Differential Effect of Vitamin D Supplementation on Natural Killer Cell Activity in Normal and Obese Mice

Abstract

Vitamin D has an immunomodulatory effect on both innate and adaptive immune responses. Vitamin D has been reported to induce differentiation of monocytes into macrophages, inhibit maturation and differentiation of dendritic cells, and suppress adaptive immunity. Although there are some in vitro studies which showed that vitamin D suppressed Natural Killer (NK) cell activity and IFN‐γ secretion, little is known about the in vivo effect of vitamin D on NK cell functions. In this study, we investigated the in vivo effect of vitamin D on NK cell functions by measuring NK activity, cytokine secretion, activating receptor expression, and degranulation capacity in mice fed control or high‐fat diets containing different levels of vitamin D. Five week‐old C57BL/6 mice were divided into 6 groups by fat amount (10% or 45% kcal fat, CD or HFD) and vitamin D content (50, 1000, or 10000 IU/kg of diet, DD, DC, or DS), then fed diets for 12 weeks. NK cell activity was assessed using radioisotope 51Cr release assay against YAC‐1 target cells. Splenocyte subpopulation was measured by flow cytometry (FACS) analysis. Intracellular expression of IFN‐γ by NK cell, CD4 T cell, and CD8 T cell and proportions of NK cells and CD8 T cells expressing NKG2D and CD107a were determined by FACS analysis upon stimulation of splenocytes with PMA (50 ng/mL)/ionomycin (0.5 μM) for 4 hours. NK activity was significantly higher in NDDS group than HFDDS group at effector to target (E:T) ratios of 100:1, 50:1, and 12.5:1. NDDS group showed significantly higher NK activity compared with NDDD and NDDC groups at E:T ratios of 100:1 and 50:1, whereas there was no significant difference in NK activity among HFDDD, HFDDC, and HFDDS groups. Splenic CD11b‐ NK cell population was significantly higher in NDDS group than HFDDS group, while splenic CD11b+ NK cell population tended to be higher in NDDS group compared with HFDDS group (p=0.057). Percentage of splenic NKT cell was significantly higher in NDDS group than HFDDS group. There were significant differences in the splenic population of CD4 T cell, CD8 T cell, and B cell between ND and HFD groups (p=0.042, p=0.001, p<0.001). Intracellular expression of IFN‐γ and surface expressions of NKG2D, and CD107a in NK cells were not affected by the different levels of vitamin D. However, there were significantly higher surface expressions of CD107a by CD8 T cells in NDDD group compared with NDDS group. In conclusion, vitamin D supplementation enhanced splenic NK activity in control mice, whereas this effect of vitamin D was not observed in high‐fat diet‐induced obese mice. Alternation in splenic NK cell population might have partially contributed to increased NK activity by vitamin D supplementation.Support or Funding InformationSupported by the grant from the National Research Foundation (NRF) of Korea (NRF‐2015R1D1A1A01059679).