Vitamin A Supplementation for Modulation of Mycobacterium Tuberculosis Immune Responses in Latent Tuberculosis

Overview

In populations with high prevalence of latent tuberculosis infection (LTBI), malnutrition (PEM) may influence incident rates of TB. PEM and specific micronutrient deficiencies compromise cell mediated immunity (CMI) and increase susceptibility to, or severity of infections. Vitamin A supplementation significantly reduces all-cause child mortality. The mechanism of the benefits of supplementation on clinical outcomes is largely unknown, but is likely to be related to an influence on the immune system. Vitamin A supplementation promotes lymphogenesis and induces a higher proportion of CD4 naïve T-cells in children. Most cases of LTBI that progress to active disease are vitamin A deficient. Vitamin A deficiency is common in most TB endemic countries. At the MRC, 32% of TBCC contacts were vitamin A deficient. Hypothesis: The investigators plan to test the hypotheses: that supplementation with vitamin A will affect the magnitude and quality of immune responses to mycobacterial antigens and progression to clinical disease.

Tuberculosis (TB) remains a significant global health problem. Approximately a third of the world's populations are infected with Mycobacterium tuberculosis and 95% of cases occur in developing countries. This enormous pool of latently infected individuals is expected to pose a major obstacle for TB control in highly endemic countries and globally. In high income, low TB burden countries, targeted testing of TB case contacts and treatment for latent TB infection (LTBI) is practised as a component of TB control strategies. However, this is not practised in high burden, low-income countries. New evidence from mathematical modelling suggests that, to meet millennium development goals, interventions against M. tuberculosis infection will be required. Children are at increased risk of rapid progression to active disease (usually within a year for infants). , Malnutrition has been identified as a major risk factor for progression to TB because of its profound effect on cellular immune function- the key host defence against TB. There are 2 types of risk associated with malnutrition: acquisition of infection and risk of infection progressing to disease. Therefore, in populations with high prevalence of latent TB infection, co-prevalent malnutrition may influence TB incidence rates. Vitamin A supplementation has been clearly shown to reduce all-cause child mortality in developing countries. Vitamin A given at recommended doses has a profound effect on improving outcomes in measles and overall childhood mortality and morbidity. The mechanism for this has been attributed to its modulation of immune responses in addition to correcting underlying deficiency. In TB patients, it is nearly impossible to determine nutritional status before disease and thus determine whether malnutrition led to TB or TB led to malnutrition. However, some studies have established a link between vitamin A deficiency and susceptibility to respiratory infections and progression from latent to active TB disease. Preschool children with symptomatic vitamin A deficiency have been found to have respiratory disease at twice the rate in non-deficient children, irrespective of anthropometric status. Getz et al found 81% of persons in a cohort with LTBI that had low levels of vitamin A developed disease compared to 30% of those with normal levels. We had previously observed a 32% prevalence of vitamin A deficiency in a subset of Tuberculosis case contact study contacts with latent TB (unpublished data). The mechanism of the benefits of vitamin A on clinical outcomes especially as related to measles is largely unknown and on tuberculosis is yet to be proven. However, it is likely to be related to an influence on the immune system.In experimental and animal models, vitamin A promotes differentiation and cytokine secretion by macrophages and may down regulate the secretion of pro-inflammatory cytokines e.g. TNF-alpha and IL-6. in children. Vitamin A supplementation has been reported to promote lymphogenesis and induce a higher proportion of CD4 naïve T-cells (CD4+ CD45RA). In addition, the quality of T-cell function may also be affected by Vitamin A.There are data indicating that IFN-gamma production is decreased in vitamin A deficient children while optimal in normal children. Immune responses of PBMCs from non-deficient children stimulated with specific antigens were biased towards more of IFN-gamma, and less of IL-10 and IL-4. This cytokine profile is reminiscent of decreased Treg differentiation and/or Th1-type immune response induced by vitamin A, which is required for protection against an intracellular pathogen such as M.tb. Indeed, data from our previous studies suggest that initial decrease in Treg induction in contacts of TB cases was associated with protection against progression to TB disease To the best of our knowledge we are unaware of any trial of vitamin A for modulation of immune responses associated with progression to active disease in children with latent TB. We will conduct a parallel group comparison of a dose of 200,000 IU Vitamin A supplementation or placebo in latently infected children aged 5-14 years to evaluate qualitative and quantitative modulation of T-cell responses and clinical disease progression.