Preterm birth affects 1 in 10 infants worldwide, with an estimated 13.4 million cases annually (Ohuma et al., 2023). It is the leading risk factor for neonatal mortality as well as long-term physical and neurodevelopmental impairments, including autism spectrum disorder and global developmental delay. Premature infants are particularly vulnerable to breathing disorders due to immature central respiratory control and underdeveloped innate immunity, which increases susceptibility to infection. Gram-positive bacterial infections are the most common in preterm infants, often acquired in neonatal intensive care units. To develop improved treatment strategies, it is critical to understand how hypoxia and infection interact to shape central immune responses.

In this study, central immune responses were examined using a novel rat model of preterm birth that combines chronic intermittent hypoxia–hyperoxia (CIHH) with Gram-positive bacterial infection. Rat pups at postnatal day 3 were exposed to 3-hour CIHH cycles for 10 days, followed by injection of peptidoglycan and lipoteichoic acid; control pups were raised in normoxia and received saline. At postnatal day 13, brain tissue was collected, and microglial morphology in key respiratory centres, the nucleus tractus solitarius (NTS) and intermediate reticular formation (IRt), was quantified using Iba-1 immunostaining, confocal imaging, and Fiji morphology analysis plugins (Clarke et al., 2021; Kim et al., 2024). Altogether, 4 experimental groups were analysed in both male and female pups: normoxia/saline, CIHH/saline, normoxia/infection, CIHH/infection.

CIHH altered microglial morphology in male pups, with perimeter and average branch length reduced by 26% and 20%, respectively, and branch number increased by 28% compared to normoxic controls. Such changes were accompanied by a shift towards hyper-ramified phenotype, with 36% of microglia displaying this morphology versus 20% amoeboid and 18% ramified. Similar effects were observed in both the NTS and IRt. In contrast, female pups showed different responses: in the NTS, cell perimeter and branch number increased by 45% and 49%, respectively, while no morphological changes were observed in the IRt.

Pups exposed to both CIHH and Gram-positive bacteria displayed microglial morphology resembling controls, suggesting opposing effects of CIHH and bacterial infection. Such interactions may impair microglial adaptation to brain milieu changes, contributing to maladaptive neural circuitry and disease phenotypes observed in preterm infants.