Functionality of Antigen-Specific T Cells that Control Viral Replication in the Central Nervous System

Spyros Kalams, M.D.

Vanderbilt University Medical Center

Funded in September, 2008: $200000 for 3 years
LAY SUMMARY . ABSTRACT . BIOGRAPHY . SELECTED PUBLICATIONS .

LAY SUMMARY

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Determining How Immune T Cells Migrate to the Brain and Control Viral Brain Infections

Researchers will examine how chronic viral infection of the brain affects the ability of immune T cells to control the infection and protect against neurological dysfunction, and then will attempt to determine whether anti-viral therapy restores immune capacity to protect the brain.  These processes will be studied in AIDS patients who are at risk of succumbing to virally-induced dementia.  

Some patients infected with HIV, the virus that causes AIDS, develop HIV dementia while others do not.  Generally in AIDS patients, the central nervous system (CNS) becomes infected as the virus reaches brain cells and replicates.  Immune T cells learn to recognize the HIV virus, however, and a subset of these cells, called cytotoxic, or “killer,” T cells, migrate to the brain. There, they directly kill virally infected brain cells while releasing proteins (called “cytokines”) that inhibit viral replication, and prevent neurological dysfunction.  In prior studies in AIDS patients, the investigators found that those with no signs of neurological dysfunction had high levels of killer T cells and low levels of virus in their cerebrospinal fluid (CSF) compared to levels in circulating blood.  They hypothesize that killer T cells migrate from the bloodstream to the brain and help control viral replication, but then under conditions of chronic viral exposure the killer T cells become exhausted and lose control of the infection, leading to dementia.  They further hypothesize that AIDS drug therapy reduces the level of virus in patients’ CSF, reverses immune dysfunction in the CNS, and protects against dementia.

Researchers will test these hypotheses, initially with 50 AIDS patients who are about to start anti-retroviral therapy.  Patients will undergo neurological and imaging evaluations.  Then, patients’ killer T cells from circulating blood and from CSF will be characterized in laboratory cultures. Investigators anticipate that patients who have a higher concentration of killer T cells in their CSF compared to plasma will show no cognitive impairment and that their killer T cells will show a greater capacity to respond to virus and to produce cytokines, than will patients whose killer T cell concentrations are higher in plasma than in CSF.  Investigators then will follow 25 of the patients, selected to represent a range of killer T cell concentrations in CSF versus plasma, to see whether anti-retroviral therapy is associated with progressive reversal of killer T cell exhaustion in CSF compared to plasma, and restoration of T cell function in the CSF.

Significance:  The study is expected to show how killer T cells function best to control viral infection and replication in the brain, and to lead to development of new immune-based therapies to control viral brain infections.       

ABSTRACT

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Functionality of Antigen-Specific T Cells that Control Viral Replication in the Central Nervous System

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.

INVESTIGATOR BIOGRAPHIES

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Spyros Kalams, M.D.

Following the reception of his M.D. from University of Connecticut Medical Center, Dr. Kalams completed his residency at Beth Israel Hospital (Boston) and an Infectious Diseases Fellowship at Massachusetts General Hospital (Boston).

Dr. Kalams is the director of viral immunology research at Vanderbilt Medical Center. The primary research focus of his laboratory is to understand the qualitative aspects of the immune response that control viral replication over the course chronic viral infections. He has assembled a cohort of HIV-infected subjects off therapy. Many of these subjects have maintained control of HIV infection, with viral loads <2,000 copies/ml, for several years. These subjects are studied in great detail to determine the magnitude of HIV-specific T cell responses with a number of functional assays, such as Elispot and intracellular cytokine staining by flow cytometry. Cell surface phenotype by flow cytometry is able to identify cells that show evidence of immune exhaustion. His laboratory is evaluating the magnitude of HIV-specific immune responses in the cerebrospinal fluid of HIV-infected individuals. The hope is to understand mechanisms of homing of HIV-specific T cells to the central nervous system and to determine whether these immune responses help protect against HIV-mediated neurological dysfunction.

SELECTED PUBLICATIONS

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Sadagopal S., Lorey S.L., Barnett L., Basham R., Lebo L., Erdem H., Haman K., Avison M., Waddell K., Haas D.W., and Kalams S.A.   Enhancement of HIV-specific CD8+ T cells in cerebrospinal fluid compared to blood among antiretroviral therapy naive HIV+ subjects.   J Virol. 2008 Aug 20.