A team of scientists from Germany and China recently conducted a large meta-analysis using transcriptome data of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) to evaluate the routes of viral entry and the characteristics of innate immune responses induced by SARS-CoV-2 infection. The study reveals that SARS-CoV-2 may enter human cells via multiple routes and that angiotensin-converting enzyme 2 (ACE-2) is not the only receptor to support viral entry. Moreover, the study finds that the innate immune responses induced by SARS-CoV-2 infection significantly vary with cell type and viral load. The study is currently available on the bioRxiv* preprint server.
SARS-CoV-2, the causative pathogen of coronavirus disease 2019 (COVID-19), is a single-stranded, positive-sense RNA virus with a genome size of about 30 kb. Other highly infectious and deadly human coronaviruses that closely relate to SARS-CoV-2 include severe acute respiratory coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV). There is existing evidence suggesting that these viruses share similar mechanisms to enter and infect human cells. Both SARS-CoV and SARS-CoV-2 bind to human ACE2 receptor through the receptor-binding domain (RBD) of their spike protein to enter host cells. Similarly, RBD of MERS-CoV spike protein binds to dipeptidyl peptidase 4 (DPP4) of the host cell for the entry. However, studies are suggesting that alternative entry routes might be present for the viral entry.
Upon viral entry, the host’s innate immune system gets activated to provide the first line of defense against the invading virus. Mechanistically, the pattern recognition receptors, such as toll-like receptors (TLRs), of the host cells recognize the molecular structure of the virus, followed by the initiation of a cascade of signaling events and subsequent production of several inflammatory cytokines, such as interferons. Interferons are an integral part of the innate immune response that targets and destroys the virus. Thus, to develop effective therapeutic interventions against deadly viruses like SARS-CoV-2, it is important to get a thorough insight into the viral entry mechanisms as well as the pattern of innate immune responses.
Current study design
The scientists analyzed a diverse set of publicly available transcriptome data of SARS-CoV-2, SARS-CoV, and MERS-CoV. These data have been obtained from different types of cells, including human lung cancer cells, lung fibroblast-derived cells, ACE2-expressing human lung cancer cells, human colorectal adenocarcinoma cells, and African green monkey kidney cells infected with SARS-CoV-2, SARS-CoV, and MERS-CoV. In addition, transcriptome data collected from COVID-19 positive patients have been analyzed. They have used these data to evaluate the routes of viral entry as well as the innate immune responses.
The scientists have observed that higher expression levels of host cell receptors, such as ACE2 or DPP4, do not influence the infection efficiency of SARS-CoV-2 or MERS-CoV. Moreover, the infection efficiency of human coronaviruses has been found to vary with cell types and viral load.
Regarding viral entry routes, they have observed that SARS-CoV-2 can enter human cells that do not express TMPRRS2 or TMPRSS4, which are cellular proteases essential for the priming of the viral spike protein. However, they have observed that for TMPRRS2 or TMPRSS4 independent entry of SARS-CoV-2, the host cells need to express cathepsin B and cathepsin L, which are endosomal proteases required for viral entry through the endocytic pathway. These observations suggest that receptor-mediated endocytosis could be an alternative route of SARS-CoV-2 entry.
Additionally, the team noticed that cells lacking ACE2 can still express small amounts of SARS-CoV-2, indicating the possibility of an alternative viral entry route. Based on the study findings, they have hypothesized that this alternative entry route could be macropinocytosis, which is a type of endocytic pathway wherein no receptor is required for viral entry. Because ACE-2 independent entry of SARS-CoV-2 has caused insufficient infection, the scientists believe that macropinocytosis cannot be a major entry route.
To determine the strength of innate immune responses against SARS-CoV-2, they analyzed the expressions of interferons, interferon-stimulated genes, and pro-inflammatory mediators in various cell types as well as in biological samples obtained from COVID-19 patients. The findings reveal that moderately and severely affected COVID-19 patients display significantly increased expressions of interferons, interferon-stimulated genes, and pro-inflammatory cytokines. In contrast, samples with inadequate viral infection have shown low levels of interferons and moderate induction of interferon-stimulated genes.
One surprising finding of the study is that in some cells, SARS-CoV-2 successfully induces the infection but fails to trigger innate immune responses. Taken together, these observations suggest that the induction of innate immune responses depends on disease severity, viral load, and cell type.
Overall, the study findings provide new insight into the mode of action of SARS-CoV-2, which could be valuable for the development of potential therapeutic interventions.
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.