Respiratory viral infections remain a leading cause of morbidity and mortality. Using a murine model of human metapneumovirus (HMPV), we identified recruitment of a C1q-expressing inflammatory monocyte population concomitant with viral clearance by adaptive immune cells. Genetic ablation of C1q led to reduced CD8+ T cell function. Production of C1q by a myeloid lineage was necessary to enhance CD8+ T cell function. Activated and dividing CD8+ T cells expressed a C1q receptor, gC1qR. Perturbation of gC1qR signaling led to altered CD8+ T cell IFN-γ production, metabolic capacity, and cell proliferation. Autopsy specimens from fatal respiratory viral infections in children demonstrated diffuse production of C1q by an interstitial population. Humans with severe COVID-19 infection also demonstrated upregulation of gC1qR on activated and rapidly dividing CD8+ T cells. Collectively, these studies implicate C1q production from monocytes as a critical regulator of CD8+ T cell function following respiratory viral infection. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Antiviral immunity; COVID-19; Complement; Human metapneumovirus.