# Effect of a gut commensal Lactobacillus strain Limosilactobacillus caviae JL20 on leptospiral whole-cell inactivated vaccine in hamsters

## TL;DR

A gut bacteria strain improves the effectiveness of a leptospirosis vaccine in hamsters by boosting immune responses and cross-protection.

## Contribution

Limosilactobacillus caviae JL20 is shown to enhance leptospiral vaccine efficacy through adjuvant-like effects and immune system activation.

## Key findings

- JL20 increased survival rates and cross-protection in hamsters against different Leptospira strains.
- JL20 boosted antibody responses and immune cell activation, including increased IL-1β, IL-6, and glycolytic activity in macrophages.
- JL20 enhanced T and B cell markers, indicating improved cellular and humoral immunity.

## Abstract

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.

Leptospirosis, a widespread zoonotic disease, remains difficult to control due to limitations of current vaccines, which often lack broad protection. This study introduces Limosilactobacillus caviae JL20, a bacterial strain that enhances the effectiveness of the leptospiral vaccine. In hamster studies, JL20 improved survival rates and extended protection against different bacterial strains. It boosted the body’s antibody response and primed immune cells in the spleen, increasing the production of key signaling molecules (IL-1β, IL-6) and cellular energy metabolism. Furthermore, JL20 activated critical immune cells: it increased activity markers on T cells and improved the function of B cells, which are essential for antibody production and immune coordination. These findings demonstrate that JL20 strengthens both the immediate and long-term immune responses triggered by the vaccine, offering a promising strategy to develop better leptospirosis vaccines.