Kang et al. (1) investigate the relations between the cytokine release syndrome (CRS), cytokines, and different acute diseases that cause CRS. Their study finds strikingly elevated levels of four proinflammatory cytokines, including IL-6, IL-8, IL-10, and MCP-1, in 91 patients with CRS associated with sepsis, acute respiratory distress syndrome (ARDS), or burns. Furthermore, the elevated levels of these cytokines are positively correlated with the increasing expression of the coagulation cascade activator plasminogen activator inhibitor-1 (PAI-1). PAI-1 is capable of causing small blood clots in blood vessels throughout the body (including pulmonary blood vessels). Importantly, the increase of PAI-1 is associated with more severe pneumonia, a common cause of death in COVID-19 patients. Finally, they find that the treatment with tocilizumab, a human monoclonal antibody that blocks IL-6 signal transduction, decreases the PAI-1 levels and alleviates severe respiratory complications in COVID-19 patients. We agree with their views on the function of IL-6 and PAI-1 in CRS. Kang et al. (1) also observe an important clinical characteristic: that the cytokines IL-6, IL-8, IL-10, and MCP-1, in the plasma of severe COVID-19 patients, are not as high as those in CRS patients with sepsis, ARDS, and burns. However, the authors did not perform statistical tests to directly investigate the associations between COVID-19 and these cytokines, which prompted us to statistically test their observations.
Instrumental variables (IVs) were introduced to explore the changing of the cytokines in COVID-19 using a two-sample Mendelian randomization (MR) analysis (2), a statistical method that has been applied for identifying the risk factors of COVID-19 (3). IVs were screened from two genome-wide association studies (GWAS): one GWAS of severe COVID-19 with respiratory failure (1,610 severe patients and 2,205 healthy controls in Italy and Spain) (4) and the other GWAS of circulating cytokines (8,293 Finnish individuals) (5). Single nucleotide polymorphisms that could be regarded as IVs must meet three standards: 1) significantly related to COVID-19 (P value < 1.0 × 10−5); 2) nonlinkage disequilibrium (R2 < 0.001); and 3) no association with cytokines (P value > 0.05). Using methods previously reported, the inverse-variance weighted (IVW) method and MR-Egger regression were conducted for main MR analysis and horizontal pleiotropy evaluation of IVs, respectively (6, 7). Based on the IVW analysis, the levels of IL-8, IL-10, and MCP-1 in COVID-19 are found to be significantly lowered in patients with CRS (Table 1). The low intercepts obtained from MR-Egger regression further indicate that the horizontal pleiotropy is unlikely to cause a bias in the levels of IL-8 and MCP-1, and IVs are unlikely to affect the level of IL-10.
Table 1.
MR analysis of the causal effect of COVID-19 on IL-6, IL-8, IL-10, and MCP-1
| Cytokines (the number of IVs) | Odds ratio based on IVW | 95% CIs based on IVW | P value based on IVW | Intercept based on MR-Egger (P value) |
| IL-6 (113) | 0.9910 | (0.9820, 1.0001) | 0.0527 | −0.0325 (9.64E-05) |
| IL-8 (116) | 0.9765 | (0.9632, 0.9899) | 0.0007 | −0.0225 (0.0582) |
| IL-10 (115) | 0.9859 | (0.9767, 0.9952) | 0.0029 | 0.0173 (0.0332) |
| MCP-1 (32) | 0.9641 | (0.9469, 0.9815) | 6.4E-05 | 0.0046 (0.6743) |
Overall, our MR analysis shows that COVID-19 induced lower levels of IL-8, IL-10, and MCP-1 than other acute diseases in CRS patients, suggesting a critical COVID-19 pathogenesis mechanism, that is, lowering the levels of these three cytokines in COVID-19 patients with CRS compared with the levels in those patents with other acute CRS-inducing diseases including bacterial sepsis, ARDS, and burns.
Acknowledgments
This work was supported by the Tou-Yan Innovation Team Program of the Heilongjiang Province (Grant 2019-15), National Natural Science Foundation of China (Grant 61871160), Young Innovative Talents in Colleges and Universities of Heilongjiang Province (Grant 2018-69), Heilongjiang Postdoctoral Fund (Grant LBH-Q20030), and a COVID-19 research grant from Michigan Medicine–Peking University Health Sciences Center Joint Institute for Clinical and Translational Research.
Footnotes
The authors declare no competing interest.
References
- 1.Kang S., et al., IL-6 trans-signaling induces plasminogen activator inhibitor-1 from vascular endothelial cells in cytokine release syndrome. Proc. Natl. Acad. Sci. U.S.A. 117, 22351–22356 (2020). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Lawlor D. A., Harbord R. M., Sterne J. A., Timpson N., Davey Smith G., Mendelian randomization: Using genes as instruments for making causal inferences in epidemiology. Stat. Med. 27, 1133–1163 (2008). [DOI] [PubMed] [Google Scholar]
- 3.Ponsford M. J., et al., Cardiometabolic traits, sepsis, and severe COVID-19: A Mendelian randomization investigation. Circulation 142, 1791–1793 (2020). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Ellinghaus D.et al.; Severe Covid-19 GWAS Group , Genomewide association study of severe Covid-19 with respiratory failure. N. Engl. J. Med. 383, 1522–1534 (2020). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.Ahola-Olli A. V., et al., Genome-wide association study identifies 27 loci influencing concentrations of circulating cytokines and growth factors. Am. J. Hum. Genet. 100, 40–50 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Burgess S., Thompson S. G., Interpreting findings from Mendelian randomization using the MR-Egger method. Eur. J. Epidemiol. 32, 377–389 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Noyce A. J.et al.; International Parkinson Disease Genomics Consortium , Estimating the causal influence of body mass index on risk of Parkinson disease: A Mendelian randomisation study. PLoS Med. 14, e1002314 (2017). [DOI] [PMC free article] [PubMed] [Google Scholar]