Background
ISCLS, first reported by Clarkson et al. in 1960 [
1], is rare disease that leads to intermittent, but ultimately self-reversing episodes of severe plasma extravasation. [
2,
3]. Although fewer than 500 cases have reported in the literature, ISCLS may be under-reported due to misdiagnosis [
4]. In addition to hypotensive shock, complications of acute ISCLS include renal failure, venous and arterial thromboses, rhabdomyolysis, and compartment syndromes resulting from extensive edema of the extremities. The 5-year survival rate has been reported to be between 73–78%, although these estimates generally antedate the widespread use of disease-sparing prophylaxis with intravenous immunoglobulins (IVIG) [
5].
ISCLS flares are frequently preceded by upper respiratory infections (URIs) or flu-like symptoms, suggesting a role for inflammation in disease pathogenesis [
4]. Increased levels of inflammatory cytokines such as C-X-C chemokine motif 10 (CXCL10), TNFα, and IL-6, and mediators of endothelial permeability (vascular endothelial growth factor [VEGFA] and angiopoietin-2 [Angpt2]) have been detected in acute ISCLS plasma [
6‐
8]. While > 85% of patients have a monoclonal gammopathy of undetermined significance (MGUS), typically of the IgG kappa isotype [
2,
5], the function(s) of the ISCLS “paraprotein” in disease pathogenesis has not been established. Monthly prophylaxis with IVIG induces disease remission in most patients and increases survival [
5,
9].
The eGCX, which is comprised of proteoglycans, glycosaminoglycan (GAG) chains, and glycoproteins, coats the surface of the vascular endothelium and serves essential functions in vascular barrier function [
10]. Microvascular endothelial cells continually secrete eCGX components, which serve as a molecular filter by constraining firm attachment of cytokines [
11]. eGCX also prevents microvascular thrombosis and limits leukocyte adhesion through interactions with plasma albumin, antithrombin-III (AT-III), and extracellular superoxide dismutase-3 (SOD3), among other proteins [
12]. Inflammation disrupts the eGCX and promotes vascular permeability, leading to shedding of soluble eGCX components such as HA and CD138 into plasma [
11,
12]. Analysis of eGCX components has been used to diagnose and monitor disease states; elevated levels of plasma HA may predict progression and severity of heart disease, diabetes, sepsis, trauma, and ischemia-reperfusion injury, among others [
13].
Previously we detected elevated serum levels of sCD138 in a single patient with active ISCLS, which normalized during disease remission [
14]. Here we present the results of long-term follow-up of this patient and analysis of eGCX components in a larger cohort of individuals with ISCLS (n = 25). Our results suggest that monitoring these elements may be useful to gauge disease activity and that restoration of eCGX function could be explored to prevent or ameliorate acute flares of ISCLS.
Discussion
Here we present evidence for eGCX remodeling in ISCLS. Although ISCLS flares release macromolecules, including proteins up to 900 kDa, into the extravascular space, we detected increased abundance of eGCX components and eGCX-degrading enzymes in circulation in acute ISCLS relative to convalescent periods, consistent with glycocalyx shedding. eGCX dysfunction could have an impact on the severity of acute vascular leakage through several mechanisms including increasing local concentrations of proinflammatory cytokines on the endothelial surface. Microvascular hyper-permeability can be a component of critical illness due to sepsis, burns, and trauma, among others. This is reflected by increased serum levels of markers of endotheliopathy [
29]. Elevated sCD138 and sTM levels predict morbidity and mortality in mechanically ventilated patients [
30]. However, severe and protracted hemoconcentration and hypoalbuminemia are uncommon in these conditions [
31]. Whether inflammation induces vascular hyper-permeability in critical illness and ISCLS through similar mechanisms is unknown. Indeed, ISCLS flares can even occur in the absence of overt inflammatory triggers.
