As COVID-19 continues to spread the world over, investigators are still trying to understand how it ravages the human body. A recent study published on the preprint server bioRxiv* in September 2020 shows that the virus is capable of infecting human embryonic cells that are destined to form the epithelial surfaces of the body, both internal and external, as well as lung cells.
This transmission electron microscope image shows SARS-CoV-2—also known as 2019-nCoV, the virus that causes COVID-19—isolated from a patient in the U.S. Virus particles are shown emerging from the surface of cells cultured in the lab. The spikes on the outer edge of the virus particles give coronaviruses their name, crown-like. Image captured and colorized at NIAID's Rocky Mountain Laboratories (RML) in Hamilton, Montana. Credit: NIAID
SARS-CoV-2 Affects Multiple Organs
Earlier studies have shown that the virus affects a number of organs in the body even though respiratory infection is the primary manifestation and the most common cause of death in COVID-19. For instance, about a quarter of all patients in the hospital showed signs of heart damage. The same number had gut symptoms, and over a third of fatal cases had evidence of viral RNA in brain tissue.
Embryonic Lung Precursor Cells are Susceptible to SARS-CoV-2
The current study focuses on providing evidence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in human embryonic cells at various stages of differentiation. The host cell receptors for this virus, namely, angiotensin-converting enzyme 2 (ACE2) and TMPRSS2, are found in embryonic tissue from the blastocyst onwards, which indicates that vertical transmission is a possibility. Some studies show that the virus can both infect and replicate in a number of cell types, including stem cells.
The researchers first showed that both ACE2 and TMPRSS2 are expressed in human lung progenitor cells (hLPCs). These were then infected with pseudoviruses expressing the SARS-CoV-2 spike protein. It was found that infection could be suppressed in the presence of breastmilk at 2 mg/mL due to the antiviral activity of the latter.
They also infected the hLPCs with the closely related pangolin coronavirus and successfully detected the viral RNA in these cells later. Treatment with either remdesivir or human breastmilk reduced the viral RNA level significantly. However, pangolin coronavirus infection led to reduced viability of hLPCs.
Embryonic Epithelial Precursor Cells are Susceptible to SARS-CoV-2
The next step was to infect early endoderm cells of the human anterior foregut with the SARS-CoV-2 pseudovirus and pangolin coronavirus strains. These cells were also positive for ACE2 and TMPRSS2 host receptor molecules.
Successful infection was signaled by luciferase activity, and goat/human breastmilk exposure/ heparin reduced the infection rate. This novel finding could indicate that a heparan sulfate receptor interacts with the classical receptors for the virus. However, both remdesivir and human breastmilk succeeded in reducing the levels of pangolin coronavirus after exposure.
Overall, therefore, hLPCs allow infection with SARS-CoV-2.
The researchers then generated human embryonic stem cells differentiating into mesoderm, endoderm, and ectoderm. They found that both endoderm and ectoderm were permissive for the SARS-CoV-2 pseudovirus. Human/goat whey protein was an inhibitor of viral activity in these cells. Human breast milk also inhibited hESCs, endoderm, and ectoderm.
Pangolin coronavirus also infected both endoderm and ectoderm cells, indicating that mesoderm lacks some essential factors for infection. Remdesivir inhibited infection in both tissues.
The researchers concluded that “the germ layer cells such as endoderm and ectoderm are also permissive for SARS-CoV-2 infection.”
Embryonic Stem Cells Non-Permissive for the Virus
They also found that hESCs did not support infection with the SARS-CoV-2 pseudovirus, but showed signs of infection with the Malayan pangolin coronavirus. These cells are also able to generate infectious viral particles. The latter was inhibited by remdesivir.
Infected cells also expressed viral protein and infectious viral particles. The infection, as well as viral replication, were both inhibited by both breastmilk and remdesivir. Heparin was identified as a potential direct inhibitor of the SARS-CoV-2 pseudovirus and pangolin coronavirus infection.
The researchers concluded, “We demonstrated that induced endoderm, ectoderm, hAFECs and hLPCs are permissive for both SARS-CoV-2 and GX_P2V infection. The hESCs are permissive for GX_P2V [pangolin coronavirus] but not SARS-CoV-2 pseudovirus infection.”
While these results show that SARS-CoV-2 pseudovirus, at least, is capable of infecting multiple cell types in the body, except human embryonic stem cells, however, this was susceptible to the pangolin coronavirus. Infected cells also expressed the viral protein and showed the presence of replicating virions. This could explain the multiple organ involvement seen with this virus. It may also indicate the researchers suggest that they act as a stable reservoir for the virus, which may integrate into the human germline. This also offers the possibility of vertical transmission.
Secondly, the pangolin strain was also observed to infect an even broader range of human tissues, which could be a red flag in terms of the latter’s ability to infect other species. In a broader sense, this could signal the danger of human coronavirus infections initiated by such zoonotic viruses.
Thirdly, the inability of the virus to infect mesoderm despite the presence of the ACE2 and TMPRSS2 receptors may explain the use of mesenchymal stem cells in COVID-19 treatment. Further study is required to uncover the full importance of these findings.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.