One percent tenofovir applied topically to humanized BLT mice and used according to the CAPRISA 004 experimental design demonstrates partial protection from vaginal HIV infection, validating the BLT model for evaluation of new microbicide candidates

In vivo platforms for analysis of HIV persistence and eradication

HIV persistence in patients undergoing antiretroviral therapy is a major impediment to the cure of HIV/AIDS. The molecular and cellular mechanisms underlying HIV persistence in vivo have not been fully elucidated. This lack of basic knowledge has hindered progress in this area. The in vivo analysis of HIV persistence and the implementation of curative strategies would benefit from animal models that accurately recapitulate key aspects of the human condition. This Review summarizes the contribution that humanized mouse models of HIV infection have made to the field of HIV cure research. Even though these models have been shown to be highly informative in many specific areas, their great potential to serve as excellent platforms for discovery in HIV pathogenesis and treatment has yet to be fully developed.

Mucosal tissue pharmacokinetics of the integrase inhibitor raltegravir in a humanized mouse model: Implications for HIV pre-exposure prophylaxis

Orally administered anti-retroviral drugs show considerable promise for HIV/AIDS pre-exposure prophylaxis (PrEP). For the success of these strategies, pharmacokinetic (PK) data defining the optimal concentration of the drug needed for protection in relevant mucosal exposure sites is essential. Here we employed a humanized mouse model to derive comprehensive PK data on the HIV integrase inhibitor raltegravir (RAL), a leading PrEP drug candidate. Under steady state conditions following oral dosing, plasma and multiple mucosal tissues were sampled simultaneously. RAL exhibited higher drug exposure in mucosal tissues relative to that in plasma with one log higher exposure in vaginal and rectal tissue and two logs higher exposure in intestinal mucosa reflecting the trends seen in the human studies. These data demonstrate the suitability of RAL for HIV PrEP and validate the utility of humanized mouse models for deriving important preclinical PK-PD data.

Modeling HIV-1 Mucosal Transmission and Prevention in Humanized Mice

The new generation humanized mice (hu-mice) that permit continuous de novo generation of human hematopoietic cells have led to novel strategies in studying HIV-1 pathogenesis, prevention and therapies. HIV-1 infection of hu-mice results in chronic viremia and CD4+ T cell loss, thus mimicking key aspects of the disease progression. In addition, the new generation hu-mice are permissive for HIV-1 sexual transmission by vaginal and rectal routes thus allowing in vivo efficacy testing of new anti-HIV-1 drugs for prevention. Two leading models are currently being used, namely the hu-HSC mice and the BLT mice. Here we describe the methodology for generating both hu-HSC and BLT mice and their use in the study of HIV-1 transmission and prevention of infection by topical and oral administration of anti-retroviral drugs. Practical aspects of the methodologies are emphasized.

CDC42 Use in Viral Cell Entry Processes by RNA Viruses

The cellular actin cytoskeleton presents a barrier that must be overcome by many viruses, and it has become increasingly apparent many viral species have developed a diverse repertoire of mechanisms to hijack cellular actin-regulating signalling pathways as part of their cell entry processes. The Rho family GTPase Cdc42 is appreciated as a key moderator of cellular actin dynamics, and the development of specific Cdc42-inhibiting agents has given us an unprecedented ability to investigate its individual role in signalling pathways. However, investigative use of said agents, and the subsequent characterisation of the role Cdc42 plays in viral entry processes has been lacking. Here, we describe the current literature on the role of Cdc42 in human immunodeficiency virus (HIV)-1 cell entry, which represents the most investigated instance of Cdc42 function in viral cell entry processes, and also review evidence of Cdc42 use in other RNA virus cell entries, demonstrating prime areas for more extensive research using similar techniques.

C5A Protects Macaques from Vaginal Simian-Human Immunodeficiency Virus Challenge

A safe and effective vaginal microbicide could decrease human immunodeficiency virus (HIV) transmission in women. Here, we evaluated the safety and microbicidal efficacy of a short amphipathic peptide, C5A, in a rhesus macaque model. We found that a vaginal application of C5A protects 89% of the macaques from a simian-human immunodeficiency virus (SHIV-162P3) challenge. We observed no signs of lesions or inflammation in animals vaginally treated with repeated C5A applications. With its noncellular cytotoxic activity and rare mechanism of action, C5A represents an attractive microbicidal candidate.

Animal and human mucosal tissue models to study HIV biomedical interventions: can we predict success?

Introduction

Preclinical testing plays an integral role in the development of HIV prevention modalities. Several models are used including humanized mice, non-human primates and human mucosal tissue cultures.

Discussion

Pharmaceutical development traditionally uses preclinical models to evaluate product safety. The HIV prevention field has extended this paradigm to include models of efficacy, encompassing humanized mice, non-human primates (typically Asian macaques) and human mucosal tissue (such as cervical and colorectal). As our understanding of the biology of HIV transmission improves and includes the influence of human behaviour/biology and co-pathogens, these models have evolved as well to address more complex questions. These three models have demonstrated the effectiveness of systemic (oral) and topical use of antiretroviral drugs. Importantly, pharmacokinetic/pharmacodynamic relationships are being developed and linked to information gathered from human clinical trials. The models are incorporating co-pathogens (bacterial and viral) and the effects of coitus (mucosal fluids) on drug distribution and efficacy. Humanized mice are being tailored in their immune reconstitution to better represent humans. Importantly, human mucosal tissue cultures are now being used in early clinical trials to provide information on product efficacy to more accurately characterize efficacious products to advance to larger clinical trials. While all of these models have made advancements in product development, each has limitations and the data need to be interpreted by keeping these limitations in mind.

Conclusions

Development and refinement of each of these models has been an iterative process and linkages to data generated among each of them and from human clinical trials are needed to determine their reliability. Preclinical testing has evolved from simply identifying products that demonstrate efficacy prior to clinical trials to defining essential pharmacokinetic/pharmacodynamic relationships under a variety of conditions and has the potential to improve product selection prior to the initiation of large-scale human clinical trials. The goal is to provide researchers with ample information to make conversant decisions that guide optimized and efficient product development.

Breast Milk of HIV-Positive Mothers Has Potent and Species-Specific In Vivo HIV-Inhibitory Activity

Despite the nutritional and health benefits of breast milk, breast milk can serve as a vector for mother-to-child HIV transmission. Most HIV-infected infants acquire HIV through breastfeeding. Paradoxically, most infants breastfed by HIV-positive women do not become infected. This is potentially attributed to anti-HIV factors in breast milk. Breast milk of HIV-negative women can inhibit HIV infection. However, the HIV-inhibitory activity of breast milk from HIV-positive mothers has not been evaluated. In addition, while significant differences in breast milk composition between transmitting and nontransmitting HIV-positive mothers have been correlated with transmission risk, the HIV-inhibitory activity of their breast milk has not been compared. This knowledge may significantly impact the design of prevention approaches in resource-limited settings that do not deny infants of HIV-positive women the health benefits of breast milk. Here, we utilized bone marrow/liver/thymus humanized mice to evaluate the in vivo HIV-inhibitory activity of breast milk obtained from HIV-positive transmitting and nontransmitting mothers. We also assessed the species specificity and biochemical characteristics of milk's in vivo HIV-inhibitory activity and its ability to inhibit other modes of HIV infection. Our results demonstrate that breast milk of HIV-positive mothers has potent HIV-inhibitory activity and indicate that breast milk can prevent multiple routes of infection. Most importantly, this activity is unique to human milk. Our results also suggest multiple factors in breast milk may contribute to its HIV-inhibitory activity. Collectively, our results support current recommendations that HIV-positive mothers in resource-limited settings exclusively breastfeed in combination with antiretroviral therapy.

IMPORTANCE

Approximately 240,000 children become infected with HIV annually, the majority via breastfeeding. Despite daily exposure to virus in breast milk, most infants breastfed by HIV-positive women do not acquire HIV. The low risk of breastfeeding-associated HIV transmission is likely due to antiviral factors in breast milk. It is well documented that breast milk of HIV-negative women can inhibit HIV infection. Here, we demonstrate, for the first time, that breast milk of HIV-positive mothers (nontransmitters and transmitters) inhibits HIV transmission. We also demonstrate that breast milk can prevent multiple routes of HIV acquisition and that this activity is unique to human milk. Collectively, our results support current guidelines which recommend that HIV-positive women in resource-limited settings exclusively breastfeed in combination with infant or maternal antiretroviral therapy.

