Relation between optical configuration and immunogenicity of synthetic polypeptides

Radical rearrangement and transfer reactions in proteins

Radical rearrangement and transfer reactions play an important role in the chemical modifications of proteins in vivo and in vitro. These reactions depend on protein sequence, as well as structure and dynamics. Frequently, these reactions have well-defined precedents in the organic chemistry literature, but their occurrence in proteins provides a stage for a number of novel and, perhaps, unexpected reaction products. This essay will provide an overview over a few representative examples of radical rearrangement and transfer reactions.

Discovery of Peptide Antibiotics Composed of d-Amino Acids

A paucity of viable programs and pipelines for the discovery of new antibiotics poses a significant public health threat. The emergence of resistant strains against vancomycin is particularly dangerous in hospital settings. Here, we report the design of enantiomeric targets based on bacterial cell wall biosynthesis precursors that allow for selection and identification of short linear, cyclic and bicyclic peptides that are composed of d-amino acids. These compounds are active against Staphylococcus aureus, Methicillin-resistant S. aureus, and vancomycin-resistant Enterococci that possess moderately high antibacterial activity and furthermore display no toxicity to both human red blood cells and mammalian cells at these concentrations. This 'mirror image phage display' approach yielded templates that can serve as scaffolds for further improvements in activity-based structural modifications. This strategy has the potential to provide a new class of antimicrobials that are metabolically stable and have the promise for oral delivery. The use of this platform combined with traditional medicinal chemistry approaches could rapidly yield large numbers of new therapeutic lead compounds.

Structure-function relationship studies of PTH(1-11) analogues containing D-amino acids

Parathyroid hormone (PTH) is an 84-amino acid peptide hormone. Produced in the parathyroid glands, it acts primarily on bone and kidney to maintain extracellular calcium levels within normal limits. It has been shown that the 1-34 amino acid fragment of PTH is sufficient to bind and activate the PTH type-I receptor. Recent investigations focusing on the interaction of N-terminal fragments of PTH with PTH type-I receptor showed that certain modifications can increase signalling potency in peptides as short as 11 amino acids. To understand the role of the side chains of all the amino acid residues in PTH(1-11), we synthesized all-D PTH, three retro-inverso analogues of the most active modified PTH(1-11), H-Aib-Val-Aib-Glu-Ile-Gln-Leu-Nle-His-Gln-Har-NH(2), and we substituted every L-AA of the latter with the corresponding D-AA, obtaining a library of PTH(1-11) analogues that were tested as agonists. The library was synthesized by SPPS, employing the Fmoc protocol. The biological tests showed that the activity of the D-Har11 analogue is of the same order of magnitude of that of the most active modified PTH(1-11). This behaviour is paralleled by an increase of the helical content on going from the D-Val(2) to the D-Har(11) analogue. This is in agreement with previous work where a correlation between activity and helical content has been demonstrated. The importance of a positively charged group in the C-terminal position is shown to be independent of the configuration of the C(alpha)-carbon.

Sequence-simplification and chimeric assembly: new models of peptide antigen modification

Sequence-simplified variants of a 15-mer peptide antigen, identified by amino acid side chains in alternating positions were synthesized introducing glycine residues alternatively in the parent peptide sequence and used to induce antibodies in rabbit. They reacted to a significant extent with anti-parent peptide antibodies, and in addition, affinity purified antibodies against these halved forms recognized with similar affinity and specificity, the starting peptide in affinity chromatography, optical biosensor and enzyme linked immunosorbent assay (ELISA) experiments, while no cross-reactivity was detected between reduced antigens. These findings suggest that a peptide antigen can display two molecular surfaces of recognition, identified by side chains of residues in alternating positions. Each surface can even take part in antigen/antibody interaction independently, thus indicating the possibility to select and assembly sequence-simplified forms belonging to different epitopes, also deriving from different molecules, to generate new structures incorporating a two-fold antigen/antibody specificity. Two "chimeric" forms were then synthesized starting from the P15 and P13 complementary peptides, both able to bind interleukin 2. These structures, showing simultaneously trans-surfaces of recognition belonging to both parent forms, have been found to retain antigenic properties against antibodies of simplified P15 derivatives showing the same molecular surface of recognition. In addition, anti-chimeric antibodies recognized both P15 and P13 starting peptides, while no cross-antibody recognition was observed between chimeric antigens.

