Thymosin treatment modulates production of interferon

Thymic Hormones and Cytokines

The discovery and the first studies on thymic hormones (TH) are historically linked with their role on the developmental events of T lymphocytes that occur within the thymus. However, paradoxically, during the most recent years, much more information about the effects of TH on mature effector cells, in relation with those indicating their real functions during the T-cell differentation, have been, in fact, available.

Here we report an update discussion concerning the possible role of the cooperation between TH and cytokines in T-cell development. Moreover we illustrate the results obtained by us and other authors, demonstrating a strict connection between the action of thymosins, a family of peptides first isolated and purified from calf thymus (1), and that of cytokines.

Thymosin α1 and cancer: action on immune effector and tumor target cells

Since it was first identified, thymosin alpha 1 (Tα1) has been characterized to have pleiotropic effects on several pathological conditions, in particular as a modulator of immune response and inflammation. Several properties exerted by Tα1 may be attributable to a direct action on lymphoid cells. Tα1 has been shown to exert an immune modulatory activity on both T cell and natural killer cell maturation and to have an effect on functions of mature lymphocytes, including stimulating cytokine production and cytotoxic T lymphocyte-mediated cytotoxic responses. In previous studies we have shown that Tα1 increases the expression of major histocompatibility complex class I surface molecules in murine and human tumor cell lines and in primary cultures of human macrophages. In the present paper, we describe preliminary data indicating that Tα1 is also capable of increasing the expression of tumor antigens in both experimental and human tumor cell lines. This effect, which is exerted at the level of the target tumor cells, represents an additional factor increasing the antitumor activity of Tα1.

Roles of thymosins in cancers and other organ systems

Thymosins are small peptides, originally identified from the thymus, but now known to be more widely distributed in many tissues and cells. Thymosins are divided into three main groups, alpha-, beta-, : and gamma-thymosins, based on their isoelectric points. alpha-thymosins (ProTalpha, Talphal) have nuclear localization and are involved in transcription and/or DNA replications; whereas beta-thymosins (Tbeta4, Tbeta10, Tbetal5) have cytoplasmic localization and show high affinity to G-actin for cell mobility. Furthermore, it is well known that both alpha- and beta-thymosins play important roles in modulating immune response, vascular biology, and cancer pathogenesis. More importantly, thymosins may have significant clinical applications. They may serve as molecular markers for the diagnosis and prognosis of certain diseases. In addition, they could be molecular targets of certain diseases or be used as therapeutic agents to treat certain diseases. However, the molecular mechanisms of action of thymosins are largely unknown. This review not only presents recent advances of basic science research of thymosins and their clinical applications but provides thoughtful views for future directions of investigation on thymosins.

Thymosin alpha 1 in the treatment of cancer: from basic research to clinical application

Many studies have explored the effects of immunotherapy, alone or in combination with conventional therapies, on both experimental and human cancers. Evidence has been provided that combined treatments with thymosin alpha 1 (T alpha 1) and low doses of interferon (IFN) or interleukin (IL)-2 are highly effective in restoring several immune responses depressed by tumor growth and/or cytostatic drugs. In addition, when combined with specific chemotherapy, they are able to increase the anti-tumor effect of chemotherapy while markedly reducing the general toxicity of the treatment. The advantages of using this combined chemo-immunotherapeutic approach in experimental and human cancers are reviewed in this issue.

Influences of surgical castration on the thymus of male rats

In previous studies, we have shown that castration of Sprague-Dawley rats enhances thymic weight through puberty whilst sex steroids reduce the castration-induced hypertrophy. In the current study, we have confirmed that castration enhances thymic growth compared to age-matched intact controls. In addition, immunoassays were used to measure thymosin alpha1 and thymosin beta4 levels in sera from intact and castrate rats. Castrate animals displayed greater sera levels of thymosins compared to sera from intact animals. To test whether the enhanced thymic weight and increased levels of thymosins observed post-castration were able to influence immune function of castrate animals, concanavalin A was used in the 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide assay to examine lymphocyte and thymocyte responses from both intact and castrate male rats. Responses of cells isolated from castrate rats demonstrated that lymphocytes and thymocytes were stimulated at low levels of concanavalin A (1.56-3.13 microg/ml for lymphocytes and 1.56 microg/ml for thymocytes) compared to the same cell types isolated from intact rats. Concentrations of concanavalin A ranging from 6.25 to 200 microg/ml produced no significant differences in response from intact and castrate animals.

