M2 tumour-associated macrophages contribute to tumour progression via legumain remodelling the extracellular matrix in diffuse large B cell lymphoma

Effects of M2 tumour-associated macrophages on the pathogenesis of diffuse large B cell lymphoma (DLBCL) are still controversial. Our data showed that the number of CD163-positive M2 macrophages correlated negatively with DLBCL prognosis. Macrophage depletion by clodronate liposomes significantly suppressed tumour growth in a xenograft mouse model of DLBCL using OCI-Ly3 cells. Moreover, M2 polarization of macrophages induced legumain expression in U937 cells. Exogenous legumain promoted degradation of fibronectin and collagen I, which was abolished by administration of a legumain inhibitor RR-11a. Overexpression of legumain in Raw 264.7 cells also induced tube formation of endothelial cells in matrigel. In the xenograft mouse model of DLBCL, decreased fibronectin and collagen I, as well as increased legumain expression and angiogenesis were found at the late stage tumours compared with early stage tumours. Co-localization of legumain and fibronectin was observed in the extracellular matrix of tumour tissues. Administration of the legumain inhibitor to the xenograft DLBCL model suppressed tumour growth, angiogenesis and collagen deposition compared with the control. Taken together, our results suggest that M2 tumour-associated macrophages affect degradation of the extracellular matrix and angiogenesis via overexpression of legumain, and therefore play an active role in the progression of DLBCL.