Exploring ex vivo peptideolysis of thymopentin and lipid-based nanocarriers towards oral formulations.

The oral delivery of medicines is the most popular route of administration for patients. However, thymopentin (TP5) is only available in the market in forms for parenteral administration. In large part, this is because of extensive peptidolytic degradation in the gastrointestinal tract (GIT), which decreases the amount of TP5 available for absorption. This study aims to understand the extent of TP5 peptideolysis and determine effective inhibitors and suitable lipid-based nanocarriers to aid in the development of an effective oral delivery formulation. Enzymatic degradation kinetics of TP5 was investigated in the presence or absence of mucosal and luminal components extracted from various parts of the rat intestine, including the duodenum, jejunum, ileum, and colon. Inhibition of TP5 enzymatic peptidolysis was screened in the presence or absence of EDTA, trypsin and chymotrypsin inhibitors from soybean (SBTCI), and bestatin. TP5 with SBTCI was loaded into lipid-based nanocarriers, including microemulsions, niosomes and solid lipid nanoparticles. These TP5-loaded nanocarriers were investigated through characterization of morphology, particle size, zeta potential, entrapment efficacy (EE%), and ex vivo rat intestinal degradation studies to select a lead formulation for a future oral drug delivery study. The degradation kinetics of TP5 followed pseudo-first-order kinetics, and the biological metabolism of TP5 was displayed in the presence of luminal contents, indicating that TP5 is sensitive to luminal enzymes. Notably, a considerable decrease in TP5 peptidolysis was found in the presence of SBTCI, bestatin, and EDTA. TP5 and SBTCI were loaded into three lipid-based delivery systems, displaying superior protection under ex vivo intestinal luminal contents and mucosal homogenates for 6 h compared with the pure drug solution. These findings suggest that using select inhibitors and lipid-based nanocarriers can decrease peptide degradation and may improve oral bioavailability of TP5 following oral administration.

Preformulation studies of thymopentin: analytical method development, physicochemical properties, kinetic degradation investigations and formulation perspective.

Thymopentin (TP5) is a synthetic pentapeptide with immunomodulatory properties. Given the previously described poor absorption of TP5, preformulation data is required to support effective formulation development. In this manuscript, an analytical method of TP5 was developed and validated to determine the aqueous solubility, stability, and Log P of TP5. Thermal properties were investigated, and chemical, physical and enzymatic degradation were evaluated. TP5 was informed to load in a microemulsion (ME) system according to the preformulation parameters and characterized for rheological behavior, droplet size, morphology and in vitro drug release. TP5 displayed high aqueous solubility (294.3 mg/mL), low Log P (-4.2) and 2% water content with a melting temperature of 193 °C. TP5 degraded rapidly in alkaline conditions, at elevated temperature, in oxidizing agents, and with UV exposure, however TP5 had a longer half-life in acidic conditions. The fastest enzymatic degradation was with Trypsin (half-life 6.3 h) compared with other digestive enzymes. The different degradation pathways followed first-order kinetics, and half-lives were obtained from the kinetic studies. The TP5 loaded ME exhibited a droplet size of 143 ± 35 nm with a Higuchi-model fitted sustained release profile for 24 h. These data justify and support the design of formulations to stabilize and enhance the absorption of TP5, with a ME formulation demonstrated.

A biuret-derived, MS-cleavable cross-linking reagent for protein structural analysis: A proof-of-principle study.

Chemical cross-linking combined with mass spectrometry (XL-MS) and computational modeling has evolved as an alternative method to derive protein 3D structures and to map protein interaction networks. Special focus has been laid recently on the development and application of cross-linkers that are cleavable by collisional activation as they yield distinct signatures in tandem mass spectra. Building on our experiences with cross-linkers containing an MS-labile urea group, we now present the biuret-based, CID-MS/MS-cleavable cross-linker imidodicarbonyl diimidazole (IDDI) and demonstrate its applicability for protein cross-linking studies based on the four model peptides angiotensin II, MRFA, substance P, and thymopentin.

Thymopentin-loaded phospholipid-based phase separation gel with long-lasting immunomodulatory effects: in vitro and in vivo studies.

