Wnt6 Is Expressed in Granulomatous Lesions of Mycobacterium tuberculosis-Infected Mice and Is Involved in Macrophage Differentiation and Proliferation

The Wnt signaling network, an ancient signaling system governing ontogeny and homeostatic processes, has recently been identified to exert immunoregulatory functions in a variety of inflammatory and infectious disease settings including tuberculosis. In this study, we show that Wnt6 is expressed in granulomatous lesions in the lung of Mycobacterium tuberculosis-infected mice. We identified foamy macrophage-like cells as the primary source of Wnt6 in the infected lung and uncovered a TLR-MyD88-NF-kappa B-dependent mode of induction in bone marrow-derived macrophages. Analysis of Wnt6-induced signal transduction revealed a pertussis toxin-sensitive, ERK-mediated, but beta-catenin-independent induction of c-Myc, a master regulator of cell proliferation. Increased Ki-67 mRNA expression levels and enhanced thymidine incorporation in Wnt6-treated macrophage cultures demonstrate a proliferation-promoting effect on murine macrophages. Further functional studies in M. tuberculosis-infected macrophages using Wnt6 conditioned medium and Wnt6-deficient macrophages uncovered a Wnt6-dependent induction of macrophage Arginase-1 and downregulation of TNF-alpha. This identifies Wnt6 as a novel factor driving macrophage polarization toward an M2-like phenotype. Taken together, these findings point to an unexpected role for Wnt6 in macrophage differentiation in the M. tuberculosis-infected lung.