S100A8/A9 Is a Marker for the Release of Neutrophil Extracellular Traps and Induces Neutrophil Activation

Abstract

Neutrophils are the most abundant innate immune cells in the circulation and they are the first cells recruited to sites of infection or inflammation. Almost half of the intracellular protein content in neutrophils consists of S100A8 and S100A9, though there has been controversy about their actual localization. Once released extracellularly, these proteins are thought to act as damage-associated molecular patterns (DAMPs), though their mechanism of action is not well understood. These S100 proteins mainly form heterodimers (S100A8/A9, also known as calprotectin) and this heterocomplex is recognized as a useful biomarker for several inflammatory diseases. We observed that S100A8/A9 is highly present in the cytoplasmic fraction of neutrophils and is not part of the granule content. Furthermore, we found that S100A8/A9 was not released in parallel with granular content but upon the formation of neutrophil extracellular traps (NETs). Accordingly, neutrophils of patients with chronic granulomatous disease, who are deficient in phorbol 12-myristate 13-acetate (PMA)-induced NETosis, did not release S100A8/A9 upon PMA stimulation. Moreover, we purified S100A8/A9 from the cytoplasmic fraction of neutrophils and found that S100A8/A9 could induce neutrophil activation, including adhesion and CD11b upregulation, indicating that this DAMP might amplify neutrophil activation.

A persistent neutrophil-associated immune signature characterises post-COVID19 pulmonary sequelae

“…The secretion of alarmins (e.g. S100A8/A9) have additionally been shown to mediate activation of dysregulated neutrophils in animal models and are released upon formation of neutrophil extracellular traps inducing neutrophil activation, adhesion and CD11b upregulation 34,35 again reinforcing the likely importance of neutrophilic in ammation in the development of post-COVID-19 lung sequelae.…”

Antimicrobial peptides: Defending the mucosal epithelial barrier

“…In the extracellular space, calprotectin is available to bind receptors ( Table 1 , Figure 2) , assemble heterodimers and other oligomeric forms [ 189 ], and sequester metal ions [ 190 ]. Soluble calprotectin may be recognized as a damage-associated molecular pattern (DAMP) or alarmin to activate neutrophils or function as an antimicrobial protein to suppress microbial growth [ 191 ]. Antimicrobial activity is increased locally when calprotectin incorporates into neutrophil extracellular traps (NETs) [ 191 ].…”

“…Soluble calprotectin may be recognized as a damage-associated molecular pattern (DAMP) or alarmin to activate neutrophils or function as an antimicrobial protein to suppress microbial growth [ 191 ]. Antimicrobial activity is increased locally when calprotectin incorporates into neutrophil extracellular traps (NETs) [ 191 ].…”

“…Calprotectin in NETs may be essential to clear infections in the tissues as observed in a mouse model of Candida albicans infection [ 67 ]. When complexed in NETs, calprotectin can activate neutrophils by upregulating CD11b and increasing cellular adhesion [ 191 ]. S100A9 released from the neutrophil can bind the neutrophil RAGE receptor and potentiate further release of NETs [ 178 ].…”

“…Inflammation is also generally accompanied by interstitial acidosis, which serves as another danger signal [ 224 ], and release of cellular calcium into the tissues [ 189 ]. In human periodontitis [ 164 , 165 ], the neutrophil-rich inflammatory environment would also be expected to contain increased extracellular levels of calprotectin [ 191 ]. The elevation in calcium promotes tetramerization of calprotectin, increasing antimicrobial activity.…”