Editorial: Antibody diversity: is it all coded for by the germ line genes?

Intrapair similarity of immunoglobulin levels in twins

Levels of immunoglobulins IgG, IgA, IgM and IgE were determined in 8 MZ and 14 DZ twin pairs at the ages of 6-11 years, 12-17 years and 15-20 years. Intrapair similarity in immunoglobulin levels was found to be higher in the MZ than in the DZ twins, especially in the case of immunoglobulins IgA and IgM.

[Lymphocyte membrane receptors mediating recognition of antigens (author's transl)]

The present article summarizes briefly our knowledge of antigen-specific B- and T-lymphocyte receptors. Antigen-specific receptors on mammalian B-lymphocytes are mainly monomeric IgM and IgD consisting of conventional immunoglobulin heavy and light chains. The nature of the T-lymphocyte receptors which can specifically recognize antigens is not yet fully defined but it seems that conventional light chains do not participate in the build up of this receptor. However, T-lymphocytes express two polypeptide chains which bear variable domains sharing idiotypic determinants with the corresponding B-lymphocytes. These Tau chains can bind the corresponding antigen.

Antibody diversity

Three important aspects of immunoglobulin gene organization and structure have emerged from studies of cloned immunoglobulin kappa chain genes. (i) Multiple variable genes are encoded separately in the genome of both immunoglobulin-producing and uncommitted (embryonic) cells, thereby establishing the evolutionary base for generating immunoglobulin diversity. (ii) These genes exist as many small, closely related families (subgroups) that share close sequence homology largely within their own subgroup. (iii) Comparison of two cloned variable gene segments derived from a single subgroup reveals a feature of their structure that distinguishes them from fixed genes (that is, globin genes) and provides, through extensive surrounding sequence homology, a large target for intergenic recombination. This last observation suggests that a simple recombination mechanism may account for their genetic instability in both germ line and somatic cells.

Quantitation of constant and variable region genes for mouse immunoglobulin lambda chains

We have synthesized and characterized cDNA complementary to purified mRNA derived from the lambda chain producing myeloma tumor, RPC-20. This cDNA is of sufficinet length to encode the constant region and a major portion of the variable region sequence of the lambda gene. In addition, the expected range of cross-hybridization of this lambda probe has been shown to extend to several different members of the closely related lambda subgroup, as well as to a member of the lambda subgroup represented by MOPC-315. Since there are a minimum of seven known members of the common lambda subgroup in addition to MOPC-315, these sequences, in accordance with the germ line hypothesis, must be represented by a minimum of eight variable region genes. Using the RPC-20 cDNA probe and hybridization kinetic analysis, this sequence was found to be represented as approximately two copies per haploid genome in DNA derived from a variety of k-and lambda-producing tumors and normal tissue. Inasmuch as the cross-hybridization range of the probe has been assessed and a minimum size of the lambda subgroup determined, this observation tends to rule out separate germ line genes corresponding to each individual lambda light chain variant. Certain reservations about these conclusions are discussed.

Reiteration frequency of immunoglobulin light chain genes: further evidence for somatic generation of antibody diversity

Methods have been developed for preparing mouse immunoglobulin light chain mRNA of better than 90% purity. Hybridization of both lambda and kappa mRNAs to excess liver DNA yielded results compatible with gene reiteration frequencies of two to three. There was no evidence of hybridization of these highly purified mRNAs to reiterated DNA, and, in fact, the kinetics of hybridization were very similar to that of purified globin mRNA. Purified lambda mRNA from tumors producing structurally different lambda chains were used in competition hybridization experiments. An unlabeled lambda mRNA competed with another, labeled lambda mRNA to the same extent as homologous unlabeled lambda mRNA. That is, base sequence homology among lambda mRNAs is so high that any lambda mRNA should cross-hybridize with all germ line variable (Vlambda) genes at least for those V-regions which are represented among myelomas. From amino-acid sequence data, it is argued that there are probably more than 25 different lambda V regions. Hence it is concluded that the number of germ line genes is too small to account for the diversity of lambda chains. A similar conclusion is drawn for kappa chains.

Evidence for somatic generation of antibody diversity

RNA preparations containing 70-80% mouse kappa-chain mRNA have been prepared. The remainder consists of many RNA species, each of which represents a small fraction of the total RNA. The kappa-chain mRNA preparation hybridizes with mouse liver DNA with bi-phasic kinetics, indicating that it consists of two fractions -"unique" and "reiterated." Competition hybridization experiments show that the homology among the unique fractions from different mRNAs is the same as the homology among the amino acid sequences of the corresponding kappa-chains. Hence, in addition to the C-region (constant-region) sequences, (most of) the V-region (variable-region) sequences are also derived from unique germ line genes. The reiterated fractions from different kappa-chain mRNAs show essentially complete homology with each other. This fraction seems to consist mostly of sequences which do not code for amino-acid sequences of the secreted polypeptide chain, i.e., the "external" section of the mRNA molecule. It is concluded that the number of germ line genes is too small to account for the observed diversity of antibody molecules.