Identification of a human immunodominant T-cell epitope of mycobacterium tuberculosis antigen PPE44

BACKGROUND: Recently our group has identified a novel antigen of Mycobacterium tuberculosis, protein PPE44, belonging to the "PPE protein" family. Although its role in infection is largely unknown, PPE44-specific immune responses were detected in mice infected with M. tuberculosis; moreover, immunization of mice with PPE44 subunit vaccines resulted in protective efficacy comparable to the one afforded by BCG against M. tuberculosis (Romano et al., Vaccine 26, 6053-6063, 2008). RESULTS: In the present paper, we investigated anti-PPE44 T-lymphocyte responses during human infection by evaluating the frequency of PPE44-specific interferon (IFN)-γ-secreting cells by ELISpot and flow cytometry in a small cohort of healthy subjects that had proven positive to PPD (PPD(+)) in vitro, in patients with active tuberculosis, in subjects vaccinated with BCG and in unvaccinated, PPD(- )healthy controls. We showed IFN-γ(+ )T cell immune responses to recombinant PPE44 in at least a very high proportion of PPD(+ )individuals tested and, to a lower extent, in subjects vaccinated with BCG. By the use of a panel of overlapping synthetic 20-mer peptides spanning the PPE44 primary amino acid sequence, we identified a strong CD4(+ )T-cell epitope, encompassed by peptide p1L (VDFGALPPEVNSARMYGGAG), in the NH(2)-terminus of the PPE44 molecule at the amino acid position 1-20. Conversely, our experiments did not provide evidence of a significant IFN-γ(+ )CD4(+ )T cell response to PPE44 or its immunodominant peptide p1L in most (7 out of 8) patients with active TB. CONCLUSIONS: Our data suggest an important immunological role of PPE44 and its immunodominant epitope p1L that could be useful in the design of anti-tuberculosis vaccines and in the immunological diagnosis of M. tuberculosis infection.