TGF-β-producing tBregs are associated with poor survival rates in PTGS2-high cancers.

The cues that cause B cells to adopt a regulatory phenotype are unknown. We previously reported on the regulatory B-T axis in mouse fibrosarcoma. This work demonstrates the presence of TGF-β-generating Bregs in the sarcoma microenvironment as well as variable gene expression depending on TGF-β levels. Breg depletion reduced tumor-evoked Bregs and progression of WEHI-164 fibrosarcoma, which was reversed by adoptive transfer of B cells. Tumor-derived PGE2 acting via EP4/STAT3 signaling is a key driver of tBreg formation, ultimately resulting in increased Tregs. Only the TGF-β+ tBregs caused T cell suppression. PGE2 activated STAT3 in a non-canonical manner since pre-treatment of B cells with JNK and NF-κB inhibitors completely restored tBreg-induced T cell suppression. The COX-2 inhibitor NS-398 prevented the generation of tumor-evoked Bregs, resulting in a lower tumor burden, fewer Tregs, and improved T cell responses. TGF-β-producing Bregs were identified in sarcomas and other cancers with elevated PGE2. The B cells from PTGS2hi cancers displayed unique Breg signatures when compared to PTGS2lo cancers. TCGA analysis showed that the Breg signature was higher in patients with high PTGS2 levels, which was also associated with a poor survival rate. In summary, we found that the tumor-derived PGE2 activates non-canonical MAPK/STAT3 in B cells, leading to the formation of TGF-β-generating Breg cells. PTGS2hi Breghi patients were associated with a poor survival rate, while the COX-2 inhibitor NS-398 prevented tBreg generation and reduced tumor burden. This could lead to the development of novel strategies for reducing tBreg production in cancer.