Combination therapy with BRMs in cancer and infectious diseases

In recent years many studies have stressed the importance of using biological response modifiers (BRMs) in the treatment of different conditions of immune-impairment correlated with ageing, cancer and infectious diseases. In particular, the use of different BRMs in conjunction with conventional therapies has been extensively explored. Our studies have demonstrated that treatment with Thymosin alpha-1 and low doses of IFN or IL-2 exert powerful biological effects both in vitro and in vivo. They are highly effective in restoring cytotoxic activities in immunosuppression induced by tumors and/or cytostatic drugs. In addition, when combined with specific chemotherapy, they are able to induce a dramatic inhibition of tumor growth in both experimental models and in humans. Immunotherapeutic treatment also has an application in controlling infectious diseases, especially those occurring in the immuno-compromised host. The advantage of using the combined immunotherapy treatment with antiviral drugs has been recently demonstrated by our group both in a murine experimental influenza model and in patients infected with HBV, HCV and HIV.

Effects of thymosin alpha1 on the development of amoeboid microglial cells in the corpus callosum of neonatal BALB/c and athymic mice

The present study investigated the effects of intraperitoneal injections of thymosin alpha1 on the supraventricular amoeboid microglial cells (SAMC) in the newborn athymic and normal BALB/c mice. The microglial cells labelled by the lectin GSA I-B4 and the antibody Mac-1 showed a 27% reduction in number in the athymic mice receiving thymosin alpha1 injections compared with those receiving vehicle injections, and a 37% reduction in BALB/c mice receiving thymosin alpha1 injections compared with those receiving vehicle injections. Some of the SAMC in both BALB/c and athymic mice receiving thymosin alpha1 injections became ramified, while the remainder still exhibited their normal amoeboid appearance with few filopodial processes. Ultrastructurally, the lectin reaction product was confined to the plasma membrane and some cytoplasmic vacuoles of labelled SAMC. In both BALB/c and athymic mice, some labelled microglial cells became slender or elongated after thymosin alpha1 injections. Also their cytoplasm was reduced and contained fewer organelles. Radioimmunoassay of the plasma of thymosin alpha1 and vehicle-injected mice showed that there was a significant increase in the cortisol level in BALB/c (P < 0.01) and athymic (P < 0.001) mice 5 days after thymosin alpha1 injections, compared with that of the control mice. The results point to a strong correlation between the reduction of SAMC and the increased level of plasma cortisol. Supporting this is the fact that cortisol is known to suppress the production of monocytes considered to be the precursors of amoeboid microglia.

A double-blind, placebo-controlled, pilot trial of thymosin alpha 1 for the treatment of chronic hepatitis C

A randomized, double-blind, placebo-controlled trial was performed to evaluate the efficacy and safety of thymosin alpha 1 (alpha 1) in treating chronic hepatitis C. Nineteen Italian patients with chronic active hepatitis C, proven by biopsy were randomly assigned to receive a six month course of thymosin alpha 1 (900 micrograms/m2 of body surface area twice weekly) or a placebo. All had HCV-RNA in their serum (by PCR), with serum ALT levels more than double the upper limit of the normal range for at least six months before enrollment. After treatment, patients were followed for an additional six months. All patients completed the trial. One patient treated with thymosin alpha 1, but no patient in the placebo group, normalized serum ALT levels by the end of the treatment. This patient, however, relapsed at the sixth month of the follow up. Overall, there were no significant changes in mean serum ALT levels in either group during the treatment or follow-up period. No patient cleared HCV-RNA. No side effects were reported except for local discomfort at the injection sites, reported by some patients treated with thymosin alpha 1. In conclusion, this regimen of thymosin alpha 1 is not effective in the treatment of chronic hepatitis C.

