High salt diet promotes immune cell egress from the small intestine to specific peripheral sites

Abstract

Immune cell trafficking is essential to fundamental physiological and pathophysiological processes. High salt diet (HSD) heightens immune responses, especially T cell responses. Models of salt-sensitive hypertension show an increased presence of immune cells in the kidney, aortic vasculature, and brain. However, it is not clear if HSD is promoting the expansion of immune cells through intrusion of non-resident cells, expansion of resident cells, or a combination of both. As the small intestine is largely responsible for first-pass absorption of nutrients and fluids, we hypothesized that HSD promotes aberrant small intestinal T cell egress to peripheral sites. To investigate T cell trafficking, we utilized the CAG::KikGreen (KikGR) mice, which ubiquitously express kikume green-red photoconvertible protein from the coral Favia favus, which upon exposure to blue light induces a permanent structural photoconversion from KikGreen to KikRed (KikRd). This model allows identification of cells, by flow cytometry, that migrated from photoconverted tissues to tissues that were not exposed to blue light, indicating trafficking. 8-12-week old male littermate KikGR mice were placed on normal salt diet (0.4% NaCl chow/normal drinking water; NSD) or high salt diet (4.0% NaCl chow/1% NaCl drinking water) for 5 weeks. Three days prior to tissue collection, the small intestine is externalized, exposed to 405nm blue light, replaced into peritoneal cavity, and mice are allowed to recover. Kidneys, small intestinal (SI) lamina propria, colonic lamina propria, spleen, peripheral blood mononuclear cells (PBMCs), renal lymph nodes (rLN), mesenteric lymph nodes (mLN), caudal/iliac lymph nodes (cLN), and subcutaneous inguinal lymph nodes (siLN) were isolated to examine systemic egress of KikRd+ CD4+ TCR+ T cells from the small intestine. Data representative of 2-3 independent experiments were analyzed by Student's T or two-tailed Mann-Whitney U test. Peripheral lymphoid structures that are involved in draining the gut, namely the mLN and cLN show 135% and 574% increases in the frequency of KikRd+ CD4+ T cells in mice on HSD, respectively, (mLN: NSD: 5.391.14 vs HSD: 12.641.93, % of CD4+, p=0.02, n=4-7/group; cLN: NSD: 9.390.71 vs HSD: 63.3510.29, % of CD4+, p=0.009, n=4-6/group). Interestingly, we also found a 159% increase in the frequency of KikRd+CD4+ T cells in the colonic lamina propria of mice on HSD (NSD: 6.920.74 vs HSD: 17.943.42, % of CD4+, p=0.02, n=4-7/group). No differences were found in the frequencies of KikRd+ CD4+ TCR+ T cells in the spleen (p=0.85, n=4-8/group), in circulating PBMCs (p=0.08, n=5-6/group), in siLN (p=0.16, n=5-8/group), kidneys (p=0.16, n=5-8/group) or rLN (n=2/group). This study suggests that SI CD4+ T cells have a migratory bias from the intestine to specific tissue sites induced by HSD. Specifically, HSD promotes small intestinal CD4+ T cell egress or recruitment to the mLN, cLN, and colonic lamina propria over other peripheral sites. Furthermore, as the cLN is adjacent, drains, and provides efferent lymphatic flow to the abdominal aorta, these data suggest potential migration or recruitment of SI CD4+ T cells to seed the aortic vasculature during HSD.