In a recent study published in the bioRxiv * prepress server, researchers at Stanford University School of Medicine revealed that the mechanism by severe acute respiratory syndrome that coronavirus 2 (SARS-CoV-2) escapes reactions cytotoxic to natural killer cells (NK).
Fund
The immune response to viral infection is significantly influenced by NK cells, which are innate lymphocytes. These cytotoxic effector cells react rapidly to viral infection by targeting and lysis of infected cells.
Studies evaluating the immune response to CoV 2019 disease (COVID-19) during the SARS-CoV-2 pandemic have found that NK cells are less common in the peripheral blood of severe patients with SARS-CoV-2 in comparison with healthy donors. In addition, the immune profile has revealed substantial transcriptional and phenotypic alterations related to the severity of peripheral NK cells that persist in the blood of patients with SARS-CoV-2.
Although several studies have shown that NK cells can inhibit SARS-CoV-2 replication in vitro, the mechanism by which NK cells react immediately to SARS-CoV-2-infected cells remains uncertain. This is especially crucial because several viruses adopt techniques to prevent them from being detected and eliminated by NK cells.
Study: SARS-CoV-2 escapes direct NK cell killing by Nsp1-mediated downregulation of NKG2D ligands
About the study
The present research aimed to evaluate the ability of SARS-CoV-2 to adjust the identification and lysis of NK cells from virus-infected cells. The team developed an in vitro model approach that analyzes the reaction of NK cells to SARS-CoV-2-infected cells using primary NK cells derived from healthy donors and SARS-CoV-2 competent for replication. To further elucidate how the balance between the detection and escape of SARS-CoV-2 leads to COVID-19, they focused on examining the direct death of SARS-CoV-2-infected target cells by NK cells.
Human adenocarcinomic basal alveolar epithelial cells (A549) – angiotensin-converting enzyme 2 (ACE2), identified and lysed by NK cells and which are infectious with SARS-CoV-2, were used as a model by the authors. to study the reaction of NK cells to COVID -19. They introduced NK cells obtained from healthy pre-activated overnight donors with interleukin 2 (IL-2) to address SARS-CoV-2-infected cells for 48 hours to understand how exposure to target cells infected with SARS-CoV-2 affects the activity of NK cells and phenotype.
The researcher assessed whether SARS-CoV-2 modifies the ability of NK cells to annihilate infected target cells. In addition, they explored how SARS-CoV-2-infected cells managed to escape being lysed and recognized by NK cells.
The authors examined the expression of NK group 2D ligand (NKG2D-L) in cells that persisted after being co-cultured with NK cells to investigate the relationship between NKG2D-L expression and death. of SARS-CoV-2 infected cells. The team then attempted to find out how SARS-CoV-2 affects NKG2D-L protein expression in SARS-CoV-2-infected cells.
NK cells respond similarly to SARS-CoV-2-infected and mock-infected target cells. A) Representative flow charts (left) and box diagram (right) showing the percentage of mNeonGreen positive A549-ACE2 cells after infection with mNeon Green SARS-CoV-2 (MOI 0.5) or medium (“simulated”) at an MOI of 0.5 for 24 or 48 hours. Bar charts represent n = 4 technical replicas ∓ SD values. B) Diagram illustrating the experimental design of NK cell destruction assays. CD) Graphs showing average fluorescence intensity (C) and% of positive NK cells (D) for eight different NK cell markers by flow cytometry after culture without targets, simulated infected targets or targets infected with SARS-CoV-2. EF) Representative flow graphs (E) and quantities (F) of the percentage of NK cells expressing CD107a and IFNγ in culture without targets, simulated infected targets or SARS-CoV-2 infected targets. Significance values were determined using a Wilcoxon graded sum test paired with Bonferroni’s correction for the multiple hypothesis test.
Results
Overall, the results of the study showed that NK cells have weak cytotoxic reactions to SARS-CoV-2-infected targets, selectively killing uninfected spectator cells. In addition, the team showed that a significant decrease in the ligands of SARS-CoV-2-infected cells for the NKG2D stimulant receptor was what caused this escape of NK cell-mediated death.
In fact, NK cells can efficiently identify and kill infected cells in the early stages of SARS-CoV-2 infection prior to NKG2D-L descent. However, NK cells lose this ability due to the expression of viral proteins within infected cells. Current data show that when introduced into culture in later time seals after SARS-CoV-2 infection, the post-expression of viral proteins that dampen the innate immune response, NK cells cannot kill cells. Effectively infected cells. Because there is a small window for the removal of infected cells before spectator cell death occurs, the researchers noted that the timing of the migration of NK cells to the site of infection could be crucial in deciding whether NK cells were pathogenic or protective in COVID-19.
Finally, the authors found that the non-structural protein 1 SARS-CoV-2 (Nsp1) causes the descent of NKG2D-L. They also illustrated that transfection with Nsp1 alone was sufficient to impart tolerance to NK cell death. This observation has crucial implications for the regulation of SARS-CoV-2 facilitated by NK cells, as the preferred evasion of infected cells with spectator cell killing could lead to the pathogenesis of SARS-CoV. -2.
Conclusions
Collectively, current research examines in depth the reaction of NK cells to SARS-CoV-2 and provides new information on the function of NK cells in COVID-19. The researchers found that SARS-CoV-2-infected cells avoid being destroyed by healthy NK cells intrinsically to the cell, leading to the preferential destruction of uninfected spectator cells. The ability of infected cells to escape NK cell identification requires that the infection last long enough to make it easier for an infected cell to display SARS-CoV-2-encoded proteins. In addition, the present results highlight the importance of studying the temporal dynamics of the NK cell reaction in SARS-CoV-2 infected cells.
In addition, the team demonstrated that lowering NKG2D-L drives the process of evading SARS-CoV-2 NK cell recognition. The main feature of the NK cell response to SARS-CoV-2, according to the findings, was the loss of NKG2D-L. The study data further demonstrated that the SARS-CoV-2 Nsp1 protein was responsible for this downregulation of the ligand and that Nsp1 itself was adequate to induce direct leakage of NK cells. Current work shows that NK cell responses to SARS-CoV-2-infected cells could be partially or completely rescued by reducing Nsp1 activity, demonstrating that this protein was an even more desirable target than previously thought. previously.
* Important news
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guided by clinical practice / health-related behavior, or treated as established information.