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

Diffuse large B cell lymphoma (DLBCL) is the most frequent Non-Hodgkin’s lymphoma, accounting for 30–40% of newly diagnosed lymphomas1. Although the current standard therapy for DLBCL cures the majority of patients, it is still a fatal malignancy that requires new targets for additional therapeutic options. Studies on the gene expression profiles of DLBCL biopsy specimens have revealed increased infiltration of macrophages into DLBCL stroma2,3. Diverse polarized subtypes of macrophages occur in the tumour microenvironment, and there is functional heterogeneity between M1 and M2 macrophages. However, the mechanism underlying the effect of M2 macrophages on the pathogenesis of DLBCL remains unclear4,5,6. Several clinical data suggest that a high density of tumour-associated macrophages (TAMs), especially M2 TAMs can predict adverse outcomes in many kinds of cancers7,8. In invasive pancreatic cancer, CD163-positive M2 TAMs, but not CD68-positive macrophages, are associated with a poor prognosis9. Similarly, a higher ratio of CD163- to CD68-positive macrophages in angioimmunoblastic T-cell Lymphoma correlates significantly with poor overall survival10. Paradoxically, Hasselblom et al. suggested that CD68-positive TAMs have no prognostic value in DLBCL11. Further study on the correlation between the clinical stage as well as other prognostic indexes and M2 TAMs is urgently needed. Legumain is an asparaginyl endopeptidase that is classified as a member of the C13 family of cysteine proteases12. Accumulating data have identified legumain expression in a variety of tumour types, suggesting a role in tumour progression13. Furthermore, studies have verified the effect of legumain on modulation of tumour- and stroma-derived components of the cancer degradome14,15. A recent study on the crystal structure revealed multi-branched and context-dependent activation processes of legumain, suggesting broad roles in addition to its primary role as an endolysosomal cysteine proteinase16. TAMs have been found to express abundant amounts of legumain. A vaccine or inhibitor against legumain significantly eliminates M2 TAMs and consequently inhibits tumour growth and metastases in murine tumour models17,18. Here, we provide evidence that a high percentage of M2 TAMs predicts poor outcomes in DLBCL patients. M2 TAMs in the lymphoma microenvironment overexpressed and secreted legumain. In addition, legumain was involved in angiogenesis and tumour progression via extracellular matrix (ECM) remodelling. Therefore, we have identified a novel therapeutic target as well as a diagnostic index for the treatment of DLBCL.