Immunotherapy

Antibody-Based therapy
Considered an efficient method for the clinical treatment of different infectious diseases, including MERS-CoV and SARS-CoV-1 (217), antibody-based immunotherapy has been studied as a potentially applicable tool to treat COVID-19. The mechanisms involved with its effects against SARS-CoV-2 are related to preventing the virus from entering the host cells, blocking its replication.
The virus entry block was studied for acting both in the cell receptor ACE2 and directly on the virus (neutralizing antibodies [nAbs]), specifically in the S1 subunit of the S protein (218–220). Regarding the blocking of ACE2 receptors, the application of some mechanisms stand out: the soluble version of ACE2 fused to an immunoglobulin Fc domain (ACE2-Fc), RDB domain attached to Fc (RDB-Fc), and receptor-targeted monoclonal antibodies (mAb) (221).
Viral neutralization by nAbs is also an immunotherapeutic approach and directly recognizes epitopic regions of SARS-CoV-2. This effect can be achieved either directly through mAbs manufactured in laboratories or by using polyclonal antibodies (pAbs) (218). nAbs act directly on the virus, preventing its infectivity by activating several pathways, such as the complement system, cell cytotoxicity, and phagocytic clearance (222–224).
The therapeutic use of mAbs has shown good outcomes, mainly due to its high specificity. Recently, several mAbs against viruses have been developed, including SARS-CoV-1 and MERS-CoV, in which the S protein is the major target described both in vitro and in vivo. According to some studies, the specific nAbs against SARS-CoV-1 RBD in the S protein could effectively block SARS-CoV-2 entry (218, 225). However, Wrapp et al. (226) tested several published SARS-CoV-1 RBD-specific nAbs and found that they do not have substantial binding to the S protein of SARS-CoV-2, suggesting that the cross-reactivity may be limited. Thus, the combination of nAbs with different viral targets and sources could improve treatment efficacy. In addition to experimental studies, to date, more than 10 clinical trials have aimed at testing human mAbs against SARS-Cov-2 (227–235), which could also represent an alternative, effective treatment.
Furthermore, some immunomodulatory mAbs have been tested in the context of COVID-19. It is remarkable that until now most of the data published regarding the use of immunomodulatory mAbs derive from studies using either anti-IL-6 or anti-IL-6R, probably because using IL-6 blockers seems promising at controlling the cytokine storm associated with the development of ARDS in more aggressive patterns of SARS-CoV-2 infection. However, clinical observations remain controversial.
Although some studies found considerable clinical improvements resulting from treatment with IL-6 blockers (236–239), others do not report any significant difference between the clinical features of groups treated with anti-IL6/IL-6R mAbs and their respective controls (without anti-IL-6/IL-6R) (240–243). These controversial results can be explained by the pleiotropic function of IL-6, which also play an important anti-inflammatory role, questioning the use of IL-6 blockade to suppress inflammation-induced tissue damage (244). Additionally, severe side effects have been associated with the use of IL-6 blockers, including enhanced hepatic enzymes, thrombocytopenia, severe bacterial and fungal infections, and sepsis (241, 245). In general, data from analyses on the use of this type of mAbs remain inconclusive (243, 246, 247).
Recent findings are optimistic, but data validation by robust scientific evidence has been hampered by the small sample size in most case reports and studies on the use of mAbs blocking other immune mediators, such as IL-1β, GM-CSF, and complement protein C5 (238, 248–250). However, seeking to verify the effectiveness of using mAbs blocking inflammatory mediators, dozens of clinical trials are currently underway.
Aiming at reducing the hyper inflammation found in the lungs of SARS-CoV-2-infected patients, different clinical studies are currently investigating the activities of mAbs anti-JAK, anti-GM-CSF, anti-GM-CSF receptor, anti-M-CSF receptor, anti-CD14, anti-IFNγ, anti-VEGF, anti-BKT, anti-CCR5, anti-IL-6, anti-IL-6 receptor, anti-TNFα, anti-IL1β, anti-IL1β receptor, and complement C5 inhibitor (220, 251, 252). Similarly, ongoing clinical trials have sought to reverse the hyper-thrombotic state found in critically ill patients by using anti-P-selectin, anti-CTGF, and factor XIIa antagonist mAbs (253, 254). Furthermore, to restore the exhausted T lymphocytes’ and NK cells’ immunity, other clinical studies applied anti-PD1 mAbs under the hypothesis of a stimulus of anti-viral response and prevention of ARDS (255–257).