Immunosuppression by D-isomers of HLA class I heavy chain (amino acid 75 to 84)-derived peptides is independent of binding to HSC70

BACKGROUND

Peptides derived from the class I heavy chain were shown to modulate immune responses in vitro and in vivo. A peptide derived from HLA-B2702 (2702.75-84) inhibited differentiation of cytotoxic T cells as well as T cell and natural killer cell-mediated cytotoxicity in vitro. Peptide-mediated immunomodulation seemed to be independent of the MHC proteins expressed by responder and stimulator cells. In vivo studies in rodents demonstrated prolongation of heart and skin allograft survival after peptide therapy. Here, the correlation between the peptide's biological activity and its amino acid sequence was analyzed using peptides derived from amino acid 75-84 of several mouse, rat, and human MHC class I proteins as well as peptides with single amino acid substitutions in the 2702.75-84 sequence.

METHODS

Peptides consisting of both L- and D-amino acids were tested for inhibition of murine and human T cell-mediated and lymphokine-activated killer cell-mediated cytotoxicity, binding to hsc70, and prolongation of heart allograft survival in vivo.

RESULTS

Replacement of glutamic acid residue (E) at position 75 with valine (V) resulted in a peptide [2702.75-84(E>V)] with increased in vitro and in vivo activity but unchanged affinity for hsc70. Surprisingly, both L- and D-isomers of 2702.75-84 and 2702.75-84(E>V) inhibited cytotoxic cells in vitro and prolonged heart allograft survival in vivo. However, as expected, the peptides consisting of D-amino acids did not bind to hsc70.

CONCLUSION

Assuming that both D- and L-isomers modulate immune responses by similar mechanisms, these results suggest that the peptides' effect is independent of binding to hsc70.

Identification of D-peptide ligands through mirror-image phage display

Genetically encoded libraries of peptides and oligonucleotides are well suited for the identification of ligands for many macromolecules. A major drawback of these techniques is that the resultant ligands are subject to degradation by naturally occurring enzymes. Here, a method is described that uses a biologically encoded library for the identification of D-peptide ligands, which should be resistant to proteolytic degradation. In this approach, a protein is synthesized in the D-amino acid configuration and used to select peptides from a phage display library expressing random L-amino acid peptides. For reasons of symmetry, the mirror images of these phage-displayed peptides interact with the target protein of the natural handedness. The value of this approach was demonstrated by the identification of a cyclic D-peptide that interacts with the Src homology 3 domain of c- SRC. Nuclear magnetic resonance studies indicate that the binding site for this D-peptide partially overlaps the site for the physiological ligands of this domain.

Topological mimicry of cross-reacting enantiomeric peptide antigens

Rabbit polyclonal antibodies against multimeric peptide antigens were found to cross-react to a significant extent with topologically related variants of the parent antigen, where the chirality of each amino acid residue (inverso derivatives), or the peptide sequence orientation (retro derivatives), was inverted or where both modifications were simultaneously introduced (retro-inverso derivatives). All peptide variants displayed similar recognition properties for antibodies and similar dose-dependent inhibitory effects on the interaction between immobilized parent antigen and corresponding antibodies. Importance of peptide side chain topology on antigenicity was evaluated analyzing the recognition properties of two sequence-simplified parent peptide variants, one lacking of the side chains in the sequence odd position and the other in even position. These two variants, prepared introducing glycine residues alternatively in the parent peptide sequence, were found to cross-react to a significant extent with the original antibody raised against the parent peptide. Analysis of molecular models of peptide enantiomeric variants in the elongated all-trans configuration suggested that the topological equivalence of alternating side chains could lead to the formation of similar recognition surfaces, thus mimicking the parent peptide antigenic structure.

A comparison of the immunogenicity of a pair of enantiomeric proteins

The immunogenicity of a folded, all D-amino acid protein, rubredoxin, has been compared with that for the corresponding L-protein enantiomer. Following multiple administrations with alum adjuvant, the L-protein induced a strong, specific IgG antibody response, whereas the D-protein did not. This relative lack of responsiveness to the D-protein cannot be attributed to rapid excretion, since it is retained at least 4 times longer than the natural L-protein. These observations provide the first direct evidence that a folded D-amino acid protein has low immunogenicity and is long lived in vivo. Proteins with such properties may be useful as molecular platforms in a variety of chemical and pharmacological applications.

