key: cord-0794483-1i6kvuof
authors: Stephen-Victor, Emmanuel; Das, Mrinmoy; Karnam, Anupama; Pitard, Bruno; Gautier, Jean-François; Bayry, Jagadeesh
title: Potential of regulatory T cell-based therapies in the management of severe COVID-19
date: 2020-07-02
journal: Eur Respir J
DOI: 10.1183/13993003.02182-2020
sha: c647b4ac7523e3101681f50ee31536e970d8f00d
doc_id: 794483
cord_uid: 1i6kvuof

In view of dysregulated immune response, cytokine storm and inflammation-induced severe lung damage in severely ill COVID-19 patients, we propose that CD4+CD25+FoxP3+ regulatory T cell-based therapies could be considered for the patient management.

"Cytokine storm", and inflammation-mediated severe lung damage and defective hemostasis are the main underlying reasons for morbidity and mortality in COVID-19 patients [1] . Therefore, several immunotherapies that target various inflammatory processes have been successfully used in COVID-19 patients and many other strategies are under evaluation [2, 3] . However, in view of dysregulated immune response in severe COVID-19 patients, we suggest that CD4 + CD25 + FoxP3 + regulatory T cell (Treg)-based strategies could be considered for the patient management.

Vigorous anti-microbial responses triggered against the pathogen can be detrimental to the host due to collateral tissue damage. Therefore, regulatory mechanisms, and in particular, Tregs are in place to ensure that inflammation is kept in check. Tregs are either thymus-derived or induced in periphery and are classically known for inducing immune tolerance, and preventing autoimmune and inflammatory diseases [4] . Tregs inhibit the activation of both innate and adaptive immune cells via inhibitory surface molecules (like cytotoxic T lymphocyte antigen-4 (CTLA-4) and lymphocyteactivation gene-3) and secretion of immunosuppressive cytokines (IL-10, TGF- and IL-35). Both Treg subsets are equally important to prevent inflammation-induced tissue damage during acute infections and to promote tissue repair as shown particularly in case of influenza infection model [4, 5] .

The current evidence suggests that the level of peripheral Tregs is prominently reduced in severely ill COVID-19 patients compared to mild patients [6] [7] [8] [9] . Though the reasons for reduced frequency of Tregs in peripheral blood is not completely understood, one of the possibilities is that Tregs might have migrated to lungs to prevent tissue damage. Detailed investigation of Tregs in the lung tissues of severe COVID-19 patients and their molecular signatures would provide insight on these questions. However, in silico analyses of CD4 + T cells from the COVID-19 patients' bronchoalveolar lavage transcriptomic data suggest that IL2 transcripts were reduced in severe cases compared to mild cases [10] . Therefore, reduced IL2 would lead to enhanced apoptosis of Tregs and is confirmed by reduced levels of FoxP3 as well. Moreover, severe COVID-19 patients have increased levels of soluble IL-2R (CD25) [6] [7] [8] probably due to inflammation-induced enhanced proteolytic cleavage of cell surface CD25. This soluble CD25 could potentially interfere with IL-2 bioavailability and signaling, and hence might further promote apoptosis of Tregs. Also, it has been shown that Middle East Respiratory Syndrome Coronavirus (MERS-CoV) could infect T cells [11] and hence direct effect of acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on the biology of Tregs cannot be ruled out.

Considering the importance of Tregs in immune homeostasis, reduction in the levels of Tregs could be one of the reasons for the hyperactivated immune system and damaged lungs in severe COVID-19 patients. Of note, depletion of Treg from the mice infected with murine coronavirus lead to increased mortality with acute encephalitis, thus highlighting the protective nature of Tregs during acute coronavirus infection [12] . It should also be noted that obesity is one of the risk factors for COVID-19, and data from obese subjects and pertinent animal models have shown that Tregs in the circulation and visceral adipose tissues are decreased compared to lean subjects, and consequently a higher state of inflammation and insulin resistance [13, 14] .