Thymopentin (TP5) is an effective immunomodulatory agent for autoimmune disease that has been used clinically for decades. However, its application is greatly limited by its extremely short half-life in vivo, poor membrane permeability and extensive metabolism in gastrointestinal tract, resulting in repeated injection and poor patient compliance. In the present study, we developed a TP5-loaded, phospholipid-based phase separation gel (PPSG) to achieve sustained drug release profile and long-lasting therapeutic effects. We firstly demonstrated the physiochemical characteristics of PPSG before and after phase transition by examining the viscosity and morphology change caused by the phase transition. Moreover, the PPSG exerted a low cytotoxicity in L929 cells and HUVECs, suggesting the biocompatibility of PPSG. A month-long drug release profile of TP5 PPSG was observed both in vitro and in vivo, revealing its sustained and controlled drug release property. Most importantly, in cyclophosphamide-induced immunosuppressive rats, a single dose of TP5 PPSG (15 mg/kg, sc) injected could normalize their T-SOD levels and CD4+/CD8+ ratio; such an immunoregulatory effect was comparable to that produced by repeated injection of TP5 solution (0.6 mg/kg per day, sc) for 14 consecutive days. Thus, TP5 PPSG has a great potential for sustained delivery of TP5 in clinical use because of its simple manufacture process, good biocompatibility and long-lasting immunomodulatory efficacy, which could greatly improve patient compliance.

A Novel MS-Cleavable Azo Cross-Linker for Peptide Structure Analysis by Free Radical Initiated Peptide Sequencing (FRIPS).

The chemical cross-linking/mass spectrometry (MS) approach is a growing research field in structural proteomics that allows gaining insights into protein conformations. It relies on creating distance constraints between cross-linked amino acid side chains that can further be used to derive protein structures. Currently, the most urgent task for designing novel cross-linking principles is an unambiguous and automated assignment of the created cross-linked products. Here, we introduce the homobifunctional, amine-reactive, and water soluble cross-linker azobisimidoester (ABI) as a prototype of a novel class of cross-linkers. The ABI-linker possesses an innovative modular scaffold combining the benefits of collisional activation lability with open shell chemistry. This MS-cleavable cross-linker can be efficiently operated via free radical initiated peptide sequencing (FRIPS) in positive ionization mode. Our proof-of-principle study challenges the gas phase behavior of the ABI-linker for the three amino acids, lysine, leucine, and isoleucine, as well as the model peptide thymopentin. The isomeric amino acids leucine and isoleucine could be discriminated by their characteristic side chain fragments. Collisional activation experiments were conducted via positive electrospray ionization (ESI) on two Orbitrap mass spectrometers. The ABI-mediated formation of odd electron product ions in MS/MS and MS3 experiments was evaluated and compared with a previously described azo-based cross-linker. All cross-linked products were amenable to automated analysis by the MeroX software, underlining the future potential of the ABI-linker for structural proteomics studies. Graphical Abstract ᅟ.

Myristic acid-modified thymopentin for enhanced plasma stability and immune-modulating activity.

Natural albumin ligand (fatty acids)-conjugated peptides can rapidly bind to circulating albumin and form complexes to control the release of peptides. The purpose of this study was to prolong the half-life and immune-modulating effects of thymopentin (TP5) by using the albumin binding strategy. We synthesized myristic acid-modified TP5 (TP5-MA) by conjugating a myristic acid-acylated lysine to a permissive site of TP5, which improved the albumin binding affinity of TP5-MA and dramatically enhanced its stability in human plasma. We observed well-preserved bioactivities of TP5-MA in RAW264.7 macrophages using a tumor necrosis factor (TNF)-α stimulation assay. Moreover, the prolonged immune-modulating effect of TP5-MA was confirmed by the normalized CD4+/CD8+ ratio in immune-depressed rat models, which resulted in a reduced administration frequency (twice per week). In general, the enhanced pharmacokinetic and pharmacodynamic properties of TP5-MA make it a promising product for the treatment of immunodeficiency diseases.

A novel accelerated in vitro release method to evaluate the release of thymopentin from PLGA microspheres.

A novel accelerated method of good correlations with "real-time" release to evaluate in vitro thymopentin release from poly (D, L-lactide-co-glycolide) (PLGA) microsphere was developed. Thymopentin-loaded microspheres were made from three types of PLGA, and peptide release was studied in various conditions. Incomplete release of peptide (<60%) from microspheres was found in accelerated testing with two typical release media. This problem was circumvented by adding organic solvents to the release media and varying the temperature in the media heating process. Release media containing three kinds of organic solvents at 50 °C were tested, respectively, and hydro-alcoholic solution was selected for further study. After the surfactant concentration (0.06%, W/V) and ethanol concentration (20%, V/V) were fixed, a gradient heating program, consisting of three stages and each stage with a different temperature, was introduced to enhance the correlations between the short- and long-term release. After adjusting the heating time of each stage, a good correlation (R(2) = 9896, formulation 8 K; R(2) = 0.9898, formulation 13 K; R(2) = 0.9886, formulation 28 K) between accelerated and "real-time" release was obtained. By optimizing the conditions as ethanol concentration and temperature gradients, this accelerated method may be appropriate for similar peptide formulations that not well correlate with "real-time" release.

