Studies on Synthetic Polypeptide Antigens

A Potent d-Protein Antagonist of VEGF-A is Nonimmunogenic, Metabolically Stable, and Longer-Circulating in Vivo

Polypeptides composed entirely of d-amino acids and the achiral amino acid glycine (d-proteins) inherently have in vivo properties that are proposed to be near-optimal for a large molecule therapeutic agent. Specifically, d-proteins are resistant to degradation by proteases and are anticipated to be nonimmunogenic. Furthermore, d-proteins are manufactured chemically and can be engineered to have other desirable properties, such as improved stability, affinity, and pharmacokinetics. Thus, a well-designed d-protein therapeutic would likely have significant advantages over l-protein drugs. Toward the goal of developing d-protein therapeutics, we previously generated RFX001.D, a d-protein antagonist of natural vascular endothelial growth factor A (VEGF-A) that inhibited binding to its receptor. However, RFX001.D is unstable at physiological temperatures (Tm = 33 °C). Here, we describe RFX037.D, a variant of RFX001.D with extreme thermal stability (Tm > 95 °C), high affinity for VEGF-A (Kd = 6 nM), and improved receptor blocking. Comparison of the two enantiomeric forms of RFX037 revealed that the d-protein is more stable in mouse, monkey, and human plasma and has a longer half-life in vivo in mice. Significantly, RFX037.D was nonimmunogenic in mice, whereas the l-enantiomer generated a strong immune response. These results confirm the potential utility of synthetic d-proteins as alternatives to therapeutic antibodies.

Definition of human apolipoprotein A-I epitopes recognized by autoantibodies present in patients with cardiovascular diseases

Autoantibodies to apolipoprotein A-I (anti-apoA-I IgG) have been shown to be both markers and mediators of cardiovascular disease, promoting atherogenesis and unstable atherosclerotic plaque. Previous studies have shown that high levels of anti-apoA-I IgGs are independently associated with major adverse cardiovascular events in patients with myocardial infarction. Autoantibody responses to apoA-I can be polyclonal and it is likely that more than one epitope may exist. To identify the specific immunoreactive peptides in apoA-I, we have developed a set of methodologies and procedures to isolate, purify, and identify novel apoA-I endogenous epitopes. First, we generated high purity apoA-I from human plasma, using thiophilic interaction chromatography followed by enzymatic digestion specifically at lysine or arginine residues. Immunoreactivity to the different peptides generated was tested by ELISA using serum obtained from patients with acute myocardial infarction and high titers of autoantibodies to native apoA-I. The immunoreactive peptides were further sequenced by mass spectrometry. Our approach successfully identified two novel immunoreactive peptides, recognized by autoantibodies from patients suffering from myocardial infarction, who contain a high titer of anti-apoA-I IgG. The discovery of these epitopes may open innovative prognostic and therapeutic opportunities potentially suitable to improve current cardiovascular risk stratification.

Experimental determination of the absolute enantioselectivity of an antibody-catalyzed Diels-Alder reaction and theoretical explorations of the origins of stereoselectivity

The exo and endo Diels-Alder adducts of p-methoxycarbonylbenzyl trans-1,3-butadiene-1-carbamate and N,N-dimethylacrylamide have been synthesized, and the absolute configurations of resolved enantiomers have been determined. On the basis of this information, the absolute enantioselectivities of the Diels-Alder reaction catalyzed by antibodies 13G5 and 4D5 as well as other catalytic antibodies elicited in the same immunizations have been established. The effects of different arrangements of catalytic residues on the structure and energetics of the possible Diels-Alder transition states were modeled quantum mechanically at the B3LYP/6-311++G**//B3LYP/6-31+G** level of theory. Flexible docking of these enantiomeric transition states in the antibody active site followed by molecular dynamics on the resulting complexes provided a prediction of the transition-state binding modes and an explanation of the origin of the observed enantioselectivity of antibody 13G5.

The autoantibody repertoire: searching for order

The spontaneously occurring autoantibodies that are associated with human diseases bear the hallmarks of a typical immune response. The repertoire of autoantibodies is surprisingly limited, however, and is the same in both humans and mice. Neither molecular mimicry nor immune dysregulation accounts for this unexpectedly narrow focus of specificities. Experimental data on the properties of the target autoantigens--such as their structure, catabolism, exposure to the immune system after cell death and recently described immunostimulatory effects on immature dendritic cells--indicate that these properties, in conjunction with the tissue microenvironment, help to select the autoantibody repertoire.

