Tunnels in Hidradenitis Suppurativa: Active Inflammatory Entities with Specific Molecular and Genetic Profiles – A Narrative Review

Abstract

Background: Hidradenitis suppurativa (HS) is a chronic, inflammatory, debilitating disorder of the pilosebaceous unit. Dermal tunnels, sinus tracts, or fistulas are unique features of HS. One may hypothesize that HS tunnels remain active and may contribute to inflammation and disease severity. Summary: Increased inflammatory infiltrate with an increased number and densities of immune cells was reported in HS tunnels. Moreover, significantly higher levels of IL-36, Il-17A, IL-17C, IL-17F, and CXCL8 mRNA compared to non-tunnel HS skin were found. Furthermore, in the lumen, a proliferative gelatinous mass consisting of inflammatory cells with similar cytokine levels as inflammatory HS lesions was found. It was also proven that HS sinus tracts are often colonized by Porphyromonas spp. and Prevotella spp. with a tendency to biofilm creation. The genetic profile of HS tunnels varies from non-tunnel HS skin, with upregulation of pro-inflammatory and downregulation of anti-inflammatory genes. Lastly, treatment with newly developed drugs targeting different subunits of IL-17 seems promising in decreasing dermal tunnel drainage and in the resolution of sinus tracts. Moreover, a higher percentage of patients treated with these drugs achieved HiSCR75 and HiSCR90.