Calcidiol, the major circulating metabolite of vitamin D, supports induction of pleiotropic antimicrobial responses in vitro. Vitamin D supplementation elevates circulating calcidiol concentrations, and thus has a potential role in the prevention and treatment of infection. The immunomodulatory effects of administering vitamin D to humans with an infectious disease have not previously been reported. To characterize these effects, we conducted a detailed longitudinal study of circulating and antigen-stimulated immune responses in ninety-five patients receiving antimicrobial therapy for pulmonary tuberculosis who were randomized to receive adjunctive high-dose vitamin D or placebo in a clinical trial, and who fulfilled criteria for per-protocol analysis. Vitamin D supplementation accelerated sputum smear conversion and enhanced treatment-induced resolution of lymphopaenia, monocytosis, hypercytokinaemia, and hyperchemokinaemia. Administration of vitamin D also suppressed antigen-stimulated proinflammatory cytokine responses, but attenuated the suppressive effect of antimicrobial therapy on antigen-stimulated secretion of IL-4, CC chemokine ligand 5, and IFN-α. We demonstrate a previously unappreciated role for vitamin D supplementation in accelerating resolution of inflammatory responses during tuberculosis treatment. Our findings suggest a potential role for adjunctive vitamin D supplementation in the treatment of pulmonary infections to accelerate resolution of inflammatory responses associated with increased risk of mortality.

By using combined positron emission and computed tomography (PET–CT), Esmail et al. show that some patients with latent tuberculosis have signs of subclinical, active disease in the lungs and a greater likelihood of progression, suggesting a spectrum of disease rather than discrete latent and active disease states. Tuberculosis is classically divided into states of latent infection and active disease. Using combined positron emission and computed tomography in 35 asymptomatic, antiretroviral-therapy-naive, HIV-1-infected adults with latent tuberculosis, we identified ten individuals with pulmonary abnormalities suggestive of subclinical, active disease who were substantially more likely to progress to clinical disease. Our findings challenge the conventional two-state paradigm and may aid future identification of biomarkers that are predictive of progression.

Immunology is a central theme when it comes to tuberculosis (TB). The outcome of human infection withMycobacterium tuberculosisis dependent on the ability of the immune response to clear or contain the infection. In cases where this fails, the bacterium replicates, disseminates within the host, and elicits a pathologic inflammatory response, and disease ensues. Clinical presentation of TB disease is remarkably heterogeneous, and the disease phenotype is largely dependent on host immune status. Onward transmission ofM. tuberculosisto new susceptible hosts is thought to depend on an excessive inflammatory response causing a breakdown of the lung matrix and formation of lung cavities. But this varies in cases of underlying immunological dysfunction: for example, HIV-1 infection is associated with less cavitation, while diabetes mellitus comorbidity is associated with increased cavitation and risk of transmission. In compliance with the central theme of immunology in tuberculosis, we rely on detection of an adaptive immune response, in the form of interferon-gamma release assays or tuberculin skin tests, to diagnose infection withM. tuberculosis. Here we review the immunology of TB in the human host, focusing on cellular and humoral adaptive immunity as well as key features of innate immune responses and the underlying immunological dysfunction which associates with human TB risk factors. Our review is restricted to human immunology, and we highlight distinctions from the immunological dogma originating from animal models of TB, which pervade the field.

Summary: Matrix metalloproteinases (MMP) can degrade all components of pulmonary extracellular matrix. Mycobacterium tuberculosis induces production of a number of these enzymes by human macrophages, and these are implicated in the pathogenesis of pulmonary cavitation in tuberculosis. The active metabolite of vitamin D, 1α,25‐dihydroxyvitamin D₃ [1α,25(OH)₂D₃], has previously been reported to inhibit secretion of MMP‐9 in human monocytes (MN), but its influence on the secretion and gene expression of MMP and tissue inhibitors of MMP (TIMP) in M. tuberculosis‐infected cells has not previously been investigated. We therefore determined the effects of 1α,25(OH)₂D₃ on expression, secretion and activity of a number of MMP and TIMP in M. tuberculosis‐infected human leucocytes; we also investigated the effect of 1α,25(OH)₂D₃ on the secretion of interleukin‐10 (IL‐10) and prostaglandin E₂ (PGE₂), both transcriptional regulators of MMP expression. We found that M. tuberculosis induced expression of MMP‐1, MMP‐7 and MMP‐10 in MN and MMP‐1 and MMP‐10 in peripheral blood mononuclear cells (PBMC). 1α,25(OH)₂D₃ significantly attenuated M. tuberculosis‐induced increases in expression of MMP‐7 and MMP‐10, and suppressed secretion of MMP‐7 by M. tuberculosis‐infected PBMC. MMP‐9 gene expression, secretion and activity were significantly inhibited by 1α,25(OH)₂D₃ irrespective of infection. In contrast, the effects of 1α,25(OH)₂D₃ on the expression of TIMP‐1, TIMP‐2 and TIMP‐3 and secretion of TIMP‐1 and TIMP‐2 were small and variable. 1α,25(OH)₂D₃ also induced secretion of IL‐10 and PGE₂ from M. tuberculosis‐infected PBMC. These findings represent a novel immunomodulatory role for 1α,25(OH)₂D₃ in M. tuberculosis infection.

In high burden settings, the risk of infection with Mycobacterium tuberculosis increases throughout childhood due to cumulative exposure. However, the risk of progressing from tuberculosis (TB) infection to disease varies by age. Young children (<5 years) have high risk of disease progression following infection. The risk falls in primary school children (5 to <10 years), but rises again during puberty. TB disease phenotype also varies by age: generally, young children have intrathoracic lymph node disease or disseminated disease, while adolescents (10 to <20 years) have adult-type pulmonary disease. TB risk also exhibits a gender difference: compared to adolescent boys, adolescent girls have an earlier rise in disease progression risk and higher TB incidence until early adulthood. Understanding why primary school children, during what we term the “Wonder Years,” have low TB risk has implications for vaccine development, therapeutic interventions, and diagnostics. To understand why this group is at low risk, we need a better comprehension of why younger children and adolescents have higher risks, and why risk varies by gender. Immunological response to M. tuberculosis is central to these issues. Host response at key stages in the immunopathological interaction with M. tuberculosis influences risk and disease phenotype. Cell numbers and function change dramatically with age and sexual maturation. Young children have poorly functioning innate cells and a Th2 skew. During the “Wonder Years,” there is a lymphocyte predominance and a Th1 skew. During puberty, neutrophils become more central to host response, and CD4+ T cells increase in number. Sex hormones (dehydroepiandrosterone, adiponectin, leptin, oestradiol, progesterone, and testosterone) profoundly affect immunity. Compared to girls, boys have a stronger Th1 profile and increased numbers of CD8+ T cells and NK cells. Girls are more Th2-skewed and elicit more enhanced inflammatory responses. Non-immunological factors (including exposure intensity, behavior, and co-infections) may impact disease. However, given the consistent patterns seen across time and geography, these factors likely are less central. Strategies to protect children and adolescents from TB may need to differ by age and sex. Further work is required to better understand the contribution of age and sex to M. tuberculosis immunity.