We have previously described a set of mutants (16.23-selected mutants) of a B lymphoblastoid cell line that are defective in the presentation of intact proteins to class II-restricted T cells, but effectively present immunogenic peptides. The mutations in these mutants are recessive in somatic cell hybrids and are not in Class II structural genes. Here, we report on a unique mutant, 5.2.4, in which a similar defect in class II-restricted antigen presentation has occurred in association with a one-megabase homozygous deletion in the class II region of the major histocompatibility complex (MHC). The defects in class II presentation among three of the 16.23-selected mutants, and between these mutants and 5.2.4, are noncomplementary in somatic cell hybrids. This suggests that the class II presentation-defective phenotype in all four mutants results from lesions in a single MHC-linked gene, a conclusion strengthened by the finding that in a hybrid made with a second, unrelated MHC deletion mutant, T2, the class II presentation defect in a 16.23-selected mutant is also not complemented. Mutant 5.2.4, in addition to its class II presentation defect, is also defective in surface expression of MHC class I molecules, most likely because its deletion encompasses the peptide supply factor 1 gene, whose function is known to be required for normal abundance of cell surface class I molecules. However, the surface abundance of class I molecules is normal in the 16.23-selected mutants, suggesting that the lesions affecting class I surface abundance and class II presentation result from mutations in different genes.

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