Macrophages exert either a detrimental or beneficial role in Multiple Sclerosis (MS) pathology, depending on their inflammatory environment. Tissue Transglutaminase (TG2), a calcium-dependent cross-linking enzyme, has been described as a novel marker for anti-inflammatory, interleukin-4 (IL-4) polarized macrophages (M(IL-4)), which represent a subpopulation of macrophages with phagocytic abilities. Since TG2 is expressed in macrophages in active human MS lesions, we questioned whether TG2 drives the differentiation of M(IL-4) into an anti-inflammatory phenotype and whether it plays a role in the phagocytosis of myelin by these cells. In macrophage-differentiated THP-1 monocytes, TG2 was increased upon IL-4 treatment. Reducing TG2 expression impairs the differentiation of M(IL-4) macrophages into an anti-inflammatory phenotype and drives them into a pro-inflammatory state. In addition, reduced TG2 expression resulted in increased presence of myelin basic protein in macrophages upon myelin exposure of M(IL-4) macrophages. Moreover, the elevated presence of an early endosome marker and equal expression of a lysosome marker compared to control macrophages, suggest that TG2 plays a role in phagosome maturation in M(IL-4) macrophages These data suggest that tuning macrophages into TG2 producing anti-inflammatory cells by IL-4 treatment may benefit effective myelin phagocytosis in e.g. demyelinating MS lesions and open avenues for successful regeneration.
Keywords: Inflammation; Macrophages; Multiple sclerosis; Myelin; Phagocytosis; Tissue Transglutaminase.
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