Genital Tenofovir Concentrations Correlate With Protection Against HIV Infection in the CAPRISA 004 Trial: Importance of Adherence for Microbicide Effectiveness

OBJECTIVE

The CAPRISA 004 trial showed that coitally dosed tenofovir 1% gel reduced HIV acquisition by 39% overall and 54% when used consistently. The objective of this analysis was to ascertain its pharmacokinetic-pharmacodynamic relationship to protect against HIV acquisition.

DESIGN

Genital and systemic tenofovir concentrations in 34 women who acquired HIV (cases) were compared with 302 randomly selected women who remained HIV uninfected (controls) during the CAPRISA 004 trial. In total, 336 cervicovaginal fluid (CVF), 55 plasma, and 23 paired cervical and vaginal tissue samples were assayed by validated methods for tenofovir and tenofovir diphosphate (tenofovir-DP) detection.

RESULTS

Tenofovir was detected in the genital tract in 8 (23.5%) cases and 119 (39.4%) controls (P = 0.076). Among those with detectable genital tract tenofovir, the median CVF concentrations were 97% lower in cases compared with controls, 476 versus 13,821 ng/mL (P = 0.107). A total of 14.7% (5/34) of cases and 32.8% (99/302) of controls were found to have tenofovir CVF concentrations above 100 ng/mL [odds ratio (OR): 0.35, P = 0.037]. At a higher threshold, 8.8% (3/34) of cases and 26.2% (79/302) of controls were found to have tenofovir CVF concentrations above 1000 ng/mL (OR: 0.27, P = 0.036). Plasma tenofovir concentrations were <1 ng/mL in all women and were detected only in controls (16.7%) and not in cases (0%), (P = 0.031). Returned used tenofovir gel applicators and CVF concentrations were correlated (Spearman r = 0.22, P = 0.001).

CONCLUSIONS

A tenofovir concentration of ≥100 ng/mL in CVF was associated with 65% (95% CI: 6% to 87%) protection against HIV, whereas a ≥1000 ng/mL concentration correlated with 76% (95% CI: 8% to 92%) protection against HIV infection.

Role of Semen on Vaginal HIV-1 Transmission and Maraviroc Protection

We used bone marrow/liver/thymus (BLT) humanized mice to establish the effect of semen on vaginal HIV infection and on the efficacy of topically applied maraviroc. Our results demonstrate that vaginal transmission of cell-free HIV occurs efficiently in the presence of semen and that topically applied maraviroc efficiently prevents HIV transmission in the presence of semen. We also show that semen has no significant effect on the transmission of transmitted/founder viruses or cell-associated viruses.

Development of water-soluble polyanionic carbosilane dendrimers as novel and highly potent topical anti-HIV-2 microbicides

The development of topical microbicide formulations for vaginal delivery to prevent HIV-2 sexual transmission is urgently needed. Second- and third-generation polyanionic carbosilane dendrimers with a silicon atom core and 16 sulfonate (G2-S16), napthylsulfonate (G2-NS16) and sulphate (G3-Sh16) end-groups have shown potent and broad-spectrum anti-HIV-1 activity. However, their antiviral activity against HIV-2 and mode of action have not been probed. Cytotoxicity, anti-HIV-2, anti-sperm and antimicrobial activities of dendrimers were determined. Analysis of combined effects of triple combinations with tenofovir and raltegravir was performed by using CalcuSyn software. We also assessed the mode of antiviral action on the inhibition of HIV-2 infection through a panel of different in vitro antiviral assays: attachment, internalization in PBMCs, inactivation and cell-based fusion. Vaginal irritation and histological analysis in female BALB/c mice were evaluated. Our results suggest that G2-S16, G2-NS16 and G3-Sh16 exert anti-HIV-2 activity at an early stage of viral replication inactivating the virus, inhibiting cell-to-cell HIV-2 transmission, and blocking the binding of gp120 to CD4, and the HIV-2 entry. Triple combinations with tenofovir and raltegravir increased the anti-HIV-2 activity, consistent with synergistic interactions (CIwt: 0.33-0.66). No vaginal irritation was detected in BALB/c mice after two consecutive applications for 2 days with 3% G2-S16. Our results have clearly shown that G2-S16, G2-NS16 and G3-Sh16 have high potency against HIV-2 infection. The modes of action confirm their multifactorial and non-specific ability, suggesting that these dendrimers deserve further studies as potential candidate microbicides to prevent vaginal/rectal HIV-1/HIV-2 transmission in humans.

Preclinical assessments of vaginal microbicide candidate safety and efficacy

Sexually transmitted infections like HIV, HPV, and HSV-2, as well as unplanned pregnancy, take a huge toll on women worldwide. Woman-initiated multipurpose prevention technologies that contain antiviral/antibacterial drugs (microbicides) and a contraceptive to simultaneously target sexually transmitted infections and unplanned pregnancy are being developed to reduce these burdens. This review will consider products that are applied topically to the vagina. Rectally administered topical microbicides in development for receptive anal intercourse are outside the scope of this review. Microbicide and microbicide/contraceptive candidates must be rigorously evaluated in preclinical models of safety and efficacy to ensure that only candidates with favorable risk benefit ratios are advanced into human clinical trials. This review describes the comprehensive set of in vitro, ex vivo, and in vivo models used to evaluate the preclinical safety and antiviral efficacy of microbicide and microbicide/contraceptive candidates.

Nanoformulations of Rilpivirine for Topical Pericoital and Systemic Coitus-Independent Administration Efficiently Prevent HIV Transmission

Vaginal HIV transmission accounts for the majority of new infections worldwide. Currently, multiple efforts to prevent HIV transmission are based on pre-exposure prophylaxis with various antiretroviral drugs. Here, we describe two novel nanoformulations of the reverse transcriptase inhibitor rilpivirine for pericoital and coitus-independent HIV prevention. Topically applied rilpivirine, encapsulated in PLGA nanoparticles, was delivered in a thermosensitive gel, which becomes solid at body temperature. PLGA nanoparticles with encapsulated rilpivirine coated the reproductive tract and offered significant protection to BLT humanized mice from a vaginal high-dose HIV-1 challenge. A different nanosuspension of crystalline rilpivirine (RPV LA), administered intramuscularly, protected BLT mice from a single vaginal high-dose HIV-1 challenge one week after drug administration. Using transmitted/founder viruses, which were previously shown to establish de novo infection in humans, we demonstrated that RPV LA offers significant protection from two consecutive high-dose HIV-1 challenges one and four weeks after drug administration. In this experiment, we also showed that, in certain cases, even in the presence of drug, HIV infection could occur without overt or detectable systemic replication until levels of drug were reduced. We also showed that infection in the presence of drug can result in acquisition of multiple viruses after subsequent exposures. These observations have important implications for the implementation of long-acting antiretroviral formulations for HIV prevention. They provide first evidence that occult infections can occur, despite the presence of sustained levels of antiretroviral drugs. Together, our results demonstrate that topically- or systemically administered rilpivirine offers significant coitus-dependent or coitus-independent protection from HIV infection.

When taken consistently, PrEP has been shown to reduce the risk of HIV infection by up to 92% in people who are at high risk. However, PrEP is much less effective if it is not taken consistently. To improve adherence to the drug regimen, several new drug delivery systems, that include novel gel formulations and long-acting delivery systems, are being evaluated. In this manuscript, we used BLT humanized mice, an in vivo model of vaginal HIV transmission, to evaluate two novel delivery systems for HIV prevention. In the first approach, we combined the highly efficient encapsulation of antiretroviral drugs into nanoparticles with a thermosensitive gel that remains liquid at room temperature and solidifies at body temperature. Our results showed that this delivery system provided significant protection from HIV vaginal infection. In a second approach, we evaluated a long-acting nanoparticle formulation for coitus-independent protection from HIV acquisition. Our results showed that a single injection of the long-acting antiviral drug also resulted in reduced HIV infection. However, protection was not complete and transmission was concealed by a significant delay in the onset of plasma viremia that could result in superinfection by two different viruses administered up to four weeks apart.

