Mature tertiary lymphoid structures in pilonidal sinus disease

Pilonidal sinus disease (PSD) is characterized by recurrent purulent abscesses or sinus tracts in the sacrococcygeal region. Although its pathophysiology remains unclear, mixed inflammatory infiltrates are seen on histopathology. We aimed to understand the spatial organization of immune cells using histologic and immunofluorescence techniques. We examined skin samples from 10 patients with chronic PSD and 5 site-matched healthy controls. H&E staining revealed dense immune infiltrates in PSD samples, including lymphoid and plasma cell aggregates. Immunofluorescence staining for T cells, B cells, and follicular dendritic cells demonstrated numerous tertiary lymphoid structures (TLSs). TLSs are organized lymphoid aggregates that are associated with more severe disease in several autoimmune diseases. We observed immature TLSs in 90%, mature TLSs in 40%, and germinal centers in 20% of PSD samples. No healthy samples contained TLSs. Samples with abundant plasma cells had higher TLS densities than samples with few plasma cells. This immune profile mirrors that described in hidradenitis suppurativa, a disease with overlapping clinical and histologic features, raising the possibility of a shared pathogenesis. In this study, we identify TLS and germinal center formation in PSD and correlate their presence with plasma cells, which may suggest an autoimmune component in PSD rather than autoinflammation alone.