Malnutrition drives infection susceptibility and dysregulated myelopoiesis that persists after refeeding intervention.

Undernutrition is one of the largest persistent global health crises, with nearly 1 billion people facing severe food insecurity. Infectious disease represents the main underlying cause of morbidity and mortality for malnourished individuals, with infection during malnutrition representing the leading cause of childhood mortality worldwide. In the face of this complex challenge, simple refeeding protocols have remained the primary treatment strategy. Although an association between undernutrition and infection susceptibility has been appreciated for over a century, the underlying mechanisms remain poorly understood and the extent to which refeeding intervention is sufficient to reverse nutritionally acquired immunodeficiency is unclear. Here we investigate how malnutrition leads to immune dysfunction and the ability of refeeding to repair it. We find that chronic malnutrition severely impairs the ability of animals to control a sub-lethal bacterial infection. Malnourished animals exhibit blunted immune cell expansion, impaired immune function, and accelerated contraction prior to pathogen clearance. While this defect is global, we find that myelopoiesis is uniquely impacted, resulting in in reduced neutrophil and monocyte numbers prior to and post-infection. Upon refeeding, we observe that animals recover body mass, size, cellularity across all major immune organs, the capacity to undergo normal immune cell expansion in response to infection, and a restoration in T cell responses. Despite this broad improvement, refed animals remain susceptible to bacterial infection, uncoupling global lymphoid atrophy from immunodeficiency. Mechanistically, we find peripheral neutrophil and monocyte numbers fail to fully recover and refed animals are unable to undergo normal emergency myelopoiesis. Altogether, this work identifies a novel cellular link between prior nutritional state and immunocompetency, highlighting dysregulated myelopoiesis as a major driver. We believe these findings illustrate how exposure to food scarcity is an immunologic variable, even post-recovery, which should be accounted for in patient medical history and current global public health policy.