The authors have declared that no competing interests exist.
Mannose binding lectins (MBL), a key molecule in our innate immune response, contributes to host defense against coronaviruses such as SARS-CoV. This article reviews the role of MBL in the innate immune response against coronavirus infections, highlights evidence of MBL’s significance, and suggests dietary MBL supplementation through increased consumption of fruits and vegetables as an accessible and viable approach to minimizing COVID-19 infection risk.
Increasing consumption of plant lectins (e.g., eating fruits and vegetables) may reduce COVID-19 risks.
The global COVID-19 pandemic continues to spread. Public health measures in the USA have had limited efficacy with masks and social distancing the primary methods for reducing the infection rate. While a lot of research is focused on a vaccine which trains the adaptive immune system to provide an acquired immunity, the innate immune system should not be overlooked in its ability to help reduce the risk of COVID-19 infection. This article reviews the role of mannose binding lectins (MBL) in the innate immune system, knowledge about the epidemiology of COVID-19, and how supplementing MBLs increases resilience against COVID-19 infection. Several studies have shown that MBLs display anti-coronavirus properties of varying efficacy. Evidence supporting the positive effects of MBL with respect to COVID-19 infections includes (a) their ability to bind to coronaviruses thereby inhibiting infectivity and activating the complement system, (b) observed infection rates varying by race and ethnicity consistent with variations in mannose binding lectin levels, and (c) consumption of more plant lectins correlated with better outcomes and lower rates of infection. Supplementing MBL through diet to enhance innate immune response may therefore reduce infection rate, improve outcomes, and increase overall public health while vaccine development continues. MBLs are common and readily accessible from plants so it is recommended that people at risk of or infected by COVID-19 consider dietary supplementation of plant MBLs by increasing consumption of fruits and vegetables. Large scale population studies should be conducted to refine these generalized recommendations towards more effective dietary recommendations.
The innate immune system is our first line of defense against infection by invading pathogens.
Multiple studies have shown that MBL deficiency increases susceptibility to infection, including influenza and SARS-CoV-1.
COVID-19 is caused by a coronavirus known as Severe Acute Respiratory Syndrome CoronaVirus 2 (SARS-CoV-2). SARS-CoV-2 was previously known by its provisional name, 2019 novel coronavirus (2019-nCoV). Previous human coronavirus infections include Severe Acute Respiratory Syndrome CoronaVirus (SARS-CoV-1), which caused the SARS outbreak in Asia between 2002 and 2004, and Middle East Respiratory Syndrome CoronaVirus (MERS-CoV).
Coronaviruses are enveloped viruses with large single-stranded RNA genomes.
S is the main target of antibodies during infection and a focus for vaccine design because it is virtually the only antigen present on the virus surface.
Site-specific glycosylation analysis suggests that the glycan shield of SARS-CoV-2, like other coronaviruses, exhibits numerous vulnerabilities.
SARS-CoV-1’s less dense glycan shield reveals a broad spectrum of underprocessed oligomannose-type glycans with Man5GlcNAc2 to Man9GlcNAc2 glycans all present.
MBLs exhibit significant activity against enveloped viruses, such as coronaviruses, as their glycosylated envelopes allow MBLs to interfere at two points of the viral life cycle: viral entry and viral shedding.
As MBL and the innate immune defense are the first line host defense against pathogens, host lectin levels (specifically, MBL levels), and composition may contribute to both susceptibility and outcome (e.g., whether an infected host exhibits symptoms or remains asymptomatic). MBL deficient individuals are more susceptible to SARS.
Presuming SARS-CoV-2 effective MBL levels are a significant factor is consistent with observations that those of certain racial and ethnic communities (including non-Hispanic Blacks, Hispanics, Latinos, and Asians) are at higher risk.
The frequencies of these genetic differences and their resulting MBL deficiencies contribute towards why people of Southeast African and South American descent are more susceptible to SARS than Caucasians as observed in CDC data. Currently, as of June 30, 2020, the CDC reports “non-Hispanic Black persons” and “Hispanic or Latinos persons” are disproportionately affected at a rate several times that of non-Hispanic white persons.
As plant lectins exhibit anti-coronavirus properties of varying efficacy, it’s likely that plant MBL supplementation is a contributing factor towards COVID-19 risk and outcomes.
An example where an active lectin supplement may be demonstrating its anti-coronaviral properties is the Nictaba lectin from the tobacco plant (
While tobacco products have clear disadvantages in increasing COVID-19 severity, their ability to reduce infection risk suggests plant lectin supplementation (e.g., consumption of fruits and vegetables) may be a viable approach to augmenting host MBL production and boosting innate immune system response to SARS-CoV-2. See
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Fruit and vegetable consumption is also correlated with income where higher incomes are associated with higher levels of fruit and vegetable consumption.
With MBL levels a critical factor in COVID-19 infection risk and outcomes, it is important to determine the appropriate amount of MBL supplementation appropriate for mitigating COVID-19 risk. Further research here is recommended as MBL blood concentrations vary widely from undetectable to as high as 10 µg/ml, and MBL deficiency criteria is not standardized with prior studies using thresholds ranging from 0.1 µg/ml to 1 µg/ml.2 A prior study found that the median serum level of MBL in Hong Kong Chinese SARS-CoV-1 patients was significantly lower (0.733 µg/ml) than that in control subjects (1.369 µg/ml); an average difference of 0.636 µg/ml.
To illustrate the viability of dietary MBL supplementation, we posit an extreme example of compensating for MBL deficiency using leek (
While research continues for a vaccine to SARS-CoV-2, we can take precautionary steps to strengthen our innate immune response thereby reducing infection risk and improving outcomes. Many plant lectins, especially mannose binding lectins, interfere with the viral life cycle of coronaviruses and facilitate recognition by the innate immune system. While mannose binding lectins are common in legumes and alliums (e.g., leeks, onions, shallots, wild garlic, chives, etc…), plant lectins exhibit a broad range of specificities with varying levels of affinity.
Further studies and experimentation should be considered to understand in more detail the impact of various MBL genotypes and MBL deficiencies with respect to COVID-19. Results from these studies may be used to identify higher risk individuals, enable better prediction of outcomes, and improve treatment methodologies. As many plant lectins exhibit anti-coronaviral properties, large scale population studies of diet and infection with respect to COVID-19 infection and outcomes should be conducted to identify foods most likely to contain lectins effective against SARS-CoV-2. Results from these studies would enable refinement of dietary recommendations for at risk groups and facilitate further study of specific SARS-CoV-2 effective lectins. Together, future studies in these directions could inform public policy decisions towards improving overall public health.
All authors have read and agreed to the published version of the manuscript. Authorship must be limited to those who have contributed substantially to the work reported. Kevin Lau is responsible for research, conception, analysis and writing of this article.
No funding was received for this research.
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