The effect on neurological function of virus-specific T cells in the central nervous system (CNS) is not well understood. It has been suggested that cytotoxic T lymphocyte (CTL) responses may contribute to neuropathogenesis during viral infections through direct cytotoxicity or release of inflammatory cytokines such as interferon-gamma (IFN-γ) or TNFα. We recently characterized HIV-specific CD8+ T cell responses in CSF among antiretroviral naïve adults with chronic HIV-1 infection, relatively high peripheral blood CD4+ T cell counts, and low plasma HIV-1 RNA concentrations. Among HIV+ individuals with no neurological symptoms and with little or no HIV-1 RNA in CSF, we found relatively higher frequencies of functional HIV-specific CTLs in CSF compared to blood (p=.0004). Our study, complemented by data from macaque models, suggests that HIV specific CTLs play an important role in controlling intrathecal viral replication and may therefore protect against HIV-associated neurocognitive impairment.
We hypothesize that, while antigen-specific T cells in CNS help protect against neurological dysfunction, under conditions of chronic antigen exposure virus-specific T cells in CNS may become functionally exhausted, leading to loss of viral control with associated CNS dysfunction.
The Specific Aims of this proposal are:
1. To characterize the role and functionality of antigen-specific T cells in CSF of HIV+ patients representing a broad range of CSF and plasma HIV-1 RNA concentrations;
2. To evaluate the effect of anti-retroviral therapy (ART) on the phenotype and functionality of antigen-specific T cells in CSF of HIV+ patients.
We will perform a detailed phenotypic and functional analysis of HIV-specific CTLs simultaneously in peripheral blood and CSF of these patients. We will use a 14-color flow cytometry panel to evaluate expression levels of exhaustion markers, such as programmed death-1 (PD-1), the proliferative senescence marker (CD57), the IL-7 receptor (CD127), as well as cellular markers for tissue homing, memory phenotype, and immune activation. We will also assess the functional capacities of antigen-specific T cells by measuring their in-vitro expansion potential and cytokine production.
We hypothesize that among HIV+ subjects with advanced disease, especially with neurocognitive impairment and relatively increased intrathecal levels of virus, the frequency of HIV-specific CTLs in CSF will be the same or lower than the frequency of these cells in peripheral blood as compared to healthier HIV-infected subjects. Alternatively, while the frequency of HIV-specific cells may still be higher in the intrathecal compartment, the phenotype of these cells will be consistent with immune exhaustion (i.e. CD57high, PD-1high, CD127low). We also predict these cells will show diminished proliferative capacity and a decreased ability to produce cytokines in response to cognate antigen ex vivo. Such findings would be consistent with a mechanism that involves loss of immune control of viral replication in the CNS, with subsequent CNS dysfunction. The longitudinal nature of these studies will allow us to determine whether anti-retroviral therapy reverses this cellular phenotype and dysfunction, and to what extent this dysfunction is reversible in the CNS compared to the periphery.
Our study will serve as a model for understanding the functionality of antigen-specific T cells in the CNS during viral infections. These evaluations may also lead in the development and evaluation of immunotherapeutic approaches to treatment of viral infections of the brain.