Mycobacterial Infection is Promoted by Neutral Sphingomyelinase 2 Regulating a Signaling Cascade Leading to Activation of ß1-Integrin.

BACKGROUND/AIMS: Mycobacteria-induced diseases, especially tuberculosis, cause more than 1 million deaths each year, which is higher than any other single bacterial pathogen. Neutral sphingomyelinase 2 (Nsm2) has been implied in many physiological processes and diseases, but the role of Nsm2 in pathogen-host interactions and mycobacterial infections has barely been studied. METHODS: We investigated the role of the Nsm2/ceramide system in systemic infection of mice and murine macrophages with Mycobacterium bovis Bacillus Calmette-Guérin (BCG) as a model for mycobacterial infection. For in vitro assays we isolated bone marrow-derived macrophages from Wildtype mice or Nsm2-heterozygous and investigated the role of Nsm2 for macrophage migration/clustering as well as the involvement of p38 mitogen-activated protein kinases (p38K), c-Jun N-terminal kinase (JNK), ß1-integrin and Rac1 activity by Western blot and microscopic studies. For in vivo assays we injected mice intravenously with BCG and analyzed infected tissues for the role of Nsm2-mediated activation of ß1-integrin in granuloma formation and bacterial burden. RESULTS: Our results reveal that BCG infection of macrophages results in rapid stimulation of Nsm2. Genetic and pharmacological studies demonstrate that Nsm2 stimulates a signaling cascade via p38K and JNK to an activation of surface ß1-integrin and Rac1 that leads to the formation of granuloma-like macrophages clusters in vitro and granuloma in vivo. Heterozygosity of Nsm2 in macrophages or antibody-mediated neutralization of active b1-integrin reduced macrophage clusters in vitro and granuloma formation in vivo. Most importantly, Nsm2 heterozygosity or treatment with neutralizing antibodies against ß1-integrin protected mice from systemic BCG infections and chronic infections of the liver and spleen. CONCLUSION: The findings indicate that the Nsm2/ ceramide system plays an important role in systemic infection of mice with mycobacteria by regulating a signaling cascade via p38K, JNK, b1-integrin and Rac1.