gms | German Medical Science

Introduction: The human calcium-binding protein S100A12 is a granulocyte-specific pro-inflammatory danger-associated molecular pattern (DAMP) molecule that can be detected at high levels in serum of patients with auto-inflammatory conditions and sepsis. However, S100A12 might also be retained in neutrophil extracellular traps (NETs) which are released during a unique form of cell death called NETosis. In this way, S100A12 could contain the activation of responder cells at local sites of inflammation. The goal of this study is to examine the role of DAMP-dependent activation of T cells by S100A12-containing NETs.

Methods: Neutrophils were isolated from healthy human volunteers. NETosis-prone neutrophils were induced by pulse (only 15 min) incubation with PMA followed by intensive washing and seeded on coverslips. Subsequent NETosis was allowed to develop for additional 16h. S100A12 bound in NETs was characterized by fluorescence microscopy. Isolated naïve T cells were added to NETting neutrophils and to untreated, apoptotic or necrotic neutrophils. To exclude a soluble factor, T cells were separated from NETs using transwell inserts. Activation of T cells after 24h and 48h was evaluated by analysis of CD69 and CD25 expression using flow cytometry.

Results: NET formation by pulse incubation with PMA was detectable as early as 3h and reached its maximum after 16h. Fluorescence microscopy analysis of isolated NETs revealed presence of antimicrobial compound LL37, histone H1, neutrophil elastase and high amount of the DAMP protein S100A12. Interestingly, co-culture of T cells with NETting neutrophils, but not with apoptotic or necrotic granulocytes, induced high levels of CD69 and CD25 expression on T cells. Also, T cell activation was significantly reduced when separated from NETs by transwell filters.

Conclusion: Pulse stimulation of neutrophils is sufficient to induce profound NETosis. S100A12 is most abundant in NET-forming granulocytes and thus could be a driving DAMP in site-directed activation of immune cells during auto-inflammatory reactions. NETs specifically induce significant T cell activation via cell contact-dependent mechanisms.