Product-dependent anti-factor VIII antibodies.

The development of anti-factor (F)VIII antibodies in haemophilia A (HA) subjects undergoing replacement therapy has been well documented. The correlation between antibody development and the FVIII product used for replacement therapy remains a subject of discussion. The aim of this study was to evaluate the presence of anti-FVIII antibodies towards three commercial rFVIII products in 34 HA subjects' plasmas. Antibodies were quantitated by a Multiplex Fluorescence Immunoassay. All plasmas contained anti-FVIII antibodies at variable concentrations ranging from 50 nm to 570 µm. Eleven of the 20 HA subjects treated with one (r)FVIII product contained inhibitory anti-FVIII antibodies (0.8-3584 BU). The inhibitory antibody titre and the molar concentrations of total antibody were mildly correlated (r(2) = 0.6). Pronounced differences in antibody recognition with the three rFVIII products were observed. For the group treated with Product 'A', the titre towards this product was 2.4-fold higher than that observed with another full-length rFVIII-containing product (Product 'B') and almost four-fold higher than that measured with a B domain-less rFVIII product (Product 'C'). For the group of 14 HA subjects treated with FVIII other than Product 'A', only one showed higher antibody titre when measured with this product. Our data suggest that the development of anti-FVIII antibodies is biased towards the product used for treatment and that a significant fraction of antibodies bind to the B domain of FVIII.

Alloantibodies to factor VIII in haemophilia.

Up to one-third of haemophilia A patients develop factor VIII (FVIII) alloantibodies (inhibitors). The Bethesda assay detects inhibitors but is relatively insensitive. Recently, a new fluorescence-based immunoassay (FLI) was developed for antibody detection. The aim of this study was to assess the prevalence of inhibitors as measured by FLI. Assays of FVIII, FVIII inhibitor by Bethesda assay with Nijmegen modification, and FVIII inhibitor by FLI were performed on adult patients with haemophilia A. Data were complete for 46 patients (median age 39), of whom 72% were severe, 7% moderate and 22% mild. The Bethesda assay was positive in only two patients (4%), while FLI was positive in 23 of 46 patients (50%), with values ranging from 0.4 to 33.7 nm (median 3.5 nm). FLI titres exceeded 7.0 nm in 19.5% of patients, all but one of whom had severe haemophilia. FLI antibody-positive patients were less likely to be HIV positive (30% vs. 70%, P = 0.02). The use of a prophylaxis regimen was associated with a lower incidence of antibody; only two of 23 patients with detectable antibody and none of those with antibody >7 nm were on a prophylaxis regimen, while nine of 23 patients without antibody were on prophylaxis, (P = 0.03). There was no difference in inhibitor presence in patients using recombinant versus plasma-derived factor. Antibodies detected by FLI are frequent in patients with haemophilia A, but are less common in those who are HIV positive or are receiving regular FVIII prophylaxis.

Immunologic quantitation of tissue factors.

The large number of conflicting reports on the presence and concentration of circulating tissue factor (TF) in blood generates uncertainties regarding its relevance to hemostasis and association with specific diseases. We believe that the source of these controversies lies in part in the assays used for TF quantitation. We have developed a highly sensitive and specific double monoclonal antibody fluorescence-based immunoassay and integrated it into the Luminex Multi-Analyte Platform. This assay, which uses physiologically relevant standard and appropriate specificity controls, measures TF antigen in recombinant products and natural sources including placenta, plasma, cell lysates and cell membranes. Comparisons of reactivity patterns of various full-length and truncated TFs on an equimolar basis revealed quantitative differences in the immune recognition of TFs by our antibodies in the order of TF 1-263 > 1-242 > 1-218 > placental TF. Despite this differential recognition, all TF species are quantifiable at concentrations < or = 2 pM. Using a calibration curve constructed with recombinant TF 1-263 and plasma from healthy individuals (n = 91), we observed the concentration of TF antigen in plasma to be substantially lower than that generally reported in the literature: TF antigen in plasma of 72 individuals (79%) was below 2 pM (quantitative limit of our assay); TF antigen levels between 2.0 and 5.0 pM could be detected in six individuals (7%); and in 14% (13 plasmas), the non-specific signal was higher than the specific signal, and thus TF levels could not be determined. These differential recognition patterns affect TF quantitation in plasma and should be considered when evaluating plasma TF-like antigen concentrations.