Increased expression of Mycobacterium tuberculosis 19 kDa lipoprotein obliterates the protective efficacy of BCG by polarizing host immune responses to the Th2 subtype.

Mycobacterium tuberculosis can not only neutralize immune effector functions, but also has the ability to modulate host-signalling cascades involved in the development of these responses. The 19 kDa antigen (Rv3763), a lipoprotein of M. tuberculosis, elicits high levels of interleukin (IL)-12 from macrophages in addition to its powerful immunomodulatory properties, leading to suppression of antigen-presentation signalling cascades. The present study was aimed at analysing the effect of overexpression of this antigen on the immunostimulatory properties of M. bovis Bacille Calmette-Guerin (BCG). We have constructed a recombinant BCG strain (rBCG19N) producing higher levels of the 19 kDa antigen in both the cytoplasmic (approximately eightfold) and extracellular (approximately fivefold) fractions as compared to the wildtype BCG. Immunization of mice with rBCG19N elicited high levels of interferon-gamma (IFN-gamma) and relatively low levels of IL-10 against the purified 19 kDa antigen. However, in response to total BCG sonicate, mice immunized with rBCG19N produced significantly high levels of IL-10 with relatively very low levels of IFN-gamma. This polarization of the host immune responses towards T-helper 2 subtype resulted in complete abrogation of the protective efficacy of BCG, when rBCG19N was used as a live vaccine against M. tuberculosis challenge in guinea pigs.