Hydrolysis of coagulation factors by circulating IgG is associated with a reduced risk for chronic allograft nephropathy in renal transplanted patients

Immunoglobulins with Non-Canonical Functions in Inflammatory and Autoimmune Disease States

Immunoglobulins are known to combine various effector mechanisms of the adaptive and the innate immune system. Classical immunoglobulin functions are associated with antigen recognition and the initiation of innate immune responses. However, in addition to classical functions, antibodies exhibit a variety of non-canonical functions related to the destruction of various pathogens due to catalytic activity and cofactor effects, the action of antibodies as agonists/antagonists of various receptors, the control of bacterial diversity of the intestine, etc. Canonical and non-canonical functions reflect the extreme human antibody repertoire and the variety of antibody types generated in the organism: antigen-specific, natural, polyreactive, broadly neutralizing, homophilic, bispecific and catalytic. The therapeutic effects of intravenous immunoglobulins (IVIg) are associated with both the canonical and non-canonical functions of antibodies. In this review, catalytic antibodies will be considered in more detail, since their formation is associated with inflammatory and autoimmune diseases. We will systematically summarize the diversity of catalytic antibodies in normal and pathological conditions. Translational perspectives of knowledge about natural antibodies for IVIg therapy will be also discussed.

B Cells and Antibodies in Transplantation

Overlooked for decades, the humoral alloimmune response is increasingly recognized as a leading cause of graft loss after transplantation. However, improvement in the diagnosis of antibody-mediated rejection has not yet translated into better outcomes for transplanted patients. After an update on B cell physiology and antibody generation, the 2015 Beaune Seminar in Transplant Research challenged the conventional view of antibody-mediated rejection pathophysiology and discussed the latest promising therapeutic approaches.

Generation of Catalytic Antibodies Is an Intrinsic Property of an Individual's Immune System: A Study on a Large Cohort of Renal Transplant Patients

Renal transplant is the treatment of choice for patients with terminal end-stage renal disease. We have previously identified low levels of catalytic IgG as a potential prognosis marker for chronic allograft rejection. The origin and physiopathological relevance of catalytic Abs is not well understood, owing to the fact that catalytic Abs have been studied in relatively small cohorts of patients with rare diseases and/or without systematic follow-up. In the current study, we have followed the evolution of the levels of catalytic IgG in a large cohort of renal transplant patients over a 2-y period. Our results demonstrate that, prior to transplant, patients with renal failure present with heterogeneous levels of IgG hydrolyzing the generic proline-phenylalanine-arginine-methylcoumarinamide (PFR-MCA) substrate. PFR-MCA hydrolysis was greater for patients' IgG than for a therapeutic preparation of pooled IgG from healthy donors. Renal transplant was marked by a drastic decrease in levels of catalytic IgG over 3 mo followed by a steady increase during the next 21 mo. Patients who displayed high levels of catalytic IgG pretransplant recovered high levels of catalytic Abs 2 y posttransplant. Interestingly, IgG-mediated hydrolysis of a model protein substrate, procoagulant factor VIII, did not correlate with that of PFR-MCA prior transplantation, whereas it did 12 mo posttransplant. Taken together, our results suggest that the level of circulating catalytic IgG under pathological conditions is an intrinsic property of each individual's immune system and that recovery of pretransplant levels of catalytic IgG is accompanied by changes in the repertoire of target Ags.

IL-10-independent regulatory B-cell subsets and mechanisms of action

Although classically B cells are known to play important roles in immune protection via humoral immunity, recently their regulatory mechanisms have been best appreciated in the context of autoimmunity. Several studies have identified different subsets of regulatory B cells that vary not only in their phenotype but also in their mechanism of action. Although the best-studied mechanism of B-cell immune regulation is IL-10 production, other IL-10-independent mechanisms have been proposed. These include maintenance of CD4(+)Foxp3(+) regulatory T cells; production of transforming growth factor-β, IL-35, IgM or adenosine or expression of PD-L1 (programmed death 1 ligand 1) or FasL (Fas ligand). Given that B-cell-targeted therapy is being increasingly used in the clinic, a complete understanding of the mechanisms whereby B cells regulate inflammation associated with specific diseases is required for designing safe and effective immunotherapies targeting B cells.

