The wasps Leptopilina heterotoma (Lh) and L. boulardi (Lb), are obligate parasites of Drosophila species. Lh, a generalist parasite is successful on many fly species, while Lb is a melanogaster specialist. Remarkably, Lh silences both the humoral and cellular immunity of its fly hosts, while Lb activates humoral but suppresses cellular immunity. This difference in virulence has been attributed to their virus-like particles (VLPs). VLPs are produced in the wasp venom glands and enter the larval hemocoel during oviposition. Morphologically, Lh and Lb VLPs are similar, with spikes emerging from central particle cores.
Our sequence-based results have uncovered VLP protein classes that suggest an essentially non-viral, microvesicle-like structure with eukaryotic and prokaryote-like proteins. We have also identified key proteins that might underlie the contrasting virulence strategies of the two wasps. p40, an abundant Lh VLP surface/spike tip protein is absent in Lb. Our previous work has shown that p40 is synthesized in venom glands and enters the larval hemocytes. Its surface localization and antibody inhibition experiments suggested that p40 mediates VLP interaction with host cell membranes and this interaction is pivotal to lamellocyte lysis.
Imaging studies show that in phagocytic hemocytes (plasmatocytes), VLPs require Rab5 to promote programmed cell death. However, they enter and kill encapsulating hemocytes (lamellocytes) by lysing cell membrane independently of Rab5. Domain analysis and in silico modeling of p40 suggests the presence of a fold from SipD/IpaD proteins of Gram negative type III secretion systems. These proteins reside at bacterial needle tips and regulate protein secretion into host cells.
VLPs are abundant in the larval dorsal vessel, within lymph gland progenitors, and around but not within posterior signaling center (PSC) cells. PSC cells are no longer tightly clustered but are instead dispersed in the body of the lobes. Lymph glands are non-responsive to wasps when PSCs are ablated and VLP staining in the lymph gland is low. These studies highlight the PSC’s role in immunity and suggest that Lh VLPs may inactivate this function.
Our studies shed light on the biotic nature of VLPs and on novel mechanisms of immune suppression likely shared by many other wasp species and active in a variety of Drosophila hosts world-wide. (M.E. Heavner & J. Ramroop contributed equally.).