What limits affinity maturation of antibodies in Xenopus--the rate of somatic mutation or the ability to select mutants?

Abstract

Although the Xenopus immunoglobulin heavy chain locus is structurally and functionally similar to mammalian IgH loci, Xenopus antibodies are limited in heterogeneity, and they mature only slightly in affinity during immune responses. During the antibody response of isogenic frogs to DNP-KLH, it and v cDNA sequences using elements of the VH1 family were cloned, sequenced and compared with germline counterparts. There were zero to four mutations per sequence, mostly single base substitutions, in the framework and CDRs 1 and 2 of VH. No mutations were found in JH. Since the point mutation rate was only 4-to 7-fold lower than that calculated for mice, affinity maturation does not seem to be limited by mutant availability. Because of a relatively low ratio of replacement to silent mutations in the CDRs and a very high ratio of GC to AT base pairs altered by mutation, it is suggested that the problem results from the absence of an effective mechanism for selecting mutants, which in turn might be related to the absence of germinal centers in Xenopus.

The Targeting of Somatic Hypermutation Closely Resembles That of Meiotic Mutation

“…Other groups have hypothesized the presence of a two-component mutator (21)(22)(23)(24), consistent with the observation that G and C are mutated more frequently in the murine cell line 18-81 (26) and the Burkitt lymphoma line Ramos (8). Furthermore, the G ⅐ C-targeting component is argued to have arisen first (or been co-opted first by SH) (22), consistent with the observations that AGC/GCT or G and C are preferentially targeted in shark (45) and Xenopus (46). The identity of the molecules involved in somatic hypermutation will surely be revealed soon, but even after their names are known, it will remain to learn how they do what they do.…”

The Nucleotide Targets of Somatic Mutation and the Role of Selection in Immunoglobulin Heavy Chains of a Teleost Fish

“…Thus, these results indicate that somatic mutation may have evolved as a mechanism to principally increase repertoire diversity. This basic mechanism continues to be phylogenetically operational as shown, for example, in the repertoire studies with sheep (65) and the apparent lack of selection in the studies with Xenopus H chains (28).…”

“…Earlier studies in xenopus and shark H chains indicated that somatic mutation occurred in these two classes of vertebrates and that there was a strong mutational bias toward G and C (28,29). Subsequent studies on shark L chains and shark NAR have shown that mutations in A and T can account for 40 -50% of the mutations.…”

Normal Somatic Hypermutation of Ig Genes in the Absence of 8-Hydroxyguanine-DNA Glycosylase

“…For example, lower species such as horned shark (31) and frogs (32) have a predominance of G:C mutations; mice deficient in mismatch repair proteins Msh2 (33)(34)(35)(36) and Msh6 (37) have a large proportion of G:C mutations; and cell lines that mutate in vitro accumulate mostly G:C mutations (38 -41). This has led to the proposal that mutations of G and C occur first, and then another pathway subsequently introduces mutations of A and T (36).…”