Involvement of the chaperone tapasin in HLA-B44 allelic losses in colorectal tumors

Tumors can exhibit selective allelic losses of HLA class I antigens as part of altered HLA phenotypes. In colorectal tumors, the HLA class I allele most frequently lost is HLA-B44, although the precise mechanism responsible for this loss has not been described to date. From a total of 95 colorectal cryopreserved tumor samples, we selected, (by immunohistochemical staining) 13 tumors with HLA-B44-negative expression. Loss of heterozygosity at 6p21.3 was demonstrated to be the cause of the negative expression in 4 cases. In the remaining 9 cases, structural analyses of microdissected tissue samples of the 3 subtypes of HLA-B44 loss in these tumors (B*4402, B*4403 and B*4405) did not reveal any mutations. However, all 3 subtypes of HLA-B44 presented in this study shared A common characteristic: the presence of an aspartic amino acid residue at position 114 in the HLA class I heavy chain. This residue has been described as determining tapasin dependence for the surface expression of these alleles and therefore for antigen presentation. We studied tapasin transcription by RT-PCR in these tumors and found tapasin downregulation in all 9 tumors samples with the HLA-B44-negative phenotype. In contrast, tapasin was normally transcribed in HLA-B44-positive colorectal tumors samples, as well as in 3 HLA-B44-negative laryngeal carcinomas and I bladder tumor. Defective tapasin transcription seems to be an alteration responsible for the absence of HLA-B44 expression in colorectal tumors, thus contributing to the generation of tumor immune escape phenotypes. (C) 2004 Wiley-Liss, Inc.