Early secretory antigen target of 6-kDa of Mycobacterium tuberculosis inhibits macrophage apoptosis and host defense via TLR2

Mycobacterium tuberculosis (M. tb) is an intracellular pathogen adept at evading the human immune system through a variety of mechanisms. During infection, M. tb secretes numerous virulence factors, including the 6 kDa early secretory antigen target (ESAT-6), which is produced by the ESX-1 secretion system. ESAT-6 plays a crucial role in host-pathogen interactions, either independently or in association with culture filtrate protein 10 (CFP-10). While some research has investigated the role of ESAT-6 in M. tb pathogenicity and vaccine development, its precise contribution to immune evasion and the cellular mechanisms involved remain poorly understood. To address this, we used cultured THP-1(A) macrophages to characterize the effects of secreted ESAT-6 on cellular host defenses and apoptosis. We found that ESAT-6 (5 μg/ml) inhibited M. tb-induced apoptosis in THP-1(A) macrophages by suppressing Toll-like receptor 2 (TLR2) through the Caspase-9/Caspase-3 pathway. Additionally, ESAT-6 reduced phagocytosis of M. tb by THP-1(A) macrophages by downregulating the production of interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-α), and interleukin-12 (IL-12). Furthermore, ESAT-6 diminished the bactericidal activity of macrophages by inducing reactive oxygen species (ROS) production. In parallel, our in silico analysis of differentially expressed genes in dendritic cells (DCs) infected with Bacille Calmette-Guérin (BCG) strains, with or without the region of difference-1 (RD1) gene, strongly suggests that ESAT-6, located within the RD1 region, modulates host defense functions and apoptosis in DCs during BCG infection. Collectively, these findings indicate that ESAT-6 plays a pivotal role in modulating the innate immune response of macrophages against M. tb by regulating macrophage recognition, phagocytosis, bactericidal activity, and apoptosis. Our study provides valuable insights into potential molecular targets for the development of innovative vaccines and therapeutic strategies against M. tb.