Hemosiderosis pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
It is found at first in the mononuclear phagocytes of the liver, bone marrow, spleen, and lymph nodes and in scattered macrophages throughout other organs. With progressive accumulation, parenchymal cells throughout the body (but principally the liver, pancreas, heart, and endocrine organs) become "bronzed" with accumulating pigment. Hemosiderosis is Iron deposition that occurs in the setting of: Genetic (ie hemochromatosis), Transfusional, Abnormal clearance/use, Increase absorption, Abnormal Hepcidin, Hemolytic anemia, Hemotropic parasites.
Pathophysiology
After the repeated episodes of a diffuse alveolar hemorrhage, the alveolar macrophages are responsible for the repeated clean up of excess blood. As the macrophages degrade the erythrocytes, the excess iron from heme degradation within the alveolar macrophages stimulates intracellular ferritin molecules. Further processing of the ferritin leads to hemosiderin complexes.9see below for more information). In the early stages of pulmonary hemosiderosis, interstitial and intra-alveolar hemorrhage predominate, with collections of both free hemosiderin and hemosiderin-filled macrophages found in the alveolar spaces and the interstitium. When the disease progresses, interstitial fibrosis ensues. Pulmonary hemosiderosis can occur either as a primary lung disorder or as the sequela to other pulmonary, cardiovascular, or immune system disorder. Based on disease characteristics, there are three types of pulmonary hemosiderosis:
Group 1 pulmonary hemosiderosis
Group 1 pulmonary hemosiderosis is defined by pulmonary hemorrhage associated with circulating anti-glomerular basement membrane (anti-GBM) antibodies. Anti-GBM diseases are small vessel vasculitis affecting the capillary system, where there are immunoglobulin and complement deposition along basement membranes of primarily the lungs and the kidneys such as in Goodpasture syndrome. Most of these patients will develop glomerular crescent formation with rapidly progressive glomerulonephritis. However, on average, 40-60% of patients with anti-GBM diseases will develop an alveolar hemorrhage. Unlike idiopathic pulmonary hemosiderosis, group 1 pulmonary hemosiderosis is stratified based on kidney biopsy, which shows linear deposits of IgG under direct immunofluorescence. Lung biopsy samples are not used in the diagnosis of anti-GBM disease because it would likely have no specific information.
Group 2 pulmonary hemosiderosis
Group 2 pulmonary hemosiderosis is defined by pulmonary hemorrhage associated with immune complex disease and immune complex formation along the alveolar basement membrane.Immune complexes are antigen-antibody complexes formed by joining IgG or IgM to a soluble antigen, which triggers complement activation. The result of immune complex formation is a break in the vascular-endothelial barrier and alveolar-epithelial barrier, leading to alveolar edema, hemorrhage, and massive infiltration of polymorphonuclear neutrophils (PMNs). This activation of PMNs and macrophages release large amounts of oxidants and proteases, leading to damage to the alveolar wall leading to potential acute lung injury and alveolar hemorrhage, which may present itself as an acute respiratory distress syndrome (ARDS). Recurrent episodes of these immune complex-mediated lung injuries inevitably lead to pulmonary scarring and fibrosis. Associated conditions, although rare, include systemic lupus erythematosus (SLE), Henoch-Schonlein purpura, Wegener’s granulomatosis, and mixed connective tissue disease.
Group 3 pulmonary hemosiderosis or Idiopathic pulmonary hemosiderosis
Group 3 pulmonary hemosiderosis is defined as pulmonary hemorrhage without known immunologic association, also known as idiopathic pulmonary hemosiderosis (IPH). As previously noted, repeated episodes of diffuse alveolar hemorrhage result in the accumulation of iron in the form of hemosiderin inside pulmonary macrophages. These recurrent episodes also lead to the thickening of alveolar basement membranes and interstitial fibrosis. It should be considered idiopathic pulmonary hemosiderosis is a diagnosis of exclusion after having ruled out primary and secondary causes of pulmonary hemosiderosis such as immune complex diseases or anti-GBM diseases.
Hemosiderin
70% of iron is found in the hemoglobin of RBCs.
30% of iron stored in the form of :
- Ferritin.
- Hemosiderin
Hemosiderin is aggregated, partially deproteinized ferritin, insoluble in the aqueous solution, and found in the liver cells, spleen, and bone marrow. On-demand, it is released slowly.
Hemosiderin formation
- The principle iron storage protein, ferritin, comprises heavy (H) and light (L) chain monomers, which co-assemble to form heteropolymers of 24 subunits. Ferritin can carry up to 4,500 iron atoms to attenuate cytosolic and nuclear-free labile iron pools (Harrison and Arosio, 1996). The H-chain subunit, owing to its ferroxidase activity, oxidizes Fe2+ to Fe3+ to enhance iron sequestration by ferritin (Muhoberac and Vidal, 2013). On the other hand, the L-subunit facilitates the iron-core formation and has a greater storage capacity than the H-subunit. All ferritins have 24 protein subunits arranged in 432 symmetry to give a hollow shell with an 80 Å diameter cavity capable of storing up to 4500 Fe(III) atoms as an inorganic complex. Autophagy is the dominant process degrading cytosolic ferritin and mitochondrial electron transport proteins in lysosomes, liberating iron, and increasing cytosolic iron levels. Protein aggregation is able to trigger autophagy, tempting the postulation that ferritin aggregates are a preliminary step to lysosomal uptake as ferritin in the cell sap finds its way into secondary lysosomes by becoming engulfed within autophagic vacuoles made by folding of large sections of endoplasmic reticulum around intracellular organelles and cell sap. These vacuoles then fuse with lysosomes to become autophagosomes, where the ingested organelles and the ferritin are subjected to digestion. Thus the fate of iron taken up from the blood and trapped by ferritin in vesicles, or cell sap ferritin engulfed by autophagic vacuoles, is digestion in secondary lysosomes. The ferritin molecules are digested with the loss of part of their protein shell to form hemosiderin.