Most newly discharged patients who recently recovered from COVID-19 produce virus-specific antibodies and T cells, suggests a study published on May 3rd in the journal Immunity, but the responses of different patients are not all the same.
While the 14 patients examined in the study showed wide-ranging immune responses, results from the 6 of them that were assessed at two weeks after discharge suggest that antibodies were maintained for at least that long.
Additional results from the study indicate which parts of the virus are most effective at triggering these immune responses and should therefore be targeted by potential vaccines.
It is not clear why immune responses varied widely across the patients. The authors say this variability may be related to the initial quantities of virus that the patients encountered, their physical states, or their microbiota.
It is also important to note that the laboratory tests that are used to detect antibodies to SARS-CoV-2 in humans still need further validation to determine their accuracy and reliability.
“These findings suggest both B and T cells participate in immune-mediated protection against the viral infection,” says co-senior study author Chen Dong of Tsinghua University.
“Our work has provided a basis for further analysis of protective immunity and for understanding the mechanism underlying the development of COVID-19, especially in severe cases. It also has implications for designing an effective vaccine to protect against infection.”
Relatively little is known about the protective immune responses induced by the disease-causing virus, SARS-CoV-2, and addressing this gap in knowledge may accelerate the development of an effective vaccine, adds co-senior study author Cheng-Feng Qin of the Academy of Military Medical Sciences in Beijing, China.
With this goal in mind, the researchers compared the immune responses of 14 COVID-19 patients who had recently become virus-free to those of six healthy donors. Eight of the patients were newly discharged, and the remaining six were follow-up patients who were discharged two weeks prior to the analyses.
Specifically, the researchers collected blood samples and assessed the levels of immunoglobulin M (IgM) antibodies, which are the first to appear in response to an infection, as well as immunoglobulin G (IgG) antibodies, which are the most common type found in blood circulation.
Compared to healthy controls, both newly discharged and follow-up patients showed higher levels of IgM and IgG antibodies that bind to the SARS-CoV-2 nucleocapsid protein, which encapsulates the viral genomic RNA, as well as the S protein’s receptor-binding domain (S-RBD), which binds to receptors on host cells during the process of viral entry.
Taken together, these findings show that COVID-19 patients can mount antibody responses to SARS-CoV-2 proteins and suggest that these antibodies are maintained for at least two weeks after discharge.
In addition, five newly discharged patients had high concentrations of neutralizing antibodies that bind to a pseudovirus expressing the SARS-CoV-2 S protein. Neutralizing antibodies prevent infectious particles from interacting with host cells. In addition, all except one follow-up patient had detectable neutralizing antibodies against the pseudovirus.
Compared to healthy controls, five newly discharged patients had higher concentrations of T cells that secrete interferon gamma (IFNγ) – a signaling molecule that plays a critical role in immunity – in response to the SARS-CoV-2 nucleocapsid protein.
These are the same patients who had high concentrations of neutralizing antibodies. In addition, three newly discharged patients showed detectable levels of IFNγ-secreting T cells specific to the SARS-CoV-2 main protease – a protein that plays a critical role in viral replication.
Meanwhile, seven newly discharged patients showed detectable levels of IFNγ-secreting T cells specific to the S-RBD of SARS-CoV-2. By contrast, only one follow-up patient had a high concentration of IFNγ-secreting T cells responsive to the nucleocapsid protein, the main protease, and S-RBD.
One finding with potential clinical relevance is that the amount of neutralizing antibodies was positively associated with IgG antibodies against S-RBD, but not with those that bind to the nucleocapsid protein.
Moreover, S-RBD induced both antibody and T cell responses. “Our results suggest that S-RBD is a promising target for SARS-CoV-2 vaccines,” says co-senior study author Fang Chen of Chui Yang Liu Hospital affiliated to Tsinghua University. “But our findings need further confirmation in a large cohort of COVID-19 patients.”
Funding: This work was supported by from the National Key Research and Development Program of China, Natural Science Foundation of China, Beijing Municipal Science and Technology, Zhejiang University Foundation, and Tsinghua University. L.N., Y.F., W.P., and C.D. have filed a provisional patent on the methodology of detecting SARS-CoV-2-specific antibody responses.
