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Gene for the human T cell differentiation antigen Leu-2/T8 is closely linked to the kappa light chain locus on chromosome 2

We have mapped the gene encoding the T cell differentiation antigen Leu- 2/T8 to human chromosome 2 by hybridization of a Leu-2/T8 complementary DNA clone to DNA from a panel of mouse-human cell hybrids. In situ hybridization further localizes the gene to the 2p1 region in close proximity to the Ig kappa light chain gene. The Leu-2/T8 gene translocates with C kappa to chromosome 8 in a Burkitt lymphoma line carrying a t(2;8) translocation. These data support the hypothesis that Leu-2/T8 is the human homologue of the mouse Lyt-2,3 antigen.

Though protein sequences with which to make a direct structural comparison between these two molecules are not presently available, biochemical evidence, tissue distribution, correlation with effector function, and blocking studies using monoclonal antibodies all make this conclusion plausible. Using classical genetic methods, the gene(s) for Lyt-2,3 have been mapped to chromosome 6, tightly linked to the mouse C, locus (13)(14)(15)(16). We were, therefore, interested in determining the chromosomal location of Leu-2/T8 and its possible linkage to the human x locus on chromosome 2 (17,18).
We have now used one such cDNA clone as a probe to determine the chromosomal localization of the Leu-2/T8 gene by hybridization to DNA from mousehuman somatic cell hybrids, and by in situ hybridization to human chromosomes.

Materials and Methods
Somatic Cell Hybrids. The generation and characterization of the mouse-human somatic cell hybrids have been described previously (18,20,21). The human chromosomal contents of these cell lines are shown in Tables I and II Southern Blots. Cellular DNA from the hybrid cell lines or from controls was thoroughly digested with Eco RI (New England Biolabs, Beverly MA), fractionated by electrophoresis through a 0.9% agarose gel, and transferred by blotting to nitrocellulose filters according to the procedure of Southern (22). Blots were hybridized with the 2 kilobase (kb) insert of a Leu-2/T8 cDNA clone labeled with [s2P]orthophosphate by nicktranslation to a specific activity of 2-4 x 108 cpm/#g. Hybridizations were for 16 h at 42°C in 40% formamide, 4x saline sodium citrate (SSC), Denhardt's solution, 20 mM Tris, pH 7.6, 0.1% sodium dodecyl sulfate (SDS), 100 #g/ml denatured herring sperm DNA, and 10% dextran sulfate. The filters were each washed twice at room temperature for 30 min with 2x SSC, 0.1% SDS, and once at 52°C for 30 min with 0.1x SSC, 0.1% SDS. The filters were then exposed to XAR-5 film with an intensifying screen for 3 d at -70°C.
In Situ Hybridization. In situ hybridizations were performed as previously described (20). The probe was a Leu-2/T8 cDNA clone containing a 2 kb insert, labeled with all four [3H]nucleoside triphosphates by nick-translation.

Results
Hybridization to DNA From Mouse-Human Somatic Cell Hybrids. We used 10 mouse-human hybrid cell lines to map the Leu-2/T8 gene within the human genome. DNA derived from these lines was digested with Eco RI, fractionated by agarose gel electrophoresis, blotted onto a nitrocellulose filter, and hybridized to a 2 kb Leu-2/T8 cDNA clone insert labeled by nick-translation. Fig. 1 shows examples of hybridization to some of the somatic cell hybrid DNA and normal human DNA. The Leu-2/T8 probe hybridized to two human-specific fragments: 7.5 kb and 4.8 kb (Fig. 1, lanes 1, 2, 8, I0, 11). Comparison of the human chromosome composition of the hybrids with the presence of Leu-2/T8-specific DNA allows assignment of the Leu-2/T8 gene to chromosome 2, since this is the only chromosome for which there is complete concordance ( Table I). None of the hybrids used contains human chromosome 21. We feel we can exclude that chromosome because three independent hybrids without detectable chromosome 21 but containing chromosome 2 have hybridizing human sequences. This conclusion is confirmed by two additional types of evidence discussed below.
To further define the location of the Leu-2/T8 gene on chromosome 2 we hybridized the Leu-2/T8 probe to DNA derived from mouse-human fusions in which the human partner was a Burkitt lymphoma cell line (JI) carrying a t(2;8) translocation. In an earlier study (18) we had shown that the Ig r locus on one chromosome 2 of this cell line is translocated to a region distal (3') to an unrearranged c-myc oncogene on chromosome 8, thus enhancing c-myc transcription. The breakpoint on chromosome 2 is on band pll, between V~ genes; regions distal to this (including the C~ gene) are translocated (18). We digested DNA derived from several JI x NP3 hybrids with Eco RI and hybridized to the previously described Leu-2/T8 cDNA probe ( Fig. 1 and Table II). Humanspecific bands appeared with JI 4-5B7 DNA (8q+), while no signal was present with JI 5-4 (8 and 2p-) DNA. The weak signal with JI 4-5B7 DNA can be explained by the low frequency (10-30%) of metaphases containing the 8q+ chromosome. These results show that the Leu-2/T8 gene maps to the short arm of chromosome 2, either distal to the CK locus or, less likely, between some of the V, genes and C~.   Leu-2/T8 cDNA probe to human metaphase chromosome preparations from peripheral blood cells of a normal male. After autoradiography, we analyzed the metaphase spreads for grain localization. Examples of two such spreads are shown in Fig. 2. Over 27% of all grains were located on the short arm of chromosome 2. Over 91% of the 2p grains were between the centromere and band 2p13, with most grains at 2p12. A histogram depicting the silver grain distribution along the human chromosomes is shown in Fig. 3. T h e short arm of chromosome 2 represents ~3.3% of the haploid genome, and our observation that >27% of the Leu-2/T8 probe hybridization is localized to the proximal half of this region is highly significant (P << 0.01). Thus, cytological hybridization localizes the Leu-2/T8 gene to the pl region of chromosome 2.

Discussion
The chromosomal location of the Leu-2/T8 gene is significant because of the close linkage to the C, locus, which has previously (18) been mapped distally on the p l 1 band of chromosome 2. This linkage to C, parallels that found for the mouse Lyt-2,3 gene(s). The reasons for the maintenance of the linkage to CK in mouse and man are not known. Note that the genes encoding the/3 chain of the mouse T cell receptor are also linked to C, on chromosome 6 (23). However, that linkage is not maintained for the human T cell receptor /3 chain genes, which are located on chromosome 7, rather than 2 (23). The close linkage of both Leu-2/T8 and Lyt-2,3 to C, lends further support to the hypothesis that these two T cell differentiation antigens are homologous structures. We have now isolated mouse clones homologous to Leu-2/T8 (Rose Zamoyska, V. P. Sukhatme, and J. R. Parnes, unpublished results) and can determine whether they in fact encode Lyt-2 and/or -3.