Novel allele-specific, post-translational reduction in HLA class I surface expression in a mutant human B cell line

Human B cell line .220 has a novel defect in HLA class I cell surface expression. Mutant .220 was derived from .184TG(r), from which both copies of HLA-A and -B are deleted, and has less surface HLA-C than .184TG(r). Transfer of class I genes into .220 revealed allele-specific reductions in surface expression: HLA-A1 and -B8 were 1 to 21% of normal; HLA-A11, -A24, and -B5 were moderately reduced; and HLA-A2, -A3, and -B7 were reduced little, if at all. Class I mRNA in .220(A1) and .220(B8) transferents is normal in size and at least normal in quantity. Surface expression of class I molecules was restored by fusing .220 transferents with mutant .174, which lacks the TAP-1 and -2 genes needed for transport of class I-binding peptides. Fusion of .220(A1) cells with β₂-microglobulin-deficient Daudi cells also fully restored surface expression of class I molecules encoded by both parental cells, indicating β₂-microglobulin is functional in .220. Pulse-chase experiments showed transgene-encoded HLA-A1 and -B8 α-chains are made in apparently normal amounts and associate with β₂-microglobulin in .220. However, post-translational processing of the HLA-A1 and -B8 molecules is retarded in or before the Golgi apparatus, and immunoprecipitable HLA-A1 molecules disappear after their synthesis. The effects of these abnormalities on surface expression of class I molecules were reversed by incubating .220(A1) and .220(B8) cells at 21°C, which greatly increased the amounts of cell surface HLA-A1 and -B8.