SARS-CoV-2 infection can activate innate and adaptive immune responses. However, uncontrolled inflammatory innate responses and impaired adaptive immune responses may lead to harmful tissue damage, both locally and systemically.
In patients with severe COVID-19, but not in patients with mild disease, lymphopenia is a common feature, with drastically reduced numbers of CD4+ T cells, CD8+ T cells, B cells and natural killer (NK) cells1,2,3,4, as well as a reduced percentage of monocytes, eosinophils and basophils3,5. An increase in neutrophil count and in the neutrophil-to-lymphocyte ratio usually indicates higher disease severity and poor clinical outcome5.
In addition, exhaustion markers, such as NKG2A, on cytotoxic lymphocytes, including NK cells and CD8+ T cells, are upregulated in patients with COVID-19. In patients who have recovered or are convalescent, the numbers of CD4+ T cells, CD8+ T cells, B cells and NK cells and the markers of exhaustion on cytotoxic lymphocytes normalize6,7. Moreover, SARS-CoV-2-specific antibodies can be detected.
Convalescent plasma containing neutralizing antibodies has been used to treat a small number of patients with severe disease, and preliminary results show clinical improvement in 5 of 5 critically ill patients with COVID-19 who developed ARDS8. High-throughput platforms, such as the large-scale single-cell RNA sequencing of B cells (enriched for B cells that produce antibodies directed at the SARS-CoV-2 spike glycoprotein) from patients who are convalescent, have allowed the identification of SARS-CoV-2-specific neutralizing antibodies.
The detection of SARS-CoV-2-specific IgM and IgG in patients provided the basis for disease diagnosis, in conjunction with RT-PCR-based tests. However, two studies, based on the analysis of 222 and 173 patients with COVID-19, respectively, reported that patients with severe disease frequently had an increased IgG response and a higher titre of total antibodies, which was associated with worse outcome5,9.
This was suggestive of possible antibody-dependent enhancement (ADE) of SARS-CoV-2 infection. The immunopathological effects of ADE have been observed in various viral infections, characterized as antibody-mediated enhancement of viral entry and induction of a severe inflammatory response.
Worryingly, it was shown that a neutralizing monoclonal antibody targeting the receptor-binding domain of the spike protein of the related Middle East respiratory syndrome (MERS) virus can enhance viral entry. A potential pathogenic effect of antibodies targeted at SARS-CoV-2 would be of major concern for vaccine development and antibody-based therapies. Additional independent large-cohort studies are needed to substantiate or dismiss this possibility.
Most patients with severe COVID-19 exhibit substantially elevated serum levels of pro-inflammatory cytokines including IL-6 and IL-1β, as well as IL-2, IL-8, IL-17, G-CSF, GM-CSF, IP10, MCP1, MIP1α (also known as CCL3) and TNF, characterized as cytokine storm1,2,3,4.
Also, C-reactive protein and D-dimer are found to be abnormally high. High levels of pro-inflammatory cytokines may lead to shock and tissue damage in the heart, liver and kidney, as well as respiratory failure or multiple organ failure.
They also mediate extensive pulmonary pathology, leading to massive infiltration of neutrophils and macrophages, diffuse alveolar damage with the formation of hyaline membranes and a diffuse thickening of the alveolar wall. Spleen atrophy and lymph node necrosis were also observed, indicative of immune-mediated damage in deceased patients.
A number of studies have trialled strategies to dampen inflammatory responses. Elevated levels of IL-6 were found to be a stable indicator of poor outcome in patients with severe COVID-19 with pneumonia and ARDS. One clinical trial (ChiCTR2000029765), using the IL-6 receptor-targeted monoclonal antibody (mAb) tocilizumab, reported quick control of fever and an improvement of respiratory function in 21 patients with severe COVID-19 treated in Anhui, China.
All patients, including two who were critically ill, have recovered and have been discharged from hospital. The efficacy of tocilizumab in treating patients with COVID-19 who develop ARDS needs to be further assessed in larger randomized controlled trials.
This encouraging clinical trial indicates that neutralizing mAbs against other pro-inflammatory cytokines may also be of use, with potential targets including IL-1, IL-17 and their respective receptors.
