Respiratory and systemic monocytes, dendritic cells, and myeloid-derived suppressor cells in COVID-19: Implications for disease severity.

Since the beginning of the SARS-CoV-2 pandemic in 2020, researchers worldwide have made efforts to understand the mechanisms behind the varying range of COVID-19 disease severity. Since the respiratory tract is the site of infection, and immune cells differ depending on their anatomical location, studying blood is not sufficient to understand the full immunopathogenesis in patients with COVID-19. It is becoming increasingly clear that monocytes, dendritic cells (DCs), and monocytic myeloid-derived suppressor cells (M-MDSCs) are involved in the immunopathology of COVID-19 and may play important roles in determining disease severity. Patients with mild COVID-19 display an early antiviral (interferon) response in the nasopharynx, expansion of activated intermediate monocytes, and low levels of M-MDSCs in blood. In contrast, patients with severe COVID-19 seem to lack an early efficient induction of interferons, and skew towards a more suppressive response in blood. This is characterized by downregulation of activation markers and decreased functional capacity of blood monocytes and DCs, reduced circulating DCs, and increased levels of HLA-DRlo CD14+ M-MDSCs. These suppressive characteristics could potentially contribute to delayed T-cell responses in severe COVID-19 cases. In contrast, airways of patients with severe COVID-19 display hyperinflammation with elevated levels of inflammatory monocytes and monocyte-derived macrophages, and reduced levels of tissue-resident alveolar macrophages. These monocyte-derived cells contribute to excess inflammation by producing cytokines and chemokines. Here, we review the current knowledge on the role of monocytes, DCs, and M-MDSCs in COVID-19 and how alterations and the anatomical distribution of these cell populations may relate to disease severity.

Shedding of infectious SARS-CoV-2 by hospitalized COVID-19 patients in relation to serum antibody responses.

BACKGROUND: Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a global pandemic. The understanding of the transmission and the duration of viral shedding in SARS-CoV-2 infection is still limited. OBJECTIVES: To assess the timeframe and potential risk of SARS-CoV-2 transmission from hospitalized COVID-19 patients in relation to antibody response. METHOD: We performed a cross-sectional study of 36 COVID-19 patients hospitalized at Karolinska University Hospital. Patients with more than 8 days of symptom duration were sampled from airways, for PCR analysis of SARS-CoV-2 RNA and in vitro culture of replicating virus. Serum SARS-CoV-2-specific immunoglobulin G (IgG) and neutralizing antibodies titers were assessed by immunofluorescence assay (IFA) and microneutralization assay. RESULTS: SARS-CoV-2 RNA was detected in airway samples in 23 patients (symptom duration median 15 days, range 9-53 days), whereas 13 patients were SARS-CoV-2 RNA negative (symptom duration median 21 days, range 10-37 days). Replicating virus was detected in samples from 4 patients at 9-16 days. All but two patients had detectable levels of SARS-CoV-2-specific IgG in serum, and SARS-CoV-2 neutralizing antibodies were detected in 33 out of 36 patients. Total SARS-CoV-2-specific IgG titers and neutralizing antibody titers were positively correlated. High levels of both total IgG and neutralizing antibody titers were observed in patients sampled later after symptom onset and in patients where replicating virus could not be detected. CONCLUSIONS: Our data suggest that the presence of SARS-Cov-2 specific antibodies in serum may indicate a lower risk of shedding infectious SARS-CoV-2 by hospitalized COVID-19 patients.

Human influenza virus infection elicits distinct patterns of monocyte and dendritic cell mobilization in blood and the nasopharynx.

During respiratory viral infections, the precise roles of monocytes and dendritic cells (DCs) in the nasopharynx in limiting infection and influencing disease severity are incompletely described. We studied circulating and nasopharyngeal monocytes and DCs in healthy controls (HCs) and in patients with mild to moderate infections (primarily influenza A virus [IAV]). As compared to HCs, patients with acute IAV infection displayed reduced DC but increased intermediate monocytes frequencies in blood, and an accumulation of most monocyte and DC subsets in the nasopharynx. IAV patients had more mature monocytes and DCs in the nasopharynx, and higher levels of TNFα, IL-6, and IFNα in plasma and the nasopharynx than HCs. In blood, monocytes were the most frequent cellular source of TNFα during IAV infection and remained responsive to additional stimulation with TLR7/8L. Immune responses in older patients skewed towards increased monocyte frequencies rather than DCs, suggesting a contributory role for monocytes in disease severity. In patients with other respiratory virus infections, we observed changes in monocyte and DC frequencies in the nasopharynx distinct from IAV patients, while differences in blood were more similar across infection groups. Using SomaScan, a high-throughput aptamer-based assay to study proteomic changes between patients and HCs, we found differential expression of innate immunity-related proteins in plasma and nasopharyngeal secretions of IAV and SARS-CoV-2 patients. Together, our findings demonstrate tissue-specific and pathogen-specific patterns of monocyte and DC function during human respiratory viral infections and highlight the importance of comparative investigations in blood and the nasopharynx.

