Updated project metadata. Both iron homeostasis and erythropoiesis are known to be affected by aging. Iron needs in mammals are met primarily by iron recycling from senescent red blood cells (RBCs), a task chiefly accomplished by red pulp macrophages (RPMs) in the spleen. Given that RPMs continuously process iron, their cellular functions might be susceptible to age-dependent decline, a possibility that has been unexplored to date. In our project, we identified a formation of undegradable iron- and heme-rich extracellular aggregates in the spleens of 10-11-month-old female mice. We further found that feeding mice an iron-reduced diet alleviates the deposition of iron in the spleen in the form of these insoluble aggregates. Here, we performed: i) label-free proteomic analysis of the aggregates that were magnetically isolated from the aged mice (maintained on a standard diet), using samples from young mice as a background control; and ii) TMT-based quantification of the differences in the aggregate protein composition between mice that aged on a standard versus iron-reduced diet. In each experiment, three biological replicates per group were analyzed.