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The global COVID-19 pandemic has claimed the lives of more than 750,000 US citizens. Dysregulation of the immune system underlies the pathogenesis of COVID-19, with inflammation mediated tissue injury to the lung in the setting of suppressed systemic immune function. To define the molecular mechanisms of immune dysfunction in COVID-19 we utilized a systems immunology approach centered on the circulating leukocyte phosphoproteome measured by mass cytometry. We find that although COVID-19 is associated with wholesale activation of a broad set of signaling pathways across myeloid and lymphoid cell populations, STAT3 phosphorylation predominated in both monocytes and T cells. STAT3 phosphorylation was tightly correlated with circulating IL-6 levels and high levels of phospho-STAT3 was associated with decreased markers of myeloid cell maturation/activation and decreased ex-vivo T cell IFN-γ production, demonstrating that during COVID-19 dysregulated cellular activation is associated with suppression of immune effector cell function. Collectively, these data reconcile the systemic inflammatory response and functional immunosuppression induced by COVID-19 and suggest STAT3 signaling may be the central pathophysiologic mechanism driving immune dysfunction in COVID-19.

Dap12 and FcRγ, the two transmembrane ITAM-containing signaling adaptors expressed in dendritic cells (DC), are implicated in the regulation of DC function. Several activating and adhesion receptors including integrins require these chains for their function in triggering downstream signaling and effector pathways, however the exact role(s) for Dap12 and FcRγ remains elusive as their loss can lead to both attenuating and enhancing effects. Here, we report that mice congenitally lacking both Dap12 and FcRγ chains (DF) show a massively enhanced effector CD8(+) T cell response to protein antigen immunization or West Nile Virus (WNV) infection. Thus, immunization of DF mice with MHCI-restricted OVA peptide leads to accumulation of IL-12-producing monocyte-derived dendritic cells (Mo-DC) in draining lymph nodes, followed by vastly enhanced generation of antigen-specific IFNγ-producing CD8(+) T cells. Moreover, DF mice show increased viral clearance in the WNV infection model. Depletion of CCR2+ monocytes/macrophages in vivo by administration anti-CCR2 antibodies or clodronate liposomes completely prevents the exaggerated CD8+ T cell response in DF mice. Mechanistically, we show that the loss of Dap12 and FcRγ-mediated signals in Mo-DC leads to a disruption of GM-CSF receptor-induced STAT5 activation resulting in upregulation of expression of IRF8, a transcription factor. Consequently, Dap12- and FcRγ-deficiency exacerbates GM-CSF-driven monocyte differentiation and production of inflammatory Mo-DC. Our data suggest a novel cross-talk between DC-ITAM and GM-CSF signaling pathways, which controls Mo-DC differentiation, IL-12 production, and CD8(+) T cell responses.