Age-related waning of in vitro Interferon-γ levels against r32kDaBCG in BCG vaccinated children

BACKGROUND Mycobacterium bovis BCG vaccine has displayed inconsistent efficacy in different trials conducted in various geographical regions. Nevertheless, it significantly reduces the risk of severe childhood tuberculosis and continues to be used to prevent tuberculosis in many countries. Many studies revealed that efficacy of vaccine wanes with age. Most of the studies were based on in vivo and in vitro responses to tuberculin. With the advent of newer tests such as in vitro interferon-gamma assays and identification of potent immunogenic mycobacterial proteins there is a need to corroborate the observations. This study aims at ascertaining the need for a booster at a later age as indicated by in vitro release of IFN-gamma while evaluating Ag85A as an antigen. METHODS Ninety healthy children who were without any clinical evidence of the disease, 45 with a BCG-scar and the remaining 45 without scar and 25 with tuberculosis were included in the study. The incidence of TB was analyzed in 216 children attending a DOTS clinic during 1996-2005. CD3+, CD4+ and CD8+ cell counts were measured by Flow cytometry. r32kDaBCG (Ag85A-BCG) protein was used to stimulate T cells in in vitro T cell responses and interferon-gamma (IFN-gamma) cytokine levels in the supernatants were measured by ELISA. RESULTS High incidence of TB was observed in age group 13-14 years followed by children in the age group 10-12 years (Chi-square 242.22; p < 0.000). T cell subsets were within the normal range in all subjects. 79% of vaccinated children showed positive proliferative responses with a mean SI value of 4.98 +/- 1.99 while only 39% of the unvaccinated and 58% of the tuberculosis children showed positive responses with mean values of 2.9 +/- 1.6 (p < 0.001) and 2.9 +/- 1.7(p < 0.057), respectively. The stimulation indices in vaccinated children decreased in the older children concurring with an increase in the incidence of TB. CONCLUSION Significantly high levels of in vitro IFN-gamma demonstrated in BCG vaccinated children in our study substantiate the observation that BCG is effective in children, but the effect may wane with age. The immunity could be boosted using modified r32kDa (Ag85A) of BCG.