A new method for peptide synthesis in the N→C direction: amide assembly through silver-promoted reaction of thioamides.

The Ag(I)-promoted coupling of amino acids and peptides with amino ester thioamides generates peptide imides without epimerisation. The peptide imides undergo regioselective hydrolysis under mild conditions to generate native peptides. This method was employed to prepare the pentapeptide thymopentin in the N→C direction, in high yield and purity.

Increased antitumor activity of tumor-specific peptide modified thymopentin.

Thymopoietin pentapeptide (thymopentin, TP5), an immunomodulatory peptide, has been successfully used as an immune system enhancer for treating immune deficiency, cancer, and infectious diseases. However, poor penetration into tumors remains a key limitation to the efficacy and application of TP5. iRGD (CRGDK/RGPD/EC) has been introduced to certain anticancer agents, and increased specific tumor penetrability of drugs and cell internalization have been observed. In the present study, we fused this iRGD fragment with the C-terminal of TP5 to yield a new product, TP5-iRGD. Cell attachment assay showed that TP5-iRGD exhibits more extensive attachment to the melanoma cell line B16F10 than wild-type TP5. Tumor cell viability assay showed that iRGD conjugation with the TP5 C-terminus increases the basal antiproliferative activity of the pentapeptide against the melanoma cell line B16F10, the human lung cancer cell line H460, and the human breast cancer cell line MCF-7. Subsequent injections of TP5-iRGD inhibited in vivo melanoma progression more efficiently than the native TP5. Murine spleen lymphocyte proliferation assay also showed that TP5-iRGD and the parent pentapeptide feature nearly identical spleen lymphocyte proliferation activities. We built an integrin αvß3 and TP5-iRGD computational binding model to investigate the mechanism by which TP5-iRGD promotes increased activity further. Conjugation with iRGD promotes binding to integrin αvß3, thereby increasing the tumor-homing efficiency of the resultant peptide. These experimental and computational observations of increased TP5-iRGD activity help broaden the usage of TP5 and reflect the great application potential of the peptide as an anticancer agent.

Molecular ageing: free radical initiated epimerization of thymopentin--a case study.

The epimerization of amino acid residues increases with age in living organisms. In the present study, the structural consequences and thermodynamic functions of the epimerization of thymopentin (TP-5), the active site of the thymic hormone thymopoietin, were studied using molecular dynamics and density functional theory methods. The results show that free radical-initiated D-amino acid formation is energetically favoured (-130 kJmol(-1)) for each residue and induces significant changes to the peptide structure. In comparison to the wild-type (each residue in the L-configuration), the radius of gyration of the D-Asp(3) epimer of the peptide decreased by 0.5 Å, and disrupted the intramolecular hydrogen bonding of the native peptide. Beyond establishing important structural, energetic and thermodynamic benchmarks and reference data for the structure of TP-5, these results disseminate the understanding of molecular ageing, the epimerization of amino acid residues.

Influence of potential inhalation carriers on stability of thymopentin in rat bronchoalveolar lavage fluid.

In the present study, the stability of thymopentin (TP5) in bronchoalveolar lavage fluid (BALF) in presence of potential excipients in inhalation formulation was investigated. The content of TP5 was determined using HPLC method. Commonly used bulking agent, dispersibility enhancers and absorption enhancers in inhalation were investigated with respect to the stability of TP5 in BALF. Finally, the stability of TP5 in two inhalation formulations based on the screening experiments was tested in BALF. The results showed that TP5 alone degraded very rapidly in BALF and zero-order enzymatic reaction with a half-life of t0.5 = 49.20 min was observed using 10 times diluted BALF. Among the amino acids examined, leucine and phenylalanine effectively inhibited the enzymolysis of TP5 with prolonged half-life of 112.7 min and 136.2 min, respectively. Nevertheless, slight but insignificant inhibition effect was witnessed for tyrosine, aspartic acid, and lysine; and negligible prevention on the degradation process of TP5 were found for lactose and mannitol. Regarding chitosan, irrespective with molecular weight, the formation of chitosan-TP5 complex improved the stability of TP5 with prolonged t0.5 by 1.8 times. However, along with the improved stability of TP5 in spray-dried chitosan microspheres, the content of TP5 in formulations was reduced to about 75% during preparation process. Thus, leucine was proved to be a prior candidate for inhalation formulation of TP5. Consequently, the results indicate the potential of leucine as carrier for pulmonary delivery of TP5 serving as both stabilizer and dispersibility enhancer.

