Improved generation of catalytic antibodies by MRL/MPJ-lpr/lpr autoimmune mice

Recent advances using FcRn overexpression in transgenic animals to overcome impediments of standard antibody technologies to improve the generation of specific antibodies

This review illustrates the salutary effects of neonatal Fc receptor (FcRn) overexpression in significantly improving humoral immune responses in the generation of antibodies for immunotherapy and diagnostics. These include: (1) improved IgG protection; (2) augmented antigen-specific humoral immune response with larger numbers of antigen specific B cells, thus offering a wider spectrum of clones; (3) generation of antibodies against weakly immunogenic antigens; (4) significant improvements in the number and substantial developments in the diversity of hybridomas. FcRn transgenesis thus confers a number of practical benefits, including faster antibody production, higher antibody yields and improved generation of hybridomas for monoclonal antibody production. Notably, these efficiencies in polyclonal antibody production were also demonstrated in FcRn transgenic rabbits. Overall, FcRn transgenic animals yield more antibodies and provide a route to the generation of antibodies against antigens of low immunogenicity that are difficult to obtain using currently available methods.

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.

Advances in monoclonal antibody technology: genetic engineering of mice, cells, and immunoglobulins

The ability to produce antibodies that are directed against specific antigens has played a crucial role in advancing scientific discoveries. Recombinant technologies have extended the application of antibodies beyond the research laboratory and into the clinic for the treatment of cancer and other diseases. Creative approaches using these technologies have been used to reduce the antibody to its minimal functional size, and/or make them bifunctional (immunotoxins), bispecific, or less immunoreactive (humanized). Additionally, mice that are engineered to generate antibodies of human genomic origin have been used to produce therapeutic antibodies and are being further developed. As the research and clinical demands for antibodies continue to increase, the development of improved resources (cell lines and animals) to improve production efficiency, generate larger repertoires, and deliver greater yields of antibodies is being explored, and advances in this area are discussed further in this review.

Selective chemotherapeutic strategies using catalytic antibodies: a common pro-moiety for antibody-directed abzyme prodrug therapy

Prodrug activation by catalytic antibodies (abzymes) conjugated with anti-tumor antibodies, called antibody-directed abzyme prodrug therapy (ADAPT), has been proposed as a strategy for site-specific drug delivery systems for anti-tumor drugs. The delivery of abzymes is achieved by making a bi-specific antibody with a monovalent catalytic antibody and a monovalent binding antibody. To achieve ADAPT, we focused on specific requirements for prodrugs and catalytic antibodies, the stability of the prodrugs against natural enzymes, and the applicability of abzymes for a wide range of prodrugs. Attention was paid to the design of a pro-moiety rather than a parent drug. As a common pro-moiety, we chose vitamin B(6), because the bulky vitamin B(6) esters are relatively stable against hydrolytic enzymes in serum. We have generated catalytic antibodies by immunization of a vitamin B(6) phosphonate transition state analog. The elicited antibodies were found to hydrolyze several anti-cancer and anti-inflammatory prodrugs with the vitamin B(6) pro-moiety. Finally, we evaluated antibody-catalyzed prodrug activation by examining the growth inhibition of human cervical cancer (HeLa) cells with the vitamin B(6) ester of butyric acid. These results suggest that the pro-moiety of vitamin B(6) ester is stable enough to resist natural enzymes in serum and is removed by the tailored catalytic antibodies. The combination of catalytic antibodies and prodrugs masked with vitamin B(6) would allow hydrophobic and highly toxic drugs to be used.

