CD4/major Histocompatibility Complex Class II Interaction Analyzed with CD4- and Lymphocyte Activation Gene-3 (LAG-3)-Ig Fusion Proteins

We analyzed CD4 major histocompatibility complex (MHC) class II interactions with CD4 and lymphocyte activation gene (LAG)-3 recombinant fusion proteins termed CD4Ig and LAG-3Ig. CD4Ig bound MHC class II molecules expressed on the cell surface only when used in the micromolar range. This weak CD4Ig binding was specific, since it was inhibited by anti-CD4 and anti-MHC class II mAb. LAG-3Ig bound MHC class II molecules with intermediate avidity (Kd = 60 nM at 37 degrees C). Using LAG-3Ig as a competitor in a CD4/MHC class II-dependent cellular adhesion assay, we showed that this recombinant molecule was able to block CD4/MHC class II interaction. In contrast, no inhibition was observed in a CD4/MHC class II-dependent T cell cytotoxicity assay. Together, these results suggest that co-engagement of the TcR with CD4 alters the CD4/MHC class II molecular interaction to become insensitive to LAG-3Ig competition.