Coronavirus disease (COVID-19) causes a serious threat to human health. Virus-like particles (VLPs) constitute a promising platform in SARS-CoV-2 vaccine development. In this study, the E, M, and S genes were cloned into multiple cloning sites of a new triple expression plasmid with one p10 promoter, two pPH promoters, and three multiple cloning sites. The plasmid was transformed into DH10 Bac^TMEscherichia coli competent cells to obtain recombinant bacmid. Then the recombinant bacmid was transfected in ExpiSf9^TM insect cells to generate recombinant baculovirus. After ExpiSf9^TM cells infection with the recombinant baculovirus, the E, M, and S proteins were expressed in insect cells. Finally, SARS-CoV-2 VLPs were self-assembled in insect cells after infection. The morphology and the size of SARS-CoV-2 VLPs are similar to the native virions.

Long-lived memory cell formation and maintenance are usually regulated by cytokines and transcriptional factors. Adjuvant effects of IL-7 have been studied in the vaccines of influenza and other pathogens. However, few studies investigated the adjuvant effects of cytokines and transcriptional factors in prolonging the immune memory induced by a tuberculosis (TB) subunit vaccine. To address this research gap, mice were treated with the Mycobacterium tuberculosis (M. tuberculosis) subunit vaccine Mtb10.4-HspX (MH) plus ESAT6-Ag85B-MPT64_<190–198>-Mtb8.4-Rv2626c (LT70), together with adeno-associated virus-mediated IL-7 or lentivirus-mediated transcriptional factor Id3, Bcl6, Bach2, and Blimp1 at 0, 2, and 4 weeks, respectively. Immune responses induced by the vaccine were examined at 25 weeks after last immunization. The results showed that adeno-associated virus-mediated IL-7 allowed the TB subunit vaccine to induce the formation of long-lived memory T cells. Meanwhile, IL-7 increased the expression of Id3, Bcl6, and bach2—the three key transcription factors for the generation of long-lived memory T cells. The adjuvant effects of transcriptional factors, together with TB fusion protein MH/LT70 vaccination, showed that both Bcl6 and Id3 increased the production of antigen-specific antibodies and long-lived memory T cells, characterized by high proliferative potential of antigen-specific CD4⁺ and CD8⁺ T cells, and IFN-γ secretion in CD4⁺ and CD8⁺ T cells, respectively, after re-exposure to the same antigen. Overall, our study suggests that IL-7 and transcriptional factors Id3 and Bcl6 help the TB subunit vaccine to induce long-term immune memory, which contributes to providing immune protection against M. tuberculosis infection.

Tuberculosis (TB), caused by Mycobacterium tuberculosis (M. tuberculosis), is among the most serious infectious diseases worldwide. Adjuvanted protein subunit vaccines have been demonstrated as a kind of promising novel vaccine. This study proposed to investigate whether cytokines interliukine-7 (IL-7) and interliukine-15 (IL-15) help TB subunit vaccines induce long-term cell-mediated immune responses, which are required for vaccination against TB. In this study, mice were immunized with the M. tuberculosis protein subunit vaccines combined with adnovirus-mediated cytokines IL-7, IL-15, IL-7-IL-15, and IL-7-Linker-IL-15 at 0, 2, and 4 weeks, respectively. Twenty weeks after the last immunization, the long-term immune responses, especially the central memory-like T cells (T_CM like cell)-mediated immune responses, were determined with the methods of cultured IFN-γ-ELISPOT, expanded secondary immune responses, cell proliferation, and protective efficacy against Mycobacterium bovis Bacilli Calmette-Guerin (BCG) challenge, etc. The results showed that the group of vaccine + rAd-IL-7-Linker-IL-15 induced a stronger long-term antigen-specific T_CM like cells-mediated immune responses and had higher protective efficacy against BCG challenge than the vaccine + rAd-vector control group, the vaccine + rAd-IL-7 and the vaccine + rAd-IL-15 groups. This study indicated that rAd-IL-7-Linker-IL-15 improved the TB subunit vaccine’s efficacy by augmenting T_CM like cells and provided long-term protective efficacy against Mycobacteria.