NK cells in COVID-19-from disease to vaccination

Natural killer cell dysfunction is associated with colorectal cancer with severe COVID-19

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 induced coronavirus disease 2019 (COVID-19) has posed a great challenge to public health worldwide and also increased susceptibility to colorectal cancer (CRC). Natural killer (NK) cells serve as the first line of defense in the host’s innate immune system, performing natural killing functions and mediating cytotoxicity against tumors and viruses. Therefore, a better understanding of NK cell cytotoxicity may facilitate the development of treatment strategies for CRC-associated with COVID-19.

AIM

To investigate the cytotoxic killing function of peripheral NK cells in patients with CRC and severe COVID-19 (CRC⁺ patients).

METHODS

The percentages of circulating NK and NKT cells in CRC⁺ and age-matched patients with CRC were analyzed using flow cytometry. NK cell cytotoxic activity (NKCA) and corresponding NK cytotoxic factor (NKCF) activity in peripheral blood mononuclear cells were evaluated using a Real-Time Cell Analyzer.

RESULTS

The numbers and percentage of peripheral NK and NKT cells in patients with CRC⁺ were lower than those in patients with CRC. Additionally, compared to patients with CRC, those with CRC⁺ had lower levels of NKCA and NKCF activity in lysed K562 cells. Positive correlations were observed between NKCA and NK cell numbers, NKCA and NK cell percentages, NKCF activity, and NK cell percentages in patients with CRC⁺. Furthermore, a negative correlation was observed between NKCA and the severity of COVID-19 in patients with CRC. The area under the receiver operating characteristic curve for NKCA was greater than those for the other indices.

CONCLUSION

CRC⁺ is associated with lower levels of peripheral NK cells and impaired natural cytotoxicity, contributing to the immunopathogenesis of severe COVID-19 rather than immune control.

Structural Basis of SARS-CoV-2 Nsp13-Derived Peptide-Mediated NK Cell Activation

As pivotal effectors of antiviral immunity, natural killer (NK) cells are crucial for controlling the spread of COVID-19. The nonstructural protein 13 of SARS-CoV-2 can encode a viral peptide (Nsp13232-240) preventing human leukocyte antigen E (HLA-E) from recognizing inhibitory receptor NKG2A, thereby activating NK cells. The underlying molecular mechanisms of Nsp13232-240 remain unclear. Therefore, we compared the interaction discrepancy between the self-peptide and Nsp13232-240, theoretically predicting its source. Results indicate that electrostatic interaction energy provides the main source of binding, and its attenuation greatly promotes binding affinity differences. Nsp13232-240 disrupts the hydrogen bond network between CD94 and HLA-E, impacting the binding of hot-spot residues, including Q112CD94 and E161HLA-E. Moreover, Nsp13232-240 breaks the salt bridges formed by K217NKG2A and K199NKG2A with HLA-E. Conformational changes induced by Nsp13232-240 lead to diminished atomic contacts and an unstable binding pattern. These findings provide novel insights into the immunomodulatory role of Nsp13232-240 and may inform future NK cell-mediated strategies targeting SARS-CoV-2.

Testosterone and estradiol reduce inflammation of human macrophages induced by anti-SARS-CoV-2 IgG

COVID-19, the disease caused by SARS-CoV-2, particularly causes severe inflammatory disease in elderly, obese, and male patients. Since both aging and obesity are associated with decreased testosterone and estradiol expression, we hypothesized that decreased hormone levels contribute to excessive inflammation in the context of COVID-19. Previously, we and others have shown that hyperinflammation in severe COVID-19 patients is induced by the production of pathogenic anti-spike IgG antibodies that activate alveolar macrophages. Therefore, we developed an in vitro assay in which we stimulated human macrophages with viral stimuli, anti-spike IgG immune complexes, and different sex hormones. Treatment with levels of testosterone reflecting young adults led to a significant reduction in TNF and IFN-γ production by human macrophages. In addition, estradiol significantly attenuated the production of a very broad panel of cytokines, including TNF, IL-1β, IL-6, IL-10, and IFN-γ. Both testosterone and estradiol reduced the expression of Fc gamma receptors IIa and III, the two main receptors responsible for anti-spike IgG-induced inflammation. Combined, these findings indicate that sex hormones reduce the inflammatory response of human alveolar macrophages to specific COVID-19-associated stimuli, thereby providing a potential immunological mechanism for the development of severe COVID-19 in both older male and female patients.

