A novel MTB-specific IL2 release assay for the diagnosis of latent Mycobacterium tuberculosis infection (LTBI). Results from a large outbreak study

Background: Transmission of Mycobacterium tuberculosis (MTB) depends on the intensity of MTB exposure, the immune status of the host and mycobacterial virulence.

Methods: We present the results of a tuberculosis contact investigation of a highly symptomatic sputum smear positive index case with pulmonary tuberculosis among a large group of well-defined asymptomatic close contacts. Immunological investigations included a tuberculin skin tests (TST) and early secretory antigenic target (ESAT)-6/culture filtrate protein (CFP)-10 specific ELISPOT with IFN-γ and a novel ESAT-6/CFP-10 specific IL-2 ELISPOT.

Results: Only six (3.4%) of 172 close contacts had a positive TST result (³ 5mm). Seven (4.2%) out of 167 were positive on either ESAT-6 or CFP-10 stimulated IFN-γ release assay.

For close contacts (≥100h) the risk to develop a positive IFN-γ release assay test response was 40.8 times higher (95% CI 6.9–240.3; p<0.0001), and the risk of TST conversion increased by 19.3 times compared to individuals with only occasional contact (<100h). However, in contrast to ELISPOT testing the TST result was strongly confounded by BCG vaccination (OR=14.6 (95% CI 1.4–150.1; p=0.02). The T-cell IL2 release assay did not improve the diagnostic accuracy of the IFN-γ release ELISPOT. Agreement of TST and ELISPOT results was only moderate (kappa=0.416).

Conclusions: Despite intense contact to a highly symptomatic index case, putative transmission of MTB only rarely occurred in this outbreak study. MTB-specific IFN-γ release ELISPOT was superior to the TST to identify close contacts likely to be infected with MTB, while an additional MTB-specific IL2 release ELISPOT did not increase the accuracy for the diagnosis of latent Mycobacterium tuberculosis infection (LTBI) in this study.