How the immune system responds to the infection is among the major questions clinicians and researchers must answer to identify therapies and develop a successful vaccine.
As the number of COVID-19 cases climbs and experience with treating the infection grows, a clear pattern is beginning to emerge: Patients with the disease caused by a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection often have lymphopenia – including very low numbers of CD4+ and CD8+ T cells.1,2
A meta-analysis of 13 studies including a total of 2,282 patients with COVID-19 showed that lymphocyte count was significantly lower in those with severe disease, and the presence of lymphopenia was associated with an almost threefold increased risk of severe COVID-19 (odds ratio = 2.99; 95% CI 1.31-6.82).2 Another study out of China found that 83.2% of 1,099 patients with COVID-19 admitted to hospitals had lymphopenia.3 Those with severe disease had particularly low lymphocyte counts compared with patients with non-severe disease. In addition, another meta-analysis available as a preprint reported that, of 2,083 severe and moderately ill patients with COVID-19, lymphopenia was significantly associated with more severe disease.4
"One question is whether the lymphopenia is induced by COVID-19 and is a parameter of the severity of the infection, or are patients who are lymphopenic particularly susceptible to a severe infection with [SARS-CoV-2]?" said David Avigan, MD, a hematologist-oncologist and immunotherapy expert at the Beth Israel Deaconess Medical Center in Boston and a professor of medicine at Harvard Medical School.
How the immune system responds to a SARS-CoV-2 infection is among the major questions clinicians and researchers must answer to identify appropriate and effective therapies, develop a long-lasting, successful vaccine, and better treat patients with COVID-19. To understand the infection's effects on the immune system and inflammation, ASH Clinical News spoke with Dr. Avigan and other clinicians and researchers who are treating and analyzing the immune response in patients with COVID-19.
Which Came First?
"When we talk about lymphopenia among patients with COVID-19, most studies have shown that it's predominantly the T lymphocytes that are depleted," explained Irini Sereti, MD, Chief of the HIV Pathogenesis Section of the Laboratory of Immunoregulation at the National Institute of Allergy and Infection Diseases. "Some studies also show that natural killer cells or B cells are somewhat depleted in severe infection as well."
But the extent of lymphocyte depletion that is caused by the viral infection is not fully understood, she added, as some of the patients with coronavirus who were analyzed were receiving corticosteroids and other medications that can affect circulating lymphocyte numbers.
"Only about 2% of the lymphocytes are represented in the peripheral blood, so we may be missing much of the picture of what is going on in the body when we only sample the blood," Dr. Sereti said. "What we can say from the data so far is that there is a significant degree of lymphopenia, especially in patients with more severe disease, that is primarily affecting the T cells," she added.
Cezmi Akdis, MD, a medical researcher and immunologist at the University of Zurich and the director of the Swiss Institute for Allergy and Asthma Research, agreed. The T lymphocytes could be decreased if the bone marrow is suppressed during a cytokine storm or if they home to the lungs in patients with pneumonia. If a patient already has a decreased lymphocyte count, he explained, a high viral dose could prevent the immune system from clearing the virus efficiently, resulting in severe disease. In this case, the levels of T cells are too low to clear the virus and can result in a cytokine storm and inflammation, eventually leading to tissue damage.
"One question is whether the lymphopenia is induced by COVID-19 and is a parameter of the severity of the infection, or are patients who are lymphopenic particularly susceptible to a severe infection with [SARS-CoV-2]."
â€”David Avigan, MD
However, both physicians acknowledged that the medical community does not have a definitive answer about what causes lymphopenia in patients infected with SARS-CoV-2.
"Most experts suspect that there is some direct effect of the viral infection on lymphocytes," said Dr. Avigan. "The lymphopenia could also be a manifestation of the inflammatory footprint that the virus causes."
SARS-CoV-2, a cytopathic virus, can induce injury and the death of those cells infected with the virus, resulting in pyroptosis, a type of pro-inflammatory programmed cell death that is linked to vascular leakage.5,6 Scientists hypothesize that the pyroptosis may trigger the inflammatory response seen in some patients with coronavirus.
Lisa F. P. Ng, PhD, a viral immunologist at the Singapore Immunology Network of the Agency for Science, Technology and Research, and colleagues recently conducted a review on immunity and inflammation seen with COVID-19 that referred to a 2005 study on SARS caused by the SARS coronavirus (SARS-CoV).7 The authors determined that SARS coronavirus particles and its RNA could be found in T cells as well as other immune cells such as monocytes and macrophages. The work suggested that SARS-CoV and possibly SARS-CoV-2 could directly infect immune cells; the infection may result in direct killing of lymphocytes and immune system dysfunction.