Thymopentin nanoparticles engineered with high loading efficiency, improved pharmacokinetic properties, and enhanced immunostimulating effect using soybean phospholipid and PHBHHx polymer.

Formulation of protein and peptide drugs with sustained release properties is crucial to enhance their therapeutic effect and minimize administration frequency. In this study, immunomodulating polymeric systems were designed by manufacturing PHBHHx nanoparticles (NPs) containing thymopentin (TP5). The release profile of the drug was studied over a period of 7 days. The PHBHHx NPs containing TP5-phospholipid (PLC) complex (TP5-PLC) displayed a spherical shape with a mean size, zeta potential, and encapsulation efficiency of 238.9 nm, -32.0 mV, and 72.81%, respectively. The cytotoxicity results showed the PHBHHx NPs had a relatively low toxicity in vitro. TP5 entrapped in the NPs could hardly release in vitro, while the NPs had longer than 7 days release duration after a single subcutaneous injection in Wistar rats. The immunodepression rat model was built to evaluate the immunomodulating effects of TP5-PLC-NPs in vivo. The results of T-lymphocyte subsets (CD3(+), CD4(+), CD8(+), and CD4(+)/CD8(+) ratio) analysis and superoxide dismutase (SOD) values suggested that TP5-PLC-NPs had stronger immunoregulation effects than TP5 solution. In conclusion, an applicable approach to markedly enhancing the loading of a water-soluble peptide into a hydrophobic polymer matrix has been introduced. Thus, TP5-PLC-NPs are promising nanomedicine systems for sustained release effects of TP5.

Synthetic modifications of the immunomodulating peptide thymopentin to confer anti-mycobacterial activity.

Effective global control of tuberculosis (TB) is increasingly threatened by the convergence of multidrug-resistant TB and the human immunodeficiency virus (HIV) infection. TB/HIV coinfections exert a tremendous burden on the host's immune system, and this has prompted the clinical use of immunomodulators to enhance host defences as an alternative therapeutic strategy. In this study, we modified the clinically used synthetic immunomodulatory pentapeptide, thymopentin (TP-5, RKDVY), with six arginine residues (RR-6, RRRRRR) at the N- and C-termini to obtain the cationic peptides, RR-11 (RKDVYRRRRRR-NH2) and RY-11 (RRRRRRRKDVY-NH2), respectively. The arginine residues conferred anti-mycobacterial activity to TP-5 in the peptides as shown by effective minimum inhibitory concentrations of 125 mg/L and killing efficiencies of >99.99% against both rifampicin-susceptible and -resistant Mycobacterium smegmatis. The immunomodulatory action of the peptides remained unaffected as shown by their ability to stimulate TNF-α production in RAW 264.7 mouse macrophage cells. A distinct change in surface morphology after peptide treatment was observed in scanning electron micrographs, while confocal microscopy and dye leakage studies suggested bacterial membrane disruption by the modified peptides. The modified peptides were non-toxic and did not cause hemolysis of rat red blood cells up to a concentration of 2000 mg/L. Moreover, RY-11 showed synergism with rifampicin and reduced the effective concentration of rifampicin, while preventing the induction of rifampicin resistance. The synthetic peptides may have a potential application in both immunocompetent and immunocompromised TB patients.

Synthesis of a precursor tripeptide Z-Asp-Val-Tyr-OH of thymopentin by chemo-enzymatic method.

The precursor tripeptide of thymopentin was synthesized by a combination of chemical and enzymatic methods. First, Val-Tyr-OH dipeptide was synthesized by a novel chemical method in two steps involving preparation of NCA-Val. Second, the linkage of the third amino acid Z-Asp-OMe to Val-Tyr-OH was completed by an enzymatic method under kinetic control. An industrial alkaline protease alcalase was used in water-organic cosolvent systems. The synthesis reaction conditions were optimized by examining the effects of several factors including organic solvents, water content, temperature, pH, and reaction time on the yield of Z-Asp-Val-Tyr-OH. The optimum condition is of pH 10.0, 35°C, acetonitrile/Na2CO3-NaHCO3 buffer system (85:15, v/v), and reaction time of 2.5 hr, which achieves tripeptide yield of more than 70%.

