Mice expressing the E7 oncogene of HPV16 in epithelium show central tolerance, and evidence of peripheral anergising tolerance, to E7-encoded cytotoxic T-lymphocyte epitopes

Abstract

In order to derive mice which expressed both the E7 open reading frame transgene of human papillomavirus type 16 in skin and MHC class 1 restriction elements for several E7-encoded cytotoxic T-lymphocyte (CTL) epitopes, K14. HPV16E7 mice which express E7 in basal keratinocytes were crossed to the F1 generation with A2.1 K(b) transgenic mice which express the MHC binding cleft domains of human HLA A*0201, and murine H-2(b). F1 mice (denoted K14E7XA2.1) expressed E7 in the thymus at least as early as 2-5 days before birth. Immunisation of FVBXA2.1 control mice (transgenic for HLA A*0201 and H-2(b) but not for E7), with two HLA A*0201-restricted epitopes of E7 and one H- 2(b)-restricted CTL epitope of E7, gave strong primary CTL responses recognising epitope-pulsed or constitutively E7-expressing syngeneic target cells. In contrast, in immunised K14E7XA2.1 mice, the CTL responses to the H- 2(b) epitope and one of the HLA A*0201 CTL epitopes were strongly down- regulated, and to the other HLA A*0201 epitope, completely abolished, as demonstrated by percentage specific killing by bulk splenocyte cultures in cytotoxicity assays, and by CTL precursor frequency analysis. In thymus- transplanted bone marrow radiation chimeras in which the immune system of K14E7XA2.1 mice was replaced by a FVBXA2.1 immune system, specific immunisation did not result in reemergence of strong E7-directed CTL responses. In agreement with these in vitro findings, specific immunisation failed to significantly alter the course of E7-associated tumour development in K14E7XA2.1 mice. These data are consistent with a model of central deletional CTL tolerance to E7-encoded epitopes recognised in the context of two distinct MHC class 1 restriction elements, and with the possibility of peripheral T-cell anergy maintained by expression of E7 in the skin.