Construction and immunogenicity evaluation of a tuberculosis subunit vaccine based on fusion protein AP2 and novel adjuvant ASM

Tuberculosis is a severe global respiratory infectious disease caused by infection with Mycobacterium tuberculosis (M. tb). Bacillus Calmette-Guérin (BCG) is currently the only approved vaccine for TB prevention; however, its protective efficacy against tuberculosis in adults is limited. To overcome the limitations of BCG, recombinant plasmids were constructed and expressed using antigens Rv2074 and Rv2642. Subsequently, a full-length gene synthesis strategy incorporating a linker was adopted to construct the fusion antigen AP2 and express its corresponding protein. Peripheral blood samples from subjects were stimulated in vitro with the fusion protein AP2 and the recombinant subunit proteins respectively. Subsequently, changes in the levels of relevant cytokines in the peripheral blood were detected using Multiplex Fluorescence Cytometric Immunoassay (MFCIA). The results demonstrated that both AP2 and its subunit antigens were capable of stimulating lymphocytes from individuals infected with M. tb to produce high levels of Th1-type cytokines. A novel subunit vaccine was formulated by combining the novel adjuvant system ASM (MPL-A, Squalane, M. vaccae lysate ) with the fusion protein AP2. After C57BL/6 mice were immunized with this vaccine, the results indicated that AP2/ASM induced high levels of antigen-specific IgG antibodies, particularly IgG2c. The IgG2c titer in the BCG+AP2/ASM group reached 1:10000, with an IgG2c/IgG1 ratio > 6, indicating a strong Th1 bias. The IFN-γ secretion in the BCG+AP2/ASM group was approximately 2-fold higher than that in the BCG group and 7.3-fold higher than in the PBS group upon AP2 stimulation. Flow cytometry analysis further revealed that the BCG+AP2/ASM group exhibited the highest numbers of IFN-γ⁺CD4⁺ T single-positive cells and TNF-α⁺CD4⁺ T single-positive cells, which were significantly greater than those in all other groups. In contrast, the numbers of IFN-γ⁺CD4⁺ T single-positive cells and TNF-α⁺CD4⁺ T single-positive cells in the Freund's adjuvant group were significantly lower than those in the novel adjuvant ASM group. This experiment demonstrates that the fusion protein AP2 induces stronger downstream T‑cell responses and cytokine secretion, and the novel adjuvant ASM provides strong immunogenicity, suggesting that AP2/ASM is a promising immunogenic candidate vaccine.