Studies ofIg gene structure and organization during the past decade have illuminated the central mechanisms of antibody diversification: the somatic rearrangement and reassortment of multiple gene segments, junctional flexibility, and point mutation . The inherited set of Ig gene segments provides a diversified genetic basis upon which these dynamic processes operate during ontogeny. Thus, the composition of these germline genes imposes a major influence on the development of the antibody repertoire.The present study examines the germline content and organization of the mouse heavy chain variable region genes (V genes) . We set out to analyze the locus in sufficient detail and resolution to provide the basis for determining the extent of inherited Vgene diversity, the evolution ofthe germline repertoire, and any functional consequences of the physical arrangement of the V gene segments .The mouse Igh locus consists of at least 100-200 V genes (1-3). V gene families, defined by nucleotide sequence relationships, comprise distinct sets of highly related V genes that can be identified by hybridization using prototypic V gene probes . This classification of V gene families (l, 4) has provided a useful framework for the study of germline V gene content, polymorphism, and utilization (1-7). Whereas the general organization of the Igh locus is 5'-V HDJ .-C -3' (8, 9), previous studies of V gene family organization have resulted in partial, relatively low resolution maps lacking consistency between reports (10-12)."Deletion mapping" takes advantage of the fact that V, gene rearrangements result in the deletion of DNA originally separating the rearranged V gene segment and the DJ .-CH region (13). We have constructed a panel of 32 pre-B cell lines, most of which have rearranged V, genes on both chromosomes. Since these cell lines were derived from Fi mice heterozygous at the Igh locus, V, gene deletions can be identified using RFLPs. V, gene analyses of 51 independently rearranged chromosomes are consistent with a single V, gene map order of nine V, gene families. The genomic stability of these cell lines and consistent deletion profiles of all 51 rearranged loci provide a high resolution V gene map that has compelling experimental support.

Somatic mutations to arginine (R) are a common feature of a subset of J558 H chain genes that code for the majority of high-affinity, anti-dsDNA antibodies in autoimmune MRL/lpr mice. To examine the consequences of such amino acid substitutions on DNA binding, we reverted three somatic mutations of a prototypic anti-dsDNA H chain gene, VH3H9, and assayed the effect of those reversions by expression in a V lambda 1 L chain-only plasmacytoma line. Reversion of R53 eliminated virtually all dsDNA binding and sharply reduced ssDNA affinity. While the complete germ-line revertant of VH3H9 retained a low level of DNA binding, the substitution of R96, a product of N base addition in the third complementarity determining region (CDR3), with glycine (G) was sufficient to abolish measureable DNA specificity. Antibodies with higher affinity for DNA were generated by introducing arginines into VH3H9 at any one of four positions where somatic mutations to arginine had been identified by sequencing other anti-dsDNA J558 H chain genes. All four arginine mutants showed affinity increments consistent with their direct involvement in DNA binding, although one such mutant, K64R, required the simultaneous reversion of an adjacent aspartic acid (D) to the germ-line glycine. Two variants with three nongerm-line arginines showed further improvements in DNA affinity suggesting that their contributions to DNA binding may be additive. Molecular modeling of antibody and mutant F(ab) structures and calculations of their electrostatic potentials were used as an aid in interpreting the results and in predicting the location and size of possible combining sites.

A rearranged murine V kappa 8/J kappa 5 L chain gene that codes for the L chain of most antibodies generated in the primary response of BALB/c mice to the antigenic site, Sb, of the hemagglutinin (HA) molecule of influenza virus A/PR/8/34 (PR8) has been cloned. Three transgenic lines were generated by microinjecting the gene. Lines Ga and L each contain a single copy of the transgene whereas line Gb contains three complete copies. Mice of the Ga lineage showed increased V kappa 8-specific mRNA levels only in spleen, but not in nonlymphoid organs and therefore displayed apparently normal lymphoid-specific regulation of the Ig transgene. B cell hybridomas generated from these mice were analyzed for rearrangements of endogenous V kappa genes. Greater than 90% of the C kappa alleles were retained in germ-line configuration in the Ga line, compared with only 0 to 18% in the L line. Thus, a wide variation in the frequency of endogenous rearrangements is seen among mice of different lineages using the same transgene construct. None of more than 150 hybridomas derived from LPS-stimulated splenic B cells of Ga mice exhibited HA-binding activity although they expressed the transgene and, in most cases, excluded endogenous V kappa rearrangements. In contrast, a large fraction of hybridomas isolated after primary immunization with PR8 were HA(Sb)-specific. This indicated that the transgene was functional but formed HA-specific antibodies with a more restricted set of H chains than previously hypothesized. The primary anti-HA response to immunization with PR8 was diminished in all lines compared with normal mice except for a slightly accelerated but transient burst of anti-HA antibody formation in two out of three lines (Ga and Gb). This early response in G lineage mice was largely specific for HA(Sb) and thus appeared to be composed of transgene-expressing antibodies. No differences in serum titers were observed in the secondary anti-HA responses to booster inoculation with PR8 between transgenic and normal mice.