In a recent study published in the Trends in Pharmacological Sciences, researchers engineered a mutant angiotensin-converting enzyme 2 (ACE2) decoy that effectively protected human lung epithelia from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection by competitively binding to its spike (S) protein. Additionally, this mutant decoy improved lung injury in mice expressing human ACE2.
There is an urgent need for novel coronavirus disease 2019 (COVID-19) therapies as the SARS-CoV-2 pandemic continues and new variants emerge. Importantly, COVID-19 vaccines are ineffective in preventing breakthrough infection from new SARS-CoV-2 variants, including its new variant of concern (VoC) Omicron.
S protein-ACE2 binding establishes SARS-CoV-2 infection on the host cell surface. Blocking this binding on the surface of cells lining the lung airways, specifically lung alveolar epithelial cells, could effectively prevent SARS-CoV-2 infection.
About the study
In the present study, researchers used deep mutagenesis to develop a mutant ACE2, termed sACE2.v2.4. This mutant ACE2 could bind S protein with a 35-fold higher affinity than wild-type (WT) ACE2 primarily due to its conformational stability.
They tested sACE2.v2.4 in vivo using transgenic mice expressing human ACE2, referred to as K18-hACE2 mice, as they used the epithelial-specific K18 protomer (K18-hACE2). SARS-CoV-2 could enter their lung epithelial cells, injured during COVID-19 infection. Overall, this animal model resembled severe clinical COVID-19, especially concerning lung histology and related features.
The researchers also determined the binding affinity of the decoy sACE22.v2.4-immunoglobulin G1 (IgG1) to S protein. They compared its binding affinity with that of clinically used anti-SARS-CoV-2 antibodies.
Notably, similar to the mutant used in the study, monoclonal antibodies (mAbs) neutralize the virus by binding and preventing its docking and uptake by ACE2 on the cell surface; additionally, they promote virus clearance.
Study findings
Similar engineered ACE2-Fc decoys have successfully contained lung damage and systemic manifestations in hamster models and improved their lung histopathology. Similarly, a study has reported that an ACE2-Fragment, crystallizable(Fc) decoy reduced pulmonary SARS-CoV-2 pseudovirus transduction in K18-hACE2 mice. Due to its Fc component, it had enhanced effector functions that promoted immune clearance of SARS-CoV-2.
The functional analyses relevant to pulmonary microvascular injury in clinical acute respiratory distress syndrome (ARDS) and COVID-19 performed during the study helped researchers investigate mutant ACE2 receptor decoys much more broadly than previous studies. Importantly, in the present study, Zhang et al. also assayed endothelial barrier dysfunction, vascular endothelial (VE)-cadherin integrity, and pulmonary edema formation.
They observed that treatment with sACE22.v2.4-IgG1 decoy restored body-weight of transgenic mice and the overall survival rate; however, it remained unclear whether this treatment reduced injury to lung epithelia. Although after 14 days (rather than seven days) of treatment, edema fluid cleared, and the lung wet/dry ratio normalized, suggesting that the lung epithelium plays a pivotal role in the recovery of mice. These findings are similar to those reported by previous studies on experimental influenza pneumonia.
Majorly due to the IgG1 Fc fragment of the decoy construct intravenous infusion of sACE22.v2.4-IgG1 reduced SARS-CoV-2 loads in the lungs of transgenic mice and enabled immune clearance of free virions and SARS-CoV-2-infected cells.
Although decoy treatment did not have any dramatic effects on the lungs of K18-hACE2 mice; there are some plausible concerns concerning the use of a decoy for treating COVID-19. For instance, they could trigger an autoimmune reaction or aberrant ACE2 activity or interfere with endogenous ACE2 signaling. These possible issues need to be tested in early phase clinical trials.
Multiple previous human clinical trials reports suggest that adverse hemodynamic changes did not occur with WT-soluble ACE2; likewise, ACE2-like enzymatic activity improved SARS-CoV-2-induced lung injury in mice and hamsters.
After inoculating K18-hACE2 mice with the more lethal P.1 variant of SARS-CoV-2, the authors noted that sACE22.v2.4-IgG1 treatment prevented mortality only when administered 12 hours (not 24 hours) post-inoculation, thus recommending an early treatment in case of a lethal variant infection.
Additionally, in vitro, sACE2.v2.4 mutant bound more strongly than WT ACE2 to the S of several SARS-CoV-2 variants. When tested against Omicron, in vitro binding of sACE2.v2.4 to Omicron S was strong, unlike mAbs that failed against Omicron due to its highly mutated S protein.
Conclusion
The sACE2.v2.4 decoy strategy showed promise against SARS-CoV-2-induced lung injury in mice. Even when used against more transmissible and infective SARS-CoV-2 VOCs, the decoy strategy worked remarkably well and the sACE2.v2.4 decoy strongly bound Omicron S.
Together, the study observations suggest that perhaps, the sACE2.v2.4 decoy will remain effective against SARS-CoV-2 variants that are yet to emerge.