Catalytic antibodies in clinical and experimental pathology: human and mouse models

Most of the data accumulated through studies on natural catalytic autoantibodies indicate that production scales up markedly in pathological abnormalities. We have previously described an increased level of DNA-hydrolyzing autoantibodies in the sera of patients with various autoimmune disorders [systemic lupus erythematosus (SLE), rheumatoid arthritis, scleroderma], HIV infection and lymphoproliferative diseases accompanied by autoimmune manifestations. In the present study, we show that an increased level of catalytic activity of autoantibodies can be observed in the sera of autoimmune mice, thus providing a fundamental insight into the medical relevance of abzymes. Polyclonal autoantibodies purified from sera of NZB/W, MRL-lpr/lpr and SJL/J mice show proteolytic and DNA-hydrolyzing activities, as opposed to those harvested from non-autoimmune BALB/c mice. The expressiveness of the catalytic activity was strongly dependent on the age of the animal. The highest levels of catalytic activity were found in the sera of mice aged between 8 and 12 months; the lowest level was typical of younger animals whose age ranged from 6 to 8 weeks. Specific inhibition assays of the catalytic activities were performed to throw light on the nature of the abzyme activity. Within a cohort of aging animals, a strong correlation between marked autoimmune abnormalities and levels of catalytic activities has been established. Nonimmunized SJL/J mice revealed specific immune responses to myelin basic protein (MBP), skeletal muscle myosin (skMyo) and cardiac myosin (Myo), and highly purified antibodies from their serum show specific proteolytic attack against the target antigens. This finding prompted us to undertake a more detailed study of specific antibody-mediated proteolysis in diseased humans. A targeted catalytic response was originally demonstrated against MBP and Myo in multiple sclerosis and myocarditis patients, respectively.

Evolution of catalytic antibody repertoire in autoimmune mice

We have attempted to efficiently obtain catalytic antibodies (catAbs) with amidase/esterase activity in the expanded sequence space of the antibody repertoire. In doing so, we used an autoimmune mouse strain, MRL/lpr, that is known to produce enhanced levels of autoantibodies. We applied different types of haptens, such as, and, that are supposed to mimic the transition state of the substrate in the ester/amide hydrolysis. Among them, hapten (2) could not be used, as it was readily broken down after synthesis. Upon immunization with hapten (1), catAbs preferentially evolved in MRL/lpr mice, but this did not happen upon immunization with haptens (3) and (4). Independently, immunization to MRL/lpr mice with successfully elicited the catAbs with the ability to activate vitamin B(6) prodrugs. The common observation seen in these two cases is that most of the catAbs derived from MRL/lpr mice by hapten (1) and half of them by hapten (5) had a Lys at H95, which is at the junctional N region between the V(H) and J(H) gene segments. Despite the conservation of Lys (H95), analyses of the N-region and utilization of the D gene segment in the heavy chain gene showed that these catAbs were from several independent clones of the same family. Studies of site-directed mutagenesis suggest that, in the catAbs elicited from hapten (1), a Lys (H95) and a His (L91) are involved in the catalytic function. Both residues are known to interact with the phosphonate moiety of hapten (1). Such studies also suggest that, in the catAbs elicited from hapten (5), a Lys (H95) and a His (H35) are involved in the catalytic function. These basic amino acids seem to be important for binding to the phosphonate hapten, as they were not changed even after extensive evolution following multiple mutations. By contrast, in normal BALB/c mice, immunization of hapten (1) resulted in eliciting catAbs in lower yield and the majority were the non-catAbs, whose sequences were quite different from those of the catAbs from MRL/lpr mice. They were clonally related to one another and most of them originated from a single clone. The positions of the interacting key residues in the CDRs that interact with the phosphorus moiety strongly differ between our catAbs and other reported catAbs with esterase/amidase activity, which were elicited by the phosphonate/phosphonamidate haptens from normal mice. Further comparison of antibodies elicited by the phosphorus haptens, such as DNA, RNA, phosphocholine, and phosphotyrosine, indicated that none of them had sequence similarity in the basic amino acids and their positions in the CDRs, except for one example, which is anti-DNA antibody elicited from C3H-lpr mice. Analysis based on the classification of canonical structures of the antibodies again suggested that our catAbs derived from MRL/lpr mice belong to an unusual class that is not listed in the literature. Taken together, the above evidence suggests that the unique catalytic subsets that existed in the initial repertoire in the MRL/lpr mice could effectively be captured by the phosphonate haptens through the interaction with the Lys at H95. In the BALB/c mice, however, another noncatalytic subset with an ability to bind only to a moiety other than the phosphonate moiety alternatively evolved, because of the lowest abundance or elimination of the catalytic subsets.

