Avian pathogenic Escherichia coli alters complement gene expression in chicken erythrocytes

1. Avian Escherichia coli (E. coli) causes significant losses in livestock by inducing morbidity and mortality. Erythrocytes, the most abundant in blood, possess dual functions of oxygen transportation and immune regulation. In recent years, the interaction between erythrocytes and the complement system has gradually become a focal point of study. However, the transcription dynamics of the complement system in chicken erythrocytes post-E. coli invasion remains unclear. 2. In this study, chicken erythrocytes and E. coli were co-cultured for 0.25–2 h to assess adhesion, analysed by indirect immunofluorescence (IIF) and scanning electron microscopy (SEM). Quantitative real-time PCR (qRT-PCR) examined differential expression of complement genes (CD93, C1q, C1s, C2, C3, C3AR1, C4, C4A, C5, C5AR1, C6, C7, C8G, CFI, MBL) in vitro using erythrocytes at 0.25–2 h and in vivo using chicks at 1, 3 and 7 d post-E. coli infection. 3. E. coli adheres to chicken erythrocytes, as observed using IF and SEM. Gene expression analysis revealed early downregulation of C4, C4A, MBL and late upregulation of CD93, C1q, C1s, C3, C3AR1, C5AR1, C6, with C5, C7, C8G downregulated at 7 dpi. C2 expression varied at each time point. 4. This study first showed E. coli adhering to erythrocytes, which activated complement genes rapidly. In vivo recovery from chickens with colibacillosis favours classical pathway activation, while lectin pathway may be inhibited, suggesting early immune down-regulation.