The Big Idea: Iron-dependent inflammation in venous disease and proposed parallels in multiple sclerosis

Impaired venous drainage of the lower extremities, mainly due to venous reflux or to venous outflow obstruction, leads to a cascade of pathological events clinically graded by the clinical class (C) of the CEAP classification (Clinical, aEtiological, Anatomical, Pathophysiological) of chronic venous disease (CVD). Varicose veins are the most frequent clinical sign in class C2. When oedema complicates varicose veins, the clinical picture is graded as C3. Pigmentation, lipodermatosclerosis and other skin changes are classified as C4. A small but significant number of the affected patients develop venous ulcers. Healed ulcers are classified as C5, whereas active ulcers are C6. Altered venous haemodynamics are a necessary but not exclusive element for explaining progression along the clinical classes to the point of skin lesion. In 1982, Browse and Burnand observed a peri-capillary fibrin deposition and speculated that cuffs act as a barrier to oxygen diffusion and nutrients, resulting in epidermal cell death. This mechanism of tissue injury has not yet been demonstrated. The fibrin cuff may be more properly considered a scaffold for tissue reparative processes. The cuff contains fibrin, but also laminin, fibronectin, tenascin, and types I and III collagen, encircling the dilated capillary vein The decline of the fibrin cuff theory over the last twenty years has led to investigation of other factors emphasizing inflammatory mechanisms as amplifiers of the insufficient venous drainage. Recent studies demonstrate a pivotal role for tissue iron accumulation in inducing and maintaining inflammation in CVD. Iron deposits in CVD cause readily visible brownish dermal areas which sometimes precede, but always surround, ulcers. The origin of increased leg iron stores is extravasation of red blood cells (erythrocytes) in conditions of significant venous stasis. Erythrocytes are degraded by the interstitial macrophages, with the released iron incorporated into ferritin. Over time, with increasing overload of iron, the structure of ferritin changes to haemosiderin. In 1988, Ackermann found a twenty-fold higher average concentration of iron in lower limbs affected by venous ulcers as compared to the upper arm of the same subjects. The phenomenon of leg haemosiderin deposits seems to be significant for the entire body, since this protein has been demonstrated in the urine of patients affected by CVD. Increased iron stores and interstitial protein extravasa- tion are potent chemo-attractants and presumably represent the initial underlying chronic inflammatory signal respon- sible for white blood-cells recruitment and migration in the matrix (Figure 2B). In 1988, Coleridge-Smith observed leukocytes trapped in the venous microcirculation second- ary to venous hypertension. This work paved the way to the investigation of the relationship between CVD and inflammation. The mechanism of white cell migration in the subcutaneous matrix was further elucidated by studies of the expression of adhesion molecules in a model of venous hypertension. Several studies confirmed the expression of these molecules, including ICAM, VCAM and selectins. Such adhesion molecules block circulating white cells on the vein wall and facilitate transmigration into the tissue. The predominant cells migrating into the extra-cellular matrix are macrophages and T-lymphocytes. Macrophages take up iron accumulated in the tissue and store it in intracellular ferritin-like structures (Figure 3B). Intra- and extra-cellular overload of iron in the tissue could potentially be dangerous for generation of free radicals due to possible release of free iron from deposits. Wenk et al.and Yeoh-Ellerton found increased iron levels in Panel A, classic fibrin cuffs (arrow) thicken veins (v) in a venous ulcer bed, 40X. Panel B, fibrin cuffs (arrow) encircles proliferated thick walled veins (v) in a peri-ventricular MS plaque, exudates from chronic leg ulcers as compared to acute wounds. They also observed significant concentrations of metabolites from oxidative stress. The final step of the pathogenetic chain leading to matrix disruption and ulcer development involves over- expression of matrix metallo-proteases (MMPs) that are not substantially balanced by their physiological tissue inhibitors (TIMPs). MMPs cause a substrate-specific degradation of matrix components, including collagen, elastin and laminin. Unrestricted MMP activity can lead to matrix break down and ulcer onset. Some experiments demonstrate that local iron overload may induce MMP hyper-activation through the so-called MMP iron-driven pathway. However, the iron hypothesis does not readily explain why leg iron deposits in CVD produce lesions only in some individuals. We hypothesized that such individual differences could be genetically determined, and investigated the role of the C282Y and H63D mutations of the HFE gene, associated with hemochromatosis in Northern European populations. C282Y mutation significantly increases the risk of ulcer in primary CVD by more than six-fold, while patients carrying the H63D variant have an earlier age of ulcer onset by almost 10 years. HFE mutations are associated with increased iron efflux from the macrophage. Our findings support the hypothesis that lesions are promoted by enhanced iron release and ROS generation.