Even people with minor illness from the coronavirus can develop antibodies that could leave them immunised for several weeks or more, according to an early French study that tested hospital staff with mild infections.
Researchers said the results, which have not yet been peer reviewed, were “encouraging” since little is known about the mechanisms of immunity against the novel coronavirus, especially in people with minor forms of the disease.
“We knew that people with severe forms of the disease developed antibodies within 15 days of the onset of symptoms,” said Arnaud Fontanet, head of the global health department at the Institut Pasteur, which conducted the research with the University Hospital in Strasbourg.
“We now know that this is also true for those who develop minor forms, even if the rates of antibodies are probably weaker.”
The study was carried out on 160 members of staff at two hospital sites in Strasbourg who had all tested positive for COVID-19 and suffered mild forms of the disease.
Two types of serological tests, which aim to look for a previous infection, indicated that almost all health workers – 153 out of 160 in one case, 159 out of 160 in the other – had developed antibodies within 15 days after the onset of infection.
Using a separate test to determine if the antibody could neutralise the virus, the study found some 98 percent of the volunteers had these antibodies between 28 and 41 days after the first signs of infection.
The researchers said in a statement on Tuesday that the neutralising activity of the antibodies appeared to increase over time.
Olivier Schwartz, head of Pasteur’s virus and immunity unit, said the objective would now be to monitor the “persistence of the antibody response and their capacity to neutralise” the virus over the longer term.
Immune response against SARS-CoV-2
The invasion and pathogenesis of SARS-CoV-2 are associated with the host immune response. The spike glycoprotein (S protein) on the viral envelop binds to its receptor, angiotensin-converting enzyme 2 (ACE2), on the surface of human cells.15, 23
An analysis of the structure of the SARS-CoV-2 S protein and its binding affinity for ACE2 using cryogenic electron microscopy and surface plasmon resonance showed that the structure of SARS-CoV-2 S protein is very similar to that of SARS, although with minor differences.28
The affinity of SARS-CoV-2 S protein binding to ACE2 is 10 to 20 times higher than that of the SARS S protein, suggesting that SARS-CoV-2 might transmit more readily from person to person.28
Innate immunity is the first line of defence against virus invasion. Viral infection of mammals activates intracellular pattern recognition receptors that sense pathogen-associated molecular patterns, such as double-stranded RNA or uncapped mRNA.
The recognition of pathogen-associated molecular patterns results in subsequent cytolytic immune responses, mainly through the type I interferons (IFN) and natural killer cells. Adaptive immunity also plays an important part in viral clearance via activated cytotoxic T cells that destroy virus-infected cells and antibody-producing B cells that target virus-specific antigens.
Patients with COVID-19, especially those with severe pneumonia, are reported to have substantially lower lymphocyte counts and higher plasma concentrations of a number of inflammatory cytokines such as IL-6 and tumor necrosis factor (TNF).27, 29, 30
Another study29 reported that CD4+ T cells, CD8+ T cells, and natural killer cells were reduced in severely ill patients compared with those with mild disease symptoms. Moreover, a substantial reduction of CD4+ T cell and CD8+ T cell counts in the peripheral blood was also observed in a patient who died.7
Notably, the proinflammatory subsets of T cells, including IL-17-producing CCR4+ CCR6+ CD4+ (T-helper 17 or Th17) cells and perforin and granulysin-expressing cytotoxic T cells were increased, which could be partly responsible for the severe immune injury in the lungs of this patient.7
The anti-viral immune response is crucial to eliminate the invading virus, but a robust and persistent anti-viral immune response might also cause massive production of inflammatory cytokines and damage to host tissues.31
The overproduction of cytokines caused by aberrant immune activation is known as a cytokine storm. In fact, in the late stages of coronavirus disease, including SARS, MERS, and COVID-19, cytokine storms are a major cause of disease progression and eventual death.7, 32, 33
Huang and colleagues27 found increased plasma concentrations of both Th1 (eg, IL-1β and IFNγ) and Th2 (eg, IL-10) cytokines. Notably, patients admitted to the intensive care unit (ICU) had higher plasma concentrations of IL-2, IL-7, IL-10, granulocyte-colony stimulating factor, IFNγ-induced protein-10 (IP-10), macrophage chemoattractant protein-1, macrophage inflammatory protein 1α, and TNF compared to those not admitted to the ICU.
Two other studies29, 34 also showed that plasma IL-6 concentrations were above the normal range in patients with severe symptoms of COVID-19 compared with healthy individuals and those with milder symptoms. Mehta and colleagues35 suggest that secondary haemophagocytic lymphohistiocytosis (sHLH) could be associated with severe COVID-19 cases.
