T-cell and serological responses to Erp, an exported Mycobacterium tuberculosis protein, in tuberculosis patients and healthy individuals

Background: The identification of antigens able to differentiate tuberculosis (TB) disease from TB infection would be valuable. Cellular and humoral immune responses to Erp (Exported repetitive protein)-a recently identified M. tuberculosis protein-have not yet been investigated in humans and may contribute to this aim. Methods: We analyzed the cellular and humoral immune responses to Erp, ESAT-6, Ag85B and PPD in TB patients, in BCG(+) individuals without infection, BCG+ individuals with latent TB infection (LTBI) and BCG-controls. We used lymphoproliferation, ELISpot IFN-gamma, cytokine production assays and detection of specific human antibodies against recombinant M. tuberculosis proteins. Results: We included 22 TB patients, 9 BCG+ individuals without TB infection, 7 LTBI and 7 BCG-controls. Erp-specific T cell counts were higher in LTBI than in the other groups. Erp-specific T cell counts were higher in LTBI subjects than TB patients (median positive frequency of 211 SFC/10(6) PBMC (range 118-2000) for LTBI subjects compared to 80 SFC/10(6) PBMC (range 50-191), p = 0.019); responses to PPD and ESAT-6 antigens did not differ between these groups. IFN-gamma secretion after Erp stimulation differed between TB patients and LTBI subjects (p = 0.02). Moreover, LTBI subjects but not TB patients or healthy subjects produced IgG3 against Erp. Conclusion: The frequencies of IFN-gamma-producing specific T cells, the IFN-gamma. secretion and the production of IgG3 after Erp stimulation are higher in LTBI subjects than in TB patients, whereas PPD and ESAT-6 are not.