Phenotypic and functional analysis of positive selection in the gamma/delta T cell lineage.

Recent evidence suggests that T cells expressing gamma/delta antigen receptors (T cell receptor [TCR]) are subject to positive selection during development. We have shown that T cells expressing a class I major histocompatibility complex (MHC)-specific gamma/delta TCR transgene (tg) are not positively selected in class I MHC-deficient, beta 2-microglobulin (beta 2m) gene knockout mice (tg+ beta 2m-). In this report, we examine phenotypic and functional parameters of gamma/delta positive selection in this transgenic model system. TCR-gamma/delta tg+ thymocytes of mature surface phenotype (heat stable antigen-, CD5hi) were found in beta 2m+ but not in beta 2m- mice. Moreover, subsets of tg+ thymocytes with the phenotype of activated T cells (interleukin [IL]2R+, CD44hi, or Mel-14lo) were also present only in the beta 2m+ mice. Cyclosporine A, which blocks positive selection of TCR-alpha/beta T cells, also inhibited gamma/delta tg+ T cell development. These results support the idea that positive selection of TCR-gamma/delta requires active TCR-mediated signal transduction. Whereas tg+ beta 2m+ thymocytes produced IL-2 and proliferated when stimulated by alloantigen, TCR engagement of tg+ beta 2m- thymocytes by antigen induced IL-2R expression but was uncoupled from the signal transduction pathway leading to IL-2 production and autocrine proliferation. Overall, these results demonstrate significant parallels between gamma/delta and alpha/beta lineage development, and suggest a general role for TCR signaling in thymic maturation.

Requirement for positive selection of gamma delta receptor-bearing T cells.

The alpha beta and gamma delta T cell receptors for antigen (TCR) delineate distinct T cell populations. TCR alpha beta-bearing thymocytes must be positively selected by binding of the TCR to major histocompatibility complex (MHC) molecules on thymic epithelium. To examine the requirement for positive selection of TCR gamma delta T cells, mice bearing a class I MHC-specific gamma delta transgene (Tg) were crossed to mice with disrupted beta 2 microglobulin (beta 2M) genes. The Tg+beta 2M- (class I MHC-) offspring had Tg+ thymocytes that did not proliferate to antigen or Tg-specific monoclonal antibody and few peripheral Tg+ cells. This is evidence for positive selection within the gamma delta T cell subset.

Activation of alveolar macrophages in asymptomatic HIV-infected individuals.

After the initial infection with HIV, there is evidence of immune dysfunction despite an apparent normal clinical state. In the context that the lung is a major site affected by opportunistic infection during the progression of this immune dysfunction, and that some components of the immune system are activated during early HIV infection, we hypothesized that there may be activation of alveolar macrophages (AM), a key component of the pulmonary host defense system, during the asymptomatic phase of HIV infection. Compared to normals, in HIV-infected individuals the class II MHC molecules DR, DQ, and DP were all expressed more frequently and in greater cell surface density on AM (p < 0.03, all comparisons), and there was increased spontaneous release of superoxide anion (O2-.) by AM (p < 0.002). To gain insight into whether the activation of the AM was an inherent property of the cells or dependent on the in vivo milieu, AM were evaluated after 24 h in culture for O2-. release. In contrast to the findings in fresh AM, after 24 h in culture, O2-. release by HIV AM was not different from normals (p > 0.7), suggesting that these AM had been activated in vivo. To assess whether IFN-gamma could be mediating these effects, mRNA levels of the IP-10 gene (a gene specifically induced by increased concentrations of IFN-gamma) were quantified in AM. Strikingly, the IP-10 gene was expressed only in AM of HIV-seropositive individuals, suggesting the AM had been exposed to IFN-gamma in vivo. Overall, these observations are consistent with the concept that the HIV-seropositive state is associated with activation of AM, in part due to local exposure to IFN-gamma.

Systemic glutathione deficiency in symptom-free HIV-seropositive individuals.

To find out whether systemic glutathione deficiency is associated with human immunodeficiency virus (HIV) infection, thus contributing to the immunodeficiency state, glutathione concentrations in venous plasma and lung epithelial lining fluid (ELF) of symptom-free HIV-seropositive and normal individuals were measured. Total and reduced glutathione concentrations in the plasma of the HIV-infected subjects were about 30% of those in the normal individuals. Concentrations of these substances in the ELF of HIV-infected subjects were about 60% of those in the controls. There was no correlation between ELF and plasma concentrations of total or reduced glutathione. Since glutathione enhances immune function, glutathione deficiency may contribute to the progressive immune dysfunction of HIV infection.