Homotypic Neonatal and Adult Inflammation Primes Adult Microglia Without Impairing Adult Breathing
Authors: Beyeler Sarah A. , Naidoo Robyn , Plunkett Deanna L. M. , Huxtable Adrianne G. 
Published: May 2022

A population of adults are predisposed to breathing deficits during subsequent illness and disease, yet it is unknown what factor(s) contribute to this adult vulnerability. Since neonatal inflammation primes adult microglia in non‐respiratory control regions and augments peripheral immune responses, we hypothesized that neonatal and adult homotypic inflammation (same inflammatory stimulus at both ages) primes male and female microglia in respiratory control regions, contributing to impairments in adult breathing during subsequent inflammatory challenges. Using lipopolysaccharide (LPS) to induce homotypic inflammation in neonates (1mg/kg LPS, P4) and adults (24µg/kg LPS, i.p.), medullary microglia were primed in adult males (neonatal LPS + adult LPS: 25±4% microglia, n=9; neonatal LPS + adult saline: 18±2% microglia, n=9; neonatal saline + adult saline: 10±1% microglia, n=9; p<0.01) and adult females (neonatal LPS + adult LPS: 25±3% microglia, n=9; neonatal LPS + adult saline: 14±2% microglia, n=6; neonatal saline + adult saline: 10±2% microglia, n=8; p<0.0001). However, spinal cord microglia were only primed in adult males (neonatal LPS + adult LPS: 13±2% microglia, n=9; neonatal LPS + adult saline: 9±2% microglia, n=9; neonatal saline + adult saline: 10±2% microglia, n=9; p<0.01) and not adult females (neonatal LPS + adult LPS: 13±2% microglia, n=9; neonatal LPS + adult saline: 9±1% microglia, n=6; neonatal saline + adult saline: 9±1% microglia, n=8; p<0.01). Thus, neonatal and adult homotypic inflammation region‐ and sex‐dependently prime microglia in respiratory control regions. To test whether primed microglia in respiratory control regions contribute to deficits in adult breathing, breathing was assessed by plethysmography in adults at two time points: during the peak inflammatory response (3hrs) and peak microglial migration and proliferation (24hrs). Neither adult eupneic breathing nor chemoreflexes (hypercapnic ventilatory response, HCVR, and hypoxic ventilatory response, HVR) were primed in males or females by neonatal and adult homotypic inflammation at either time point. Interestingly, adult female chemoreflexes were time‐dependently increased by neonatal and adult homotypic inflammation. In adult females, neonatal and adult homotypic inflammation increased the peak HCVR at 3hrs (neonatal LPS + adult LPS: 298±112% ∆VE from baseline, n=6) compared to 24hrs (neonatal LPS + adult LPS: 169±41% ∆VE from baseline, n=6, p=0.04), and increased the peak HVR at 3hrs (neonatal LPS + adult LPS: 159±37% ∆VE from baseline) compared to 24hrs (neonatal LPS + adult LPS: 62±23% ∆VE from baseline, n=6, p=0.02). Thus, adult female chemoreflexes were time‐dependently impacted by the combination of neonatal and adult homotypic inflammation, suggesting time‐specific effects of adult inflammation. Despite no significant impact of primed microglia on baseline breathing and chemoreflexes, primed respiratory control microglia have the potential to significantly impair respiratory control during more severe inflammatory challenges associated with illness and disease. Further, sex differences in primed respiratory control microglia suggest sex‐specific vulnerability to adult ventilatory control disorders.