Vitamin D3 contribution to systemic and local control of Salmonella Typhimurium infection in in vivo mouse-model

Introduction Salmonella enterica serovar Typhimurium (STM) represents a major public health problem causing severe disease in humans worldwide, ranging from self-limiting to severe gastroenteritis. Recently, the antimicrobial effect of Vitamin D3 (VitD3), and particularly of its active metabolite 1,25-dihydroxyvitamin D3, has been shown in mice models and humans, by regulating both innate and adaptive immunity. However, evidence of the efficacy of VitD3 supplementation in treating human gastrointestinal infection remains scant. Owing to the above, we investigated the role of VitD3 in the context of an experimental STM infection in a controlled mouse model to assess its impact on infection severity and systemic and enteric immune dynamics. Materials and Methods In detail, 6–8-week-old male C57BL/6 mice were fed ad libitum for 7 weeks with (Group A, n=15) or without (Group B, n=15) supplementation of VitD3 (75,000 IU/kg). Following 24h of streptomycin treatment, 10 mice from each group were orally infected with 104 CFU/0.2 ml of STM wild type strain 14028, obtaining Group A1 and Group B1. Uninfected groups (A3 and B3, 5 mice each) remained as controls. Mice were euthanized 3 days post-infection and samples were collected for the following analyses: complete blood counts (CBC) (flow cytometry); bacterial colonization (CFU counts) and IL-1alfa expression in spleens (qPCR); proximal colon sections (semi-quantitative histological assessment of inflammation). Comparison between groups A1 and B1 was made by T-test, setting a significant threshold if p≤0.05. Results Our results showed that A1 mice exhibited significantly improved survival rate and reduced splenic STM colonization and inflammation, compared to B1 mice. In addition, A1 mice showed focal (28.6%) or diffuse (71.4%) moderate colitis, while all B1 mice showed diffuse severe lymphoplasmacytic infiltration. Flow cytometry analysis showed a significant reduction in CD8+ T-cells (CD3+CD4-CD56-), macrophages (F4/80+) and neutrophils (CD11b+ Ly6g+) in A1 compared to B1 mice, besides a slight increase in B lymphocytes (B220+). Discussion and Conclusions These preliminary findings confirm that VitD3 supplementation reduces the severity of STM infection by decreasing bacterial colonization and general inflammation in mice, as evidenced by lower splenic CFU counts, reduced splenic and enteric inflammation, and altered immune cell profiles. The observed reduction in inflammatory immune cells and the slight increase in B lymphocytes suggest that VitD3 modulates the immune response to favour the control of infection, both systemically and locally. These results evidence the role of VitD3 as an effective immunomodulatory molecule in combating STM infection, encouraging its use as a complementary therapeutic agent for the treatment of human gastrointestinal infections.