Down-regulation of cell surface CD4 molecule expression induced by anti-CD4 antibodies in human T lymphocytes

Antigenic modulation was defined as the down-regulation of a cell surface antigen expression induced by exposure to specific antibody. We investigated the modulation of CD4 surface expression in human peripheral blood lymphocytes incubated in vitro with anti-CD4 monoclonal antibodies (mAbs). Modulation of surface CD4 was achieved at 37 degrees C, but not at 4 degrees C, with five different murine anti-CD4 mAbs of IgG1 and IgG2a subclasses, with different epitope specificities. Modulation was dose dependent with a maximum at nonsaturating mAb concentration. It was reversible upon culture in mAb-free medium. It was accelerated and amplified in the presence of monocytes or after cross-linking of anti-CD4 mAbs. It could be induced with solid phase anti-CD4 mAbs, but not with soluble F(ab')2 fragments. Its magnitude was identical on all CD4+ lymphocytes. It was associated with a moderate down-regulation of CD2 and CD3 but not of CD8 and HLA class I surface expression. Modulation was slightly augmented by addition of inhibitors of the endosome/lysosome pathway but not by protein synthesis inhibitors. The anti-CD4 mAb initially bound to cell surface was no longer detectable after 24 hr of culture. Most of surface CD4 proteins complexed with antibody were rapidly internalized and transiently replaced by CD4 from an intracytoplasmic pool and then no longer were expressed. CD4 mRNA was moderately decreased in cells incubated with anti-CD4 mAb while beta-actin and beta 2-microglobulin mRNAs remained at stable levels. It was concluded that down-regulation of CD4 surface expression induced by anti-CD4 mAb concerned only a part of CD4 molecules and was associated with a decreased synthesis. The delay required to achieve maximal modulation is likely to reflect exhaustion of the intracytoplasmic recycling pool of CD4 molecules.