Treatment with interleukin-7 restores host defense against Pneumocystis in CD4⁺ T-lymphocytedepleted mice

Pneumocystis pneumonia (PCP) is a major cause of morbidity and mortality in patients with HIV infection. CD4⁺ T lymphocytes are critical for host defense against this infection, but in the absence of CD4⁺ T lymphocytes, CD8⁺ T lymphocytes may provide limited host defense. The cytokine interleukin-7 (IL-7) functions to enhance lymphocyte proliferation, survival, and recruitment of immune cells to sites of infection. However, there is little known about the role of IL-7 in PCP or its potential use as an immunotherapeutic agent. We hypothesized that treatment with recombinant human IL-7 (rhIL-7) would augment host defense against Pneumocystis and accelerate pathogen clearance in CD4-depleted mice. Control and CD4-depleted mice were infected with Pneumocystis, and rhIL-7 was administered via intraperitoneal injection. Our studies indicate that endogenous murine IL-7 is part of the normal host response to Pneumocystis murina and that administration of rhIL-7 markedly enhanced clearance of Pneumocystis in CD4-depleted mice. Additionally, we observed increased recruitment of CD8⁺ T lymphocytes to the lungs and decreased apoptosis of pulmonary CD8⁺ T lymphocytes in rhIL-7-treated animals compared to those in untreated mice. The antiapoptotic effect of rhIL-7 was associated with increased levels of Bcl-2 protein in T lymphocytes. rhIL-7 immunotherapy in CD4-depleted mice also increased the number of gamma interferon (IFN-γ)-positive CD8⁺ central memory T lymphocytes in the lungs. We conclude that rhIL-7 has a potent therapeutic effect in the treatment of murine Pneumocystis pneumonia in CD4-depleted mice. This therapeutic effect is mediated through enhanced recruitment of CD8⁺ T cells and decreased apoptosis of lung T lymphocytes, with a preferential action on central memory CD8⁺ T lymphocytes.