Innate lymphoid cells and disease tolerance in SARS-CoV-2 infection

Age-dependent NK cell dysfunctions in severe COVID-19 patients

Natural Killer (NK) cells are key innate effectors of antiviral immune response, and their activity changes in ageing and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Here, we investigated the age-related changes of NK cell phenotype and function during SARS-CoV-2 infection, by comparing adult and elderly patients both requiring mechanical ventilation. Adult patients had a reduced number of total NK cells, while elderly showed a peculiar skewing of NK cell subsets towards the CD56lowCD16high and CD56neg phenotypes, expressing activation markers and check-point inhibitory receptors. Although NK cell degranulation ability is significantly compromised in both cohorts, IFN-γ production is impaired only in adult patients in a TGF-β-dependent manner. This inhibitory effect was associated with a shorter hospitalization time of adult patients suggesting a role for TGF-β in preventing an excessive NK cell activation and systemic inflammation. Our data highlight an age-dependent role of NK cells in shaping SARS-CoV-2 infection toward a pathophysiological evolution.

The deciphering of the immune cells and marker signature in COVID-19 pathogenesis: An update

The precise interaction between the immune system and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is critical in deciphering the pathogenesis of coronavirus disease 2019 (COVID-19) and is also vital for developing novel therapeutic tools, including monoclonal antibodies, antivirals drugs, and vaccines. Viral infections need innate and adaptive immune reactions since the various immune components, such as neutrophils, macrophages, CD4⁺ T, CD8⁺ T, and B lymphocytes, play different roles in various infections. Consequently, the characterization of innate and adaptive immune reactions toward SARS-CoV-2 is crucial for defining the pathogenicity of COVID-19. In this study, we explain what is currently understood concerning the conventional immune reactions to SARS-CoV-2 infection to shed light on the protective and pathogenic role of immune response in this case. Also, in particular, we investigate the in-depth roles of other immune mediators, including neutrophil elastase, serum amyloid A, and syndecan, in the immunopathogenesis of COVID-19.

Immune modulation as a consequence of SARS-CoV-2 infection

Erroneous immune responses in COVID-19 could have detrimental effects, which makes investigation of immune network underlying COVID-19 pathogenesis a requisite. This study aimed to investigate COVID-19 related alterations within the frame of innate and adaptive immunity. Thirty-four patients clinically diagnosed with mild, moderate and severe COVID-19 disease were enrolled in this study. Decreased ILC1 and increased ILC2 subsets were detected in mild and moderate patients compared to healthy controls. NK cell subsets and cytotoxic capacity of NK cells were decreased in severe patients. Moreover, CD3⁺ T cells were reduced in severe patients and a negative correlation was found between CD3⁺ T cells and D-dimer levels. Likewise, moderate and severe patients showed diminished CD3⁺CD8⁺ T cells. Unlike T and NK cells, plasmablast and plasma cells were elevated in patients and IgG and IgA levels were particularly increased in severe patients. Severe patients also showed elevated serum levels of pro-inflammatory cytokines such as TNF-α, IL-6 and IL-8, reduced intracellular IFN-γ and increased intracellular IL-10 levels. Our findings emphasize that SARS-CoV-2 infection significantly alters immune responses and innate and acquired immunity are differentially modulated in line with the clinical severity of the disease. Elevation of IL-10 levels in NK cells and reduction of CD3⁺ and CD8⁺ T cells in severe patients might be considered as a protective response against the harmful effect of cytokine storm seen in COVID-19.

The molecular mechanism of SARS-CoV-2 evading host antiviral innate immunity

The newly identified Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has resulted in a global health emergency (COVID-19) because of its rapid spread and high mortality. Since the virus epidemic, many pathogenic mechanisms have been revealed, and virus-related vaccines have been successfully developed and applied in clinical practice. However, the pandemic is still developing, and new mutations are still emerging. Virus pathogenicity is closely related to the immune status of the host. As innate immunity is the body’s first defense against viruses, understanding the inhibitory effect of SARS-CoV-2 on innate immunity is of great significance for determining the target of antiviral intervention. This review summarizes the molecular mechanism by which SARS-CoV-2 escapes the host immune system, including suppressing innate immune production and blocking adaptive immune priming. Here, on the one hand, we devoted ourselves to summarizing the combined action of innate immune cells, cytokines, and chemokines to fine-tune the outcome of SARS-CoV-2 infection and the related immunopathogenesis. On the other hand, we focused on the effects of the SARS-CoV-2 on innate immunity, including enhancing viral adhesion, increasing the rate of virus invasion, inhibiting the transcription and translation of immune-related mRNA, increasing cellular mRNA degradation, and inhibiting protein transmembrane transport. This review on the underlying mechanism should provide theoretical support for developing future molecular targeted drugs against SARS-CoV-2. Nevertheless, SARS-CoV-2 is a completely new virus, and people’s understanding of it is in the process of rapid growth, and various new studies are also being carried out. Although we strive to make our review as inclusive as possible, there may still be incompleteness.

