylation of the unliganded human GR at Ser-226 but not Ser-211, unlike dexamethasone, which induced hyperphosphorylation at both serine residues. Ser-226 is shown to be required for the ligand-independent GR-mediated repression of IL-6 in response to TNFα. Taken together, these results support a model whereby the unliganded GR attenuates TNFα-stimulated IL-6 transcription by a mechanism involving selective phosphorylation and recruitment of the unliganded GR and GRIP-1 to the IL-6 promoter. These findings suggest the presence of a novel autoregulatory mechanism that may prevent overproduction of IL-6 in the endocervix, possibly protecting against negative effects of excessive inflammation.

Innate and adaptive immunity at mucosal surfaces of the female reproductive tract: stratification and integration of immune protection against the transmission of sexually transmitted infections

This review examines the multiple levels of pre-existing immunity in the upper and lower female reproductive tract. In addition, we highlight the need for further research of innate and adaptive immune protection of mucosal surfaces in the female reproductive tract. Innate mechanisms include the mucus lining, a tight epithelial barrier and the secretion of antimicrobial peptides and cytokines by epithelial and innate immune cells. Stimulation of the innate immune system also serves to bridge the adaptive arm resulting in the generation of pathogen-specific humoral and cell-mediated immunity. Less understood are the multiple components that act in a coordinated way to provide a network of ongoing protection. Innate and adaptive immunity in the human female reproductive tract are influenced by the stage of menstrual cycle and are directly regulated by the sex steroid hormones, progesterone and estradiol. Furthermore, the effect of hormones on immunity is mediated both directly on immune and epithelial cells and indirectly by stimulating growth factor secretion from stromal cells. The goal of this review is to focus on the diverse aspects of the innate and adaptive immune systems that contribute to a unique network of protection throughout the female reproductive tract.

Preventing mucosal HIV transmission with topical microbicides: challenges and opportunities

A combination of prevention and treatment modalities will be needed to successfully control the global spread of HIV. Microbicides, drug products topically applied to mucosal surfaces to prevent HIV infection, are one of these biomedical interventions that hold great promise. In order to be efficacious, microbicides must overcome several challenges imposed by the mucosal microenvironment they intend to protect and the mischievous human immunodeficiency virus with its enormous capacity to adapt. Recent data, however, supports the establishment of the primo-infection by only a small number of founder viruses, which are highly vulnerable to microbicidal intervention at the initial stages of mucosal invasion. The biological foundation of these challenges and opportunities in microbicide development is explored in this review. This article forms part of a special supplement on presentations covering HIV transmission and microbicides, based on the symposium "Trends in Microbicide Formulations", held on 25 and 26 January 2010, Arlington, VA.

Microbicides in the prevention of HIV infection: current status and future directions

More than 28 years since the first cases of HIV/AIDS, there is still no cure or vaccine. The worst affected region is sub-Saharan Africa and, increasingly, it is young women who are bearing the brunt of the epidemic. Consequently, there is an urgent need for HIV prevention options for women in developing countries. Microbicides are topical products that can be used vaginally by women to impede sexual transmission of HIV and thus represent one of the most promising prevention strategies. Efficacy trials with early nonspecific microbicide gels have so far been unsuccessful, but the field has now switched its focus to products containing highly potent and highly specific antiretroviral drugs that are easier to use, and can be formulated in a variety of dosage forms to suit individual and regional preferences. However, these products have their own challenges, with a greater likelihood of absorption, and the potential for systemic toxicities or the development of resistance in infected individuals who are unaware of their HIV status. The conduct of clinical trials is complex for all microbicides, with limited availability of trial sites, difficulties in dose selection and safety monitoring, and a lack of a truly objective measure of adherence. Once a microbicide has been shown to be safe and effective, there will need to be a clear pathway to regulatory approval, and the successful launch of a product will depend on having in place appropriate methods for distribution to the women who need it, along with a strategy for ensuring that they use it correctly. This will require substantial effort in terms of education and community engagement, and these activities need to be initiated well in advance of microbicide rollout.

Phosphorothioate 2' deoxyribose oligomers as microbicides that inhibit human immunodeficiency virus type 1 (HIV-1) infection and block Toll-like receptor 7 (TLR7) and TLR9 triggering by HIV-1

