OBJECTIVE To investigate the immunoprotective effects of the active protein mixture derived from the novel Mycobacterium tuberculosis fusion strain "B/R."

METHODS C57BL/6 mice were randomly divided into four groups: PBS, inactivated B/R strain, "B/R" strain active protein mixture and BCG. Mice in each group were subcutaneously administered the corresponding agents. Serum antibodies and cytokines were measured. At 8 weeks post-immunization, mice were challenged with a high-dose BCG. Four weeks later, lung histopathology and bacterial burden were evaluated. At 8, 12 and 16 weeks post-immunization, the proportion of memory T cells in the spleen and cytokine levels in splenocyte supernatants were assessed.

RESULTS Serum immunoglobulin （Ig） G levels in all immunized groups were higher than those in the PBS group. The "B/R" strain active protein mixture induced IgG production, though its effect was weaker than that of the BCG group. The IgG2a/IgG1 ratio was ＞1 in all groups, indicating a predominantly cellular immune response. Interferon-γ （IFN-γ） levels in all immunized groups were higher than those in the PBS group. Except for the inactivated B/R strain group at week 2, interleukin （IL）-2 levels in all other groups were higher than those in the PBS group, with the "B/R" strain active protein mixture group showing the highest IL-2 level at week 2. IL-4 expression exhibited an opposite trend, and a Th1-biased immune response was induced in all immunized groups. After high-dose BCG infection, no significant macroscopic abnormalities were observed in lung tissues across groups. Microscopic pathological damage and bacterial counts in lung tissues were lower than those in the PBS group. Flow cytometry revealed higher proportions of CD₄⁺T_CM and CD₄⁺T_EM in splenic lymphocytes across immunized groups compared to the PBS group. The "B/R" strain active protein mixture and BCG groups showed higher proportions of CD₈⁺T_CM and T_EM cells than the PBS group, with the former exhibiting slightly lower CD₈⁺T_CM levels at week 16. The trends in IFN-γ and IL-2 levels in cultured splenocyte supernatants were largely consistent with those in serum.

CONCLUSION The immunoprotective efficacy of the active protein mixture from the novel M. tuberculosis fusion strain "B/R" is comparable to BCG and may act earlier and faster upon pathogen stimulation.