Innate immunity: the first line of defense against SARS-CoV-2

The coronavirus disease 2019 (COVID-19) pandemic, caused by severe acute respiratory syndrome coronavirus (SARS-CoV)-2, continues to cause substantial morbidity and mortality. While most infections are mild, some patients experience severe and potentially fatal systemic inflammation, tissue damage, cytokine storm and acute respiratory distress syndrome. The innate immune system acts as the first line of defense, sensing the virus through pattern recognition receptors and activating inflammatory pathways that promote viral clearance. Here, we discuss innate immune processes involved in SARS-CoV-2 recognition and the resultant inflammation. Improved understanding of how the innate immune system detects and responds to SARS-CoV-2 will help identify targeted therapeutic modalities that mitigate severe disease and improve patient outcomes.

Innate and Adaptive Immune Assessment at Admission to Predict Clinical Outcome in COVID-19 Patients

During the COVID-19 pandemic, many studies have been carried out to evaluate different immune system components to search for prognostic biomarkers of the disease. A broad multiparametric antibody panel of cellular and humoral components of the innate and the adaptative immune response in patients with active SARS-CoV-2 infection has been evaluated in this study. A total of 155 patients were studied at admission into our center and were categorized according to the requirement of oxygen therapy as mild or severe (the latter being those with the requirement). The patients with severe disease were older and had high ferritin, D-dimer, C-reactive protein, troponin, interleukin-6 (IL-6) levels, and neutrophilia with lymphopenia at admission. Moreover, the patients with mild symptoms had significantly increased circulating non-classical monocytes, innate lymphoid cells, and regulatory NK cells. In contrast, severe patients had a low frequency of Th1 and regulatory T cells with increased activated and exhausted CD8 phenotype (CD8⁺CD38⁺HLADR⁺ and CD8⁺CD27^-CD28^-, respectively). The predictive model included age, ferritin, D-dimer, lymph counts, C4, CD8⁺CD27^-CD28^-, and non-classical monocytes in the logistic regression analysis. The model predicted severity with an area under the curve of 78%. Both innate and adaptive immune parameters could be considered potential predictive biomarkers of the prognosis of COVID-19 disease.

Innate Immune Response to SARS-CoV-2 Infection: From Cells to Soluble Mediators

The vulnerability of humankind to SARS-CoV-2 in the absence of a pre-existing immunity, the unpredictability of the infection outcome, and the high transmissibility, broad tissue tropism, and ability to exploit and subvert the immune response pose a major challenge and are likely perpetuating the COVID-19 pandemic. Nevertheless, this peculiar infectious scenario provides researchers with a unique opportunity for studying, with the latest immunological techniques and understandings, the immune response in SARS-CoV-2 naïve versus recovered subjects as well as in SARS-CoV-2 vaccinees. Interestingly, the current understanding of COVID-19 indicates that the combined action of innate immune cells, cytokines, and chemokines fine-tunes the outcome of SARS-CoV-2 infection and the related immunopathogenesis. Indeed, the emerging picture clearly shows that the excessive inflammatory response against this virus is among the main causes of disease severity in COVID-19 patients. In this review, the innate immune response to SARS-CoV-2 infection is described not only in light of its capacity to influence the adaptive immune response towards a protective phenotype but also with the intent to point out the multiple strategies exploited by SARS-CoV-2 to antagonize host antiviral response and, finally, to outline inborn errors predisposing individuals to COVID-19 disease severity.

Regulation and Function of ILC3s in Pulmonary Infections

Lower respiratory infections are among the leading causes of morbidity and mortality worldwide. These potentially deadly infections are further exacerbated due to the growing incidence of antimicrobial resistance. To combat these infections there is a need to better understand immune mechanisms that promote microbial clearance. This need in the context of lung infections has been further heightened with the emergence of SARS-CoV-2. Group 3 innate lymphoid cells (ILC3s) are a recently discovered tissue resident innate immune cell found at mucosal sites that respond rapidly in the event of an infection. ILC3s have clear roles in regulating mucosal immunity and tissue homeostasis in the intestine, though the immunological functions in lungs remain unclear. It has been demonstrated in both viral and bacterial pneumonia that stimulated ILC3s secrete the cytokines IL-17 and IL-22 to promote both microbial clearance as well as tissue repair. In this review, we will evaluate regulation of ILC3s during inflammation and discuss recent studies that examine ILC3 function in the context of both bacterial and viral pulmonary infections.