Elevated Systemic Pentraxin-3 Is Associated With Complement Consumption in the Acute Phase of Thrombotic Microangiopathies

Serum from half of patients with immune thrombocytopenia trigger macrophage phagocytosis of platelets

Humoral antiplatelet factors, such as autoantibodies, are thought to primarily clear platelets by triggering macrophage phagocytosis in immune thrombocytopenia (ITP). However, there are few studies characterizing the capacity and mechanisms of humoral factor-triggered macrophage phagocytosis of platelets using specimens from patients with ITP. Here, we assessed sera from a cohort of 24 patients with ITP for the capacity to trigger macrophage phagocytosis of normal donor platelets and characterized the contribution of humoral factors to phagocytosis. Sera that produced a phagocytosis magnitude greater than a normal human serum mean + 2 standard deviations were considered phagocytosis-positive. Overall, 42% (8/19) of MHC I alloantibody-negative ITP sera were phagocytosis-positive. The indirect monoclonal antibody immobilization of platelet antigens assay was used to detect immunoglobulin G (IgG) autoantibodies to glycoproteins (GP)IIb/IIIa, GPIb/IX, and GPIa/IIa. Autoantibody-positive sera triggered a higher mean magnitude of phagocytosis than autoantibody-negative sera. Phagocytosis correlated inversely with platelet counts among autoantibody-positive patients but not among autoantibody-negative patients. Select phagocytosis-positive sera were separated into IgG-purified and -depleted fractions via protein G and reassessed for phagocytosis. Phagocytosis was largely retained in the purified IgG fractions. In addition, we assessed serum concentrations of C-reactive protein, serum amyloid P, and pentraxin 3 as potential phagocytosis modulators. Pentraxin 3 concentrations correlated inversely with platelet counts among patients positive for autoantibodies. Taken together, sera from approximately half of the patients with ITP studied triggered macrophage phagocytosis of platelets beyond a normal level. An important role for antiplatelet autoantibodies in phagocytosis is supported; a role for pentraxins such as pentraxin 3 may be suggested.

Distinction of early complement classical and lectin pathway activation via quantification of C1s/C1-INH and MASP-1/C1-INH complexes using novel ELISAs

The most commonly used markers to assess complement activation are split products that are produced through activation of all three pathways and are located downstream of C3. In contrast, C4d derives from the cleavage of C4 and indicates either classical (CP) or lectin pathway (LP) activation. Although C4d is perfectly able to distinguish between CP/LP and alternative pathway (AP) activation, no well-established markers are available to differentiate between early CP and LP activation. Active enzymes of both pathways (C1s/C1r for the CP, MASP-1/MASP-2 for the LP) are regulated by C1 esterase inhibitor (C1-INH) through the formation of covalent complexes. Aim of this study was to develop validated immunoassays detecting C1s/C1-INH and MASP-1/C1-INH complex levels. Measurement of the complexes reveals information about the involvement of the respective pathways in complement-mediated diseases. Two sandwich ELISAs detecting C1s/C1-INH and MASP-1/C1-INH complex were developed and tested thoroughly, and it was investigated whether C1s/C1-INH and MASP-1/C1-INH complexes could serve as markers for either early CP or LP activation. In addition, a reference range for these complexes in healthy adults was defined, and the assays were clinically validated utilizing samples of 414 COVID-19 patients and 96 healthy controls. The immunoassays can reliably measure C1s/C1-INH and MASP-1/C1-INH complex concentrations in EDTA plasma from healthy and diseased individuals. Both complex levels are increased in serum when activated with zymosan, making them suitable markers for early classical and early lectin pathway activation. Furthermore, measurements of C1-INH complexes in 96 healthy adults showed normally distributed C1s/C1-INH complex levels with a physiological concentration of 1846 ± 1060 ng/mL (mean ± 2SD) and right-skewed distribution of MASP-1/C1-INH complex levels with a median concentration of 36.9 (13.18 - 87.89) ng/mL (2.5-97.5 percentile range), while levels of both complexes were increased in COVID-19 patients (p<0.0001). The newly developed assays measure C1-INH complex levels in an accurate way. C1s/C1-INH and MASP-1/C1-INH complexes are suitable markers to assess early classical and lectin pathway activation. An initial reference range was set and first studies showed that these markers have added value for investigating and unraveling complement activation in human disease.

Interaction of the Factor H Family Proteins FHR-1 and FHR-5 With DNA and Dead Cells: Implications for the Regulation of Complement Activation and Opsonization

Complement plays an essential role in the opsonophagocytic clearance of apoptotic/necrotic cells. Dysregulation of this process may lead to inflammatory and autoimmune diseases. Factor H (FH), a major soluble complement inhibitor, binds to dead cells and inhibits excessive complement activation on their surface, preventing lysis, and the release of intracellular material, including DNA. The FH-related (FHR) proteins share common ligands with FH, due to their homology with this complement regulator, but they lack the domains that mediate the complement inhibitory activity of FH. Because their roles in complement regulation is controversial and incompletely understood, we studied the interaction of FHR-1 and FHR-5 with DNA and dead cells and investigated whether they influence the regulatory role of FH and the complement activation on DNA and dead cells. FH, FHR-1, and FHR-5 bound to both plasmid DNA and human genomic DNA, where both FHR proteins inhibited FH-DNA interaction. The FH cofactor activity was inhibited by FHR-1 and FHR-5 due to the reduced binding of FH to DNA in the presence of the FHRs. Both FHRs caused increased complement activation on DNA. FHR-1 and FHR-5 bound to late apoptotic and necrotic cells and recruited monomeric C-reactive protein and pentraxin 3, and vice versa. Interactions of the FHRs with pentraxins resulted in enhanced activation of both the classical and the alternative complement pathways on dead cells when exposed to human serum. Altogether, our results demonstrate that FHR-1 and FHR-5 are competitive inhibitors of FH on DNA; moreover, FHR-pentraxin interactions promote opsonization of dead cells.