Bee's immune system and their vaccine
Good news: the U.S. Department of Agriculture approved the first insect vaccine! It is well known that the honeybee population has been decreasing throughout the last decades; insecticides and habitat loss play a big part in this, as well as disease. Honeybees are not just important for producing honey; they are also used widely for pollination when other pollinators are scarce, and they are the most economically valuable pollinators for monocultures.
Honeybee (Apis mellifera) Picture by Kathy Keatley Garvey, UC Davis
What differentiates bees in a colony
Young bees tend to perform maintenance tasks while older bees go foraging. Hormones control this role change, and scientists have not yet identified other compounds. Some studies have shown that mature bees can delay young bees’ maturation according to the colony’s needs! One of the compounds that control this behaviour change is (at least in part) ethyl oleate, which acts as an inhibitor to delay age at onset foraging in young bees. Ethyl oleate is much higher in forager bees than in nurse/worker bees, and when the lack of foragers reduces ethyl oleate exposure, younger bees will mature much quicker. This change provides an adaptive colony response. When this article was published, other honey bee pheromones were already identified (QMP and BP; ethyl oleate is part of BP). Still, other parts of BP are not found in forager bees, or levels do not differ among different bee classes, and these hormones also showed the same effect in younger bees. This study also suggests that ethyl oleate is spread by trophallaxis, a form of bees to exchange food and information. When bees lick the queen, they pass on some of the queen’s essence while they exchange food. This shows that the queen is well (or not) and suppresses ovary development in worker bees. This also accelerates the spread of disease, especially if the pathogen resides in the bee’s gut.
But things are more complex; bees vary among species, but how much? Bees are complex, in fact, research by Robinson Lab at the University of Illinois has been studying variations among bees. One study compared how bees become foragers with bees from Africa and Europe. Surprise, surprise, all bees showed different levels of sensitivity to these hormones that influence the age at onset foraging and different levels of sensitivity to social inhibition. Natural selection made bees from different parts of the world more sensitive to the physiological processes associated with these hormones. Of course, this was just one study with bees from two different places; however, it shows how different bees are amongst themselves, just like humans! Another study by the same lab used comparative genomics (when scientists compare the whole DNA of different organisms to find things in common, for example) to study if honeybees and humans share some molecular mechanisms in similar superficial behaviours. Humans and honeybees showed strong similarities in genes associated with social responsiveness. Bees that were more socially unresponsive had certain gene groups that were more transcribed (active) than bees that were more socially responsive. These gene groups are associated with autism spectrum disorders (ASD) in humans; this study used the SFARI (Simons Foundation Autism Research Initiative) database, which contains all genes that influence ASD. These bees did not show to not be incompetent at surviving; they just reacted less to certain stimuli. This is very interesting, especially if we can think of how similar we are to insects and how they can be used to study human disorders. Bees are as complex (molecularly speaking) as humans, which needs to be considered when studying them.
Immune system
Bees are social insects; because of this, they have evolved to have group-level and individual defences against pathogens. When honeybee’s genome was studied, they found that this species of bees only has roughly one-third as many genes in 17 gene families involved in insect immunity (comparing with sequenced genomes of Drosophila and Anopheles - these are model insect organisms, so they are better understood (since more research has been done on them)). Why did honeybees evolve to reduce the number of genes involved in immunity? Evans et al., in 2006, hypothesised that this might reflect the strength of social barriers to disease (they are so strong fighting a pathogen as a group that they do not need to be that strong individually) or because bees are attacked by a limited set of highly coevolved pathogens.
Because bees have been in groups for so long, they evolved together and developed strategies to fight disease. One of these strategies is when worker bees identify infected larvae, they remove them from the healthy brood and discard them. Another is that they build nests with antimicrobial materials and the transference of immune traits among bees in the same group. The latter is particularly important since living in a group as a social species does increase the transmission rate.
In 2012, Bull et al. studied how age might impact the survival rates of honeybees when infected with the fungus Metarhizium anisopliae. Bees from both groups were infected with the pathogen, and the expression of genes that regulate the immune systems was measured. ‘House’ bees (younger bees) had more upregulated genes than older counterparts, but forager bees showed better survival rates when infected with the pathogen. This study also concluded that there is an increase in the expression of genes associated with microbial proteins during bee development.
Royal jelly also plays a big role (and ultimately is what the vaccine is based on). Royal jelly is a glandular secretion made by a subset of worker bees fed to the queen and young larvae; this jelly has antimicrobial compounds. Like worker bees, the queen can transfer pathogen fragments into developing eggs, giving immunity or better immune responses to pathogens; this is called trans-generational immune priming. The protein that carries the fragments of pathogens is called vitellogenin, which is also used by worker bees to synthesise royal jelly.
The effects of insecticides on honeybees have been studied for some time now. Exposure to sublethal levels of some insecticides, like neonicotinoids, may affect reproduction, development and physiology. Even at low doses, neonicotinoids lead to impaired learning and homing behaviour (the ability of some animals to know how to return home); and also, make bees more susceptible to certain pathogens such as microsporidians, deformed wing virus and black queen cell virus.
