Trained immunity agonists given to neonatal mice fail to protect against leptospiral infection in adulthood and may cause adverse effects

Background and Objectives Trained immunity (TI) refers to a non-specific, long-lasting protective immune response that occurs following initial stimulation of the immune system and thought to be largely mediated by functional reprogramming of myeloid cells. TI has been demonstrated in BCG-vaccinated infants and can be induced in human cells and adult mice via agonists of pattern recognition receptors (PRR), such as β-glucan or MDP (a muropeptide that activates NOD2). However, its induction in neonates remains poorly understood. Previously, we demonstrated that the synthetic TLR2-NOD2 dual agonist CL429 enhances antimicrobial functions in adult mice and protects against subsequent infection with Leptospira interrogans , a zoonotic pathogen. We also demonstrated the immediate protective benefits of NOD2 stimulation in neonates against Cryptosporidium, a zoonotic pathogen that affects young animals in livestock herds. However, whether NOD2 agonists can induce TI in neonates and protect them at adulthood is unknown. Methods and Results Here, we investigated whether exposure of neonatal mice to PRR agonists (CL429, MDP or β-glucan), administered intraperitoneally at one week interval at 7/14 or 14/21 days of age, could enhance inflammatory cytokines production after ex vivo restimulation and confer long-term protection into adulthood. Surprisingly, none of the treatments enhanced ex vivo cytokine responses in adulthood after restimulation, nor did they confer protection against experimental leptospirosis. Instead, MDP-treated neonates exhibited 50% mortality following adult infection, revealing an unexpected detrimental effect. Conclusion These findings demonstrate that these PRR agonists fail to induce protective TI against Leptospira when administered to neonatal mice, challenging assumptions derived from adult models. Furthermore, our study reveals the risks of administering immunostimulants during the early stages of life, and highlights unanticipated and potentially harmful, PRR- and age-specific mechanisms of immune system modulation.