The β chain of the HLA-DQ molecule is shorter by eight amino acid residues than other major histocompatibility complex class II β chains due to elimination of the fifth exon coding for part of the cytoplasmic domain. This elimination is caused by one base substitution in the splice accepter site of the exon. We found that two HLA-DQB1 alleles, DQB1*0503 and DQB1*0601, did not have this substitution, and the exon was utilized in these two alleles. However, two forms of HLA-DQB mRNA, with or without exon 5, were generated in Epstein-Barr virus-transformed cell lines homozygous for DQB1*0503 or DQB1*0601, indicating alternative mRNA splicing. The alternative splicing of DQB1*0601 mRNA was also found in peripheral blood lymphocytes and L cell transfectants. To investigate the functional relevance of the allele-specific long cytoplasmic tail of HLA-DQ β chain, we developed three types of L cell transfectants expressing exclusively the HLA-DQw6 molecules with short cytoplasmic tail, long cytoplasmic tail, or both forms of the β chain, and used them as antigen presenting cells for streptococcal cell wall antigen-specific T cell lines. These three types of transfectants could function almost equally well as antigen presenting cells. It was thus demonstrated that both forms of HLA-DQ β chain, with or without eight amino acid residues coded for by the exon 5, can be associated with the HLA-DQ α chain, be expressed on the cell surface, and function as restriction molecules in antigen recognition by the CD4+ T cells.
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