HLH is a disease entity characterised by an uncontrolled cytokine storm and expansion of tissue macrophages or histiocytes that exhibit haemophagocytic activity.36 HLH can result from genetic defects in cytolytic pathways (familial or primary HLH) or other diseases such as infection, malignancy, and rheumatic disease (sHLH).37
In 1952, Farquhar and Claireaux first described cytokine storm in patients with HLH.38 The characteristics of HLH, including hypercytokinaemia, unremitting fever, cytopenias, hyperferritinaemia, and multi-organ damage, are commonly seen in seriously ill patients with COVID-19.27, 35
It is suggested that alveolar macrophages expressing ACE2 are the primary target cells for SARS-CoV-2 infection. These activated macrophages may play an important part in HLH-like cytokine storm during COVID-19.39 Thus, early identification and appropriate treatment of this hyperinflammatory status is important for reducing the mortality of patients with COVID-19. 35
Potential immunotherapy in COVID-19
Evidence has shown that asymptomatic COVID-19 carriers can transmit the disease to others and that the virus has a wider range of incubation time than initially thought (0–24 days).4
In addition, the virus displays a high infectivity. If the virus continues to mutate to lower its pathogenicity, there is a high possibility that it might coexist with humans. Therefore, there is an urgent need to develop therapies to treat SARS-CoV-2.
Repurposing of approved drugs is commonly employed to fight against newly emerged diseases, such as COVID-19, as these drugs have known pharmacokinetic and safety profiles. Due to the importance of immune imbalance in the pathogenesis of SARS-CoV-2 infection, several immune-modulating drugs that regulate different aspects of inflammation (table) are being tested for their efficacy in the treatment of severe COVID-19.
Hyperinflammation is an important determinant of disease outcome in COVID-19, and immunosuppression might be beneficial to reduce the mortality in patients with severe symptoms.7, 35 Therefore, early identification of such patients is crucial. It has been proposed that laboratory tests of ferritin, lymphocyte or leukocyte counts, platelet counts, erythrocyte counts, and sedimentation rate could be used to screen patients at high risk of hyperinflammation.
Application of the HScore, used for the evaluation of patients with sHLH, was recommended by Mehta and colleagues35 to identify patients with COVID-19 at high risk of hyperinflammation. The HScore combines both laboratory and clinical parameters, including serum aspartate aminotransferase, triglycerides, fibrinogen, ferritin, cytopaenias, body temperature, organomegaly, haemophagocytosis on bone marrow aspirate, and signs of immunosuppression.35
In addition, evaluation of cytokine profiles and immune cell subsets has important implications for selecting appropriate immunosuppressants (eg, tocilizumab could be considered in patients with high concentrations of serum IL-6). Given the fact that anti-viral immunity is required to recover from COVID-19, the pros and cons of using an immunosuppressant on these patients should be carefully considered.
The severity of the hyperinflammation and viral load or replication status needs to be taken into consideration. One way to avoid the suppression of anti-viral immunity is to choose selective instead of broad immunosuppressive drugs.
The timing of treatment is also crucial to reduce the side-effects of immunosuppression; unfortunately there is not yet any definitive evidence with regard to the appropriate timing of administration of these agents. Further studies are required to determine the appropriate timing and routes of drug administration.
TableRepurposing of immune-modulating therapies for COVID-19
| Mechanism of action | |
|---|---|
| csDMARDs | |
| Chloroquine or hydroxychloroquine | Interference with ACE2 to block virus invasion; increase of endosomal pH required for virus fusion; mild immune suppression |
| Glucocorticoids | Suppression of immune and inflammatory responses |
| Leflunomide | Inhibition of virus replication |
| Thalidomide | Reduction of inflammatory cell infiltration; reduction of cytokine storm; reduction of lung damage and pulmonary interstitial fibrosis |
| bDMARDs | |
| Tocilizumab | Blockade of IL-6 receptor and its downstream signalling pathways |
| Anakinra | Blockade of IL-1 receptor and its downstream signalling pathways |
| tsDMARDs | |
| Baricitinib | JAK inhibitor; blockade of viral invasion through the inhibition of AAK1; immune suppression |
| Ruxolitinib | JAK inhibitor; immune suppression |
| Cell therapy | |
| Stem cells | Suppression of inflammation; proviral silencing |
| Plasma therapy | |
| Convalescent plasma | Promotion of virus elimination via virus-specific antibodies |
AAK1=AP2-associated protein kinase 1. ACE2=angiotensin-converting enzyme 2. bDMARDs=biologic disease-modifying anti-rheumatic drugs. csDMARDs=conventional synthetic disease-modifying anti-rheumatic drugs. IL=interleukin. JAK=Janus kinase. tsDMARDs=targeted synthetic disease-modifying anti-rheumatic drugs.
Biological immuno-modulating drugs
IL-6 is a key inflammatory cytokine that has a critical part in inflammatory cytokine storm and is elevated in patients with COVID-19.29, 34 Tocilizumab, a recombinant humanised monoclonal antibody against the IL-6 receptor, is widely used in treatment for autoimmune diseases, such as rheumatoid arthritis.40
In patients with COVID-19, IL-6-producing CD14+ CD16+ inflammatory monocytes were significantly increased, and numbers of these cells were further increased in patients with COVID-19 admitted to the ICU.41
The authors of this study proposed that hyperactivated Th1 cells producing granulocyte-macrophage colony stimulating factor (GM-CSF) and IFNγ in the lung promote IL-6-producing monocytes through release of GM-CSF, suggesting that both IL-6 and GM-CSF might be potential therapeutic targets in patients with COVID-19.41
Tocilizumab is a first-line drug for the treatment of cytokine release syndrome (a rapid and massive release of cytokines into the blood from immune cells, usually caused by immunotherapy), especially in patients with comorbidities. In terms of mechanism, tocilizumab binds to both the membrane and soluble forms of IL-6 receptor, thereby suppressing the JAK-signal transducer and activator of transcription (STAT) signalling pathway and production of downstream inflammatory molecules.42, 43
There are many ongoing trials assessing the efficacy of tocilizumab in COVID-19 (appendix p 1). However, animal studies have shown that IL-6 is required for the clearance of viruses and control of pulmonary inflammation.44 Therefore, clinicians should pay close attention to the possibility that blocking IL-6 could interfere with viral clearance or exacerbate lung inflammation.
