Characterization of conventional and plasmacytoid dendritic cells in swine secondary lymphoid organs and blood

Dendritic cells (DCs) act as antigen presenting cells that bridge innate and adaptive immune systems with the unique capacity to initiate primary T-cell responses and efficiently stimulate memory responses. In pig, little information is available about these cells in secondary lymphoid organs, the place where T cell activation usually occurs. As increased knowledge on DC is a necessary prerequisite to further understand their role in response to microbial infection or in protection after vaccination, we investigated the DC types that would be present in tonsil, spleen and non-subcutaneous lymph nodes in the steady state. One population was composed of CD172a(+)CD11R1(+)CD1(+/-)CD80/86(+/-) cells and would correspond to conventional DCs (cDC), while the other one was composed of CD172a(+)CD4(+)CD1(+/-)CD80/86(+/-) cells and would correspond to plasmacytoid DCs (pDC). These subsets were also detected in blood but spleen was the tissue with the higher frequency of such DCs. In lymphoid organs, most of cDC and pDC were in an immature status, as revealed by the low percentage of cells expressing the co-stimulatory molecule CD80/86. However, expression of that marker by 5% of DCs in organs and up to 15% in blood, together with lower expression of CD1a and expression of CD208, would indicate a partial activation and/or semi-maturation. Interestingly, 8% of tonsil pDC and 15% of blood pDC were shown to secrete IFN-alpha, while 18-20% of cDC expressed TNF-alpha in these tissues. Both cell types also expressed IL-12 and IL-10 in the steady state. Measurements of IFN-alpha, TNF-alpha, IL-12 and IL-10 levels in serum confirmed their production within immune homeostasis, whereas IL-6, IL-18 and IFN-gamma could not be detected. Altogether, these data complete knowledge on porcine immune system cells and will be a useful tool for further in vivo studies on porcine DC role in peripheral tolerance induction and in immune responses to pathogens.