Mild and Asymptomatic COVID-19 Convalescents Present Long-Term Endotype of Immunosuppression Associated With Neutrophil Subsets Possessing Regulatory Functions

Abstract

The SARS-CoV-2 infection [coronavirus disease 2019 (COVID-19)] is associated with severe lymphopenia and impaired immune response, including expansion of myeloid cells with regulatory functions, e.g., so-called low-density neutrophils, containing granulocytic myeloid-derived suppressor cells (LDNs/PMN-MDSCs). These cells have been described in both infections and cancer and are known for their immunosuppressive activity. In the case of COVID-19, long-term complications have been frequently observed (long-COVID). In this context, we aimed to investigate the immune response of COVID-19 convalescents after a mild or asymptomatic course of disease. We enrolled 13 convalescents who underwent a mild or asymptomatic infection with SARS-CoV-2, confirmed by a positive result of the PCR test, and 13 healthy donors without SARS-CoV-2 infection in the past. Whole blood was used for T-cell subpopulation and LDNs/PMN-MDSCs analysis. LDNs/PMN-MDSCs and normal density neutrophils (NDNs) were sorted out by FACS and used for T-cell proliferation assay with autologous T cells activated with anti-CD3 mAb. Serum samples were used for the detection of anti-SARS-CoV-2 neutralizing IgG and GM-CSF concentration. Our results showed that in convalescents, even 3 months after infection, an elevated level of LDNs/PMN-MDSCs is still maintained in the blood, which correlates negatively with the level of CD8+ and double-negative T cells. Moreover, LDNs/PMN-MDSCs and NDNs showed a tendency for affecting the production of anti-SARS-CoV-2 S1 neutralizing antibodies. Surprisingly, our data showed that in addition to LDNs/PMN-MDSCs, NDNs from convalescents also inhibit proliferation of autologous T cells. Additionally, in the convalescent sera, we detected significantly higher concentrations of GM-CSF, indicating the role of emergency granulopoiesis. We conclude that in mild or asymptomatic COVID-19 convalescents, the neutrophil dysfunction, including propagation of PD-L1-positive LDNs/PMN-MDSCs and NDNs, is responsible for long-term endotype of immunosuppression.

Low-Density Granulocytes in Immune-Mediated Inflammatory Diseases

“…Patients with COVID-19 are also susceptible to venous and arterial thrombosis [ 72 , 73 ] and demonstrate features of autoimmunity [ 74 , 75 ]. Frequency of LDGs increases significantly in COVID-19 patients and correlates with disease severity [ 76 – 78 ], supporting earlier transcriptomic data disclosing the presence of neutrophils in PBMC from COVID-19 patients [ 79 , 80 ]. LDGs have been found to suppress the proliferation of autologous T cells in ex vivo assays and may contribute to lymphopenia and impaired immune response during severe SARS-CoV-2 infection [ 77 , 78 , 80 ].…”

“…Frequency of LDGs increases significantly in COVID-19 patients and correlates with disease severity [ 76 – 78 ], supporting earlier transcriptomic data disclosing the presence of neutrophils in PBMC from COVID-19 patients [ 79 , 80 ]. LDGs have been found to suppress the proliferation of autologous T cells in ex vivo assays and may contribute to lymphopenia and impaired immune response during severe SARS-CoV-2 infection [ 77 , 78 , 80 ]. Nonetheless, LDGs from severe COVID-19 patients, especially LDGs with intermediate CD16 expression (CD16 int LDGs), have been found to be prone to form NETs and aggregate with platelets, which promote alveolar cell apoptosis and thrombogenic coagulopathy [ 81 , 82 ].…”

Myeloid-derived suppressor cells in COVID-19: A review

“…The frequency of MDSCs was reported as high as 90% of the total circulating mononuclear cells in patients with severe disease and up to 25% in patients with mild or moderate disease [ 67 ]. Moreover, MDSCs levels have been shown to remain elevated after recovery from mild and even asymptomatic COVID-19 [ 68 ], likely contributing to the recently described post-COVID-19 immunodepression [ 69 ].…”

“… No change in e-MDSC (proportion) No [ 2 ] 14 N = 58 (COVID-19: 13 moderate, 37 severe; HD: 8) PB No Fresh M-MDSC: HLA-DR -/dim CD11b + CD14 + CD15 − PMN-MDSC: HLA-DR -/dim CD11b + CD14 − CD15 + Increase in M-MDSC and PMN-MDSC (proportion, in patients as compared to HD) No [ 84 ] 15 N = 26 (COVID-19: 13 convalescent; HD: 13) PB Yes Fresh M-MDSC: Lin − HLA-DR -/dim CD11b + CD14 + CD15 − PMN-MDSC: Lin − HLA-DR -/dim CD11b + CD14 − CD15 + e-MDSC: Lin − HLA-DR − CD11b + CD14 − CD15 − Increase in PMN-MDSC (proportion, in convalescent patients as compared to HD). No change in M-MDSC and e-MDSC (proportion) Inhibition of antigen-nonspecific T cell proliferation [ 68 ] 16 N = 67 (COVID-19: 26 mild, 15 severe; HD: 26) PB Yes Fresh M-MDSC: HLA-DR − CD11b + CD66b − CD14 + CD15 − PMN-MDSC: HLA-DR − CD11b + CD66b + CD14 − CD15 + Increase in M-MDSC and PMN-MDSC (proportion, in patients as compared to HD; increase in PMN-MDSC in mild patients as compared to severe patients) No [ 70 ] 17 N = 120 (COVID-19: 27 mild, 47 severe, 46 deceased) PB No Fresh M-MDSC: HLA-DR − CD14 + Increase in M-MDSC (proportion, in deceased patients as compared to mild and severe patients) No [ 3 ] 18 N = 80 (COVID-19: 40 ICU discharged, 40 ICU deceased) PB No Fresh <...>…”

“…There was no evidence of increased numbers of e-MDSCs in the peripheral blood of COVID-19 patients [ 2 , 68 , 72 , 75 , 76 ].…”