Functional characterization of small intestinal lamina propria dendritic cells in Th17 differentiation

Abstract

The intestine is constantly exposed to various antigens derived from commensal microorganisms and food and contains large numbers of Th17 and regulatory T (Treg) cells. Intestinal lamina propria DCs (LP-DCs) are expected to play unique roles in establishing these T cell populations in the gut compared to DCs in other organs. Previous studies reported distinct tolerogenic properties of intestinal mucosal DCs in the Treg development, but other distinct features of LP-DCs remain unclear. To investigate the functional characteristics of LP-DCs, I compared the genome-wide mRNA expression of small intestinal LP-DCs and splenic DCs (SP-DCs) in the presence or absence of zymosan or CpG DNA stimulation. SP-DCs specifically exhibited the increase of the genes associated with anti-viral responses regardless of the stimuli. On the other hand, I found that zymosan led to the specific upregulation of IL-6 in LP-DCs, but not in SP-DCs. In a sharp contrast, IL-12p40 was produced exclusively from activated SP-DCs. This cytokine expression profile of each DC population was not limited to the stimulation with zymosan but was also observed after stimulation with agonists for various Toll-like receptors (TLRs). In agreement with the ability to produce a high level of IL-6, zymosan-activated LP-DCs, but not SP-DCs, induced in vitro differentiation of naïve CD4+ T cells into Th17 cells in the presence of TGF-β, and oral delivery of zymosan significantly increased small intestinal Th17 cells in vivo. Therefore, small intestinal DCs are intrinsically programmed with the unique cytokine production characteristic which facilitates the Th17 cell differentiation. To elucidate the underlying mechanisms of distinct cytokine profiles of LP-DCs, I examined the expression levels of pattern recognition receptors (PRRs) and their role in mediating the unique cytokine production pattern. I also investigated the role of cyclooxygenase-2 (COX-2) in regulating cytokine profiles of DCs because COX-2 expression pattern mirrored that of IL-6 and prostaglandin E2 (PGE2), one of the products of COX-2 enzyme, is known to induce more IL-6 and less IL-12 production in bone marrow-derived DCs. I found that PGE2 level was higher in the small intestine than the spleen and exogenous PGE2 enhanced IL-6 production and reduced IL-12p40 secretion from SP-DCs. Moreover, a COX inhibitor treatment significantly reduced PGE2 level in the small intestine and abolished the zymosan-mediated induction of small intestinal Th17 cells in mice. These findings suggest that the relatively high levels of COX2 and PGE2 in the small intestinal environment contribute to establish the unique characteristics of LP-DCs. Taken together, the results presented in this thesis suggest that LP-DCs promote Th17 cell accumulation in the small intestine through superior IL-6 production.