Several V genes participate in the early phenyloxazolone response in various combinations

Toward selective covalent inactivation of pathogenic antibodies: a phosphate diester analog of vasoactive intestinal peptide that inactivates catalytic autoantibodies

We report the selective inactivation of proteolytic antibodies (Abs) to an autoantigen, the neuropeptide vasoactive intestinal peptide (VIP), by a covalently reactive analog (CRA) of VIP containing an electrophilic phosphonate diester at the Lys(20) residue. The VIP-CRA was bound irreversibly by a monoclonal Ab that catalyzes the hydrolysis of VIP. The reaction with the VIP-CRA proceeded more rapidly than with a hapten CRA devoid of the VIP sequence. The covalent binding occurred preferentially at the light chain subunit of the Ab. Covalent VIP-CRA binding was inhibited by VIP devoid of the phosphonate diester group. These results indicate the importance of noncovalent VIP recognition in guiding Ab nucleophilic attack on the phosphonate group. Consistent with the covalent binding data, the VIP-CRA inhibited catalysis by the recombinant light chain of this Ab with potency greater than the hapten-CRA. Catalytic hydrolysis of VIP by a polyclonal VIPase autoantibody preparation that cleaves multiple peptide bonds located between residues 7 and 22 essentially was inhibited completely by the VIP-CRA, suggesting that the electrophilic phosphonate at Lys(20) enjoys sufficient conformational freedom to react covalently with Abs that cleave different peptide bonds in VIP. These results suggest a novel route to antigen-specific covalent targeting of pathogenic Abs.

Genetic analysis of the maternally induced affinity enhancement in the non-Ox1 idiotypic antibody repertoire of the primary immune response to 2-phenyloxazolone

The early phases of ontogeny are decisive for the development of the B-cell repertoire. Here, we demonstrate that maternal tertiary immunization of BALB/c mice with 2-phenyloxazolone (phOx) caused a drastic alteration of the primary antigen-specific repertoire of the offspring. Maternal tertiary immunization or quaternary antibodies, which exhibited an extremely weak cross-reactivity with the major Ox1 idiotype (IdOx1), induced a change in the proportion of IdOx1/non-IdOx1 antiphOx antibodies in the F1 and F2 primary repertoire. The observed variability in the level of IdOx1 expression (10-90%) exceeded even the seemingly genetically based differences between various mouse strains. In comparison with the non-IdOx1 of control mice, half of the non-IdOx1 antibodies showed a 5-100-fold enhanced affinity. Sixty per cent of these antibodies exhibited an affinity identical to that of IdOx1 antibodies, which are normally of the highest affinity, while the remaining 40% exceeded even that of IdOx1 by a factor of 10. The non-IdOx1 were encoded by VH/VL genes and/or combinations thereof which are either new, hitherto unobserved in the antiphOx response, or typical of memory responses in normal mice. The significance of these data is discussed with respect to the possibility that maternal antibodies, which are acquired through multiple immune maturation processes, may have an epigenetic (non-Mendelian) inheritable potential for the offspring.

Repertoire shift occurs during the memory maintenance phase of the immune responses and is not affinity-driven

We have examined the immunoglobulin variable gene usage and antibody affinities during the memory maintenance phase of the immune response to the hapten phenyl-oxazolone. Hapten-specific hybridomas representing the memory population were generated 4-6 months postimmunization. The V-gene expression of these hybridomas was determined by reverse transcriptase-polymerase chain reaction screening and antibody affinities were estimated by biointeraction analysis (BIA) using the BIAcore biosensor. Our results show that the V-gene repertoire has already been shifted during the memory maintenance phase of the immune response, i.e. prior to a second antigenic challenge, and did not entail any advantages in terms of antigen-binding capacities. Our results concur with the view that antibody affinities are modulated mainly through differences in dissociation rates rather than in association rates, and the implications of this with respect to affinity maturation is discussed.

