Making sense of COVID-induced loss of smell

The coronavirus SARS-CoV-2 has led to more than six million confirmed deaths worldwide to date throughout the course of the COVID-19 pandemic. While SARS-CoV-2 enters the body through the respiratory system into the lungs, it can also induce damage in other organs. For instance, the sense of smell, which is mediated by the olfactory sensory neurons in our nose along with our brain, is lost in some COVID patients. How this virus affects our ability to smell is a puzzling question, and one that has been investigated by a team led by Dr. Zazhytska in the Lomvardas lab at Columbia University. They have tirelessly worked on solving this puzzle throughout the COVID shutdown period, and their discoveries, which have recently been published in the journal Cell, have started to provide some key answers.

We can smell the scents around us because the olfactory receptors in our olfactory sensory neurons bind to odorant molecules, relay the information through signaling molecules, and eventually signal to our brain (Figure 1). Dr. Zazhytska and her colleagues found that SARS-CoV-2 was rarely detected in the olfactory sensory neurons themselves, indicating that the virus probably doesn’t gain access to our brain through these sensory neurons. In fact, the most commonly infected cells are the neighboring sustentacular cells (Figure 1b), which are important in maintaining the health of the layer of olfactory cells, including the neurons. If the sustentacular cells die, the sensory neurons can be exposed to a stressful environment without support. Thus, the shutdown of the olfactory system might be an indirect effect of SARS-CoV-2 infection.

Figure 1 The basic structure of the olfactory system.
(A) Signal transduction in olfactory sensory neurons. The cell membrane separates the interior of the cell (cell cytoplasm, bottom) from the outside environment (top).
(B) Anatomy of cells in the nose that are involved in smell perception.
(Figure was made using BioRender).

There are about four hundred olfactory receptor genes scattered across our genome, and each neuron only expresses one of them. This stringent setup is achieved by interactions between multiple chromosomes that bring all the dispersed olfactory receptor genes together and form a cluster in the nucleus of the neuron. This cluster arrangement of olfactory receptor genes allows the gene expression machinery to access and turn on only one receptor at a time. Remarkably, Dr. Zazhytska and her colleagues discovered that this organization is disrupted dramatically after SARS-CoV-2 infection in both hamsters and humans. Infected individuals also show reduced expression of not only receptor genes, but also key molecules that are involved in smell perception, likely as consequences of the disrupted organization.

Interestingly, when the team of scientists exposed uninfected hamsters to UV-treated serum from SARS-CoV-2 infected hamsters, which no longer contain virus, they still observed this same disorganization of olfactory receptor genes in the animals. This observation suggests that not the virus itself but some other circulating molecule(s) trigger the abnormal organization. Identifying these molecules may provide potential treatments for COVID-induced loss of smell, as well as other diseases that can affect our olfaction, including early onset Alzheimer’s disease.

Edited by: Sam Rossano, Eric Smith, James Lee, Trang Nguyen, Maaike Schilperoort

Covid-19 and Immunity after Organ Transplant

Over the last two years, the SARS-CoV-2 (Covid-19) pandemic has been at the forefront of media coverage. Hospitals have been overwhelmed, full cities locked down, travel banned, and we are all desperately waiting for a return to the normalcy that immunity promises. However, the development and retention of immunity can be dependent on the individual, and Covid-19 has been particularly daunting to individuals  with weakened immune systems (people who are immunocompromised). These individuals are at an increased risk of succumbing to Covid-19. Overall, it has been easy to identify the individuals that fall into this risk category. However, there has been limited research on the immunity of individuals that  have undergone organ transplants. In a new article by Dr. Mithil Soni, researchers have identified the effects of a solid organ transplant (SOT) on the development and retention of immunity to a plethora of viruses including SARS-CoV-2. SOTs are transplants that  include the kidney, liver, heart, lungs, intestines, and pancreas .

 Dr. Soni and colleagues also focus on the immunity generated by T cells, or cells beyond antibodies that play a role in killing viruses that enter the body. In this study, they focused on the immune response of one patient, a 33-year old male  suffering from erythropoietic protoporphyria, a genetic metabolic disorder that results in excessive liver damage. This subject underwent a SOT and received a liver. When undergoing a SOT, individuals are usually put through a stringent course of immunosuppressants to prevent organ rejection, which places them in the category of immunocompromised.  To their surprise, during a check-up this patient was found to have antibodies for SARS-CoV-2, indicating a previous Covid-19 infection without any serious symptoms. The ability to overcome Covid-19 with minimal symptoms while being classified as immunocompromised intrigued Dr. Soni and colleagues, and the patient agreed to provide his blood for further testing.  

The team went on to test the patient’s immune response to many infections that usually impact immunocompromised individuals. They tested the blood’s immune response to cytomegalovirus and BK virus, two viral infections that immunocompromised and SOT patients are prone to. They also tested the response to Epstein-Barr virus, which can cause Mononucleosis. From the blood, Dr. Soni and colleagues were able to collect and grow the T cells in their lab, exposed them to viruses, and measured their release of cytokines, proteins that are important for a strong immune response. They found a very strong T cell immune response against both cytomegalovirus and BK virus. They also tested the immune response to SARS CoV-2 and other coronaviruses and found a similar level T cell immune response as seen with cytomegalovirus and BK virus. 

These findings overall indicated that the SOT patient continued to have a robust immune response to multiple viruses despite the immunocompromised status. This study shows that it is possible to have robust immune responses to viruses including SARS CoV-2 in an immunocompromised state such as seen after a SOT. However, this research is based on a single case study. To truly understand T-cell memory and activity in immunocompromised individuals much more research has to be done. This means Dr. Soni and colleagues still have their work cut out for them and are actively expanding the research done here. Their next immediate steps are to repeat this study with blood from a larger group of healthy and immunocompromised individuals in the hopes that they will eventually be able to answer the question of SOT and immunity.

Figure: Depiction of increased immunity after SOT.  Top: Liver transplant. Bottom: Expected T cell activity in response to virus vs actual T cell activity in response to virus. 


Dr. Mithil Soni, is a previous Postdoctoral Research Fellow and current Associate Research Scientist at Columbia University.

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