B-cell-mediated strategies to fight chronic allograft rejection

Solid organs have been transplanted for decades. Since the improvement in graft selection and in medical and surgical procedures, the likelihood of graft function after 1 year is now close to 90%. Nonetheless even well-matched recipients continue to need medications for the rest of their lives hence adverse side effects and enhanced morbidity. Understanding Immune rejection mechanisms, is of increasing importance since the greater use of living-unrelated donors and genetically unmatched individuals. Chronic rejection is devoted to T-cells, however the role of B-cells in rejection has been appreciated recently by the observation that B-cell depletion improve graft survival. By contrast however, B-cells can be beneficial to the grafted tissue. This protective effect is secondary to either the secretion of protective antibodies or the induction of B-cells that restrain excessive inflammatory responses, chiefly by local provision of IL-10, or inhibit effector T-cells by direct cellular interactions. As a proof of concept B-cell-mediated infectious transplantation tolerance could be achieved in animal models, and evidence emerged that the presence of such B-cells in transplanted patients correlate with a favorable outcome. Among these populations, regulatory B-cells constitute a recently described population. These cells may develop as a feedback mechanism to prevent uncontrolled reactivity to antigens and inflammatory stimuli. The difficult task for the clinician, is to quantify the respective ratios and functions of “tolerant” vs. effector B-cells within a transplanted organ, at a given time point in order to modulate B-cell-directed therapy. Several receptors at the B-cell membrane as well as signaling molecules, can now be targeted for this purpose. Understanding the temporal expansion of regulatory B-cells in grafted patients and the stimuli that activate them will help in the future to implement specific strategies aimed at fighting chronic allograft rejection.

IVIg treatment reduces catalytic antibody titers of renal transplanted patients

Catalytic antibodies are immunoglobulins endowed with enzymatic activity. Catalytic IgG has been reported in several human autoimmune and inflammatory diseases. In particular, low levels of catalytic IgG have been proposed as a prognostic marker for chronic allograft rejection in patients undergoing kidney transplant. Kidney allograft is a treatment of choice for patients with end-stage renal failure. Intravenous immunoglobulins, a therapeutic pool of human IgG, is used in patients with donor-specific antibodies, alone or in conjunction with other immunosuppressive treatments, to desensitize the patients and prevent the development of acute graft rejection. Here, we followed for a period of 24 months the levels of catalytic IgG towards the synthetic peptide Pro-Phe-Arg-methylcoumarinimide in a large cohort of patients undergoing kidney transplantation. Twenty-four percent of the patients received IVIg at the time of transplantation. Our results demonstrate a marked reduction in levels of catalytic antibodies in all patients three months following kidney transplant. The decrease was significantly pronounced in patients receiving adjunct IVIg therapy. The results suggests that prevention of acute graft rejection using intravenous immunoglobulins induces a transient reduction in the levels of catalytic IgG, thus potentially jeopardizing the use of levels of catalytic antibodies as a prognosis marker for chronic allograft nephropathy.

Hydrolysis of factor VIII mediated by catalytic antibodies occurs in haemophilia A patients with or without factor VIII inhibitors

The major complication of the substitutive treatment of haemophilia A (HA) is the development of antifactor VIII (FVIII) antibodies. Most of these antibodies neutralize FVIII procoagulant activity, and are identified as FVIII inhibitor. A subgroup of these antibodies, 'catalytic antibodies', catalyses the FVIII hydrolysis. We investigated the frequency and the activity of catalytic antibodies, according to the phenotype of HA and the presence or absence of FVIII inhibitor. IgG from 16 patients with inhibitor and 17 patients without inhibitor were purified. Rates of FVIII hydrolysis and inhibitor titres were evaluated. Anti-FVIII catalytic antibodies were detected in 63.6% of patients with HA, irrespective of the HA phenotype and the presence of FVIII inhibitor. The frequency was significantly higher for severe HA patients (73.3%) and patients with inhibitor (87.5%), but their FVIII-proteolytic activity was not significantly different from patients with mild or moderate HA and patients without inhibitor. The evolution of both catalytic and inhibitory activities was studied for 11 patients with FVIII inhibitor. We observed two profiles. In the profile 1, 18.2% of patients, the catalytic activity and the inhibitor titre coevolved. In contrast, a dissociated evolution of these two parameters was observed in 72.8% patients in profile 2. These data confirm the importance of anti-FVIII catalytic activity in patients with severe, moderate and mild HA. Interestingly, most of the patients presented a dissociated profile, suggesting that anti-FVIII antibodies might not systematically act as FVIII inhibitors.

