Differences in mutational patterns between rheumatoid factors in health and disease are related to variable heavy chain family and germ‐line gene usage

Abstract

The sequences of the heavy chain variable (V_H) segment and dissociation constants (K_d) of 14 IgM rheumatoid factors (RF) derived from 11 different germ‐line gene segments from five healthy immunized donors (HID) are described. We extend a previous analysis of two clones from one donor using only the germ‐line segment DP‐10. In the present study, the mutation patterns of these new RF and the two earlier reported HID RF clones are analyzed in relation to V_H family, germ‐line origin, and K_d. The panel of HID RF is further compared with 33 previously described IgM RF from patients with rheumatoid arthritis (RA). There is a high rate of mutation in the panel of HID RF (mean of ten mutations/V_H). RF originating in RA patients have a comparable mutation rate (mean of 11 mutations/V_H), suggesting that hypermutation of IgM RF is not disease related. The HID RF have, however, a significantly lower affinity for IgG than the RA RF. We found that the structural basis of the differences between HID and RA RF is related to V_H family usage. RF of the V_H1 family use very similar germ‐line genes in HID and RA patients. HID RF of the V_H1 family have, however, a low ratio of replacement‐to‐silent (R : S) mutations of only 0.41 in the heavy chain complementarity region (CDR_H) 1 and 2. This is statistically significantly lower than the corresponding ratio of 3.14 in the V_H1 RA RF. In contrast, RF of the V_H3 family from HID and RA patients have very similar R : S ratios of 1.75 and 1.71 in CDR_H1 and 2, respectively. The V_H3 RA RF are, however, predominantly encoded by genes not encoding any HID RF. Thus, both repertoire differences and hypermutation resulting in significantly lower R : S ratios can be observed in RF from HID compared with RA RF.