Antigen Drives Very Low Affinity B Cells to Become Plasmacytes and Enter Germinal Centers

In the first week of the primary immune response to the (4-hydroxy-3-nitrophenyl)acetyl (NP) hapten, plasmacytic foci and germinal centers (GCs) in C57BL/6 mice are comprised of polyclonal populations of B lymphocytes bearing the λ1 L-chain (λ1+). The Ig H-chains of these early populations of B cells are encoded by a variety of VH and D exons undiversified by hypermutation while later, oligoclonal populations are dominated by mutated rearrangements of the VH186.2 and DFL16.1 gene segments. To assess directly Ab affinities within these defined splenic microenvironments, representative VDJ rearrangements were recovered from B cells participating in the early immune response to NP, inserted into Ig H-chain expression cassettes, and transfected into J558L (H−; λ1+) myeloma cells. These transfectoma Abs expressed a remarkably wide range of measured affinities (Ka = 5 × 104-1.3 × 106 M−1) for NP. VDJs recovered from both foci and early GCs generated comparable affinities, suggesting that initial differentiation into these compartments occurs stochastically. We conclude that Ag normally activates B cells bearing an unexpectedly wide spectrum of Ab affinities and that this initial, promiscuous clonal activation is followed by affinity-driven competition to determine survival and clonal expansion within GCs and entry into the memory and bone marrow plasmacyte compartments.

Kinetics of somatic mutation in lymph node germinal centres

Somatic mutation activity in immunoglobulin V kappa genes during the response to the hapten 2-phenyl-5-oxazolone was measured in lymph node B-cell populations at various timepoints after footpad immunization. When the V kappa Ox1 genes rearranged to the J kappa 5 segment were amplified from genomic DNA using the polymerase chain reaction and sequenced, somatic mutations could be detected as early as day 4 after immunization. Somatic mutations were also detected after sequencing RNA from oxazolone-specific hybridomas derived from lymph node cells at day 4 after immunization. These early mutations were found mostly in cells with a germinal centre phenotype. No indication of selection at the population level by apoptosis was detected until day 7 after immunization. These results suggest somatic mutations can be induced very early during the immune response in lymph node cells, prior to the peak of clonal expansion and selection with regard to antigen binding.

Specific V gene usage in anti-phosphorylcholine IgE antibodies

Three hybridomas from phosphorylcholine(PC)-KLH immunized BALB/c mice producing IgE antibodies against the PC hapten were investigated for their fine specificity to the hapten and usage of V gene segments in H- and L-chains. All three IgE antibodies recognize the entire azophenyl-PC hapten. They are T15 Id negative and do not bind to the natural PC determinant expressed by the Streptococcus carbohydrate R36A. T15 Id positive IgE antibodies could neither be elicited by immunization in detectable amounts nor generated by the cell fusion technique. By using the Southern blot technique and nucleotide sequence analysis of PCR amplified VHDJH and VLJL rearrangements, we have demonstrated that the three IgE anti-PC hybridomas use the VH1-DSP2-JH2, the VHOX1-DSP2-JH3 or the VH36-60-D-JH2 gene segment combinations for the H chain together with the V kappa 1C-J kappa 1, V kappa 1C-J kappa 2 or V lambda 1-J lambda 1 genes for the L chains. Except for the VH36-60, the same gene segments were found in different combinations in anti-PC antibodies of other Ig classes than IgE. However, high rates of somatic mutations are expressed in both VH1 of the H chain and in V kappa 1C of the L chain. The VH36-60 is expressed in antibodies with the major Id of the azophenyl-arsonate (Ars) response and VHOX1 generally contributes to the phenyl-oxazolone specificity. This suggests that these V genes are involved in the recognition of the azophenyl moiety of the coupled PC hapten. Thus PC-KLH specific IgE antibodies utilize mutated VH1 and/or VH/VL gene segment combinations which are involved in binding of the azophenyl spacer. These IgE are therefore specific for azophenyl-phosphorylcholine, unlike antibodies normally expressed against the Streptococcus PC determinant in mice. The genetic diversity and the high mutation rates indicate that the specific B cells develop later in the immune response. Thus, they represent newly generated specificities of so-called group II anti-PC antibodies and are not isotype-switch descendants from already existing T15 Id positive IgM antibodies.

