Histocompatibility leukocyte
antigen (HLA)-A2 is used as a restricting
element to present several
melanoma-associated
antigen (MAA)-derived
peptides to cytotoxic T lymphocytes (CTLs).
HLA-A2 antigen is selectively lost in primary
melanoma lesions and more frequently in
metastases. Only scanty information is available about the molecular mechanisms underlying this abnormality, in spite of its potentially negative impact on the
clinical course of the disease and on the outcome of T cell-based
immunotherapy. Therefore, in this study we have shown that the selective
HLA-A2 antigen loss in
melanoma cells 624MEL28 is caused by a splicing defect of
HLA-A2 pre-mRNA because of a base substitution at the 5'
splice donor site of intron 2 of the
HLA-A2 gene. As a result,
HLA-A2 transcripts are spliced to two aberrant forms, one with exon 2 skipping and the other with intron 2 retention. The latter is not translated because of an early
premature stop codon in the retained intron. In contrast, the transcript with exon 2 skipping is translated to a truncated
HLA-A2 heavy chain without the alpha(1) domain. Such a
polypeptide is synthesized in vitro but is not detectable in cells, probably because of the low steady state level of the corresponding
mRNA and the low translation efficiency. These results indicate that a single mutational event in an HLA class I gene is sufficient for loss of the corresponding allele. This may account, at least in part, for the high frequency of selective HLA class I allele loss in
melanoma cells. Our conclusion emphasizes the need to implement active specific
immunotherapy with a combination of
peptides presented by various HLA class I alleles. This strategy may counteract the ability of
melanoma cells with selective HLA class I allele loss to escape from immune recognition.