In vivo analysis of Nef's role in HIV-1 replication, systemic T cell activation and CD4(+) T cell loss

Background

Nef is a multifunctional HIV-1 protein critical for progression to AIDS. Humans infected with nef(−) HIV-1 have greatly delayed or no disease consequences. We have contrasted nef(−) and nef(+) infection of BLT humanized mice to better characterize Nef’s pathogenic effects.

Results

Mice were inoculated with CCR5-tropic HIV-1_JRCSF (JRCSF) or JRCSF with an irreversibly inactivated nef (JRCSFNefdd). In peripheral blood (PB), JRCSF exhibited high levels of viral RNA (peak viral loads of 4.71 × 10⁶ ± 1.23 × 10⁶ copies/ml) and a progressive, 75% loss of CD4⁺ T cells over 17 weeks. Similar losses were observed in CD4⁺ T cells from bone marrow, spleen, lymph node, lung and liver but thymocytes were not significantly decreased. JRCSFNefdd also had high peak viral loads (2.31 × 10⁶ ± 1.67 × 10⁶) but induced no loss of PB CD4⁺ T cells. In organs, JRCSFNefdd produced small, but significant, reductions in CD4⁺ T cell levels and did not affect the level of thymocytes. Uninfected mice have low levels of HLA-DR⁺CD38⁺CD8⁺ T cells in blood (1–2%). Six weeks post inoculation, JRCSF infection resulted in significantly elevated levels of activated CD8⁺ T cells (6.37 ± 1.07%). T cell activation coincided with PB CD4⁺ T cell loss which suggests a common Nef-dependent mechanism. At 12 weeks, in JRCSF infected animals PB T cell activation sharply increased to 19.7 ± 2.9% then subsided to 5.4 ± 1.4% at 14 weeks. HLA-DR⁺CD38⁺CD8⁺ T cell levels in JRCSFNefdd infected mice did not rise above 1–2% despite sustained high levels of viremia. Interestingly, we also noted that in mice engrafted with human tissue expressing a putative protective HLA-B allele (B42:01), JRCSFNefdd exhibited a substantial (200-fold) reduced viral load compared to JRCSF.

Conclusions

Nef expression was necessary for both systemic T cell activation and substantial CD4⁺ T cell loss from blood and tissues. JRCSFNefdd infection did not activate CD8⁺ T cells or reduce the level of CD4⁺ T cells in blood but did result in a small Nef-independent decrease in CD4⁺ T cells in organs. These observations strongly support the conclusion that viral pathogenicity is mostly driven by Nef. We also observed for the first time substantial host-specific suppression of HIV-1 replication in a small animal infection model.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0187-z) contains supplementary material, which is available to authorized users.

Use of partial least squares path modelling to assess the willingness of Chinese female sex workers to participate in a microbicide trial

OBJECTIVES

To investigate the willingness of Chinese female sex workers (FSWs) to participate (WTP) in a clinical trial of microbicides; to explore the potential hindrances and facilitating factors; and to provide support for future microbicide clinical trials by tailoring their design to better meet the specific needs of FSWs.

STUDY DESIGN

Cross-sectional study.

METHODS

In total, 404 FSWs were investigated using structured questionnaires. Exploratory factor analysis and partial least squares path modelling were used to explore the correlations between several influencing factors and WTP.

RESULTS

The WTP of FSWs enrolled in this study was high (53.47%, 216/404). Possible benefits from enrolment in the trial were positively associated with WTP, while concern about a hypothetical microbicide, potential physical harm, economic loss from participation, and fear of family or social isolation were negatively associated with WTP.

CONCLUSION

FSWs are appropriate participants in microbicide clinical trials, and are likely to benefit from effective microbicides. In a microbicide clinical trial, it is imperative to ensure protection of the rights, dignity, safety, confidentiality and welfare of FSW participants.

The Vaginal Acquisition and Dissemination of HIV-1 Infection in a Novel Transgenic Mouse Model Is Facilitated by Coinfection with Herpes Simplex Virus 2 and Is Inhibited by Microbicide Treatment

Epidemiological studies have demonstrated that herpes simplex virus 2 (HSV-2) infection significantly increases the risk of HIV-1 acquisition, thereby contributing to the expanding HIV-1 epidemic. To investigate whether HSV-2 infection directly facilitates mucosal HIV-1 acquisition, we used our transgenic hCD4/R5/cT1 mouse model which circumvents major entry and transcription blocks preventing murine HIV-1 infection by targeting transgenic expression of human CD4, CCR5, and cyclin T1 genes to CD4(+) T cells and myeloid-committed cells. Productive infection of mucosal leukocytes, predominantly CD4(+) T cells, was detected in all hCD4/R5/cT1 mice intravaginally challenged with an HIV-1 infectious molecular clone, HIV-Du151.2env-NLuc, which expresses an env gene (C.Du151.2) cloned from an acute heterosexually infected woman and a NanoLuc luciferase reporter gene. Lower genital tract HIV-1 infection after HIV-Du151.2env-NLuc intravaginal challenge was increased ~4-fold in hCD4/R5/cT1 mice coinfected with HSV-2. Furthermore, HIV-1 dissemination to draining lymph nodes was detected only in HSV-2-coinfected mice. HSV-2 infection stimulated local infiltration and activation of CD4(+) T cells and dendritic cells, likely contributing to the enhanced HIV-1 infection and dissemination in HSV-2-coinfected mice. We then used this model to demonstrate that a novel gel containing tenofovir disoproxil fumarate (TDF), the more potent prodrug of tenofovir (TFV), but not the TFV microbicide gel utilized in the recent CAPRISA 004, VOICE (Vaginal and Oral Interventions to Control the Epidemic), and FACTS 001 clinical trials, was effective as preexposure prophylaxis (PrEP) to completely prevent vaginal HIV-1 infection in almost half of HSV-2-coinfected mice. These results also support utilization of hCD4/R5/cT1 mice as a highly reproducible immunocompetent preclinical model to evaluate HIV-1 acquisition across the female genital tract.

IMPORTANCE

Multiple epidemiological studies have reported that genital herpes simplex virus 2 (HSV-2) infection increases the risk of HIV-1 sexual acquisition by severalfold. Understanding the underlying mechanisms by which HSV-2 facilitates HIV-1 infection and optimizing the efficacy of therapies to inhibit HIV-1 infection during HSV-2 coinfection should contribute to reducing HIV-1 transmission. Using our novel transgenic hCD4/R5/cT1 mouse model infectible with HIV-1, we demonstrated that HSV-2 infection enhances vaginal transmission and dissemination of HIV-1 infection while stimulating recruitment and activation of CD4(+) T cells and dendritic cells in the lower genital tract. HIV acquisition by hCD4/R5/cT1 mice vaginally coinfected with HSV-2 could be completely prevented in almost half the mice by preexposure prophylaxis (PrEP) with a novel gel containing tenofovir disoproxil fumarate (TDF), the tenofovir prodrug, but not with the tenofovir microbicide gel utilized in CAPRISA-004, VOICE, and FACTS-001 clinical trials. The hCD4/R5/cT1 mice represent a new preclinical mouse model to evaluate vaginal HIV-1 acquisition.

BLT humanized mice as a small animal model of HIV infection

Humanized mice are valuable models for the research and development of vaccine strategies and therapeutic interventions to control or eradicate HIV. The BLT humanized mouse model is particularly promising because the combination of transplantation of human fetal pluripotent hematopoietic stem cells with surgical engraftment of human fetal thymic tissue results in improved T cell reconstitution, maturation, and selection. To date, the BLT humanized mouse model has been used to study many aspects of HIV infection including prevention, mucosal transmission, HIV-specific innate and adaptive immunity, viral latency, and novel antiretroviral and immune-based therapies for suppression and reservoir eradication. Here we describe recent advances and applications of the BLT humanized mouse model of HIV infection and discuss opportunities to further improve this valuable small animal model.

