Association of vitamin D deficiency, season of the year, and latent tuberculosis infection among household contacts

Abstract

Objectives: Vitamin D (VD) enhances the immune response against Mycobacterium tuberculosis in vitro, and VD deficiency has been described in patients with active tuberculosis (TB). However, the role of hypovitaminosis D in the pathogenesis of early TB infection acquisition is unclear. We aimed to evaluate the association of VD deficiency, season of the year, and latent TB infection in household contacts (HHC), given that this is a potentially modifiable condition often related to nutritional deficiencies and lack of sun exposure.

Methods: We prospectively enrolled new pulmonary TB cases (n = 107) and their HHC (n = 144) over a 2-year period in Santiago, Chile. We compared plasma 25-hydroxycholecalciferol (25OHD) levels and examined the influence of season, ethnic background, living conditions, and country of origin.

Results: Over 77% of TB cases and 62.6% of HHC had VD deficiency (<20 ng/ml). Median 25OHD concentration was significantly lower in TB cases than in HHC (11.7 vs. 18.2 ng/ml, p<0.0001). Migrants HHC had lower 25OHD levels than non-migrants (14.6 vs. 19.0 ng/ml, p = 0.026), and a trend towards a higher burden of latent TB infection (52.9% vs. 35.2%, p = 0.066). Multivariate analysis found VD deficiency in HHC was strongly associated with being sampled in winter/spring (adOR 25.68, 95%CI 7.35-89.7), corresponding to the seasons with lowest solar radiation exposure. Spring enrollment-compared with other seasons-was the chief risk factor for latent TB infection in HHC (adOR 3.14, 95%CI 1.28-7.69).

Conclusions: Hypovitaminosis D was highly prevalent in TB cases and also in HHC. A marked seasonality was found for both VD levels and latent TB in HHC, with winter being the season with lowest VD levels and spring the season with the highest risk of latent TB infection.

Fig 1. Plasma levels of 25OHD and inflammatory parameters in active pulmonary TB cases, household contacts (HHC) and non-HHC subjects.

A) 25OHD levels in active pulmonary TB cases (n = 92), household contacts (HHC) with latent TB infection (QFT(+), n = 55), HHC without latent TB infection (QFT(-), n = 84) and non-HHC subjects (n = 31). B) Ultrasensitive C-reactive protein (CRP) in active pulmonary TB cases (n = 23), HHC with latent TB infection (QFT(+), n = 33), HHC without latent TB infection (QFT(-), n = 53) and non-HHC subjects (n = 31). C) Interleukin 6 (IL-6) in active pulmonary TB cases (n = 79), HHC with latent TB infection (QFT(+), n = 36), HHC without latent TB infection (QFT(-), n = 32) and non-HHC subjects (n = 19). D) TNF-α in active pulmonary TB cases (n = 79), HHC with latent TB infection (QFT(+), n = 36), HHC without latent TB infection (QFT(-), n = 32) and non-HHC subjects (n = 19). (*** = p< 0.001, ** = p<0.01, * = p<0.05). Horizontal line represents the median of each subject group.

Fig 2. Plasma levels of 25OHD in household contacts (HHC) (n = 139) and proportion of HHC with latent TB infection (LTBI) in each season.

Median value with IQR is shown. The proportion of LTBI found in HHC was higher in spring (62.5%) than in other seasons (32.7%). (*** p = 0.004).

Fig 3. A schematic model for the impact of the seasonality and levels of Vitamin D (VD) metabolites on the risk of being infected with M. tuberculosis and the protective mechanisms of the immune response.

(A) During spring/summer season, sunny days favor outdoor activities, decrease close contact with people with TB and increase sun light exposure promoting enhanced skin production of 7-dehydrocholesterol (7DHC), the precursor of active VD, and the synthesis of D3 metabolites first in liver and then in kidney. The main production of 1,25-dihydroxyvitamin D3 (1,25(OH)D3) occurs in the kidney but it can also be produced by inflammatory cells during an immune response to infections [10,11,23]. Alveolar macrophages recognize molecules associated with M. tuberculosis, such as the mycobacterial lipoprotein LpqH, through Toll-like receptors (TLRs) such as TLR2/1 and the co-receptor CD14 expressed on their cell surface [20,24]. Engagement of these receptors induce a cell signaling pathway that include AMPK and p38 MAPK activation, which leads to upregulation of CYP27B1 hydroxylase and the conversion of 25OHD into 1,25(OH)D3 [24]. Given that immune cells can also express the VD receptor (VDR), 1,25(OH)D3 binds to heterodimer formed between the VDR and the retinoid X receptor (RXR) and translocate into the nucleus where this transcription factor complex specifically recognizes DNA sequences named VD response elements (VDRE) leading to the production of antimicrobial peptides, such as LL-37 (cathelicidin) and ß-defensin 2 (BD2) [24]. Cathelicidin induced by 1,25(OH)D3 drives the elimination of engulfed mycobacteria by promoting fusion of autophagosomes containing mycobacteria with lysosomes [24,25]. M. tuberculosis infection and 1,25(OH)D3 induce IL-1ß gene expression, which binds via IL-1ß receptor to epithelial alveolar cells to promote the expression of BD2. The release of BD2 along with 1,25(OH)D3 contributes to control mycobacterial proliferation in the macrophage [26]. (B) During autumn/winter season, cold, cloudy and rainy days promote indoor lifestyle and enhanced contact with people with TB, particularly under overcrowded living conditions. Reduced sun light exposure leads to significantly low skin production of 7DHC and consequently reduced synthesis of D3 metabolites in liver and kidney, resulting in detrimental innate immune response against M. tuberculosis. Concomitantly, reduced levels of cathelicidin impairs autophagy process. Overall, the combination of these factors would favor a higher susceptibility of being infected.