Parasites have evolved to attack host immune systems using a variety of weapons known as virulence factors. In addition to helping parasites evade host defenses, virulence factors can also help us understand how immune systems function. In a recent edition of PNAS, Nathan Mortimer and colleagues from Emory University uncovered previously unknown immune mechanisms by exploring how certain parasitoid wasp virulence factors overwhelm fruit fly defenses.
Parasitoid wasps inject eggs and venom into invertebrate hosts; the eggs hatch and the larvae then consume the host before emerging as adult wasps. But not all hosts are passive victims; larval fruit flies, for example, mount an effective response. After infection by a wasp, the fly immune system up-regulates blood cell production and encapsulates the egg with specialized blood cells. To better understand the mechanisms of this response, Mortimer and colleagues enlisted the help of a little-studied parasitoid fly, Ganaspis sp.1. These wasps are interesting because they are so deadly. The team found that adult Ganaspis emerged from nearly 100% of infected animals in over a dozen species of fruit flies. This is in contrast to other ‘avirulent’ wasp species (e.g. Leptopilina clavipes) that have emergence rates of only a few per cent after infection.
To address whether Ganaspis evades or actively suppresses the fly immune system, the team used mutant flies that spontaneously encapsulate their own tissues. When these mutants were infected by Ganaspis, self-encapsulation was reduced. Furthermore, after Ganaspis attack, wild-type fly larvae showed the same increase in blood cell production that accompanies infection by L. clavipes. These results show that fly larvae try to mount a response to infection by Ganaspis, but a virulence factor prevents them from encapsulating the eggs.
To understand how Ganaspis disables encapsulation, Mortimer and colleagues first used transcriptomics and mass spectrometry to identify the most abundant proteins in wasp venom. Surprisingly, the most common venom protein is a large pump (sarco/endoplasmic reticulum ATPase, SERCA) that downregulates intracellular calcium by moving it into internal stores. This prompted the team to examine calcium levels in isolated blood cells in response to wasp venom. Wasp venom triggered a decrease in calcium in one type of blood cell (plasmatocyte). This response was abolished by addition of a pharmacological inhibitor of SERCA. The team also compared calcium levels in plasmatocytes after attack by L. clavipes and Ganaspis. Attacks by L. clavipes triggered a transient burst of calcium in plasmatocytes; these calcium spikes were not present in plasmatocytes after Ganaspis attack. When these calcium bursts were genetically inhibited, larvae lost the ability to encapsulate the normally easy-to-handle L. clavipes eggs. In contrast, genetically increasing intracellular calcium levels enhanced the number of larvae that encapsulated the tough-to-tackle Ganaspis eggs. Overall, these experiments show that bursts of calcium in plasmatocytes are required to initiate the immune response to wasp attack. They also show that the reason why Ganaspis is so deadly is because it has a calcium pump in its venom that prevents the calcium-mediated response.
The work of Mortimer and colleagues demonstrates the value of taking a comparative approach to the study of immune system function. By studying a diverse array of co-evolving parasites and their hosts, researchers can uncover new mechanisms of immune function.