Expansion of Drosophila haemocytes using a conditional GeneSwitch driver affects larval haemocyte function, but does not modulate adult lifespan or survival after severe infection.

Macrophages are responsible for diverse and fundamental functions in vertebrates. Drosophila blood cells (haemocytes) are dominated by cells bearing a striking homology to vertebrate macrophages (plasmatocytes). The importance of haemocytes has been demonstrated previously, with immune and developmental phenotypes observed upon haemocyte ablation. Here, we show that we can increase Hemolectin (Hml)-positive cell numbers using a constitutively active form of ras and ablate Hml-positive cell numbers using the pro-apoptotic transgene bax. However, compared with larvae, total blood cell numbers in adults were not significantly affected by experimental expansion or ablation, implying the existence of feedback mechanisms regulating haemocyte numbers. No effect on lifespan was observed from driving ras and bax in Hml-positive cells via a conditional approach (Hml-GeneSwitch). Using constitutive expression, we observed differences in lifespan; however, we attribute this to differences in genetic background. Additionally, no effect of either transgene was observed upon infection with a high dose of two different bacterial species, although pupal lethality was observed upon expansion of Hml-positive cells in a self-encapsulation mutant genetic background. The latter confirms that changes in Hml-positive cell numbers can result in phenotypes. The lack of adult phenotypes could be due to the strength of experimental manipulations or compensation via feedback mechanisms operating to regulate total blood cell numbers. Our study demonstrates the importance of conditional approaches to modulate haemocyte cell numbers, allowing for more precise study of innate immune function. This strategy could be especially fruitful to uncover mechanisms regulating total blood cell numbers across development and ageing.