Quantitative analysis of infiltrating immune cells and bovine papillomavirus type 1 E2-positive cells in equine sarcoids

•The distributions of CD4+, CD8+, FoxP3+, RORγt−, CD206+, CD14+ and BPV-1 E2+ cells were determined in equine sarcoids.•Immunofluorescence and quantitative automated high content analysis were applied to determine cell distributions.•BPV-1 E2+ cells were increased in lesional tissue of equine sarcoids.•Increased densities of cytotoxic and T helper cells were detected in lesional and perilesional tissue of equine sarcoids.•Although the overall Th17 response was attenuated, immunosuppressive Tregs were not increased in lesional tissue. Sarcoids are the most frequently observed skin tumours in equids and consist of cutaneous accumulations of transformed fibroblasts. Their aetiopathogenesis is closely linked to a presumably abortive infection by bovine papillomavirus (BPV) types 1 and 2. In cattle, dermal fibropapillomas induced by BPV1/2 usually regress spontaneously due to a local, cell-mediated, immune response; however, equids appear to lack an effective immune response to BPV1/2 and mechanisms of immune evasion have been postulated. As a consequence, equine sarcoids tend to persist and are prone to recur. In this study, cryosections were analysed by immunofluorescent staining and a high content analysis system to determine the presence and distribution of CD4+, CD8+, FoxP3+, RORγt−, CD206+ and CD14+ cells, along with expression of the BPV1 early regulatory protein E2. A higher density of cells was positive for BPV1 E2+ within the transformed tissue than in perilesional tissue or normal skin of horses with sarcoids and control horses. The proportion of CD8+ cytotoxic T cells was significantly increased in perilesional and lesional tissues, whereas CD4+ T helper cells were present in higher density only in lesional tissue compared to normal skin from horses with and without sarcoids. The proportion of pro-inflammatory CD4+FoxP3+RORγt+ regulatory T cells was decreased in sarcoid tissue compared to perilesional, distant and control tissue. There were no significant differences in densities of CD4+FoxP3+ RORγt− regulatory T cells between sarcoids and control tissues. Equine sarcoids are characterised by infiltrations of CD8+ and CD4+ T cells, with decreased representation by pro-inflammatory CD4+FoxP3+RORγt+ regulatory T cells.