A recent observational study45 from China reported that tocilizumab treatment in severe COVID-19 cases resulted in improvement in COVID-19 symptoms, peripheral oxygen saturation, and lymphopenia within a few days. A substantial remission of lung lesion opacity in chest CT scan was observed in 95% of patients (19 of 20) after 5 days of treatment, and all patients were discharged after an average of 15.1 days of hospital stay.45
Blockade of the IL-1 pathway is used for the treatment of some hyperinflammation conditions. The IL-1 receptor antagonist anakinra is approved for rheumatoid arthritis, Still’s disease, and cryopyrin-associated periodic syndrome.
A phase 3 randomised controlled trial (RCT) for severe sepsis reported that treatment with anakinra was associated with a significantly lower 28-day mortality in patients who were septic with hyperinflammation, without increased adverse events.46
A retrospective analysis47 of 44 patients with sHLH who were treated with anakinra indicated that treatment with anakinra resulted in a 57% decrease of ferritin concentrations, and early initiation of anakinra was associated with reduced mortality.
Since IL-1 was reported to be increased in some patients with COVID-19,27 blockade of IL-1 seems a reasonable approach for the treatment of hyperinflammation in these patients.35 Several trials of anakinra are currently underway, including a phase 2/3 clinical trial evaluating the efficacy and safety of anakinra and emapalumab (IFNγ inhibitor) in reducing hyperinflammation and respiratory distress in patients with COVID-19 (NCT04324021; appendix p 1).
Targeted synthetic immunosuppressants
Baricitinib is a small molecule compound that selectively inhibits the kinase activity of JAK1 and JAK2. Baricitinib can be used in combination with one or more TNF inhibitors and is approved for the treatment of rheumatoid arthritis48 and psoriatic arthritis.49
Through searching the BenevolentAI database, Richardson and colleagues50 predicted that baricitinib might effectively reduce the ability of SARS-CoV-2 virus to infect lung cells.51 As noted, SARS-CoV-2 binds to the ACE2 receptor on host cells and enters lung cells through receptor-mediated endocytosis. ACE2 is widely expressed in several tissues, including renal, vascular, heart, and lung.
High concentrations of ACE2 expression on pulmonary AT2 alveolar epithelial cells makes these cells particularly susceptible to SARS-CoV-2 infection.52 AP2-associated protein kinase 1 (AAK1) regulates endocytosis via phosphorylation of the clathrin adaptor protein AP2.
Richardson and colleagues identified six high-affinity AAK1 inhibitors from 47 clinical candidates in the BenevolentAI database. Baricitinib was then further selected based on its relatively mild side-effects and the feasibility to achieve effective concentrations in the blood. In addition, baricitinib can also bind to cyclin G-related kinases, which also regulate receptor-mediated endocytosis.
The immunosuppressive function of baricitinib might also be of benefit to the hyperactive immune status in severe cases of COVID-19 where immune-mediated lung injury and ARDS might occur.
Ruxolitinib, another oral JAK1 and JAK2 inhibitor approved specifically for the treatment of myelofibrosis, has been used for the treatment of sHLH. Ruxolitinib was shown to rapidly improve respiratory, liver, and haemodynamic function in an 11-year-old boy with refractory HLH,53 and to substantially improve serum ferritin, lactate dehydrogenase, fibrinogen, and liver function in a 38-year-old female patient with refractory Epstein-Barr virus-related sHLH.54
An open-label clinical trial55 showed that ruxolitinib was well tolerated and manageable for treating sHLH, with symptoms and cytopenias improved in all (n=5) patients within the first week of ruxolitinib treatment. Concentrations of ferritin, soluble IL-2 receptor, and STAT1 phosphorylation were also reduced after the administration of ruxolitinib.55
Animal studies showed that inhibition of JAK1 and JAK2 using ruxolitinib improved weight loss, organomegaly, anaemia, thrombocytopenia, hypercytokinaemia, and tissue inflammation in animal models of both primary HLH and sHLH by reducing STAT1-dependent CD8+ T-cell expansion.56
Considering the similar hyperinflammatory nature of sHLH and severe COVID-19, JAK1 and JAK2 inhibitors such as baricitinib and ruxolitinib could be potential treatments for the hyperinflammation seen in COVID-19.50 Several registered RCTs are evaluating the efficacy of ruxolitinib and baricitinib in the treatment of COVID-19 (appendix p 2).
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