Production and function of cytokines in natural and acquired immunity to Candida albicans infection

Host resistance against infections caused by the yeast Candida albicans is mediated predominantly by polymorphonuclear leukocytes and macrophages. Antigens of Candida stimulate lymphocyte proliferation and cytokine synthesis, and in both humans and mice, these cytokines enhance the candidacidal functions of the phagocytic cells. In systemic candidiasis in mice, cytokine production has been found to be a function of the CD4+ T helper (Th) cells. The Th1 subset of these cells, characterized by the production of gamma interferon and interleukin-2, is associated with macrophage activation and enhanced resistance against reinfection, whereas the Th2 subset, which produces interleukins-4, -6, and -10, is linked to the development of chronic disease. However, other models have generated divergent data. Mucosal infection generally elicits Th1-type cytokine responses and protection from systemic challenge, and identification of cytokine mRNA present in infected tissues of mice that develop mild or severe lesions does not show pure Th1- or Th2-type responses. Furthermore, antigens of C. albicans, mannan in particular, can induce suppressor cells that modulate both specific and nonspecific cellular and humoral immune responses, and there is an emerging body of evidence that molecular mimicry may affect the efficiency of anti-Candida responses within defined genetic contexts.

Thymosin stimulates interleukin-6 production from rat spleen cells in vitro

Thymosin fraction 5 (TF5) is a partially purified preparation of bovine thymus that affects the differentiation and function of T-cells in vitro. Interleukin-6 (IL-6) is a pleiotropic cytokine that induces terminal maturation of B-cells and T-cell activation and differentiation. Although TF5 had previously been shown to stimulate the production of a number of lymphokines, its effects on IL-6 were not known. In this study we determined the effect of TF5 on IL-6 production from rat spleen cells in vitro. TF5 (100 micrograms/ml) stimulated IL-6 production from splenocytes (0.75-3.0 x 10(5) cells/well) in the presence of 0.008-0.2 micrograms/well of the T-cell mitogen concanavalin-A (con-A) by 10-20 fold during a 72 h incubation period. Dose-response studies demonstrated that 10 micrograms/ml of TF5 was the lowest concentration capable of enhancing IL-6 production. The ability of TF5 to stimulate IL-6 production in the presence of con-A could be demonstrated within 24 h of incubation; longer incubation periods (48-72 h) correlated with further enhancements of IL-6 production. Partial purification of the IL-6-inducing activity from TF5 resulted in three subfractions possessing activity in the presence of con-A (MB2, MB3, MB7) and one in the absence of con-A (MB2). The previously characterized thymosin peptides T alpha 1 and T beta 4 had no effect on IL-6 production in the absence or presence of mitogen. This study reports a new biological activity for TF5 and suggests that a novel constituent of TF5 may enhance the production of IL-6 from spleen cells.

Low thymosin alpha-1 concentrations in patients chronically infected with the hepatitis B virus

Serum concentrations of thymosin alpha-1 (TA-1), an immunoregulatory peptide, were assayed in patients with chronic hepatitis B infection and volunteer blood donor controls. Coded samples were tested using a radioimmunoassay. The TA-1 concentrations in HBsAg-positive patients averaged 1555 pg/ml, whereas in the control group, the mean TA-1 in serum was 2159 pg/ml (P less than 0.001). Similar results were found for age- and sex-matched pairs. The TA-1 concentration was not associated with HBe antigen/antibody status or with values of liver-associated enzymes. Analysis of T-cell subsets in nine HBsAg carriers revealed significantly decreased CD4 (helper)-cells, both in absolute numbers and as the percentage of total lymphocytes present. These data suggest that individuals with chronic hepatitis B infection have a relative deficiency of the immunomodulatory peptide TA-1, which may contribute to the characteristics of the resultant T-cell population.

Biochemical identification of thymosin alpha-1: its phylogenetic distribution and evolutionary implications

1. Thymosin alpha-1, like reactivity, was found in several different species (insects, crab, protozoan, fungus and bacteria) by radioimmunoassay and immune fluorescence and as an extracellular product from the bacterial genus Mycobacterium. 2. Biochemically, thymosin alpha-1 has been isolated from combined crab visceral and nervous tissue by reverse phase HPLC. 3. The identification of thymosin alpha-1 in lower life forms suggests a more generalized exocrine origin in unicellular organisms prior to the development of the immune system or exocrine differentiation.

