A Closer Look: Social Immunity

by Clarence Collison

Individual honey bees of all ages and castes have developed mechanisms to limit the impacts of their pathogens.

Insect social life is generally associated with increased exposure to pathogens and the risk of disease transmission, due to factors such as high population density, frequent physical contact and reduced genetic variability (Baracchi et al. 2012). The stable, high levels of humidity and temperature of their brood nests, result in suitable environments for the development of microorganisms including pathogens (Baracchi et al. 2011). Honey bees are attacked by numerous parasites and pathogens toward which they present a variety of individual and group-level defenses (Evans and Spivak 2010). Behaviors that reduce colony-level parasite and disease loads are termed “social immunity.”

Individual honey bees of all ages and castes have developed mechanisms to limit the impacts of their pathogens. These mechanisms involve resisting pathogens, by building barriers to infection or mounting defense responses once infection has occurred or tolerating pathogens, by compensating for the energetic costs or tissue damage caused by either these pathogens or the bee’s own immune responses. Mechanical, physiological, and immune defenses provide the classic route for resisting pathogens (Evans and Spivak 2010). Mechanical barriers include the insect cuticle and epithelial layers, which in many cases prevent microbes from adhering to or entering the body. Physiological inhibitors to microbial invasion can include changes in the pH and other chemical conditions of the insect gut (Crailsheim and Riessberger-Galle 2001). Honey bees are known to mount an induced immune response to wounding or pathogen exposure (Evans et al. 2006).

Most physiological immune responses are internal and targeted at organisms that have invaded the body. Responses to the pathogen may include producing antimicrobial peptides and lysozymes that either inhibit the growth of microorganisms or kill them. Similarly, their blood cells phagocytose single-celled parasites, whereas larger invaders are encapsulated in a layer of blood cells that are melanized, sealing off the invader from the host’s body. These are examples of personal immunity in which the challenged individual is the main beneficiary of the immune response (Cotter and Kilner 2011).

“Social immunity,” describes how individual behaviors of colony members effectively reduce disease and parasite transmission at the colony level (Cremer et al. 2007; Simone-Finstrom and Spivak 2010). These behaviors range from more common acts like grooming of nestmates and removal of dead material from the main nest area (undertaking behavior) to “social fever” in honey bees that is used to kill pathogens (Starks et al. 2000) and the detection and removal of pre-infectious diseased or parasitized brood (hygienic behavior). Hygienic behavior is an antiseptic behavior and differs from undertaking (the removal of dead adult nestmates) and grooming (the removal of foreign objects and pathogens from oneself (autogrooming) or from another adult in the nest (allogrooming) (Wilson-Rich et al. 2009).
In contrast to individual immunity, social immunity describes colony-level anti-parasite/pathogen protection, achieved by the cooperation of all colony members, collectively avoiding, controlling or eliminating infections and reducing parasite load. The nature of these defenses are that they cannot be performed efficiently by single individuals but depend strictly on the cooperation of multiple individuals (Cremer and Sixt 2009).
The most virulent, colony killing, bacterial agents are Paenibacillus larvae causing American foulbrood (AFB) and European foulbrood (EFB) associated bacteria. Besides the innate immune defense mechanisms, honey bees have developed behavioral defenses to combat these infections. Foraging for antimicrobial plant compounds plays a key role for this “social immunity” behavior. Secondary plant metabolites in floral nectar are known for their antimicrobial effects. Yet these compounds are highly plant specific, and the effects on bee health will depend on the floral origin of the honey produced. As worker bees not only feed themselves, but also larvae and other colony members, honey is a prime candidate acting as self-medication agent in honey bee colonies to prevent or decrease infections. Erler et al. (2014) tested eight AFB and EFB bacterial strains and the growth inhibitory activity of three honey types. Using a high-throughput cell growth assay, they showed that all honeys have high growth inhibitory activity and the two monofloral honeys appeared to be strain specific. The specificity of the monofloral honeys and the strong antimicrobial potential of the polyfloral honey suggest that the diversity of honeys in the honey stores of a colony may be highly adaptive for its “social immunity” against the highly diverse suite of pathogens encountered in nature.

