Based on the previous experience in experimental animals and man, it may be concluded that TLI provides a useful method of immunosuppression and, under certain conditions, induces permanent and specific tolerance to BM and organ allografts, even across major histocompatibility barriers. Experience regarding the applicability of TLI to clinical BMT for malignant as well as nonmalignant disorders, as an adjunct to other immunosuppressive treatment modalities, is preliminary, but appears encouraging. Further studies are necessary to assess the relative advantages over the alternative methods of immunosuppression currently available.
Thymopoietin is a polypeptide hormone of the thymus consisting of 49 amino acids. The pentapeptide thymopentin (TP-5) Arg-Lys-Asp-Val-Tyr, corresponding to amino acids 32-36 of thymopoietin, appears to represent the active site of thymopoietin in that it has all the biological activities of the native hormone. Thymopoietin is secreted by epithelial cells of the thymus and is pleiotropic in action, affecting neuromuscular transmission, induction of early T cell differentiation and immune regulation. The immuno-regulatory actions of thymopentin on peripheral T cells are mediated by intracellular cyclic GMP elevations in contrast to the intracellular cyclic AMP elevations induced in precursor T cells that trigger their further differentiation to T cells. Thymopoietin and thymopentin have the biological characteristics of being immunonormalizing in a number of animal model systems of immune dysbalance. These include immune dysbalances induced by thymectomy or the thymic involution associated with aging or by other procedures in thymus-intact animals. The normalizing action of thymopentin, whether the immune dysbalance be in the direction of hyper- or hyporesponsiveness, points to its potential utility in human diseases characterized by immune dysbalance.
The effect of thymopentin on the mortality rate of mice treated with lethal doses (LD90) of herpes virus 2 and on the cytotoxic T cell activity after sublethal doses (LD10) of herpes virus was investigated in two series of experiments. Doses of 1, 0.1 or 0.01 ng of thymopentin per g/mouse were administered i.p. in each experiment, either 3 days before, 3 days (66 h) after, or 3 and 6 days after the herpes virus infection. The cumulative mortality rate was evaluated 10 days after the infection. Cytotoxic T cell activity was measured 3, 7 and 14 days after the infection. The 0.1-ng dose of thymopentin reduced the mortality rate to less than 50% (p = 0.0000) if it was administered 3 days before the infection. A single injection of any dose after infection did not reduce the mortality at all, while two injections of 0.1 ng reduced it by about 25% (p = 0.0038). A 1-ng dose showed a mild but significant reduction (p = 0.0313) if it was applied 3 days before the infection. The cytotoxic T cell activity was either not influenced or significantly modified (p < 0.05), i.e. increased or decreased as compared to the control, depending on the dose and timing of thymopentin. A correlation between increased cytotoxic T cell activity and protection against mortality can be demonstrated, while no protection was observed in dose regimens where the cytotoxic T cell activity became reduced. The results are discussed in connection with earlier clinical studies in which the beneficial effect of thymopentin has been demonstrated in frequently relapsing herpes labialis and herpes genitalis patients.
High-performance liquid chromatography (HPLC) was used to evaluate the purity and batch-to-batch consistency of four commercially available thymic extracts and thymopentin, the synthetic pentapeptide corresponding to the biologically active region of the thymic hormone thymopoietin. The thymus extracts were all extremely heterogeneous, differing one from the other. Additionally, they showed high batch-to-batch variation. In contrast, thymopentin was homogeneous and this homogeneity was consistent in different batches.
Three pilot studies testing thymopentin in AIDS patients are presented. One study included 5 patients with the full-blown syndrome, all treated with 50 mg thymopentin 3 times a week by intravenous slow infusion; no immunologically nor clinically positive results were observed, indicating that the T cell pool in such patients is severely depleted. Six other patients with the prodromal stage of AIDS were treated 1 month with 50 mg thymopentin administered as an intravenous bolus injection 3 times weekly and thereafter for another month with same dose regimen as intravenous slow infusions. The patients on infusion therapy experienced statistically significant immunological improvements; these positive findings were paralleled with an improvement of the patients' clinical condition. These positive responses persisted for an average of 8 months. In another group of 5 pre-AIDS patients thymopentin was administered via the subcutaneous route using 15 mg 3 times weekly; only 1 patient revealed immunological and clinical improvement. In summary, only patients with the pre-AIDS syndrome are likely to benefit from immunomodulation therapy with thymopentin, and the mode of administration seems to be crucial.
