Today it is generally accepted that B cells require cognate interactions with CD4 IntroductionThe most serious complication in replacement therapy with FVIII products is the development of neutralizing antibodies against FVIII (FVIII inhibitors), which is observed in approximately 25% to 30% of patients with severe hemophilia A. 1 Although several genetic 2 and nongenetic 3 factors that contribute to the risk for patients to develop these antibodies have been identified, why some patients develop antibodies while others do not remains largely unknown.Today it is generally accepted that B cells require cognate interactions with CD4 ϩ T cells to develop high-affinity antibodies against protein antigens. 4,5 In line with this perception, several lines of evidence have supported the involvement of CD4 ϩ T cells in the generation of antibody responses against FVIII in patients with hemophilia A and in murine hemophilia models. 6,7 CD4 ϩ T cells express T-cell receptors that recognize antigen-derived peptides (CD4 ϩ T-cell epitopes) presented by MHC class II molecules, which are expressed on specialized antigen-presenting cells. 8 Structural features of both the MHC class II molecule and the peptide determine the specificity of CD4 ϩ T cells that can bind to the MHC class II-peptide complex. 8,9 The conditions under which CD4 ϩ T cells interact with this complex determine whether the immune system reacts with nonresponsiveness, is activated to develop specific antibodies, or is tolerized to suppress antibody responses. 9,10 Therefore, it is crucial to understand which FVIII peptides are presented by MHC class II complexes under conditions of FVIII replacement therapy and how CD4 ϩ T cells interact with MHC class II-FVIII peptide complexes expressed by antigenpresenting cells. The available information on FVIII peptides presented in the context of specific human MHC class II molecules is limited. Several studies used peripheral blood cells of patients and healthy controls 11 to identify CD4 ϩ T-cell epitopes in the A2 domain, 12 A3 domain, 13 and C2 domain of FVIII. 14 However, these studies lack information on the specific MHC class II molecules associated with the FVIII peptides identified. Jacquemin et al identified T-cell epitopes of FVIII using CD4 ϩ T-cell clones isolated from a mild hemophilia A patient carrying an Arg2150His mutation in the C1 domain of FVIII. 15 All clones recognized FVIII peptides encompassing residue Arg2150. Peptides were presented by HLA-DRB1*0401/HLA-DRB4*01 or HLA-DRDRB1*1501/ HLA-DRB5*0101. One of the peptides identified was a promiscuous epitope that bound to several different HLA-DR proteins. James et al used MHC class II tetramers to analyze FVIII-specific CD4 ϩ T cells obtained from a mild hemophilia A patient carrying an Ala2201Pro mutation in the C2 domain of FVIII. 16 Responses of CD4 ϩ T cells to sequences containing Ala2201 (wild-type), Pro2201 (hemophilic), and other predicted T-cell epitopes were evaluated and resulted in the identification of an HLA-DRB1*0101 restricted T-ce...

SummaryMonoclonal antibodies are essential therapeutics and diagnostics in a large number of diseases. Moreover, they are essential tools in all sectors of life sciences. Although the great majority of monoclonal antibodies currently in use are of mouse origin, the use of human B cells to generate monoclonal antibodies is increasing as new techniques to tap the human B cell repertoire are rapidly emerging. Cloned lines of immortalized human B cells are ideal sources of monoclonal antibodies. In this review, we summarize our studies to the regulation of the replicative life span, differentiation, and maturation of B cells that led to the development of a platform that uses immortalization of human B cells by in vitro genetic modification for antibody development. We describe a number of human antibodies that were isolated using this platform and the application of the technique in other species. We also discuss the use of immortalized B cells as antigen‐presenting cells for the discovery of tumor neoantigens.

Key Points AT1413 is a monoclonal antibody isolated from a cured patient with AML that recognizes CD43s, a novel epitope expressed by AML and MDS blasts. AT1413 eliminates CD43s-expressing leukemic blasts in vitro and in vivo and may have potential as a therapeutic antibody.

Replacement of the missing factor VIII (FVIII) is the current standard of care for patients with hemophilia A. However, the short half-life of FVIII makes frequent treatment necessary. Current efforts focus on the development of longer-acting FVIII concentrates by introducing chemical and genetic modifications to the protein. Any modification of the FVIII protein, however, risks increasing its immunogenic potential to induce neutralizing antibodies (FVIII inhibitors), and this is one of the major complications in current therapy. It would be highly desirable to identify candidates with a high risk for increased immunogenicity before entering clinical development to minimize the risk of exposing patients to such altered FVIII proteins. In the present study, we describe a transgenic mouse line that expresses a human F8 cDNA. This mouse is immunologically tolerant to therapeutic doses of native human FVIII but is able to mount an antibody response when challenged with a modified FVIII protein that possesses altered immunogenic properties. In this situation, immunologic tolerance breaks down and antibodies develop that recognize both the modified and the native human FVIII. The applicability of this new model for preclinical immunogenicity assessment of new FVIII molecules and its potential use for basic research are discussed. (Blood. 2011; 118(13):3698-3707) IntroductionHemophilia A is an X-linked bleeding disorder that is caused by reduced function or lack of clotting factor VIII (FVIII). 1 Replacement of the missing protein is the current standard of care for patients. However, the short half-life of FVIII, ϳ 7-17 hours, 2 makes frequent treatment necessary. Current efforts focus on the development of longer-acting FVIII concentrates, which should decrease the required treatment frequency and therefore improve the quality of life for patients. Recently described approaches in the development of longer-acting concentrates are based on strategies that have been successfully applied to other therapeutic proteins: chemical modifications such as the addition of polyethylene glycol (PEG) polymers, polysialic acids, or hydroxyethyl starch 3,4 ; alternative formulations with PEG-modified liposomes 3 ; and fusion to the Fc part of human IgG. 5 In addition, molecular modifications of the FVIII protein aimed at increasing the duration of its cofactor activity or reducing its clearance in vivo have been reported. 3 Any chemical or molecular modification of the FVIII protein can potentially increase its immunogenic potential. Modifications could generate neo-epitopes for both B and T cells or may induce altered structures that could bind and trigger receptors expressed on cells of the innate immune system, thereby amplifying potential anti-FVIII antibody responses. 6 Finally, modifications could generate repetitive epitopes for B cells that might cause the activation and differentiation of B cells and the subsequent production of antibodies without the requirement for T-cell help. 7,8 Therefore, the potential impact that any...