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 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. 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.

Evolutionary pathway of the Beijing lineage of Mycobacterium tuberculosis based on genomic deletions and mutT genes polymorphisms.

Among the genotypes that prevail in the modern spectrum of Mycobacterium tuberculosis strains, the Beijing genotype is the one that causes major concern, as it is geographically widespread and it is considered hypervirulent. Comparative genomic studies have shown that Beijing strains have principally evolved through mechanisms of deletion of chromosomal regions, designated regions of difference (RD), and mutations. In this paper, we aimed to determine the evolutionary history of Beijing strains through the analysis of polymorphisms generated by deletions of large specific sequences, i.e., RD105, RD181, RD150, and RD142, and by single nucleotide substitutions in genes mutT4 and mutT2, coding for DNA repair enzymes. Based on the molecular characteristics of a collection of Beijing strains recently isolated in Tuscany, Italy, we propose a phylogenetic reconstruction of the Beijing family. According to our model, the Beijing family evolved from a M. tuberculosis progenitor following deletion of the RD207 region, an event responsible for the loss of spacers 1-34 in the direct repeat (DR) locus. The major lineages of the Beijing family then evolved via subsequent deletions of regions RD105, RD181 and RD150. In the most ancient evolutionary lineages genes mutT4 and mutT2 were in wild type configuration; the mutT4 mutation was acquired subsequent to the RD181 deletion in a progenitor strain that, in turn, gave rise to a sublineage bearing the mutT2 mutation. Within the major branches of the Beijing family, deletion of additional spacers in the DR locus led to evolution of sublineages characterized by different spoligotypes. Our evolutionary model of the Beijing family provides a deeper framework than previously proposed for epidemiologic and phylogenetic studies of circulating M. tuberculosis Beijing strains, thus allowing a more systematic and comprehensive evaluation of the relevance of Beijing strain variability.