Th1 type CD4+T cell responses involving IFN-gamma, TNF and NO production are critical in the control of Mycobacterium tuberculosis infection. The development of T lymphocytes with this phenotype is known to be dependent on the production of Interleukin (IL)-12 and mice lacking this cytokine are consequently highly susceptible to M. tuberculosis. In previous studies we showed that 5-lipoxygenase-dependent (5-LO-dependent) lipoxins regulate host IL-12 production in vivo and influence host resistance to Toxoplasma gondii. In studies performed during the report period, Andre Bafica, in collaboration with Julio Aliberti at Duke, established these lipid mediators as key chemical regulators of resistance to M. tuberculosis infection. High levels of lipoxin A4 (LXA4) were detected in sera from infected WT but not 5-LO-deficient mice. Moreover, lungs from M. tuberculosis-infected 5-LO -/- animals showed increased IL-12, IFN-gamma, and NO synthase 2 (NOS2) mRNA levels compared with the same tissues in WT mice. Similarly, splenocyte recall responses were enhanced in mycobacteria-infected 5-LO -/- versus WT mice. Importantly, bacterial burdens in 5-LO-/- lungs were significantly lower than those from WT mice, and this enhancement in the resistance of the 5-LO-/- animals to M. tuberculosis was completely prevented by administration of a stable LXA4 analog. Together our results demonstrated that lipoxins negatively regulate protective Th1 responses against mycobacterial infection in vivo and suggest that the inhibition of lipoxin biosynthesis could serve as a strategy for enhancing host resistance to M. tuberculosis. Although IL-12 is clearly critical for the induction of IFN-gamma-dependent host control of M. tuberculosis, the role of the cytokine in the maintenance of host resistance and pulmonary Th1 effector function has been unclear. To address this question, Carl Feng reconstituted IL-12p40-deficient mice with IL-12 during the first 4 wk of infection and then assessed the effects of cytokine withdrawal. Although IL-12 administration initially resulted in restricted mycobacterial growth and prolonged survival, the reconstituted animals eventually succumbed to infection. This breakdown in bacterial control was accompanied by a marked reduction in the numbers of IFN-gamma-producing CD4+ T cells in lungs. Moreover, whereas CD4+ T cells isolated from chronically infected wild-type mice expanded and transferred long-term protection to M. tuberculosis-challenged RAG-/-mice, they failed to do so in IL-12p40-deficient RAG-/-recipients and were clearly reduced in frequency within pulmonary granulomas in the latter animals. These studies established that continuous IL-12 production is necessary for maintenance of the pulmonary Th1 cells required for host control of persistent M. tuberculosis infection and suggest that breakdown of this mechanism could be a contributing factor in reactivated disease.
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