Investigating the presence of neutrophil extracellular traps in septal and lobular cutaneous panniculitides

BACKGROUND: Panniculitides are a heterogeneous group of inflammatory dermatoses involving the subcutaneous fatty tissue. Histologically, they are classified into septal and lobular panniculitis, according to the predominant localization of the inflammatory infiltrate. Neutrophils are frequently found in panniculitis, mainly at the early stages. Here, we investigated whether neutrophils contribute to various types of cutaneous panniculitis by releasing neutrophil extracellular traps (NETs). MATERIALS AND METHODS: Formalin-fixed paraffin-embedded skin biopsies from 25 patients with panniculitis were included in the study. Our cohort was divided into n = 10 erythema nodosum (septal panniculitis) and n = 15 lobular panniculitis, including n = 7 lupus panniculitis, n = 1 pancreatic panniculitis, n = 1 Weber-Christian disease, n = 1 deep fungal infection, n = 2 lipodermatosclerosis, and three cases did not have an identified etiology. The presence of neutrophils and NETs was assessed by double immunofluorescence using antibodies against elastase, a neutrophilic marker, and citrullinated histone 3, a marker of NETs. RESULTS: The mean percentages (±SEM) of elastase-positive neutrophils showing NETs were 44% ± 3% in erythema nodosum and 43% ± 7% in lobular panniculitis. The difference was not statistically significant and reflects the implication of NETs not only in severe scarring lobular panniculitis but also in benign non-scarring self-remitting reactive inflammation such as erythema nodosum. In tissues, NETs were located in the interlobular septa in erythema nodosum and in the inflamed fat lobules in lobular panniculitis. CONCLUSIONS: NETs are massively present in septal and lobular subtypes of panniculitides, suggesting their involvement in tissue damage.