Heme and hemolysis in innate immunity: adding insult to injury

Heme is a vital, iron-containing prosthetic molecule present in a variety of proteins, of which hemoglobin is the most abundant. While the reactivity afforded by its central iron ion is essential for many cellular processes, it renders heme a potentially damaging molecule upon its release from hemeproteins, as it can catalyze the generation of reactive oxygen species. Severe intravascular hemolysis results in the leakage of vast amounts of hemoglobin, and subsequently, heme into the plasma. As such, heme is increasingly recognized as a major driving force for hemolysis-associated pathology including an increased risk for bacterial infections, due to its pro-oxidant, cytotoxic and immunomodulatory effects. Here, we provide a succinct review of recent, significant developments on how heme can influence innate immune functions, ranging from the maintenance of iron homeostasis by macrophages, the modulation of inflammatory responses, to its role in altering resistance mechanisms against bacterial infections.