A novel IgM intranasal intervention against SARS-CoV-2

Statement of Significance

IgM and IgA constitute the first line of defense in the respiratory tract against mucosal pathogens. These immunoglobulins assemble into pentamers and dimers respectively. A recent publication in focus highlights the discovery of IgM-14, a novel IgM molecule for prophylactic as well as therapeutic treatment against the emerging SARS-CoV-2 variants.

Since early 2020, coronavirus disease of 2019 (COVID-19) has turned into a global pandemic causing millions of deaths worldwide. The high infectivity of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) resulted in rapid spread in several countries, thereby affecting the total global population. SARS-CoV-2 is composed of a positive-sense single stranded genomic RNA, which is more prone for mutation accumulation than DNA viruses. However, coronaviruses encode an enzyme that corrects faulty replication thereby makes fewer mutations than most RNA viruses [1]. Therefore, in most cases, a mutation accumulation would happen by the process of natural selection by increasing the frequency of a mutant that confer a competitive advantage for either replication, transmission or evading immunity than those that reduce the viral fitness. The viral replicas with variations in their genomic sequence that confer a fitness advantage either in epidemiologic, immunologic or pathogenic properties are termed variants of concern (VOCs).

SARS-CoV-2 enters into a human cell through its spike (S) protein binding to angiotensin-converting enzyme 2 (ACE2). Neutralizing antibodies that target the S protein, in particular the viral receptor-binding domain (RBD), have been developed to treat COVID-19. To date the neutralizing human monoclonal antibody treatments approved in the USA for emergency use against the battle with SARS-CoV-2 have shown suboptimal efficacy with the emerging VOCs [2]. Moreover, all of these emergency use antibody treatments are of IgG1 isotype that have to be administered through intravenous (IV) infusion [3]. The levels of IgG antibodies after such IV infusion are 200–500 times lower in the mucosal compartments than those in serum [4]. This results in administration of high doses (i.e., up to 8 g) to achieve antiviral activity. Despite such high doses, the antiviral effect is minimal in the respiratory tract compared to the placebo controls [5]. In a recent study published in Nature [6], Ku et al. at the University of Texas demonstrate that an intranasal (IN) deliverable engineered antibody (IgM-14) shows promise for both prophylactic and therapeutic treatment against the original SARS-CoV-2 and the emerging resistant VOCs (Fig. 1A and B).

Figure 1.

Depiction of IgM-14 nasal spray neutralizing the SARS-CoV-2 in the respiratory tract. (A) The S protein binds to ACE2 receptor, which is neutralized in the presence of IgM-14. (B) A schematic of the pentameric IgM, highlighting the variable fragment (F_v) i.e., the specific region of the antibody that targets the RBD of the S protein which upon binding neutralizes the SARS-CoV-2. 1C: A surface view representation of the S protein (PDB: 6VYB), showing the open and closed conformation of the RBD domain. RBD is magnified to show its binding interface with the ACE2 (PDB: 6MOJ) and the epitope of the antibodies CoV2-14, CoV2-06 and CoV2-09. CoV2-14 is highlighted to show its overlap with the exact RBD-ACE2 interface at the backside of RBD. The structural models were generated using PyMOL software (The PyMOL Molecular Graphics System, Version 1.2r3pre, Schrödinger, LLC). (A) and (B) were created with BioRender.com.

The study shows that an antibody, IgM-14, engineered from a previously isolated antibody (CoV2-14) by phage display technology [7], demonstrated extremely efficient antiviral activity in mice respiratory tract, especially when delivered as a nasal spray. Evaluation of the bio-distribution of IN administered IgM-14 antibody in mice showed minimal antibody exposure in all the organs of the mice except the nasal cavity and the lungs, where the antibody persisted for up to 168 h with no observable side effects. After examining several dosage levels, one single IN dose as low as of 0.044 and 0.4 mg/kg of IgM-14 was determined to have prophylactic and therapeutic efficacy respectively against SARS-CoV-2 in mice. Although in general there might be an enhanced activity of IgMs than IgGs due to avidity, the optimal epitope recognition by the parental IgG-14 antibody contributed greatly towards the more than 230-fold potency of the engineered IgM-14’s activity. Compared with other engineered and isolated antibodies recognizing different epitopes, the above-mentioned monoclonal antibody CoV2-14 had a higher potency or antiviral activity especially due to its recognition of the critical epitope on the RBD of the S protein (Fig. 1C). The RBD of the S protein is flexible in nature and gets extended or exposed during its interaction with the ACE2 receptor of the human cells. By computational molecular docking and epitope mapping via alanine scanning, it was demonstrated that all of the critical epitope residues of the CoV2-14 lie within the receptor binding motif (RBM) of the RBD. These epitope residues overlap precisely with a critical portion of the interface that is recognized by the ACE2 receptor to establish a molecular interaction or binding of the virus to the host cell, thereby increasing the steric clashes drastically unlike the other selected antibodies, which are not positioned exactly on the interface of ACE2 interaction within the RBM rather very close to the interface. Nevertheless, in future a high-resolution complex structure would be needed to gain the precise mechanistic understanding of epitope recognition by CoV2-14, which is currently obtained by molecular docking and alanine scanning experiments. Apart from IgM-14 displaying enhanced efficiency against the original SARS-CoV-2, it has a superior antiviral activity against the emerging resistant VOCs compared to the parental IgG-14 and the rest of the antibodies. Though the exact mechanistic reason for the increased potency of IgM-14, compared with its parental IgG-14, has not been explored thoroughly. One could easily theorize that the overall functional affinity (avidity) of a pentameric IgM would affect the efficiency in a cumulative fashion as compared to a monomeric IgG subtype along with its critical epitope recognition ability. In mice, IN delivery of the IgM-14 showed an efficient targeting of the respiratory tract and the airways. Furthermore, IgM-14 efficiently prevented respiratory infections, primary site of infection and reduced viral loads to undetectable levels in the lungs of mice 2 days post infection. Investigations of preclinical pharmacokinetics for IgM-14 in rats and mice by IN dosage of 2 mg/kg twice daily for up to 5 consecutive days were performed, which displayed no side effects and all animals survived with no change in body weight until the study termination. Apart from the virus neutralization activity, the natural IgM’s possess several protective properties in vivo such as providing protection from uncontrolled inflammation, contribution and maintenance of the immunological balance, efficient disposal of dead and senescent cells, and role in early activation of the complement system. Given that IgM-14 shows an efficient protection from SARS-CoV2, information regarding its effector function potency in vivo would be very useful to obtain a mechanistic insight of the antibody’s efficacy.

