New conformationally restricted analog of the immunosuppressory mini-domain of HLA-DQ and its biological properties

A novel immunosuppressory peptide originating from the ubiquitin sequence

Ubiquitin is a conservative polypeptide present in every eukaryotic cell. Apart from its involvement in proteasomal degradation and other intracellular signal pathways, it was suggested to play an important role as the extracellular immunomodulator and antimicrobial agent. Moreover, ubiquitin-derived peptides were shown to express significant biological activities. Our previous studies showed a high immunosuppressive potency of the ubiquitin peptic hydrolysate in which we identified over 70 different peptides. The present work focuses on synthesizing the most abundant of these peptides and investigating their immunomodulatory potency. The peptide VKTLTGKTI possessed the highest immunosuppressory activity in AFC experiments, comparable to the previously described LEDGRTLSDY sequence (a previously discovered ubiquitin-derived peptide). Moreover, some of the investigated peptides expressed immunostimulatory effects. These findings support the idea that ubiquitin, together with products of its degradation, could represent a self-regulating immunoregulatory system. Peptide VKTLTGKTI was also tested for its activity to prolong the skin graft survival in mice. The results showed that the investigated peptide significantly extended the skin transplant rejection time, therefore it could be considered as a potential supplementary medicine in the post-transplantation therapy. Moreover, we synthesized two analogs of investigated peptides, first designed to mimic the non-linear epitope consisting of ubiquitin 16-21 and ubiquitin 52-57 fragments, and second designed to mimic the ubiquitin 5-13 hairpin. We also tested their immunosuppressory activity in in vitro experiments.

Effects of peptides, with emphasis on feeding, pain, and behavior A 5-year (1999-2003) review of publications in Peptides

Novel effects of naturally occurring peptides are continuing to be discovered, and their mechanisms of actions as well as interactions with other substances, organs, and systems have been elucidated. Synthetic analogs may have actions similar or antagonistic to the endogenous peptides, and both the native peptides and analogs have potential as drugs or drug targets. The journal Peptides publishes many leading articles on the structure-activity relationship of peptides as well as outstanding reviews on some families of peptides. Complementary to the reviews, here we extract information from the original papers published during the past five years in Peptides (1999-2003) to summarize the effects of different classes of peptides, their modulation by other chemicals and various pathophysiological states, and the mechanisms by which the effects are exerted. Special attention is given to peptides related to feeding, pain, and other behaviors. By presenting in condensed form the effects of peptides which are essential for systems biology, we hope that this summary of existing knowledge will encourage additional novel research to be presented in Peptides.

Threonine at position 6 is not essential for the immunosuppressive activity of HLA-DQ(β164–172)-hexapeptide

It has been previously found that the nonapeptide fragment of human leukocyte antigen (HLA)-DQ molecule, located in the beta chain 164-172 with the Thr-Pro-Gln-Arg-Gly-Asp-Val-Tyr-Thr sequence, suppresses the immune response. The hexapeptide: Arg-Gly-Asp-Val-Tyr-Thr was the shortest fragment of HLA-DQ showing both cellular and humoral immunosuppressive activity, while the analog deprived of the last amino acid (Arg-Gly-Asp-Val-Tyr) showed very weak stimulatory activity with respect to the humoral immune response. This suggested that the threonine residue in the hexapeptide plays an essential role in immunosuppression. In this study, the role of the side chain of threonine residue was scrutinized in a series of synthetic analogs in which the Thr residue was substituted by various amino acids, amides and methyl ester. The synthesized peptides were evaluated for their immunosuppressive activity. Our results indicate that the substitutions did not significantly affect the immunomodulatory properties, revealing that the threonine side chain is not critical for the immunosuppressive potency of the peptides. Interestingly, a simple analogue, pentapeptide amide H-Arg-Gly-Asp-Val-Tyr-NH2 possessed high immunosuppressive potency, comparable to that of cyclosporine.

Dimerization of the immunosuppressive peptide fragment of HLA-DR molecule enhances its potency

Our previous studies revealed that the nonapeptide fragment of HLA-DR molecule, located in the beta chain 164-172 with the VPRSGEVYT sequence, suppresses the immune responses. The sequence is located on the exposed molecule loop, therefore it may be involved in the interactions with other proteins. We suggested that the loop may serve as a functional epitope on the HLA class II surface for intermolecular binding, and that possible mechanism of biological action of the synthesized peptides is associated with interfering of adhesion of HLA class II molecules to their coreceptors. It has been postulated that oligomerization of the coreceptors is required for stable binding to class II HLA. Based on the crystal dimeric structure of HLA-DR molecules, we designed, and synthesized molecules able to induce the putative coreceptors dimerization. The synthesized series of compounds consisted of two VPRSGEVYT sequences linked through their C-termini by spacers of different length: (VPRSGEVYTGn)2K-NH2 ( n = 4-6). The results demonstrate that the dimerization of the nonapeptide fragment of HLA-DR results in enhanced immunosuppressory properties.

Immunosuppressory activity of ubiquitin fragments containing retro-RGD sequence

A peptide fragment corresponding to the ubiquitin(50-59) sequence (LEDGRTLSDY) (U50-59) possesses a very high immunosuppressory activity, comparable to that of cyclosporine, both in the cellular and humoral immune responses. We found that the pentapeptide DGRTL (U52-56) is the shortest, effective immunosuppressory fragment of ubiquitin, although its potency is weaker than that of U50-59. Replacement of each consecutive residue with alanine in U52-56 allowed identification of essential amino acids involved in the immunosuppression. We also evaluated the roles of its N- and C-terminal groups by their acetylation and/or amidation, respectively. The active sequence is located in the external loop of the molecule and therefore it may serve as an important functional epitope for intermolecular binding. Based on the crystal structure of ubiquitin molecule, we designed and synthesized the cyclic analogue with a restricted conformation, cyclo(Glt-Gln-Leu-Glu-Asp-Gly-Arg-Thr-Leu-Ser-Asp-Lys)-NH2 (Glt = glutaryl) by reacting the C-terminal Lys side chain with the glutarylated N-terminus. The peptide was designed to mimic the ubiquitin(48-59) loop, in order to obtain the ligand that may interact with hypothetical receptors of the loop. The cyclization product selectively but strongly suppresses the cellular immune response. The results indicate that the 48-59 loop may serve as an important functional epitope in the ubiquitin molecule for intermolecular binding.