MIV-150/zinc acetate gel inhibits cell-associated simian-human immunodeficiency virus reverse transcriptase infection in a macaque vaginal explant model

The transmission of both cell-free and cell-associated immunodeficiency viruses has been demonstrated directly in multiple animal species and possibly occurs in humans, as suggested by genotyping of the infecting human immunodeficiency virus (HIV) in acutely infected women and in semen from their partners. Therefore, a microbicide may need to block both mechanisms of HIV transmission to achieve maximum efficacy. To date, most of the preclinical evaluation of candidate microbicides has been performed using cell-free HIV. New models of mucosal transmission of cell-associated HIV are needed to evaluate candidate microbicide performance. The MIV-150/zinc acetate/carrageenan (MZC) gel protects Depo-Provera-treated macaques against cell-free simian-human immunodeficiency virus reverse transcriptase (SHIV-RT) infection when applied vaginally up to 8 h before challenge. We recently demonstrated the potent activity of MZC gel against cell-free SHIV-RT in macaque vaginal explants. In the current study, we established a cell-associated SHIV-RT infection model of macaque vaginal tissues and tested the activity of MZC gel in this model. MZC gel protected tissues against cell-associated SHIV-RT infection when present at the time of viral exposure or when applied up to 4 days prior to viral challenge. These data support clinical testing of the MZC gel. Overall, our ex vivo model of cell-associated SHIV-RT infection in macaque vaginal mucosa complements the cell-free infection models, providing tools for prioritization of products that block both modes of HIV transmission.

Studies of retroviral infection in humanized mice

Many important aspects of human retroviral infections cannot be fully evaluated using only in vitro systems or unmodified animal models. An alternative approach involves the use of humanized mice, which consist of immunodeficient mice that have been transplanted with human cells and/or tissues. Certain humanized mouse models can support robust infection with human retroviruses including different strains of human immunodeficiency virus (HIV) and human T cell leukemia virus (HTLV). These models have provided wide-ranging insights into retroviral biology, including detailed information on primary infection, in vivo replication and pathogenesis, latent/persistent reservoir formation, and novel therapeutic interventions. Here we describe the humanized mouse models that are most commonly utilized to study retroviral infections, and outline some of the important discoveries that these models have produced during several decades of intensive research.

Nanotech-derived topical microbicides for HIV prevention: the road to clinical development

More than three decades since its discovery, HIV infection remains one of the most aggressive epidemics worldwide, with more than 35 million people infected. In sub-Saharan Africa, heterosexual transmissions represent nearly 80% of new infections, with 50% of these occurring in women. In an effort to stop the dramatic spread of the HIV epidemic, new preventive treatments, such as microbicides, have been developed. Nanotechnology has revolutionized this field by designing and engineering novel highly effective nano-sized materials as microbicide candidates. This review illustrates the most recent advances in nanotech-derived HIV prevention strategies, as well as the main steps required to translate promising in vitro results into clinical trials.

Latest animal models for anti-HIV drug discovery

INTRODUCTION

HIV research is limited by the fact that lentiviruses are highly species specific. The need for appropriate models to promote research has led to the development of many elaborate surrogate animal models.

AREAS COVERED

This review looks at the history of animal models for HIV research. Although natural animal lentivirus infections and chimeric viruses such as chimera between HIV and simian immunodeficiency virus and simian-tropic HIV are briefly discussed, the main focus is on small animal models, including the complex design of the 'humanized' mouse. The review also traces the historic evolution and milestones as well as depicting current models and future prospects for HIV research.

EXPERT OPINION

HIV research is a complex and challenging task that is highly manpower-, money- and time-consuming. Besides factors such as hypervariability and latency, the lack of appropriate animal models that exhibit and recapitulate the entire infectious process of HIV, is one of the reasons behind the failure to eliminate the lentivirus from the human population. This obstacle has led to the exploitation and further development of many sophisticated surrogate animal models for HIV research. While there is no animal model that perfectly mirrors and mimics HIV infections in humans, there are a variety of host species and viruses that complement each other. Combining the insights from each model, and critically comparing the results obtained with data from human clinical trials should help expand our understanding of HIV pathogenesis and drive future drug development.

[Microbicides for preventing sexually transmitted infections: Current status and strategies for preclinical evaluation of new candidates]

Microbicides are a new tool, still under investigation, which could help prevent infection by the human immunodeficiency virus (HIV) and other sexually transmitted infections (STIs). Increasing evidence shows that the complexity of sexual transmission of viral pathogens requires the identification of compounds able to block the early events during the cycle of viral infection. In this manuscript we provide a comprehensive review of the different microbicide strategies that have been studied or are currently being considered for STI prevention, particularly emphasizing those having the potential to block HIV infection. The manuscript also reviews the complex process that is required to conduct future clinical studies in humans and concludes with a brief discussion of the strategies that could be part of the immediate future in microbicide research.

Hypogammaglobulinemia in BLT humanized mice--an animal model of primary antibody deficiency

Primary antibody deficiencies present clinically as reduced or absent plasma antibodies without another identified disorder that could explain the low immunoglobulin levels. Bone marrow-liver-thymus (BLT) humanized mice also exhibit primary antibody deficiency or hypogammaglobulinemia. Comprehensive characterization of B cell development and differentiation in BLT mice revealed other key parallels with primary immunodeficiency patients. We found that B cell ontogeny was normal in the bone marrow of BLT mice but observed an absence of switched memory B cells in the periphery. PC-KLH immunizations led to the presence of switched memory B cells in immunized BLT mice although plasma cells producing PC- or KLH- specific IgG were not detected in tissues. Overall, we have identified the following parallels between the humoral immune systems of primary antibody deficiency patients and those in BLT mice that make this in vivo model a robust and translational experimental platform for gaining a greater understanding of this heterogeneous array of humoral immunodeficiency disorders in humans: (i) hypogammaglobulinemia; (ii) normal B cell ontogeny in bone marrow; and (iii) poor antigen-specific IgG response to immunization. Furthermore, the development of strategies to overcome these humoral immune aberrations in BLT mice may in turn provide insights into the pathogenesis of some primary antibody deficiency patients which could lead to novel clinical interventions for improved humoral immune function.

HIV pre-exposure prophylaxis: mucosal tissue drug distribution of RT inhibitor Tenofovir and entry inhibitor Maraviroc in a humanized mouse model

Pre-exposure prophylaxis (PrEP) strategies utilizing anti-retroviral drugs show considerable promise for HIV prevention. However there is insufficient pharmacokinetic (PK) data on drug concentrations required for protection at the relevant mucosal tissues where the infection is initiated. Here we evaluated the utility of a humanized mouse model to derive PK data on two leading drugs, the RT inhibitor Tenofovir (TFV) and CCR5 inhibitor Maraviroc (MVC). Following oral dosing, both the drugs and the intracellular active TFV-diphosphate could be detected in vaginal, rectal and intestinal tissues. The drug exposures (AUC₂₄ h) were found to be higher in vaginal tissue compared to plasma with even higher levels detected in rectal and intestinal tissues. The overall trends of drug concentrations seen in humanized mice reflect those seen in the human thus establishing the utility of this model complementing the present non-human primate (NHP) models for future pre-clinical evaluations of promising HIV PrEP drug candidates.

Targeted cytotoxic therapy kills persisting HIV infected cells during ART

Antiretroviral therapy (ART) can reduce HIV levels in plasma to undetectable levels, but rather little is known about the effects of ART outside of the peripheral blood regarding persistent virus production in tissue reservoirs. Understanding the dynamics of ART-induced reductions in viral RNA (vRNA) levels throughout the body is important for the development of strategies to eradicate infectious HIV from patients. Essential to a successful eradication therapy is a component capable of killing persisting HIV infected cells during ART. Therefore, we determined the in vivo efficacy of a targeted cytotoxic therapy to kill infected cells that persist despite long-term ART. For this purpose, we first characterized the impact of ART on HIV RNA levels in multiple organs of bone marrow-liver-thymus (BLT) humanized mice and found that antiretroviral drug penetration and activity was sufficient to reduce, but not eliminate, HIV production in each tissue tested. For targeted cytotoxic killing of these persistent vRNA(+) cells, we treated BLT mice undergoing ART with an HIV-specific immunotoxin. We found that compared to ART alone, this agent profoundly depleted productively infected cells systemically. These results offer proof-of-concept that targeted cytotoxic therapies can be effective components of HIV eradication strategies.