Because of dysregulated immune response in severe COVID-19 patients, we propose that Tregs have therapeutic potential in the patient management. Adoptive transfer of ex vivo expanded polyclonal Tregs has been used recently to treat autoimmune and inflammatory diseases [15] . But, polyclonal Treg therapy is time consuming, requiring nearly two weeks to expand sufficient quantities of viable clinical-grade Tregs for the immunotherapy. However, unlike autoimmune diseases, COVID-19 patients need an instant therapy to prevent morbidity and mortality. Therefore, adoptive transfer autologous polyclonal Treg therapy is not a viable option in COVID-19 patients. Also, the approach is not economically feasible for an infectious disease. Alternatively, allogeneic HLA-matched umbilical cord-derived Tregs are under exploration for inflammatory conditions [16] (Ongoing clinical trials: NCT02932826, NCT03011021) ( Figure 1 ). In view of drawbacks with autologous Treg therapy, allogeneic matched cord Tregs could be considered for severe COVID-19 patients.

Another strategy is to boost Tregs in vivo. Low dose IL-2 has been used to specifically induce Treg expansion in vivo in type 1 diabetes, and other autoimmune and inflammatory diseases [17] . The high affinity of CD25 towards IL-2 would lead to selective Treg expansion. But severe COVID-19 patients display increased levels of soluble IL-2R [6] [7] [8] that could potentially scavenge IL-2. Other reports have also demonstrated that severe COVID-19 patients have increased IL-2 levels [1] and despite having higher IL-2, Tregs were lower in severe COVID-19 patients. All these arguments suggest that low dose IL-2 therapy might not be beneficial in COVID-19 patients. Nevertheless, a clinical trial is planned with low-dose IL-2 for acute respiratory distress syndrome related to COVID-19 (NCT04357444) and data from this trial will provide a valuable information on the feasibility of this strategy not only for COVID-19 but also for other acute viral diseases. Other Treg expansion strategies like IL-2 complexed with monoclonal antibodies to selectively trigger Treg activation [18] or to induce STAT5 phosphorylation and Treg expansion [19] have been explored in pre-clinical models. But they are not yet tested in clinic and in view of their unproven efficacy in the patients, it is too risky to use them in COVID-19 patients.

In view of various shortcomings with either adoptive Treg therapy or in vivo Treg expansion strategies, therapy with Treg-derived immunoregulatory molecules, in particular CTLA-4 might hold the potential for controlling the inflammation in severe COVID-19 patients (Figure 1 ). At cellular level, CTLA-4 interacts with B7 family members (CD80 and CD86) on innate cells. By transendocytosis and degradation of B7 molecules inside CTLA-4-expressing cells, CTLA-4 reduces costimulatory signals for T cells [20] . The recombinant Fc-fused CTLA-4 protein abatacept has been used since several years for the immunotherapy of systematic autoimmune diseases. Though our knowledge on the mechanisms of this fusion protein is still incomplete, current literature suggest that by interacting with B7 molecules on antigen presenting cells, abatacept interferes with CD28mediated T cell signaling and activation. Additionally, abatacept might affect activation of innate cells like monocytes and dendritic cells, and enhance Tregs though data on the Treg function are limited and conflicting [21, 22] .

While data is not available regarding abatacept therapy in COVID-19 patients, recent longitudinal observational studies on the incidence of COVID-19 in abatacept-treated patients provide a pointer towards its therapeutic potential in severely ill COVID-19 patients. An epidemiological survey performed in the large tertiary hospital of Barcelona indicated that abatacept-treated patients (42 patients among the cohort of 959 biologic and synthetic disease modifying anti-rheumatic drugs (DMARD)-treated patients) exhibited lowest frequency of COVID-19-compatible symptoms [23] . Similar data were also obtained from a hospital in Madrid, which noticed that none of the 27 abatacept-treated patients (among 802 DMARD-treated patients) admitted to hospital for COVID-19 symptoms [24] . Furthermore, among 779 biologic DMARD-treated patients from Siena, Italy, only two were tested positive for COVID-19 and notably none of the 55 abatacept-treated patients in this cohort experienced COVID-19 [25] .

