HLA-Promiscuous Th1-Cell Reactivity of MPT64 (Rv1980c), a Major Secreted Antigen of Mycobacterium tuberculosis, in Healthy Subjects

Objective: To determine HLA-promiscuous Th1-cell reactivity of MPT64 (Rv1980c) in healthy humans. Materials and Methods: Peripheral blood mononuclear cells (PBMCs) were obtained from healthy subjects (n = 61) and HLA typed genomically. PBMCs were tested for Th1-cell reactivity (antigen-induced proliferation and interferon-γ secretion) with complex antigens (whole cells, culture filtrate and cell walls) and single secreted antigens (Ag85B, MPT64 and MPB70) of Mycobacterium tuberculosis. In addition, culture-filtrate-induced T-cell lines were established from PBMCs of 14 donors and tested with the above antigens in Th1-cell assays. Furthermore, 3 T-cell lines were tested for cytotoxic activity against MPT64-pulsed monocytes/macrophages, and a T-cell line was analysed for HLA restriction in antigen presentation using anti-HLA class I and class II monoclonal antibodies and HLA-DR-typed antigen-presenting cells. Results: PBMCs showed strong Th1-cell reactivity with all of the complex mycobacterial antigens, whereas MPT64 induced moderate Th1-cell reactivity, which was comparable to the reactivity induced by other previously characterized secreted antigens of M. tuberculosis, i.e. Ag85B and MBP70. Furthermore, HLA heterogeneity of the responding donors suggested that MPT64 was presented to Th1 cells promiscuously. Testing of the T-cell lines confirmed that Th1 cells contributed to the promiscuous antigen-specific reactivity observed with PBMCs and exhibited cytotoxic activity against MPT64-pulsed monocytes/macrophages. In addition, a T-cell line investigated for HLA restriction analysis showed that MPT64 was presented to T cells in association with HLA-DR molecules in a promiscuous manner. Conclusion: MPT64 is promiscuously recognized by human Th1 cells with cytotoxic activity and therefore deserves consideration as a candidate vaccine against tuberculosis in humans.