Exploring trained immunity to complement vaccination against Streptococcus suis in swine.

Streptococcus suis is a major pathogen in swine, causing septicaemia, meningitis, and arthritis. Effective control is complicated by the presence of multiple serotypes, genetic heterogeneity, and interference of maternal antibodies with vaccine efficacy. Moreover, the emergence of antibiotic-resistant strains poses additional challenges. Trained immunity, a process of innate immune memory induced by certain stimuli, has been proposed as a novel approach to enhance host defense mechanisms. This study aimed to evaluate whether a mycobacterial-derived immunomodulator (dpB) administered with a specific S. suis vaccine could improve protection in pigs. Thirty-two weaned pigs were allocated into four groups: vaccine alone, vaccine and live dpB, vaccine and inactivated dpB, and non-immunized controls. After two immunizations, animals were challenged with a S. suis serotype 2 strain. Immune responses were assessed through ex vivo blood stimulation, cytokine measurement and phagocytosis assays. The group receiving vaccine and inactivated dpB presented a better response, including reduced bacterial loads in tissues, less severe lung lesions and elevated pre-challenge cytokine levels. Clinically, these animals showed milder symptoms and lower bacterial dissemination post-challenge. While trained immunity remains a relatively new concept in veterinary immunology, these results suggest that inactivated dpB, when combined with specific vaccination, could serve as an effective complementary strategy to improve protection against S. suis. Further research is needed to elucidate the underlying mechanisms and to establish its potential role in comprehensive disease prevention programs.