Professor Akiko, an immunologist at Yale University, mainly introduced the immune mechanism of long COVID and the related research work in this speech. I benefited a lot from this speech and will talk about my harvest and thinking from the following aspects.
1. Long COVID clinical case definitions:
There is no unified definition yet.
l WHO: Post COVID-19 condition occurs in individuals with a history of probable or confirmed SARS-CoV-2 infection, usually 3 months from the onset of COVID-19 with symptoms and that last for at least 2 months and cannot be explained by an alternative diagnosis. Common symptoms include fatigue, shortness of breath, cognitive dysfunction but also others and generally have an impact on everyday functioning. Symptoms may be new onset following initial recovery from an acute COVID-19 episode or persist from the initial illness. Symptoms may also fluctuate or relapse over time.
l CDC: Post-COVID conditions are a wide range of new, returning, or ongoing health problems people can experience four or more weeks after first being infected with the virus that causes COVID-19. Even people who did not have COVID-19 symptoms in the days or weeks after they were infected can have post-COVID conditions. These conditions can present as different types and combinations of health problems for different lengths of time.
l NICE: Post-COVID-19 syndrome for people who still have symptoms for more than 12 weeks after the start of acute symptoms. The guideline also makes recommendations for clinical investigations of patients presenting with new or ongoing symptoms 4 weeks or later after acute infection.
2. Incidence of long COVID
There are many studies in this area, mainly divided into two cases. One is severe patients who need to be hospitalized or even ICU rescue. After their recovery, the incidence of long COVID is as high as 50%. For those with mild or asymptomatic infection, they may gradually develop long COVID symptoms after three months of infection, and the risk is about 5-30%.
3. Symptoms of long COVID
The symptoms of long COVID are very complex, there are more than two hundred on the symptom list, mainly including fatigue, shortness of breath, memory impairment, digestive tract symptoms, and related symptoms of many other organs. Across studies, the most common symptom was fatigue, followed by cough, headache, muscle pain, loss of taste/smell, sore throat, shortness of breath, etc.
Thinking: Obviously, it is impossible for every patient to have more than two hundred symptoms. Some have one or two, and some have more than ten, so doctors need specialized knowledge to make an accurate diagnosis. Maybe we need a diagnosis and treatment guide for long COVID, so that doctors could deal with it. Given the high incidence of long COVID and the wide range of symptoms, it is likely to be a so-called "post-pandemic" and our current healthcare system is not adequate to deal with the millions of long COVID patients. Long COVID patients need a coordinated medical system of various specialties, such as cardiovascular, neurology, respiratory, dermatology, etc., in order to efficiently diagnose and treat different symptoms of long COVID patients. It is necessary to establish this kind of long COVID diagnosis and treatment institutions with multidisciplinary experts, and various specialists can quickly receive long COVID patients with corresponding symptoms, but there are not many such specialized institutions at present.
4. The immune mechanism of long COVID
(1) Persistent viral infection
That is, viral RNA or protein resides in certain tissues in the body for a long time and triggers chronic inflammation. Many studies have reported that viral antigens or RNA can be detected in various tissues, especially in the intestine, lymph nodes and other tissues.
(2) Autoimmunity
l Molecular simulation theory
When the host makes an immune response against viral antigens, it can cross-react with the host's own components and cause autoimmunity, which breaks the orderly and stable state of the body.
l Bystander effect
Activated autoreactive T cells and virus-specific T cells can initiate the bystander effect and cause immune damage.
(3) Intestinal flora imbalance or latent virus reactivation
Immune dysregulation may allow other pathogens that acutely infected individuals already carry, such as EBV virus, to reactivate and drive the disease itself.
(4) Tissue damage
Long COVID occurs when the virus and/or host immunity cause irreparable damage to tissue, or cause an inflammatory response.
Thinking: Regarding these four etiological hypotheses, is the relationship between them exclusive? Can they exist at the same time? Or is it sequential? For example, persistent viral infection can cause immunosuppression in patients through molecular mimicry mechanisms or bystander effects, leading to the reactivation of certain latent viruses. Understanding these pathogenic mechanisms horizontally or vertically is beneficial to implement precise treatment on patients.
5. Possible downstream drivers of long COVID
(1) Microclots
SARS-CoV-2 protein drives a series of coagulation processes. Knowing the components and formation of microclots is important for further understanding of long COVID according to molecular basis .
