Platelets play a major role in the processes of primary hemostasis and pathological inflammation-induced thrombosis. In the mid-2000s, several studies expanded the role of these particular cells, placing them in the “immune continuum” and thus changing the understanding of their function in both innate and adaptive immune responses. Among the many receptors they express on their surface, platelets express Toll-Like Receptors (TLRs), key receptors in the inflammatory cell–cell reaction and in the interaction between innate and adaptive immunity. In response to an infectious stimulus, platelets will become differentially activated. Platelet activation is variable depending on whether platelets are activated by a hemostatic or pathogen stimulus. This review highlights the role that platelets play in platelet modulation count and adaptative immune response during viral infection.

Soluble CD40 ligand (sCD40L) is a molecule known for its thromboinflammatory properties and may act as a biomarker for platelet activation. Platelets are the principal producers of sCD40L, which is recognized for its impact on platelet function. However, its contribution to the platelet hyperreactivity observed in SARS-CoV-2 infection remains poorly understood. During viral infection, platelets function as crucial intermediaries, engaging with both viruses and leukocytes; and serve as a substantial source of inflammatory mediators, promoting thromboinflammation and immunothrombosis. While platelet hyperactivation is associated with the severity and mortality of COVID-19, the precise function of sCD40L in this setting remains inadequately defined. This study examined the role of platelet-derived sCD40L in platelet activation, aggregation, and thrombosis associated with COVID-19. Platelets from blood samples of 160 patients—102 with non-severe cases and 58 with severe cases—demonstrated heightened activation and aggregation, as well as elevated sCD40L release. In a mouse thrombosis model, sCD40L intensified thrombus development. These findings underscore the essential function of platelet-derived sCD40L in the pathophysiology of COVID-19 and endorse the therapeutic potential of targeting CD40L-mediated pathways to mitigate thromboinflammatory consequences.

Despite the prevalence of fucosylated IgG in plasma, specific IgGs with low core fucosylation sporadically emerge in response to virus infections and blood cell alloantigens. This low fucosylation of IgG is implicated in the pathogenesis of SARS-CoV-2 and dengue infections. In COVID-19, the presence of IgGs with low core fucosylation (afucosylated IgGs) targeting spike protein predicts disease progression to a severe form and actively mediates this progression. This study reveals that SARS-CoV-2 infection of megakaryocytes promotes the generation of pathogenic afucosylated anti-spike IgGs, leading to outcomes, such as pulmonary vascular thrombosis, acute lung injury, and mortality in FcγRIIa-transgenic mice. Platelets from mice injected with virus-infected human megakaryocytes express significant activation biomarkers, indicating a direct link between the immune response and platelet activation. Mice injected with virus-infected human megakaryocytes demonstrate an elevated rate of thrombus formation induced by FeCl₃ (4%) and a reduction in bleeding time, emphasizing the intricate interplay of viral infection, immune response, and hemostatic complications. Treatment with inhibitors targeting FcγRIIa, serotonin, or complement anaphylatoxins of mice injected with spike-expressing MKs successfully prevents observed platelet activation, thrombus formation, and bleeding abnormalities, offering potential therapeutic strategies for managing severe outcomes associated with afucosylated IgGs in COVID-19 and related disorders.

More than 90% of total human plasma immunoglobulin G (IgG) is found in a fucosylated form, but specific IgGs with low core fucosylation (afucosylated IgGs) are found in response to infections with enveloped viruses and to alloantigens on blood cells. Afucosylated IgGs mediate immunopathology in severe COVID-19 and dengue fever in humans. In COVID-19, the early formation of non-neutralizing afucosylated IgG against the spike protein predicts and directly mediates disease progression to severe form. IgG lacking core fucosylation causes dramatically increased antibody-dependent cellular toxicity mediated by intense FcγR-mediated stimulation of macrophages, monocytes, natural killer cells, and platelets. The mechanism and the context within which afucosylated IgG formation occurs in response to enveloped virus antigens have remained elusive thus far in COVID-19, dengue fever, and other infections. This study demonstrates that administration of human bone marrow megakaryocytes infected by SARS-CoV-2 into the circulation of K18-hACE2 transgenic mice drives the formation of pathogenic afucosylated anti-spike IgG antibodies, and is sufficient to reproduce severe COVID-19 manifestations of pulmonary vascular thrombosis, acute lung injury, and death in mice.

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