Identification of macrophage cell-surface binding sites for cationized bovine serum albumin

Autoimmune diseases are characterized by the presence of autoantibodies often restricted to host proteins exhibiting charge rich domains. Charged polypeptides elicit strong immune responses, and cationized bovine serum albumin and other cationic proteins are significantly more immunogenic than their less charged counterparts. These phenomena may involve enhanced protein uptake by macrophages, resulting in greater processing and presentation of antigenic peptide-MHC complexes to T-cells. We compared macrophage cell-surface binding and uptake of native and cationized bovine serum albumin. Specific binding of [125I]cationized bovine serum albumin to THP-1 macrophages in vitro was 11-16 fold greater than for native albumin. Half-maximal inhibition of [125I]cationized albumin binding was observed at 10-7M ligand. The specificity of [125I]cationized bovine serum albumin binding and uptake was further studied in terms of competitive inhibition of proteolysis by proteins of varying charge content. Cationized bovine serum albumin, but not native albumin, inhibited proteolysis of [125I]cBSA. Calf thymus histones also inhibited cBSA degradation. High concentration of myelin basic protein was moderately effective at blocking cBSA degradation, while myoglobin and beta lactalbumin showed no inhibition. These results indicate that specific cell-surface binding sites which occur on macrophages may mediate selective uptake of certain proteins with highly charged domains including some autoantigens.

Effect of the chemical structure of N-(2-hydroxypropyl)methacrylamide copolymers on their ability to induce antibody formation in inbred strains of mice

The homopolymer of N-(2-hydroxypropyl)methacrylamide (HPMA) and copolymers of HPMA differing in oligopeptide side chains (-Gly-Gly-OH; -Acap-Phe-OH; -Acap-Leu-HMDA and -Gly-Phe-Tyr-OH) or in their content (1%, 3.5% and 8.4% mole of -Gly-Gly-OH side chains) were investigated with respect to their ability to induce antibody formation and mitogenic reaction in inbred strains of mice. The dependence on the antigen dose, on composition of the side chain and on the genetic background of the immunized organism was defined. It was demonstrated that the specificity of the antibody formed is predominantly directed against oligopeptide side chains, though some part of the antibody is also produced against hydroxypropyl chains. Neither the homopolymer nor the copolymers behave in the tissue culture as mitogens.

Immune responses to some proteins and synthetic polypeptides in inbred strains of rats

The antibody responses to DNP-bovine gamma globulin, bovine gamma globulin, human serum albumin, ovalbumin, pepsin and five synthetic polypeptides were examined in strains of inbred rats representative of eight common major histocompatibility complex (RTI) haplotypes. With each antigen the antibody response varied considerably among strains, and the data provide many potential strain and antigen combinations with which to study the genetic control of the immune response.

Induction of immunological tolerance to the penicilloyl antigenic determinant. I. Evaluation of penicilloylated amino acid polymers and copolymers in mice

Antibody responses to the penicilloyl (BPO) group, the major antigenic determinant of penicillin allergy induced in C3H mice by penicilloylated bovine gamma globulin in complete Freund's adjuvant, were reduced or abolished by various amino acid polymers and copolymers of different composition and size carrying BPO groups. Tolerogenic treatment was effective before or after primary immunization and also during anamnestic responses. The unresponsive state was of long duration and persisted even after several booster injections when efficient tolerogens were used. Among the most promising tolerogens are fully penicilloylated oligolysines with a molecular weight below 10 000.

The genetic control of the antibody response in inbred rats

The antibody response of genetically inbred rats to poly(Glu52Lys33Tyr15) is controlled by a complex polygenic system which includes at least two autosomal genes and a sex influence, which may also be genetically determined. The genetic control of the quantity, binding constants, and specificity of the antibody formed linked to the major histocompatibility locus. Factors other than the major genetic ones and the sex influence also affect the quantity of antibody formed, since animals of the same genotype can make significantly different amounts of antibody, depending upon the crosses by which they acquire the major histocompatibility alleles. After immunization with poly(Glu52Lys33Tyr15) the low responders make fewer antibody-producing cells, are not capable of mounting a delayed hypersensitivity reaction to the polypeptide and appear to be deficient in their ability to produce the specific IgM antibody. Immunization of the low responders with antigen aggregated with methylated bovine serum albumin enhances the quantity of antibody formed, increases the binding constants and crossreactivity of the antibody and enhances the delayed hypersensitivity response. In contrast to the findings with the L-amino acid polypeptide, there does not appear to be any genetic control over the antibody response to the D-amino acid enantiomorph poly(DGlu52DLys33DTyr15), which is minimal in all strains.

Maternal-fetal interaction and immunological memory

Female rats of the poorly responding, inbred F344 strain were immunized with poly(Glu(52)Lys(33)Tyr(15)) aggregated with methylated bovine serum albumin, and then they were mated. The first and second litters in the F(1) generation and in the F(2) generation showed an enhanced immune response. When poly-lysine was used as the aggregating agent, enhancement occurred in only the first litter of the F(1) generation and in the F(2) generation. In both cases, antigen was transmitted from the immunized female to her offspring, where it localized in the bone marrow and, in a few cases, in the thymus and spleen also. The transplacental passage of antigen is probably the basis for the enhanced antibody response, which is a manifestation of immunological memory.