Identification of productive and transforming anal intraepithelial neoplasia using immunohistochemical markers p16INK4a and HPV E4

Background: Immunohistochemical (IHC) biomarkers enable standardised, reproducible grading of anal intraepithelial neoplasia (AIN) based on progression of HPV infection from transient productive infection to complete neoplastic transformation. We investigated the expression of IHC biomarker HPV E4, a marker of productive HPV infection, in different grades of AIN caused by low-risk (lr) and high-risk (hr) HPV infection. Methods: A reference grading of AIN was established using expert, consensus, subjective HE diagnoses supported by p16 expression for 183 anal biopsies: normal squamous epithelium 37, AIN1 67, AIN2 43, AIN3 36. Causative HPV genotype of the worst lesion was identified using laser capture microdissection and SPF10-PCR. Results: Most AIN1 lesions were caused by lrHPV (72%) whereas most AIN2 (84%) and AIN3 (92%) was caused by hrHPV. None of the normal biopsies showed E4 positivity and 49% (33/67) of AIN1, 56% (24/43) of AIN2 and 3% (1/36) of AIN3 showed E4 positivity. While the large majority of E4 positive AIN1 (26/33) and AIN2 (16/24) lesions showed extensive positivity, the E4 positive AIN3 lesion showed only focal, superficial E4 positivity. HPV E4 positivity was seen in lesions caused by lr and hr HPV. Conclusions: Our results show that when using HPV E4 in addition to p16, productive infections among both low-grade and high-grade AIN can be identified. Studies that further explore progression and regression in transforming (E4 negative) and productive (E4 positive) high-grade AIN, are necessary.