Fig. 6.

Ii expression influences the endocytosis of class II molecules. Cell surface proteins expressed on the fibroblast cell lines M1.DR4 and M1.DR4.Ii, were biotinylated with sulfo-NHS-SS-biotin. The cells were warmed to allow internalization of these biotin-labeled surface molecules, followed by cleavage of residual biotin from cell surface proteins with a glutathione solution. The biotin-labeled endocytosed class II molecules and transferrin receptors (TfR) were then detected in a capture ELISA, and the percent endocytosis was calculated as described in Materials and Methods. Data is a representative of at least three separate experiments and expressed as percent class II endocytosis ± SEM.