Human immune system development and survival of non-obese diabetic (NOD)-scid IL2rγ(null) (NSG) mice engrafted with human thymus and autologous haematopoietic stem cells

Immunodeficient mice bearing targeted mutations in the IL2rg gene and engrafted with human immune systems are effective tools for the study of human haematopoiesis, immunity, infectious disease and transplantation biology. The most robust human immune model is generated by implantation of human fetal thymic and liver tissues in irradiated recipients followed by intravenous injection of autologous fetal liver haematopoietic stem cells [often referred to as the BLT (bone marrow, liver, thymus) model]. To evaluate the non-obese diabetic (NOD)-scid IL2rγ(null) (NSG)-BLT model, we have assessed various engraftment parameters and how these parameters influence the longevity of NSG-BLT mice. We observed that irradiation and subrenal capsule implantation of thymus/liver fragments was optimal for generating human immune systems. However, after 4 months, a high number of NSG-BLT mice develop a fatal graft-versus-host disease (GVHD)-like syndrome, which correlates with the activation of human T cells and increased levels of human immunoglobulin (Ig). Onset of GVHD was not delayed in NSG mice lacking murine major histocompatibility complex (MHC) classes I or II and was not associated with a loss of human regulatory T cells or absence of intrathymic cells of mouse origin (mouse CD45(+) ). Our findings demonstrate that NSG-BLT mice develop robust human immune systems, but that the experimental window for these mice may be limited by the development of GVHD-like pathological changes.

Animal models for microbicide safety and efficacy testing

PURPOSE OF REVIEW

Early studies have cast doubt on the utility of animal models for predicting success or failure of HIV-prevention strategies, but results of multiple human phase 3 microbicide trials, and interrogations into the discrepancies between human and animal model trials, indicate that animal models were, and are, predictive of safety and efficacy of microbicide candidates.

RECENT FINDINGS

Recent studies have shown that topically applied vaginal gels, and oral prophylaxis using single or combination antiretrovirals are indeed effective in preventing sexual HIV transmission in humans, and all of these successes were predicted in animal models. Further, prior discrepancies between animal and human results are finally being deciphered as inadequacies in study design in the model, or quite often, noncompliance in human trials, the latter being increasingly recognized as a major problem in human microbicide trials.

SUMMARY

Successful microbicide studies in humans have validated results in animal models, and several ongoing studies are further investigating questions of tissue distribution, duration of efficacy, and continued safety with repeated application of these, and other promising microbicide candidates in both murine and nonhuman primate models. Now that we finally have positive correlations with prevention strategies and protection from HIV transmission, we can retrospectively validate animal models for their ability to predict these results, and more importantly, prospectively use these models to select and advance even safer, more effective, and importantly, more durable microbicide candidates into human trials.

Overcoming current limitations in humanized mouse research

Immunodeficient mice engrafted with human cells and tissues have provided an exciting alternative to in vitro studies with human tissues and nonhuman primates for the study of human immunobiology. A major breakthrough in the early 2000s was the introduction of a targeted mutation in the interleukin 2 (IL-2) receptor common gamma chain (IL2rg(null)) into mice that were already deficient in T and B cells. Among other immune defects, natural killer (NK) cells are disrupted in these mice, permitting efficient engraftment with human hematopoietic cells that generate a functional human immune system. These humanized mouse models are becoming increasingly important for preclinical studies of human immunity, hematopoiesis, tissue regeneration, cancer, and infectious diseases. In particular, humanized mice have enabled studies of the pathogenesis of human-specific pathogens, including human immunodeficiency virus type 1, Epstein Barr virus, and Salmonella typhi. However, there are a number of limitations in the currently available humanized mouse models. Investigators are continuing to identify molecular mechanisms underlying the remaining defects in the engrafted human immune system and are generating "next generation" models to overcome these final deficiencies. This article provides an overview of some of the emerging models of humanized mice, their use in the study of infectious diseases, and some of the remaining limitations that are currently being addressed.

In vivo analysis of highly conserved Nef activities in HIV-1 replication and pathogenesis

BACKGROUND

The HIV-1 accessory protein, Nef, is decisive for progression to AIDS. In vitro characterization of the protein has described many Nef activities of unknown in vivo significance including CD4 downregulation and a number of activities that depend on Nef interacting with host SH3 domain proteins. Here, we use the BLT humanized mouse model of HIV-1 infection to assess their impact on viral replication and pathogenesis and the selection pressure to restore these activities using enforced in vivo evolution.

RESULTS

We followed the evolution of HIV-1LAI (LAI) with a frame-shifted nef (LAINeffs) during infection of BLT mice. LAINeffs was rapidly replaced in blood by virus with short deletions in nef that restored the open reading frame (LAINeffs∆-1 and LAINeffs∆-13). Subsequently, LAINeffs∆-1 was often replaced by wild type LAI. Unexpectedly, LAINeffs∆-1 and LAINeffs∆-13 Nefs were specifically defective for CD4 downregulation activity. Viruses with these mutant nefs were used to infect BLT mice. LAINeffs∆-1 and LAINeffs∆-13 exhibited three-fold reduced viral replication (compared to LAI) and a 50% reduction of systemic CD4+ T cells (>90% for LAI) demonstrating the importance of CD4 downregulation. These results also demonstrate that functions other than CD4 downregulation enhanced viral replication and pathogenesis of LAINeffs∆-1 and LAINeffs∆-13 compared to LAINeffs. To gain insight into the nature of these activities, we constructed the double mutant P72A/P75A. Multiple Nef activities can be negated by mutating the SH3 domain binding site (P72Q73V74P75L76R77) to P72A/P75A and this mutation does not affect CD4 downregulation. Virus with nef mutated to P72A/P75A closely resembled the wild-type virus in vivo as viral replication and pathogenesis was not significantly altered. Unlike LAINeffs described above, the P72A/P75A mutation had a very weak tendency to revert to wild type sequence.

CONCLUSIONS

The in vivo phenotype of Nef is significantly dependent on CD4 downregulation but minimally on the numerous Nef activities that require an intact SH3 domain binding motif. These results suggest that CD4 downregulation plus one or more unknown Nef activities contribute to enhanced viral replication and pathogenesis and are suitable targets for anti-HIV therapy. Enforced evolution studies in BLT mice will greatly facilitate identification of these critical activities.

In vivo analysis of highly conserved Nef activities in HIV-1 replication and pathogenesis

BACKGROUND

The HIV-1 accessory protein, Nef, is decisive for progression to AIDS. In vitro characterization of the protein has described many Nef activities of unknown in vivo significance including CD4 downregulation and a number of activities that depend on Nef interacting with host SH3 domain proteins. Here, we use the BLT humanized mouse model of HIV-1 infection to assess their impact on viral replication and pathogenesis and the selection pressure to restore these activities using enforced in vivo evolution.

RESULTS

We followed the evolution of HIV-1LAI (LAI) with a frame-shifted nef (LAINeffs) during infection of BLT mice. LAINeffs was rapidly replaced in blood by virus with short deletions in nef that restored the open reading frame (LAINeffs∆-1 and LAINeffs∆-13). Subsequently, LAINeffs∆-1 was often replaced by wild type LAI. Unexpectedly, LAINeffs∆-1 and LAINeffs∆-13 Nefs were specifically defective for CD4 downregulation activity. Viruses with these mutant nefs were used to infect BLT mice. LAINeffs∆-1 and LAINeffs∆-13 exhibited three-fold reduced viral replication (compared to LAI) and a 50% reduction of systemic CD4+ T cells (>90% for LAI) demonstrating the importance of CD4 downregulation. These results also demonstrate that functions other than CD4 downregulation enhanced viral replication and pathogenesis of LAINeffs∆-1 and LAINeffs∆-13 compared to LAINeffs. To gain insight into the nature of these activities, we constructed the double mutant P72A/P75A. Multiple Nef activities can be negated by mutating the SH3 domain binding site (P72Q73V74P75L76R77) to P72A/P75A and this mutation does not affect CD4 downregulation. Virus with nef mutated to P72A/P75A closely resembled the wild-type virus in vivo as viral replication and pathogenesis was not significantly altered. Unlike LAINeffs described above, the P72A/P75A mutation had a very weak tendency to revert to wild type sequence.

