Studies in the Pathogenesis of Systemic Capillary Leak Syndrome

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Immune dysregulation may contribute to the pathophysiologic findings seen in SCLS. A MGUS, a premalignant precursor to multiple myeloma (MM), is a condition characterized by a clonal plasma cell population that secretes monoclonal immunoglobulins (Ig, also referred to as a paraprotein). MGUS is detectable in a majority of SCLS cases. Several patients with SCLS in whom MGUS evolved into myeloma or plasma cell leukemia experienced fewer capillary leak episodes after chemotherapy for their hematopoietic disorder. These findings suggest that the monoclonal paraprotein from the dysregulated plasma cell population may be the direct or indirect source of the pathophysiologic findings observed. Current studies are focused on identifying the target antigen of SCLS monoclonal Ig, if any, using high-throughput protein arrays. We are also characterizing the transcriptome of blood cell RNA and the proteome of SCLS serum/plasma, both pre- and post-attack, to determine whether specific biomarkers of acute SCLS symptoms and/or etiological factors can be identified. We have now evaluated more than 75 patients with a confirmed diagnosis of SCLS under this protocol in the last 12 years. We are the primary referral center in the U.S. for SCLS. Circulating permeability factors, vascular endothelial growth factor (VEGF), angiopoietin 2 (Angpt-2), CXCL10, CCL2, and IL-6, were elevated in episodic SCLS sera compared to remission sera. These findings suggest that angiogenic proteins and proinflammatory cytokines that induce endothelial cell (EC) hyper-permeability may contribute to transient EC barrier dysfunction around SCLS flares. The transient episodes of hypotensive shock and anasarca in SCLS are thought to arise from reversible microvascular barrier dysfunction. Application of episodic but not convalescent SCLS sera to human microvascular ECs caused vascular endothelial cadherin internalization, disruption of interendothelial junctions, actin stress fiber formation, and increased permeability in complementary functional assays. EC contraction and temporary attenuation of adherens junctions may thus permit leakage of solutes and proteins into the extravascular space during acute episodes. In fiscal year 21, we continued to study mechanisms underlying the discovery that the skin microvasculature and endothelial cell lines from SCLS patients are hyper-responsive to routine inflammatory mediators such as VEGF and histamine. Preliminary data suggest that SCLS cells have a primary metabolic abnormality that renders them more susceptible to these mediators. The role of specific gene defects in SCLS, if any, is unknown; e.g. whether the endothelium is genetically programmed for hyper-responsiveness to routine stimuli. There are no consistent familial aggregations in SCLS, and whole exome sequencing of unrelated adults with SCLS and several parent-child trios failed to reveal a uniform exotic etiology. Using Affymetrix Single Nucleotide Polymorphism (SNP) microarrays, we performed the first genome-wide SNP analysis of SCLS in a cohort of 12 disease subjects and 18 controls. From unbiased high-density mapping of single-nucleotide polymorphisms (SNPs), a small genetic interval, 3p25.3, was identified as the highest-ranking candidate susceptibility locus (p 10-6) with an odds ratio of 41. Odds ratios (7-41) and p values (10-4 and 10-6) for the top SCLS-associated variants were outsized for such a small sample size. These results imply high penetrance for a rare disease allele that remains to be identified. Currently, we are performing whole genome sequencing of DNA from patients to test the hypothesis that they are prone to exaggerated responses to otherwise mundane inflammatory stressors due to underlying genetic defect(s). In FY23, we reported that two patients with SCLS had either remission of symptoms or reduced treatment requirements following autologous stem cell transplantation for myeloma, suggesting a role for the monoclonal gammopathy in idiopathic systemic capillary leak syndrome pathogenesis. In FY23, we identified a patient with a history of pneumococcal bacteremia and SCLS, who carried a heterozygous loss of function variant in IRAK4 with associated defects in TLR signaling. Finally, in FY23 we continued a collaboration with PharmAbcine, Inc. to study a monoclonal antibody, 4E2, which targets the Angpt1/2 receptor, Tie2. 4E2 is a novel agonistic antibody that binds to Tie2 receptor and promotes stabilization of leaky blood vessels in tumor models in mice. 4E2 activated Tie2 in SCLS patient-derived endothelial cells independently of Angpts and fortified barrier function at homeostasis, and dampened permeability induced by inflammatory stressors. 4E2 reduced mortality in the SJL/J mouse model of SCLS induced by systemic histamine challenge and diminished vascular leakage associated with respiratory viral infection. These findings support a critical role for Tie2 dysregulation in ISCLS and highlight a viable therapeutic approach to this catastrophic disorder.