Impact of inducible co-stimulatory molecule (ICOS) on T-cell responses and protection against Mycobacterium tuberculosis infection.

Even though Mycobacterium tuberculosis (Mtb) remains one of the top microbial killers, more than 90% of the 2 billion infected individuals never develop active tuberculosis (TB), indicating efficient immune control of infection in these individuals. Immune mechanisms promoting either control or reactivation of TB are incompletely understood. Kinetic analyses of T-cell responses against Mtb in C57BL/6 mice revealed surface expression of inducible co-stimulatory molecule (ICOS) on >30% of all CD4(+) T cells, suggesting a pivotal role of this costimulatory molecule of the CD28 family in TB control. Surprisingly, Mtb-infected ICOS(-/-) mice showed lower bacterial burden during the late chronic stage of infection as compared to WT controls. ICOS deficiency resulted in a reduced Mtb-specific CD8(+) T-cell response during late-stage infection. In contrast, the polyclonal CD4(+) Th1 response against Mtb was increased, most likely caused by diminished numbers and frequencies of Tregs. Thus, by altering effector T-cell populations differentially, ICOS signaling modulates TB control in the late stage of infection.

CXCL5-secreting pulmonary epithelial cells drive destructive neutrophilic inflammation in tuberculosis.

Successful host defense against numerous pulmonary infections depends on bacterial clearance by polymorphonuclear leukocytes (PMNs); however, excessive PMN accumulation can result in life-threatening lung injury. Local expression of CXC chemokines is critical for PMN recruitment. The impact of chemokine-dependent PMN recruitment during pulmonary Mycobacterium tuberculosis infection is not fully understood. Here, we analyzed expression of genes encoding CXC chemokines in M. tuberculosis-infected murine lung tissue and found that M. tuberculosis infection promotes upregulation of Cxcr2 and its ligand Cxcl5. To determine the contribution of CXCL5 in pulmonary PMN recruitment, we generated Cxcl5(-/-) mice and analyzed their immune response against M. tuberculosis. Both Cxcr2(-/-) mice and Cxcl5(-/-) mice, which are deficient for only one of numerous CXCR2 ligands, exhibited enhanced survival compared with that of WT mice following high-dose M. tuberculosis infection. The resistance of Cxcl5(-/-) mice to M. tuberculosis infection was not due to heightened M. tuberculosis clearance but was the result of impaired PMN recruitment, which reduced pulmonary inflammation. Lung epithelial cells were the main source of CXCL5 upon M. tuberculosis infection, and secretion of CXCL5 was reduced by blocking TLR2 signaling. Together, our data indicate that TLR2-induced epithelial-derived CXCL5 is critical for PMN-driven destructive inflammation in pulmonary tuberculosis.