Amino acid residue substitution at t-cell determinant-flanking sites in β-lactoglobulin modulates antigen presentation to t cells through subtle conformational change

We compared T-cell responses to regions in residues 21-40 of A and B variants of bovine milk β-lactoglobulin (β-LG) that vary by two different amino acid residues at 64 and 118. Results showed that T cells from C57/BL6 and C3H/HeN mice immunized with peptide 21-40 or BALB/c mice immunized with peptide 21-32 or 25-40 responded more vigorously to β-LG B than to β-LG A. This difference in response to 25-40 in BALB/c mice was not observed when β-LGs B and A were denatured, suggesting that the conformation difference affects display of the determinant 25-40. Reactivity of anti-β-LG monoclonal antibodies and molecular modeling using molecular dynamics calculations revealed subtle differences in the three-dimensional structure of these two variants. Furthermore, substitution of two amino acid residues at sites distant from the T-cell determinant induced differential determinant display on antigen-presenting cells, possibly due to subtle conformational changes in β-LG.