CD151 identifies an NK cell subset that is enriched in COVID-19 patients and correlates with disease severity

Severe coronavirus disease 2019 (COVID-19) often leads to acute respiratory distress syndrome and multi-organ dysfunction, driven by a dysregulated immune response, including a cytokine storm with elevated proinflammatory cytokine levels. Natural killer (NK) cells are part of the innate immune system with a fundamental role in the defense against viral infections. However, during COVID-19 acute infection, they exhibit an altered phenotype and impaired functionality contributing to the immunopathogenesis of the disease. In this work, we have studied a cohort of patients with COVID-19 (ranging from mild to severe) by analyzing IL-15, TGF-β, PlGF and GDF-15 plasma levels and performing multiparametric flow cytometry studies. Our results revealed that severe COVID-19 patients exhibited high levels of IL-15, PlGF and GDF-15, along with an enrichment of an NK cell subset expressing the CD151 tetraspanin, which correlated with IL-15 plasma levels and disease severity. In patients, these CD151+ NK cells displayed a more activated phenotype characterized by an increased expression of HLA-DR, CD38 and granzyme B, a distinct receptor repertoire, with lower levels of CD160 and CD31 and higher levels of CD55 and, remarkably, a higher expression of tissue-resident markers CD103 and the NK cell decidual marker CD9. Last of all, in individuals with severe disease, we identified an expansion of a CD151brightCD9+ NK cell subset, suggesting that these cells play a specific role in COVID-19. Altogether, our findings suggest that CD151+ NK cells may have a relevant role in COVID-19 immunopathogenesis.

Tissue-specific features of innate lymphoid cells in antiviral defense

Innate lymphocytes (ILCs) rapidly respond to and protect against invading pathogens and cancer. ILCs include natural killer (NK) cells, ILC1s, ILC2s, ILC3s, and lymphoid tissue inducer (LTi) cells and include type I, type II, and type III immune cells. While NK cells have been well recognized for their role in antiviral immunity, other ILC subtypes are emerging as players in antiviral defense. Each ILC subset has specialized functions that uniquely impact the antiviral immunity and health of the host depending on the tissue microenvironment. This review focuses on the specialized functions of each ILC subtype and their roles in antiviral immune responses across tissues. Several viruses within infection-prone tissues will be highlighted to provide an overview of the extent of the ILC immunity within tissues and emphasize common versus virus-specific responses.

Delineating immune variation between adult and children COVID-19 cases and associations with disease severity

The SARS-CoV-2 pandemic has emphasized the need to explore how variations in the immune system relate to the severity of the disease. This study aimed to explore inter-individual variation in response to SARS-CoV-2 infection by comparing T cell, B cell, and innate cell immune subsets among primary infected children and adults (i.e., those who had never experienced SARS-CoV-2 infection nor received vaccination previously), with varying disease severity after infection. We also examined immune subset kinetics in convalescent individuals compared to those with persistent infection to identify possible markers of immune dysfunction. Distinct immune subset differences were observed between infected adults and children, as well as among adult cases with mild, moderate, and severe disease. IgM memory B cells were absent in moderate and severe cases whereas frequencies of B cells with a lack of surface immunoglobulin expression were significantly higher in severe cases. Interestingly, these immune subsets remained stable during recovery implying that these subsets could be associated with underlying baseline immune variation. Our results offer insights into the potential immune markers associated with severe COVID-19 and provide a foundation for future research in this area.

Viral escape from NK-cell-mediated immunosurveillance: A lesson for cancer immunotherapy?

Natural killer (NK) cells are innate lymphocytes that participate in immune responses against virus-infected cells and tumors. As a countermeasure, viruses and tumors employ strategies to evade NK-cell-mediated immunosurveillance. In this review, we examine immune evasion strategies employed by viruses, focusing on examples from human CMV and severe acute respiratory syndrome coronavirus 2. We explore selected viral evasion mechanisms categorized into three classes: (1) providing ligands for the inhibitory receptor NKG2A, (2) downregulating ligands for the activating receptor NKG2D, and (3) inducing the immunosuppressive cytokine transforming growth factor (TGF)-β. For each class, we draw parallels between immune evasion by viruses and tumors, reviewing potential opportunities for overcoming evasion in cancer therapy. We suggest that in-depth investigations of host-pathogen interactions between viruses and NK cells will not only deepen our understanding of viral immune evasion but also shed light on how NK cells counter such evasion attempts. Thus, due to the parallels of immune evasion by viruses and tumors, we propose that insights gained from antiviral NK-cell responses may serve as valuable lessons that can be leveraged for designing future cancer immunotherapies.