Exploiting antibodies as catalysts: potential therapeutic applications

In this review, we explore recent developments in the generation of catalytic antibodies and their potential in therapy.

Molecular evolution of catalytic antibodies in autoimmune mice

Catalytic Abs (catAbs) preferentially evolved in autoimmune MRL/MPJ-lpr/lpr (MRL/lpr) mice upon immunization with the phosphonate transition-state analogue (TSA), but this did not happen in normal BALB/c mice. The majority of the catAbs from MRL/lpr mice were from several independent clones of the same family. Most of them had a lysine at position 95 in the heavy chain (H95), which is at the junctional region. This residue, which interacts with the phosphonate moiety of the TSA and presumably is involved in the catalytic activity, was not changed even after expansive evolution following multiple mutations. By contrast, the majority that arose from BALB/c mice were the non-catAbs, which were quite different in the sequence from the catAbs from MRL/lpr mice, but they were clonally related to one another, so most of them were originated from a single clone. In the MRL/lpr mice, the catalytic subsets that existed in the initial repertoire were effectively captured by the phosphonyl oxygens in the TSA by interacting with the lysine at H95. In the BALB/c mice, however, another noncatalytic subset with only the binding capability directed to a moiety other than the phosphonate moiety was alternatively evolved, because of the lowest abundance or elimination of the catalytic subsets.

Phosphonate ester probes for proteolytic antibodies

The reactivity of phosphonate ester probes with several available proteolytic antibody (Ab) fragments was characterized. Irreversible, active site-directed inhibition of the peptidase activity was evident. Stable phosphonate diester-Ab adducts were resolved by column chromatography and denaturing electrophoresis. Biotinylated phosphonate esters were applied for chemical capture of phage particles displaying Fv and light chain repertoires. Selected Ab fragments displayed enriched catalytic activity inhibitable by the selection reagent. Somewhat unexpectedly, a phosphonate monoester also formed stable adducts with the Abs. Improved catalytic activity of phage Abs selected by monoester binding was evident. Turnover values (kcat) for a selected Fv construct and a light chain against their preferred model peptide substrates were 0.5 and 0.2 min(-1), respectively, and the corresponding Michaelis-Menten constants (Km) were 10 and 8 microm. The covalent reactivity of Abs with phosphonate esters suggests their ability to recapitulate the catalytic mechanism utilized by classical serine proteases.

Overpassing an aberrant V(kappa) gene to sequence an anti-idiotypic abzyme with (beta)-lactamase-like activity that could have a linkage with autoimmune diseases

A monoclonal antibody 9G4H9 that exhibits a beta-lactamase-like activity was previously obtained in accordance with the idiotypic network theory. This abzyme presents the most catalytic efficiency in amidase activity described in literature (kcat = 0.9 min-1). Some reports have demonstrated that functionality as complex as catalysis may be mimicked in this way. Comparison of the catalytic properties of both enzyme and abzyme previously allowed us to obtain better knowledge about 9G4H9 abzymatic machinery. In attempt to characterize this abzyme, the variable regions of kappa and heavy chain were cloned. We present a 'universal' method to clone the correct Vkappa gene to bypass aberrant Vkappa (abVkappa) produced by MOPC-21-derived hybridomas. Sequences obtained are compared in the GenBank database. The VH and Vkappa genes present some important sequence homology with autoantibodies suggesting a direct relationship between catalytic anti-idiotypic antibody and autoimmunity.