A model of anti-protein affinity maturation antibodies: a thermodynamic approach.

ACIERNO, JUAN PABLO; GOLDBAUM, FERNANDO ALBERTO; CAUERFF, ANA

Copacabana, Rio de Janeiro, Brasil

Workshop; 1st Workshop on Biocalorimetry and Biological Thermodynamics; 2006

The study of the molecular basis of affinity maturation mechanism it is not only important to understand these aspects of the humoral immune response but also for biotechnological and biomedical applications since optimizing the affinity of antibodies to subnanomolar levels could be of great importance in those disciplines. Two monoclonal antibodies (mAbs) were elicited against lysozyme (HEL), the first, D44.1 was obtained after a short immunization plan, and the second, the mAb F10.6.6 was obtained after a long term immunization plan. These two mAbs are derived from the same germlines genes, and they recognize the same epitope on HEL surface, but with F10.6.6 having ~103 increase in affinity for antigen, compared to D44.1. Our goal is to understand why two very similar mAbs behaves so different when binding to HEL is studied. Previously solved crystal structures of the free and complexed Fab fragments from both antibodies were analyzed. Table I DGº DHº TDSº F10.6.6 -51,65 -65,76 -14,11 D44.1 -40,60 -38,36 2,24 (all units: kcal/mol) F10.6.6 showed a 1.7 times larger enthalpy component, that can be explained by the larger amount of contacts made by the F10.6.6 to HEL observed in the crystal structures (Cauerhff et al, PNAS 2004). On the other side, we observed that the F10.6.6-HEL reaction had an entropic component around 10 times higher than D44.1. This could be caused by the loss of contacts between the VL and VH domains of the antibodies. Upon the complex formation, F10.6.6 looses around 40 VL-VH contacts, as the D44.1 looses 10 contacts.