COVALENT DIMERIZATION OF CD28 CTLA-4 AND OLIGOMERIZATION OF CD80/CD86REGULATE T-CELL COSTIMULATORY INTERACTIONS/

T lymphocyte receptors CD28 and CTLA-4 bind costimulatory molecules CD 80 (B7-1) and CD86 (B7-2) on antigen presenting cells and regulate T c ell activation, While distinct functional roles have been ascribed to each of these molecules, little is known about how they interact, To b etter characterize these interactions, we have used surface plasmon re sonance to perform equilibrium and kinetic binding analyses of extrace llular fragments of CD28/CTLA-4/CD80/CD86. We show that CTLA-4 and CD2 8 binding are both characterized by rapid kinetic on-rates and rapid d issociation rates, Native disulfide-linked homodimers of CD28 and CTLA -4 bound with two kinetically distinct binding sites, one of high avid ity and slow dissociation and one of low avidity and more rapid dissoc iation, Monomeric CTLA-4 bound only with low affinity and rapid dissoc iation, Therefore, covalent dimerization of CTLA-4 is required for its high avidity binding. Oligomerization of CD80/CD86 is also required f or high avidity CTLA-4 binding since CTLA-4 bound with low avidity to monomeric CD86, This contrasts with the ability of CD80/CD86 on antige n presenting cells to bind CTLA4Ig with high avidity and predicts thei r organization as oligomers or clusters that permit multivalent bindin g. Thus, covalent receptor dimerization and ligand oligomerization are two key features of the CD28/CTLA-4/CD80/CD86 receptor System that co ntrol ligand binding and may regulate signal transduction by controlli ng the duration of receptor occupancy.