Allergenic epitopes of bovine alpha S1-casein recognized by human IgE and IgG.

B-cell epitopes of bovine alpha S1-casein, one of the major allergens of cow's milk, were identified by a screening method based on synthetic peptides. According to the known amino acid sequence of alpha S1-casein, a set of 188 overlapping sequential decapeptides shifted by one amino acid was manually synthesized on polyethylene pins by the 9-fluorenyl-methoxycarbonyl (Fmoc) method. Peptides were screened by an enzyme-linked immunosorbent assay (ELISA) specific for human IgE and IgG. Bound antibodies were detected by successive incubation with up to three polyclonal antibodies, the last one conjugated to horseradish peroxidase. Tested sera were from 15 patients with acute clinical reactions to cow's milk and IgE-specific reactions to bovine alpha-casein in the ELISA and immunoblot. Sera from 10 healthy subjects without remarkable reactions to cow's milk proteins were used as controls. All sera from allergic subjects showed reactions with three regions of alpha S1-casein, corresponding to amino acids 19-30, 93-98, and 141-150. Furthermore, individual sera showed reactions with other parts of the protein. No essential differences in the epitope specificity of IgE and IgG were found. Inhibition of IgE binding to alpha S1-casein with soluble synthetic peptides confirmed the results and revealed peptide CN-2 as the most inhibiting one.