Effects Of A Local Auxiliary Protein On The Two-Dimensional Affinity Of A Tcr-Peptide Mhc Interaction

The affinity of T-cell receptors (TCRs) for major histocompatibility complex molecules (MHCs) presenting cognate antigens likely determines whether T cells initiate immune responses, or not. There exist few measurements of two-dimensional (2D) TCR-MHC interactions, and the effect of auxiliary proteins on binding is unexplored. Here, Jurkat T-cells expressing the MHC molecule HLA-DQ8-glia-alpha 1 and the ligand of an adhesion protein (rat CD2) were allowed to bind supported lipid bilayers (SLBs) presenting fluorescently labelled L3-12 TCR and rat CD2, allowing measurements of binding unconfounded by cell signaling effects or co-receptor binding. The 2D K-d for L3-12 TCR binding to HLA-DQ8-glia-alpha 1, of 14 +/- 5 molecules/mu m(2) (mean +/- s.d.), was only marginally influenced by including CD2 up to similar to 200 bound molecules/mu m(2) but higher CD2 densities reduced the affinity up to 1.9-fold. CeU-SLB contact size increased steadily with ligand density without affecting binding for contacts at up to similar to 20% of total cell area, but beyond this lamellipodia appeared, giving an apparent increase in bound receptors of up to 50%. Our findings show how parameters other than the specific protein-protein interaction can influence binding behavior at cell-cell contacts.