These observations though from the small cohorts and possibly patients were practicing strict social distancing, the data on abatacept-treated patients are not surprising in view of documented clinical benefits of abatacept in autoimmune diseases and mechanisms of action of CTLA-4 to mitigate inflammatory responses. Various immunotherapies that target "cytokine storm" and prevent lung damage have gained prominence in the management of severe COVID-19 patients. Among Tregbased therapeutic approaches, CTLA-4-based therapies appear to be attractive to overcome hyperinflammatory state of severely ill COVID-19 patients. Clinicaltrials.gov registry shows that 47 studies are registered for the evaluation of tocilizumab (a humanized monoclonal antibody to IL-6 receptor α)/sarilumab (a fully human monoclonal antibody to IL-6 receptor α) in COVID-19 patients and 14 studies are registered for anakinra (an IL-1 receptor antagonist). However, no clinical trials are registered for abatacept therapy in COVID-19 patients. Initial clinical trials were mainly focused on the management of 'cytokine storm' and in particular, the role of IL-6 and IL-1. Also, data on the Tregs in COVID-19 patients were scarce during early period of COVID-19 pandemic. These points might explain why abatacept has not been considered yet for the management of COVID-19 patients. As more data are emerging now on the fate of Tregs in COVID-19 patients, aforementioned clinical observations could be taken as a basis for initiating randomized trials on CTLA-4-Fc (abatacept) therapy in moderate or severe COVID-19 patients. Though abatacept therapy in autoimmune patients was not associated with predisposition to infections, selection of a dosage and window of treatment are critical to strike a balance between inflammation and protective immune response to SARS-CoV-2. We believe that early treatment with abatacept might not benefit COVID-19 patients as it would curtail effective protective immune responses against SARS-CoV-2. For the initial exploration, the treatment regimen adapted for the tocilizumab therapy of COVID-19 patients or even abatacept regimen used in DMARD treatment of rheumatic diseases could be considered. Tregs and their functions are compromised in severe COVID-19 patients engendering unrestrained immune cell activation. Dysregulated antigen-presenting cells (APCs) insinuate tissue inflammation and immunopathology by secreting inflammatory cytokines and activating T celldependent immune response. We suggest that either adoptive transfer of allogenic Tregs or use of Treg-derived molecules like CTLA-4 (abatacept) might block activation of APC and costimulatory pathways. Such therapies have potential to curtail tissue inflammation and immunopathology leading to better management of COVID-19 patients.

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Ongoing Clinical Trials for the Management of the COVID-19 Pandemic

Adjunct immunotherapies for the management of severely ill COVID-19 patients

Human FOXP3(+) Regulatory T cell heterogeneity and function in autoimmunity and cancer

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Dysregulation of immune response in patients with COVID-19 in Wuhan, China

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High-dimensional immune profiling by mass cytometry revealed immunosuppression and dysfunction of immunity in COVID-19 patients

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Middle east respiratory syndrome coronavirus efficiently infects human primary t lymphocytes and activates the extrinsic and intrinsic apoptosis pathways

Role of regulatory t cells in coronavirus-induced acute encephalitis

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Circulating regulatory t cells are reduced in obesity and may identify subjects at increased metabolic and cardiovascular risk

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Trans-endocytosis of CD80 and CD86: a molecular basis for the cell-extrinsic function of CTLA-4

CTLA-4Ig) treatment reduces t cell apoptosis and regulatory T cell suppression in patients with rheumatoid arthritis

Regulatory T-cell dynamics with abatacept treatment in rheumatoid arthritis

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Support statement: Partly supported by grants from Agence Nationale de la Recherche, France (Appel Flash COVID-19-COVIMUNE and ANR-19-CE17-0021(BASIN)).