CONCLUSIONS

The in vivo phenotype of Nef is significantly dependent on CD4 downregulation but minimally on the numerous Nef activities that require an intact SH3 domain binding motif. These results suggest that CD4 downregulation plus one or more unknown Nef activities contribute to enhanced viral replication and pathogenesis and are suitable targets for anti-HIV therapy. Enforced evolution studies in BLT mice will greatly facilitate identification of these critical activities.

HIV-1 infection, response to treatment and establishment of viral latency in a novel humanized T cell-only mouse (TOM) model

Background

The major targets of HIV infection in humans are CD4⁺ T cells. CD4⁺ T cell depletion is a hallmark of AIDS. Previously, the SCID-hu thy/liv model was used to study the effect of HIV on thymopoeisis in vivo. However, these mice did not develop high levels of peripheral T cell reconstitution and required invasive surgery for infection and analysis. Here, we describe a novel variant of this model in which thy/liv implantation results in systemic reconstitution with human T cells in the absence of any other human hematopoietic lineages.

Results

NOD/SCID-hu thy/liv and NSG-hu thy/liv mice were created by implanting human fetal thymus and liver tissues under the kidney capsule of either NOD/SCID or NSG mice. In contrast to NOD/SCID-hu thy/liv mice that show little or no human cells in peripheral blood or tissues, substantial systemic human reconstitution occurs in NSG-hu thy/liv. These mice are exclusively reconstituted with human T cells (i.e. T-cell only mice or TOM). Despite substantial levels of human T cells no signs of graft-versus-host disease (GVHD) were noted in these mice over a period of 14 months. TOM are readily infected after parenteral exposure to HIV-1. HIV replication is sustained in peripheral blood at high levels and results in modest reduction of CD4⁺ T cells. HIV-1 replication in TOM responds to daily administration of combination antiretroviral therapy (ART) resulting in strong suppression of virus replication as determined by undetectable viral load in plasma. Latently HIV infected resting CD4⁺ T cells can be isolated from suppressed mice that can be induced to express HIV ex-vivo upon activation demonstrating the establishment of latency in vivo.

Conclusions

NSG-hu thy/liv mice are systemically reconstituted with human T cells. No other human lymphoid lineages are present in these mice (i.e. monocytes/macrophages, B cells and DC are all absent). These T cell only mice do not develop GVHD, are susceptible to HIV-1 infection and can efficiently maintain virus replication. HIV infected TOM undergoing ART harbor latently infected, resting CD4⁺ T cells.

Intravaginal rings as delivery systems for microbicides and multipurpose prevention technologies

There is a renewed interest in delivering pharmaceutical products via intravaginal rings (IVRs). IVRs are flexible torus-shaped drug delivery systems that can be easily inserted and removed by the woman and that provide both sustained and controlled drug release, lasting for several weeks to several months. In terms of women's health care products, it has been established that IVRs effectively deliver contraceptive steroids and steroids for the treatment of postmenopausal vaginal atrophy. A novel application for IVRs is the delivery of antiretroviral drugs for the prevention of human immunodeficiency virus (HIV) genital infection. Microbicides are antiviral drugs delivered topically for HIV prevention. Recent reviews of microbicide IVRs have focused on technologies in development and optimizing ring design. IVRs have several advantages, including the ability to deliver sustained drug doses for long periods of time while bypassing first pass metabolism in the gut. IVRs are discreet, woman-controlled, and do not require a trained provider for placement or fitting. Previous data support that women and their male sexual partners find IVRs highly acceptable. Multipurpose prevention technology (MPT) products provide protection against unintended/mistimed pregnancy and reproductive tract infections, including HIV. Several MPT IVRs are currently in development. Early clinical testing of new microbicide and MPT IVRs will require a focus on safety, pharmacokinetics and pharmacodynamics. Specifically, IVRs will have to deliver tissue concentrations of drugs that are pharmacodynamically active, do not cause mucosal alterations or inflammation, and do not change the resident microbiota. The emergence of resistance to antiretrovirals will need to be investigated. IVRs should not disrupt intercourse or have high rates of expulsion. Herein, we reviewed the microbicide and MPT IVRs currently in development, with a focus on the clinical aspects of IVR assessment and the challenges facing microbicide and MPT IVR product development, clinical testing, and implementation. The information in this review was drawn from PubMed searches and a recent microbicide/MPT product development workshop organized by CONRAD.

Transmission of chimeric HIV by mating in conventional mice: prevention by pre-exposure antiretroviral therapy and reduced susceptibility during estrus

Heterosexual transmission accounts for the majority of new human immunodeficiency virus (HIV) cases worldwide. The current approach to investigate HIV heterosexual transmission in animals involves application of virus stock to the vaginal surface, a method that does not reproduce the physiological conditions of vaginal intercourse that influence the rate of transmission. We have previously described efficient infection of conventional mice using EcoHIV/NL4-3 and EcoHIV/NDK, chimeric HIV molecular clones constructed to express all HIV structural and regulatory genes except envelope, which is replaced by a rodent-tropic envelope gene. Here we investigated whether EcoHIV/NDK-infected male mice transmit virus to females during coitus, and the sensitivity of this transmission to HIV pre-exposure prophylaxis and the estrus state. Our general approach was to allow mating between EcoHIV/NDK-infected male mice and uninfected females for 1–7 nights. At 1–6 weeks after mating, mice were euthanized and virus burdens were measured by quantitative PCR (qPCR) amplification of HIV RNA or DNA in peritoneal macrophages, inguinal lymph node cells, spleen cells or vas deferens, or by ELISA for antibodies to HIV Gag. We found that 70–100% of female mice mated to EcoHIV/NDK-infected males acquired infection. Pericoital treatment of females with either 2′,3′-dideoxcytidine (ddC) or tenofovir largely prevented their EcoHIV/NDK infection by mating (P<0.05 and P<0.003, respectively). In males, T cells were dispensable for virus transmission. The rate of EcoHIV/NDK sexual transmission to females in estrus declined sharply (P=0.003) but their infection by injection was unaffected, indicating that the local environment in the female reproductive tract influences susceptibility to HIV. We conclude that this system of EcoHIV/NDK transmission during mouse mating reproduces key features of heterosexual transmission of HIV in humans and can be used to investigate its biology and control.

Humanized mice for the study of infectious diseases

Many of the pathogens that cause human infectious diseases do not infect rodents or other mammalian species. Small animal models that allow studies of the pathogenesis of these agents and evaluation of drug efficacy are critical for identifying ways to prevent and treat human infectious diseases. Immunodeficient mice engrafted with functional human cells and tissues, termed 'humanized' mice, represent a critical pre-clinical bridge for in vivo studies of human pathogens. Recent advances in the development of humanized mice have allowed in vivo studies of multiple human infectious agents providing novel insights into their pathogenesis that was otherwise not possible.

Rectal transmission of transmitted/founder HIV-1 is efficiently prevented by topical 1% tenofovir in BLT humanized mice

Rectal microbicides are being developed to prevent new HIV infections in both men and women. We focused our in vivo preclinical efficacy study on rectally-applied tenofovir. BLT humanized mice (n = 43) were rectally inoculated with either the primary isolate HIV-1JRCSF or the MSM-derived transmitted/founder (T/F) virus HIV-1THRO within 30 minutes following treatment with topical 1% tenofovir or vehicle. Under our experimental conditions, in the absence of drug treatment we observed 50% and 60% rectal transmission by HIV-1JRCSF and HIV-1THRO, respectively. Topical tenofovir reduced rectal transmission to 8% (1/12; log rank p = 0.03) for HIV-1JRCSF and 0% (0/6; log rank p = 0.02) for HIV-1THRO. This is the first demonstration that any human T/F HIV-1 rectally infects humanized mice and that transmission of the T/F virus can be efficiently blocked by rectally applied 1% tenofovir. These results obtained in BLT mice, along with recent ex vivo, Phase 1 trial and non-human primate reports, provide a critically important step forward in the development of tenofovir-based rectal microbicides.

