Primary in vivo responses to ovalbumin. Probing the predictive value of the Kb binding motif

CD8+ cytolytic T cells recognize Ag presented by MHC class I molecules on the surface of target cells. It is known that presenting cells process nascent protein into Peptides of approximately eight to nine Amino acids which bind to the peptide groove of MHC class I and are transported to the cell surface. Recently, several laboratories have postulated that each MHC class I haplotype has a binding motif of at least two Amino acids nested within the peptide. One such motif is XXXXF/YXXL which binds to the mouse MHC class I molecule, H2-Kb, and can be found in the known antigenic peptide from OVA at Amino acids 257-264. By using the motif to scan OVA five Peptides were found that fit this pattern, OVA 11-18, OVA 55-62, OVA 107-114, OVA 176-183, and OVA 257-264. Binding studies revealed that three out of the five Peptides (OVA 55-62, OVA 176-183, and OVA 257-264) bind to MHC class I. To test the natural antigenicity of the predicted Peptides, C57BL/6 mice were immunized with OVA containing immunostimulating complexes to elicit a MHC class I-driven response to naturally processed OVA. The cytolytic potential of the responding T cell population was tested in vitro by using EL-4 cells preincubated with the predicted synthetic Peptides as targets. The known antigenic peptide OVA 257-264 elicited a strong response; however, OVA 176-183 was also recognized while the remaining three were not recognized. The CTL response did not strictly correlate with the ability of the selected Peptides to bind Kb, for example, OVA 55-62 was able to bind Kb efficiently, yet elicited no cytolytic response. In addition, the plasticity of the peptide-binding motif was probed by making amino acid substitutions, and as a result the motif proved to be more flexible than previously suspected. This represents the first report of a Kb-associated CTL epitope within OVA other than OVA 257-264. It also demonstrates the predictive quality of the Kb-binding motif; however, not all predicted Peptides were recognized by primary OVA-induced CTL, implying more rules of processing and binding are needed.