Primary cell cultures of immunocytes: a new tool for the in vitro study of insect-microbe interactions

Several microorganisms inhabit insects’ body and exhibit a range of interactions with the host immune system depending on their pathogenic, mutualistic, commensal or saprophytic nature. In particular, when a strict coevolution with the host occurred and lead to a strong genome reduction, some microorganisms are able to escape insect defense responses. It is the case of some insect-borne plant pathogens for which the ability to elicitate or suppress the immune responses has been proved. Moreover immunocytes have been shown to interact with symbionts and play a role in the vectorial capacity of insects. In this scenario, primary cell cultures of phytoplasma vectors’ immunocytes have been proposed as a new tool for studying the interactions between the pathogen and the host as well as the interplay between symbionts and immunocytes, in order to better understand insect vector competence. Once the best culture medium has been identified, immunocytes of the psyllid vectors Cacopsylla melanoneura and C. pyri and of the leafhopper Euscelidius variegatus were kept alive for more than 2 months with mitosis activities observed 2 weeks post culture, while adhesion and phagocytosis activities were confirmed by functionality test. In situ hybridization revealed that the defensin gene is actively transcribed in cultured E. variegatus immunocytes, while cecropins were not recorded in this species. Bacterial challenges revealed the induction of defensine gene only after Staphilococcus aureus challenge, but not after Escherichia coli and Asaia spp. exposure. The possibility to culture insect vector immunocytes and to analyze their ability in synthetize some antimicrobial peptides, opens new opportunities for the study of insect-microbe interactions. In particular, the chance of plant pathogens to induce or modulate the immune responses of the host could provide potential targets for the management of insect vectors in the future.