"The T lymphocytes are dying, disappearing rapidly in some patients with COVID-19," said Dr. Akdis. "This has also been observed with other coronavirus infections – SARS and Middle East respiratory syndrome – and appears to be due to pyroptosis, although we don't understand the mechanism yet."
Hypoxia-induced lymphocyte death also possibly occurs, according to Dr. Sereti, another process linked to inflammatory cytokines that are detrimental to the T lymphocyte population.
Understanding the ways lymphocytes are depleted in individuals infected with the novel coronavirus is vital, particularly if the lymphopenia could be targeted using novel or existing drugs, the experts told ASH Clinical News.
Lymphopenia as a Biomarker?
Regardless of how the lymphocyte population is depleted, many published studies show that among patients with COVID-19, lymphopenia correlates with a greater disease severity.
"In our experience at Beth Israel, we noticed that patients who have a more compromised immune system may have a harder time with this infection," said Dr. Avigan. "We certainly had initial concerns about patients with hematologic malignancies because they had what was felt to be a higher susceptibility to this infection and the potential for greater severity of COVID-19 due to immunosuppression," he continued.
Recent reports from China and the U.S. estimate that patients with cancer are three to five times more likely to develop severe COVID-19, and, among patients with hematologic malignancies, case fatality rates have reached up to 37%.8
In particular, prolonged lymphopenia lasting for several days – but not acute transient lymphopenia that is observed at initial presentation but quickly resolves – may be an important prognostic factor to identify patients likely to develop severe COVID-19 disease, according to Dr. Sereti.
"Lymphopenia may have clinical utility as a marker to stratify patients by disease severity," said Dr. Avigan.
Dr. Akdis agreed. He and colleagues at the Zhongnan Hospital of Wuhan University in China analyzed 289 patients hospitalized with COVID-19 to identify risk factors for severity and mortality. According to Dr. Akdis, the preliminary analysis revealed that a continuing increase of leukocyte and neutrophil counts, sustained lymphopenia, progressing decrease in platelet counts, and a high neutrophil-to-lymphocyte ratio were associated with in-hospital death.9
A study from the Beijing Ditan Hospital in China, available as a preprint, prospectively analyzed 61 patients with SARS-CoV-2 admitted to the hospital in January 2020 for prognostic factors of severe COVID-19 illness.10 As with Dr. Akdis' analysis, these researchers also identified a high neutrophil-to-lymphocyte ratio as an independent risk factor for severe illness. Older age (>50 years) and a neutrophil-to-lymphocyte ratio of â‰¥3.13 were associated with severe COVID-19 illness. The study authors recommended that patients with these factors have rapid access to the intensive care unit at a hospital.
Timing and quality of the immune response appear to be important for the course of disease and its severity.
"The timing for development of the naturalizing antibody response is likely a decisive factor for the body's ability to clear the infection," said Dr. Akdis. "If the response is too slow, the lymphopenia ensues and there could be little chance for developing a robust neutralizing antibody response. The helper and cytotoxic T cells are getting lost and there is no help for antibody production."
"What we don't fully understand is whether the inability to respond to the viral infection is a consequence of the lymphopenia in patients with COVID-19," said Dr. Avigan. "Does the lymphopenia prevent an individual from mounting a robust antibody response, for example? There are many studies ongoing to understand what roles different parts of the immune system – the cellular and humoral components – play in creating protection and effective responses to fight the virus."
Dr. Sereti and her NIH colleagues are currently conducting such a study, collecting samples from patients at various stages of their disease to capture what the immune system looks like in response to a coronavirus infection. They hope to figure out a comprehensive picture of individual immune responses to the virus. "Comparing the immune profiles of the elderly, who generally have weaker lymphocytes, with the immune profiles of children, for example, could provide particularly good clues because we know that children generally have mild disease," she explained.
Thus far, study results suggest that lymphopenia may be a useful prognostic marker, but additional prospective studies are needed to validate these results. "If there is viral-mediated inflammation, the lymphocyte depletion may be an initial step in this cascade and, in that way, lymphopenia may be an early marker to help guide clinical care to try to prevent further damaging inflammation," said Dr. Avigan.
Treating Viral Infection and Lymphopenia
While researchers try to understand why lymphopenia develops in patients with COVID-19, clinicians are learning which treatments are appropriate for which patients – and when. Therapeutic options for which there is some evidence of efficacy include antiviral drugs such as remdesivir and anti-inflammatory drugs such as low-dose dexamethasone.
