Identification of the components, targets, and magnitude of an effective immune response to HIV are important steps toward the development of effective vaccines or immunotherapies. Although patients with normal CD4+ T cell counts and low levels of plasma virus are a heterogeneous group, a small subgroup of patients with truly non-progressive HIV infection and restriction of virus replication likely holds important clues to the basis of an effective immune response to HIV. A small subpopulation (fewer than 0.8% of HIV infected individuals) shows no signs of progression over a 10-year period. We have assembled a stringently defined cohort of such patients, termed long-term nonprogressors (LTNP), with non-progressive disease. Many of these patients have been infected for 20 years in the absence of antiretroviral therapy with no CD4+ T cell decline, and plasma viral RNA below 50 copies per milliliter. Cells from these patients are being used to systematically dissect the mechanisms of immune mediated restriction of virus replication. HIV-specific T cell responses of these patients have been studied in extreme detail. Prior work has indicated that there is a dramatic association with the HLA B*5701 allele and that the immune response is highly focused on peptides restricted by this allele. More recently we have found that this focus is specific to HIV, and is not found in the response to other pathogens such as Hepatitis C virus or Cytomegalovirus. LTNP patients do not differ in the frequency of HIV-specific T cells nor in the ability to recognize the autologous virus, when compared to progressors. Thus far the only difference in the HIV-specific immune response that distinguishes LTNP from progressors is the maintenance of HIV-specific CD8+ T cells with a high proliferative capacity. This proliferation parallels perforin expression. In addition, we have recently demonstrated that this increase in proliferation is coupled to extraordinary killing in vitro. CD8+ T cells specific for 3 or fewer HIV peptides can kill the majority of targets in less than one hour. Strong HIV specific CD4+ T cell proliferative responses are demonstrable in these LTNP patients. However, they are not demonstrable in the majority of infected patients with progressive disease. Several explanations have been offered for this phenomenon, including depletion of HIV-specific cells by direct infection and depletion of IL-2 expressing CCR7+ central memory cells. We used 11-color flow cytometry to compare the frequency, phenotype, and cytokine secretion of HIV-specific CD4+ T cells to those specific for tetanus, influenza, CMV, Epstein-Barr virus, and adenovirus in LTNP and progressor cohorts. In patients on antiretroviral therapy, the frequency, phenotype and function of HIV-specific CD4+ T cells were similar to those specific for these other pathogens. Proliferation, IL-2 production and CCR7 expression were all diminished by viremia during an interruption of antiretroviral therapy. We also observed that viremia was accompanied by a decrease in production of TNF-a, IL-1B and IL-6 by monocytes. However, these changes did not cause diminished proliferation of CD4+ T cells. Rather, diminished proliferation of HIV-specific CD4+ T cells in vitro was completely attributable to diminished IL-2 production. Thus diminished proliferation, IL-2 production, and CCR7 expression are an effect, and not a cause, of high levels of viral replication. Taken together, during the chronic phase of infection we do not detect differences in the HIV-specific CD4+ T cells between patients with or without immunologic control of HIV that would suggest a critical defect in this response.
These findings raise a number of fundamental issues for the potential use of the T cell response to HIV in prophylactic vaccines or immunotherapies. Because most individuals fail to restrict HIV replication despite a broad, high frequency CD8+ T cell response, simply stimulating a high frequency response via a therapeutic or prophylactic vaccine will not necessarily result in durable immunologic control. Thus, it is unclear whether CD8+ T cells can be sufficiently primed such that qualitative changes in the CD8+ T cell response can be avoided. In addition, it is unclear how long after HIV infection of vaccinees these qualitative changes might occur and immunologic restriction of viral replication is lost. Further, it is unclear whether these qualitative changes can be reversed in infected individuals. A deeper understanding of the basis of immunologic control in LTNP and the loss of immunologic control in progressors is likely to provide information that is critical for use of the cellular immune response in each of these settings.

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