Thirteen patients who were on chronic hemodialysis for renal failure received booster vaccination with 40 μg HB-Vax while receiving thymopentin as adjuvant therapy. A 50-mg dose of thymopentin was administered subcutaneously 3 times weekly for 3 weeks and vaccination was given after the first week’s treatment. Seven of the 13 patients had never developed any measurable anti-HBs titers in spite of 4–5 previous vaccinations, although 6 out of these 7 had used interferon as adjuvans to their last vaccination. The remaining 6 patients were hyporesponders, but the antibodies had vanished in all before the study was started. Twelve out of the 13 patients developed anti-HBs after revaccination while on thymopentin adjuvant therapy. The antibody production started in most cases within 2 weeks after the booster HB-Vax and showed an increasing trend for up to 6 weeks. Protective titers persisted in the responder cases for the duration of the study (2–3 months). Thus thymopentin may represent a valuable tool to achieve a successful hepatitis vaccination in hemodialysis patients.
Utilization of the hapten-modified self system has increased our understanding of how accessory cells influence immune responsiveness. While the ability to elicit various responses is partially dependent upon accessory cell phenotype, functional differences seem also to be important. By comparing the functional capabilities of purified lymphoid and myeloid accessory cells with tumor cell correlates, we have found that certain hapten-modified cells which express Ia and produce IL 1 are capable of augmenting PFC responses after intravenous administration. These results would suggest that haptenmodified accessory cell carriers which lack Ia or do not deliver secondary signals such as IL 1 (possibly as a result of the haptenation procedure) are responsible for inducing tolerance and suppressor cells. Future work will focus on the mechanism by which P388AD.2 and other tumor cells can elicit augmentation or tolerance in vitro.
This paper reviews all available data on thymo-pentin derived from the extensive preclinical safety program; most of those studies have been concluded, only the carcinogenicity studies are nearing completion. The overview also compiles the safety parameters generated from patients treated with thymopentin for different clinical conditions; in some cases treatment lasted for 12–24 months (56 patients). Different doses and modes of administration were used. The percentage of patients with side effects was comparable to the incidences in the placebo groups. Thymopentin was also well tolerated when administered con-comitantly with a long list of drags given for other reasons. The overall conclusion in that thymopentin is a safe compound.
We developed a working hypothesis and relevant models based on it, which can be applied to investigations of immunoregulatory processes. Oligopeptides effecting signalling functions (activators or supressors of lymphocytes, macrophages, and other immune system cells) are postulated to emerge from protein precursors by limited proteolytic cleavage close to the appropriate cell receptors (the distance between the sites of formation and action of peptides is comparable with the cell dimensions). The limited proteolysis reactions presumably occur during cooperation of immunocompetent cells involving a mutual (tête-à-tête) exchange of information between lymphocytes or lymphocytes and macrophages; the resulting oligopeptides (called tetines or cellular hormones) and/or their precursors are transferred by direct contact via immunosynapsis from cell to cell. We used the signature and equivocation principles of information theory, as well as data on the quasicyclic structures of active sites of peptides and proteins, to further our knowledge of consecutive limited proteolysis and quasicyclization reactions in conjunction with the processes of tetine formation.
The safety data collected from 196 patients treated with thymopentin during the clinical development of this compound in Europe are reviewed and compared with the incidence of adverse drug reactions experienced when using a commercial nonsteroid anti-inflammatory drug. Quantitatively side effects are reported in the same range for both drugs, however, they appear to be quite different when classified by body systems. Some patients complained about somnolence, and it is speculated whether this symptom might be related to thymopentin’s known effect on the neuromuscular junction. The general conclusion is that thymopentin is an extremely well tolerated drug when used according to the dose regimens recommended.
