Although the mechanisms underlying β2m amyloid fibril formation and deposition have not been fully elucidated, significant advances have been achieved. Native β2m is the major structural component of β2m amyloid fibrils. In addition, just as with other types of amyloidosis, there are several other associated molecules including glycosaminoglycans (GAGs), particularly heparan sulfate and chondroitin sulfate, proteoglycans (PGs) such as chondroitin sulfate proteoglycan, apolipoprotein E (ApoE), serum amyloid P component, alpha2-macroglobulin, and plasma proteinase inhibitors [12].

In the clinical setting, the earliest deposition of β2m amyloid is observed in the cartilage tissue that contains numerous PGs such as aggrecan, biglycan, decorin, and lumican [12]. Decorin is also the component of the tendinous tissue present in the carpal tunnel. Several in vitro and in vivo studies that have helped elucidate the potential roles of these molecules in the formation of β2m amyloid fibrils will be reviewed.

In vitro studies suggest that there are three phases of β2m amyloid fibril formation: nucleation, extension, and stabilization. β2m amyloid fibril formation occurs at a low pH in vitro. Partial unfolding of β2m is thought to be a prerequisite to its assembly into amyloid fibrils. A pH of 2.0–3.0 appears to be optimal for promoting the extension of β2m amyloid fibrils. Indeed, some PGs, especially biglycan, can induce the polymerization of acid-denatured β2m amyloid fibrils. Moreover, low concentration of trifluoroethanol and very small concentration of sodium dodecyl sulfate can induce partial unfolding of β2m amyloid fibrils, resulting in the extension of β2m amyloid fibrils at a neutral pH as well as fibril stabilization. Some GAGs, especially heparin, can also enhance fibril extension in the presence of trifluoroethanol at neutral pH [13]. Finally, ApoE, GAGs, and PGs can form stable complexes with fibrils playing a role in the stabilization of β2m amyloid fibrils.

A hypothesized model for the deposition of β2m amyloid fibrils in vivo has been proposed [12] (Fig. 6.2), whereby fibril formation takes place through a nucleation-dependent polymerization model, followed by several molecular interactions that lead to stabilization of the fibrils, rendering them resistant to proteolysis.

The earliest stage of β2m amyloid deposition occurs in the cartilage, followed by extension to the capsule and synovium. To explain this specific tissue involvement, it is important to gain understanding of the biochemical composition and physiology of these tissues. In the joint space, the synovial fluid is composed primarily of hyaluronan and sulfated GAGs and is produced by synovial fibroblasts, recirculating into the blood stream via the lymphatic system. The synovial membrane consists of an interstitium or subintima filled with GAGs, matrix proteins, and fibroblasts. Synoviocytes are classified as macrophage-like and fibroblast-like resident cells based on morphology and function. Macrophage-like synoviocytes tend to produce cytokines (e.g., interleukin-1β [IL-1β] and tumor necrosis factor-α [TNF-α]), whereas fibroblast-like synoviocytes produce matrix proteins (e.g., fibronectin, laminin, and collagen), GAGs (both nonsulfated [hyaluronan] and sulfated [e.g., chondroitin-6 sulfate, dermatan sulfate, and heparin sulfate]), and matrix metalloproteinases (MMPs) (e.g., collagenase and stromelysin). In the normal joint, the majority of the cells residing in the synovial intimal lining are fibroblast-like, but this ratio may vary in inflammatory joint disorders. Macrophage-like synoviocytes in the intimal lining appear to originate from the bone marrow, whereas intimal fibroblasts-like cells originate from the subintima [14]. The cartilage is composed of chondrocytes, type-2 collagen, and GAGs, the most abundant of which is aggregan.

β2m has specific affinity for collagen, which is dose-dependent. This may explain its predilection to deposit in articular structures where collagen is abundant and also explain why in situ, heparan sulfate is the most common GAG component of β2m amyloid. Interestingly, native β2m in solution is a highly soluble monomeric protein, which is incapable of amyloid generation under physiological conditions at neutral pH. However, β2m amyloid proteins obtained from patients undergoing treatment with chronic dialysis are not uniformly water soluble suggesting heterogeneity in terms of superstructure [15]. Other factors may be involved after β2m deposition, including modification by advanced glycation endproducts (AGEs) [16] such as imidazolone, carboxymethyl lysine, and pentosidine, molecules known to accumulate in uremia.

Advanced glycation endproducts have pro-inflammatory properties and induce macrophage chemotaxis with the release of pro-inflammatory cytokines, thereby contributing to joint inflammation. Furthermore, AGE-modified β2m appears to interact with the AGE receptor present on monocytes and macrophages, thus stimulating the release of platelet-derived growth factors, IL-6, TNF-α, and IL-1β. AGE-modified β2m can also induce bone resorption by stimulating collagenase synthesis, leading to collagen degradation and connective tissue breakdown, resulting in the creation of a potential nidus for β2m deposition. Moreover, AGE-modified β2m can decrease the synthesis of type-1 collagen by fibroblasts. After deposition, β2m activates synovial fibroblasts to produce MMPs such as stromelysin, which cause articular destruction [17]. Serum stromelysin is elevated in patients with dialysis-associated amyloidosis, thereby linking β2m-deposition with destructive arthropathy [18]. Moreover, β2m stimulates mRNA and protein synthesis of IL-6 by osteoblasts, a potent bone-resorbing cytokine. Therefore, theoretically both hormonal and local regulatory factors can aggravate the deleterious effects of β2m on bone resorption [17]. β2m also appears to have osteoclastogenic effects that may be relevant for the bone cystic formations observed in β2m amyloidosis [19]. Interestingly, one study detected non-fibrillar β2m aggregates in the spleen and heart, which suggests that accumulation of the protein may precede amyloidogenesis, at least in the nonskeletal tissues [20].

