SALs for the direct determination of HLA restriction. (A) Schematic demonstrating the model system. Mouse fibroblasts are stably transfected with a single HLA molecule, and Pepmix is added to these cells in culture. After incubation, Pepmix is removed so that the only antigen present is what is bound to and presented by the SAL. SALs are then cocultured with VSTs and intracellular flow cytometry is performed. (B-C) Two retrospective examples of using SALs to determine HLA restriction and correlate with clinical outcomes. In panel B, the coculture of a VST product given to patient UPN-69 shows IFN-γ positivity only with DRB1*03:01 SALs, indicating restriction through that allele. The patient and product did not match at DRB1*03:01 and had no clinical response. By contrast, in Figure 2C, a coculture of the VST product given to patient UPN-32 shows restriction by SALs through both DRB1*03:01 and DRB1*07:01. The patient and product matched at DRB1*07:01 and had a robust clinical response.