(2) Mitochondrial dysfunction
A study at Yale University suggests that long COVID patients tested by cardiopulmonary exercise have normal cardiopulmonary function and circulating oxygen levels, but oxygen extraction and utilization of tissues are impaired.
If cardiopulmonary dysfunction causes hypoxia, hyperbaric oxygen therapy can be used. However, if it is a mitochondrial disorder, other therapies to repair mitochondria are needed, and hyperbaric oxygen therapy may not be effective. Understanding the level of circulating oxygen and the level of tissue utilization is important information for hyperbaric oxygen therapy.
6. Animal studies of long COVID
Humanized mice were injected with the AAV-hACE2 vector through the trachea, so that the virus entry receptor is only expressed in the respiratory tract, and then a small dose of SARS-CoV-2 was infected intranasally, so that the mice have only mild respiratory tract infection. No significant weight loss or behavioral changes were observed in the infected mice. In addition, prolonged secretion of cytokines was found in blood as well as CSF, especially CCL11 in CSF, 7 days/week after infection. At the same time, mild SARS-CoV-2 infection of the respiratory tract was observed to cause long-term changes in the central nervous system, including a large number of reactive microglia in the white matter, decreased oligodendrocytes, loss of myelination of neuronal axons and decreased neuroblasts in the hippocampus. Interestingly, a similar increase in reactive microglia was observed in autopsy specimens.
Thinking: This animal modeling method is worth learning. On the one hand, the human immune gene is knocked into the mice seat, which imitates the key components of the human immune system. At the same time, the created model has only mild respiratory tract infection, which is consistent with natural infection. In addition, are the physiopathological changes in the brain related to some mediator or the vagus nerve? Because the vagus nerve can sense inflammation in the body, it can be transmitted to the brain by activating glia, and there are many nuclei in the brain stem that regulate disease behavior and autonomic function, etc.
7. Clinical study of Long COVID
(1) Studies have found that IFN-β and IFN-λ levels are elevated in long COVID patients 8 months after infection, compared with matched control groups (patients who no longer have symptoms 4-8 months after infection) , suggesting that there may be some kind of pathogen-associated molecular pattern which stimulates these antiviral cytokines, possibly SARS-COV-2 RNA, or something else.
(2) Clinical symptoms were collected and analyzed by proteomic and immune signatures. The study found 4 expected risk factors for long COVID: type 2 diabetes, SARS-COV-2 RNA viremia, EBV viremia, and autoantibodies (typical in lupus disease)
(3) 4 cohorts of long COVID-19 patients
1) Mount Sinal-Yale cross sectional long COVID cohort
To study the changes of immune and physiological factors in the early stage of the pandemic, and explore the differences between long COVID and convalescent controls through patient symptom surveys, flow cytometry, proteomic analysis, autoantibody and SARS-CoV-2 antibody analysis.
2) Yale RECOVERY longitudinal long COVID vaccine cohort
Blood and saliva samples were collected from patients before and after vaccination to study how the vaccine affects the symptoms and immune responses of patients with long COVID. The study showed that 60% of patients improved after vaccination, while 18.7% felt worse after vaccination.
Significance: not only to see how the vaccine response is, but also to understand what factors can make the response to vaccination positive/negative, in addition, with longitudinal collection of samples from patients with different responses (improved/unchanged/worsened) to the vaccine, it is possible to develop agents that trigger similar reactions and provide references for clinical treatment from the perspective of immunology.
3) Yale LISTEN Longitudinal symptom-based long COVID cohort
Six-step method: community (population composition), consent (sharing medical records), collecting health data, surveying data, collecting biological samples (saliva, blood), data analysis
4) Long IMPACT Longitudinal COVID inpatient follow up
Medical records, immunophenotypic information, symptoms and biological samples 2 years after discharge were collected from patients since first treatment, and long-term outcomes were predicted.
Taken together, the clinical cohort study pipeline includes: establishment of patient cohorts, flow cytometry analysis of cell types and different activation populations, in-depth analysis (cytokines, autoantibody T cell receptors, sequencing metabolomics, hormone analysis), multimodal data analysis. And clinical research includes mastering global epidemiology, understanding risk factors, exploring biomarkers of different endotypes, and designing clinical trials based on the pathophysiology of each endotype.
8. Implications of long COVID research for ME/CSF
Long COVID has many overlapping symptoms and mechanisms with ME/CSF, as well as different symptoms and pathogens, but both have symptoms of fatigue. There are four different pathogenesis of long COVID, and it may be possible to share biomarkers with ME/CFS, which will also help ME/CSF patients get treatment.
