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

Effective humoral immune responses require well-orchestrated cellular interactions between B and T follicular helper (Tfh) cells. Whether this interaction is impaired and associated with COVID-19 disease severity is unknown. Here, longitudinal acute and convalescent blood samples from 49 COVID-19 patients across mild to severe disease were analysed. We found that during acute infection activated and SARS-CoV-2-specific circulating Tfh (cTfh) cell frequencies expanded with increasing disease severity. The frequency of activated and SARS-CoV-2-specific cTfh cells correlated with plasmablast frequencies and SARS-CoV-2 antibody titers, avidity and neutralization. Furthermore, cTfh cells but not other memory CD4 T cells, isolated from severe patients induced more pronounced differentiation of autologous plasmablast and antibody production in vitro compared to cTfh cells isolated from mild patients. However, the development of virus-specific cTfh cells was delayed in patients that displayed or later developed severe disease compared to those that maintained a mild or moderate disease. This correlated with a delayed induction of high-avidity and neutralizing virus-specific antibodies. Our study therefore suggests that impaired generation of functional virus-specific cTfh cells delays the production of high-quality antibodies to combat the infection at an early stage and thereby enabling progression to more severe COVID-19 disease.

Distinct lung-homing receptor expression and activation profiles on NK cell and T cell subsets in COVID-19 and influenza (preprint)

Respiratory viral infections with SARS-CoV-2 or influenza viruses commonly induce a strong infiltration of immune cells into the lung, with potential detrimental effects on the integrity of the lung tissue. Despite comprising the largest fractions of circulating lymphocytes in the lung, little is known about how blood natural killer (NK) cells and T cell subsets are equipped for lung-homing in COVID-19 and influenza. Using 28-colour flow cytometry and re-analysis of published RNA-seq datasets, we provide a detailed comparative analysis of NK cells and T cells in peripheral blood from moderately sick COVID-19 and influenza patients, focusing on the expression of chemokine receptors known to be involved in leukocyte recruitment to the lung. The results reveal a predominant role for CXCR3, CXCR6, and CCR5 in COVID-19 and influenza patients, mirrored by scRNA-seq signatures in peripheral blood and bronchoalveolar lavage from publicly available datasets. NK cells and T cells expressing lung-homing receptors displayed stronger phenotypic signs of activation as compared to cells lacking lung-homing receptors, and activation was overall stronger in influenza as compared to COVID-19. Together, our results indicate migration of functionally competent CXCR3 + , CXCR6 + , and/or CCR5 + NK cells and T cells to the lungs in moderate COVID-19 and influenza patients, identifying potential common targets for future therapeutic interventions in respiratory viral infections. Author summary The composition of in particular CXCR3 + and/or CXCR6 + NK cells and T cells is altered in peripheral blood upon infection with SARS-CoV-2 or influenza virus in patients with moderate disease. Lung-homing receptor-expression is biased towards phenotypically activated NK cells and T cells, suggesting a functional role for these cells co-expressing in particular CXCR3 and/or CXCR6 upon homing towards the lung.

Airway antibodies wane rapidly after COVID-19 but B cell memory is generated across disease severity (preprint)

Understanding immune responses following SARS-CoV-2 infection in relation to COVID-19 severity is critical to predicting the effects of long-term immunological memory on viral spread. Here we longitudinally assessed systemic and airway immune responses against SARS-CoV-2 in a well-characterized cohort of 147 infected individuals representing the full spectrum of COVID-19 severity; from asymptomatic infection to fatal disease. High systemic and airway antibody responses were elicited in patients with moderate to severe disease, and while systemic IgG levels were maintained after acute disease, airway IgG and IgA declined significantly. In contrast, individuals with mild symptoms showed significantly lower antibody responses but their levels of antigen-specific memory B cells were comparable with those observed in patients with moderate to severe disease. This suggests that antibodies in the airways may not be maintained at levels that prevent local virus entry upon re-exposure and therefore protection via activation of the memory B cell pool is critical.

Shedding of infectious SARS-CoV-2 from airways in hospitalized COVID-19 patients in relation to serum antibody responses (preprint)

To understand the risk of transmission of SARS-CoV-2 in hospitalized COVID-19 patients we simultaneously assessed the presence of SARS-CoV-2 RNA, live infectious virus in the airways, and virus-specific IgG and neutralizing antibodies in sera in 36 hospitalized COVID-19 patients. SARS-CoV-2 could be cultured from four patients, all with low or undetectable antibody response. Our data suggests that the level of SARS-CoV-2 antibodies may correlate to risk for shedding live SARS-CoV-2 virus in hospitalized COVID-19 patients.

Functional myeloid-derived suppressor cells expand in blood but not airways of COVID-19 patients and predict disease severity (preprint)

The immunopathology of COVID-19 remains enigmatic, exhibiting immunodysregulation and T cell lymphopenia. Monocytic myeloid-derived suppressor cells (M-MDSC) are T cell suppressors that expand in inflammatory conditions, but their role in acute respiratory infections remains unclear. We studied blood and airways of COVID-19 patients across disease severity at multiple timepoints. M-MDSC frequencies were elevated in blood but not in nasopharyngeal or endotracheal aspirates of COVID-19 patients compared to controls. M-MDSCs isolated from COVID-19 patients suppressed T cell proliferation and IFN{gamma} production partly via an arginase-1 (Arg-1) dependent mechanism. Furthermore, patients showed increased Arg-1 and IL-6 plasma levels. COVID-19 patients had fewer T cells, and displayed downregulated expression of the CD3{zeta} chain. Ordinal regression showed that early M-MDSC frequency predicted subsequent disease severity. In conclusion, M-MDSCs expand in blood of COVID-19 patients, suppress T cells and strongly associate with disease severity, suggesting a role for M-MDSCs in the dysregulated COVID-19 immune response.