Splenic clearance of heated red blood cells: Congestion, sequestration, erythrophagocytosis and erythropoiesis and the role of the splenic lobule

Abstract

The mechanism by which the spleen clears damaged red blood cells (RBC) from the blood is based on the unique open circulation of the spleen which allows blood to leave the confines of the closed vascular system and enter the filtration beds, or pulp cords. Damaged RBC are sequestered in these specialized vascular beds and then destroyed by resident macrophages. Neither the open circulation nor the clearance process are well understood. The aim of this thesis was to provide fundamental information on the structure and function of the spleen and its behavior during the clearance process. Heated RBC (HRBC) were used as a simplified model of damaged RBC. This model avoided complicating factors such as anemia, infection and chronicity and so could be used to identify those aspects of clearance that are specific responses to damaged RBC. A single bolus of isologous donor HRBC was injected into a series of mice and the onset, progression and resolution of a discrete episode of HRBC destruction in the spleen was studied by transmission electron microscopy, light microscopy and tissue culture. Within minutes of injection, vascular changes including hyperemia and changes in interendothelial slit patency congested the filtration beds, creating a low shear environment in which large numbers of RBC came in direct contact with macrophages and reticular cells. While RBC were temporarily trapped in congested filtration beds, HRBC adhered to reticular cells and thus became sequestered there. Macrophages culled most HRBC within two hours with the assistance of reticular cells and barrier cells. HRBC clearance stimulated splenic erythropoiesis and monocytopoiesis. Erythropoiesis occurred in the absence of anemia and elevated serum erythropoietin and was ultimately ineffective. The splenic lobule was central to the clearance process and its anatomy was explored in detail. The vascular, cellular and proliferative responses to damaged RBC were strikingly similar to those of inflammation and tissue repair. We hypothesize that clearance is a modified form of inflammation and show how inflammatory processes are modified by the complex anatomy of the splenic lobule to remove damaged cells from the blood, thereby sparing the circulation from harmful effects of intravascular inflammation