Role of Reserve Cells in Metaplasia and the Development of Human Papillomavirus–Associated High-Grade Squamous Intraepithelial Lesions at the Cervical Transformation Zone

Squamous cervical cancers generally arise as a result of persistent infection with high-risk human papillomaviruses (hrHPVs) and occur near the squamocolumnar junction (SCJ) and within the transformation zone (TZ). The susceptibility of the TZ to HPV-related carcinogenesis appears linked to epithelial cell plasticity, with squamous metaplasia originating from a specialized stem cell population at this site. Two alternative cell populations have been implicated: keratin (K)7+ve cuboidal cells located at the SCJ vs a more broadly distributed K17+ve cervical reserve cell population. To distinguish between the hypotheses, we utilized multiplex immunofluorescence and large-scale digital imaging to map cell populations at the TZ of 165 women with and without hrHPV infections. Our results did not reveal a distinct population of K7+ cuboidal cells at the SCJ but found instead that the cuboidal and columnar cells of the TZ express K7 and K8 throughout and lack the p63 transcription factor required for epithelial stratification. Squamous metaplasia and reserve cells, which are defined by their subcolumnar location and pattern of biomarker expression (K5/K17/P63), were conspicuous at cervical crypt entrances within the TZ extending proximally toward the endocervix. In HPV-infected tissue, crypt-entrance regions with thin high-grade squamous intraepithelial lesion pathology showed prominent expression of hrHPV E6/E7 mRNA, as detected by fluorescence in situ hybridization, and p16/MCM expression, with infection also apparent in neighboring reserve cells. In some instances, normal/uninfected reserve cells (E6/E7 mRNA-ve) and squamous metaplasia were not only seen close to these regions of hrHPV infection but also extended well beyond the infected area both laterally and by depth. Our results confirm that the reserve cells underneath the columnar epithelia at TZ have the potential to undergo malignant squamous transformation via reserve cell proliferation, in agreement with previous histopathological studies. These translational findings highlight the importance of understanding the molecular biology of the epithelial sites where HPV cancers develop and suggest that in high-risk individuals, treatment strategies should target a wider area than previously thought.