A growing appreciation is emerging of the beneficial role of vitamin D for health and resistance against infectious diseases, including tuberculosis. However, research has predominantly focused on murine and human species and functional data in bovines is limited. Therefore, the objective of this study was to assess the microbicidal activity and immunoregulatory effect of the vitamin D metabolite 1,25(OH)D on bovine peripheral blood leukocytes (PBL) in response to Mycobacterium bovis BCG (BCG) infection using a combination of functional assays and gene expression profiling. Blood from Holstein-Friesian bull calves with low circulating levels of 25(OH)D was stimulated with 1,25(OH)D for 2 h, and then infected with M. bovis BCG. Results showed that 1,25(OH)D supplementation significantly increased BCG killing by on average 16 %, although responses varied between 1 % and 38 % killing. Serial cell subset depletion was then performed on PBL prior to 1,25(OH)D incubation and BCG infected as before to analyse the contribution of major cell types to mycobacterial growth control. Specific antibodies and either magnetic cell separation or density gradient centrifugation of monocytes, granulocytes, CD3+, CD4+, and CD8+ T lymphocytes were used to capture each cell subset. Results showed that depletion of granulocytes had the greatest impact on BCG growth, leading to a significant enhancement of bacterial colonies. In contrast, depletion of CD4 or CD8 T cells individually, or in combination (CD3), had no impact on mycobacterial growth control. In agreement with our previous data, 1,25(OH)D significantly increased bacterial killing in PBL, in monocyte depleted samples, and a similar trend was observed in the granulocyte depleted subset. In addition, specific analysis of sorted neutrophils treated with 1,25(OH)D showed an enhanced microbicidal activity against both BCG and a virulent strain of M. bovis. Lastly, data showed that 1,25(OH)D stimulation increased reactive oxygen species (ROS) production and the expression of genes encoding host defence peptides (HDP) and pathogen recognition receptors (PRRs), factors that play an important role in the microbicidal activity against mycobacteria. In conclusion, the vitamin D metabolite 1,25(OH)D improves antimycobacterial killing in bovine PBLs via the synergistic activity of monocytes and granulocytes and enhanced activation of innate immunity.
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