Not all Group B Streptococci are alike: macrophage responses reveal inflammatory and immune-evasive strains

Group B Streptococcus (GBS) remains a leading cause of neonatal sepsis and meningitis despite preventive strategies. Disease severity and clinical presentation vary widely and are influenced by strain-specific virulence traits. Macrophages are key innate immune sentinels during GBS infection; however, how genetically distinct clinical isolates differentially and dynamically reprogram macrophage inflammatory, immunoregulatory, metabolic, and cell death responses remains poorly understood.

Human THP-1 macrophages were infected with 12 fully characterized clinical GBS isolates representing multiple serotypes, sequence types, clinical presentations, and neonatal gestational ages. Cytokine and chemokine production was quantified at 3 and 24 hours post-infection using LEGENDplex bead-based immunoassays. Caspase-1 activity was measured by bioluminescence, and expression of inflammatory, immunoregulatory, metabolic, and cell death-associated genes were assessed by RT-qPCR at 4 and 24 hours. Data were analyzed using appropriate statistical tests, and multidimensional responses were integrated using radar plot visualization.

Macrophage responses to GBS were highly isolate-specific and varied over time. Serotype Ia and Ib isolates triggered rapid inflammasome-associated activation with early IL-1β and IL-18 release and high caspase-1 activity, consistent with pyroptosis. In contrast, serotype II and especially hypervirulent serotype III isolates showed minimal early inflammasome activation but induced delayed immunoregulatory programs marked by elevated IL-10 and ACOD1 expression. Reciprocal analyses revealed an inverse relationship between ACOD1 and IL-1β and a positive association between ACOD1 and IL-10, indicating coordinated immunometabolic regulation. Serotype-specific glycolytic gene expression signatures appeared at later time points. Stratification by clinical metadata showed that isolates from preterm infants induced stronger early inflammatory responses.

This study shows that GBS pathogenicity is not a uniform species-level trait but reflects isolate-specific abilities to reprogram macrophage immunity. By integrating temporal resolution with strain diversity, it provides a mechanistic framework linking macrophage immune trajectories to preterm birth-associated inflammation and heterogeneous outcomes in neonatal infections.