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

Abstract

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/106 PBMC (range 118-2000) for LTBI subjects compared to 80 SFC/106 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.

Figure 1

Proliferative responses to the specific mycobacterial antigens, PPD, Erp, ESAT-6 and Ag85B were evaluated in 3 groups: BCG⁺ subjects without infection, with latent TB infection (LTBI) and in TB patients (TB), measured by lymphoproliferation assay. A logarithmic scale is used and horizontal bars indicate positive values.

Figure 2

Numbers of antigen-specific T cells in the 4 groups:non BCG-vaccinated controls (BCG^-), BCG⁺ subjects without infection and with latent TB infection (LTBI), and in TB patients (TB), measured by ELISpot IFN-γ assay after stimulation with PPD, Erp, ESAT-6 and Ag85B. A logarithmic scale is used and horizontal bars indicate positive values.

Figure 3

Cytokine productions (IFN-γ or ●, TNF-α or ▲ and IL-10 or ■) were represented for the 3 groups: BCG⁺ subjects without infection, with latent TB infection (LTBI) and in TB patients (TB).

Figure 4

Subtypes of IgG were represented in column bar after Erp, ESAT-6 and Ag85B stimulation in the 4 groups: non BCG-vaccinated controls (BCG^-), BCG⁺ subjects without infection, with latent TB infection (LTBI) and in TB patients (TB) in the order: IgG1, IgG2, IgG3, IgG4.