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

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 NH2-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.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.Tuberculosis (TB) is the most significant bacterial infection of humans worldwide involving an estimated 2 billion people, that is one third of the world's population [1]. The host's immune system plays a central role in the progression of TB infection; it is in fact estimated that about 5-10% of individuals that become infected with Mycobacterium tuberculosis develop active pulmonary TB and become infectious, while the large majority develop latent infection due to the immunological containment of infection in specific granulomas where tubercle bacilli do not multiply, but persist in a dormant state without provoking any clinical symptoms [2,3]. Latent TB may undergo reactivation when the immune system is less ef