Constant domain-regulated antibody catalysis

Some antibodies contain variable (V) domain catalytic sites. We report the superior amide and peptide bond-hydrolyzing activity of the same heavy and light chain V domains expressed in the IgM constant domain scaffold compared with the IgG scaffold. The superior catalytic activity of recombinant IgM was evident using two substrates, a small model peptide that is hydrolyzed without involvement of high affinity epitope binding, and HIV gp120, which is recognized specifically by noncovalent means prior to the hydrolytic reaction. The catalytic activity was inhibited by an electrophilic phosphonate diester, consistent with a nucleophilic catalytic mechanism. All 13 monoclonal IgMs tested displayed robust hydrolytic activities varying over a 91-fold range, consistent with expression of the catalytic functions at distinct levels by different V domains. The catalytic activity of polyclonal IgM was superior to polyclonal IgG from the same sera, indicating that on average IgMs express the catalytic function at levels greater than IgGs. The findings indicate a favorable effect of the remote IgM constant domain scaffold on the integrity of the V-domain catalytic site and provide a structural basis for conceiving antibody catalysis as a first line immune function expressed at high levels prior to development of mature IgG class antibodies.

Need for a paradigm shift in therapeutic approaches to CNS injury

Irreversible damage to the nervous system can result from many causes including trauma, disruption of blood supply, pathogen infection or neurodegenerative disease. Common features following CNS injury include a disruption of axons, neuron death and injury, local B-cell and microglial activation, and the synthesis of pathogenic autoantibodies. CNS injury results in a pervasive inhibitory microenvironment that hinders regeneration. Current approaches to eliminate the inhibitory environment have met with limited success. These results argue for a paradigm shift in therapeutic approaches to CNS injury. Targeting CNS cells (neurons, oligodendrocytes and astrocytes) themselves may drive CNS repair. For example, our group and others have demonstrated that autoreactive antibodies can participate in aspects of CNS regeneration, including remyelination. We have developed recombinant autoreactive natural human IgM antibodies with the therapeutic potential for CNS repair in several neurologic diseases.

Acquired hemophilia A

Acquired hemophilia A is a rare but often life-threatening hemorrhagic disorder characterized by an autoantibody directed against coagulation factor VIII. Fifty per cent of cases are idiopathic whereas the remaining 50% are associated with pregnancy, autoimmune conditions, malignancies and drugs. In this review the actual knowledge on diagnostic and therapeutic aspects of this disease will be summarized.

Autoantibodies with enzymatic properties in human autoimmune diseases

Immunoglobulins (Ig) or antibodies are heavy plasma proteins, with sugar chains added to amino acid residues by N-linked glycosylation and occasionally by O-linked glycosylation. The versatility of antibodies is demonstrated by the various functions that they mediate such as neutralization, agglutination, fixation with activation of complement and activation of effector cells. In addition to this plethora of functions, some antibodies express enzymatic activity. Antibodies endowed with enzymatic properties have been described in human autoimmune manifestations in a variety of disorders such as autoimmune thyroiditis, systemic erythematosus (SLE), scleroderma, rheumatoid arthritis (RA), multiple sclerosis (MS) and acquired hemophilia (AH). Antibodies isolated from these conditions were able to specifically hydrolyze thyroglobulin, DNA, RNA, myelin basic protein (MBP), and factor VIII (FVIII) or factor IX (FIX), respectively. The therapeutic relevance of these findings is discussed.

Proteolytic antibodies activate factor IX in patients with acquired hemophilia

Acquired hemophilia is a rare bleeding disorder characterized by the spontaneous occurrence of inhibitory antibodies against endogenous factor VIII (FVIII). IgG from some patients with acquired hemophilia hydrolyze FVIII. Because of the complex etiology of the disease, no clinical parameter, including the presence of FVIII-hydrolyzing IgG, has been associated with patient's survival or death. Here, we demonstrate the presence of anti-FIX antibodies in acquired hemophilia patients. IgG from some patients were found to hydrolyze FIX. In most cases, IgG-mediated FIX-hydrolysis resulted in FIX activation. IgG-mediated hydrolysis of FIX thus led to the significant generation of activated FIX in 25 of 65 patients. Based on the estimated kinetic parameters, patients' IgG activated up to 0.3nM FIX in 24 hours, an amount that restored thrombin generation in vitro provided the presence of more than or equal to 3% residual FVIII activity in plasma. This work identifies proteolytic IgG as novel molecules able to activate FIX under pathologic conditions. IgG-mediated FIX activation is a prevalent phenomenon among acquired hemophilia patients. The presence of FIX-activating IgG may partly compensate for the antibody-mediated inhibition of endogenous FVIII in restoring thrombin generation. This clinical trial was registered at www.clinicaltrials.gov as #NCT00213473.

