Review 1: "Outcome of SARS-CoV-2 infection linked to MAIT cell activation and cytotoxicity: evidence for an IL-18 dependent mechanism"

RR:C19 Evidence Scale rating by reviewer:

• Strong. The main study claims are very well-justified by the data and analytic methods used. There is little room for doubt that the study produced has very similar results and conclusions as compared with the hypothetical ideal study. The study’s main claims should be considered conclusive and actionable without reservation.

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Review:

Severe COVID-19 pathology and death is a direct consequence of both SARS-CoV-2 and the accompanying immune response, characterised by dysregulated cytokine production. As MAIT cells are exquisite cytokine and virus responsive cells abundant in the peripheral blood and lungs ^1-3, Flament et al. ⁴ sought to characterise their circulating phenotype in COVID-19 infection. 

This paper uses whole blood and PBMCs from an impressive cohort of 182 patients; samples from 51 moderate, 51 severe and 82 age and BMI-matched healthy controls were used from four sites during the early pandemic. The main robust finding is a reduction in peripheral MAIT cell frequency as a fraction of T cells correlating with severity of disease. This replicates contemporaneous results from other groups in COVID-19 ^5,6, and is supported by similar results in other virus and bacterial infections ^1,7,8. The major strength of this study is the larger cohort size and attempts to match confounders such as age and co-morbidities known to influence MAIT cell frequency.

Similar to other studies ^5,6, Flament et al. phenotype peripheral MAIT cells and confirm activation in COVID-19 patients. Unsupervised analysis of albeit preselected flow parameters identifies unstimulated MAIT cell CD69 and CD56 expression as key correlates of severity. Given the appreciable 41% mortality, they report a sensitive correlation of MAIT cell activation (CD69 expression) with fatal outcomes. Although not unique to MAIT cells, the correlation is more striking than other cytokine responsive cell types and through multiparameter assessment correlated with other clinical measures of severity, including oxygenation, SAPSII score and CRP. This data is in agreement with another recent study by Parott et al. ⁵, but conflicts with a study by Jouan et al. ⁶ which found MAIT cell activation to correlate with improved PaO₂/FiO₂. This could be due to differences in the study cohorts and timing of blood samples; Jouan et al. used samples from the early pandemic, with very low mortality overall and potentially different standards of care.

To understand the basis for the MAIT cell activation, Flament et al. correlated this data with serum cytokines to find an association with IL-6, IL-8, IL-10, IL-15 and IL-18 levels.  IL-18 in particular continues to rise with duration of severe disease in ICU patients, with those patients sampled at later time points having increased mortality and increased secondary bacterial infections as well as higher MAIT activation. Currently, the specificity of these findings to COVID, and severe COVID, rather than critical illness is unclear.

Finally, using an in vitro co-culture model they suggest SARS-CoV-2 infected macrophages activate MAIT cells in an early type I IFN and late IL-18 dependent manner. These cytokines are important in RNA and DNA virus mediated MAIT activation ^2,9, although such in vitro experiments can be impacted by virus and cell preparations. The kinetics of the response and comparisons with clinical serum data are more complex.

Overall, this and other studies paint a picture where MAIT cell activation and frequency are sensitive biomarker of severe illness, including COVID-19. One would potentially expect comparable observations for other cytokine-responsive T cells, including iNKT and Vd2⁺gdT cells. Heterogeneous sampling may explain conflicting predictors of mortality, and further studies following patients longitudinally together with disease controls would be required to confirm whether MAIT cell activation is indeed a predictor of disease severity – or merely an indicator of critical illness. Furthermore mechanisms of peripheral MAIT cell depletion, and their protective or pathogenic role in tissues would need to be addressed.

References

1.     Provine, N. M. & Klenerman, P. MAIT Cells in Health and Disease. Annu. Rev. Immunol. 38, annurev–immunol–080719–015428–228 (2020).

2.     van Wilgenburg, B. et al. MAIT cells are activated during human viral infections. Nat Commun 7, 11653 (2016).

3.     Wilgenburg, B. V. et al. MAIT cells contribute to protection against lethal influenza infection in vivo. Nat Commun 9, 4706 (2018).

4.     Flament, H. et al. Outcome of SARS-CoV-2 infection linked to MAIT cell activation and cytotoxicity: evidence for an IL-18 dependent mechanism. medRxiv 2020.08.31.20185082 (2020). doi:10.1101/2020.08.31.20185082

5.     Parrot, T. et al. MAIT cell activation and dynamics associated with COVID-19 disease severity. Sci Immunol 5, eabe1670 (2020).

6.     Jouan, Y. et al. Phenotypical and functional alteration of unconventional T cells in severe COVID-19 patients. J Exp Med 217, 467 (2020).

7.     Toubal, A., Nel, I., Lotersztajn, S. & Lehuen, A. Mucosal-associated invariant T cells and disease. Nat Rev Immunol 178, 1–657 (2019).

8.     Godfrey, D. I., Koay, H. F., McCluskey, J. & Gherardin, N. A. The biology and functional importance of MAIT cells. Nat. Immunol. 20, 1110–1128 (2019).

9.     Provine, N. M. et al. Activation of MAIT cells plays a critical role in viral vector vaccine immunogenicity. bioRxiv 7, 661397 (2019).