SARS-CoV-2 spike and nucleocapsid proteins fail to activate human dendritic cells or gamma delta T cells

gamma delta T cells are thought to contribute to immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), but the mechanisms by which they are activated by the virus are unknown. Using flow cytometry, we investigated if the two most abundant viral structural proteins, spike and nucleocapsid, can activate human gamma delta T cell subsets, directly or in the presence of dendritic cells (DC). Both proteins failed to induce interferon-gamma production by V delta 1 or V delta 2 T cells within fresh mononuclear cells or lines of expanded gamma delta T cells generated from healthy donors, but the same proteins stimulated CD3(+) cells from COVID-19 patients. The nucleocapsid protein stimulated interleukin-12 production by DC and downstream interferon-gamma production by co-cultured V delta 1 and V delta 2 T cells, but protease digestion and use of an alternative nucleocapsid preparation indicated that this activity was due to contaminating non-protein material. Thus, SARS-CoV-2 spike and nucleocapsid proteins do not have stimulatory activity for DC or gamma delta T cells. We propose that gamma delta T cell activation in COVID-19 patients is mediated by immune recognition of viral RNA or other structural proteins by gamma delta T cells, or by other immune cells, such as DC, that produce gamma delta T cell-stimulatory ligands or cytokines.