The molecular mechanisms
Because everything that happens to us is molecules interacting with each other, bees can not be any different. The figure below shows the signalling pathways in bees linked to immune responses.
Signalling Pathways in honeybees. In Drosophila, some pathways are preferable for specific pathogens. Gram-negative bacteria tend to trigger the Imd pathway while Gram-positive bacteria tend to trigger the Toll pathway.
It is also in what they eat.
In 2010, Alaux et al. studied how honeybees immune systems responded to different dietary protein quantities (monofloral pollen), and qualities (polyfloral pollen) can shape a bee’s immunocompetence (whether the bees were able to trigger a normal immune response). This was done by measuring haemocyte concentration, fat body content and phenoloxidase activity for individual immunity. For social immunity, they measured the glucose oxidase activity bees secrete to their food to sterilise the feed and the colony. Glucose oxidase is an enzyme that catalyses the oxidation of β-d-glucose to gluconic acid and hydrogen peroxide (the latter has antiseptic properties). Limited nutrition or a decrease in protein content did not affect phenoloxidase activity. Increased protein quality (polyfloral pollen) increased immunocompetence levels; the polyfloral pollen diet induced higher glucose oxidase activity than monofloral diets, including protein-richer diets! Eating more diversely helps bees’ immune systems, and resource abundance and diversity directly impact bees’ health! Also, when bees had a low protein diet, glucose oxidase activity was greatly affected, indicating that bees might invest more resources into sterilising the food to protect the collective brood (instead of individual immunity). This makes sense since honeybees have a limited number of genes to defend themselves individually against disease, making social defence more important to ensure survival.
Bee’s microbiome
Microbiomes have been shown to be very critical for the health of many animals, and bees are no exception, even though their relationship is poorly understood. Horak et al. studied the effects of the symbiont Snodgrassella alvi, a bacteria that has been shown to affect bee gene expression! They did this by infecting honeybees with and without S. alvi with the pathogen Serratia marcescens (alive bacteria and heat-killed S. marcescens). The honeybees’ immune system responded differently in alive and heat-killed S. marcescens, the latter triggering a more extensive immune response. The Toll pathway was preferable for this pathogen over the Imd pathway, and the symbiont’s protective effect seemed to be the cause of increased survival rates of bees infected with the heat-killed pathogen and increase of pathogen clearance suggesting a key role in colonisation resistance when the symbiont is present in bee’s gut. S. alvi is very competent; it helps bees survive infection by out-competing the pathogen for resources, space, and direct antagonism. This study also suggests that A.alvi is not seen as a pathogen by the bee’s immune system by forming a biofilm that camouflages it from the immune system. This symbiont appears to control gene expression by reducing the expression of Imd pathway genes. When bees had a mix of alive and heat-killed S.alvi, the Toll pathway seemed to favour bees with only heat-killed S.alvi (but a great decrease in Imd pathway gene expression). The reason why heat-killed bacteria had a sharper immune response is not clear. Still, the authors hypothesised that this might happen because heat-killed bacteria cannot form biofilms, so they might have a greater chance of spreading throughout the gut and interact more with the bee’s immune system.
American Foulbrood
American Foulbrood (AF) is one of the most common bee pathogens, and even after more than a century of research, it is still one of the most fatal bee diseases worldwide. A spore-forming, gram-positive bacteria called Paenibacillus larvae causes this disease. When a colony is infected, the only solution is to destroy the infected colonies, usually by burning the hives, contaminated hive material, and antibiotics (even though the last is illegal in the UK for the moment).
Dalan Animal Health
Dalan Animal Health was the company that, earlier this year, had the first vaccine for American foulbrood approved in America. The vaccine contains dead P.larvae (the pathogen) that, once digested in the bees’ gut, it deposit in the ovaries of the bee, giving immunity to the larvae. And no, the bees will not be injected with a tiny syringe; the vaccine will be delivered mixed with the sugar feed given to queen bees. The immune system of honeybees is not like ours; they are insects and do not produce antibodies like humans, so the delivery of such drugs and treatments has to be different. After digesting the vaccine, the queen bees give origin to immune bee larvae.
This started in 2015 when scientists identified a specific protein that boosts bee offspring’s immune response.
When the development of this vaccine started, there was no regulation for insect vaccines because this was the first one. Dalan has to show ‘proof of safety, purity and a certain degree of efficiency.’ says Ms Kleiser from Dalan Animal Health. This vaccine is a breakthrough and hopefully will be the first in a market that many people thought it did not exist.
Alternative to a vaccine
Scientists at The University of Texas developed genetically engineered bacteria to protect bees from varroa mites and deformed wing viruses. These bacteria are hosted in the bee’s gut and pump out medicine that protects bees from these pathogens. This was the first time someone treated a disease by genetically engineering a bee’s microbiome. Varroa mites that fed on treated bees were 70% more likely to die by day 10, and treated bees were 36% more likely to survive on day 10.
