Fig. 1. Antibody uptake is dependent upon the presence of the CD8 lumenal domain. (A) CD8-CIMPR-expressing cells were mixed with untransfected HeLa cells and incubated with anti-CD8 continuously for 3 hours. After fixation, the cells were labelled with anti-SNX1 (Snx1) antibodies and fluorescently labelled anti-mouse antibodies. The anti-CD8 antibodies are detectable only in some cells, even at high exposure. (B) CD8-CIMPR or untransfected HeLa cells (Helas) were seeded at similar density into a 24-well tissue culture dish. Four wells of each cell line were incubated with anti-CD8 for 3 hours, fixed, permeabilised and then incubated with ¹²⁵I-protein-A. Four wells of each were incubated without anti-CD8 before fixation and incubation with ¹²⁵I-protein-A and the other four wells were lysed in SDS-PAGE sample buffer and subjected to SDS-PAGE. The transferin receptor (TfnR) and CD8-CIMPR were detected by western blotting (shown in the inset). Only cells expressing the CD8-CIMPR and incubated with the anti-CD8 antibody bound the ¹²⁵I-protein-A.