The alternative binding site for protein A in the Fab fragment of immunoglobulins

Twenty-six new human or murine monoclonal immunoglobulins (IgM, IgA, murine IgG1 or human IgG3) with a known V-region sequence were tested for alternative (non-Fc) binding to Staphylococcal protein A. Seven of them did not bind at all. Four immunoglobulins (all mouse IgG1) were bound but easily eluted (at pH 6). They were probably bound via the Fc part. All eleven were classified as negative for alternative binding. Fifteen immunoglobulins were found to bind more firmly; they came off the protein A column at pH 4-3 (alternative binders). Amino acid sequences of immunoglobulins that have been typed in the present work or earlier (25 binders and 26 non-binders) were compared. The light chain, the C region of the heavy chain and the D and JH segments look irrelevant for alternative binding. The N-terminal portion (amino acids 1-94) of the H chain probably forms the ligand of protein A. A peptide making the ligand cannot be reliably localized within this stretch but binder proteins had a high homology in residues 6-29. All mouse immunoglobulins expressing VH genes of families J606 or S107 were alternative binders; those expressing other families were non-binders.

Fine specificity and VJ usage of light chains in antibodies to the phosphorylcholine hapten

In the memory response to the phosphorylcholine hapten (PC) two major groups of anti-PC antibodies with different fine specificities are elicited. Group I antibodies are mainly PC specific, whereas Group II antibodies are comprised of two specificities directed against the phenyl-PC and the phenyl moiety of the PC hapten. The VL gene usage of 17 monoclonal memory anti-PC antibodies were investigated by Southern blot analysis and nucleotide sequencing. Six out of eight Group I memory PC-specific antibodies used the same VK22-JK5 rearrangement as the major T15 primary response idiotype. One expressed a mutated JK1 and one employed another VK22 gene family member. A shift in specificity from PC (Group I) towards phenyl-PC (Group II) was accompanied with the usage of either VK1C-JK1 or VK1A-JK5 rearrangements. The phenyl-specific Group II antibodies expressed the V lambda 1-J lambda 1 L chain rearrangement in combination with VH M141 expressing H chains. In this specific segment of Group II antibodies most mutations were found. Thus four different VL genes were found to contribute to the fine specificity of memory response antibodies to the PC hapten in a clear structure-function relationship. The diversified fine specificity in the memory response derives mainly from the usage of different L chains with particular VJ rearrangements in combination with VH of the dominant initial response clonotype and is not primarily due to mutational events.

The same few V genes account for a majority of oxazolone antibodies in most mouse strains

The early primary anti-phenyloxazolone antibodies of 12 mouse strains were studied by determining proportions of two defined subsets id495 (the classical phOx idiotype) and id350. Id495-positive antibodies bear an H chain encoded by VHOx1 gene (family Q52) and an L chain usually coded for by VKOx1 but occasionally by other VK genes. Id350-positive antibodies are encoded by a VK gene VK45.1, and usually by a VH gene of the S107 family. All 12 strains (representing nine H-chain and four kappa-chain haplotypes) produced id350-positive anti-phOx antibodies. While id495 is the predominant major subset in the BALB/c response (originally studied), id350 seems to be the predominant subset of early anti-phOx antibodies in the mouse species. The combined proportion of the two subsets varied from ca. 50 to almost 100% of the total in all strains except C57BL.

