Macromolecular charge and reticuloendothelial function: Comparison between the kinetics of administered native and cationized ferritins and the corresponding immune complexes in the mouse

In order to evaluate the role of macromolecular charge on uptake by the reticuloendothelial system (RES), kinetic studies were carried out following the intravenous administration of ¹²⁵I-labelled native ferritin (NF, pI 4.5) or cationized ferritin (CF, pI 7) to Swiss-Webster female mice subsequently killed at 2, 4, 8, 24 and 36 hr later. The same experiments were performed following the administration of radiolabelled (¹²⁵I) native ferritin immune complexes (NFIC, pI 5-6.5) and cationized ferritin immune complexes (CFIC, pI (6.5-7.5). These complexes were prepared in five-times antigen excess by combination of affinity-purified anti-ferritin IgG-¹²⁵I with NF or CF. A striking difference between the plasma clearance of NF and that of CF was observed in that the former was rapidly eliminated within 8 hr whereas the latter persisted in the circulation at 24 hr. This was associated with a significant increase in the uptake of NF by the liver, spleen, and kidney. No differences were observed in blood cell-associated radioactivity. Immunohistochemical studies confirmed the presence of increased amounts of NF in Kupffer cells and splenic phagocytes. Thus, the uptake of ferritin by components of the RES is highly dependent upon its pI. The present data may be explained by differences in diffusibility of CF and NF or alternatively by differential interactions with the cell surface in vivo. Contrary to the prior investigation carried out with the antigens alone, the plasma clearance and organ (liver, spleen and kidney) kinetic studies of NFIC and CFIC were similar. In addition, immunohistochemical studies demonstrated that the uptakes of NFIC (pI 5-6.5), CFIC (pI 6.5-7.5) and CFIC (pI 7-9) by Kupffer cells and splenic phagocytes were similar. As further confirmation for similarity in binding to Fc receptors of human polymorphonuclear leucocytes, Scatchard analysis failed to demonstrate any differences between NFIC and CFIC. These studies provide evidence that, within the range employed in this investigation, the charge of ferritin within the immune complex (and hence the charge of the complex itself) does not affect its uptake by receptors of phagocytic cells. In contrast, the uptake of ferritin, which is not Fc or C3 receptor dependent, is clearly conditioned by electrostatic charge.