Delayed generation of functional virus-specific circulating T follicular helper cells correlates with severe COVID-19.

Effective humoral immune responses require well-orchestrated B and T follicular helper (Tfh) cell interactions. Whether these interactions are impaired and associated with COVID-19 disease severity is unclear. Here, longitudinal blood samples across COVID-19 disease severity are analysed. We find that during acute infection SARS-CoV-2-specific circulating Tfh (cTfh) cells expand with disease severity. SARS-CoV-2-specific cTfh cell frequencies correlate with plasmablast frequencies and SARS-CoV-2 antibody titers, avidity and neutralization. Furthermore, cTfh cells but not other memory CD4 T cells, from severe patients better induce plasmablast differentiation and antibody production compared to cTfh cells from mild patients. However, virus-specific cTfh cell development is delayed in patients that display or later develop severe disease compared to those with mild disease, which correlates with delayed induction of high-avidity neutralizing antibodies. Our study suggests that impaired generation of functional virus-specific cTfh cells delays high-quality antibody production at an early stage, potentially enabling progression to severe disease.

Comparison of Lung-Homing Receptor Expression and Activation Profiles on NK Cell and T Cell Subsets in COVID-19 and Influenza.

Respiratory viral infections with SARS-CoV-2 and influenza viruses commonly induce a strong infiltration of immune cells into the human lung, with potential detrimental effects on the integrity of the lung tissue. Despite comprising the largest fractions of circulating lymphocytes in the lung, rather little is known about how peripheral blood natural killer (NK) cell and T cell subsets are equipped for lung-homing in COVID-19 and influenza. Here, we provide a detailed comparative analysis of NK cells and T cells in patients infected with SARS-CoV-2 or influenza virus, focusing on the protein and gene expression of chemokine receptors known to be involved in recruitment to the lung. For this, we used 28-colour flow cytometry as well as re-analysis of a publicly available single-cell RNA-seq dataset from bronchoalveolar lavage (BAL) fluid. Frequencies of NK cells and T cells expressing CXCR3, CXCR6, and CCR5 were altered in peripheral blood of COVID-19 and influenza patients, in line with increased transcript expression of CXCR3, CXCR6, and CCR5 and their respective ligands in BAL fluid. NK cells and T cells expressing lung-homing receptors displayed stronger phenotypic signs of activation compared to cells lacking lung-homing receptors, and activation was overall stronger in influenza compared to COVID-19. Together, our results indicate a role for CXCR3+, CXCR6+, and/or CCR5+ NK cells and T cells that potentially migrate to the lungs in moderate COVID-19 and influenza patients, identifying common targets for future therapeutic interventions in respiratory viral infections.

Functional monocytic myeloid-derived suppressor cells increase in blood but not airways and predict COVID-19 severity.

The immunopathology of coronavirus disease 2019 (COVID-19) remains enigmatic, causing immunodysregulation and T cell lymphopenia. Monocytic myeloid-derived suppressor cells (M-MDSCs) are T cell suppressors that expand in inflammatory conditions, but their role in acute respiratory infections remains unclear. We studied the blood and airways of patients with COVID-19 across disease severities at multiple time points. M-MDSC frequencies were elevated in blood but not in nasopharyngeal or endotracheal aspirates of patients with COVID-19 compared with healthy controls. M-MDSCs isolated from patients with COVID-19 suppressed T cell proliferation and IFN-γ production partly via an arginase 1-dependent (Arg-1-dependent) mechanism. Furthermore, patients showed increased Arg-1 and IL-6 plasma levels. Patients with COVID-19 had fewer T cells and downregulated expression of the CD3ζ chain. Ordinal regression showed that early M-MDSC frequency predicted subsequent disease severity. In conclusion, M-MDSCs expanded in the blood of patients with COVID-19, suppressed T cells, and were strongly associated with disease severity, indicating a role for M-MDSCs in the dysregulated COVID-19 immune response.