Intein-mediated expression, purification, and characterization of thymosin α1-thymopentin fusion peptide in Escherichia coli.

Thymosin α1-thymopentin (Tα1-TP5) fusion peptide has been proved to be an immune regulator based on its higher immunoregulatory activity than Tα1 and TP5. To obtain Tα1-TP5 more effectively and economically, Tα1-TP5 was genetically fused to a self-cleaving intein-chitin binding domain tag for purification via chitin beads in Escherichia coli. After affinity purification, the target peptide was released from the chitin beads via self-cleaving intein ((INTervening protEIN) induced by dithiothreitol. Further, Tα1-TP5 was purified by Superdex 30 and identified by Tricine-SDS-PAGE and electrospray ionization-mass spectrometry. Finally, about 7.6 mg Tα1-TP5 purified from the soluble fraction and inclusion bodies was obtained from 1 L culture media. The purity was 95% after a series of chromatographic purification steps. In vitro, the purified Tα1-TP5 could stimulate the proliferation of mouse splenic lymphocytes. Overall, this work demonstrated that Tα1-TP5 was purified with low cost and high efficiency, greatly expanding its potential use as an immune regulator.

Box-Behnken optimization design and enhanced oral bioavailability of thymopentin-loaded poly (butyl cyanoacrylate) nanoparticles.

This study was done to prepare thymopentin (TP5)-loaded poly (butyl cyanoacrylate) nanoparticles (TP5-PBCA-NPs) and evaluate thier efficacy for oral delivery. TP5-PBCA-NPs were prepared by emulsion polymerization, and the formulation was optimized based on Box-Behnken experimental design. The physico-chemical characteristics of TP5-PBCA-NPs were evaluated using transmission electron microscopy (TEM), malvern zetasizer, Fourier transform infra-red spectroscopy (FT-IR) and differential scanning calorimetry (DSC). The encapsulation efficiency, enzymatic degradation and release behavior of TP5-PBCA-NPs in various media were evaluated using a high-performance liquid chromatography (HPLC) method. The pharmacodynamic studies on oral administration of TP5-PBCA-NPs were performed in FACScan flow cytometry. An optimum formulation consisted of 0.7% poloxamer 188 (Pol), 0.6% dextran-70 (Dex), 0.1% sodium metabisulfite (Sm), 0.1% TP5 and 1% (v/v) n-butyl cyanoacrylate. The particle size and zeta potential of optimized TP5-PBCA-NPs was 212 nm and -22.6 mV respectively with 82.45% encapsulation efficiency. TP5 was entrapped inside the nanoparticles in molecular dispersion form. The release of TP5 from PBCA-NPs was pH dependent; the cumulative release percentage in 0.1 M HCI for 4 hours was less than 16% while it was more than 80% in pH6.8 PBS. The PBCA-NPs could efficiently protect TP5 from enzymatic degradation; the remained percentage of TP5 encapsulated in PBCA-NPs was 58.40% after incubated with trypsin in pH6.8 PBS for 4 h while it was only 32.29% for free drug. In the oral administration study in vivo, the lowered T-lymphocyte subsets values were significantly increased and the raised CD4+/CD8+ ratio was evidently reduced compared with that of TP5 solution (p < 0.05), and the improvement of bioavailability was dose-dependent. These results indicated that the PBCA nanoparticles may be a promising carrier for oral delivery of TP5.

Preparation, characterization and in vivo pharmacodynamic evaluation of thymopentin loaded poly(lactide acid)/poly(lactide-co-glycolide acid) implants.

To avoid the clinical inconvenience of repeated injection of the immune modulator thymopentin (TP5), biodegradable implants comprising a mixed polymer matrix of poly(lactide acid) (PLA) and poly(lactide-co-glycolide acid) (PLGA) were produced using a simple extrusion method. Drug release from these TP5-loaded implants was characterized both in vitro and in vivo. Pharmacodynamic studies were carried out in immunosuppressed rats using the ratio of CD4(+)/CD8(+) cells, determined by flow cytometry, as an index of immunity. The results indicated that the entrapment efficiency of the implants was greater than 98%, but the release rate of TP5 depended on the drug loading. Implants containing less than 10% TP5 showed consistent release over 30 days, with low burst-release both in vitro and in vivo. Improved immunity and survival rates were observed in rats treated by TP5 injection and in rats given middle-to-high dose implants. When the release of TP5 exceeded 0.1 mg/kg body weight/day the CD4(+)/CD8(+) ratios increased in the 3 weeks after implantation, reaching a maximum (91.6% of the normal level) by the end of the third week. The TP5-loaded implants presented here provide a promising alternative to injections and the results support the further development of controlled-release TP5 formulations.