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.

Characteristics, mechanisms of action, and epitope mapping of anti-factor VIII antibodies

The development of anti-factor VIII (FVIII) antibodies (Abs), also called inhibitors, is currently one of the most serious complications arising during the treatment of hemophilia A patients. Improved prevention and eradication of these Abs remain a challenge both for clinicians and scientists. Numerous studies in the literature have reported on their epitope specificity, on their mechanism of FVIII inactivation, as well as on the methods used for their detection. In this review, we summarize the current knowledge on the nature (isotypes, kinetic properties), epitope properties, and mechanisms of action of anti-FVIII Abs. Furthermore, we present methods for detection and epitope characterization of anti-FVIII Abs with emphasis on the Luminex technique susceptible to facilitate the monitoring of changes in the epitope specificity of these Abs.

In vivo sensitized and in vitro activated B cells mediate tumor regression in cancer adoptive immunotherapy

Adoptive cellular immunotherapy utilizing tumor-reactive T cells has proven to be a promising strategy for cancer treatment. However, we hypothesize that successful treatment strategies will have to appropriately stimulate not only cellular immunity, but also humoral immunity. We previously reported that B cells in tumor-draining lymph nodes (TDLNs) may function as APCs. In this study, we identified TDLN B cells as effector cells in an adoptive immunotherapy model. In vivo primed and in vitro activated TDLN B cells alone mediated effective (p < 0.05) tumor regression after adoptive transfer into two histologically distinct murine pulmonary metastatic tumor models. Prior lymphodepletion of the host with either chemotherapy or whole-body irradiation augmented the therapeutic efficacy of the adoptively transferred TDLN B cells in the treatment of s.c. tumors as well as metastatic pulmonary tumors. Furthermore, B cell plus T cell transfers resulted in substantially more efficient antitumor responses than B cells or T cells alone (p < 0.05). Activated TDLN B cells conferred strong humoral responses to tumor. This was evident by the production of IgM, IgG, and IgG2b, which bound specifically to tumor cells and led to specific tumor cell lysis in the presence of complement. Collectively, these data indicate that in vivo primed and in vitro activated B cells can be employed as effector cells for cancer therapy. The synergistic antitumor efficacy of cotransferred activated B effector cells and T effector cells represents a novel approach for cancer adoptive immunotherapy.

Immunoglobulin A with protease activity secreted in human milk activates PAR-2 receptors, of intestinal epithelial cells HT-29, and promotes beta-defensin-2 expression

Secretory antibodies of the immunoglobulin A (sIgA) class constitute the first line of antigen-specific immune protection against pathogens and other antigens at mucosal surfaces. Although initially perceived as potentially deleterious, catalytic antibodies have been proposed to participate in the removal of metabolic wastes and in protection against infection. Here we show that the presence of sIgA endowed with serine protease-like hydrolytic activity in milk strongly correlates with PAR-2 activation in human intestinal epithelial cells. F(ab')(2) fragments of sIgA activated the epithelial cells in culture to produce beta-defensin-2 (hBD2). Intracellular Ca(2+) mobilization was induced by treatment with (1) sIgA-F(ab')(2) fragments; (2) trypsin, a recognized PAR-2 agonist; or (3) a synthetic PAR-2 agonist peptide (SLIGKV). The co-treatment with a synthetic PAR-2 antagonist peptide (FSLLRY) and sIgA-F(ab')(2) fragments eliminates the latter's effect; nevertheless, cells were not refractory to subsequent stimulation with sIgA-F(ab')(2) fragments. Both the induction of hBD-2 expression in epithelial cells and the increase in intracellular [Ca(2+)] stimulated by sIgA-F(ab')(2) fragments were inhibited by treatment with serine protease inhibitors or pertussis toxin (PTX). These findings suggest that catalytic antibodies can activate intestinal epithelial cells through G-protein-coupled PAR-2, and could actively participate in the immune system of breastfed babies inducing the production of peptides related to innate defense, such as defensins.