Identification of Liver Endothelial Cells as the Primary Site of IgM Catabolism in the Rat

Rat IgMs, both monoclonal protein from ascites fluid and total serum IgM, were purified by sequential gel filtration and metal chelate affinity chromatography on immobilized zinc-iminodiacetate. Two monoclonal IgMs, IR202 and IR968, chromatographed identically on gel filtration, but required different pHs for elution from the zinc affinity column. IR202 behaved like a euglobulin, being readily precipitated in low-ionic-strength buffers, while IR968 remained soluble under these conditions. IgM was isolated from serum in 30-50% yield by chromatographic procedures similar to those used for the monoclonal proteins, and 20-30% of the isolated serum IgM was precipitable as a euglobulin. The half-life of both monoclonal and serum euglobulin IgMs was 0.8 days, while the polyclonal globulin and IR968 had half-lives of 1.8 and 2.8 days, respectively, in the rat circulation. The tissue and cellular sites of catabolism of the monoclonal IgMs were determined after labeling with the residualizing label, dilactitol-[125I]tyramine. For both proteins the liver was identified as the major tissue site of catabolism, accounting for 60-80% of degraded protein in the body. When liver was fractionated into parenchymal and nonparenchymal cells (NPC), the NPC were found to account for 86 and 69% of protein recovered in liver, for IR202 and IR968, respectively. Separation of NPC into endothelial (EC) and Kupffer cell populations by elutriation centrifugation revealed that EC contained the majority, ∼70% of total NPC radioactivity from either IgM. Based on the ratios of endocytic indices (μl of plasma/106 cells/day) for each cell type, the EC also had a higher efficiency for uptake of both IgMs, approximately threefold greater, than for the fluid phase marker, polyvinylpyrrolidone, or for rat serum albumin. We conclude that hepatic EC are a major site of IgM catabolism, regardless of the heterogeneity in physical and biological properties of various IgM populations.