Immunoregulation by ESAT-6: From Pathogenesis of Tuberculosis to Potential Anti-Inflammatory and Anti-Rejection...

The early secreted antigenic target of 6 kDa (ESAT-6), a main effector molecule of the ESX-1 secretion system, is identified as a virulence determinant and immunoregulatory protein of Mycobacterium tuberculosis (Mtb), affecting the interaction between host immune cells and pathogens. ESAT-6 facilitates the survival of mycobacteria and their cell-to-cell spreading through membrane-permeabilizing activity and the regulation of host immune cell functions. In this review, we first summarize the recent knowledge of the roles of ESAT-6 in the survival of bacteria, phagosomal escape, and pathogenicity during Mtb infection. Then, we focused on its complex immunomodulatory effects on different immune cells, such as macrophages, dendritic cells, neutrophils, and T cells, accentuating its capability to either facilitate or inhibit immune responses through different signaling pathways. While our review has summarized its main roles in immunopathology in the context of tuberculosis, we additionally search for emerging evidence indicating that ESAT-6 has anti-inflammatory and immunosuppressive properties. Particularly, we discuss recent preclinical studies showing its capability to suppress transplant rejection and alloimmunity, probably via the induction of regulatory T cells. Nevertheless, the potential clinical use of ESAT-6 remains uncertain and needs further verification by comprehensive preclinical and clinical studies. Thus, we propose that ESAT-6 may be exploited to ameliorate immunopathology in TB infection and to suppress immune-mediated inflammation or transplant rejection as well.