Immunological effects of an arginine side chain contaminating synthetically prepared peptides

The side chain, 4-methoxy-2,3,6-trimethylbenzenesulphonyl (Mtr), is a protective group coupled to arginine to mask the omega-nitrogen, in order to protect the guanidino function during peptide synthesis by the 9-fluorenylmethoxycarbonyl (Fmoc) procedure (Walker, 1994). This group is removed at the completion of peptide synthesis; however, the cleavage process can be incomplete. We have found that animals injected with a mixed population of pure, i.e. unmodified, and Mtr-containing MBP peptides have an immunodominant humoral response to the Mtr-bearing peptide. This response is dependent on the characteristics of the MBP peptide involved. For two MBP peptides, the Mtr-containing peptide had increased binding to antibody over pure peptide. For two other peptides, only the Mtr-containing peptide bound antibody while the unmodified peptide did not. In a separate system involving a polyclonal response to an unrelated peptide from beta2-microglobulin (beta2 m), the dominance of the Mtr group was also evident. These results provide further evidence that a small side chain on a single amino acid in a peptide can markedly alter the immunogenicity and antigenicity of that peptide for antibody reactivity. This evidence emphasizes the need for a critical awareness of each component of peptide synthesis and its potential to alter the immunoreactivity of the final product.