In summary, the clinical manifestations of dialysis-associated amyloidosis are likely the result of complex in situ inflammatory reactions, induced by monocytes and macrophages, synovial cells, osteoblasts, and osteoclasts in response to β2m, AGE, and AGE-modified β2m. Inflammatory local bone destruction can also precipitate the migration of inflammatory cells, resulting in the release of more cytokines and further tissue destruction [16]. The pathogenesis of β2m amyloidosis is summarized in Fig. 6.3.

Although the retention of β2m is a prerequisite for the development of β2m amyloidosis [21], there are other patient- and dialysis-related risk factors. Advanced age and duration of dialysis are important predictors of this complication. Indeed, β2m amyloidosis is seen earlier in older patients despite shorter duration of dialysis. In one study from Japan, among patients receiving dialysis for 20–24, 25–29, and ≥ 30 years, the incidence of orthopedic surgical interventions related to dialysis-associated amyloidosis was 25 %, 66 %, and 78 %, respectively [22]. Carboxymethyl lysine, which is an AGE found on proteins and lipids as a result of oxidative stress and chemical glycation, accumulates with aging and in uremia, and serum levels have been shown to predict development of the carpal tunnel syndrome, a clinical manifestation of dialysis-associated amyloidosis [23]. Measures of oxidative stress, including levels of superoxide dismutase and malonyldialdehyde, have also been associated with dialysis-associated amyloidosis [24].

In terms of dialysis-related factors, cellulose-based dialyzer membranes, previously used in conventional hemodialysis, have small pores that are impermeable to β2m and are considered less compatible with blood components resulting in activation of several inflammatory pathways including the complement system [25]. These dialyzers were believed to increase the intradialytic generation of β2m, thereby contributing to further accumulation of this molecule. Synthetic polymers, which were subsequently developed, are more permeable to β2m and are known as high-flux dialyzers. These filters also possess more biocompatible properties and have a lower complement-activating potential. Although the removal of circulating β2m can be enhanced by the use of high-flux dialyzers, and other novel therapies such as hemodiafiltration, frequent hemodialysis, and hemoperfusion, β2m continues to accumulate over time. The predicted annual net retention of β2m according to varying duration and frequency of high-flux hemodialysis is shown in Fig. 6.4. The use of high-flux dialyzer would be expected to delay but not prevent the development of β2m amyloidosis. An additional potential dialysis-related contributor to β2m accumulation is the contamination of dialysate water with bacterial products. This can induce host inflammatory responses during dialysis including cytokine production, which may accelerate the course of dialysis-associated amyloidosis. Several studies have demonstrated that the use of ultrapure dialysate, which has undergone a terminal filtration process aimed at removal of bacterial products, results in a reduction of cytokine production and circulating levels of IL-6 and C-reactive protein [26]. However, the potential long-term salutary effect of this strategy on the development of β2m amyloidosis is at present unknown, although in a recent cross-sectional study of 147 patients, the prevalence of suspected or confirmed dialysis-associated amyloidosis was 68 % among patients dialyzed with low-flux dialyzers compared to 28 % among those dialyzed with high-flux dialyzers and against ultrapure dialysate [23].

In summary, advanced age, the uremic milieu, and dialysis-related variables including dialyzer characteristics and the dialysate water purity may impact the molecular composition of the cartilage and other connective tissues, providing optimal conditions for β2m amyloid fibril formation and the resulting associated morbidity [27].

The clinical diagnosis of β2m amyloidosis is primarily suspected on the basis of the history. Patients rarely display symptoms until they have received dialysis for at least 5 years [34], presenting with symptoms of the carpal tunnel syndrome, shoulder pain, and typically, cervical radioculopathy or myelopathy.

On physical examination, findings of the carpal tunnel syndrome include diminished pin-prick sensation in the median nerve distribution or thenar muscle atrophy. The Hoffman-Tinel test and Phalen test may increase the sensitivity and specificity of early detection of the carpal tunnel syndrome. Joint swelling can be found in chronic arthropathy, and limited shoulder range of motion may reflect amyloid-related rotator cuff tears. Cervical tenderness and radioculopathy usually reflect more destructive spondyloarthropathy, and some patients may develop pathological fracture of long bones [29]. β2m amyloid deposition in the myocardium may result in congestive heart failure, and its accumulation in bowel tissue has been associated with reports of intestinal obstruction and bleeding.

If β2m amyloidosis is suspected based on Congo red positivity of tissue; other types of amyloidoses such as AA, AL or ATTR cannot be excluded until the presence of β2m in the deposits is confirmed by immunostaining using anti-β2m antibodies or by proteomic methods. Furthermore, other causes of bone disease in dialysis patients need to be ruled out including that associated with secondary hyperparathyroidism.