Resumen de Crosstalk between innate and adaptive immunity in teleost fish : The role of dendritic cells and B cell regulatory cytokines
Dendritic cells (DCs) are the most specialized antigen presenting cells (APCs), being a key link between innate and acquired immunity. A previous study in our group identified a cell subset that expresses CD8α and major histocompatibility complex II (MHC II) on the cell membrane in rainbow trout skin (Oncorhynchus mykiss) (CD8+ DCs). These CD8+ cells shown phenotypical and functional characteristics of cross-presenting DCs such us expression of CD8, CD103, CD141, Batf3, and others. In this work, we report the identification of homologue populations in rainbow trout gills and intestine. Consequently, we have carried out a broad phenotypic and functional characterization of these new populations that includes the confirmation of novel capacities for DCs in teleost. Interestingly, phenotypic and functional differences were found between the three populations suggesting that the location of DCs strongly shape their mature stage and their immune capacities. Despite the differences found among these populations, some common characteristics were identified in all subset such as the expression of cross presenting markers and regulatory B cell cytokines belonging to the tumor necrosis factor (TNF) superfamily of ligands such us B cell activating factor (BAFF) a proliferation-inducing ligand (APRIL) and BAFF and APRIL like molecule (BALM).
These members of the TNF superfamily are produced mainly by innate cells like DCs and play and important role in activation and differentiation of B cells. For this reason, we have studied how these cytokines and their potential receptors are regulated throughout an inflammation model. To carry this out, we have studied the response of the peritoneal cavity to an intraperitoneal injection (i.p) with viral hemorrhagic septicemia virus (VHSV), given that previous studies had described that this inflammatory process is mainly mediated by IgM+ B cells. We first performed a transcriptional analysis in total leukocytes from peritoneum, observing an up regulation of the BAFF and BAFF receptor (BAFF-R) mRNA levels after the (i.p) of VHSV. On the other hand, when we analyzed the regulation of these genes in sorted IgM+ B cells, we observed increases in the levels of transcription of BAFF, APRIL, BALM and their receptors. These results reveal that these cytokines as well as their receptors play a key role in peritoneal inflammation processes mediated by IgM+ B cells.
Finally, having demonstrated that fish DCs also produce APRIL, we studied the functionality of this cytokine on IgM+ B cells, using in this case splenic B cells. We have compared the effects produced by APRIL on IgM+ B cells to those previously reported for BAFF, to increase our knowledge on the relation between these phylogenetically close cytokines. In the current work, we have demonstrated that APRIL induces an exclusive proliferation of IgM+ B cells, increases the number of IgM secreting cells and also increases the levels on MHC II on the surface of these cells as well as their antigen processing capacity.
The results of this study increase our knowledge concerning the crosstalk between innate and adaptive immune responses and on how B cells are regulated by innate cells such as DCs. This knowledge will be essential for the rational design of vaccines, one of the most important goals of aquaculture at the moment
