Erythrophagocytosis by plasmacytoid dendritic cells and monocytes is enhanced during inflammation

BACKGROUNDGeneration of antibodies against red blood cell (RBC) antigens can be a clinically significant problem. The underlying mechanisms that regulate the production of RBC antibodies are only partially understood; however, factors such as inflammation significantly increase the rates of RBC antibody generation. Humoral alloimmunization begins with consumption of transfused RBCs by antigen-presenting cells (APCs). Recently, it has become appreciated that there are multiple different types of APCs. The relative contribution of APC subsets to RBC antibodies has not been described in either the quiescent or the inflamed states. STUDY DESIGN AND METHODSTo evaluate the types of APCs that consume RBCs, and how inflammation affects this process, C56Bl/6 mice were treated with polyinosinic-polycytidylic acid (poly(I:C)) to induce an inflammatory response and/or were transfused with 3,3-dihexadecyloxacarbocyanine perchlorate-labeled syngeneic RBCs. Erythrophagocytosis (both at baseline and during inflammation) was analyzed for different subsets of macrophages (M), dendritic cells (DCs), B cells, and monocytes, by a combined approach using flow cytometry and fluorescent microscopy technology. RESULTSIn four independent experiments, erythrophagocytosis at baseline was predominately performed by red pulp M phi; however, during inflammation both plasmacytoid DCs (pDCs) and monocytes increased RBC consumption. Furthermore, pDCs up regulated MHC-II and activation markers CD80 and CD86. In addition to changing patterns of erythrophagocytosis, inflammation also led to a significant decrease in CD11c+ conventional DC populations and an increase in granulocytes. CONCLUSIONSThe nature of APCs that consume transfused RBCs is changed by inflammation. Given that APCs initiate humoral immune responses, these findings provide potential mechanistic insight into how inflammation regulates RBC alloimmunization.