The antigenicity of sequential polypetides. II. The antigenicity of some sequential polymers including several related to collagen

The antigenicity in guinea-pigs of a series of sequential polymers has been studied. Almost all the polymers had a repeating sequence of three amino acids and with few exceptions glycine was one of the three residues and proline or a proline homologue one of the others. As a result the majority of the peptides used probably possessed at least in part, the collagen type of triple helix, although with the exception of (Gly-Pro-Gly)_n all these polymers were antigenic inducing both a humoral and cell-mediated response.

With (D-Ala-Gly-D-Pro)_n no humoral response was obtained but there were excellent cell-mediated reactions. The mutual cross-reactivity, both humoral and cell-mediated, between (Gly-Pro-Ala)_n and (Pro-Ala-Gly)_n emphasized the importance of the sequence in the body of the polymers rather than the sequence at the free ends in determining specificity. This is confirmed by the absence of cross-reactivity of both of these polymers with (Ala-Pro-Gly)_n.

With a series of samples of (Ala-Gly-Pro)_n of increasing single chain weight average molecular weight ranging from ca 2,000 to ca 15,000 the most immunogenic for cell-mediated reactions was that of lowest molecular weight, but for humoral responses immunogenicity ran parallel with increasing molecular weight with the exception of the polymer of highest molecular weight, which was almost insoluble.

Differences in immune response to synthetic antigens in two inbred strains of guinea-pigs

The study of the immunogenicity of some linear and multichain synthetic polypetides in guinea-pigs, of inbred strains 2 and 13 led to their division into three categories: (a) immunogenic in both inbred strains: linear copolymer of tyrosine, glutamic acid and alanine; (b) immunogenic in strain 13 and negative in strain 2: linear copolymer of tyrosine and glutamic acid; and (c) immunogenic in strain 2 and negative in strain 13: linear and branched copolymers containing lysine. No immune response was detected in strain 2 guinea-pigs to the copolymer of tyrosine, glutamic acid and lysine, composed only of the D-optical isomers.

The immune response, or lack of response, in F₁ hybrids of the inbred strains was identical with that of inbred strain 2 suggesting that the genetic determinants of this strain are dominant.

No cross-reactions were observed at the delayed stage in inbred strain 2 between linear and multichain copolymers of similar composition.

Inhibition, no-effect or enhancement of immune responses following injection of mixtures of immunogenic and non-immunogenic synthetic polypeptides

The phenomenon of antigenic competition has been investigated by the use of synthetic compounds and guinea-pigs of inbred strain 2 as experimental animals. The effect of addition to the immunizing mixture of immunogenic and non-immunogenic synthetic compounds has been studied.

Antigenic competition has been demonstrated between the synthetic antigen 252,p(Tyr,Glu,Lys) and DNP-p(Lys) or 28,pdlAla-p(Tyr,Glu)--pLys.

The non-immunogenic compounds 251,p(dTyr,dGlu,dLys), 33,pTyr-pdlAla--pLys, and p(Tyr) have been found respectively to inhibit, enhance, or not affect the immune response to 252,p(Tyr,Glu,Lys). Two weak antigens, namely 509,p(Tyr,Glu,)-pdlAla--pLys and p(Lys) also enhanced the response to 252,p(Tyr,Glu,Lys). Possible explanations for these findings are discussed.

Mechanism of delayed reactions

Arsanilic acid conjugates of polymers of L-typrosine, glutamic acid, and alanine are immunogenic and can elicit hapten-specific, delayed-hypersensitivity reactions in sensitized guinea pigs. Conjugates of the D-amino acid polymers are neither immunogenic nor capable of eliciting delayed reactions. Mixtures of small amounts of conjugates capable of eliciting a delayed reaction with larger amounts of D-amino acid polymer conjugates produce only small delayed reactions. I suggest that the delayed reaction is an active response requiring the continued participation of immunogenic material in sensitized animals; it is not the reaction of preformed antibody-like material with the antigenic determinant.