PrEP and Protective Immune Responses Against HIV-1

Human immunodeficiency virus (HIV) infection poses a public health threat both in the U.S. as well as worldwide. A role for regulatory T cells, potent negative regulators of the adaptive and innate immune responses, is poorly understood in the case of viral infection in general and in HIV infection in particular. Therefore, in addition to the work described above, a continuation of the work I did during my postdoctoral studies, we also plan to extend our studies of regulatory T cells into the role(s) they play during HIV infection and vaccine responses.

Pre-exposure prophylaxis (PrEP), in which HIV-1 uninfected persons use oral or topical antiretrovirals to protect against sexual HIV-1 acquisition, is one of the most promising new HIV-1 prevention strategies. Highly exposed, persistently HIV-1 uninfected individuals (e.g., sex workers, partners of HIV-1 infected persons, infants born to HIV-1 infected women) have been studied for immunologic responses that might signal protection against HIV-1, which are the types of immune responses essential for an effective HIV-1 vaccine.  It has been hypothesized that PrEP, by aborting infections through direct anti-viral activity, may allow HIV-1 specific immune responses to develop in exposed persons.  In non-human primate models, T cell responses in blood recognizing SHIV antigens were achieved in a high proportion (≥50%) of animals receiving PrEP. By inhibiting replication, PrEP may permit persistent HIV-1 exposure to be an antigenic source of immunologic priming – much as a first dose of a vaccine – allowing development of protective adaptive anti-HIV-1 responses. Thus, PrEP may serve as a “chemotherapeutic” vaccine: a PrEP-primed immune response could enhance the antiviral activity and efficacy of PrEP, blunt early HIV-1 replication for PrEP breakthrough infections, and provide anti-HIV-1 protection after PrEP is discontinued.  Thus, PrEP could ultimately enhance immunogenicity and efficacy of a partially-effective HIV-1 vaccine.

The Partners PrEP Study provides a unique and ideal opportunity to characterize anti-HIV-1 immune responses in persons on PrEP (compared to those randomized to placebo) and to explore mechanisms of PrEP protection. These findings will inform future studies of PrEP as “chemovaccination” with candidate HIV-1 vaccines. In the Partners PrEP Study, we have prospectively cryopreserved peripheral blood mononuclear cells (PBMCs) for HIV-1 immunologic studies.  We propose to explore anti-HIV-1 adaptive and innate responses in HIV-1 exposed but uninfected persons receiving PrEP, evaluate the relationship between anti-HIV-1 immune responses and HIV-1 protection against HIV-1 and control of viral replication in breakthrough infections, and, in an exploratory study, evaluate immune responses in mucosal samples obtained from persons on PrEP.

Regulatory T-cell modulation of immunity to mucosal viral infections

Regulatory T cells are well known for their role in dampening the immune responses to self-antigens and thereby helping to prevent autoimmune disease. Additionally, recent work has highlighted the importance of regulatory T cells in immunity to infectious disease. Thus, the question arises as to how regulatory T cells can participate in both of these goals that are so important to human survival – preventing damaging immune responses to self while simultaneously permitting immune responses to be generated to fight foreign infectious agents. Previous studies have pointed to a role for regulatory T cells in limiting late immune responses to various infectious agents, thereby minimizing immune response-induced tissue damage while preventing or diminishing pathogen clearance. However, we recently demonstrated a novel and unexpected role for regulatory T cells in facilitating early immune responses to genital herpes simplex-2 (HSV-2) infection in orchestrating the timely trafficking of immune effector cells to the site of infection where they can fight infection. Therefore, this unexpected finding of an immune response-promoting function of regulatory T cells has subsequently led to several new lines of work that will be addressed in the lab.

Specifically, we will first address the role of regulatory T cells during the immune response to primary and secondary challenge with genital HSV-2 infection. This has important implications for vaccine design for sexually transmitted viruses, since how the population of regulatory T cells could change and perhaps impact the “memory” immune response that is generated subsequent to a primary antigenic challenge such as a vaccine is currently unknown. Secondly, we will extend our studies of the role of regulatory T cells during a second common mucosal infection that is considered to be a major public health threat- influenza virus infection. Thirdly, following up on work from our previous studies, we will characterize the mechanisms by which regulatory T cells modulate dendritic cell function during mucosal viral infections. Finally, we will determine if regulatory T cells must be antigen-specific in order to respond to genital HSV-2 infection. Results from these studies will help to reveal the roles of regulatory T cells during various types of mucosal viral infections at different mucosal surfaces of the body, as well as the different roles that regulatory T cells could play at various phases of the immune response to viruses. We expect that knowledge gained from this research program will assist in the generation of improved clinical interventions for mucosal viral infections, including vaccines. Our previously published work as well as recently generated and unpublished data in the lab suggests that regulatory T cells play several important roles in the immune response to viruses; thus, gaining an understanding of exactly what these cells do as well as where and when in the course of both a primary and secondary immune response is vital for designing future vaccines that will allow regulatory T cells to effectively assist in generating and maintaining protective immune responses.

