Differential Expression of Ovine Innate Immune Genes by Preterm and Neonatal Lung Epithelia Infected with Respiratory Syncytial Virus

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Abstract
Preterm infants have increased susceptibility to severe manifestations of respiratory syncytial virus (RSV) infection. The cause(s) for this age-dependent vulnerability is/are not well-defined, but alterations in innate immune products have been implicated. In sheep, RSV disease severity has similar age-dependent characteristics and sheep have several related innate molecules for study during pulmonary infection including surfactant protein A (SP-A), surfactant protein D (SP-D), sheep beta defensin 1 (SBD1), monocyte chemotactic protein 1 (MCP1), and Toll-like receptor 4 (TLR4). However, the in vivo cellular gene expression as a response to RSV infection is poorly understood. In this study, the effect of RSV infection on expression of these innate immune genes was determined for bovine RSV-infected (bRSV+ fluorescence) epithelial cells, adjacent cells lacking bRSV antigen (adjoining cells lacking fluorescence), and control cells from non-infected lung using laser capture microdissection (LCM) and real-time RT-PCR. Control lambs had increased expression of innate immune molecules in full term (term) compared to preterm epithelia with statistical significance in SBD1, SP-D, and TLR4 mRNA. Infected cells (bRSV+ fluorescent cells) had consistently higher mRNA levels of SP-A (preterm and term), MCP1 (preterm and term), and SP-D (preterm). Interestingly, bRSV- cells of infected term lambs had significantly reduced SP-D mRNA expression compared to bRSV+ and control epithelia, suggesting that RSV infected cells may regulate the adjacent epithelial SP-D expression. This study defines specific innate immune components (e.g., SBD1, SP-D, and TLR4) that have differential age-dependent expression in the airway epithelia. Furthermore, cellular bRSV infection enhanced certain innate immune components while suppressing adjacent cellular SP-D expression in term animals. These in vivo gene expression results provide a framework for future studies on age-dependent susceptibility to RSV and RSV pathogenesis.