More recently, the passive administration of pAbs has also been tested in COVID-19 patients (222–224, 258–267), also known as convalescent plasma (CP) or immune plasma, which is already used effectively and safely in the treatment of other severe acute respiratory syndrome infections of viral etiology, such as SARS, MERS, and H1N1, and offers only a short-term but rapid immunity to the susceptible individuals (268).
A strict criterion to select the CP donor states that the individual must show clinical recovery and test negative for the virus presence. Thus, after being confirmed, a high titer of neutralizing antibodies against SARS-CoV-2 must be stored in blood banks (269, 270).
Some reviews related to patients who received transfusion with CP showed a reduction in viral load, improvement in clinical symptoms, better radiological findings, and improved survival (260, 261, 271–273). In addition, after having received CP containing nAbs, COVID-19 patients had significant improvements from the beginning of treatment (until 22 days), presenting lower fever, decreased viral load, and higher nAbs levels. Further, 60% of the patients were discharged one month after the treatment (271). Better outcomes were found in early administration of CP (before SARS-CoV-2 seroconversion), preferably on day 5, for obtaining maximum efficacy (268).
More recently, Li et al. found no statistically significant clinical improvement or mortality reduction in a randomized clinical trial with CP-treated COVID-19 patients (274). However, the authors reported that CP treatment is potentially beneficial to critically ill patients by suggesting a possible antiviral efficiency of high titer of nAbs. Notably, there are clinical controversies, ethical issues, and potential risks associated with convalescent plasma therapy (275), such as the possibility of ADE development, exacerbating the disease severity, and causing a significant illness in future exposure to coronaviruses infection (268, 276, 277) (REF). Divergences between studies may be caused by variations in the composition of CP, which is highly variable and includes a variety of blood-derived components, timing of CP administration, titer of the specific antibody in administered plasma, and presence of blood borne pathogens (268). Nonetheless, understanding the efficacy and safety of CP therapy relies on the completion of the ongoing clinical trials.
Another therapeutic strategy using antibodies is intravenous immunoglobulin (IVIg) that contains polyclonal IgG isolated from healthy donors, which can be further enhanced by using IgG antibodies collected from recovered COVID-19 patients in the same geographical region as the patient. Results have been mostly positive, although many of these therapies have not been formally evaluated through a randomized, double-blind, placebo-controlled clinical trial (278). According to recent studies, IVIg can be used effectively in early stages of SARS-CoV-2 infection (before the initiation of systemic damage), reducing the use of mechanical ventilation, preventing the progression of pulmonary lesions, and promoting early recovery (268). Also, cross-neutralization activity was shown against SARS-CoV-2 in commercial IVIg manufactured prior to the COVID-19 pandemic and are currently under evaluation as potential therapies for COVID-19 (279). Thus, intravenous use of immunoglobulins can prove helpful in therapy against SARS-CoV-2, however, adjustments in the therapeutic regimen are necessary for all IVIg possibilities, as well as a complete understanding of the possible adverse effects, such as the risk of ADE (278, 279), that are being studied in more than 10 ongoing clinical trials.
Some works have shown that therapies focusing on the interaction between SARS-CoV-1 and the ACE2 receptor may be extended for use in SARS-CoV-2 patients as an immunotherapy tool (218). However, other authors refute this idea based on the fact that recent studies showed limited cross-neutralization between SARS-CoV-1 antibodies and SARS-CoV-2 (280, 281). Furthermore, it was shown that SARS-CoV-2 S protein binds ACE2 with a higher affinity than SARS-CoV-1, suggesting that such interaction differs between the two viruses (266).

Immune Cell-Based Therapy
In addition to antibody-based therapies, scientists have been studying immune cell-based therapies as a tool to combat COVID-19, focusing especially on NK and T cells. The importance of NK cells as the first antiviral responders can be seen in patients with NK cell deficiency and immunocompromised individuals who have increased susceptibility to viral infections (282). In this sense, Market et al. (282) gathered the main reports so far addressing potential therapies focusing on mediating NK cell activity to mitigate the immunopathological consequences of COVID-19, and consequently lighten the load on our health systems.
Some ongoing clinical trials have been studying the use of NK cell therapy through different approaches. A randomized phase I/II trial studied the infusions of CYNK-001 cells, an allogeneic off-the-shelf cell therapy enriched for CD56+/CD3- NK cells expanded from human placental CD34+ cells in 86 hospitalized patients with moderate COVID-19 disease (283). Another randomized phase I/II study explored the use of NKG2D-ACE2 CAR-NK cells with each common, severe, and critical type COVID-19. The authors hypothesize that these cells target the S protein of SARS-CoV-2 and NKG2DL on the surface of infected cells with ACE2 and NKG2D, respectively, seeking out the elimination of SARS-CoV-2 virus particles and their infected cells (284).
