Rv0646c, an esterase from M. tuberculosis, up-regulates the host immune response in THP-1 macrophages cells

The genome sequence of Mycobacterium tuberculosis revealed the presence of several hydrolases involved in lipid metabolism including the members of Lip gene family. Rv0646c (LipG) is one of them. It is annotated as putative esterase/lipase because of the presence of consensus sequence 'GXSXG.' The gene was cloned, expressed, and purified in E. coli. It showed 22 U/mg specific activity with pNP-butyrate as a preferred substrate. However, it actively worked on substrates with short chain. The enzyme was optimally active at 50 °C/pH 8.0 and also stable up to 50 °C and in a lower pH range (pH 6-8). The K_m, V_max, and catalytic efficiency of the enzyme were calculated to be 500 µM, 58.82 µmoles/min/ml, and 3.92 µM/min, respectively. Homology modeling of Rv0646c revealed the presence of a canonical putative catalytic triad (Ser123, His279, and Asp251). The esterase activity was abolished in the presence of serine hydrolase inhibitors, THL and PMSF. Various antigenic epitopes were predicted in Rv0646c. The protein mounted significantly high antibody response against the sera of extrapulmonary and MDR-TB patients. Rv0646c up-regulated the production of various pro-inflammatory cytokines (TNF-α and IFN-γ), chemokine (IL-8), and nitric oxide in THP-1-derived macrophages. The secretion of IL-6 from macrophages was also found to be elevated in response to Rv0646c. The treatment resulted in the increased level of reactive oxygen species. Conclusively, Rv0646c could be classified as esterase having vast immunogenic property by eliciting strong humoral response as well as cell-mediated immunity.