Synergistic effect of thymosin alpha 1 and alpha beta-interferon on NK activity in tumor-bearing mice

We have investigated the possibility of thymosin alpha 1 (TH) cooperating with alpha beta-interferon (IFN) in boosting natural killer (NK) activity in tumor-bearing, immunosuppressed mice in vivo. Treatment with a single injection of 30,000 IU of IFN 24 h before testing enhanced NK activity in tumor-bearing mice if the IFN was administered 9 days after tumor inoculation, when the animals have normal NK responsiveness. On the other hand, the same treatment led to lower or no improvement of NK responses if the treatment was given 13 or 17 days after tumor inoculation, at a time when tumor growth causes immunosuppression. However, combination treatment with TH (200 micrograms/kg) for 4 days, followed by IFN was found to restore normal NK cell activity. Selective depletion of antigen-positive cells showed that killer cells stimulated by combination treatment with TH and IFN seem to bear phenotypic characteristics of NK cells. These studies provide the first documentation of a novel combination approach to reconstitution of immunosuppressed tumor-bearing mice using TH and IFN. We hypothesize that TH restores NK boosting activity by IFN by effecting the differentiation/induction of precursor populations of IFN-responsive cells.

Production of human B and T cell growth factors is enhanced by thymic hormones

The thymic preparations thymosin fraction 5 (TF5) and synthetic thymosin alpha 1 (T alpha 1) were examined for their ability to enhance growth factor production by human peripheral blood mononuclear cells (PBMC). The results showed that both TF5 and T alpha 1 were capable of enhancing the production of a B cell growth factor (BCGF-12kD) and T cell growth factor (TCGF; IL-2). Enhancement by T alpha 1 could be obtained at 100-200-fold lower concentrations than that seen with TF5. In contrast, no enhancement of growth factor production was obtained with control preparations of non-thymic tissue extracts at any concentrations used. It was observed that stimulation of BCGF-12kD and IL-2 was most significantly obtained when the PBMC were activated with lectin. Furthermore, no direct effect of thymic hormones on test B and T cells was observed. These observations provide the first direct evidence that production of B cell growth factors can be enhanced by thymic hormones. In addition, these studies suggest that thymic hormones may regulate B cell responses by acting on mature activated T lymphocytes.

In vitro effects of thymosin and lithium on lymphoproliferative responses of normal donors and HIV seropositive male homosexuals with AIDS-related complex

The in vitro effects of thymosin fraction 5 (TF5) and lithium chloride (LiCl) on the ability of peripheral blood mononuclear cells (PBMC) obtained from 37 normal male donors and 33 male patients with AIDS-related complex (ARC) to respond to alloantigenic stimulation (mixed leukocyte reaction, MLR) and to produce interleukin 2 (IL-2) in response to mitogens were studied. TF5 significantly increased MLR responses in normal donors (P less than 0.01) and in a group of 33 ARC patients with depressed cellular immunity (P less than 0.05). Similar effects were observed when LiCl was added to the MLR assays in both the normal and the ARC patient groups. Furthermore, TF5 and LiCl exhibited additive immunoenhancing properties. In 10 normal donors TF5 enhanced phytohemaggutinin (PHA)-induced IL-2 production as well as IL-2 production in response to pokeweed mitogen (PWM) (P less than 0.02). TF5-mediated enhancement of IL-2 production by PBMC obtained from ARC patients was observed in response to both mitogens, i.e., PHA and PWM. Additionally, LiCl increased PHA-induced IL-2 production in both normal subjects and ARC patients. LiCl and TF5 together had an additive effect in the enhancement of IL-2 production in both groups of subjects. Our data extend previous observations regarding the immunoregulatory activities of TF5 and LiCl and provide evidence that PBMC obtained from ARC patients have the potential to respond in vitro to these agents. The significance of these findings is discussed.

On the molecular size of thymosins

The immunoregulatory polypeptide prothymosin alpha and its biologically active N-terminal fragment thymosin alpha 1m, with relative molecular masses of 12,500 and 3108 respectively, were found to behave as oligomers (trimers to hexamers) in gel-filtration measurements. This phenomenon of an apparent association of polypeptides has been reported for other thymosins--parathymosin alpha, thymosin beta 4 and thymosin beta 10. In contrast, sedimentation equilibrium ultracentrifugation shows that thymosin alpha 1 is a monomer with a relative molecular mass of 3000 +/- 200. Measurement of the diffusion coefficient as 221 micron2/s suggests that the molecule is approximately spherical. The implications for the molecular species of prothymosin alpha, parathymosin alpha, and beta-thymosins are discussed.

