ARTHRITIS-RELATED B-CELL EPITOPES IN COLLAGEN-II ARE CONFORMATION-DEPENDENT AND STERICALLY PRIVILEGED IN ACCESSIBLE SITES OF CARTILAGE COLLAGEN FIBRILS

In collagen-induced arthritis, a murine autoimmune model for rheumatoi d arthritis, immunization with native but not heat-denatured cartilage -specific collagen type II (CII) induces a B cell response that largel y contributes to arthritogenicity. Previously, we have shown that mono clonal antibodies established from arthritis prone DBA/1 mice require the triple-helical conformation of their epitopes for antigen recognit ion. Here, we present a novel approach to characterize arthritis-relat ed conformational epitopes by preparing a panel of 130 chimeric collag en X/CII molecules. The insertion of a series of CII cassettes into th e triple-helical recombinant collagen X allowed for the first time the identification of five triple-helical immunodominant domains of 5-11 amino acid length, to which 75% of 36 monoclonal antibodies bound. A c onsensus motif, ''R G hydrophobic,'' was found in all immunodominant e pitopes. The antibodies were encoded by a certain combination of V-gen es in germline configuration, indicating a role of the consensus motif in V-gene selection. The immunodominant domains are spread over the e ntire monomeric CII molecule with no apparent order; however, a highly organized arrangement became apparent when the CII molecules were dis played in the quarter-staggered assembly within a fibril. This discret e epitope organization most likely reflects structural constraints tha t restrict the exposure of CII epitopes on the surface of heterotypica lly assembled cartilage fibrils. Thus, our data suggest a preimmune B cell selection process that is biased by the accessibility of CII dete rminants in the intact cartilage tissue.