Porcine tonsils are the portal of entry of several critical pathogens of the pig, including PRRSV and CSFV. Amongst other cell types, cells of the myeloid lineage, including macrophages and dendritic cells, are responsible for the uptake of pathogens, antigen recognition and orchestrating the adaptive immune response.
Several myeloid cell populations have been identified in porcine blood, which include populations of cDCs, pDC and monocytes. Myeloid cells in porcine tissues have received less attention. In order to further define myeloid cells resident in the porcine tonsil we firstly excluded all cells expressing typical lineage markers from the dissociated tonsillar cells. The remaining enriched cells were then stained with an 8 colour flow cytometry panel including Live/Dead, lineage antibodies (CD21, CD8a, CD3, IgM), CD4, CD14, MHC-II, CD172a, CD163 and CADm1. The 5 cell populations identified were putatively assigned as, macrophages, pDCs (lin-CD172alow, MHC-IIneg/low, CD4+), cDC1 (lin-,CD172alow MHC-IIhigh CADm1high), cDC2 (lin-CD172ahigh, MHC-IIhigh, CADm1low/neg) and CD14+ DCs (lin-CD172ahigh, MHC-IIhigh, CD14low). Across all 5 subsets, freshly isolated cells expressed both MHC class II and CD80/86, and this expression increased substantially following 4 hour culture without stimuli, with highest levels consistently associated with cDC1, cDC2 and CD14+ DCs. Furthermore, CD14+ DCs (and macrophages) also showed the highest antigen processing capacity. As previously demonstrated in human and mouse, porcine myeloid cell subsets also showed differential ability to respond to different TLR agonists; macrophages and CD14+ DCs responded primarily to LPS (TLR4 agonist), which was aligned with monocytes. cDC1 DCs secreted TNF-α following polyI:C (TLR3 agonist) and IL-12 after CpG (TLR9 agonist) stimulation indicating parallels between pig and human cDC1 counterparts. Analysis of TLR and DC signature gene expression is currently being assessed by qRT-PCR to characterise the myeloid populations further.