Immune CD8+ T Cells Prevent Reactivation of Toxoplasma gondii Infection in the Immunocompromised Host

Toxoplasma gondii remains a serious cause of morbidity and mortality in individuals that are immunosuppressed, patients with AIDS in particular. The cellular immune response, especially by gamma interferon (IFN-γ)-producing CD8+ T cells, is an essential component of protective immunity against the parasite. In the present study the role of CD8+ T cells during the reactivation of Toxoplasma infection in an immunocompromised murine model was evaluated. Chronically infected mice were challenged with LP-BM5 virus, and the kinetics of CD8+ T-cell function was studied. At 10 weeks after viral infection, mice showed obvious signs of systemic illness and began to die. At this stage, CD8+ T cells were unresponsive to antigenic stimulation and unable to kill Toxoplasma-infected targets. IFN-γ production by the CD8+ T cells from dual-infected animals reached background levels, and a dramatic fall in the frequency of precursor cytotoxic T lymphocytes was observed. Histopathological analysis of the tissues demonstrated signs of disseminated toxoplasmosis as a result of reactivation of infection. However, treatment of the dual-infected animals with immune CD8+ T cells at 5 weeks post-LP-BM5 challenge prevented the reactivation of toxoplasmosis, and mice continued to live. Our study for the first time demonstrates a therapeutic role for CD8+ T cells against an opportunistic infection in an immunocompromised state