Another example of a behavioral disease resistance mechanism is the collection and use of plant resins. Honey bees forage for plant-produced resins with antimicrobial properties and incorporate them into their nest architecture. These resins are brought back to the colony where they are mixed with varying amounts of wax and utilized as propolis (Simone et al. 2009; Simone-Finstrom and Spivak 2010). This use of resins can reduce chronic elevation of an individual bee’s immune response. Since high activation of individual immunity can impose colony-level fitness costs, collection of resins may benefit both the individual and colony fitness. Simone-Finstrom and Spivak (2012) presented evidence that honey bee colonies may self-medicate with plant resins in response to a fungal infection. Self-medication is generally defined as an individual responding to infection by ingesting or harvesting non-nutritive compounds or plant materials. They showed that colonies increase resin foraging rates after a challenge with a fungal parasite (Ascophaera apis which causes chalkbrood). Additionally, colonies experimentally enriched with resin had decreased infection intensities of this fungal parasite. If considered self-medication, this is a particularly unique example because it operates at the colony level. Most instances of self-medication involve pharmacophagy, whereby individuals change their diet in response to direct infection with a parasite. In this case with honey bees, resins are not ingested but used within the hive by adult bees exposed to fungal spores. Thus the colony, as the unit of selection, may be responding to infection through self-medication by increasing the number of individuals that forage for resin.

As the antimicrobial venom peptides of Apis mellifera are present both on the cuticle of adult bees and on the nest wax it has recently been suggested that these substances act as a social antiseptic device. Baracchi et al. (2011) confirmed the idea that the venom functions are well beyond the classical sterotype of defence against predators. The presence of antimicrobial peptides on the comb wax and on the cuticle of workers represents a good example of “collective immunity” and a component of the “social immunity,” respectively.
Several bee pathogens are sensitive to temperature, and the individual bee or the colony may create a “fever” to kill nosema (Martín-Hernández 2009; Campbell et al. 2010) and chalkbrood (Starks et al. 2000). Behavioral fever is a common response to an infection in many animals. Honey bees maintain elevated temperatures in the brood nest (Seeley 1985) to accelerate brood development and to facilitate defense against predators. Honey bees engulf invading hornets in defensive balls, which they heat to lethal temperatures (Ono et al. 1995). Starks et al. (2000) also identified an additional defensive function of elevating nest temperature: honey bees generate a brood comb fever in response to colonial infection by the heat-sensitive pathogen Ascosphaera apis (causative agent of chalkbrood). This response occurs before larvae are killed, suggesting that either honey bee workers detect the infection before symptoms are visible, or that larvae communicate the ingestion of the pathogen.

Social life is generally associated with an increased risk of disease transmission, but at the same time it allows behavioral defense at both the individual and collective level. Bees infected with deformed-wing virus were introduced into observation hives; through behavioral observations and chemical analysis of cuticular hydrocarbons from healthy and infected bees, Baracchi et al. 2012 offers the first evidence that colonies can detect and remove infected adult bees, probably by recognizing the cuticular hydrocarbon profiles of sick individuals. They also found that health-compromised colonies were less efficient at defending themselves against infected bees, thus facing an ever increasing risk of epidemics. This new antiseptic behavior was interpreted as an adaptation at the colony level and one which should be considered as an element of the social immunity system of the bee hive.

Rueppell et al. (2010) challenged honey bee foragers with prolonged CO2 narcosis or by feeding with the cytostatic drug hydroxyurea. Both treatments resulted in increased mortality but also caused the surviving foragers to abandon their social function and remove themselves from their colony, resulting in altruistic suicide. A simple model suggests that altruistic self removal by sick social insect workers to prevent disease transmission is expected under most biologically plausible conditions. Altruistic self-removal appears to be a potentially widespread mechanism of social immunity.

Le Conte et al. (2011) identified a set of genes involved in social immunity by analyzing the brain transcriptome of highly Varroa-hygienic bees, who efficiently detect and remove brood infected with Varroa mites. The function of these candidate genes does not seem to support a higher olfactory sensitivity in hygienic bees, as previously hypothesized. However, comparing their genomic profile with those from other behaviors suggests a link with brood care and the highly Varroa-hygienic Africanized honey bees. These results represent a first step toward the identification of genes involved in social immunity.

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Clarence Collison is an Emeritus Professor of Entomology and Department Head Emeritus of Entomology and Plant Pathology at Mississippi State University, Mississippi State, MS.