Date Published: July 3, 2018
Publisher: Public Library of Science
Author(s): Emily MacLean Scott, Lauren A. Solomon, Courtney Davidson, Jessica Storie, Nami Shrestha Palikhe, Lisa Cameron, Joao P. B. Viola.
CRTh2 (encoded by PTGDR2) is a G-protein coupled receptor expressed by Th2 cells as well as eosinophils, basophils and innate lymphoid cells (ILC)2s. Activation of CRTh2, by its ligand prostaglandin (PG)D2, mediates production of type 2 cytokines (IL-4, IL-5 and IL-13), chemotaxis and inhibition of apoptosis. As such, the PGD2-CRTh2 pathway is considered important to the development and maintenance of allergic inflammation. Expression of CRTh2 is mediated by the transcription factor GATA3 during Th2 cell differentiation and within ILC2s. Other than this, relatively little is known regarding the cellular and molecular mechanisms regulating expression of CRTh2. Here, we show using primary human Th2 cells that activation (24hrs) through TCR crosslinking (αCD3/αCD28) reduced expression of both mRNA and surface levels of CRTh2 assessed by flow cytometry and qRT-PCR. This effect took more than 4 hours and expression was recovered following removal of activation. EMSA analysis revealed that GATA3 and NFAT1 can bind independently to overlapping sites within a CRTh2 promoter probe. NFAT1 over-expression resulted in loss of GATA3-mediated CRTh2 promoter activity, while inhibition of NFAT using a peptide inhibitor (VIVIT) coincided with recovery of CRTh2 expression. Collectively these data indicate that expression of CRTh2 is regulated through the competitive action of GATA3 and NFAT1. Though prolonged activation led to NFAT1-mediated downregulation, CRTh2 was re-expressed when stimulus was removed suggesting this is a dynamic mechanism and may play a role in PGD2-CRTh2 mediated allergic inflammation.
CRTh2 (chemoattractant receptor homologous molecule expressed on Th2 cells) is a seven transmembrane spanning receptor for prostaglandin D2 (PGD2) , a lipid mediator released from allergen/IgE activated mast cells  and macrophages following microbial activation . Activation of CRTh2 (encoded by PTGDR2) mediates chemotaxis , production of pro-allergic cytokines [1, 4, 5] and inhibition of apoptosis . CRTh2 expression by human CD4+ T helper lymphocytes is considered the most reliable marker of Th2 cells [7–11], but CRTh2 is also expressed by eosinophils, basophils [11, 12] and a subset of innate lymphoid cells (ILC2) . Together Th2 cells and ILC2s orchestrate development of allergic inflammation through production of IL-4, IL-5 and IL-13 [14, 15] which induces production of IgE, inflammatory cell infiltration to sites of exposure and tissue remodeling .
This study shows that expression of CRTh2 by CD4+ T cells is decreased following T cell activation through engagement of CD3 and CD28. While this observation was previously reported [4, 8], it received little attention. Our report is the first to investigate the underlying transcriptional regulation and to discuss the potential role of this mechanism in immune responses. Since both T cell activation and CRTh2 signaling induce Th2 cytokine expression , this was somewhat counterintuitive. However, our kinetic analysis revealed that while surface expression of CRTh2 was reduced by 24 hours, it was maintained up to 4 hours after activation, indicating there is a window of opportunity after activation in which CRTh2 remains on the surface of Th2 cells available to respond to PGD2.