Animal models of antiretroviral prophylaxis for HIV prevention

PURPOSE OF REVIEW

Oral and topical pre-exposure prophylaxis (PrEP) with antiretroviral drugs are novel biomedical interventions recently found to prevent HIV transmission among high-risk populations. In this review, we outline lessons learned from animal studies and discuss next steps in preclinical PrEP research including the study of new PrEP modalities, pharmacologic correlates of protection, and biological factors that may modulate PrEP efficacy.

RECENT FINDINGS

Studies using macaque or humanized mice models of mucosal simian immunodeficiency virus (SIV), HIV, or simian/human immunodeficiency virus (SHIV) transmission have provided efficacy data against rectal and vaginal infection. A multitude of oral and topical PrEP regimens including drugs such as tenofovir (TFV), tenofovir disoproxil fumarate (TDF) and emtricitabine (FTC) were tested against either wild-type or drug-resistant viruses. These models have also helped define prophylactic windows of protection of nondaily dosing and are being used increasingly to study pharmacokinetic and pharmacodynamic relationships.

SUMMARY

As human data from PrEP trials validate animal models or help fine tune them, it is expected that these models will play increasingly important roles in PrEP development as the field extends into new drug classes and combinations, episodic dosing, and novel long-acting drug formulations. By providing both efficacy and pharmacologic information these models can define correlates and mechanisms of protection, inform dose selection, and advance the most promising PrEP candidates and dosing modalities.

HIV restriction by APOBEC3 in humanized mice

Innate immune restriction factors represent important specialized barriers to zoonotic transmission of viruses. Significant consideration has been given to their possible use for therapeutic benefit. The apolipoprotein B mRNA editing enzyme catalytic polypeptide 3 (APOBEC3) family of cytidine deaminases are potent immune defense molecules capable of efficiently restricting endogenous retroelements as well as a broad range of viruses including Human Immunodeficiency virus (HIV), Hepatitis B virus (HBV), Human Papilloma virus (HPV), and Human T Cell Leukemia virus (HTLV). The best characterized members of this family are APOBEC3G (A3G) and APOBEC3F (A3F) and their restriction of HIV. HIV has evolved to counteract these powerful restriction factors by encoding an accessory gene designated viral infectivity factor (vif). Here we demonstrate that APOBEC3 efficiently restricts CCR5-tropic HIV in the absence of Vif. However, our results also show that CXCR4-tropic HIV can escape from APOBEC3 restriction and replicate in vivo independent of Vif. Molecular analysis identified thymocytes as cells with reduced A3G and A3F expression. Direct injection of vif-defective HIV into the thymus resulted in viral replication and dissemination detected by plasma viral load analysis; however, vif-defective viruses remained sensitive to APOBEC3 restriction as extensive G to A mutation was observed in proviral DNA recovered from other organs. Remarkably, HIV replication persisted despite the inability of HIV to develop resistance to APOBEC3 in the absence of Vif. Our results provide novel insight into a highly specific subset of cells that potentially circumvent the action of APOBEC3; however our results also demonstrate the massive inactivation of CCR5-tropic HIV in the absence of Vif.

New generation humanized mice for virus research: comparative aspects and future prospects

Work with human specific viruses will greatly benefit from the use of an in vivo system that provides human target cells and tissues in a physiological setting. In this regard humanized mice (hu-Mice) have played an important role in our understanding of viral pathogenesis and testing of therapeutic strategies. Limitations with earlier versions of hu-Mice that lacked a functioning human immune system are currently being overcome. The new generation hu-Mouse models are capable of multilineage human hematopoiesis and generate T cells, B cells, macrophages and dendritic cells required for an adaptive human immune response. Now any human specific pathogen that can infect humanized mice can be studied in the context of ongoing infection and immune responses. Two leading humanized mouse models are currently employed: the hu-HSC model is created by transplantation of human hematopoietic stem cells (HSC), whereas the BLT mouse model is prepared by transplantation of human fetal liver, thymus and HSC. A number of human specific viruses such as HIV-1, dengue, EBV and HCV are being studied intensively in these systems. Both models permit infection by mucosal routes with viruses such as HIV-1 thus allowing transmission prevention studies. Cellular and humoral immune responses are seen in both the models. While there is efficient antigen specific IgM production, IgG responses are suboptimal due to inefficient immunoglobulin class switching. With the maturation of T cells occurring in the autologous human thymus, BLT mice permit human HLA restricted T cell responses in contrast to hu-HSC mice. However, the strength of the immune responses needs further improvement in both models to reach the levels seen in humans. The scope of hu-Mice use is further broadened by transplantation of additional tissues like human liver thus permitting immunopathogenesis studies on hepatotropic viruses such as HCV. Numerous studies that encompass antivirals, gene therapy, viral evolution, and the generation of human monoclonal antibodies have been conducted with promising results in these mice. For further improvement of the new hu-Mouse models, ongoing work is focused on generating new strains of immunodeficient mice transgenic for human HLA molecules to strengthen immune responses and human cytokines and growth factors to improve human cell reconstitution and their homeostatic maintenance.

In vivo blockade of the PD-1 receptor suppresses HIV-1 viral loads and improves CD4+ T cell levels in humanized mice

The programmed death-1 (PD-1) pathway limits the function of virus-specific T cells during chronic infection. We previously showed that blockade of the PD-1 pathway increases HIV-1-associated T cell function in vitro. However, the effect of PD-1 blockade on HIV-1 disease progression in vivo has not been examined. As in humans, HIV-1-infected humanized BALB/c-Rag2(-/-)γc(-/-) (Rag-hu) mice express elevated levels of PD-1 on T cells during chronic infection. To examine the effect of PD-1 blockade on disease progression, Rag-hu mice with chronic HIV-1 infection were treated with a blocking mAb directed against programmed cell death-1 ligand-1, the ligand for PD-1. Programmed cell death-1 ligand-1-treated Rag-hu mice exhibited a progressive decrease in the HIV-1 plasma viral load, with a 7-fold decrease by day 7, a 20-fold decrease by day 14, a 178-fold decrease by day 21, and a 269-fold decrease by day 28 postinitiation of treatment. By day 7, the percentage of CD4(+) T cells was statistically higher in the treated compared with the untreated group, and this trend was sustained throughout the 28-d treatment period. Moreover, there was a strong inverse correlation between plasma viral load and the percentage of both CD4(+) (r = -0.66; p < 0.0001) and CD8(+) (r = -0.64; p < 0.0001) T cells in the treated mice but not the untreated mice. This study provides "proof of concept" that humanized mice can be used to examine the effects of immunotherapeutic interventions on HIV-1 infection. Furthermore, to our knowledge, these data demonstrate for the first time that blockade of the PD-1 pathway reduces HIV-1 viral loads.

Biology of mucosally transmitted sexual infection-translating the basic science into novel HIV intervention: a workshop summary

A group of over 200 international scientists came together on April 15 in Sydney, Australia just before the 2012 International Microbicides Conference as a part of a workshop to address the basic concepts and factors that modulate HIV infection at the mucosal surface. The meeting focused on defining the interaction between virus, prevailing host physiology, microbiota, and innate and adaptive immune responses and how they combine to impact the outcome at the moment of potential viral transmission. Speakers examined the biology of HIV entry during transmission, innate and natural antiviral mechanisms at the mucosa, microbicide efficacy, pharmacokinetic, and pharmacodynamics, animal models, and opportunities for combining HIV prevention strategies. Other viral infection models both in vivo and in vitro were considered for the insights they provided into HIV transmission events. The workshop raised important questions that we need to answer to further our basic understanding of host and viral factors influencing HIV transmission to inform the development of novel prevention strategies.

