Use of HLA loss mutants to analyse the structure of the human major histocompatibility complex

Class I loci of the human major histocompatibility complex (MHC) encode 44,000-molecular weight polypeptides that associate nonconvalently with β-microglobulin¹. Included in this category are the HLA-A, -B and -C loci (Fig. 1a), which are extensively polymorphic². HLA-specific cDNA clones^3,4 now allow this polymorphism to be studied at the DNA level. However, the lack of sufficient amino acid sequence data for all but a few of the class 1 antigens presents a major difficulty in the allelic assignment of those DNA clones obtained in man^3-5 and mouse ^6-8. HLA loss variants produced with deletion-inducing ionizing radiation offer a means of partially circumventing the time-consuming and difficult task of sequencing HLA class I gene products for identifying restriction enzyme digest fragments of DNA and recombinant DNA clones. P.K. et al.⁹ used γ-ray irradiation followed by immunoselection to obtain many mutants that no longer expressed one or more HLA specificities from the human lymphoblastoid cell line LCL 721. Only the expression of HLA-B8 was lost in one class of mutant, while expressions of HLA-B8 and at least one additional cis- linked HLA allele were lost in mutants of the second class. Some mutants lost expression of the entire haplotype, that is, HLA-A1, -B8 and -DR3 (ref. 9) as well as SB (refs 4, 10 and unpublished results). Recently, anti-HLA-B5 and anti-HLA-A2 sera have been used to isolate mutants that have lost expressions of one or more loci of the haplotype on the other chromosome 6 of LCL 721 (Table 1)¹⁰. Data presented here illustrate how one cDNA clone of an HLA class I gene, pHLA-1, in combination with γ-ray-induced HLA loss variants, will be used to identify recombinant DNA HLA clones and to correlate individual DNA restriction fragments with expression of specific HLA alleles.