Variation of the expression of Mycobacterium tuberculosis ppe44 gene among clinical isolates

PPE44 is a member of the Mycobacterium tuberculosis PPE proteins, a polymorphic family of 69 glycine-rich proteins that predictively represent a source of antigenic variation. We have studied the genetic diversity of gene ppe44 among clinical isolates. No genomic polymorphism of ppe44 was found by a PCR-RFLP assay employing three restriction enzymes. Nucleotide sequencing of gene ppe44 of a number of isolates, selected to represent the major phylogenetic lineages of M. tuberculosis, showed no nucleotide substitution, with the exception of isolates of Beijing genotype. These findings indicate that gene ppe44 is basically conserved among M. tuberculosis strains. Expression of gene ppe44 was then determined at the transcriptional level by a real-time RT-PCR assay. Extremely high quantitative variations in ppe44 expression were found among the isolates; ppe44 expression of Beijing strains was significantly higher than non-Beijing strains. To test whether differential expression of gene ppe44 has the potential to provide a dynamic antigen display, antibodies to PPE44 were titered in sera of M. tuberculosis-infected subjects. Variation of antibody response to PPE44 was found with regard to both antibody titers and the proportion of responding subjects. These results indicate that differential expression of genes ppe could influence host’s immune responsiveness, thus having implications in the immunopathogenesis of tuberculosis