Antibodies raised against peptide fragments of bovine alpha s1-casein cross-react with the intact protein only when the peptides contain both B and T cell determinants.

In order to compare the immune response to peptides with that to the native protein, we used bovine alpha s1-casein (alpha s1-CN), a major milk protein whose conformation seems to be in a highly disordered state in solution, as the model antigen. Firstly, the T and B cell determinants on this protein were localized by a T cell proliferation assay and an enzyme-linked immunosorbent assay (ELISA), using 13 synthetic overlapping peptides encompassing the entire sequence of alpha s1-CN. T cells from alpha s1-CN-primed C3H/He mice, a high responder to alpha s1-CN, strongly proliferated in the presence of peptides 91-110, 61-80 and 46-65. In addition, peptides 151-170, 136-155 and 106-125 were also found to contain T cell determinants. On the other hand, peptides 181-199, 46-65, 76-95 and 106-125 were generally found to be immunodominant B cell determinants, while peptides 121-140, 136-155, 91-110 and 151-170 also had antibody-binding activity. The peptides were then tested for their ability to elicit a specific antibody. This revealed that only peptides 91-110, 106-125, 136-155 and 46-65 were able to produce specific antibodies that bound to the native protein as well as the peptides themselves. These peptides contained both B and T cell determinants on the intact protein. Thus, we confirmed that a peptide corresponding to both T and B cell determinants on alpha s1-CN was capable of eliciting a specific antibody that reacted with the protein as well as the peptide itself.