Oligomeric state makes important contributions to the signaling mechanisms of costimulatory molecules. In this study we address the biological relevance of the disulfide-linked dimeric structure of CD28. Fluorescence Resonance Energy Transfer (FRET) demonstrates that removal of the interdomain disulfide bond (C123) does not interfere with the formation of CD28 oligomers on the cell surface. Although the C123S mutant shows 40% lower binding affinity to the ligand B7-1, it is able to costimulate anti-CD3-induced IL-2 production but at a lower level (150%) compared to the wild-type (270%). Interestingly, binding to B7-2 was not affected. Thus, the covalently linked dimeric structure of CD28 represents an important mechanistic determinant for the optimal costimulatory activity in the immunological synapse.
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