A new study published online on the preprint server medRxiv* in May 2020 reports that specific changes in the way the human immune system responds to COVID-19 determine whether the individual develops moderate or severe illness.
Novel Coronavirus SARS-CoV-2 This scanning electron microscope image shows SARS-CoV-2 (yellow)—also known as 2019-nCoV, the virus that causes COVID-19—isolated from a patient in the U.S., emerging from the surface of cells (blue/pink) cultured in the lab. Image captured and colorized at NIAID's Rocky Mountain Laboratories (RML) in Hamilton, Montana. Credit: NIAID
The COVID-19 pandemic has affected most of the world’s inhabited regions, though to varying extents. It has taken the lives of over a tenth of those affected, in some regions, through the development of a severe and acute multi-organ dysfunction in which acute severe respiratory distress is prominent. This often-terminal syndrome occurs in up to 8% of all cases.
The mechanism by which the virus causes respiratory failure is largely undetermined even now, which has limited the development of proper therapies and management plans.
Many scientists think the cytokine storm syndrome is at the root of COVID-19-linked respiratory failure. This is based on the secretion of a high level of pro-inflammatory cytokines, causing inflammatory cells to be recruited and tissue damage to occur in the lung. The cytokines thought to be elevated in the lungs include IL-2, IL-7, IL-10, GCSF, IP-10, MCP-1, MIP-1α, and TNFα.
This data came from a small study comparing a few COVID-19 patients in intensive care units (ICU) to others who had less severe illness. However, a deep understanding of the inflammatory milieu is essential to properly understand how cytokines contribute to the severity of the disease.
Influenza-related acute respiratory failure is also thought to be due to a significantly elevated level of inflammation and cytokines, amounting to a cytokine storm. The current study is aimed at achieving a comparison of the inflammatory response in a group of severely ill influenza patients tested in 2019-2020, with those occurring in severe COVID-19.
How the Study Was Done
The number of COVID-19-positive patients tested by PCR was 79, with the comparison group being 26 influenza patients who were recruited in the 15 months just before the COVID-19 epidemic in the St. Louis region. The latter group was markedly older than the first or the controls.
Among the COVID-19 patients, 27% died in hospital vs. 8% of the influenza patients. The number of pre-existing diseases like diabetes and cardiovascular disease was comparable in both groups.
Reduction in Cell Counts
Both COVID-19 and influenza patients had grossly reduced numbers of all types of both CD4 and CD8 T cells and CD19 B cells. These three populations generally comprise most of the peripheral blood mononuclear cells (PBMCs). The number of activated CD4 T cells was lower in influenza patients, whereas monocyte numbers were significantly reduced in COVID-19, but one type was elevated in acute influenza. However, the number of plasmablasts in the bloodstream was much higher in the COVID-19 group.
Uneven Cytokine Expression
Cytokine expression in COVID-19 was seen in two distinct forms: in one group, very much the minority, 3 of 79 patients had extreme inflammation, with over 17/35 cytokines measured in the study present at very high levels. This is practically the definition of a cytokine storm.
On the other hand, the remaining COVID-19 patients had lower levels for 28/35 cytokines, including IL-9 and GM-CSF. Those that were significantly higher in the COVID-19 group were IL-6 and IL-8. In other words, most patients with COVID-19 showed a lower level of inflammation except for certain cytokines.
The existence of clusters of cytokines, which can be traced to prior events in the individual’s medical history, such as herpes virus infection, can confuse the changes in cytokine levels. To overcome this, the researchers used modular informatics to organize cytokines into clusters that are regulated simultaneously. This helped generate a more accurate picture of the condition-linked expression of each cytokine.
Among these clusters, two modules, designated Module 1 and Module 2, were interesting as their levels dropped as the risk of COVID-19 infection rose. The unexpected finding here is that Module 2 contains both IL-6 and IL-8, which are often found to be increased in severely ill COVID-19 patients. In the current study, they were present at higher levels in the COVID-19 patients compared to the influenza cohort.
Nonetheless, as part of the module, neither of these showed significant association with COVID-19.
Targeted Suppression of Some Inflammatory Cytokines
The analytic model suggests that higher inflammation is more characteristically a part of influenza, while COVID-19 is actually defined by lesser inflammation. Secondly, a uniformly higher level of expression was likely with higher levels of inflammation in influenza, unlike the selective rise in certain inflammatory chemicals seen with COVID-19, along with a lower level of inflammation.
A more detailed analysis showed that both Modules 1 and 2 were linked to severe disease in both influenza and COVID-19, leading to death or mechanical ventilation. The strongest associations were with high IL1-RA and IL-6 levels, which increased the odds of an adverse outcome by about 4-fold, in both groups and in the COVID-19 cohort alone.
The scientists view this as indicating the induction of these specific inflammatory pathways in the most severe COVID-19 patients, even while the overall level of inflammation is lower in these patients.
Specific Immune Pathways in COVID-19 vs. Influenza
The next step was to understand the transcription patterns in COVID-19 patients with respiratory failure so that the investigators could uncover the primary regulatory signal that decided which way the immune response was headed. This was because immune activation often results in immunosuppression as a consequence of negative feedback.
Using almost 30,000 cells from 3 patients with COVID-19, 3 with influenza, and one control, the researchers identified 22 supposed clusters of transcription molecules. These were classified into major cell groups, which were then studied for differences in either condition.
They found that several immune pathways, like IFN-alpha and IFN-gamma pathways, were specifically expressed at higher levels among influenza patient-derived cells, including B cells, CD8 cells, Tregs, and some types of macrophages and monocytes. On the other hand, pathways that are involved in the metabolism and proliferation of cells are seen to be expressed highly in cells from COVID-19 patients.
In other words, the immune cells and pathways found at higher levels in COVID-19 patients vary widely from those in influenza patients. The far more common phenotype of immune variation in COVID-19 patients was termed “targeted immunosuppression” by these workers.
This consisted of very high levels of IL-6 and IL-8, coupled with an otherwise uniformly lower level of inflammation and an almost zero level of interferon-mediated response. Overall reductions in cell number, especially in monocytes but also lymphocytes, with a higher number of plasmablasts, is also characteristic. The latter may reflect viral abundance and persistence in the host.
Cortisol and IL-6
The interferon-specific immunosuppression is significant in that IFN-gamma is very important to produce effector responses type I, which may result in low antiviral activity. The researchers also hypothesize that this uneven immunosuppression could be due to excessive cortisol production.
IL-6 is known to boost cortisol levels via a number of mechanisms. Secondly, in COVID-19, the virus may reduce ACE2 levels, increasing angiotensin II levels, which in turn increases IL-6 and cortisol production.
Finally, glucocorticoids (GCs) are either produced at a higher rate or the cells become more sensitive to them after infection with this virus. Thus, the researchers summarize the implications of their epic study: “Together these data suggest a feed-forward amplification of IL-6 and GC signaling, paired with a profound suppression of other potentially protective immune functions through GC-induced apoptosis and suppression of key antiviral pathways. GCs can also drive other pathological phenotypes, including clotting dysfunction, consistent with severe manifestations of COVID-19.”
This could help use specific IL-6 blockers and GC inhibitors in most COVID-19 patients who will show this immune phenotype, to achieve a more uniform and beneficial reduction in inflammation.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.