Immune Mechanisms of Flavivirus Control: Project 5 – The Role of Regulatory T cells in Immune Control of Flavivirus Infection

Flaviviruses present a major public health problem in the US and worldwide. A role of regulatory T cells (Tregs), potent negative regulators of the adaptive and innate immune responses, in viral infection in general, and in flavivirus infection in particular, is poorly understood. We hypothesize that there are distinct qualitative and quantitative requirements for diverse manifestations of Treg function affecting virus specific responses and associated pathology. These manifestations might vary depending on time during the course of viral infection, localization (secondary lymphoid organs vs. peripheral tissues), and the specific virus type. In this application, we will employ a well-established experimental model of West Nile virus (WNV) infection in mice to investigate a role for Treg cells in flavivirus infection. In our studies, we will test the aforementioned hypotheses by taking advantage of Foxp3gfp and Foxp3DTR knock-in mice, harboring GFP-marked or a human diphtheria toxin receptor (DTR)-decorated Treg subset, respectively, and therefore, allowing for isolation and efficient ablation of Tregs upon DT treatment. We will investigate a role for Treg localization and TCR specificity in distinct aspects of Treg function during WNV infection. In these studies we will utilize monoclonal Treg cells expressing transgene-encoded TCR for adoptive transfers into Treg-ablated mice and Treg-specific disruption of major mechanisms of migration to the infected tissue, i.e. brain. The multi-project objectives of this highly collaborative project are interdependent and will benefit from studies proposed in three Specific Aims. We plan to investigate the effects of WNV infection on Treg homeostasis, trafficking, and suppressive function (Aim1), the role of Tregs in adaptive immunity to WNV infection (Aim 2), and the mechanisms of Tregs sensing of the presence of WNV infection (Aim 3). These studies are closely integrated with four other projects in this U19-funded center to provide a comprehensive understanding of cell intrinsic mechanisms of innate and adaptive immunity to flaviruses with the cell-extrinsic negative regulation of these responses by regulatory T cells.

Systems Immunogenetics of Biodefence Pathogens in the Collaborative Cross: Project 4 – Systems Immunogenetics of West Nile Virus Infection: Adaptive Immunity

West Nile virus (WNV) is a neurotropic virus well documented to require innate immunity, humoral immunity, as well as T-cell mediated immunity in order to generate effective protection and viral control. Due to the varied and complex nature of the immune responses generated in response to WNV infection, there are naturally complex and varied genetic networks involved in immunity to WNV as well, though to date, we lack a complete understanding of how host genetic diversity impacts the orchestration of immunity to virus infection. Thus, the goal of this project is to use a systems immunology approach to identify candidate host genes and genetic networks that control regulation of adaptive immunity, as well as the interplay between innate and adaptive immunity, to West Nile virus infection, thereby providing us with a better understanding of how adaptive immunity to neurotropic viruses is regulated at the genetic level. In order to accomplish this goal, we plan to use the Collaborative Cross (CC) mice, a highly diverse recombinant inbred mouse panel designed to promote the identification and characterization of multiple interacting genes underlying complex phenotypes, such as host immunity to viral infection. The use of this unique genetic tool will therefore allow us to identify multiple immunoregulatory genes and genetic networks that control various aspects of WNV-induced adaptive immunity, and in conjunction with collaborators, who are seeking to identify genes regulating innate immunity to WNV, to link innate and adaptive immune networks in response to WNV. This will ultimately allow us to not only validate these targets in human systems, but also to generate more effective animal models of virus infection so that we can better simulate human diversity and make further discoveries that will allow us to create better vaccines and therapies for West Nile and other neuroinvasive viral diseases.

Through the findings of this interactive program, we will significantly advance our understanding of how genes and genetic networks regulate the adaptive immune response to virus infection, both globally as well as specifically in the case of neuroinvasive infections. This will likely lead to improved treatment and therapeutics for various clinically important infections.