A cyclic peptide accelerates the loading of peptide antigens in major histocompatibility complex class II molecules

Spectroscopic characterization of eupalitin-3-O-β-D-galactopyranoside from Boerhavia procumbens: In vivo hepato-protective potential in rat model

The liver is one of the most important organs responsible for detoxifying biomolecules and xenobiotics. Herein, we report the isolation, characterization, and hepatoprotective effect of the Boerhavia procumbens-derived eupalitin-3-O-β-D-galactopyranoside (EGP) compound. The structure of the EGP compound was deduced by using NMR spectroscopic techniques and mass spectrometry. The EGP hepatoprotective activities were evaluated with HepG2 cell viability and LDH assays in vitro, and CCl4-induced toxicity was investigated in vivo in the rat model. Compared to the CCl4-treated group, cells exposed to the EGP compound at 200 µg/ml showed increased cell viability (60.52 ± 1.22 %) and decreased LDH levels (23.81 ± 1.89 U/ml). The serum levels of SGPT, SGOT, ALP, and total bilirubin in the CCl4-treated group were substantially higher than those in the control group (64 ± 1.89 U/ml, 86 ± 1.47 U/ml, 252.6 ± 2.96 U/ml, and 5.45 ± 0.32 mg/dl, respectively). When compared to animals treated with CCl4 alone, the EGP compound's in vivo hepatoprotective effect at 60 mg/kg with CCl4 significantly (p < 0.01) decreased the levels of SGPT and SGOT (26 ± 1.34 U/ml and 42.92 ± 1.6 U/ml) respectively. Furthermore, our histological study showed a significant response in restoring and maintaining the normal morphological appearance of the liver. Thus, our results show that the EGP compound is a promising and novel lead molecule for better hepatotoxicity control and therapy.

Small organic molecules accelerate the expansion of regulatory T cells

The regulatory T cells (T_reg cells) expressing CD4 + CD25 + FOXP3 + markers are indispensable for the initiation of immune homeostasis and tolerance to self-antigens in both mice and humans. A decrease in regulatory T cells leads to various autoimmune pathologies. Herein, we report three low molecular weight, small organic molecules as a new series of T_reg proliferators TRP-1-3. These small molecules were tested for their proliferative effect on regulatory T cells. It was found that TRP-1 (Oleracein E) strongly accelerates the T_reg proliferation in vitro in a concentration-dependent manner. The effect was evident for all subsets of T_reg cells tested, including naturally occurring, thymus-derived and peripherally-induced or adaptive T_reg, indicating an effect independent of the maturation site. Importantly, increased T_reg cells numbers by TRP-1 correlated with improved CD4 + CD25 + FOXP3 + expression in vitro, while propidium iodide-based staining showed low TRP-1-induced cytotoxicity. Molecular docking plus simulation studies of these TRP-1-3 with IL-2R, mTOR and TCR receptors suggest a TCR-based T_reg cells activation mechanism. Because of its high T_reg cells activities and low cellular cytotoxicity, TRP-1-3 may be useful in stimulating ex-vivo/in-vivo, T_reg cell-specific responses for therapeutic applications.

Mechanistic understanding and significance of small peptides interaction with MHC class II molecules for therapeutic applications

Major histocompatibility complex (MHC) class II molecules are expressed by antigen-presenting cells and stimulate CD4(+) T cells, which initiate humoral immune responses. Over the past decade, interest has developed to therapeutically impact the peptides to be exposed to CD4(+) T cells. Structurally diverse small molecules have been discovered that act on the endogenous peptide exchanger HLA-DM by different mechanisms. Exogenously delivered peptides are highly susceptible to proteolytic cleavage in vivo; however, it is only when successfully incorporated into stable MHC II-peptide complexes that these peptides can induce an immune response. Many of the small molecules so far discovered have highlighted the molecular interactions mediating the formation of MHC II-peptide complexes. As potential drugs, these small molecules open new therapeutic approaches to modulate MHC II antigen presentation pathways and influence the quality and specificity of immune responses. This review briefly introduces how CD4(+) T cells recognize antigen when displayed by MHC class II molecules, as well as MHC class II-peptide-loading pathways, structural basis of peptide binding and stabilization of the peptide-MHC complexes. We discuss the concept of MHC-loading enhancers, how they could modulate immune responses and how these molecules have been identified. Finally, we suggest mechanisms whereby MHC-loading enhancers could act upon MHC class II molecules.

Total synthesis, structural, and biological evaluation of stylissatin A and related analogs

The natural product cyclic peptide stylissatin A (1a) was reported to inhibit nitric oxide production in LPS-stimulated murine macrophage RAW 264.7 cells. In the current study, solid-phase total synthesis of stylissatin A was performed by using a safety-catch linker and yielded the peptide with a trans-Phe(7) -Pro(6) linkage, whereas the natural product is the cis rotamer at this position as evidenced by a marked difference in NMR chemical shifts. In order to preclude the possibility of 1b being an epimer of the natural product, we repeated the synthesis using d-allo-Ile in place of l-Ile and a different site for macrocyclization. The resulting product (d-allo-Ile(2) )-stylissatin A (1c) was also found to have the trans-Phe(7) -Pro(6) peptide conformations like rotamer 1b. Applying the second route to the synthesis of stylissatin A itself, we obtained stylissatin A natural rotamer 1a accompanied by rotamer 1b as the major product. Rotamers 1a, 1b, and the epimer 1c were separable by HPLC, and 1a was found to match the natural product in structure and biological activity. Six related analogs 2-7 of stylissatin A were synthesized on Wang resin and characterized by spectral analysis. The natural product (1a), the rotamer (1b), and (d-allo-Ile(2) )-stylissatin A (1c) exhibited significant inhibition of NO(.) . Further investigations were focused on 1b, which also inhibited proliferation of T-cells and inflammatory cytokine IL-2 production. The analogs 2-7 weakly inhibited NO(.) production, but strongly inhibited IL-2 cytokine production compared with synthetic peptide 1b. All analogs inhibited the proliferation of T-cells, with analog 7 having the strongest effect. In the analogs, the Pro(6) residue was replaced by Glu/Ala, and the SAR indicates that the nature of this residue plays a role in the biological function of these peptides. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

On Peptides and Altered Peptide Ligands: From Origin, Mode of Action and Design to Clinical Application (Immunotherapy)

T lymphocytes equipped with clonotypic T cell antigen receptors (TCR) recognize immunogenic peptides only when presented in the context of their own major histocompatibility complex (MHC) molecules. Peptide loading to MHC molecules occurs in intracellular compartments (ER for class I and MIIC for class II molecules) and relies on the interaction of the respective peptides and peptide binding pockets on MHC molecules. Those peptide residues not engaged in MHC binding point towards the TCR screening for possible peptide MHC complex binding partners. Natural or intentional modification of both MHC binding registers and TCR interacting residues of peptides - leading to the formation of altered peptide ligands (APLs) - might alter the way peptides interact with TCRs and hence influence subsequent T cell activation events, and consequently T cell effector functions. This review article summarizes how APLs were detected and first described, current concepts of how APLs modify T cellular signaling, which biological mechanisms might force the generation of APLs in vivo, and how peptides and APLs might be used for the benefit of patients suffering from allergic or autoimmune diseases.