2003; baxter - Supplements - Haematologica

[Immunobiology of Tolerance Induction]review paperMurine models for the study offactor VIII inhibitorshaematologica 2003; 88(suppl. n. 12):55-63http://www.haematologica.org/free/immunotolerance2001.pdfBIRGIT M. REIPERT,*° MARIA SASGARY,* CHRISTINA HAUSL,°ELISABETH MAIER,* RAFI U. AHMAD,* PETER L. TURECEK,*HANS P. SCHWARZ*°*Baxter BioScience, Vienna, Austria; °Center forBioMolecular Therapeutics, Vienna, AustriaCorrespondence: Hans Peter Schwarz, MD, Baxter,BioScience Industriestraße 67 A-1220 Vienna, Austria.Phone: international +43.1.20100-2067. Fax: international+43.1.20100-525. E-mail: schwarh@baxter.comRegulation of the immune response to a complexprotein antigen like factor VIII (FVIII)requires interaction between different typesof immune cells and migration of these cellsbetween several compartments of the immunesystem. Access to such cells and the compartmentsthey traverse is limited in humans for obviousethical reasons. Hence, in vivo animal modelsare needed to understand the mechanisms ofantibody formation and to develop new therapeuticapproaches for inducing immune toleranceto FVIII in humans. As imperfect as any animalmodel is, important advances in human therapyhave resulted from appropriate use of suchmodels. Our present knowledge of how neutralizingantibodies develop against FVIII is based predominantlyon clinical findings in hemophilicpatients. The presence of FVIII inhibitors makespatients refractory to replacement therapy and is,therefore, a serious complication of modern treatment.The risk of patients with severe hemophiliaA developing anti-FVIII antibodies is clearlyassociated with the type of FVIII gene mutationthey have. Patients affected with nonsense mutations,large deletions and intrachromosomalrecombinations (inversions) in the FVIII geneshow the highest inhibitor incidence 1-3 (HAM-STeRS: The hemophilia A mutation, structure,test, resource site: http://europium.csc.mrc.ac.uk).These mutations are predicted to cause a completedeficit of any endogenous FVIII production.Therefore, human FVIII is probably a protein foreignto the immune system in these patients.Other mutations such as missense mutations andsmall deletions are associated with a much lowerincidence of inhibitor formation. 1-3 In these cases,non-functional FVIII antigen may circulate inthe blood 4 and render the immune system tolerantto the mutant FVIII protein. Native FVIII, presentin FVIII products could, therefore, be recognizedas an altered self protein by the immunesystem in these patients. Apart from the type ofFVIII gene mutation, other genetic factors such asthe HLA haplotype seem to be important for thedevelopment of FVIII inhibitors. To investigatehow important the HLA haplotype is in an animalmodel, animals with different genetic backgrounds,e.g. different inbred strains of mice, areneeded.The application route and dose of an antigendetermines the subsequent immune response. Inpatients, FVIII is given intravenously. Therefore,the spleen is probably the major location for thedevelopment of anti-FVIII immune responses.An animal model should reflect this and developdetectable anti-FVIII immune responses afterintravenous injection of FVIII doses equivalent tothe doses used for patients.Factor VIII is a protein antigen with some extrafeatures that distinguish it from other proteinantigens. In its activated form, activated FVIII(FVIIIa), is an essential cofactor in blood coagulation.In the presence of negatively chargedphospholipids, FVIIIa directly interacts with theserine protease factor IXa and forms a complexthat converts factor X into activated factor X(factor Xa). Factor Xa, in the presence of activatedfactor V (factor Va), phospholipids andcalcium ions, is then able to convert prothrombinto its active enzymatic form, thrombin. Apartfrom their function in blood coagulation, bothfactor Xa and thrombin have been shown in vitroto activate one or more of the protease-activatedreceptors (PAR receptors) expressed onendothelial cells. 5-7 This activation inducesproinflammatory stimuli. Therefore, each timeFVIII is given it is possible that it induces proinflammatorystimuli that directly influence theregulation of the immune response to FVIII. Ananimal model in which a severe bleeding diathesisis associated with a lack of FVIII would beoptimal for immunological studies.Given the diversity of the disease and the heterogeneityof the genetic background of thepatients with hemophilia A, who develop anti-FVIII antibodies, no single animal model couldcover all aspects of the immune response againstFVIII. Therefore, different models are needed toresearch the various facets of the response.Murine models in which the mechanism andhaematologica vol. 88(supplement n. 12):september 2003