Crystal structure of an anti-anti-idiotype shows it to be self-complementary

The structure of the Fab fragment of the mouse anti-anti-idiotypic monoclonal antibody (mAb) GH1002 was solved by X-ray crystallography. mAb GH1002 was elicited with the syngeneic anti-idiotype mAb MK2-23 which mimics the determinant defined by anti-human high molecular weight-melanoma associated antigen (HMW-MAA) mAb 763.74. The Fab fragments of mAb GH1002 exist in the crystal as dimers related by crystallographic 2-fold axes. The interface between dyad-related Fab fragments is formed primarily by interaction of the hypervariable loops of one with the other. The self-interaction of Fab fragments of anti-anti-idiotypic mAb GH1002 through their combining sites is extremely tight and intricate, closely resembling that observed in structures of id-anti-id complexes, and comparable in terms of total contact area, charge complementarity, and number of hydrogen bonds. The self-complementarity of the antibody observed here could be coincidental and thus reflect some non-specific binding capability. It might, on the other hand, be immunologically relevant and exemplify a certain degree of evolved self complementarity characteristic of antibodies participating in idiotypic cascades.