Born Immunity

How honey bees pass on immunity to their young.

A single egg of the honey bee (A. mellifera)

Alex Wild

Because invertebrates don’t have antibodies—which carry vertebrate immune systems’ memories—researchers believed insects could not acquire specific disease immunity or pass it to their young. Although examples of insects capable of both functions have been emerging recently, precisely how they transfer immunity to the next generation has been a mystery—until now.

Immunologist Dalial Freitak was searching for the carrier of that immune message when she joined the University of Helsinki in Finland. There, she heard molecular biologist Heli Salmela describe her work on vitellogenin, an egg-yolk protein that many egg-laying insects, including honey bees (Apis mellifera), deliver to developing eggs. Also found in bee blood, or hemolymph, vitellogenin can bind to specific molecular structures on pathogens’ cell walls. Those disease markers can induce immune responses in insects.

Queen honey bee ovaries (left) take up bacterial material (shown in fluorescence at right) bound by the egg-yolk protein vitellogenin.

Heli Salmela and Dalial Freitak
Along with Gro Amdam at Arizona State University, Freitak and Salmela decided to explore what role vitellogenin might play in transferring disease signals to young bees. First, they used multiple methods to confirm that vitellogenin actually binds to the bacteria Paenibacillus larvae, which causes the fatal American foulbrood disease in honey bee larvae, and to Escherichia coli, representing bacteria with a fundamentally different type of cell wall structure. Then, they examined how vitellogenin interacted with the signature molecular patterns on each pathogen’s cell walls.

Next, the researchers incubated dissected ovaries of honey bee queens with fluorescent-dyed bits of E. coli, both with and without vitellogenin. They observed whether the fluorescent bacteria entered egg tissue. They repeated the experiment using other proteins found in bee hemolymph.

The team verified not only that vitellogenin is capable of sticking to signature fragments of disease-causing P. larvae and E. coli, but also that this protein was necessary for carrying the fragments—the inducers of immune response—to developing eggs. They concluded that when honey bee queens are exposed to bacteria, vitellogenin carries digested bacterial bits to bee egg tissue, preparing young bees to fend off future infection. This discovery could pave the way for new ways to protect important pollinating insects and other egg-laying species, or to control pest populations. (PLOS Pathogens)