Airway antibodies emerge according to COVID-19 severity and wane rapidly but reappear after SARS-CoV-2 vaccination.

Understanding the presence and durability of antibodies against SARS-CoV-2 in the airways is required to provide insights into the ability of individuals to neutralize the virus locally and prevent viral spread. Here, we longitudinally assessed both systemic and airway immune responses upon SARS-CoV-2 infection in a clinically well-characterized cohort of 147 infected individuals representing the full spectrum of COVID-19 severity, from asymptomatic infection to fatal disease. In addition, we evaluated how SARS-CoV-2 vaccination influenced the antibody responses in a subset of these individuals during convalescence as compared with naive individuals. Not only systemic but also airway antibody responses correlated with the degree of COVID-19 disease severity. However, although systemic IgG levels were durable for up to 8 months, airway IgG and IgA declined significantly within 3 months. After vaccination, there was an increase in both systemic and airway antibodies, in particular IgG, often exceeding the levels found during acute disease. In contrast, naive individuals showed low airway antibodies after vaccination. In the former COVID-19 patients, airway antibody levels were significantly elevated after the boost vaccination, highlighting the importance of prime and boost vaccinations for previously infected individuals to obtain optimal mucosal protection.

TOX is expressed by exhausted and polyfunctional human effector memory CD8+ T cells.

CD8+ T cell exhaustion is a hallmark of many cancers and chronic infections. In mice, T cell factor 1 (TCF-1) maintains exhausted CD8+ T cell responses, whereas thymocyte selection-associated HMG box (TOX) is required for the epigenetic remodeling and survival of exhausted CD8+ T cells. However, it has remained unclear to what extent these transcription factors play analogous roles in humans. In this study, we mapped the expression of TOX and TCF-1 as a function of differentiation and specificity in the human CD8+ T cell landscape. Here, we demonstrate that circulating TOX+ CD8+ T cells exist in most humans, but that TOX is not exclusively associated with exhaustion. Effector memory CD8+ T cells generally expressed TOX, whereas naive and early-differentiated memory CD8+ T cells generally expressed TCF-1. Cytolytic gene and protein expression signatures were also defined by the expression of TOX. In the context of a relentless immune challenge, exhausted HIV-specific CD8+ T cells commonly expressed TOX, often in clusters with various activation markers and inhibitory receptors, and expressed less TCF-1. However, polyfunctional memory CD8+ T cells specific for cytomegalovirus (CMV) or Epstein-Barr virus (EBV) also expressed TOX, either with or without TCF-1. A similar phenotype was observed among HIV-specific CD8+ T cells from individuals who maintained exceptional immune control of viral replication. Collectively, these data demonstrate that TOX is expressed by most circulating effector memory CD8+ T cell subsets and not exclusively linked to exhaustion.

Influenza A Virus Infection Induces Hyperresponsiveness in Human Lung Tissue-Resident and Peripheral Blood NK Cells.

NK cells in the human lung respond to influenza A virus- (IAV-) infected target cells. However, the detailed functional capacity of human lung and peripheral blood NK cells remains to be determined in IAV and other respiratory viral infections. Here, we investigated the effects of IAV infection on human lung and peripheral blood NK cells in vitro and ex vivo following clinical infection. IAV infection of lung- and peripheral blood-derived mononuclear cells in vitro induced NK cell hyperresponsiveness to K562 target cells, including increased degranulation and cytokine production particularly in the CD56brightCD16- subset of NK cells. Furthermore, lung CD16- NK cells showed increased IAV-mediated but target cell-independent activation compared to CD16+ lung NK cells or total NK cells in peripheral blood. IAV infection rendered peripheral blood NK cells responsive toward the normally NK cell-resistant lung epithelial cell line A549, indicating that NK cell activation during IAV infection could contribute to killing of surrounding non-infected epithelial cells. In vivo, peripheral blood CD56dimCD16+ and CD56brightCD16- NK cells were primed during acute IAV infection, and a small subset of CD16-CD49a+CXCR3+ NK cells could be identified, with CD49a and CXCR3 potentially promoting homing to and tissue-retention in the lung during acute infection. Together, we show that IAV respiratory viral infections prime otherwise hyporesponsive lung NK cells, indicating that both CD16+ and CD16- NK cells including CD16-CD49a+ tissue-resident NK cells could contribute to host immunity but possibly also tissue damage in clinical IAV infection.