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Category: Viruses and Viral Pathogenesis
T-Lymphocyte Immune Responses in HIV Infection, Page 1 of 2
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In most viral infections, the cell-mediated immune response plays a vital role in arresting or eliminating the infectious agent. For HIV, this antiviral activity involves both the innate and adaptive immune systems. This chapter provides a review of the adaptive T-cell immune responses that appear to be directed against HIV through recognition of viral proteins associated with HIV or virus-infected cells. Since HIV infection disturbs the immune system, some of the findings also have direct relevance to this discussion. In addition, the noncytotoxic anti-HIV activity of CD8+ cells that appears to be an innate immune activity is reviewed. The chapter concludes with a discussion of T regulatory cells.
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Differentiation of CD4+ lymphocytes. This overall development of various T-cell subsets is mirrored as well in the formation of CD8+ T-cell subsets. Naïve cells after exposure to antigen develop into effector cells, which in terms of CD4+ cells are of the TH1 or TH2 type ( Table 11.2 ). Effector cells can revert to effector/memory cells or resting (central) memory cells in the presence of low antigen. With reexposure to antigens, the T effector memory cells quickly respond and evolve into terminal effector cells. These cells are shortlived with high antigen specificity and, particularly in the case of CD8+ cells, show cytotoxic activity. Figure from reference 1340.
Generation of the T-cell repertoire. (a) Undifferentiated lymphoid stem cells (prothymocytes), as they pass through the thymus, express CD4 and CD8 molecules. Their T-cell receptor (TCR) genes are rearranged and expressed, and the resulting thymocytes undergo negative and positive selection. T-cell clones released into circulation have a diverse TCR pattern, indicating their potential for polyclonal responses. (b) With destruction of T cells by HIV, a limited T-cell repertoire may exist which, when allowed to regenerate through a variety of therapeutic interventions, may not recover the entire repertoire initially present in the host. (c) The end-stage T-cell repertoire may be limited so that it cannot react quickly to a particular antigen. In a sense, as shown here, the T-cell repertoire could not respond in a way that would require the spelling of ZEBRA. Nevertheless, by other processes, it might be able to respond (although more slowly) by recognizing the antigen as “a horse with white and black stripes.” Ideas derived from figures and discussions provided by H. Cliff Lane.
Cytokine-driven type 1 cell differentiation. Bacteria, protozoa, probably viruses, and helminths can induce the production of IL-12 by macrophages or other cells. IL-12 then induces differentiation of naïve or TH0 cells into a TH1 subset, which encourages cell-mediated immunity. Other pathogens, particularly helminths, stimulate IL-4 production by cells not yet defined. IL-4 induces TH0 cells toward TH2 cell differentiation. The TH2 cells mediate help for antibody production. What determines the predominant TH1 (type 1) or TH2 (type 2) response in the host is not known. Modified from reference 4023 with permission. Copyright 1993 AAAS.
Cross-regulation of the TH1 and TH2 subsets of CD4+ cells. The production of certain cytokines by TH1 and TH2 cells can suppress production of cytokines by the corresponding cell subset (3786). From reference 2520 with permission.
Effect of CD8+ cells and CD8+ cell antiviral factor (CAF) on parameters of HIV replication. The CD8+ cell noncytotoxic antiviral response blocks viral replication, as indicated by decreased reverse transcriptase (RT) activity, viral protein expression measured by immunofluorescent antibody (IFA) techniques, and in situ RNA production. This activity has no effect on the number of infected cells in the culture. The antiviral effect is observed as well in a reduction in unspliced (us), single-spliced (ss), and double-spliced (ds) HIV RNA levels compared to a normal expression of β-actin RNA. Finally, the suppressing effect of CD8+ cells or CAF does not affect the basal-level expression of HIV LTR-driven transcription but blocks induction of this transcription by HIV, simian virus 40 tat expression, or phorbol myristate acetate (PMA) using cells in which the HIV LTR has been linked to a reporter gene (2738).
Effect of cytokines on the CD8+ cell noncytotoxic anti-HIV response. CD8+ cells were stimulated in the presence of type 1 or type 2 cytokines for 3 days. After being washed, they were tested for their ability to suppress HIV replication in acutely infected CD4+ lymphocytes. From reference 258 with permission.
Costimulation of CD8+ T cells. The lymphocyte response is optimal following the interaction of the T-cell receptor (TCR) with an MHC class I molecule associated with antigen, together with the interaction of the CD28 molecules on the T cells with the B7 molecules on antigen-presenting cells [APC]) (e.g., macrophages and dendritic cells). The response appears to be mediated by increased IL-2 production and expression of the IL-2 receptor (IL-2R) (663, 3282). A role of IL-15 produced by the APC has also been noted (635).
When CD8+ cells are stimulated in the presence of antibodies to the T-cell receptor (TCR) complex (anti-CD3 antibody) together with antibodies to the costimulatory molecule CD28, their ability to suppress HIV replication in infected CD4+ cells is increased both in asymptomatic individuals and particularly in AIDS patients (progressors). From reference 255 with permission. © 1997 The American Association of Immunologists, Inc.
Quantity of CD8+ cell antiviral factor (CAF) produced by CD8+ cells. Dilutions of CAF-containing fluid from cultured CD8+ cells from an asymptomatic individual indicate that a 1:4 dilution will still show a 50% reduction in HIV replication as measured by virus reverse transcriptase (RT) activity in the culture fluid. Fluids from CD8+ cells of healthy uninfected individuals or those with AIDS do not show evidence of CAF production.
Comparisons of assays for T-cell responses a
Characterization of CD4+ T helper cell subsets
Human CD4+ T-cell subset heterogeneity a
Human CD8 T-cell subset heterogeneity
Anti-HIV responses by CD8+ T cells
Possible mechanisms of virus resistance to anti-HIV CTL activity a
Possible evidence for detrimental effects of CD8+ CTL activity in HIV infection a
Characteristics of the CD8+ cell noncytotoxic anti-HIV response (CNAR)
CD8+ cell noncytotoxic antiviral response is part of the innate immune system
Potential clinical value of CD8+ cell noncytotoxic antiviral activity in HIV infection
Characteristics of the CD8+ cell anti-HIV factor a
Proteins lacking identity to the CD8+ cell antiviral factor (CAF)
Natural human factors with anti-HIV activity
Characteristics of human CD4+ T regulatory cells a