Monokine induced by interferon gamma and IFN-γ response to a fusion protein of Mycobacterium tuberculosis ESAT-6 and CFP-10 in Brazilian tuberculosis patients

IFN-γ responses to Mycobacterium tuberculosis antigens ESAT-6 and CFP-10 have been proposed as specific markers of M. tuberculosis infection. Monokine induced by gamma interferon (MIG/CXCL9) has been shown to be expressed by IFN-γ stimulated mononuclear cells and to attract activated T-cells through the chemokine receptor CXCR3. Since MIG is induced early in the response to IFN-γ, measuring MIG may provide an interesting marker to assess downstream IFN-γ induced responses, in contrast to assays that mainly focus on quantifying production of IFN-γ per se. We, therefore, investigated MIG and IFN-γ responses to a fusion protein of ESAT-6 and CFP-10, and compared responses to the conserved mycobacterial antigen 85B (Ag85B) and purified protein derivative (PPD) of M. tuberculosis, in 29 BCG vaccinee controls and 24 TB patients. IFN-γ secreting cells were determined by ELISPOT, and MIG production was measured by ELISA and flow cytometry. Production of MIG in response to ESAT-6/CFP-10, Ag85B and PPD correlated overall with increased numbers of IFN-γ secreting cells (r = 0.55, P < 0.0001). A significant increase was noted among patients compared to controls in the secretion of IFN-γ and MIG following stimulation with ESAT-6/CFP-10 or PPD (P < 0.05). Moreover, MIG intracellular expression was higher in TB patients compared to BCG vaccinees (P < 0.05) in response to ESAT-6/CFP-10 or PPD. We conclude that MIG production correlates significantly with enhanced T-cell IFN-γ production induced by M. tuberculosis-specific antigens ESAT-6/CFP-10. These results point to MIG as a potential novel biomarker that may be helpful in assessing downstream responses induced by IFN-γ in TB.