Thymosins and anti-thymosins: properties and clinical applications

For years, scientists have searched for ways to trigger the body's own defenses against cancer and other diseases associated with abnormal immunity. This search has led to the discovery of a number of important new biological and chemical substances that augment, direct or restore many of the normal defenses of the body. These substances are in essence the natural drugs of the body that endow us with immunity and resistance to disease. Now called biological response modifiers (BRMs), most of these 'new medicines', such as thymosins, lymphokines, and interferons, occur naturally in the body, while others, synthetic immunomodulators and thymomimetic agents (drugs that mimic thymic function) have been created in the laboratory. Previously, therapeutic drug development in this area relied upon chemical synthesis or introduction of bacterial adjuvants, or modified viral compounds and substances, which were foreign to the body. Therefore, they did not and do not rely upon or use the body's natural immune and biological response systems for protection against disease, function and response to the environment. Although scientists have known about BRMs for years, isolating and purifying them so that they could be used to treat diseases has been extremely difficult. Many of these substances, such as the lymphokines, occur in the body in minute amounts and normally do not circulate in the blood. The development of new technologies for isolation and large scale synthesis, e.g. solid phase peptide synthesis, high-pressure liquid chromatography microsequencing and genetic engineering, has now permitted scientists to isolate, purify, and synthesize BRMs in sufficiently large quantities to allow human clinical trials. In this paper we will focus on the potential clinical applications of the thymosins and anti-thymosins.

Modulation of interleukin 2 receptor expression on normal human lymphocytes by thymic hormones

The expression of interleukin 2 receptors (IL-2R) is a critical step leading to normal lymphocyte proliferation. Since thymosin fraction 5 (TF5), a thymic hormone preparation, enhances lymphoproliferative responses of human cells, we examined the effects of TF5 on the expression of IL-2R on mitogen-stimulated human lymphocytes. TF5 significantly increased the percentage and antigen density of cells expressing IL-2R after stimulation with an optimal concentration of phytohemagglutinin (PHA) when the cells from the same donor exhibited suboptimal responses to PHA alone. The same effect was observed with a suboptimal PHA concentration and with OKT3 monoclonal antibody stimulation. Thymosin alpha 1, a synthetic polypeptide originally isolated in its native form from TF5, was also able to increase IL-2R expression in response to PHA, suggesting that it is the active species in TF5. The enhancement of IL-2R expression was paralleled by increased proliferative responses. Increased IL-2R expression appears to be the direct effect of thymic hormones, since abrogation of interleukin 2 production by cyclosporin A did not affect TF5-mediated enhancement of PHA-induced IL-2R expression. These results point to a physiological role of thymic hormones in the maintenance of normal levels of IL-2R expression. This immunoregulatory activity of thymic hormones might be relevant in the treatment of conditions where there is decreased IL-2R expression, such as the acquired immune-deficiency syndrome, or in the restoration of normal IL-2R expression to lymphocytes from aged individuals.

A radioimmunoassay for thymosin alpha-1

A new radioimmunoassay (RIA) is described for the quantitation of thymosin alpha-1 (alpha-1). The assay employs an antiserum specific for the COOH-terminal half segment 15-28 of alpha-1, synthetic alpha-1-(15-28) as the hormone standard, and a radioiodinated N alpha-acetyltryrosyl-alpha-1-(15-28) as the tracer. Since alpha-1-(1-28) lacks a phenolic ring for direct radioiodination, the N alpha-acetyltyrosyl-alpha-1-(15-28) was synthesized by the solid-phase method. The peptide bears a Tyr in place of Lys in position 14 of the natural peptide. It showed full alpha-1-(15-28) immunoreactivity and its radioiodinated derivative served as tracer in the RIA. An anti-alpha-1-(15-28) antiserum was raised in a rabbit and was shown to recognize alpha-1-(15-28) or its tyrosyl analogue, and the peptide, alpha-1-(1-28). But it did not recognize other thymic hormones or the biologically active segment 32-36 of thymopoietin, or structurally unrelated peptides. It could also detect natural alpha-1 cross-reacting material in the cytoplasm of cultured human thymic epithelial cells as measured by indirect immunofluorescence. In the RIA, as little as 9 pg of alpha-1-(15-28) equivalents in a 50 microliter sample could be detected. In addition, alpha-1-(1-28)-like immunoreactivity was quantitated in 6 human thymus homogenates and ranged from 0.5 to 4.5 ng/mg of protein.