Inhalable microparticles as carriers for pulmonary delivery of thymopentin-loaded solid lipid nanoparticles.

PURPOSE: Microparticles containing solid lipid nanoparticles (SLNs) are receiving increased attention as carriers for the lung delivery of the SLNs. Thus, we aim to prepare the hybrid microparticles and thoroughly evaluate their feasibility for the pulmonary drug delivery. METHODS: The microparticles were prepared by co-spray-drying the thymopentin (TP5)-loaded SLNs with bulking agents. Thereafter, we systematically estimated the potential of the microparticles as the carriers for the pulmonary delivery of the SLNs, including the investigations of their characteristics, aerodynamic properties, pharmacokinetics and pharmacodynamics. RESULTS: The spherical and hollow microparticles presented a size of 4.1 +/- 0.1 microm and a low tap density of 0.175 +/- 0.02 g/cm(3). In addition, the microparticles showed a high aerosolization efficiency (emitted dose of 98.0% +/- 1.23% and respirable fraction of 51.07% +/- 1.21%). Furthermore, the SLNs could be easily recovered from the microparticles without essential changes on their characteristics and the drug release behavior. The pharmacokinetic and pharmacodynamic studies suggested that, compared to i.v. TP5 solution, the bioavailability and therapeutic efficacy of TP5 were remarkably strengthened after the pulmonary administration of the microparticles. CONCLUSIONS: Taken together, we believe the microparticles were suitable for inhalation and possessed an ample potential for the pulmonary delivery of the SLNs.

Novel thymopentin release systems prepared from bioresorbable PLA-PEG-PLA hydrogels.

Copolymers were synthesized by ring opening polymerization of l- or d-lactide in the presence of dihydroxyl PEG with molar mass of 6000, 12,000 and 20,000, using zinc lactate as catalyst. Bioresorbable hydrogels were obtained by mixing PLLA-PEG-PLLA and PDLA-PEG-PDLA aqueous solutions due to stereocomplexation between PLLA and PDLA chains. Rheological measurements show that the hydrogels present typical viscoelastic behaviors, although degradation could occur during the gelation process. Thymopentin was taken as a model drug to evaluate the potential of PLA-PEG-PLA hydrogels as carrier of hydrophilic drugs. Various parameters such as copolymer concentration, drug load, copolymer composition and the difference between sol and gel were considered. The release profiles are characterized by an initial burst followed by slower release. Higher copolymer concentration leads to slower release rate and less burst effect due to more compact structure which disfavors drug diffusion. Similarly, higher molar mass of the copolymers disfavors the release of TP5, and hydrogels composed of both PLLA/PEG and PDLA/PEG present slower release rates than single copolymer solutions. In contrast, drug load exhibits little influence on the release profiles due to the high water solubility of TP5. In all cases, nearly 80% of TP5 is released. In vivo studies proved the potential of TP5 containing hydrogels, especially those with a concentration of 25%. Both the CD4(+)/CD8(+) ratio and the morphology of thymus indicate the immunization efficacy of the TP5 release systems based on PLA/PEG hydrogels.

Direct binding of thymopentin to surface class II major histocompatibility complex in living cells.

The molecular analysis of thymopentin (TP5)/class II major histocompatibility complex (MHC II) complexes has been basically understood; however, the mechanism by which TP5-MHC II complexes are formed is largely unexplored. Compared with Epstein-Barr virus (EBV)-transformed B cells expressing human leucocyte antigen DR (HLA-DR), no fluorescent signal was observed on the DR-deficient cell line. This indicates that FITC labeled TP5 (FITC-TP5) is genuinely bound to HLA-DR. The binding specificity was confirmed by incubating FITC-TP5 with unlabeled TP5 and HA peptide as well as mAb for DR molecules. In addition, the binding appeared to be rapid in living antigen-presenting cell (APC), which implies that TP5 is demonstrated on APC surface and does not require processing before associating with DR. Additional support for this surface binding arises from the observation that pretreatment of cells with a variety of metabolic inhibitors failed to decrease the level of TP5/DR complexes. However, temperature has an effect on the rate of binding between TP5 and DR molecules, which is well consistent with the qualitative predication of transition state theory. The formation of antigenic complexes is accelerated at acidic pH, which shows that the formation of TP5/DR complexes is a pH-dependent process.