Accumulation of somatic mutants in the B cell compartment after primary immunization with a T cell-dependent antigen

The accumulation of somatic mutants in splenic B lymphocytes early after primary immunization with the hapten (4-hydroxy-3-nitro-phenyl)acetyl (NP) coupled to chicken gamma globulin (CG) was determined. Rearranged V186.2 heavy chain genes were amplified by the polymerase chain reaction from genomic DNA and subjected to nucleotide sequence analysis. Somatic antibody mutants become detectable on day 6 after immunization, and most of the somatic mutations accumulating in the memory compartment are introduced until day 14. At this time strong selection for mutants expressing high binding affinity for NP is apparent. Extrapolation from the mutation frequency increases between day 6 and day 14 to the previously determined mutation frequency at week 6 (Weiss. U. and Rajewsky, K., J. Exp. Med. 1990, 172: 1681) leads to the prediction that the process of mutant generation ceases to operate around day 22 after primary immunization.

Contact sensitivity to oxazolone in the chicken: evidence for Arthus type hypersensitivity of the cutaneous reaction

Cutaneous hypersensitivity reaction can be induced in chickens by skin painting with oxazolone, 33 mg/Kg of body weight (KBW). The B cell contribution to the generation of the cutaneous reaction has been a matter of controversy. In an attempt to characterize this reaction we placed special interest on the possibility that the nature of this reaction could be Arthus type hypersensitivity. From the kinetics study on the cutaneous hypersensitivity after challenge with oxazolonated egg-albumin (EA-OX) it was excluded that the nature of this reaction could be delayed type hypersensitivity. Immune sera transfer experiments demonstrated that the cutaneous reaction was antibody dependent. Serum anti-oxazolone antibody titers in sensitized chickens were assayed by antiglobulin haemagglutination, using oxazolone coupled sheep erythrocytes (OX-SRBC). High titres of IgG were found in contact sensitized chickens. Furthermore this cutaneous reaction was characterized by neutrophils, inflammatory edema, rare thrombotic occlusion of small venules and on absence of monocytes. The utilization of complete Freunds' adjuvant (CFA) given at sensitization demonstrated that CFA enhanced oxazolone antibodies in the sera of immunized chickens without a correlated increase in the intensity of the cutaneous reaction to EA-OX. Animals sensitized to oxazolone (33 mg/KBW) without CFA and challenged intravenously seven days later with oxazolone coupled to autologous chicken red blood cells (OX-CRBC) died from anaphylactic shock; instead animals with the same treatment but with CFA given at sensitization did not die from anaphylactic shock. Taken collectively it was concluded that the cutaneous reaction to oxazolone in the chicken can be categorized as Arthus hypersensitivity. The relationship between cutaneous Arthus reaction and anaphylactic shock in chickens sensitized to oxazolone is discussed.

V genes of oxazolone antibodies in 10 strains of mice

One pair of V genes (V kappa 45.1 and V11) code for a great portion of phenyloxazolone (anti-phOx) antibodies in 10 strains of mice. This combination replaces the first-known major combination VHOx1-V kappa Ox1 in some strains, and is important in most strains. C57BL/10 and SJL mice have an additional subset of antibodies encoded by genes V kappa 45.1 and V13 (a relative of V11). All three genes involved (V kappa 45.1, V11 and V13) have "allelic" variation. Four alleles of V11 were found, one in Igh haplotypes a, c and g, the second in haplotypes d, j and n, the third in b, and the fourth in f. The most distant alleles d, j, n and f had 10 nucleotide differences out of 429 determined (97.7% homology). Only one allele of the V13 gene was found from anti-phOx hybridomas but two others have been published. Three alleles of the V kappa 45.1 gene were found; one in NZB mice (Ig kappa haplotype b) another in CE (haplotype f), and the third in eight strains including representatives of three Ig kappa haplotypes (a, c and e). The three alleles had greater than 99.0% homology. The V11 and V13 genes that code for anti-phOx antibodies in C57BL/10 and SJL mice were different from the related genes found from the C57BL/10 germ line. C57BL/10 mice must have a chromosome bearing two V11 and two V13 genes. RF mice were found to have two V11 genes, and both code for anti-phOx antibodies. Our data show that the majority of antibodies in the anti-phOx response are encoded by the same restricted collection of V genes in most mouse strains. Antibody responses appear to be no less heritable than other functions of the body.

