HLA-restricted cytotoxic T-lymphocyte (CTL) recognition of human papillomavirus (HPV) oncogene products may be important in the control of the HPV infections associated with the development of cervical cancer. We have identified, in HLA-B7 individuals, a consistent variation in the HPV16 E6 oncoprotein sequence, which alters an HLA-B7 peptide binding epitope in a way likely to influence immune recognition by CTLs. These results illustrate a biologically relevant mechanism for escape from immune surveillance of HPV16 in HLA-B7 individuals. Thus, both HLA type and HPV16 strain variation need to be considered in the screening of at-risk individuals and for the rational design of anti-HPV vaccines.

Summary The development of cervical carcinoma is strongly associated with specific types of human papillomaviruses (HPVs). A role for cellular immunity in cervical disease is supported by the increased occurrence of HPV-associated lesions in immunosuppressed individuals. Upon viral infection or malignant transformation, ensuing alterations in gene expression result in the generation of novel sets of peptides which can form complexes with specific HLA class I heavy chains and P2-microglobulin. These are then expressed at the cell surface as potential targets for specific T cells. In this study of 100 carcinomas HLA-A and -B class I expression by the tumour cells was down-regulated at one or more alleles in at least 73% of cervical carcinomas. Interference with the transporter associated with antigen presentation (TAP), which translocates cytosolic peptides from endogenously synthesised proteins (e.g. viral) into the lumen of the endoplasmic reticulum was found in 38% of the HLA class I down-regulated tumours. Loss of expression for common HLA class I alleles ranged from 36% to 71%, and such changes might be expected to influence specific immunogenic peptide presentation and consequent immune recognition. These results underline the importance of single as well as multiple allelic loss in cervical neoplasia and have important implications for attempts to intervene immunologically in cervical cancer.

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The consistent dysregulation of HLA expression in cervical neoplasia is likely to influence the natural history of the disease and prospects for cell-mediated vaccine therapies. We have studied the underlying mechanisms in eight new cervical cancer cell lines derived from primary tumour biopsies. At least five independent mechanisms leading to changes in HLA expression were seen: HLA class I allelic transcription but no protein; abnormal HLA class I allelic transcription; no HLA-B locus transcription; loss of heterozygosity (LOH); no gammaIFN-mediated upregulation of HLA class I expression, and/or no interferon-gamma (gammaIFN)-mediated HLA class II induction. These were evident in different combinations in 7/8 cell lines showing that multiple, mostly irreversible mechanisms not overridden by gammaIFN, are responsible for HLA dysregulation in cervical neoplasia. Point mutations were responsible for lack of HLA-A2 expression in two cases. In cell line 808, the mutation encodes a stop codon in exon 3; in cell line 778, mutation of the first intron acceptor site leads to use of an alternative AG site in exon 2, resulting in a frameshift and a stop codon after the translation of only 38 amino acids. Tumour cells showing specific HLA class I loss may have selective advantage in the face of tumour-specific cytotoxic T cells (CTL). Such immune escape mechanisms present a major obstacle for the success of CTL-mediated therapies in cervical cancer.