Adult Drosophila Lack Hematopoiesis but Rely on a Blood Cell Reservoir at the Respiratory Epithelia to Relay Infection Signals to Surrounding Tissues.

The use of adult Drosophila melanogaster as a model for hematopoiesis or organismal immunity has been debated. Addressing this question, we identify an extensive reservoir of blood cells (hemocytes) at the respiratory epithelia (tracheal air sacs) of the thorax and head. Lineage tracing and functional analyses demonstrate that the majority of adult hemocytes are phagocytic macrophages (plasmatocytes) from the embryonic lineage that parallels vertebrate tissue macrophages. Surprisingly, we find no sign of adult hemocyte expansion. Instead, hemocytes play a role in relaying an innate immune response to the blood cell reservoir: through Imd signaling and the Jak/Stat pathway ligand Upd3, hemocytes act as sentinels of bacterial infection, inducing expression of the antimicrobial peptide Drosocin in respiratory epithelia and colocalizing fat body domains. Drosocin expression in turn promotes animal survival after infection. Our work identifies a multi-signal relay of organismal humoral immunity, establishing adult Drosophila as model for inter-organ immunity. • Adult Drosophila have an extensive blood cell reservoir at the respiratory epithelia • No sign of hematopoiesis in adult Drosophila , even after bacterial infection • Blood cells act as sentinels of infection via PGRP-LC/Imd/Upd3 signaling • In tissues of the reservoir, this induces Drosocin expression to promote survival Sanchez Bosch et al. study hemocytes (blood cells) in adult Drosophila. They do not find hematopoiesis in the adult system, but they do identify a major hemocyte reservoir at the respiratory epithelia and colocalizing fat body. Following bacterial infection, hemocytes relay signals to the tissues of the reservoir to mediate a protective humoral immune response. [ABSTRACT FROM AUTHOR]