Emergency granulopoiesis is a hematopoietic program of stem cell-driven 
neutrophil production used to counteract immune cell exhaustion following 
infection. Shigella flexneri is a Gram-negative enteroinvasive pathogen 
controlled by neutrophils. In this study, we use a Shigella-zebrafish (Danio 
rerio) infection model to investigate emergency granulopoiesis in vivo We show 
that stem cell-driven neutrophil production occurs in response to Shigella 
infection and requires macrophage-independent signaling by granulocyte 
colony-stimulating factor (Gcsf). To test whether emergency granulopoiesis can 
function beyond homoeostasis to enhance innate immunity, we developed a 
reinfection assay using zebrafish larvae that have not yet developed an adaptive 
immune system. Strikingly, larvae primed with a sublethal dose of Shigella are 
protected against a secondary lethal dose of Shigella in a type III secretion 
system (T3SS)-dependent manner. Collectively, these results highlight a new role 
for emergency granulopoiesis in boosting host defense and demonstrate that 
zebrafish larvae can be a valuable in vivo model to investigate innate immune 
memory.IMPORTANCEShigella is an important human pathogen of the gut. Emergency 
granulopoiesis is the enhanced production of neutrophils by hematopoietic stem 
and progenitor cells (HSPCs) upon infection and is widely considered a 
homoeostatic mechanism for replacing exhausted leukocytes. In this study, we 
developed a Shigella-zebrafish infection model to investigate stem cell-driven 
emergency granulopoiesis. We discovered that zebrafish initiate granulopoiesis 
in response to Shigella infection, via macrophage-independent signaling of 
granulocyte colony-stimulating factor (Gcsf). Strikingly, larvae primed with a 
sublethal dose of Shigella are protected against a secondary lethal dose of 
Shigella in a type III secretion system (T3SS)-dependent manner. Taken together, 
we show that zebrafish infection can be used to capture Shigella-mediated stem 
cell-driven granulopoiesis and provide a new model system to study stem cell 
biology in vivo Our results also highlight the potential of manipulating stem 
cell-driven granulopoiesis to boost innate immunity and combat infectious 
disease.