CGRP Suppresses Protective SiglecFhi Neutrophil Development in Neonatal Group B Streptococcus Pneumonia.

Neonatal pneumonia, a leading cause of morbidity and mortality, is frequently caused by Group B Streptococcus (GBS). The mechanisms underlying protective immunity to this pathogen in the neonatal lung remain incompletely understood. Using a clinically relevant neonatal mouse model of GBS pneumonia, we investigated the immune mechanisms influencing disease severity. We demonstrate that neutrophils are effectively recruited to the lungs of infected neonates, but their phenotype differs with disease severity. In pups with moderate disease, we observe significant infiltration of SiglecF^hi neutrophils, a phenotype associated with enhanced phagocytic capacity and bacterial clearance. In contrast, pups with severe disease failed to develop SiglecF^hi neutrophils, resulting in reduced bacterial clearance and worsened pathology. We further show that severity is associated with increased expression of calcitonin gene-related peptide (CGRP) in the lungs. CGRP suppressed neutrophil activation into the SiglecF^hi phenotype, thereby limiting their antibacterial function. Our findings show that GBS exploits the neuroimmune axis to evade host immunity through CGRP-mediated suppression of neutrophil activation.