Synthetic peptide libraries in the determination of T cell epitopes and peptide binding specificity of class I molecules.

Major histocompatibility complex (MHC) class I molecules combine with short peptides of defined length and sequence. Here we describe an approach that may be used in the analysis of peptide preference of different allelic MHC class I molecules, and in the determination of T cell epitopes. We produced synthetic "peptide libraries" of limited complexity by standard peptide chemistry. Using these peptide mixtures we show that H-2 Kb molecules can accommodate both 8- and 9-residue peptides, whereas Db molecules are unable to combine with peptides shorter than 9 amino acids present in these libraries. When these peptide mixtures are used to provide "fingerprints" of Db molecules and mutants thereof, both loss and gain of the ability to combine with certain peptides is observed. For the Kbm1 mutant a strong influence of amino acid substitutions in class I molecules on the peptides selected is observed. In these synthetic peptide mixtures, the presence of a specific T cell epitope, known to be represented once, can be detected. This approach may be extended to the identification of new T cell epitopes from larger peptide libraries.

Peptide selection by MHC class I molecules.

Synthetic peptides have been used to sensitize target cells and thereby screen for epitopes recognized by T cells. Most epitopes of cytotoxic T lymphocytes can be mimicked by synthetic peptides of 12-15 amino acids. Although in specific cases, truncations of peptides improves sensitization of target cells, no optimum length for binding to major histocompatibility complex (MHC) class I molecules has been defined. We have now analysed synthetic peptide captured by empty MHC class I molecules of the mutant cell line RMA-S. We found that class I molecules preferentially bound short peptides (nine amino acids) and selectively bound these peptides even when they were a minor component in a mixture of longer peptides. These results may help to explain the difference in size restriction of T-cell epitopes between experiments with synthetic peptides and those with naturally processed peptides.

Cisplatin-induced changes of selenium levels and glutathione peroxidase activities in blood of testis tumor patients.

Haematocrit and glutathione peroxidase activity in blood, as well as selenium levels in blood, erythrocytes and plasma, were determined in 15 patients during four courses of cisplatin combination treatment for testicular teratoma. The haematocrit steadily declined, necessitating frequent blood transfusions during or after treatment. For patients without blood transfusions during treatment the reduction of the haematocrit averaged 40%. Glutathione peroxidase activity in blood declined also; for patients without blood transfusion the reduction was 30%, which is fully explained by the decrease of the haematocrit. The enzyme activity per volume of erythrocytes remained constant during the treatment. Erythrocyte selenium level did not change significantly, but plasma selenium levels of all patients dropped within each course of chemotherapy, and progressively with each subsequent course. Between cycles the levels were largely restored to almost normal values. These results may be explained by a decreasing availability of selenium in the body to maintain the normal plasma level, due to increased retention of cisplatin in tissues and subsequent alteration of selenium metabolism.

Selenium levels in blood and plasma, and glutathione peroxidase activity in blood of breast cancer patients during adjuvant treatment with cyclophosphamide, methotrexate and 5-fluorouracil.

Glutathione peroxidase activity in whole blood and selenium levels in whole blood and plasma from breast cancer patients, were measured during combination chemotherapy with cyclophosphamide, methotrexate and 5-fluorouracil. No significant change in either glutathione peroxidase activity or selenium levels was observed. Comparison with matched controls showed no significant differences for either parameter.

Inhibition of amino acid incorporation in a cell-free system and inhibition of protein synthesis in cultured cells by reaction products of selenite and thiols.

Reaction products of selenite with thiols were tested for an inhibitory effect on amino acid incorporation in a cell-free system derived from rat liver and on protein synthesis in intact P815 and L1210 cells. In the cell-free system maximum inhibition, up to 96%, was reached at about 10 microM selenium. In intact cells inhibitory effect varied depending on which reaction product or cell line was used. Maximum inhibition was obtained after 30 min of incubation with selenium concentrations ranging from 0.25 microM to over 7 microM. Selenite itself also inhibited protein synthesis of L1210 cells, but only after 90 min of incubation and starting at selenium concentrations of 2 microM. Inhibition of protein synthesis in intact cells was followed by cell death. Pre-incubation of the reaction products of a monothiol (2-propanethiol) and of a vicinal dithiol (2,3-dimercapto-1-propanol) in culture medium showed a rapid decrease of the inhibitory capability of the product from the monothiol, but not of the product from the dithiol. The results indicate that selenite and a thiol react to form products which have differential toxic effects to cells in vitro.

Studies on the inhibition of protein synthesis by selenodiglutathione.

Amino acid incorporation in a cell-free system derived from rat liver has previously been found to be inhibited by GSSeSG (selenodiglutathione). In the present experiments the effect of GSSeSG on protein synthesis in 3T3-f cells, on growth and protein synthesis in Escherichia coli, and on amino acid incorporation in a cell-free system derived from E. coli, was studied. GSSeSG inhibits the incorporation of [3H]leucine into protein by 3T3-f cells. This inhibition cannot be reversed by removing GSSeSG and is correlated with the uptake of GSSeSG. Sodium selenite (Na2SeO3) and oxidized glutathione had no inhibitory effect in this system. [3H]Uridine or [3H]thymidine incorporation into RNA or DNA was not inhibited, indicating that the primary action of GSSeSG was on protein synthesis. GSSeSG did not influence the growth of E. coli in a synthetic medium, although enhanced amino acid incorporation was observed. In the cell-free system derived from E. coli, amino acid incorporation was not changed by GSSeSG, indicating that elongation factor G, in contrast to elongation factor 2 of mammalian cell systems, is not blocked by GSSeSG.

Amino acid incorporation in a cell-free system derived from rat liver studied with the aid of selenodiglutathione.

Selenodiglutathione (GSSeSG), a potent inhibitor of elongation factor 2 (EF2) has been used to study amino acid incorporation in a rat liver cell-free system. While translocation of the ribosomes was inhibited by GSSeSG, ribosomes with a free acceptor site were still capable of incorporating one amino acid residue. From this the average number of amino acids incorporated per ribosomes was calculated to be 2--5. In this respect virtually no difference has been observed between ribosomes present on small or large aggregates. The time required for one translocation by all active ribosomes, and the time required for the incorporation of one amino acid (starting with aminoacyl-tRNA or amino acids) has also been determined. By incubation under conditions for amino acid incorporation, part of the ribosomes were completely inactivated whereas the rest remained as active as at the start of the incubation.

Elongation factor 2 as the target of the reaction product between sodium selenite and glutathione (GSSeSG) in the inhibiting of amino acid incorporation in vitro.

The product of the reaction between sodium selenite and glutathione, designated as selenodiglutathione (GSSeSG), nearly completely inhibits amino acid incorporation from [14C]leucyl-tRNA by free polyribosomes isolated from rat liver. The mechanism of this inhibition was studied on the basis of the following three findings. Glutathione decomposes GSSeSG to harmless products; GSSeSG acts instantaneously on some component of the complete incubation system during preparation of the incubation vessels (at 0 degrees C); once GSSeSG has reacted its inhibitory effect cannot be reversed by glutathione. Accordingly, the effect of GSSeSG on the various steps of the amino acid incorporation process was studied by varying the sequence of additions of the reaction components, GSSeSG and GSH. The results of these and other experiments showed elongation factor 2 to be target of GSSeSG. The GSSeSG-B blocked factor could be regenerated by reduction with glutathione reductase and NADPH.