Abstract
Honey bees as other insects rely on the innate immune system for protection against diseases. The innate immune system includes the circulating hemocytes (immune cells) that clear pathogens from hemolymph (blood) by phagocytosis, nodulation or encapsulation. Honey bee hemocyte numbers have been linked to hemolymph levels of vitellogenin. Vitellogenin is a multifunctional protein with immune-supportive functions identified in a range of species, including the honey bee. Hemocyte numbers can increase via mitosis, and this recruitment process can be important for immune system function and maintenance. Here, we tested if hemocyte mediated phagocytosis differs among the physiologically different honey bee worker castes (nurses, foragers and winter bees), and study possible interactions with vitellogenin and hemocyte recruitment. To this end, we adapted phagocytosis assays, which—together with confocal microscopy and flow cytometry—allow qualitative and quantitative assessment of hemocyte performance. We found that nurses are more efficient in phagocytic uptake than both foragers and winter bees. We detected vitellogenin within the hemocytes, and found that winter bees have the highest numbers of vitellogenin-positive hemocytes. Connections between phagocytosis, hemocyte-vitellogenin and mitosis were worker caste dependent. Our results demonstrate that the phagocytic performance of immune cells differs significantly between honey bee worker castes, and support increased immune competence in nurses as compared to forager bees. Our data, moreover, provides support for roles of vitellogenin in hemocyte activity.
Highlights
The honey bee (Apis mellifera) is an important pollinator
Observations of the uptake of larger bead numbers by a single hemocyte was rare (Fig 1C). Similar for both labeling techniques, we found that about 3% of hemocytes were positive for the two markers (Fig 2A, Ttest: Nlatex beads = 11, NCM-Dil = 15, F = 2.003, P = 0.27)
We tested if immune activation by E.coli 2h prior to latex bead injection affects the level of detectable phagocytosis
Summary
The honey bee (Apis mellifera) is an important pollinator. The alarming decline of managed colonies has spurred much research into disease epidemics and mechanisms to counteract pathogens [1,2,3]. Social honey bees show a flexible division of labor among female workers that are all functionally sterile and assist the queen. Phagocytic performance of honey bee workers typically start as nest bees that nurse the brood and other colony members, including the queen. The physiological changes associated with worker caste progression from nurses to foragers include changes in their nutrient storage [5], hormonal regulation [6, 7], oxidative stress resilience [8], mortality and aging dynamics [5, 9,10,11] as well as in immune pathways [6, 9, 12]. Brood-less, over-wintering colonies are maintained by another worker caste—winter (diutinus) bees, which actively engage in thermoregulation and have the longest worker-lifespan (reviewed by [13])
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