The unregulated profile of the immune response in critically ill COVID-19 patients may be due to the reduction of Treg cells, which culminates in excessive release of inflammatory mediators and cytokine storms (153, 191–193). Thus, the use of adoptive transfer of these cells as a measure of inflammatory control in critically ill patients is a promising therapeutic approach. The infusion of autologous polyclonal Treg has already been used to treat inflammatory diseases, such as type 1 diabetes (285), however the use of autologous cells takes a long time, due to the period necessary for differentiation and clonal expansion, making this an unviable and costly method for infectious diseases, as is the case with COVID-19 (286).
A viable alternative is the use of allogeneic human leukocyte antigen-matched umbilical cord-derived Tregs (UBC-Treg) which can be widely expanded and used on a larger scale. A recent case study used 1x108 administration of UBC-Treg in two patients with COVID-19 who had severe respiratory failure, and both demonstrated significant clinical improvement and reduced inflammatory markers four days after starting treatment (287).
There are currently two clinical trials underway that aim to infuse Treg cells in patients with severe COVID-19 and ARDS. The first one is a multi-center, prospective, double-blinded, placebo-controlled phase 1 randomized clinical trial, which has 45 patients who will receive cryopreserved UBC-Treg (288). The second one is a randomized, double-blind, placebo-controlled phase 2 study with 88 participants who will receive off-the-shelf allogeneic hybrid Treg/Th2 cells (RAPA-501-ALLO). RAPA-501-ALLO cells will be generated from healthy donors, cryopreserved, banked, and made available for off-the-shelf therapy. The cells are manipulated ex vivo to differentiate into two anti-inflammatory phenotypes simultaneously, generating hybrid Treg/Th2 cells, with the potential to reduce inflammation and mediate a protective effect on tissues (289).
In addition to therapeutic approaches using Treg cell infusion, another three clinical trials are underway with the aim of evaluating treatment using specific SARS-CoV-2 T cells isolated from individuals who recovered from COVID-19 (290–292). The use of virus-specific T cells for off-the-shelf treatment has been used in several viral infections, such as cytomegalovirus, HHV6, adenoviruses, Ebola virus, and BK virus (293–296). Although vaccination provides T cells-based virus-specific immunity, the path to its development is long, so the use of adoptive cell transfer techniques from healthy individuals who recovered from COVID-19 and developed an effective cell response is probably the fastest way to treat critically ill individuals (297). Besides that, as mentioned before, asymptomatic or mild symptomatic patients may possibly mount robust SARS-CoV-2 specific CD8+ T cell responses (200, 201), therefore, the use of these individuals’ cells to treat critically ill patients with COVID-19 can be a promising tool.
The clinical use of IL-7 has been implemented in the treatment of cancer patients and infectious diseases, mainly with the objective of improving the immune response by stimulating the generation of lymphocytes (298, 299). In addition, IL-7 administration has been reported to increase CD4 + and CD8 + T lymphocyte counts without inducing the production of pro-inflammatory mediators, making it a promising method of recovering immune function in patients with disorders related to cytokine storms, such as sepsis and COVID-19 (300).
In a case study conducted by Monneret et al. (301), compassionate administration of IL-7 to a patient with severe COVID-19 significantly improved total lymphocyte count and HLA-DR expression in circulating monocytes four days after administration of the first dose. The patient also showed a significant improvement in lung involvement and negative viral load. Another study conducted by Laterre et al. (302), who administered IL-7 to COVID-19 patients found that there was a significant improvement in the lymphocyte count after starting treatment, in addition, the patients did not show any change in TNF-α levels, IL-1β, and IL-12p70, which may indicate that IL-7 therapy may be safe for patients with severe inflammatory changes. Thus, the use of IL-7-based immunotherapy can be an important tool to be used in future clinical trials in patients with severe lymphopenia.
Therefore, the data available to date do not ensure the success of immunotherapy applied in patients with COVID-19, thus, further studies specifically targeting SARS-CoV-2 should be performed to provide more specific data. However, immunotherapy is effective and of immediate use, being of short duration. This approach also presented limitations, such as the possibility of abnormal reactions and other serious risks, such as induction of severe acute lung injury or ADE (225). Although we are living through a unique moment in science, with some mismatched information and novel, important discoveries being made every day, immunotherapy seems to be a possibly effective option to help patients until an effective, safe vaccine or treatment is developed.