Moreover, small-molecule inhibitors of their downstream signalling components may hold promise for blocking cytokine storm-related immunopathology. In addition to the cytokine-based pathology in patients with severe COVID-19, complement activation has also been observed, indicating that complement inhibitors, if used at an early stage of the infection, may attenuate the inflammatory damage. Hopefully these approaches will be approved into clinical trials to benefit the patients.
Another approach to alleviate COVID-19-related immunopathology involves mesenchymal stem cells (MSCs), which exert anti-inflammatory and anti-apoptotic effects, can repair pulmonary epithelial cell damage and promote alveolar fluid clearance. Encouraged by preclinical and clinical studies that confirmed their safety and efficacy in non-COVID-19-related pathologies, clinical trials of MSC-based therapy in patients with severe COVID-19 have been initiated in China and two trials are currently ongoing.
To further help our fight against COVID-19, prognostic biomarkers need to be identified for patients at high risk of developing ARDS or multiple organ failure. Age (above 50 years) has emerged as one independent risk factor for severe disease, raising concerns about the feasibility of generating a potent vaccine to induce efficient cellular and humoral responses in this population.
In addition, it appears that patients with COVID-19 and hypertension or diabetes are more likely to develop severe disease. Delineating the mechanisms behind these chronic diseases for worsening disease outcome, as well as a better understanding of SARS-COV-2 immune-escape mechanisms, may provide clues for the clinical management of the severe cases.
It is of utmost importance that successful standardized treatment protocols for severe cases are recommended globally to fight the COVID-19 pandemic. The combined use of anti-inflammatory and antiviral drugs may be more effective than using either modality alone.
Based on in vitro evidence for inhibiting SARS-CoV-2 replication and blocking SARS-CoV-2 infection-induced pro-inflammatory cytokine production10, a Chinese traditional medicine has demonstrated clinical efficacy (Nanshan Zhong, personal communication).
Another, so-far under-investigated pathogenic factor that may affect therapeutic outcome involves stress-induced disorders of the neuroendocrine–immune crosstalk. It is well known that cytokines released in the context of innate immune responses to viral infections can induce the neuroendocrine system to release glucocorticoids and other peptides, which can impair immune responses.
Infectious SARS-CoV-2 viral particles have been isolated from respiratory, faecal and urine samples. Whether SARS-CoV-2 can infect the central nervous system, facilitating the release of inflammation-induced pathological neuroendocrine mediators that impact on respiratory function and ARDS pathogenesis, warrants investigation.
- Huang, C. et al. Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China. Lancet 395, 497–506 (2020).
- Xu, Z. et al. Pathological findings of COVID-19 associated with acute respiratory distress syndrome. Lancet Respir. Med. 8, 420–422 (2020).
- Qin, C. et al. Dysregulation of immune response in patients with COVID-19 in Wuhan, China. Clin. Infect. Dis. https://doi.org/10.1093/cid/ciaa248 (2020).
- Shi, Y. et al. Immunopathological characteristics of coronavirus disease 2019 cases in Guangzhou, China. Preprint at medRxiv https://doi.org/10.1101/2020.03.12.20034736 (2020).
- Zhang, B. et al. Immune phenotyping based on neutrophil-to-lymphocyte ratio and IgG predicts disease severity and outcome for patients with COVID-19. Preprint at medRxiv https://doi.org/10.1101/2020.03.12.20035048 (2020).
- Chen, X. et al. Restoration of leukomonocyte counts is associated with viral clearance in COVID-19 hospitalized patients. Preprint at medRxiv https://doi.org/10.1101/2020.03.03.20030437 (2020).
- Zheng, M. et al. Functional exhaustion of antiviral lymphocytes in COVID-19 patients. Cell. Mol. Immunol. https://doi.org/10.1038/s41423-020-0402-2 (2020).
- Shen, C. et al. Treatment of 5 critically ill patients with COVID-19 with convalescent plasma. JAMA https://doi.org/10.1001/jama.2020.4783 (2020).
- Zhao, J. et al. Antibody responses to SARS-CoV-2 in patients of novel coronavirus disease 2019. Clin. Infect. Dis. https://doi.org/10.1093/cid/ciaa344 (2020).
- Runfeng, L. et al. Lianhuaqingwen exerts anti-viral and anti-inflammatory activity against novel coronavirus (SARS-CoV-2). Pharmacol. Res. https://doi.org/10.1016/j.phrs.2020.104761 (2020).