Topical microbicides may prove to be an important strategy for preventing human immunodeficiency virus type 1 (HIV-1) transmission. We examined the safety and efficacy of sequence-nonspecific phosphorothioate 2' deoxyribose oligomers as potential novel microbicides. A short, 13-mer poly(T) phosphorothioate oligodeoxynucleotide (OPB-T) significantly inhibited infection of primary peripheral blood mononuclear cells (PBMC) by high-titer HIV-1(Ba-L) and simian immunodeficiency virus mac251 (SIV(mac251)). Continuous exposure of human vaginal and foreskin tissue explants to OPB-T showed no toxicity. An abasic 14-mer phosphorothioate 2' deoxyribose backbone (PDB) demonstrated enhanced anti-HIV-1 activity relative to OPB-T and other homo-oligodeoxynucleotide analogs. When PDB was used to pretreat HIV-1, PDB was effective against R5 and X4 isolates at a half-maximal inhibitory concentration (IC(50)) of <1 μM in both PBMC and P4-R5 MAGI cell infections. PDB also reduced HIV-1 infectivity following the binding of virus to target cells. This novel topical microbicide candidate exhibited an excellent in vitro safety profile in human PBMC and endocervical epithelial cells. PDB also retained activity in hydroxyethylcellulose gel at pH 4.4 and after transition to a neutral pH and was stable in this formulation for 30 days at room temperature. Furthermore, the compound displayed potent antiviral activity following incubation with a Lactobacillus strain derived from normal vaginal flora. Most importantly, PDB can inhibit HIV-1-induced alpha interferon production. Phosphorothioate 2' deoxyribose oligomers may therefore be promising microbicide candidates that inhibit HIV-1 infection and also dampen the inflammation which is critical for the initial spread of the virus.

Pattern recognition via the toll-like receptor system in the human female genital tract

The mucosal surface of the female genital tract is a complex biosystem, which provides a barrier against the outside world and participates in both innate and acquired immune defense systems. This mucosal compartment has adapted to a dynamic, non-sterile environment challenged by a variety of antigenic/inflammatory stimuli associated with sexual intercourse and endogenous vaginal microbiota. Rapid innate immune defenses against microbial infection usually involve the recognition of invading pathogens by specific pattern-recognition receptors recently attributed to the family of Toll-like receptors (TLRs). TLRs recognize conserved pathogen-associated molecular patterns (PAMPs) synthesized by microorganisms including bacteria, fungi, parasites, and viruses as well as endogenous ligands associated with cell damage. Members of the TLR family, which includes 10 human TLRs identified to date, recognize distinct PAMPs produced by various bacterial, fungal, and viral pathogens. The available literature regarding the innate immune system of the female genital tract during human reproductive processes was reviewed in order to identify studies specifically related to the expression and function of TLRs under normal as well as pathological conditions. Increased understanding of these molecules may provide insight into site-specific immunoregulatory mechanisms in the female reproductive tract.

Development and characterization of a three-dimensional organotypic human vaginal epithelial cell model

We have developed an in vitro human vaginal epithelial cell (EC) model using the innovative rotating wall vessel (RWV) bioreactor technology that recapitulates in vivo structural and functional properties, including a stratified squamous epithelium with microvilli, tight junctions, microfolds, and mucus. This three-dimensional (3-D) vaginal model provides a platform for high-throughput toxicity testing of candidate microbicides targeted to combat sexually transmitted infections, effectively complementing and extending existing testing systems such as surgical explants or animal models. Vaginal ECs were grown on porous, collagen-coated microcarrier beads in a rotating, low fluid-shear environment; use of RWV bioreactor technology generated 3-D vaginal EC aggregates. Immunofluorescence and scanning and transmission electron microscopy confirmed differentiation and polarization of the 3-D EC aggregates among multiple cell layers and identified ultrastructural features important for nutrient absorption, cell-cell interactions, and pathogen defense. After treatment with a variety of toll-like receptor (TLR) agonists, cytokine production was quantified by cytometric bead array, confirming that TLRs 2, 3, 5, and 6 were expressed and functional. The 3-D vaginal aggregates were more resistant to nonoxynol-9 (N-9), a contraceptive and previous microbicide candidate, when compared to two-dimensional monolayers of the same cell line. A dose-dependent production of tumor necrosis factor-related apoptosis-inducing ligand and interleukin-1 receptor antagonist, biomarkers of cervicovaginal inflammation, correlated to microbicide toxicity in the 3-D model following N-9 treatment. These results indicate that this 3-D vaginal model could be used as a complementary tool for screening microbicide compounds for safety and efficacy, thus improving success in clinical trials.

Enhancement of human immunodeficiency virus type 1 replication is not intrinsic to all polyanion-based microbicides

Polyanion-based microbicides have been developed to prevent the sexual transmission of human immunodeficiency virus (HIV). Recent data suggest that polyanions have the capacity to enhance HIV type 1 (HIV-1) replication at threshold antiviral concentrations. Evaluation of the microbicide candidates SPL7013 and PRO 2000 revealed no specific enhancement of two CCR5 HIV-1 strains in human peripheral blood mononuclear cells compared to enfuvirtide (Fuzeon). The enhancement effect is likely to be a function of the assay conditions and is not an intrinsic property of these polyanions.