Differences in pulmonary innate lymphoid cells are dependent on mouse age, sex and strain

Innate lymphoid cells (ILC) are resident in the lung and are involved in both the maintenance of homeostasis and the pathogenesis of respiratory diseases. In this study, murine lung ILC were characterised using flow cytometry and the impact of mouse age, sex and strain were assessed. Lung ILC were found as early as postnatal day 4 and numbers peaked at 2 weeks, and then decreased as the lung matured. During postnatal lung development, ILC expressed differential amounts of ILC2-associated cell surface antigens including ST2, CD90.2 and ICOS. Using Il5venusIl13td-tomato dual reporter mice, neonates were found to have increased constitutive IL-13 expression compared to adult mice. Neonates and adults had similar ratios of IL-5+CD45+ leukocytes, however, these cells were mostly composed of ILC in neonates and T cells in adults. Sex-specific differences in ILC numbers were also observed, with females having greater numbers of lung ILC than males in both neonatal and adult mice. Female lung ILC also expressed higher levels of ICOS and decreased KLRG1. Mouse strain also impacted on lung ILC with BALB/c mice having more ILC in the lung and increased expression of ST2 and ICOS compared with C57BL/6J mice. Collectively, these data show that lung ILC numbers, cell surface antigen expression, IL-5 and IL-13 levels differed between neonatal and adult lung ILC. Additionally, cell surface antigens commonly used for ILC2 quantification, such as ST2, CD90.2, and ICOS, differ depending on age, sex and strain and these are important considerations for consistent universal identification of lung ILC2.