Diagnostic potential of recombinant Mycobacterium tuberculosis PcaA antigen and its enhancement of protective efficacy as a subunit vaccine booster following BCG priming

Abstract

Bacillus Calmette-Guérin (BCG) is the only vaccine currently used in clinical practice to prevent tuberculosis (TB), however, it remains limited in preventing latent infection, TB reactivation, and providing comprehensive protection. In this study, the pcaA gene, an antigen associated with persistent infection, was selected, and the recombinant plasmid pET28a-PcaA was successfully constructed for protein expression and purification. The specificity of the antigen was further verified using chemiluminescence, enzyme-linked immunosorbent assay (ELISA), flow cytometry, and mycobacterial growth inhibition assay (MGIA). Significant differences were observed in the expression levels of IFN-γ, IL-2, IL-8, and IgG in the peripheral blood of patients with Mycobacterium tuberculosis (M. tb) following stimulation with the PcaA antigen in the Active Tuberculosis (ATB) and Latent Tuberculosis Infection (LTBI) groups. An IL-8 combined diagnostic model could effectively distinguish between ATB and LTBI, while anti-PcaA IgG demonstrated strong performance in ruling out M. tb infection. Recombinant PcaA protein (rPcaA) was formulated with liposome dimethyl dioctadecylammonium bromide (DDA) / colloidal manganese salt (MnJ)/DM to immunize mice. Serum-specific antibody levels, cytokines secreted by splenocytes, and the number of multifunctional T cells in splenocytes were assessed. The results indicated that the BCG + rPcaA-DM vaccine group exhibited significantly elevated levels of Th1-type cytokines, antibody titers, and the frequencies of IFN-γ⁺/TNF-α⁺ single and double-positive CD4⁺ and CD8⁺ T cells compared to the BCG group. Furthermore, splenocytes and lung cells from immunized mice significantly inhibited mycobacterial growth. These findings suggest that the rPcaA-DM vaccine, as a BCG booster, significantly enhances Th1 polarization and provides robust protective efficacy, with potential to prevent LTBI progression to ATB.