The process of scientists trying to understand how the coronavirus operates has been likened to detectives investigating a crime. To better understand the perpetrator, they first had to uncover its modus operandi in the human body, i.e. which cells it targets, and why.

These hotspots have recently been mapped in a study published in Cell Reports, where 28 SARS-CoV-2 and coronavirus-associated receptors and factors (SCARFS) were investigated as "accomplices" to the virus, serving as gateways for infection into various organ cells.

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Cause of death

Let's start with cause of death. Post-mortems confirm that victims had major lung damage inflicted by Covid-19-induced pneumonia. It also wreaks havoc on the heart, kidney, liver and gastrointestinal tract, and has been proven to havesome neurological impact on the brain.

"What causes the wide range of clinical phenotypes observed in people infected with SARS-CoV-2 is not yet understood," write the investigative scientists.

"It remains unclear which of these pathologies are caused by direct infection of the organs affected or indirect effects mediated by systemic inflammatory responses or comorbidities. A prerequisite to resolving these questions is to gain a better understanding of the tropism of the virus, i.e. which tissues and cell types are permissive to SARS-CoV-2 infection."

Partners in crime

This means that some cells have more coronavirus-friendly receptors than others, and are more willing to "invite the wolf in", especially when primed bycellular protease. In the case of this virus, scientists have already identified ACE2 receptors andTMPRSS2 protease as the most common "partners in crime".

But the researchers say there have to be more players at work due to the virus's high infection rate, which is what they undertook to investigate.

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Following your nose

For them, the real battle for infection takes place in the nose, more specifically, the nasal epithelium.

"The nasal epithelium expresses various combinations of factors that, in principle, could facilitate SARS-CoV-2 infection, but it also expresses robust basal levels of resistant factors, which may act as a strong protective barrier in this tissue."

Age may also have an impact on the nasal epithelium's ability to fight off the infection, where the protease undergoes a shift in regulation in young individuals.

Moving through the rest of the body, goblet and absorptive cells in the intestines and proximal tubule cells in the kidneys are more welcoming of the virus, with infection also found in the brain and lungs.

Men are the main target

Male parts like spermatogonial cells in the testes and prostate endocrine cells are also easily targeted by the coronavirus, which might explain men's vulnerability to the Covid-19. In contrast, in women, the ovaries and their cells are highly unlikely to be infected.

However, embryonic and placental development are at moderate risk of infection in pregnant women, but further research is needed.

"Because the basal expression level of these factors determines, at least in part, the tropism of the virus, this information is foundational to predict which tissues are more vulnerable to infection. These data are also important to guide and prioritise clinical interventions and pathological studies, including biopsies."

They highlight, however, that SCARF expression within and between individuals can be influenced by genetics and environmental factors and their "map" might not be representative of everyone.

Still, tracing the virus's potential footsteps throughout the body also reveals potential secret routes it might use to jump from host to host. With this guide, we might be able to finally catch up to the virus and stop it in its tracks.

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Compiled by Gabi Zietsman

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Mapping Covid-19's tracks as it attacks the body - Health24

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