Nutritional iron deficiency elicits profound rewiring of red pulp macrophage functions via high FPN and SYK-mediated signaling

Iron deficiency is a globally prevalent nutritional disorder with significant socio-medical impacts, yet the mechanisms by which different cell types adapt to iron restriction remain poorly understood. Splenic red pulp macrophages (RPMs) manage the highest iron flux in the body by recycling aged red blood cells (RBCs) through erythrophagocytosis. In this study, we investigated the adaptive mechanisms employed by RPMs under systemic iron deficiency. We found that RPMs from mildly anemic, dietary iron-deficient (ID) mice exhibit a surprising enhancement in erythrophagocytic capacity, accompanied by elevated cytoplasmic labile iron levels, reduced iron stores, and increased iron-export capacity via ferroportin (FPN). Proteomic profiling, validated by flow cytometry, revealed increased mass and activity of both lysosomes and mitochondria in RPMs from ID mice, along with upregulated expression of apoptotic cell receptors. Further assays identified a marked increase in mitochondrial respiration in ID RPMs, a unique response that contrasts with prior reports of impaired mitochondrial function in iron-deficient cells. These responses occurred independently of classical M2 polarization markers. Using both in vivo and cellular models, we identified the low hepcidin–high FPN axis as a key driver of the phagocytic and metabolic rewiring in RPMs. Additionally, calcium signaling emerged as a likely mediator of this axis. Pharmacological inhibition of calcium signaling effectors pinpointed SYK kinase as a molecular switch governing the high-FPN-induced functional reprogramming of macrophages. Short-term SYK inhibition reduced the enhanced erythrophagocytic function of RPMs during iron deficiency, impairing iron utilization for extramedullary splenic erythropoiesis. Together, our findings highlight metabolic reprogramming in RPMs as a strategic adaptation that optimizes their iron recycling and utilization capacity in response to systemic iron scarcity.