This study could be extrapolated to view IgM-14 as a chemical mask against the original SARS-CoV-2 and the emerging resistant VOCs, especially due to the ability to use the antibody as a nasal spray. The strong data showing its efficacy in preventing the rate of infections and in reduction of the lung’s viral load within 2 days convincingly supports use of IgM-14 as a promising treatment for COVID-19. Regardless, there are few limitations that surround treatments using IgM as a drug such as the developability. Both the high-expression and GMP quality purification of IgM have been traditionally considered as difficult processes [8]. But with the recent advances in the IgM production there seems to be an improvement; for example, IgM-14 was produced at a decent-titre levels i.e., >1 g/L with good purity and stability. Recent developments such as mixed-mode chromatography with anion exchange chromatography were employed in the case of engineered IgM-14. To date, >20 different IgM’s obtained from various sources such as rat, mouse and humans have been tested in clinical trials. These IgM antibodies target a range of antigens relating to infectious diseases, cancers and autoimmune diseases. Recently an engineered bispecific IgM antibody, IGM2323 (ClinicalTrials.gov identifier: NCT04082936), which is been tested in clinical trials has gained some attention in the field of oncology. This asymmetric bispecific antibody contains two different binding domains targeting two different antigens i.e., it has one CD3-binding domain for every ten CD20-binding domains CD20 [9]. So far there are no adverse effects reported for any of the IgMs tested in clinical trials; nevertheless, there is always a theoretical concern that multivalent antibodies could have an off-target binding, which might be of very low affinity but enhances due to high-avidity thereby contributing to unexpected toxicities or side effects. Also, there are some controversial opinions concerning the serological testing of COVID-19 patients, which could be summarized as IgG being of higher importance as a large number of deceased COVID-19 positive patients had lower IgG levels than IgM levels [10]. A key reason for this is due to the nature of these immune responses i.e., IgM mainly provides an immediate protection against any foreign pathogen through its high-avidity but rather less specificity for that particular pathogen. Whereas IgG provides a long-term specific protection and memory against that specific infectious agent thereby preventing any case of reinfection in an extremely fast and efficient manner. The longevity of the antibodies was also taken into consideration for these controversial claims such as shorter lifespan of IgMs to that of IgGs, which provide the long-term sustained immunity against the virus. Given that IgM-14 is an artificial supplement, having a shorter half-life could be perceived beneficial in controlling any undesired side effects. Also, based on the requirement there is always a possibility to engineer the antibody to increase the serum half. It must be noted that the study provided an elegant illustration of time-dependent IgM distribution in various tissues after the IN administration in mice but did not mention a direct evaluation or comparison of IgG and IgM serum half-lives. The typical shorter half-life of IgM might be one of the reasons the study must have emphasized on IN delivery.

Apart from the neutralizing antibodies produced by B cells, CD8 T cells also play a very crucial role in the clearance and long-term memory against the viral pathogens in the respiratory tract. Therefore, such IgM supplementation would certainly slow down the infectivity of any of the emerging resistant VOCs, providing time for antigen processing by the immune system and generating potent IgG and CD8 T cell responses before the virus causes a widespread damage to a certain individual. In conclusion, given the drastic situation of VOCs creating havoc, mirroring the events of early 2020, IN treatments especially such as IgM-14 are of high interest. Moreover, the demonstration in the paper covering the issues of efficacy, scalability and safety should be taken into account to carefully consider the future plan of actions in the realm of IgM-based antibody therapeutics against COVID-19 and future virus pandemics.

FUNDING

This work is supported by the NIH Intramural Targeted Anti-COVID-19 (ITAC) Program (ZIA BC 011943) and the Intramural Research Program of NIH, NCI CCR Antibody Engineering Program (ZIC BC 011891).

DATA AVAILABILITY

The data included in this News & Views are available upon request from the corresponding author.

CONFLICT OF INTEREST STATEMENT

M.H. is the Editor-in-Chief of the journal and is blinded from reviewing or making decisions on the manuscript. The authors declare no other conflicts of interest to this work.