Supplementary Material for: The P2X7 Receptor Is Overexpressed in the Lesional Skin of Subjects Affected by Hidradenitis Suppurativa: A Preliminary Study

Background: P2X receptors (P2XRs) are plasma membrane channels involved in the modulation of immune responses. The role of the P2X7 receptor (P2X7R) has never been investigated in hidradenitis suppurativa (HS), which is a recurrent skin disease characterized by inflammatory nodules, scarring, and suppuration. Objective: Our aim was to investigate by immunohistochemistry (IHC) P2X7R, NLRP3 (NOD-like receptor family, pyrin domain-containing 3), and interleukin-1β (IL-1β) expression in HS lesions compared to healthy control (HC) skin. Method: The intensity of IHC immunostaining was semi-quantitatively graded for keratinocytes, neutrophils, lymphocytes, and monocytes. Statistical significance was assessed by the Mann-Whitney U test, Cohen’s κ coefficient, and χ2 test. Results: A total of 59 samples, 31 from HS and 28 from HC, were collected and analysed. In skin keratinocytes, lymphocytes, and monocytes, but not in neutrophils, P2X7R and NLRP3 protein expression was significantly increased in HS versus the HC group. IL-1β protein expression was also higher in HS versus the HC group both in skin keratinocytes and in the inflammatory infiltrate. Cohen’s κ correlation coefficients for the expression of P2X7R versus NLRP3 or IL-1β in skin keratinocytes were significant (κ = 0.43 and 0.34, respectively). The same association between P2X7R and NLRP3 or IL-1β was confirmed by χ2 tests. Conclusion: P2X7R, NLRP3, and IL-1β are overexpressed, and therefore the entire P2X7R/NLRP3/IL-1β pro-inflammatory axis is likely overactive in the skin of HS patients. This observation might provide clues to the pathogenesis of this disease and suggest novel therapies and markers of disease activity.