Interleukin-7 or interleukin-15 enhances survival of Mycobacterium tuberculosis-infected mice

Both antigen-presenting cells and immune effector cells are required to effectively eradicate or contain Mycobacterium tuberculosis-infected cells. A variety of cytokines are involved to ensure productive "cross talk" between macrophages and T lymphocytes. For instance, infection of macrophages with mycobacteria leads to effective interleukin-7 (IL-7) and IL-15 secretion, and both cytokines are able to maintain strong cellular immune responses of alpha/beta and gamma/delta T cells. Here we show that either cytokine is able to enhance survival of M. tuberculosis-infected BALB/c mice significantly compared to application of IL-2, IL-4, or phosphate-buffered saline (as a control). Enhanced survival could be achieved only when IL-7 or IL-15 was delivered as a treatment (i.e., 3 weeks postinfection), not when it was administered at the time of infection. Increased survival of M. tuberculosis-infected animals was observed following passive transfer of spleen cells harvested from M. tuberculosis-infected, IL-7- or IL-15-treated animals, but not after transfer of spleen cells obtained from mice which received either cytokine alone. Histological examination revealed that IL-7 and IL-15 failed to significantly impact on the number and composition of granulomas formed or the bacterial load. Our data indicated that administration of IL-7 or IL-15 to M. tuberculosis-treated animals resulted in a qualitatively different cellular immune response in spleen cells as reflected by increased tumor necrosis factor alpha and decreased gamma interferon secretion in response to M. tuberculosis-infected antigen-presenting cells.