A Novel B-Cell-Targeted Adjuvant APRIL Improves the Breadth and Persistence of Anti-SARS-CoV-2 Immunity

Background: A broad, robust, and long-lasting antibody response is essential for effective anti-SARS-CoV-2 immunity and the prevention of breakthrough infections. As a key member of the tumor necrosis factor superfamily, a proliferation-inducing ligand (APRIL) participates in multiple B cell fate processes, including B cell survival, class-switch recombination, and plasma cell (PC) differentiation, but its potential utility as a SARS-CoV-2 vaccine adjuvant remains largely unexplored. Methods: Balb/c mice were immunized with recombinant XBB.1.5 S protein formulated with alum, with or without the addition of APRIL. Serum, bronchoalveolar lavage fluid (BALF), and fecal samples were collected for measuring antigen-specific IgG/IgA titers and antibody avidity by ELISA. Neutralization against Delta, BA.2.12.1, BF.7, XBB.1.5, and JN.1 was determined in HEK293-ACE2 cells, while cellular responses were assessed by flow cytometry and Enzyme-Linked Immunospot (ELISPOT). Gene expressions were quantified by RT-PCR. Results: APRIL augmented the immunogenicity of XBB.1.5 S in a dose-dependent manner. It promoted the generation of both systemic and mucosal antibodies, broadened the binding breadth and avidity, and conferred superior neutralizing activity against multiple variants. During a 20-week post-immunization monitoring period, mice maintained effective antibody titers and retained the capacity to neutralize diverse variants. Mechanistically, APRIL was associated with upregulated AICDA and UNG mRNA expressions, accompanied by expansion of the B cell repertoire, differentiation into PCs and antigen-specific memory B cells, and maintenance of high-avidity long-lived PCs. Conclusions: APRIL acts as an effective adjuvant that elicits broad-spectrum, long-lasting immunity against SARS-CoV-2 variants. It represents a promising immunostimulatory component for optimizing the performance of recombinant spike protein vaccines.