Evolution of the immunoglobulin heavy chain variable region (Igh-V) locus in the genus Mus

The evolution of the mouse immunoglobulin heavy chain variable region (Igh-V) locus was investigated by the comprehensive analysis of variable region (Vh) gene family content and restriction fragment polymorphism in the genus Mus. The examination of natural Mus domesticus populations suggests an important role for recombination in the generation of the considerable restriction fragment polymorphism found at the Igh-V locus. Although the sizes of individual Vh gene families vary widely both within and between different Mus species, evolutionary trends of Vh gene family copy number are revealed by the analysis of homologues of mouse Vh gene families in Rattus and Peromyscus. Processes of duplication, deletion, and sequence divergence all contribute to the evolution of Vh gene copy number. Certain Vh gene families have expanded or contracted differently in the various muroid lineages examined. Collectively, these findings suggest that the evolution of individual Vh family size is not driven by strong selective pressure but is relatively neutral, and that gene flow, rather than selection, serves to maintain the high level of restriction fragment polymorphism seen in M. domesticus.

Anti-DNA antibodies, autoimmunity and the immunoglobulin repertoire

The advent of hybridoma and recombinant DNA technology about a decade ago has allowed a detailed analysis the structure, properties and molecular genetics of antibodies. These techniques, combined with studies of idiotypes and of Abelson-transformed and other cell lines, have resulted in major findings which are of particular importance to both the normal immune system and to autoimmunity. The rearrangement and expression of antibody genes in the normal immune system are discussed first, as a background for an appreciation of the significance of the molecular genetics of autoantibodies. We then turn to autoantibody genes, with an emphasis on anti-DNA antibodies and their role in the autoimmune disease, systemic lupus erythematosus. A model for the genetics of lupus which includes a possible role for Ig genes is considered.

Genetic control of early antibody responses

By using a pair of strains that have similar VK haplotypes but different VH haplotypes (e.g. BALB/c and C57BL) it is possible to demonstrate VH-controlled genetic differences in antibodies. By using a pair that have similar VH haplotypes but different VK haplotypes (RF and BALB/c) it is possible to demonstrate VK-controlled genetic differences in antibodies. A plausible explanation for the high frequency of certain V-gene combinations in the primary response is a high affinity of the product without somatic mutations. The products of two such major primary response combinations (VHOx1/VKOX1(H3) and VH186.2/V lambda 1) have an affinity for the immunogen well above 10(6). One combination of V genes, VHOx1/VKOx1(H3) has a major role in the primary anti-phOx response of several mouse strains - the product is idiotype 260. C57BL/10 mice lack the VHOx1 and RF mice the VKOx1(H3) gene. They use the remaining partner of the pair for the response in combination with other genes, but the affinity of the product is lower than the affinity of id. 260. Concordantly, the frequency of these "half-idiotypes" is lower in the primary response than the frequency of the full combination (23% and 16% instead of 50%). When the product of a V-gene combination is very frequent in the primary response, the affinity for the immunogen must be high, but the reverse is not always true. The product of a combination can have an unusually high affinity but the frequency is low. The simplest explanation then is that the frequency of available virgin B cells is low. It can be low because of a low rearrangement frequency of one of the V genes, VH or VK. Another possibility is that only a small proportion of B cells that have the particular combination rearranged can be recruited to the response. We have discussed an example where strict heavy chain CDR3 requirements must strongly limit available B cells.

A predominant idiotype independent of specificity, or Ig and H-2 allotypes, is found in the primary but not the secondary murine antibody response to lysozyme