A radioimmunoassay for thymosin alpha 1 that detects the native polypeptide, prothymosin alpha

A radioimmunoassay, developed for thymosin alpha 1, can also be utilized for the quantitation of the intact native polypeptide, prothymosin alpha, which contains the thymosin alpha 1 sequence at its NH2-terminus (Haritos et al., 1984a). The major epitope was characterized and found to include residues 1-10 at the NH2-terminus of thymosin alpha 1. As little as 5 pmol of prothymosin alpha can be detected in tissue extracts with this radioimmunoassay.

Mechanism of action of thymosin. II. Effects of aspirin and thymosin on enhancement of IL-2 production

Recent studies have shown that thymosin fraction 5 (TF5) enhances production of interleukin-2 (IL-2) by phytohemagglutinin (PHA)-stimulated normal human peripheral blood mononuclear leukocytes (PBL). In this study we sought to determine whether this effect of TF5 might be mediated via the cyclooxygenase or lipoxygenase pathways. Our studies demonstrate that aspirin, an inhibitor of the cyclooxygenase pathway, given in vivo, or added to cultures in vitro, results in two-fold increased IL-2 production by PHA-stimulated PBL. This increase is comparable to that seen when PBL are cultured in vitro with TF5. When aspirin and TF5 are added simultaneously to PBL in the presence of PHA, an additive response is seen. An inhibitor of the lipoxygenase pathway, 15-hydroxyeicosatetraenoic acid, did not significantly change IL-2 production by PBL or influence the enhancement by TF5. Augmentation of IL-2 production by aspirin and/or TF5 was prevented by monocyte depletion of the PBL population. These results are interpreted as demonstrating (a) that TF5 and aspirin augment, by distinct mechanisms, IL-2 production by normal human PBL, (b) that the effects of both of these agents are mediated directly or indirectly via a monocyte population and (c) that aspirin, in addition to its analgesic and anti-inflammatory properties, may act as a modulator of immunological responsiveness, either alone or in combination with other biological response modifiers such as thymosin.

In vitro immune modulation by thymosin alpha 1 in patients with head and neck squamous cell carcinoma

To determine if patients with head and neck squamous cell carcinoma represent an appropriate population for immune reconstitution with thymosin alpha 1, leukocyte migration inhibition (LMI) in response to phytohemagglutinin was measured in 24 previously untreated patients with head and neck cancer, and the in vitro effects of thymosin alpha 1 on migration inhibition were assessed. Compared to normal subjects, LMI was impaired in the head and neck cancer patients. Thymosin alpha 1, in vitro, was associated with improvement in LMI in the cancer patients. Improvements in migration response with thymosin alpha 1 appeared to be independent of levels of various T-lymphocyte subpopulations. However, patients with a normal LMI response had lower suppressor/cytotoxic cell levels than normal subjects or patients with impaired LMI. The findings confirm prior reports of the effects of thymic hormones on lymphokine production in vitro and provide rationale for further clinical studies of thymosin alpha 1 in patients with head and neck squamous cell carcinoma.

Primary structure of rat thymus prothymosin alpha

The primary structure of prothymosin alpha from rat thymus, containing 113 amino acid residues, is reported as follows: (formula; see text) The sequence of the first 28 amino acids at the NH2 terminus is identical to that of calf thymosin alpha 1. The dicarboxylic amino acids, which account for nearly half of the total residues in prothymosin alpha, are largely clustered in the central portion of the polypeptide chain. The polypeptide contains no aromatic or sulfur-containing amino acids. A computer analysis of the three-dimensional structure based on the primary sequence suggests that the molecule is composed of at least five alpha-helical regions interrupted by one short extended chain and three short random coils.

Thymosin increases production of T-cell growth factor by normal human peripheral blood lymphocytes

The in vitro incubation of phytohemagglutinin-stimulated peripheral blood lymphocytes with thymosin results in a marked and reproducible increase in production of T-cell growth factor, which is dose dependent and most pronounced in the first 24 hr of culture. Incubation of lymphocytes with thymosin alone failed to induce any production of T-cell growth factor. The biological activity of thymosin fraction 5 cannot be attributed to the activity of thymosin alpha 1, one of the well-characterized peptide components of fraction 5. These data provide the basis for (i) a potential mechanism for the in vivo immunorestorative effects of thymosin in primary and secondary immunodeficiencies and (ii) identification of an additional, but as yet undefined, immunoregulatory component of thymosin fraction 5.

