Cellular and molecular mechanisms regulating the development of tertiary lymphoid structures in tumor

Abstract

Abstract Tertiary lymphoid structures (TLS) have been described in association with several tumor types, and they are usually associated with enhanced patient survival. However, mechanisms regulating their formation remain unknown. We found that intra- and peri-tumoral TLS develop in intraperitoneal (IP) but not subcutaneously (SC) implanted tumors. These TLS are characterized by a robust accumulation of B cells, and an associated reticular network of podoplanin+ fibroblasts expressing high levels of CXCL12, CXCL13, and the B-cell survival factors BAFF and APRIL. These results suggest that IP tumor-associated fibroblasts are polarized into organizer cells that orchestrate TLS formation. We’ve previously demonstrated that CD8 T effectors induce the development of lymph node-like vasculature in IP tumors. To determine whether these cells also regulate TLS development, we evaluated IP tumors grown in Rag1−/− mice and Rag1−/− mice repleted with bulk CD8 T cells. CD8 T cells upregulated the number of podoplanin+ fibroblasts and CXCL12 expression. However, CXCL13, BAFF and APRIL expression was not dependent on CD8 T cells. Thus, the representation of podoplanin+ fibroblasts and their expression of CXCL12 is regulated by adaptive immunity, while their expression of CXCL13, BAFF and APRIL is mediated by an innate element within the IP tumor microenvironment. We also found that increasing the load of the model tumor antigen ovalbumin led to an increased formation of peri-tumoral TLS with a classical organization, on top of the non-classical intratumoral structures evident at a low antigen load. These results identify novel mechanisms regulating different elements involved in organizing tumor-associated TLS.