Variable region correlates of group b allotypes: amino acid sequence studies of b9 L chains from homogeneous antibodies

Kappa-chain allotypes and isotypes in the rabbit: cDNA sequences of clones encoding b9 suggest an evolutionary pathway and possible role of the interdomain disulfide bond in quantitative allotype expression

The constant regions of rabbit kappa light chains are unusual because the sequences of the allotypic forms can differ more from each other than do some variable regions with which they associate. We report the nucleic acid sequence of a full-length cDNA clone of b9 allotype and show comparisons to available sequences of the rabbit kappa allotypes b4, b5, and bas-N4. Our analyses suggest that the primordial rabbit kappa gene encoded a bas-like sequence. They also reveal a surprising difference in the position of the variable region cysteine that forms the interdomain disulfide bond that is unique to most rabbit kappa chains. One b9 cDNA sequence lacks the usual cysteine-80 and instead encodes cysteine-108, which in three-dimensional models appears capable of forming the interdomain disulfide bond with cysteine-171 in the constant region. A partial sequence of a second b9 clone encodes both cysteine-80 and cysteine-108; the translation product of this clone could have a free reactive sulfhydryl group that might lead to an unstable nonfunctional Ig molecule. The fact that pre-B cells with b9 kappa chains do not differentiate and expand into productive Ig-producing cells with frequencies comparable to the other allotypes may be explained if a substantial proportion of the gene products have a free sulfhydryl group. Our sequence results suggest that in cells differentiating to produce kappa light chains of b9 allotype the number and location of the cysteines influence immunoglobulin expression.

Partial amino acid sequence of a rabbit immunoglobulin light chain of allotype b5

The amino acid sequence of a rabbit immunoglobulin light chain of allotype b5 has been nearly completed. A comparison of its structure with that of light chains of allotypes b4, b6, and b9 confirms that the constant regions of these various kappa chains differ by 20-35%. The substitutions are clustered in parts of the second half of the chain, and the b5 form bears more resemblance to the b6 chain than to any other, in good agreement with previous serological data. The analysis of the variable region reveals the existence of certain allotype-associated residues which have also been reported in other b5 chains, but not in proteins of the other allotypes. An examination of the rabbit light chain sequences between positions 96 and 107 suggests that this portion of the chain may be encoded separately by a joining "J" DNA segment, as has been described previously for murine and human immunoglobulins. In the rabbit, however, these J kappa regions appear to differ from one allotype to another. Together with the extensive variations of the constant regions, these data suggest that the rabbit kappa gene organization more closely resembles the murine gamma system (four different C gamma genes each flanked by its J segment) than the murine kappa system (a single C kappa gene).

Monoclonal antibodies specific for the b5 allotype of rabbit kappa light chains

The mouse monoclonal antibodies (mAb), 3B5 and 4B5, which recognize rabbit kappa light chains bearing the b5 allotype, were produced from separate fusions. The specificity of the mAbs was determined by solid-phase inhibition radioimmunoassay. Nonimmune sera of 15 b5b5 rabbits of various heavy chain haplotypes inhibited the binding of both mAbs to b5 IgG, whereas 20 sera from rabbits not expressing the b5 allotype were not inhibitory. In addition, the binding of both mAbs was inhibited by purified b5 light chains, but not by b4 light chains. The b5 epitope recognized by the mAbs was shown by sequential precipitation to be present on all b5-bearing molecules that are defined by an alloantiserum produced in a b4b4 rabbit. Antibody 4B5 forms strong precipitin bands with b5 serum and Ig in gel diffusion assays. An anomalous reaction of nonidentity was observed when mAb was compared to rabbit anti-b5 antiserum and a hypothesis to explain this phenomenon is proposed.

Isolation and characterization of a mouse-rabbit hybridoma

Immunoglobulin was obtained from a hybridoma cell line, which was a reclone of a hybrid between a rabbit cell and mouse myeloma cell (X63-Ag8). The immunoglobulin, isolated from the cell culture medium was found to be homogeneous by isoelectrofocusing and immunoelectrophoresis and consisted of mouse heavy and rabbit light chains, linked by disulphide bonds. All immunoglobulin molecules carried both mouse and rabbit determinants; mouse determinants were associated only with the heavy chains while rabbit determinants were only associated with the light chains. The rabbit light chains were of Ab4 allotypical specificity, but possess only some of the Ab4 determinants normally present in Ab4/Ab4 animals. It was suggested that the restriction in allotypical specificity may be a general property of light chain Ab allotypes; the normal serum immunoglobulins may be heterogeneous with respect to the allotypic determinants and any one molecule may possess only a proportion of determinants detected by a conventional anti-allotype antiserum.