[The Ly-Li system, a new locus of the HLA complex]

Antibodies raised through immunization of volunteers not differing for serologically defined HLA-A, B and C antigens enabled us to define since 1975 a new antigenic system controlled by the HLA complex. These new allo-antigens, designated Ly-Li, are expressed on B lymphocytes but are absent from T lymphocytes, platelets, erythrocytes and fibroblasts. Multiple alleles belong to the Ly-Li differentiation antigen system. The gene frequencies of three alleles thus far detected are 0.1558 for Li2; 0.1867 for Li3 and 0.122 for Li4. Like in the serologically defined HLA antigen systems, "inclusions" were observed also in Ly-Li system, suggesting the existence of private and public specificities. Among 48 families, 23 were informative in showing that the Ly-Li locus segregated with the HLA complex. Data on three families with recombinant haplotypes between HLA-B and D, and between Bf and HLA-D, indicated that the Ly-Li locus was near HLA-D (possibly identical with HLA-D). Anti-Ly-Li antibodies inhibited cellular proliferation in mixed leucocyte culture (MLC) primarily when directed against the antigens of stimulator cells. There was a good correlation between Ly-Li and HLA-D alleles, particularly between Li2 and DW5 (r = 0.70). Usually, HLA-D specificities were "included" in the related Ly-Li specificities, but not vice-versa. In contrast, there was a higher correlation between Ly-Li specificities and those detected by the PLT (primed lymphocyte test). The Ly-Li system seems to be of great importance for the functional characterization of lymphocyte subpopulations, for the selection of the best donor in organ transplantation, and for the investigation of susceptibility genes in diseases associated with HLA-D.