Introduction: Dysregulations in haemostasis and inflammation is a classical feature of patients with moderate to severe COVID-19. Information on their interaction is scarce in the clinical setting. Most clinical studies have chosen control groups without respiratory syndromes, such as healthy persons.

Methods: We have included patients from the normal floor and from ICU with acute respiratory symptoms and SARS-CoV-2 negative (control) and SARS-CoV-2 positive (COVID) status. In freshly withdrawn blood, we assessed ADP-, TRAP- and AA (arachidonic acid)-induced platelet reactivity by impedance aggregometry, a broad leukocyte profile and platelet-leukocyte aggregates by flow cytometry, and a typical panel of cytokines by bead-based multiplex array (BioLegend).

Results: ADP-, TRAP- and AA-induced platelet reactivity was higher in patients with COVID than without COVID (ADP: 70U [52,8; 88] vs. 44U [26,5; 62], p=0,0027; TRAP: 80,5U [57,5; 112,3] vs. 63U [46; 83], p<0,0001; ASPI 67U [48,8; 89,3] vs. 47U [28; 86,5], p=0,0186). A larger fraction of CD4+ T helper cells, classical and intermediate monocytes and neutrophil granulocytes formed aggregates with platelets in COVID patients as compared to controls (CD4+ T helper cells: 16,49% [13,915; 18,07] vs. 12,4% [9,54; 15,71], p=0,0038; classical monocytes 24,20 [18,75; 28,89] vs. 17,86% [14,84; 25,84], p=0,0079; intermediate monocytes 27,17% [19,63; 39,08] vs. 19,23% [14,84; 25,84], p=0,0153). The cytokines IL2, IL7, IL6, IL8, IL10, IL1R-antagonist, MCP1, GCSF, INFa2, INFg, CXCL10, TNFa, and CXCL10 were all higher in COVID than control patients. Importantly, COVID survivors exhibited lower cytokines levels than COVID non-survivors, underlining the association between cytokine burst and worse clinical outcome (IL1R-antagonist 1,0506pg/ml [0,2974; 9,6535] vs. 0,1701pg/ml [0,0820; 0,3905], p>0,001; IL8 0,3414pg/ml [0,1497;0,6212] vs. 0,1323pg/ml [0,1038;0,1747], p>0,001; CXL10 1,6754pg/ml [0,7253;2,6041] vs. 0,2453pg/ml [0,0767;0,6569], p=0,004). Higher levels of IL1RA, IL-6, CXCL8 and CXCL10 were correlated with higher mean platelet volume, potentially linking increased platelet reactivity to acute activation of innate immunity. Higher granularity of neutrophils and CD8+ cells, but not other leukocyte subtypes, were also correlated with D-dimer levels. D-dimer level correlated with the fraction of CD4 and classical monocytes forming aggregates with platelets. TRAP-induced platelet aggregation and mean platelet volume correlated positively with the SOFA score, reflecting that platelet hyperreagibility contributes to a worse clinical outcome.

Conclusion: Distinct bidirectional activation mechanisms between platelets and inflammatory cell types are observed in COVID as compared to SARS-CoV2-negative patients with acute respiratory syndrome. Increased inflammatory cytokines and agonist-induced platelet aggregation indicates a worse clinical outcome and increased mortality in patients with moderate to severe COVID-19.