Endogenous viral antigen processing generates pepvtide-specific MHC class I cell-surface clusters

Sensitivity is essential in CD8+ T-cell killing of virus-infected cells and tumor cells. Although the affinity of the T-cell receptor (TCR) for antigen is relatively low, the avidity of T cell-antigen-presenting cell interactions is greatly enhanced by increasing the valence of the interaction. It is known that TCRs cluster into protein islands after engaging their cognate antigen (peptides bound to MHC molecules). Here, we show that mouse K^b class I molecules segregate into preformed, long-lasting (hours) clusters on the antigen-presenting cell surface based on their bound viral peptide. Peptide-specific K^b clustering occurs when source antigens are expressed by vaccinia or vesicular stomatitis virus, either as proteasome-liberated precursors or free intracellular peptides. By contrast, K^b-peptide complexes generated by incubating cells with synthetic peptides are extensively intermingled on the cell surface. Peptide-specific complex sorting is first detected in the Golgi complex, and compromised by removing the K^b cytoplasmic tail. Peptide-specific clustering is associated with increased T-cell sensitivity: on a per-complex basis, endogenous SIINFEKL activates T cells more efficiently than synthetic SIINFEKL, and wild-type K^b presents endogenous SIINFEKL more efficiently than tailless K^b. We propose that endogenous processing generates peptide-specific clusters of class I molecules to maximize the sensitivity and speed of T-cell immunosurveillance.