THE ROLE OF THE PROSTAGLANDIN D2 RECEPTOR, CRTH2, IN THE REGULATION OF TYPE 2 IMMUNE RESPONSES AT MUCOSAL SURFACES

Abstract

Type 2 immunity is important in the generation of protective immune responses for worm clearance as well as for the repair and regenerative process that occurs following chronic infection and disease. While previous studies show that cytokines play key roles in regulating Type 2 inflammation, the role of bioactive lipid mediators such as prostaglandin D2 (PGD2) at various mucosal surfaces and how they interact with the other activating cytokine pathways in vivo has not being fully explored. In this thesis, the biology of the PGD2 and its receptor (R) chemoattractant receptor-homologous molecule expressed on T helper Type 2 (Th2) cells (CRTH2) and their intersection with cytokine-dependent effects was investigated during Type 2 inflammation in the lung and intestine. First, the interaction between the activating cytokine pathway, interleukin (IL)-33-ST2, and the bioactive lipid mediator pathway, prostaglandin D2-CRTH2, in the airway mucosa was examined. The data from these results demonstrates that CRTH2-dependent effects lie downstream of IL-33, directly affecting the migration of group 2 innate lymphoid cells (ILC2s) into inflamed lung tissues. Following this, the role of the PGD2-CRTH2 pathway in regulating mucosal responses during helminth-induced intestinal Type 2 inflammation were assessed. We show that PGD2 and its receptor CRTH2, known for their pro-inflammatory role during chronic Type 2 inflammation in the lung, have a suppressive role during helminth-induced Type 2 intestinal inflammation. CRTH2 deficient mice and chimeric mice with CRTH2 deficiency in the non-hematopoietic system infected with Nippostrongylus brasiliensis cleared their worms more efficiently and had increased intestinal mucin responses compared to their wild type mice (WT) controls. Single cell RNA sequencing revealed an enrichment for secretory lineage epithelial cells in the small intestine of CRTH2 deficient compared to wild type mice. Critically, small intestinal epithelial cells (IECs) and specifically goblet and tuft cells expressed the gene that encodes for CRTH2, Gpr44, and murine small intestinal organoids stimulated with Type 2 cytokines downregulated expression of goblet cell- and tuft cell-associated genes following culture with PGD2, in a CRTH2-dependent fashion. Together, these results demonstrate an interaction between the activating IL-33 pathway and the PGD2-CRTH2 pathway for regulation of Type 2 inflammation in the airway mucosa and highlight a novel regulatory effect of the PGD2-CRTH2 pathway during helminth-induced Type 2 intestinal inflammation. These studies may inform the development and use of therapies for treatment of Type 2 inflammatory diseases.