Removal of just the three N-terminal residues Lys-Val-Phe (TIP) on hen egg white lysozyme (HEL), by aminopeptidase cleavage, eliminates an antigenic determinant which is a recurrent and dominant focus of primary but not secondary antibody responses to HEL in a variety of mouse strains. We have generated an anti-idiotypic rabbit antiserum against such a TIP-dependent monoclonal antibody (mAb). This antiserum reacts with many different primary anti-HEL mAb, but fails to react with all of a variety of secondary anti-HEL mAb. The idiotype defined by this antiserum, termed IdXE, is a common feature of early anti-HEL antibody responses but does not appear in secondary responses. Although the presence of IdXE and TIP dependence is correlated in primary responses, studies of idiotype expression on mAb and on plaque-forming cells (PFC) using mixed erythrocyte monolayers clearly show that at the single-cell level the properties are separable, i.e., not all TIP-recognizing PFC display IdXE and a sizable proportion of cells producing non-TIP-dependent antibodies are IdXE+. The restricted idiotypy and specificity of early antibody responses to HEL occur in each of eight diverse mouse strains tested: it is not associated with a particular MHC haplotype, heavy chain allotype or light chain allotype. The finding of such strain-independent restriction in the early response pattern to a typical protein antigen is novel and suggests the involvement of highly conserved, potent regulatory mechanisms which are manifested as a limitation in the initial expression of the available repertoire.

Combinatorial association of V genes: one VH gene codes for three non-cross-reactive monoclonal antibodies each specific for a different antigen (phoxazolone, NP or gat)

Two anti-phenyloxazolone (phOx3) and one anti-GAT MAbs from C57BL mice are shown to be coded by VH gene 186.2. This gene has been found earlier to code for several anti-NP (NNP) antibodies (Bothwell et al., 1981) and anti-GT antibodies (Rocca-Serra et al., 1983; Carmack and Pincus, 1986). The L chain partner of the VH 186.2 gene is different in anti-NP and anti-GAT antibodies (Bothwell et al., 1981; Rocca-Serra et al., 1983; Carmack and Pincus, 1986); in anti-phOx antibodies two new unrelated kappa chain V regions were found. Both of the new VK genes involved code frequently for anti-phOx antibodies in BALB/c mice but then with different VH genes. We tested five 186.2-coded antibodies for cross-reactions. Four antibodies were specific, one bound only to NNP, one only to phOx and two only to GT (GAT). The fifth antibody (anti-phOx) bound also to NNP, GAT and ABA-HOP though probably with a low affinity. This is the first demonstration that one V gene can code for three different antibody specificities. It emphasizes the role of the combinatorial element in antibody diversity.

Duplications and deletions of Vh genes in inbred strains of mice

The evolution of variable region (Vh) gene family copy number and polymorphism was investigated by the analysis of the immunoglobulin heavy chain variable region (Igh-V) locus in 74 inbred strains and substrains of mice. Several strains were found to have slight differences from Igh-V haplotypes previously identified, usually involving the gain or loss of one or a few members of a single Vh gene family. These results indicate that the evolution of copy number in the mouse Igh-V locus proceeds largely by the accumulation of incremental changes, reflecting the clustered organization of the mouse Igh-V locus. We have found no evidence of very large or frequent duplication or deletion events indicative of rapid expansion or contraction processes. The existence of one or more particularly large Vh gene families most likely reflects random copy number variation, rather than selection for the amplification of their members. The identification of strains with recombinant Vh gene arrays demonstrates that recombination, both within and between haplotypes, appears to be the predominant mechanism generating the high restriction fragment length polymorphism in the Igh-V locus.

V genes of the primary antibody response of C57BL/10 mice to the hapten phenyloxazolone

The mRNA of ten monoclonal phenyloxazolone (phOx) antibodies originating from the primary (day 7) response of C57BL/10 mice were partially sequenced. The sequences were analyzed together with those of two previously published antibodies. The C57BL response does not have a predominant subset of antibodies like the BALB/c response has (VH-Ox1/V kappa-Ox1 JK5). Probably, C57BL mice lack the VH-Ox1 gene and, as a consequence, their V kappa-Ox1 gene does not have a main role in the anti-phOx response. Five V kappa and six VH genes were found to participate. All five V kappa genes or their "alleles" had previously been found from the BALB/c response to 2-phenyloxazolone (phOx). On the other hand, the two strains use different VH genes for the anti-phOx response. Most C57BL antibodies were coded by VH genes of group 1 which has only minor role in the BALB/c response. The remaining VH genes were from group 7. Our data show that one V kappa segment (e.g. V kappa-Ox1) can code for anti-phOx antibodies with several, even widely different, VH genes. On the other hand, they emphasize the role of certain VH/VL gene combinations for the anti-phOx specificity. Thus, VH genes of group 7 were found to code for anti-phOx antibodies only together with the V kappa 45.1 gene.

