Gene Expression in B and T Lymphocytes: (1) Evolution of Rat Cκ Alleles (2) The T-cell Receptor Problem

The amino acid sequence of the two kappa chain constant region allotypes found in inbred rat strains indicated that these alleles are very different and therefore may have had an unusual evolutionary history. To understand the evolution of these genes, serologic tests were performed to determine if inbred rats express latent or unexpected Cκ alleles. They apparently do not do so. Wild Norway rats were tested, and it was found that the laboratory strains do not represent a subset of the rat Cκ polymorphism. Further tests indicated that only one of the two serologic specificities could be found in related rodent species. The structure of the T cell antigen-binding receptor is a major controversial issue in immunology. It has been asserted that the T cell antigen-receptor is homologous to immunoglobulins, and one popular theory contends that VH genes are responsible for the specificity of the receptor. We tested these theories by hybridizing immunoglobulin DNA probes to RNA and DNA from cloned T cells. First, we determined that the Cλ, Jκ, Cκ, JH, Cµ and Cα genes and the sequences involved in heavy chain class switching are not rearranged in a T helper, a cytotoxic T cell and a T lymphoma. These cells also do not transcribe Cκ, Cλ, JH, Cµ and Cα RNA. Second, a cDNA clone encoding heavy chain variable region characteristic of most B cells which respond to the antigen GAT was isolated and sequenced. Poly(A)+ RNA was prepared from 12 cloned T lymphocytes specific for GAT. While six of these T cells display antigenic determinants present on immunoglobulins that bind GAT, none of them contained a transcript homologous to the cDNA probe. Finally, using a random primer, large cDNA libraries (105-106 colonies) were constructed from three T-cell hybridomas. These libraries were screened by two separate, well-characterized methods which should permit the detection of all or most VH gene segments. No VH cDNA colonies were found by these methods. Therefore immunoglobulin gene segments are not likely to be part of the T cell antigen receptor. The I-J serologic specificity has been reported to be present on T cell-derived antigen-binding molecules. Cosmid clones have been previously obtained containing all the sequences between the I-A and I-E subregions of the murine major histocompatibility complex, where I-J has been genetically mapped. The putative I-J DNA does not, however, hybridize to RNA from I-J positive suppressor T cells. Also, suppressor T lymphocytes do not rearrange this DNA. Therefore the I-J coding sequences must map elsewhere.