The Lymphotoxin β Receptor Is Critically Involved in Controlling Infections with the Intracellular Pathogens Mycobacterium tuberculosis and Listeria monocytogenes

Containment of intracellularly viable microorganisms requires an intricate cooperation between macrophages and T cells, the most potent mediators known to date being IFN-gamma and TNF. To identify novel mechanisms involved in combating intracellular infections, experiments were performed in mice with selective defects in the lymphotoxin (LT)/LT beta R pathway. When mice deficient in LT alpha or LT beta were challenged intranasally with Mycobacterium tuberculosis, they showed a significant increase in bacterial loads in lungs and livers compared with wild-type mice, suggesting a role for LT alpha beta heterotrimers in resistance to infection. Indeed, mice deficient in the receptor for LT alpha(1)beta(2) heterotrimers (LT beta R-knockout (KO) mice) also had significantly higher numbers of M. tuberculosis in infected lungs and exhibited widespread pulmonary necrosis already by day 35 after intranasal infection. Furthermore, LT beta R-KO mice were dramatically more susceptible than wild-type mice to i.p. infection with Listeria monocytogenes. Compared with wild-type mice, LT beta R-KO mice had similar transcript levels of TNF and IFN-gamma and recruited similar numbers of CD3(+) T cells inside granulomatous lesions in M. tuberculosis-infected lungs. Flow cytometry revealed that the LT beta R is expressed on pulmonary macrophages obtained after digestion of M. tuberculosis-infected lungs. LT beta R-KO mice showed delayed expression of inducible NO synthase protein in granuloma macrophages, implicating deficient macrophage activation as the most likely cause for enhanced susceptibility of these mice to intracellular infections. Since LIGHT-KO mice proved to be equally resistant to M. tuberculosis infection as wild-type mice, these data demonstrate that signaling of LT alpha(1)beta(2) heterotrimers via the LT beta R is an essential prerequisite for containment of intracellular pathogens.