Calprotectin Impairs Platelet Survival in Patients With Primary Antiphospholipid Syndrome.

OBJECTIVE: While thrombosis and pregnancy loss are the best-known clinical features of antiphospholipid syndrome (APS), many patients also exhibit "extra-criteria" manifestations, such as thrombocytopenia. The mechanisms that drive APS thrombocytopenia are not completely understood, and no clinical biomarkers are available for predicting antiphospholipid antibody (aPL)-mediated thrombocytopenia. Calprotectin is a heterodimer of S100A8 and S100A9 that is abundant in the neutrophil cytoplasm and released upon proinflammatory neutrophil activation. Here, we sought to evaluate the presence, clinical associations, and potential mechanistic roles of circulating calprotectin in a cohort of primary APS and aPL-positive patients. METHODS: Levels of circulating calprotectin were determined in plasma by the QUANTA Flash chemiluminescent assay. A viability dye-based platelet assay was used to assess the potential impact of calprotectin on aPL-mediated thrombocytopenia. RESULTS: Circulating calprotectin was measured in 112 patients with primary APS and 30 aPL-positive (without APS criteria manifestations or lupus) patients as compared to patients with lupus (without APS), patients with unprovoked venous thrombosis (without aPL), and healthy controls. Levels of calprotectin were higher in patients with primary APS and aPL-positive patients compared to healthy controls. After adjustment for age and sex, calprotectin level correlated positively with absolute neutrophil count (r = 0.41, P < 0.001), positively with C-reactive protein level (r = 0.34, P = 0.002), and negatively with platelet count (r = -0.24, P = 0.004). Mechanistically, we found that calprotectin provoked aPL-mediated thrombocytopenia by engaging platelet surface toll-like receptor 4 and activating the NLRP3-inflammasome, thereby reducing platelet viability in a caspase-1-dependent manner. CONCLUSION: These data suggest that calprotectin has the potential to be a functional biomarker and a new therapeutic target for APS thrombocytopenia.

Circulating Calprotectin as a Predictive and Severity Biomarker in Patients with COVID-19.

BACKGROUND: New tools for the assessment and prediction of the severity of hospitalized COVID-19 patients can help direct limited resources to patients with the greatest need. Circulating levels of calprotectin (S100A8/S100A9) reflect inflammatory activity in multiple conditions, and have been described as being elevated in COVID-19 patients, but their measurement is not routinely utilized. The aim of our study was to assess the practical and predictive value of measuring circulating calprotectin levels in patients at admission and during their hospitalization. METHODS: Circulating calprotectin levels were measured in 157 hospitalized patients with COVID-19 using an automated quantitative chemiluminescent assay. RESULTS: Circulating calprotectin levels were strongly correlated with changing respiratory supplementation needs of patients. The overall trajectory of circulating calprotectin levels generally correlated with patient improvement or deterioration. CONCLUSIONS: Routine measurement of circulating calprotectin levels may offer a valuable tool to assess and monitor hospitalized patients with COVID-19, as well as other acute inflammatory conditions.

Autoantibodies stabilize neutrophil extracellular traps in COVID-19.

The release of neutrophil extracellular traps (NETs) by hyperactive neutrophils is recognized to play an important role in the thromboinflammatory milieu inherent to severe presentations of COVID-19. At the same time, a variety of functional autoantibodies have been observed in individuals with severe COVID-19, where they likely contribute to immunopathology. Here, we aimed to determine the extent to which autoantibodies might target NETs in COVID-19 and, if detected, to elucidate their potential functions and clinical associations. We measured anti-NET antibodies in 328 individuals hospitalized with COVID-19 alongside 48 healthy controls. We found high anti-NET activity in the IgG and IgM fractions of 27% and 60% of patients, respectively. There was a strong correlation between anti-NET IgG and anti-NET IgM. Both anti-NET IgG and anti-NET IgM tracked with high levels of circulating NETs, impaired oxygenation efficiency, and high circulating D-dimer. Furthermore, patients who required mechanical ventilation had a greater burden of anti-NET antibodies than did those not requiring oxygen supplementation. Levels of anti-NET IgG (and, to a lesser extent, anti-NET IgM) demonstrated an inverse correlation with the efficiency of NET degradation by COVID-19 sera. Furthermore, purified IgG from COVID-19 sera with high levels of anti-NET antibodies impaired the ability of healthy control serum to degrade NETs. In summary, many individuals hospitalized with COVID-19 have anti-NET antibodies, which likely impair NET clearance and may potentiate SARS-CoV-2-mediated thromboinflammation.