Mycoplasma infection is a leading cause of pneumonia worldwide and can lead to other respiratory complications. A component of mycoplasma respiratory diseases is immunopathologic, suggesting that lymphocyte activation is a key event in the progression of these chronic inflammatory diseases. The present study delineates the changes in T cell populations and their activation after mycoplasma infection and determines their association with the pathogenesis of murine Mycoplasma respiratory disease, due to Mycoplasma pulmonis infection. Increases in T cell population numbers in lungs and lower respiratory lymph nodes were associated with the development of mycoplasma respiratory disease. Although both pulmonary Th and CD8+ T cells increased after mycoplasma infection, there was a preferential expansion of Th cells. Mycoplasma-specific Th2 responses were dominant in lower respiratory lymph nodes, while Th1 responses predominated in spleen. However, both mycoplasma-specific Th1 and Th2 cytokine (IL-4 and IFN-γ) responses were present in the lungs, with Th1 cell activation as a major component of the pulmonary Th cell response. Although a smaller component of the T cell response, mycoplasma-specific CD8+ T cells were also a significant component of pulmonary lymphoid responses. In vivo depletion of CD8+ T cells resulted in dramatically more severe pulmonary disease, while depletion of CD4+ T cells reduced its severity, but there was no change in mycoplasma numbers in lungs after cell depletion. Thus, mycoplasma-specific Th1 and CD8+ T cell activation in the lung plays a critical regulatory role in development of immunopathologic reactions in Mycoplasma respiratory disease.

The purpose of this study was to examine the 100-fold difference in mycoplasma levels in lungs of gamma interferon knockout (IFN-␥ ؊/؊ ) mice compared to those seen with wild-type BALB/c mice at 3 days postinfection. NK cells secreted IFN-␥; however, their cytotoxic granule extracts failed to kill mycoplasma. We found a conundrum: the clearance of organisms was as effective in NK

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Mycoplasma lipoproteins are recognized by Toll-like receptors (TLR), but TLRs' role in responses to infection are unknown. Mycoplasma pulmonis is a naturally occurring respiratory pathogen in mice. In the current study, we used TLR-transfected HEK cells and TLR2−/− bone marrow-derived dendritic cells to demonstrate TLR2-mediated events are important in the initial host-mycoplasma interactions promoting cytokine responses. As we found alveolar macrophages expressed TLR1, TLR2 and TLR6 mRNAs, a role for TLR2 in innate immune clearance in lungs was examined. Three days post-infection, TLR2−/− mice had higher M. pulmonis numbers in lungs, but not in nasal passages. However, TLR2−/− mice had higher lung cytokine levels, indicating TLR2-independent mechanisms are also involved in host responses. Thus, TLR2 plays a critical role in the ability of innate immunity to determine M. pulmonis numbers in the lung, and it is likely that early after respiratory infection that TLR2 recognition of M. pulmonis triggers initial cytokine responses of host cells.