The effect of chemical fixation with paraformaldehyde, glutardialdehyde or methanol on immunofluorescence staining of neutrophils and neutrophil extracellular traps

The formation of neutrophil extracellular traps (NETs) is known as an important part of the innate immune response. Still, some mechanisms regarding their formation and role during a disease are not completely understood yet. To visualize NETs by immunofluorescence microscopy, a chemical fixation is required. Therefore, this study focused on the effect of chemical fixatives on immunofluorescence staining of selected neutrophil and NET-markers, including myeloperoxidase (MPO), DNA/histone-1-complexes and citrullinated histone H3 (H3cit). Neutrophils isolated from fresh human blood were stimulated with phorbol-12-myristate 13-acetate (PMA) to induce NETs and fixed with paraformaldehyde (PFA, 4%), glutardialdehyde (GA, 5%) or methanol (MeOH, 100%) using different incubation times depending on the used fixative. We found that different fixation times with PFA had no effect on the staining intensity of MPO or DNA/histone-1-complex antibodies. For the staining of H3cit, fixation with PFA for 24 h decreased the signal intensity whereas 30 min fixation time had no effect. In contrast, glutardialdehyde induced a high amount of autofluorescence, and the fixation with 100% MeOH resulted in visible cellular damage. Therefore, we recommend 15-30 min PFA fixation for the respective stainings. Our results provide a solid basis for future experiments to study neutrophil activation and NET-formation.