Assessment of the relationship between serum 25-­hydroxyvitamin D3, human alveolar macrophage function, and in vitro and in vivo outcomes of M. tuberculosis exposure

Abstract

Approximately 50% of individuals may be resistant to infection with Mycobacterium tuberculosis (M.tb), the causative agent of tuberculosis (TB). Pulmonary innate immunity is likely to be important in shaping this, although it is unclear how this is determined in the human lung.

Ethnicity influences infection rates following exposure to TB, and is associated with differential phenotypes in established TB disease; the latter is driven by variation in serum 25-hydroxyvitamin D3 [25(OH)D3]. Through genomic vitamin D-response elements, 25(OH)D3 can influence gene transcription, potentially linking changes in serum 25(OH)D3, and innate pulmonary immune function.

Aim 1 of this project involved the recruitment of a cohort of precisely-defined individuals in recent close contact with smear-positive pulmonary TB. Their exposure to TB was quantified, along with variables associated with TB transmission. Vitamin D metabolites were measured in serum at the point of recruitment, and associations with outcome were tested. Elevation in serum 25(OH)D3 was associated with resistance to TB infection, and not confounded by other transmission factors.

Aim 2 involved the development of an experimental pathway where human alveolar macrophages (AMs) could be acquired and identified, and then infected intracellularly with the virulent M.tb strain H37Rv. The concentration of 25(OH)D3 in the serum to which these cells were exposed was varied; a higher concentration was associated with enhanced growth control of H37Rv.

Aim 3 involved the isolation of RNA from human AMs, so that gene expression in response to intracellular H37Rv could be assessed using qPCR and RNAseq. The AM was able to upregulate the genes required to activate 25(OH)D3 within the cell, but growth control in response to higher serum 25(OH)D3 exposure was not associated with transcription of the anti-mycobacterial gene cathelicidin. RNAseq suggested networks of genes, possibly involving eicosanoid metabolism, may be more important in the 25(OH)D3-driven AM response to virulent mycobacteria.