Baseline sCD138 levels in ISCLS were above those typically found in healthy subjects, suggesting ongoing eGCX dysfunction in the absence of overt clinical symptoms. Thus, rather than using a single, uniform cutoff value for sCD138 in individual patients, clinicians may need to evaluate a constellation of biomarkers such as sCD138, HA, and Angpt2 in individual patients to assess episode severity and/or progression. The lack of further increase in sCD138 in acute sera in our relatively small cohort may relate specifically to the variable timing of sample collection. A subset of samples was obtained at or near the onset of acute clinical symptoms while serial measurements in several patients revealed that sCD138 levels were highest during the recovery (post-leak) phase, during which hypervolemia can occur due to mobilization of administered IV fluids from peripheral tissues. Expanded plasma volume has been previously identified as a risk factor for CD138 shedding [
32]. Variability in acute sCD138 levels could also be due to the wide range of episode severity. Several samples were collected from patients whose clinical symptoms were mild and resolved quickly without hospitalization. Increased sCD138 levels may also relate specifically to the presence of MGUS. Monoclonal plasma cells produce and/or shed abnormally high amounts of sCD138, and changes in serum sCD138 levels not only correlate with progression to myeloma but also reflect therapeutic efficacy [
24]. Accordingly, we observed a strong correlation between sCD138 and paraprotein levels in ISCLS.
Although the abundance of several eGCX elements (e.g. HA, sCD138) were increased in the acute phase, relative quantities of endothelial cell-derived proteins that might suggest endothelial cell injury (such as TM) were not significantly increased in active disease. Cleavage of TM by neutrophil elastase and other inflammation-related proteases leads to shedding into plasma [
16]; increased sTM levels are found in critically ill patients with sepsis or trauma, and levels correlate with the extent of organ dysfunction and mortality [
33]. These findings are also consistent with our previous observations that acute ISCLS sera did not induce cytotoxicity when applied to normal endothelial cells [
6].
Other eGCX-related proteins may be shed into circulation during flares. GPC1 is the most well-studied glypican component of the eGCX and has been shown to protect against endothelial dysfunction related to blood vessel stiffness [
34]. GPC 1, 3, and 4 are elevated in plasma of patients with sepsis and correlate with markers of disease severity, organ failure, and eGCX damage [
35]. GPC1 was not measured by our array, but both GPC2 and GPC6 were elevated in acute ISCLS plasma. Although the function of GPC2 and 6 in endothelial cells has not been well studied, survey data indicate endothelial cell expression (
https://www.proteinatlas.org/ENSG00000213420-GPC2/celltype and
https://www.proteinatlas.org/ENSG00000183098-GPC6/celltype). ISCLS endothelial cells may exhibit aberrant patterns of GPC expression.
Increased abundance of eGCX-degrading proteases, including MMPs, ADAMs, and ADAMTS family members were detected in ISCLS plasma. These factors regulate eGCX composition during inflammation; MMP9 and 13 induce CD138 shedding by cleaving its ectodomain [
36]. ISCLS-associated proinflammatory cytokines including TNFα and IL-1β, among others, increase expression of eGCX degrading proteases in endothelial cells [
10]. Previously we documented widespread neutrophil degranulation in acute ISCLS [
15], suggesting another cellular source of proteases. Notably, we did not visualize grossly abnormal carbohydrate content or expression of genes encoding eGCX degrading enzymes (
Hyal1, Hpse) in cultured ISCLS endothelial cells at baseline. Thus, features of ISCLS flares not recapitulated in vitro, including increased shear stress due to hemoconcentration and increased production of reactive oxygen species (ROS) due to ischemia could adversely affect endothelial synthesis and/or degradation of eGCX elements.
Fortification of the eGCX may be a potential therapeutic approach for acute ISCLS. Low molecular weight heparin (Lovenox) is an FDA-approved anticoagulant that also reduces eGCX shedding of glycans in inflammatory states by competitively inhibiting heparanase [
37]. Sulodexide, a natural mixture of GAGs including heparan sulfate and dermatan sulfate extracted from human GI tract, has shown promise in restoring eGCX components and decreasing permeability of retinal microvasculature in patients with type 2 diabetes [
38].
Finally, although disease flares in our patients were prototypical, an unusual clinical aspect of one case was the finding of retinal holes or rifts. Although it is unclear whether the visual disturbance was a manifestation of active ISCLS, symptoms coincided with a period in which the patient was non-compliant with treatment and improved once terbutaline therapy was reinstated and IVIG prophylaxis was started. Visual disturbances in association with acute ISCLS flares are uncommon although isolated cases of macular edema [
39] and ischemic optic neuropathy [
40] have been described in case reports. Patients with ISCLS may be at increased risk for ophthalmologic complications due to hyper-coagulability associated with hemoconentration and decreased perfusion resulting from hypotensive shock [
2].
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.