A genetic analysis of the cellular immune response in Drosophila melanogaster larvae: Competence, proliferation, and differentiation of immune effector cells.
We can improve our understanding of the genetic regulation of mammalian innate cellular immunity if we can employ an animal model that does not have an adaptive immune system and that can be subjected to genetic, molecular, physiological, and statistical analysis. Drosophila melanogaster may provide such a model. To our knowledge Drosophila does not have adaptive immunity, and has been a useful system for genetic inquiry for over a century. However, while its humoral immune system has been relatively well investigated over the past two decades, and has demonstrated significant molecular and genetic homology to its mammalian counterpart, nonetheless very little is known about its cellular immune system. In pursuit of a better understanding of innate cellular immunity in D. melanogaster, we analyzed the effects of specific genetic mutations on the ability of the larval hematopoietic system to respond to parasite infection. We provide evidence that, just as occurs in mammals, the hematopoietic tissues of D. melanogaster larvae respond to an immune challenge with the proliferation, differentiation, and mobilization of immune effector cells. In doing so here, we implicate fruit fly homologues of members of the JAK-STAT and NF-kappaB pathways, which are already known to contribute to mammalian and Drosophila hematopoiesis and immunity. Furthermore, we provide evidence that the acquisition of cellular immune competence in the larval lymph gland is developmentally regulated. Finally, we present a protocol for the quantification of phenol oxidase activity in insect hemolymph. We observed a correlation between the ebony mutation, which produces flies with an excess of melanin in their cuticles, and a significant reduction in phenol oxidase activity in larval hemolymph, providing another suggestive piece of evidence linking melanin to immunity.