Increased cytolytic T lymphocyte activity and decreased B7 responsiveness are associated with CD28 down-regulation on CD8+ T cells from HIV-infected subjects

The CD28 receptor on CD4⁺ and CD8⁺ T cells interacts with B7 molecules on antigen-presenting cells (APC) to generate essential costimulatory signals. The cytolytic potential of CD8⁺ T cells could be linked to CD28 expression. Since HIV induces dysfunction of both CD4⁺ and CD8⁺ T cells, we evaluated CD28 expression and function in both subsets during HIV infection. CD28 expression on CD8⁺ T cells from HIV+ subjects was strongly reduced in a disease stage-related fashion. CD28^-CD8⁺ T cells preferentially expressed CD57 and CD11b, but lacked CD26 and IL-2Rα. The CD8+ T cells from the patients showed a significantly reduced proliferative response to co-stimulation with cell-bound anti-CD3 and B7. Nevertheless, when stimulated with plate-fixed anti-CD3, CD8+ T cells from HIV-infected subjects proliferated normally, and normal levels of IL-2Rα nod transferrin-receptor could be induced on CD28^-CD8⁺ T cells from the patients. In addition, stimulation with plate-fixed anti-CD3 induced proliferative responses in highly purified CD28^-CD8⁺ T cells from both HIV- and HIV+ persons. Furthermore, the increased cytotoxic activity of peripheral blood mononuclear cells (PBMC) from HIV+ subjects, measured in an anti-CD3 redirected assay, was predominantly exerted by CD28^-CD57⁺ T cells. CD4⁺ T cells from the patients showed a slight but significant CD28 down-regulation and were slightly hyporesponsive to B7 co-stimulation. Decrease of CD28 on CD8⁺ T cells from HIV+ subjects is associated with an impaired response to co-stimulation via B7. CD28^-CD8⁺ T cells from seropositives, however, are not completely inert, since they contain in vivo activated CTL and they can be additionally activated through a B7-independent stimulation.