In late June, scientists at the University of Oxford in the U.K. announced results from the RECOVERY study, which compared a range of possible treatments with usual care among 2,104 patients hospitalized with COVID-19. The commonly used corticosteroid dexamethasone reduced deaths by one-third among ventilated patients (rate ratio [RR] = 0.65; 95% CI 0.48-0.88; p=0.0003) and by one-fifth in patients receiving only oxygen (RR=0.80; 95% CI 0.67-0.96; p=0.0021), compared with usual care.11
"It appears that the immune response to the virus is what is killing the tissues, not the virus itself," Dr. Akdis commented. "If we can suppress the dysregulated immune response and inflammation, then patients can get better before being admitted to the intensive care unit or requiring ventilation."
Drs. Sereti and Avigan concurred. "At the point when there is severe inflammation, or when the trajectory goes toward severe inflammation, it makes sense to suppress the immune response," Dr. Sereti said.
In contrast, experts hypothesize that immune stimulation could be effective during earlier stages of the disease. One such therapeutic option being explored in clinical trials is interleukin-7 (IL-7), a cytokine required for the development of T lymphocytes. According to Dr. Sereti, IL-7 should also be considered in combination with a vaccine, as a booster of a vaccine response – especially in older patients, who are more likely to have a weaker immune response to some vaccines.12
For a disease that only emerged in December 2019, the worldwide clinical and scientific community has learned a lot about the pathophysiology of a SARS-CoV-2 infection and how to care for patients with severe manifestations of COVID-19. However, there is still much to be learned.
"What is the best timing and sequence of therapies and whether there are patients for whom anti-inflammatory treatment would exacerbate their disease – we don't know," Dr. Sereti said.
"Again, the question about COVID-19–related lymphopenia is whether it is a measure of an individual's immune competence, which speaks to whether they can fight off the virus, or is it a surrogate for a certain kind of inflammation?" said Dr. Avigan. The explanation may be complicated. "We may find that the answer is different for different individuals." â€”By Anna Azvolinsky
- Chen G, Wu D, Guo W, et al. Clinical and immunological features of severe and moderate coronavirus disease 2019. J Clin Invest. 2020;130:2620-2629.
- Zhao Q, Meng M, Kumar R, et al. Lymphopenia is associated with severe coronavirus disease 2019 (COVID-19) infections: A systemic review and meta-analysis. Int J Infect Dis. 2020;96:131-135.
- Guan WJ, Ni ZY, Hu Y, et al. Clinical characteristics of coronavirus disease 2019 in China. N Engl J Med. 2020;382:1708-1720.
- Brown RAC, Barnard J, Harris-Skillman E, et al. Lymphocytopaenia is associated with severe SARS-CoV-2 disease: a systematic review and meta-analysis of clinical data. medRxiv preprint. April 17, 2020.
- Chen IY, Moriyama M, Chang MF, Ichinohe T. Severe acute respiratory syndrome coronavirus viroporin 3a activates the NLRP3 inflammasome. Front Microbiol. 2019;10:50.
- Yang M. Cell pyroptosis, a potential pathogenic mechanism of 2019-nCoV infection. SSRN. 2020 January 29.
- Gu J, Gong E, Zhang B, et al. Multiple organ infection and the pathogenesis of SARS. J Exp Med. 2005;202:415-424.
- Zeidan AM, Boddu PC, Patnaik MM, et al. Special considerations in the management of adult patients with acute leukaemias and myeloid neoplasms in the COVID-19 era: recommendations from a panel of international experts. Lancet Haematol. 2020 June 18. [Epub ahead of print]
- Zhang JJ, Cao YY, Tan G, et al. Clinical, radiological and laboratory characteristics and risk factors for severity and mortality of 289 hospitalized COVID-19 patients. Authorea. 2020 June 8.
- Liu J, Liu Y, Xiang P, et al. Neutrophil-to-lymphocyte ratio predicts severe illness patients with 2019 novel coronavirus in the early stage. medRxiv preprint. February 12, 2020.
- University of Oxford press release. Low-cost dexamethasone reduces death by up to one third in hospitalised patients with severe respiratory complications of COVID-19." Accessed June 16, 2020, from https://www.recoverytrial.net/news/low-cost-dexamethasone-reduces-death-by-up-to-one-third-in-hospitalised-patients-with-severe-respiratory-complications-of-covid-19.
- ClinicalTrials.gov. InterLeukin-7 (CYT107) to Improve Clinical Outcomes in Lymphopenic pAtients With COVID-19 Infection UK Cohort (ILIAD-7-UK). Accessed June 25, 2020, from https://clinicaltrials.gov/ct2/show/NCT04379076.