The 6-Kilodalton Early Secreted Antigenic Target-Responsive, Asymptomatic Contacts of Tuberculosis Patients Express Elevated Levels of Interleukin-4 and Reduced Levels of Gamma Interferon

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Abstract
It is well known that the majority of healthy individuals exposed toMycobacterium tuberculosisdo not become clinically ill. We have previously shown that in recently exposed healthy contacts of tuberculosis (TB) patients, a strong immune response to theM. tuberculosis6-kDa early secreted antigenic target (ESAT-6) virulence factor correlated with a higher risk of subsequent disease, although the mechanism was unclear at that time. Inspired by recent reports that elevated expression of interleukin-4 (IL-4) in health care workers exposed toM. tuberculosisalso correlated with a higher risk of their subsequently developing disease, we examined expression of IL-4, its competitive antagonist IL-4δ2, and gamma interferon (IFN-γ) in healthy household contacts of TB patients from Ethiopia. We then compared cytokine expression to their recognition of ESAT-6 (which is largely restricted to members of the tuberculosis complex and which serves as a reliable marker of infection) or to Ag85A (an antigen that is conserved among the mycobacteria and serves as a nonspecific control). Our study shows that in these recently exposed individuals, there is a correlation between a strong response to ESAT-6 and elevated expression of IL-4. Further, elevated expression of IL-4 is associated with lower expression of its antagonistic splice variant IL-4δ2 and with the Th1 cytokine IFN-γ, suggesting that in these at-risk individuals, immunity is skewed away from a protective Th1 response, even before the development of clinical symptoms.

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