Activation of memory and virgin B cell clones in hyperimmune animals

To study the long-term memory response, BALB/c mice were allowed to rest for over a year after a secondary immunization with the hapten 2-phenyl-oxazol-5-one (phOx). For the tertiary immunization two different protocols were used. In one protocol mice were injected i.v. and 3 days later spleen cells were fused to a nonproducing hybridoma line. PhOx-specific hybridomas were established and the sequence of the heavy and light chain mRNA was determined. This tertiary response resembled the diversity pattern of the secondary response with a further increase both in somatic mutations and in the average dissociation constant. The high number of somatic mutations demonstrates the persistence of memory B cell clones over a long time period. In the second protocol mice were boosted with an i.p. injection of alumprecipitated antigen phOx and 7 or 14 days later spleen cells were fused. Sequence analysis of heavy and light chain mRNA showed that these tertiary response antibody molecules had surprisingly few somatic mutations, indicating an activation of virgin B cell clones in these hyperimmunized animals. The maturation of these newly stimulated B cell clones seems to follow somewhat similar rules to those found for the primary response. It appears therefore that the two immunization protocols reflect the response of memory and virgin B cells, respectively.

Evidence for a new murine immunoglobulin heavy chain variable region gene family

In the course of a previous study addressing autoantibody generation in murine models for generalized autoimmune disorders, we observed a nonfunctional immunoglobulin heavy chain transcript in a B cell hybridoma from lupus-prone MRL-Mp-lpr/lpr mice. This transcript, the cDNA sequence of which is presented here, is of general interest for murine immunoglobulin genetics as it cannot be ascribed to any known heavy chain variable region gene (Vh) family by nucleic acid sequence homology criteria and, hence, may represent a new Vh gene family. The sequence showed about equal nucleotide similarity (70-73%) to 3 other Vh gene families (S107, J606, 7183) and less than 65% similarity to members of the remaining 6 Vh gene families. Nucleic acid sequence comparisons with unpublished immunoglobulin sequences uncovered a highly homologous (greater than 95%) functional Vh transcript indicating that the suggested Vh gene family also encodes expressed antibody molecules.

Somatic mutation in anti-phosphorylcholine antibodies

A detailed analysis of the genes and proteins that participate in the murine immune response to PC has provided key insights at the structural level into the phenomenon of somatic mutation in B cells. Most anti-PC antibodies are encoded by 1 VH gene of the S107 subfamily, and 3 VK genes, VKT15 of the VK22 subfamily, VKM3 from the VK8 subfamily, and VK167 from the VK24 subfamily. No mutation was detected in these genes until the 2nd wk after immunization, indicating that mutation is under developmental control. The protein sequences of 73 heavy and light chains derived from the secondary response support the concept of developmental activation of mutation after antigen stimulation. No mutation was found in the IgM antibodies, whereas half of the IgG and IgA antibodies had mutation. Most of the mutated antibodies had higher affinity for antigen than their germline counterparts, which suggests that the major role of somatic mutation is to increase affinity rather than to create new specificities. Nucleotide sequencing established two hallmarks of mutation in immunoglobulin genes: mutations are targeted to a 1 kilobase region surrounding and including the rearranged variable gene, and they occur at an extraordinary frequency of 10(-2) nucleotide substitutions. Mutation is probably caused by DNA repair, and may occur during error-prone repair of nicked DNA around the variable gene or during mismatch repair of misaligned structural intermediates. The elucidation of this remarkable mechanism clearly requires studies of a more dynamic character. Two major questions that need to be answered are: what targets mutation to the variable gene, and what enzymes are involved?