Antiviral and immunological effects of tenofovir microbicide in vaginal herpes simplex virus 2 infection

The anti-HIV microbicide, tenofovir (TFV) gel, has been shown to decrease HIV-1 acquisition by 39% and reduce herpes simplex virus 2 (HSV-2) transmission by 51%. We evaluated the effect of a 1% TFV gel on genital HSV-2 infection in a mouse vaginal challenge model. In vitro plaque assays and luminex multiplex bead analysis were used, respectively, to measure postinfection vaginal viral shedding (day 1) and cytokine secretion (day 2). To further investigate the anti-HSV-2 properties, we evaluated the direct antiviral effect of TFV and the oral prodrug tenofovir disoproxil fumerate (TDF) in cell culture. Compared to placebo-treated mice, TFV-treated mice had significantly lower clinical scores, developed later genital lesions, and showed reduced vaginal viral shedding. Furthermore, the levels of IFN-γ, IL-2, TNF-α, and other cytokines were altered in the vaginal fluid following topical tenofovir treatment and subsequent HSV-2 challenge. Finally, we found that both TFV and TDF inhibited HSV-2 infection in vitro; TDF showed a 50-fold greater potency than TFV. In conclusion, we confirmed that the microbicide TFV had direct anti-HSV-2 effects in a murine vaginal challenge model. Therefore, this model would be suitable for evaluating present and future microbicide candidates. Furthermore, the present study warrants further investigation of TDF in microbicides.

Rectal microbicide development

PURPOSE OF REVIEW

Individuals practicing unprotected receptive anal intercourse are at particularly high risk of HIV infection. Men who have sex with men (MSM) in the developed and developing world continue to have disproportionate and increasing levels of HIV infection. The past few years have seen important progress in demonstrating the efficacy of oral pre-exposure prophylaxis (PrEP), vaginal microbicides, and treatment as prevention, but there has also been significant progress in the development of rectal microbicides. The purpose of this review is to summarize the status of rectal microbicide research and to identify opportunities, challenges, and future directions in this important field of HIV prevention.

RECENT FINDINGS

Recent phase 1 rectal microbicide studies have characterized the safety, acceptability, compartmental pharmacokinetics, and pharmacodynamics of both UC781 and tenofovir gels. The tenofovir gel formulation used in vaginal studies was not well tolerated in the rectum and newer rectal-specific formulations have been developed and evaluated in phase 1 studies.

SUMMARY

Complex phase 1 studies have provided important data on candidate rectal microbicides. Tenofovir gel is poised to move into phase 2 evaluation and it is possible that a phase 2B/3 effectiveness study could be initiated in the next 2-3 years.

Inefficient vaginal transmission of tenofovir-resistant HIV-1

Transmission of drug-resistant HIV has been postulated to be a threat to current first-line antiretroviral therapy (ART) regimens and the efficacy of several antiretroviral-based preexposure prophylaxis (PrEP) strategies being tested. Here we evaluated the effect of the common tenofovir (TFV) resistance mutation K65R on vaginal HIV transmission. Our results demonstrate that despite no overt loss of overall replication competence in vivo, this mutation results in significantly reduced mucosal transmission. When transmitted, the mutant virus eventually reverted to the wild type in 2 of 3 animals examined.

Humanized mice for immune system investigation: progress, promise and challenges

Significant advances in our understanding of the in vivo functions of human cells and tissues and the human immune system have resulted from the development of 'humanized' mouse strains that are based on severely immunodeficient mice with mutations in the interleukin-2 receptor common γ-chain locus. These mouse strains support the engraftment of a functional human immune system and permit detailed analyses of human immune biology, development and functions. In this Review, we discuss recent advances in the development and utilization of humanized mice, the lessons learnt, the remaining challenges and the promise of using humanized mice for the in vivo study of human immunology.

Animal models in virus research: their utility and limitations

Viral diseases are important threats to public health worldwide. With the number of emerging viral diseases increasing the last decades, there is a growing need for appropriate animal models for virus studies. The relevance of animal models can be limited in terms of mimicking human pathophysiology. In this review, we discuss the utility of animal models for studies of influenza A viruses, HIV and SARS-CoV in light of viral emergence, assessment of infection and transmission risks, and regulatory decision making. We address their relevance and limitations. The susceptibility, immune responses, pathogenesis, and pharmacokinetics may differ between the various animal models. These complexities may thwart translating results from animal experiments to the humans. Within these constraints, animal models are very informative for studying virus immunopathology and transmission modes and for translation of virus research into clinical benefit. Insight in the limitations of the various models may facilitate further improvements of the models.

IL-2 receptor γ-chain molecule is critical for intestinal T-cell reconstitution in humanized mice

Intestinal immune cells are important in host defense, yet the determinants for human lymphoid homeostasis in the intestines are poorly understood. In contrast, lymphoid homeostasis has been studied extensively in mice, where the requirement for a functional common γ-chain molecule has been established. We hypothesized that humanized mice could offer insights into human intestinal lymphoid homeostasis if generated in a strain with an intact mouse common γ-chain molecule. To address this hypothesis, we used three mouse strains (non-obese diabetic (NOD)/severe-combined immunodeficient (SCID) (N/S); NOD/SCID γ-chain(-/-) (NSG); and Rag2(-/-) γ-chain(-/-) (DKO)) and two humanization techniques (bone marrow liver thymus (BLT) and human CD34(+) cell bone marrow transplant of newborn mice (hu)) to generate four common types of humanized mice: N/S-BLT, NSG-BLT, NSG-hu, and DKO-hu mice. The highest levels of intestinal human T cells throughout the small and large intestines were observed in N/S-BLT mice, which have an intact common γ-chain molecule. Furthermore, the small intestine lamina propria T-cell populations of N/S-BLT mice exhibit a human intestine-specific surface phenotype. Thus, the extensive intestinal immune reconstitution of N/S-BLT mice was both quantitatively and qualitatively better when compared with the other models tested such that N/S-BLT mice are well suited for the analysis of human intestinal lymphocyte trafficking and human-specific diseases affecting the intestines.

Simian immunodeficiency virus-Vpx for improving integrase defective lentiviral vector-based vaccines

Background

Integrase defective lentiviral vectors (IDLV) represent a promising delivery system for immunization purposes. Human dendritic cells (DC) are the main cell types mediating the immune response and are readily transduced by IDLV, allowing effective triggering of in vitro expansion of antigen-specific primed CD8+ T cells. However, IDLV expression in transduced DC is at lower levels than those of the integrase (IN) competent counterpart, thus requiring further improvement of IDLV for future use in the clinic.

Results

In this paper we show that the addition of simian immunodeficiency (SIV)-Vpx protein in the vector preparation greatly improves transduction of human and simian DC, but not of murine DC, thus increasing the ability of transduced DC to act as functional antigen presenting cells, in the absence of integrated vector sequences. Importantly, the presence of SIV-Vpx allows for using lower dose of input IDLV during in vitro transduction, thus further improving the IDLV safety profile.

Conclusions

These results have significant implications for the development of IDLV-based vaccines.

Controlling the HIV/AIDS epidemic: current status and global challenges

This review provides an overview of the current status of the global HIV pandemic and strategies to bring it under control. It updates numerous preventive approaches including behavioral interventions, male circumcision (MC), pre- and post-exposure prophylaxis (PREP and PEP), vaccines, and microbicides. The manuscript summarizes current anti-retroviral treatment options, their impact in the western world, and difficulties faced by emerging and resource-limited nations in providing and maintaining appropriate treatment regimens. Current clinical and pre-clinical approaches toward a cure for HIV are described, including new drug compounds that target viral reservoirs and gene therapy approaches aimed at altering susceptibility to HIV infection. Recent progress in vaccine development is summarized, including novel approaches and new discoveries.

Microbicides: still a long road to success

The development of efficient microbicides, the topically applied compounds that protect uninfected individuals from acquiring HIV-1, is a promising strategy to contain HIV-1 epidemics. Such microbicides should of course possess anti-HIV-1 activity, but they should also act against other genital pathogens, which facilitate HIV-1 transmission. The new trend in microbicide strategy is to use drugs currently used in HIV-1 therapy. The success of this strategy is mixed so far and is impaired by our limited knowledge of the basic mechanisms of HIV-1 transmission as well as by the inadequacy of the systems in which microbicides are tested in preclinical studies.