The mechanisms by which GVHD and resistance to engraftment complicate bone marrow transplantation require further study and clarification. Yet the insights from the model systems presented here already have definite implications for human marrow transplantation. For example, the genetic basis of resistance to engraftment is clear. When marrow is transplanted into haploidentical recipients, resistance to engraftment and autologous host revovery is more likely to result than if donor and recipient were matched at the MHC. Engraftment could be increased in such haploidentical recipients by increasing the TBI dose in the transplant preparation regimen. Many centers have already increased TBI dose in an effort to prevent the recurrence of initial malignancies; such an increase would also aid marrow engraftment in haploidentical recipients. It has been suggested that T cell depletion of the marrow may also adversely affect engraftment and that the addition of T cells or T cell products back to the marrow might reverse such an effect. Experimental evidence for such approaches is not yet conclusive, however, and the re-addition of T cells to the marrow certainly increases the risk of GVHD. Our studies suggest that therapeutic strategies designed to ameliorate GVHD may have to account for NK phenomena. If T cells mediate GVHD via soluble factors such as IL-2 and γ-interferon which activate NK cells, then reversal of GVHD may require the disruption of NK function as well as T cell function. The use of monoclonal anti-T cell antibodies in vivo might therefore suffice as GVHD prophylaxis or as therapy in the very early stages of the disease, but anti-NK monoclonal antibodies might be needed to reverse an already established GVHD process. Further clarification of the origin of NK cells in GVHD will help direct therapeutic intervention. If the cells are of donor origin then removal of NK cells as well as T cells from the bone marrow could be attempted; if they are of host origin, then altering transplant conditioning regimens to decrease NK activity would likely prove useful. The in vivo model systems discussed here may serve not only to increase our understanding of the pathogenetic mechanisms attendant on bone marrow transplantation but may allow the evaluation of novel therapeutic strategies.
Eight patients suffering from active rheumatoid arthritis were treated with thymopentin, 50 mg, administered as fractionated intravenous infusion over 10 min 3 times weekly for 3–20 weeks. Seven patients experienced clear-cut amelioration of symptoms and signs after just 2–4 weeks of treatment, and this improvement lasted for 6–8 weeks after thymopentin had been discontinued. Comparable positive results were observed in 2 patients who later continued thymopentin therapy by subcutaneous administration. Several subcutaneous dose regimens were tried; the optimal response appears to be achieved with 100–150 mg three times weekly and 150–200 mg twice a week, respectively. Because the subcutaneous therapeutic approach is more attractive to the patient — and also more practical for the physician — it should be investigated further.
A number of complex interrelating mechanisms contributing to progressive growth of an immunogenic tumor and its evasion of immune destruction are exemplified in the model of the UV-induced tumor 1591 (fig. 3). The effects of (i) carcinogen-induced immune suppression, (ii) age-related changes in immune competence, (iii) generation and immunoselection of tumor cell variants which do not express target tumor antigen and (iv) tumor-induced immune suppression have been documented. The findings may help to assess the problems and the possibilities of immunotherapy of cancer. The problems reside in the many avenues of escape available to the tumor. The possibilities for immunotherapy derive from the fact that progressively growing tumors may still retain immunogenic antigens and therefore be susceptible to immune attack.
Peripheral human blood lymphocytes from healthy blood donors were investigated in vitro to observe the influence of different doses of thymopentin on nonstimulated proliferation, candidin-stimulated proliferation, and phytohemagglutinin (PHA)-induced interleukin 2 (IL2) production. Concentrations of thymopentin ranging from 0.01 to 10,000 ng/ml were used. The proliferation response in nonstimulated cultures was significantly higher in the presence of 0.01, 1, 10, 1,000, and 10,000 ng/ml of thymopentin. There was no significant increase with 0.1 or 100 ng/ml of thymopentin. Thus, three separate peaks were present in these unstimulated cultures: at a concentration of 0.01 ng/ml; between 1 and 10 ng/ml, and between 1,000 and 10,000 ng/ml. These peaks possibly represent three different subpopulations with different sensitivities to different concentrations of thymopentin. Candidin-induced proliferation was significantly higher only at concentrations of 1 and 10 ng/ml of thymopentin, corresponding to the second peak in the unstimulated culture. No other thymopentin concentrations induced significant increase in the candidin-stimulated cultures. No IL2 production was observed in the unstimulated cultures, even in the presence of thymopentin. On the contrary, preincubation with different concentrations of thymopentin influenced PHA-induced IL2 production. A significant increase in the IL2 level was observed in the supernatant of the cultures if 1,000 ng/ml of thymopentin was used in the preculture period. This concentration corresponds to the third peak in the unstimulated cultures. No significant changes were observed with other concentrations of thymopentin. As the measured value of IL2 is a result of a balance between IL2 production and utilization, the above-mentioned findings need further investigation. The study demonstrated the immunomodulatory character of thymopentin in at least two ways: the dose dependency of the proliferative responses, and the observation that only induced production of IL2 can be influenced by thymopentin, while thymopentin itself will not induce changes in IL2 levels.
