Programmed cell death pathways coordinate neutrophil and macrophage clearance in zebrafish and are differentially exploited by Salmonella Typhimurium

Programmed cell death (PCD) is essential for immune cell homeostasis and host defense, yet its role in neutrophil and macrophage elimination during bacterial infections remains poorly understood. Using the zebrafish model, which offers unique in vivo imaging and genetic manipulation advantages, we dissected the contribution of pyroptosis, apoptosis, and necroptosis to the regulation of neutrophil and macrophage fate during homeostasis and infection with Salmonella enterica serovar Typhimurium (ST). Under basal conditions, all three PCD pathways cooperated to control immune cell turnover. Upon infection, zebrafish larvae mounted a type III secretion system (T3SS)-independent emergency myelopoietic response that increased myeloid cell numbers. However, the pathogen rapidly counteracted this response by promoting neutrophil death through Nlrp3-mediated pyroptosis and Caspase-3-dependent apoptosis, and macrophage killing via Ripk1-dependent necroptosis-both driven by its T3SS. While blocking pyroptosis prevented neutrophil loss, it also increased host susceptibility due to impaired bacterial clearance, whereas inhibition of apoptosis or necroptosis enhanced resistance, as these pathways are dispensable for controlling infection. These findings demonstrate how ST exploits distinct PDC mechanisms to evade innate immunity and underscore their differential potential as therapeutic targets in intracellular bacterial infections.