Fibronectin modulates formation of PF4/heparin complexes and is a potential factor for reducing risk of developing HIT.

Heparin-induced thrombocytopenia (HIT) is caused by platelet-activating anti-platelet factor 4 (PF4)/heparin antibodies. Platelet activation assays that use "washed" platelets are more sensitive for detecting HIT antibodies than platelet-rich plasma (PRP)-based assays. Moreover, heparin-exposed patients vary considerably with respect to the risk of PF4/heparin immunization and, among antibody-positive patients, the risk of subsequent "breakthrough" of clinical HIT with manifestation of thrombocytopenia. We used washed platelets and PRP, standard laboratory HIT tests, and physicochemical methods to identify a plasma factor interfering with PF4/heparin complexes and anti-PF4/heparin antibody-platelet interaction, thus explaining differences in functional assays. To investigate a modulating risk for PF4/heparin immunization and breakthrough of HIT, we also tested 89 plasmas from 2 serosurveillance trials. Fibronectin levels were measured in 4 patient groups exhibiting different degrees of heparin-dependent immunization and expression of HIT. The heat-labile plasma protein, fibronectin, inhibited PF4 binding to platelets in a dose-dependent fashion, particularly in washed (vs PRP) systems. Fibronectin also inhibited PF4/heparin binding to platelets, anti-PF4/heparin antibody binding to PF4/heparin complexes, and anti-PF4/heparin antibody-induced platelet activation as a result of PF4/heparin complex disruption. In addition, plasma fibronectin levels increased progressively among the following 4 patient groups: enzyme-linked immunosorbent assay (ELISA)+/serotonin-release assay (SRA)+/HIT+ < ELISA+/SRA+/HIT- ∼ ELISA+/SRA-/HIT- < ELISA-/SRA-/HIT-. Altogether, these findings suggest that fibronectin interferes with PF4/heparin complex formation and anti-PF4/heparin antibody-induced platelet activation. Reduced fibronectin levels in washed platelet assays help to explain the greater sensitivity of washed platelet (vs PRP) assays for HIT. More importantly, lower plasma fibronectin levels could represent a risk factor for PF4/heparin immunization and clinical breakthrough of HIT.

IL-24 is a common and specific autoantigen of IgE in patients with chronic spontaneous urticaria.

BACKGROUND: The efficacy of omalizumab (anti-IgE) and increased IgE levels in patients with chronic spontaneous urticaria (CSU) suggest autoallergic mechanisms. OBJECTIVE: We sought to identify autoallergic targets of IgE in patients with CSU. METHODS: Serum samples of patients with CSU together with those of patients with idiopathic anaphylaxis and healthy control subjects (7 of each) were screened for IgE autoantibodies by using an array of more than 9000 proteins. Sera of 1062 patients with CSU and 482 healthy control subjects were used in an IgE-anti-IL-24-specific ELISA to investigate the association of IgE-anti-IL-24 and CSU. RESULTS: By using array analyses, more than 200 IgE autoantigens were found in patients with CSU that were not found in control subjects. Of the 31 IgE autoantigens detected in more than 70% of patients, 8 were soluble or membrane bound and expressed in the skin. Of these, only IgE autoantibodies to IL-24 were found in all patients with CSU. In vitro studies showed IL-24 to release histamine from human mast cells sensitized with purified IgE of patients with CSU but not control subjects. By using ELISA, mean ± SD levels of IgE-anti-IL-24 were 0.52 ± 0.24 IU/mL in patients with CSU and 0.27 ± 0.08 IU/mL in control subjects, with 80% of patients with CSU but only 20% of control subjects having levels greater than 0.33 IU/mL (P < .0001). IgE-anti-IL-24 showed acceptable predictive properties for CSU, with a likelihood ratio of 3.9. Clinically, IgE-anti-IL-24 levels showed an association with disease activity, as assessed by the urticaria activity score and with reduced basophil counts. CONCLUSION: Our findings show that patients with CSU frequently exhibit IgE autoantibodies against many autoantigens and that IL-24 is a common, specific, and functional autoantigen of IgE antibodies in patients with CSU.