A potent neutralizing monoclonal antibody can discriminate amongst IFNγ from various primates with greater specificity than can the human IFNγ receptor complex

Abstract

A monoclonal antibody (AF2) generated against recombinant human interferonγ (IFNγ) exhibited potent IFNγ neutralizing activity and prevented human IFNγ from binding to the cell surface IFNγ receptor complex. The AF2 antibody also neutralized IFNγ from higher primates (superfamily Hominoidea) but did not react with IFNγ from rhesus or other primates in the suborder Anthropoidea. IFNγ from all primates tested, however, could signal via the human IFNγ receptor complex, as indicated by the ability to upregulate the level of MHC class II molecule expression on the surface of a responsive human cell line. We cloned and sequenced the IFNγ gene from chimpanzee, gorilla, orangutan, and gibbon, and compared those with the previously reported IFNγ sequences of human, rhesus, baboon and marmoset. This comparison revealed that, of the primate IFNγs that were not reactive with AF2, rhesus IFNγ was most homologous to human IFNγ, differing at only nine amino acids and containing a one amino acid deletion. Comparing the sequence of human IFNγ with that of rhesus IFNγ suggested residues of the human IFNγ molecule that were involved in the formation of the epitope recognized by the AF2 antibody. Constructing human/rhesus chimeric IFNγ molecules, combined with site-directed mutagenesis of both human and rhesus IFNγ revealed that this epitope was dependent upon two non-contiguous amino acids that are juxtaposed in the tertiary structure of IFNγ. The determinant recognized by AF2 antibody resides in a portion of IFNγ that is proximal to, but distinct from the surface that interacts with the IFNγ receptor. Therefore, this neutralizing monoclonal antibody reacts with a conformational determinant that distinguishes primate IFNγs serologically, but not functionally.