COVID-19 plasma exosomes promote pro-inflammatory immune responses in peripheral blood mononuclear cells

Elevated serum cytokine production in COVID-19 patients associates with disease progression and severity. However, the stimulus that initiates cytokine production in patients remains to be fully revealed. Virus-infected cells can release virus-associated exosomes, extracellular vesicles (EVs) of endocytic origin, into the blood to deliver viral cargoes able to regulate immune responses. Here, we report that plasma exosomes from COVID-19 patients contain SARS-CoV-2 RNA. COVID-19 plasma exosomes stimulated robust production of interleukin-6 (IL-6), IL-8, tumor necrosis factor-α (TNF-α), and other inflammatory cytokines and chemokines in human peripheral mononuclear cells (PBMCs). Exosome depletion abolished these stimulated responses. COVID-19 plasma exosomes induced pro-inflammatory responses in CD4+ T cells, CD8+ T cells, and CD14+ monocytes, but not significantly in regulatory T cells, Th17 T cells, or memory T cells. COVID-19 plasma exosomes carry viral double-stranded RNA (dsRNA) intermediates, protect the dsRNA cargo from RNase, and deliver the dsRNA to recipient cells. COVID-19 plasma exosomes significantly increase expression of endosomal toll-like receptors 3 (TLR3), TLR7, TLR8, and TLR9 in peripheral T cells and monocytes. Inhibition of TLR3 by a specific pharmacological inhibitor considerably reduced production of cytokines and chemokines in CD4+ and CD8+ T cells, but not in CD14+ monocytes, highlighting divergent signaling pathways of immune cells in response to COVID-19 plasma exosomes. Our results indicate a novel model of crosstalk between SARS-CoV-2 infection and immune responses able to contribute to elevated cytokine production associated with COVID-19 progression, severity, and long-haul symptoms.