Abstract
The Drosophila larval cellular immune response involves cells (hemocytes) that can be recruited from a hematopoietic organ located behind the brain, as well as a sessile population of cells found just underneath the larval cuticle arranged in a segmental pattern. By using two Rac1 GTPase effector-loop mutants together with epistasis studies, we show that Rac1 requires the Drosophila melanogaster Jun N-terminal kinase Basket (Bsk), as well as stable actin formation to recruit the sessile hemocyte population. We show that actin stabilization is necessary for Rac1-induced hemocyte activation by lowering cofilin (encoded by the twinstar gene tsr) expression in blood cells. Removing Bsk by RNAi suppressed Rac1-induced release of sessile hemocytes. RNAi against Bsk also suppressed Rac1 induction of lamellocytes, a specialized population of hemocytes necessary for the encapsulation of invading pathogens. Furthermore, Rac1 and Bsk are involved in regulating the formation of actin- and focal adhesion kinase (FAK)-rich placodes in hemocytes. Lastly, Rac1 and Bsk are both required for the proper encapsulation of eggs from the parasitoid wasp Leptipolina boulardi. From these data we conclude that Rac1 induces Bsk activity and stable actin formation for cellular immune activation, leading to sessile hemocyte release and an increase in the number of circulating hemocytes.
Highlights
The Drosophila melanogaster larval cellular immune response involves circulating immune surveillance cells known as hemocytes
In Drosophila, larval hemocytes develop in the lymph gland, a hematopoietic organ consisting of multiple pairs of lobes located behind the brain (Meister, 2004)
We show that Rac1 and Bsk are involved in the regulation of cellular adhesions in activated hemocytes
Summary
The Drosophila melanogaster larval cellular immune response involves circulating immune surveillance cells known as hemocytes. In Drosophila, larval hemocytes develop in the lymph gland, a hematopoietic organ consisting of multiple pairs of lobes located behind the brain (Meister, 2004). Three basic types of hemocytes can be identified, plasmatocytes, lamellocytes and crystal cells. The largest and normally least abundant hemocytes are the lamellocytes. They are involved in the encapsulation of invading pathogens and are rarely seen in healthy larvae but become enriched when larvae are parasitized (Carton and Nappi, 1997; Lanot et al, 2001; Sorrentino et al, 2002). Crystal cells secrete components of the phenol oxidase cascade, which is involved in melanization of invading organisms and in wound repair (reviewed in Meister, 2004)
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