Modulation of endogenous prostaglandins by thymosin-alpha 1 in lymphocytes

The effects of thymosin-alpha 1 on the stimulation of specific release of prostaglandin E2 (PGE2) from splenic lymphocytes and thymocytes were studied. Experiments were also performed to study in parallel the absolute levels of thymosin-alpha 1 in the blood and the induction of serum FTS activity and of azathioprine sensitivity of spleen cells from adult thymectomized (ATx) mice. A significant difference in the release of PGE2 between normal splenocytes and splenocytes from ATx mice was observed. Thymosin-alpha 1 at certain concentrations was able to stimulate PGE2 release from lymphocytes of ATx mice while inhibiting release in lymphocytes of normal mice. Also, thymocytes were stimulated to release PGE2 after incubation with alpha 1 in a manner similar to that seen in spleen cells of ATx mice. Approximately the same concentrations of alpha 1 was found to also correct the low azathioprine sensitivity of splenocytes from ATx mice. Determinations of FTS-like activity in the blood and the pharmacokinetics of alpha 1 after administration of this synthetic molecule show a clear dissociation. A maximum peak of alpha 1 activity was obtained after 1 hr, while maximal FTS-like activity was observed after 24 hr. The inhibition of the induction by alpha 1 of FTS-like activity and of Thy 1.2 antigen by indomethacin suggests that the action of alpha 1 requires prostaglandin biosynthesis.

Enhancement of murine thymocyte cytotoxic T cell responses by thymosin

Incubation of murine thymocytes with thymosin Fraction 5 (F5) results in a twofold enhancement of the cytotoxic T lymphocyte response (CTL). The assay exhibits requirements for optimal concentrations of thymosin (100 micrograms/ml) and optimal responder/stimulator ratios. Enhancement of CTL activity can be demonstrated in several responder/stimulator strain combinations. The data indicate that thymosin F5 acts via the responder thymocyte population rather than the stimulator cells, since comparable effects were obtained using nude spleen stimulator cells devoid of mature T cells. This system provides a useful bioassay for identifying the component peptides of thymosin F5 which promote thymocyte differentiation and/or maturation and for elucidating the mechanisms of action of the biologically active thymosin peptides.

Protective activity of thymosin against opportunistic infections in animal models

Animal models for opportunistic infections were developed by using mice immunosuppressed by 5-FU. These mice were susceptible to various microorganisms, while normal mice had greater tolerance to such microbial infections. In these models, thymosin alpha 1 was found to protect mice against lethal infections with Candida albicans, Listeria monocytogenes, Pseudomonas aeruginosa, and Serratia marcescens when it was administered during 5-FU treatment prior to the infections. Thymosin alpha 1 was effective in some infections at 0.4-400 micrograms/kg/day IP, about 1/100 of the dose required for thymosin fraction 5. Activity was also demonstrated against L-monocytogenes and Ps. aeruginosa by counting the viable bacteria in the liver after infection. The protective activity against Candida, elimination of which macrophages were essential, was abrogated by anti-thymocyte serum and/or carrageenan, indicating that thymosin alpha 1 serves to maintain the functions of macrophages by reducing the damage to T cells by 5-FU. On the other hand, the activity against Pseudomonas infection was not affected by anti-thymocyte serum or carrageenan. It is probable that thymosin alpha 1 also exerts its effect on neutrophils without participation of T cells and macrophages.

Induction and augmentation of mitogen-induced immune interferon production in human peripheral blood lymphocytes by N alpha-desacetylthymosin alpha 1

Treatment of human peripheral blood lymphocytes with cloned N alpha-desacetylthymosin alpha 1 induced interferon production. The kinetics is similar to that of mitogen-induced interferon induction. N alpha-desacetylthymosin alpha 1, in combination with mitogen, augments the amount of interferon produced. This interferon is immune interferon (IFN-gamma) as determined by sensitivity to pH 2, lack of neutralization by antibodies to IFN-alpha or IFN-beta and absence of activity of MDBK cells. Although the mechanism of induction of IFN-gamma by N alpha-desacetylthymosin alpha 1 is unclear, this compound is not mitogenic at concentrations causing IFN-gamma production. These results indicate that thymic factors may also participate in the regulation of IFN-gamma-production.