The use of tractable model organisms such as Drosophila melanogaster and the powerful genetic tools they offer has contributed significantly to recent advances in comprehension of innate immunity, notably phagocytosis. To date, several phagocytic transmembrane receptors, opsonins, and ligands have been discovered in Drosophila melanogaster. Many of them belong to the Nimrod family, which includes both secreted and transmembrane proteins containing several NIM repeats. Genetic and biochemical studies have revealed the roles of transmembrane Nimrod receptors such as Draper, Simu and Eater in phagocytosis. However, the function of the five secreted Nimrod members, the Nimrod B proteins, remains unstudied.
The goals of my thesis were to characterize the function of the secreted Nimrod B proteins using newly generated CRISPR-Cas9 null mutations, focusing on NimB5, NimB4, and NimB1 genes. In this work, we first found that the secreted protein, NimB5, is produced in the fat body upon nutrient scarcity downstream of metabolic sensors and ecdysone signaling. We provide evidence that NimB5 binds to hemocytes to down-regulate their proliferation and adhesion. Blocking this signaling loop results in conditional lethality when larvae are raised on a poor diet due to excessive hemocyte numbers and insufficient energy storage. This pointed to a role of NimB5 in the allocation of resources to blood cells, tailoring the investment in the immune system to metabolic resource availability.
In the second part of this thesis, we analyzed the function of NimB4 to reveal its crucial role in clearance of apoptotic cells. NimB4 is expressed by macrophages and glial cells, the two main types of phagocytes in Drosophila. Our study points to a role of NimB4 in phagosome maturation, more specifically in the fusion between the phagosome and lysosomes. We propose that similar to bridging molecules, NimB4 binds to apoptotic corpses to engage a phagosome maturation program dedicated to efferocytosis.
Finally, in the last part of the thesis, we present preliminary results on the role of NimB1 in the Drosophila immune system. NimB1 is expressed mainly in hemocytes and regulates hemocyte number. Additionally, NimB1 shares several characteristics with NimB4, notably the ability to bind to apoptotic cells. As such, we hypothesize that it could play a similar function in the phagocytosis of apoptotic cells. However, the exact role of NimB1 in phagocytosis is still unclear, and additional work is necessary to understand if NimB1 plays a role in the uptake of apoptotic cells or phagosome maturation.
Collectively, this thesis has revealed that the NimB proteins play specific roles in efferocytosis or in blood cell number regulation. Thus, this work extended our knowledge on the Drosophila cellular immune response by providing new insight on the conserved Nimrod family of proteins.