Date Published: October 25, 2016
Publisher: Public Library of Science
Author(s): Aurélien Guillou, Katia Troha, Hui Wang, Nathalie C. Franc, Nicolas Buchon, Mary O’Riordan.
Phagocytosis is an ancient mechanism central to both tissue homeostasis and immune defense. Both the identity of the receptors that mediate bacterial phagocytosis and the nature of the interactions between phagocytosis and other defense mechanisms remain elusive. Here, we report that Croquemort (Crq), a Drosophila member of the CD36 family of scavenger receptors, is required for microbial phagocytosis and efficient bacterial clearance. Flies mutant for crq are susceptible to environmental microbes during development and succumb to a variety of microbial infections as adults. Crq acts parallel to the Toll and Imd pathways to eliminate bacteria via phagocytosis. crq mutant flies exhibit enhanced and prolonged immune and cytokine induction accompanied by premature gut dysplasia and decreased lifespan. The chronic state of immune activation in crq mutant flies is further regulated by negative regulators of the Imd pathway. Altogether, our data demonstrate that Crq plays a key role in maintaining immune and organismal homeostasis.
Mounting appropriate immune responses against pathogens is critical for the survival of all animals. Mechanisms to both eliminate microbes and resolve infection by returning the immune system to basal activity are necessary to maintain an adequate and balanced immune response [1,2]. Alterations in these responses can lead to immune deficiency or auto-inflammation [3–5]. Yet, to date, how these mechanisms are coordinated upon infection remains unclear.
Our study shows that Crq is required for the engulfment of microbes by plasmatocytes and their clearance, and that the mild immune deficiency due to crq mutation is associated with increased susceptibility to infection, defects in immune homeostasis, gut hyperplasia, and decreased lifespan (S7 Fig). We have also re-confirmed a role for crq in apoptotic cell clearance, although the phagocytosis defect of crqko plasmatocytes is less severe than what had been previously observed with two lethal crq deficiency mutants, Df(2L)al and Df(2L)XW88 . A possible explanation is that these deficiencies may have deleted at least one other gene required for apoptotic cell clearance. Additionally, morphological defects associated with secondary mutations could have exacerbated the crq phagocytosis defect by preventing efficient plasmatocyte migration to apoptotic cells. These same deficiency mutants had been assessed qualitatively for phagocytosis of bacteria by injecting embryos with E. coli or S. aureus; their plasmatocytes had no obvious defect in their ability to engulf these bacteria . However, a role for crq in phagocytosis of S. aureus, but not that of E. coli, was subsequently proposed based on S2 cell phagocytosis assays following knock-down of crq by RNAi . Here, we show that crq is required in vivo for uptake and phagosome maturation of both S. aureus and E. coli. A simple explanation of this discrepancy with E. coli could be that knocking down crq by RNAi is not sufficient to affect its role in E. coli phagocytosis (but sufficient to affect its role in S. aureus phagocytosis), and that completely abrogating crq expression by in vivo knock-out leads to a stronger phenotype with both bacteria. Our in vivo data in crqko flies further demonstrate that crq is required to resist multiple microbial infections, such as Ecc15, E. faecalis, B. bassiana, and C. albicans. These data therefore argue that crq plays a more general role in microbial phagocytosis than was previously anticipated. Our previous experiments to test whether crq is required for bacterial phagocytosis in embryos were qualitative rather than quantitative, and did not allow us to identify a role for crq at that stage . In contrast, the experiments we now report in adult crqko flies are quantitative and allowed us to identify a delay in phagocytosis, followed by a defect in bacterial clearance in crqko hemocytes. A possible explanation for this discrepancy would be that hemocytes may differ in their expression profile, behavior, and phagocytic ability at various developmental stages due to differences in their microenvironment and/or sensitivity to stimuli. Accordingly, it has recently been shown that the phagocytic activity of embryonic hemocytes acts as a priming mechanism, increasing the ability of primed cells to phagocytose bacteria at later stages . It is therefore possible that embryonic, larval and adult hemocytes display very different levels of priming and bacterial phagocytic activity, and that crq is required mostly in larval/adult bacterial phagocytosis. Alternatively, a potential defect in phagocytosis of bacteria by embryonic hemocytes of the crq deficiencies may have been suppressed by the deletion of (an)other gene(s) in that genomic region.