Genetic Mapping of a Human Class II Antigen β -Chain cDNA Clone to the SB Region of the HLA Complex

A class II antigen β -chain cDNA clone was isolated from a human B-cell cDNA library by using as a probe the murine I-Aβ gene. This cDNA clone, pHAβ , was shown to be distinct from the DCβ - and DRβ -related loci by DNA sequence analysis, thus suggesting that it might correspond to a third polymorphic human class II locus, SB, which encodes secondary B-cell antigens. Genetic mapping of this β -chain cDNA clone to the SB region was performed by the blot hybridization procedure. We showed that (i) within panels of HLA-DR homozygous human B-cell lines and of unrelated individuals who have been typed for HLA antigens, differential mobility of DNA fragments segregated with distinct SB genotypes; (ii) γ -ray-induced deletion mutants that have lost the expression of DR or DC/MT antigens but maintain SB expression preserved a pattern consistent with (a) their SB phenotype and (b) the genetic independence of the SB locus with respect to DR and DC/MT; and (iii) within an informative family, two siblings differing only for one allele at the SB locus (because of the occurrence of an internal recombination between DR and GLO) and otherwise HLA identical exhibited a restriction enzyme polymorphism linked to the SB locus. Therefore, all available data are compatible with identity between HAβ and SBβ .