AN ISOTYPE SWITCHED AND SOMATICALLY MUTATED RHEUMATOID-FACTOR CLONE ISOLATED FROM A MRL-LPR LPR MOUSE EXHIBITS LIMITED INTRACLONAL AFFINITYMATURATION/

Employing site-directed mutagenesis we have reconstructed and expresse d the germ-line precursor of an expanded rheumatoid factor (RF) clone. This RF clone, designated clone F, was isolated from an autoimmune MR L/MpJ-lpr/lpr mouse. Most of the clone members were extensively mutate d and isotyped-switched. The predominant isotype of clone F was gamma 3. The RF bound specifically to the MRL gamma 2a allotype (lgh-1(j)) b ut not to the B6 gamma 2a allotype (Igh-1(b)). The germ-line antibody was also found to bind gamma 2a in an RF assay. The affinities of the germ-line RF and representative members of the clone were measured in an ELISA-based equilibrium binding assay. The dissociation constant (K -d) of the germ-line RF was 2.5 x 10(-6) M. All of the expressed clone members had affinities within a two- to sixfold range of the germ lin e, indicating that the mechanisms of somatic hypermutation and selecti on resulted in only limited affinity maturation of this autoantibody c lone.