Megakaryocytes Internalize and Are Activated by Immune Complexes.

BACKGROUND

Receptors that bind antibodies are essential for protective adaptive immune responses against antibody-opsonized pathogens, yet their engagement by antibody-autoantigen complexes can drive chronic inflammation in autoimmune diseases. Megakaryocytes, the precursor cells of platelets, express such receptors. However, their response to immunoglobulin G antibodies remains unclear.

METHODS

We used both systemic lupus erythematosus and COVID-19 as relevant examples of autoimmune and infection-driven contexts in which antibodies are involved to characterize human and mouse megakaryocyte responses.

RESULTS

We found that megakaryocytes internalized immune complexes composed of autoantigens or SARS-CoV-2. In both human and mouse megakaryocytes, immune complexes triggered the release of chemokines and procoagulant extracellular vesicles. This process required FcγRIIA (Fc gamma receptor IIA) engagement, downstream Syk (spleen tyrosine kinase) signaling, and protein translation. A detailed analysis revealed that megakaryocyte-derived extracellular vesicles did not contain organelles and were largely indistinguishable from a subset of small-sized extracellular vesicles released by activated platelets. In FcγRIIA-transgenic mice, we analyzed megakaryocytes in both the bone marrow and lungs in a lupus model, whereas megakaryocytes were examined in the lungs in a COVID-19 model. In all cases, immunoglobulins were detected in close proximity to FcγRIIA-expressing megakaryocytes. Notably, the chemokine CXCL2 (C-X-C motif ligand-2) was increased in FcγRIIA-expressing mice under disease conditions. Tissue spatial analysis revealed that CXCL2 predominantly localized to megakaryocytes, supporting these cells as a major source. Furthermore, SARS-CoV-2 stimulated megakaryocytes to release CXCL2 only in the presence of IgG from SARS-CoV-2 immune individuals, and this response was strictly dependent on FcγRIIA expression.

CONCLUSIONS

These findings suggest that megakaryocytes contribute to adaptive immune responses through FcγRIIA-mediated signaling.