Abstract
Only mismatch repair (MMR)-deficient colorectal cancer (CRC) appears to respond well to programmed death (PD)-1 inhibition at the present time. Emerging evidence suggests a role for micro-environmental factors such as CD25+ cells modulating response to PD-1 inhibition. In the ApcMin/+ model of familial adenomatous polyposis (MMR-proficient CRC), increased Cyclooxygenase-2 (Cox-2) expression by cells which include alternatively activated mononuclear phagocytes promotes intestinal tumorigenesis by mechanisms which may include immune suppression. To gain insight into this, we compared regulatory T cell (Treg ) populations between ApcMin/+ and wild-type mice prior to and after the phase of increased intestinal Cox-2-dependent prostaglandin E2 (PGE2 ) production. There was no difference in systemic Treg function or numbers between ApcMin/+ and wild-type mice. However, increased numbers of small intestinal CD25+ Tregs were observed with increased Cox-2 activity in the absence of any difference in the expression of Tgf-β or Tslp between ApcMin/+ and wild-type mice. Cox-2 inhibitor therapy (Celecoxib) reversed the increase in ApcMin/+ intestinal CD25+ Treg numbers, without decreasing numbers of CD25+ systemic Tregs . Forkhead box protein 3 (FoxP3+ ) and Cox-2+ cells were co-localized to the interstitium of adenomas of Apcmin/+ mice. These results suggest selective dependence of an 'activated Treg ' phenotype on paracrine Cox-2 activity in ApcMin/+ small intestine. For therapeutic potential, further studies are required to evaluate the relevance of these findings to human cancer as well as the functional significance of CD25+ intestinal Tregs in cancer.