Biophysical and immunological characterization of the ESX-4 system ESAT-6 family proteins Rv3444c and Rv3445c from Mycobacterium tuberculosis H37Rv

The ESAT-6 family proteins of Mycobacterium tuberculosis are regarded as the key mediators in mycobacterial virulence and are largely considered as antigens that can improve TB vaccines and diagnostics. We have characterized Rv3444c and Rv3445c proteins of the ESX-4 system of ESAT-6 family of M. tuberculosis H37Rv, and have experimentally established that these two proteins interact to form a heterodimeric complex. Complex formation resulted in induction of α-helical conformation and stability against chemical denaturation. To evaluate the immunogenic potential, we have immunized mice with Rv3444c or Rv3445c along with Freund's incomplete adjuvant (FIA). Immunization with Rv3444c-FIA or Rv3445c-FIA resulted in long term humoral responses. Re-stimulation of splenocytes from immunized mice resulted in significant lymphocyte proliferation with induction of TNF-α and IL-6. Further, the humoral responses to Rv3444c and Rv3445c antigens in Indian patients with active pulmonary TB (n = 44), and healthy individuals (n = 20), were investigated. Compared to healthy individuals, high levels of IgG against Rv3444c and Rv3445c were observed in TB patient's sera, indicating that these proteins are actively produced during the active phase of TB. Cellular immune responses to these proteins in active pulmonary TB patients (n = 5) were also investigated using peripheral blood mononuclear cells (PBMCs). Both the proteins induce significant lymphocyte proliferation and up-regulate the induction of TNF-α and IL-6 in TB patients.