“Double-duty” dendritic cells in Xenopus laevis: Early vertebrate splenic APCs resemble conventional dendritic cells and limit native antigen presentation to B cells during a humoral response

XL cells are class II^hi, Ig-bearing cells found in splenic B cell follicles of the amphibian Xenopus laevis, bearing native antigen after immunization. XL cells are proposed to be myeloid cells serving the functions of both classical dendritic cells (cDC) and mammalian follicular dendritic cells (FDC). Here, we compare the function and gene expression of XL cells with peritoneal macrophages (pMac), which are also class II^hi/Ig-bearing cells. While pMacs were esterase-positive, adherent to plastic, and highly phagocytic, XL cells, like mammalian DC, adhered weakly to plastic, were esterase-low-to-negative, and were weakly phagocytic. Splenic red pulp macrophages, like pMacs, were esterase-positive. Both pMacs and XL cells bound to complement activated on zymosan particles. RNAseq was also done with IgY FACS-purified pMacs and XL cells. The pMac transcriptome was consistent with the classical macrophage lineage with high Mafb, myeloperoxidase, and CEBPA. In contrast, XL cells expressed cDC markers, chemokines CXCL13 and CCL19/CCL21, and the FDC marker VCAM1, consistent with their presumed function of attracting lymphocytes and presenting antigen. Mammalian FDC can present native antigen up to one year within B cell follicles, while ectotherms lack FDC but have myeloid cells that trap native antigen for unknown lengths of time. As shown previously, XL cells bear fluorescent antigen on their surface 16 days after immunization. However, we now show that antigen disappears by D30. Our data suggest that the myeloid XL cells indeed act as APC during a humoral immune response, but do not serve as an antigen depot to sustain memory B cells.