The most abundant viral mRNA species in HPV 11-infected tissue consists of two exons, joining a segment of open reading frame (ORF) E1 to ORF E4, potentially encoding the E1 up angle E4 protein. The L1 ORF encodes the major capsid protein of HPV. Our previous studies demonstrated colocalization of the HPV 11 E1 up angle E4 and L1 proteins within the same cells of HPV 11-infected human foreskin implants grown in athymic mice (the mouse xenograft system) and removed 12 weeks after implantation. Prior studies have demonstrated E1 up angle E4 transcripts early in infection and throughout the HPV 11-infected epithelium, while L1 transcripts are detected later, and in a subset of E1 up angle E4 mRNA-positive differentiated epithelial cells. Therefore, E1 up angle E4 protein may be produced at an earlier time point or in less differentiated cells than the L1 protein. To study these questions, athymic mice were implanted with HPV 11-infected human foreskin fragments. Mice were sacrificed at 1-week intervals beginning 2 weeks after implantation of tissue. The E1 up angle E4 and L1 proteins colocalized to the same differentiated epithelial cells or to tight clusters of cells in differentiated epithelial layers of HPV 11-infected implants. The E1 up angle E4 and L1 proteins were first detected 4 weeks after implantation. E1 up angle E4 protein was detected in the region of the cell membrane and cytoplasm, and never in the nucleus. L1 protein was only detected in the nucleus. Both proteins were detected in implants containing high viral copy numbers. No specific histologic changes were uniformly associated with detection of these proteins. The tight coupling of the E1 up angle E4 and L1 proteins at multiple time points suggests that expression of both proteins is necessary to complete the virus life cycle.
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