Defining the role of natural killer cells in COVID-19

PROJECT SUMMARY Severe COVID-19 induces strong alterations in the peripheral immune system. Some immune cell types take on a protective role in this disease, while others contribute to disease pathology. One cell type whose functional role in COVID-19 is not yet known is the natural killer (NK) cell. I previously demonstrated that NK cells, which can mediate antiviral activity, are strongly altered by severe COVID-19. I have also generated preliminary data demonstrating that SARS-CoV-2 modulates the expression of ligands for the NK cell activating receptor NKG2D and allows infected cells to avoid NK cell killing; however, the mechanisms underlying this remain unexplored. Moreover, although the NK cells of COVID-19 patients are poor mediators of cytotoxicity against tumor target cells, the functional responses of NK cells from COVID-19 patients to SARS-CoV-2-infected cells have not been examined. The proposed research seeks to resolve these critical gaps in our knowledge of the immune response to SARS-CoV-2 by investigating the mechanisms underlying the phenotype and function of NK cells in COVID-19. To do this, I will 1) Define the mechanisms by which SARS-CoV-2 modulates NK cell responses; 2) Characterize the functional responses of NK cells from COVID-19 patients with a wide range of disease severities against target cells infected with SARS-CoV-2; and 3) Elucidate the role of monocyte-NK cell crosstalk in driving NK cell exhaustion in severe COVID-19. I hypothesize that the experiments described in my proposal will show that prolonged stimulation of peripheral NK cells by monocytes contributes to exhaustion in the NK cells of severe COVID-19 patients, driving poor functional responses against SARS-CoV-2-infected target cells that are exacerbated by immune evasion mechanisms mediated by SARS-CoV-2. To test this hypothesis, I will utilize NK cells and monocytes from a large cohort of COVID-19 patients with disease severities ranging from mildly symptomatic to fatal. I will perform functional assays that test the responses of these NK cells against target cells infected with replication-competent SARS-CoV-2 in Stanford's BSL3 facilities. I will also assess the ability of monocytes from severe COVID-19 patients to induce activation and exhaustion in healthy NK cells by establishing a co-culture system with these two cell types. Overall, my experiments will uncover the basis for the NK cell phenotype observed in COVID-19, the effects of this phenotype on NK cell responses to SARS-CoV-2-infected cells, and the mechanisms by which SARS-CoV-2 modulates the susceptibility of infected cells to NK cell cytotoxicity. Thoroughly interrogating the responses of NK cells to SARS-CoV-2 will help to determine whether the role of NK cells in COVID-19 is protective or pathological and will further our collective understanding of NK cell biology. Moreover, the identification of receptors involved in the modulation of NK cell activation will inform the development of therapeutic strategies for COVID-19.