Varied immune response to FVIII: presence of proteolytic antibodies directed to factor VIII in different human pathologies

The versatility of antibodies is demonstrated by the various functions that they mediate such as neutralization, agglutination, fixation of the complement and its activation, and activation of effector cells. In addition to this plethora of functions, antibodies are capable of expressing enzymatic activity. Antibodies with catalytic function are a result of the productive interplay between the highly evolved machinery of the immune system and the chemical framework used to induce them (antigens). Catalytic antibodies are immunoglobulins with an ability to catalyze the reactions involving the antigen for which they are specific. Catalytic immunoglobulins of the IgM and IgG isotypes have been detected in the serum of healthy donors. In addition, catalytic immunoglobulins of the IgA isotype have been detected in the milk of healthy mothers. Conversely, antigen-specific hydrolytic antibodies have been reported in a number of inflammatory, autoimmune, and neoplastic disorders. The pathophysiological occurrence and relevance of catalytic antibodies remains a debated issue. Through the description of the hydrolysis of coagulation factor VIII as model target antigen, we propose that catalytic antibodies directed to the coagulation factor VIII may play a beneficial or a deleterious role depending on the immuno-inflammatory condition under which they occur.

[Catalytically active antibodies (abzymes) in human milk]

The review is focused on the analysis of published data and the results obtained by the authors about the catalytic activity of antibodies (abzymes) of human colostrum and milk. Possible mechanisms of origination of these abzymes and their potential role in the regulation of biological activity of human milk compounds are considered. A hypothesis about the role of secretoty abzymes in non-specific humoral defense for the epithelial cells against viral infections is proposed.

Catalytic IgG from Patients with Hemophilia A Inactivate Therapeutic Factor VIII

Factor VIII (FVIII) inhibitors are anti-FVIII IgG that arise in up to 50% of the patients with hemophilia A, upon therapeutic administration of exogenous FVIII. Factor VIII inhibitors neutralize the activity of the administered FVIII by sterically hindering its interaction with molecules of the coagulation cascade, or by forming immune complexes with FVIII and accelerating its clearance from the circulation. We have shown previously that a subset of anti-factor VIII IgG hydrolyzes FVIII. FVIII-hydrolyzing IgG are detected in over 50% of inhibitor-positive patients with severe hemophilia A, and are not found in inhibitor-negative patients. Although human proficient catalytic Abs have been described in a number of inflammatory and autoimmune disorders, their pathological relevance remains elusive. We demonstrate here that the kinetics of FVIII degradation by FVIII-hydrolyzing IgG are compatible with a pathogenic role for IgG catalysts. We also report that FVIII-hydrolyzing IgG from each patient exhibit multiple cleavage sites on FVIII and that, while the specificity of cleavage varies from one patient to another, catalytic IgG preferentially hydrolyze peptide bonds containing basic amino acids.

Theory of proteolytic antibody occurrence

Antibodies (Abs) with proteolytic and other catalytic activities have been characterized in the blood and mucosal secretions of humans and experimental animals. The catalytic activity can be traced to nucleophilic sites of innate origin located in Ab germline variable regions. Discoveries of the natural chemical reactivity of Abs were initially met with bewilderment, as the notion had taken hold that catalytic activities can be introduced into Abs by artificial means, but somatically operative selection pressures are designed only to adapt non-covalent Ab binding to antigen ground states. Unsurprisingly, initial efforts to engineer Abs with catalytic activity were oriented towards improving the non-covalent binding at the atoms immediately within the transition state reaction center. Slowly, however, dogmatic approaches to Ab catalysis have given way to the realization that efficient and specific catalytic Abs can be prepared by improving the natural nucleophilic reactivity combined with non-covalent recognition of epitope regions remote from the reaction center. The field remains beset, however, with controversy. This article attempts to provide a rational basis for natural Ab catalysis, in the hope that understanding this phenomenon will stimulate medical and basic science advances in the field.

Human milk antibodies with polysaccharide kinase activity

It was shown for the first time that a small fraction of milk secretory IgA (sIgA) is tightly bound to oligosaccharides (oligoSACs) and polysaccharides (polySACs). The ability of sIgA to phosphorylate oligo- and polysaccharides was shown to be an intrinsic property of this antibody. In contrast to known kinases, sIgAs with polysaccharide kinase activity can transfer phosphoryl group to oligo- and polysaccharides not only from [gamma-(32)P]ATP but can also use [(32)P]orthophosphate as a substrate of phosphorylation reaction. An extremely unusual property of polysaccharide kinase Abs is their high affinity for orthophosphate (K(m) = 15-77 microM), and orthophosphate is a better substrate than ATP. Two first examples of natural abzymes (Abzs) with synthetic activity were milk sIgA with protein and lipid kinase activities. Polysaccharide kinase sIgA of human milk is the third example of natural antibodies (Abs) with synthetic activity.

Mechanism and functional role of antibody catalysis

The light (L) chain of a model antibody (Ab) was deduced to contain a serine protease-like catalytic site capable of cleaving peptide bonds. The catalytic site is encoded by a germline VL gene. The catalytic activity can potentially be improved by somatic sequence diversification and pairing of the L chain with the appropriate heavy chain. Autoimmune disease is associated with increased synthesis of antigen (Ag)-specific Abs, but the reasons for this phenomenon are not known. Only recently has attention turned to the functional role of the catalytic function. Preliminary studies confirm that the catalytic cleavage of peptide bonds is a more potent means to achieve Ag neutralization, compared to reversible Ag binding. Administration of a monoclonal Ab to VIP in experimental animals induces an inflammatory response in the airways, suggesting that catalytic autoantibodies to this peptide found in airway disease and lupus are capable of causing airway dysfunction. The phenomenon of autoantibody catalysis can potentially be applied to isolate efficient catalysts directed against tumor or microbial Ags by exposing the autoimmune repertoire to such Ags or their analogs capable of recruiting the germline VL gene encoding the catalytic site.

Proteolytic activity of an antibody light chain

The light chain (L chain) of a mAb raised against unactivated vasoactive intestinal peptide (VIP) hydrolyzed this peptide, whereas the heavy chain (H chain) and an irrelevant L chain were without activity. The reaction kinetics were consistent with efficient substrate recognition by the anti-VIP L chain compared with conventional proteases. The L chain cleaved four peptide bonds clustered between residues 16 and 21 in VIP. Mixtures of the L chain with its H chain partner displayed reduced hydrolytic activity compared with the free L chain, suggesting that the H chain is a modulator of the catalytic activity. These observations suggest: 1) the immune system can generate catalytic sites in the L chain subunit of Abs found in response to polypeptide Ags, and 2) free L chains found in vivo could display an Ag-specific catalytic function.