Monoclonal antibodies to the synthetic adjuvant muramyl dipeptide: characterization of the specificity

Monoclonal antibodies to MDP were prepared by hybridization of NSO myeloma cells with spleen cells of BALB/c mice immunized with MDP conjugated to methyl-BSA. Hybridomas secreting anti-MDP antibodies were selected by the binding activity of their supernates to MDP-A--L using a radioimmunoassay. After cloning in soft agar, the specificities of monoclonal anti-MDP antibodies were assayed by an inhibition of ELISA with various derivatives of MDP. Fine structural analysis of specificity for one such clone (2-4) is reported. This antibody recognizes the N-acetyl-muramic acid (N-Ac-Mur) linked to the dipeptide but not N-Ac-Mur or/and dipeptide alone. The N-Ac group on muramic acid is an important antigenic determinant and the glycopeptide linkage seems to be crucial in presenting the sugar moiety. Conservative substitution of L-Ala (i.e. by L-Ser or L-Val) had no effect on the binding ability to the antibody whereas a radical change, i.e. replacement of L-Ala by L-Pro or N-methyl-L-Ala completely abolished the antigenicity of the molecule. There was no clear correlation between biological activities of various derivatives of MDP and their ability to react with this antibody. Some possible hypotheses explaining this lack of correlation are presented.