As COVID-19 makes its way through the population it is important to consider how we can help our immune system cope with viral infections. In my previous article I described how our immune system generally deals with viral infections. In this article, I will discuss the specific role of vitamin D in viral infections.
Vitamin D deficiency is recognized as a global public health problem and has linked with a variety of diseases including an increase in severity of viral infections. There are a few key ways that vitamin D has an impact:
Induction of antimicrobial peptides
Antimicrobial peptides play an important role in the innate (non-specific) early immune response. Antimicrobial peptides can directly or indirectly kill microbes and they also stimulate the monocyte/macrophage response. Macrophages are immune cells that engulf infectious organisms as well as dead cells and they “present” antigens on their surface in order to activate specific immune cells like T cells. Vitamin D induces cathelicidin antimicrobial peptide, which then increases the recruitment of monocytes, neutrophils and then CD8+ T cells to infected areas. It is thought that vitamin D also plays a role in the pathway that increases expression of epidermal growth factor in respiratory airways, which increases the amount of immune cells that can migrate from the blood vessels to the infected area. Vitamin D promotes the clearance of respiratory pathogens by inducing apoptosis….this is a process where our infected cells kill themselves in order to prevent virus particles from being able to replicate. Vitamin D induces the production of a molecule called LL37, which inhibits viral infection by blocking viral entry into cells or by suppressing viral activity.
Immune regulatory functions
Vitamin D is potent immune regulator. When we have any kind of an infection, we need to have a balanced immune response so that the immune system reacts strongly enough to kill the infection but not so strongly that it damages our own body. An over-reacting immune system is just as deadly as an immunocompromised one, therefore immune regulation is extremely important. A cytokine storm is a situation where the immune system over-produces cytokines too quickly causing blood vessels to become too leaky and excess fluid can enter the tissues like the lungs affecting breathing, excessive fever response, low blood pressure, rapid heart rate, and can cause death. Vitamin D modulates immune responses by selectively reducing inflammatory cytokine production and immune cell infiltration so that the inflammatory response doesn’t get out of control. Low vitamin D has been shown to reduce the number of activated cytotoxic T cells, which are the main adaptive cells that kill virus-infected cells. Vitamin D is also associated with increasing the number of T regulatory cells that helps to control the immune response so that it does not over-react. Lastly, vitamin D influences lower respiratory tract infections by reducing inflammation, which is important for not having excessive swelling and mucus production.
Induction of autophagy and apoptosis
Autophagy is a process where cells break down organelles in order to recycle materials and replace old and damaged cell components and apoptosis is the process of programmed cell death in cells that are old or infected cells….vitamin D signalling affects those pathways. Apoptosis is an important process to regulated during viral infection. Viruses can only replicate inside cells so it is important for our T cells and Natural killer cells to kill virus-infected cells as well as for infected cells to induce programmed cells death in order to greatly reduce the viral load and get an infection under control.
Respiratory epithelial cells and respiratory resident immune cells like monocytes and macrophages express the vitamin D receptor and plays a significant role in protecting us against viral infections, including people with respiratory diseases like chronic bronchitis and asthma. In a previous study a 10nmol/L blood level increase in vitamin D was associated with 7% reduction in respiratory infections. In the midst of the COVID pandemic, vitamin D supplementation may be an option to mitigate the severity of infection.
See this post for doses of vitamin D supplementation.