Sequence of primers used for qPCR assays.

Leptospirosis is a global zoonotic threat caused by pathogenic Leptospira, and it remains challenging to combat because of persistent bottlenecks in vaccine development. The lack of well-defined protective antigens across Leptospira serovars continues to necessitate reliance on whole-cell inactivated vaccines, despite their recognized limitations: suboptimal efficacy and the absence of cross-serovar protection. In this study, we presented a Limosilactobacillus caviae (L. caviae) JL20 that significantly potentiated leptospiral vaccine efficacy through adjuvant-like effects. Survival assessment of hamsters demonstrated that JL20 enhances both vaccine efficacy and cross-protection. Oral JL20 significantly increased vaccine-induced cross-reactive binding antibody titers and total IgG antibody responses. In addition, JL20 exerted a priming effect in splenic macrophages, augmenting the expression of IL-1β and IL-6 in response to leptospiral vaccine stimulation, with a parallel enhancement in glycolytic activity. In vivo experiments demonstrated that JL20 significantly upregulated the expression of surface molecules CD38, CD69, and CD25 on T cells, as well as the production of the cytokine IL-2. JL20 enhanced the surface expression of key markers—including CD40, CD80, CD86, and MHC-II—on B cells. These effects indicate that JL20 enhances both cellular and humoral immune responses of leptospiral vaccine.