Development and function of chicken XCR1 + conventional dendritic cells

Conventional dendritic cells (cDCs) are antigen-presenting cells (APCs) that play a central role in linking innate and adaptive immunity. cDCs have been well described in a number of different mammalian species, but remain poorly characterised in the chicken. In this study, we use previously described chicken cDC specific reagents, a novel gene-edited chicken line and single-cell RNA sequencing (scRNAseq) to characterise chicken splenic cDCs. In contrast to mammals, scRNAseq analysis indicates that the chicken spleen contains a single, chemokine receptor XCR1 expressing, cDC subset. By sexual maturity the XCR1 cDC population is the most abundant mononuclear phagocyte cell subset in the chicken spleen. scRNAseq analysis revealed substantial heterogeneity within the chicken splenic XCR1 cDC population. Immature MHC class II (MHCII) XCR1 cDCs expressed a range of viral resistance genes. Maturation to MHCII XCR1 cDCs was associated with reduced expression of anti-viral gene expression and increased expression of genes related to antigen presentation via the MHCII and cross-presentation pathways. To visualise and transiently ablate chicken XCR1 cDCs , we generated -iCaspase9-RFP chickens using a CRISPR-Cas9 knockin transgenesis approach to precisely edit the locus, replacing the XCR1 coding region with genes for a fluorescent protein (TagRFP), and inducible Caspase 9. After inducible ablation, the chicken spleen is initially repopulated by immature CD1.1 XCR1 cDCs. XCR1 cDCs are abundant in the splenic red pulp, in close association with CD8 T-cells. Knockout of XCR1 prevented this clustering of cDCs with CD8 T-cells. Taken together these data indicate a conserved role for chicken and mammalian XCR1 cDCs in driving CD8 T-cells responses.