Thymosin corrects the abnormal DNA synthetic response of NZB mouse thymocytes

Prothymosin Alpha and Immune Responses: Are We Close to Potential Clinical Applications?

The thymus gland produces soluble molecules, which mediate significant immune functions. The first biologically active thymic extract was thymosin fraction V, the fractionation of which led to the isolation of a series of immunoactive polypeptides, including prothymosin alpha (proTα). ProTα displays a dual role, intracellularly as a survival and proliferation mediator and extracellularly as a biological response modifier. Accordingly, inside the cell, proTα is implicated in crucial intracellular circuits and may serve as a surrogate tumor biomarker, but when found outside the cell, it could be used as a therapeutic agent for treating immune system deficiencies. In fact, proTα possesses pleiotropic adjuvant activity and a series of immunomodulatory effects (eg, anticancer, antiviral, neuroprotective, cardioprotective). Moreover, several reports suggest that the variable activity of proTα might be exerted through different parts of the molecule. We first reported that the main immunoactive region of proTα is the carboxy-terminal decapeptide proTα(100-109). In conjunction with data from others, we also revealed that proTα and proTα(100-109) signal through Toll-like receptor 4. Although their precise molecular mechanism of action is yet not fully elucidated, proTα and proTα(100-109) are viewed as candidate adjuvants for cancer immunotherapy. Here, we present a historical overview on the discovery and isolation of thymosins with emphasis on proTα and data on some immune-related new activities of the polypeptide and smaller immunostimulatory peptides thereof. Finally, we propose a compiled scenario on proTα's mode of action, which could eventually contribute to its clinical application.

Prothymosin alpha: a ubiquitous polypeptide with potential use in cancer diagnosis and therapy

The thymus is a central lymphoid organ with crucial role in generating T cells and maintaining homeostasis of the immune system. More than 30 peptides, initially referred to as "thymic hormones," are produced by this gland. Although the majority of them have not been proven to be thymus-specific, thymic peptides comprise an effective group of regulators, mediating important immune functions. Thymosin fraction five (TFV) was the first thymic extract shown to stimulate lymphocyte proliferation and differentiation. Subsequent fractionation of TFV led to the isolation and characterization of a series of immunoactive peptides/polypeptides, members of the thymosin family. Extensive research on prothymosin α (proTα) and thymosin α1 (Tα1) showed that they are of clinical significance and potential medical use. They may serve as molecular markers for cancer prognosis and/or as therapeutic agents for treating immunodeficiencies, autoimmune diseases and malignancies. Although the molecular mechanisms underlying their effect are yet not fully elucidated, proTα and Tα1 could be considered as candidates for cancer immunotherapy. In this review, we will focus in principle on the eventual clinical utility of proTα, both as a tumor biomarker and in triggering anticancer immune responses. Considering the experience acquired via the use of Tα1 to treat cancer patients, we will also discuss potential approaches for the future introduction of proTα into the clinical setting.

Appearance of systemic lupus erythematosus after thymectomy: four case reports and review of the literature

The appearance of systemic lupus erythematosus (SLE) after thymectomy (or thymomectomy) is presented in four patients together with a comparative review of additional reports found in a Medline search for the years 1966-94 in the English and French literature. Fourteen women and two men of average age of 39 years (range 11-66 years) at presentation, developed SLE after thymectomy (11 patients) or thymomectomy (five patients). Half developed SLE within 3 years after surgery (range 3 months to 18 years). The most common SLE manifestation was polyarthritis occurring in 15 of 16 patients either at presentation or during the first year. Other frequent manifestations included skin rashes, fever, cytopenias and pleuritis. Two rare manifestations of SLE, optic neuritis and transverse myelitis, were reported in two patients. Thymic hormone activity was measured in one patient and was undetectable compared with normal controls. HLA studies in eight patients showed the combination of A1, B8 in four. In conclusion, the appearance of SLE after thymectomy or thymomectomy appears to be more than a coincidence. It may provide insights into the pathogenesis of SLE.

Survey of thymic hormone effects on physical and immunological parameters in C57BL/6NNia mice of different ages

Immunosenescence occurs with aging, which is seen in decline in response to mitogens PHA, ConA, decline in cell-mediated immunity, increase in anemia, and increase in autoimmune antibodies to erythrocytes and DNA. These studies compared FTS, TP5, TM4, and TF5 in C57BL/6NNia mice. Mice aged 4, 26, 52, 78 and 104 wk were treated with various hormones 5x/wk for 3 wk and monitored for hormonal effects on weight; hematocrit; peripheral blood, spleen, and thymic cell numbers; spleen and peripheral blood cell mitogen responses to PHA, ConA, LPS; IgM hemolysin autoantibody; and cell-mediated cytotoxicity to P815 allogenic cells. Hormone treatments altered mitogen responses, enhanced IgM hemolysin autoantibody production, and modulated cell-mediated immune responses. The effects were not consistent for every hormone. There was a tendency for enhancement in younger mice and suppression in older animals. Treatment with FTS showed the greatest changes in either enhancing or suppressing the different parameters measured. The hormonal effects appeared to be age specific in that certain activities were altered for certain age groups but not in others. Hormone treatment did not restore any immune parameters in old mice to the level of young animals. In general, the different hormones did not consistently produce the same effects in C57BL/6NNia mice of different age groups. Even though all animals received from National Institutes on Aging (NIA) animal models program were held under strictly controlled conditions, intrinsic variations between cohorts of different ages are difficult to control. Cohorts of aging animals tested at different times might be intrinsically different. This inherent variability in the cohorts could affect the range of activity, specificity and reproducibility of hormone effects in vivo. Most importantly, it should be emphasized that cross-sectional data identifies age differences rather than age changes. There is no assurance that age changes in any individual or in all subpopulations follow this pattern. In our studies only healthy animals were used. Old, sick, or tumor-bearing animals were culled out prior to being sent to us. Therefore, the 78- and 104-wk-old mice represent selected healthy cohorts. The age changes that take place can be answered only from repeated measurements made in the same individual over time.

Rapid isolation of thymosin beta 4 from thymosin fraction 5 by preparative high-performance liquid chromatography

We have developed a rapid, efficient, and reproducible two-step method for the purification of thymosin beta 4 (T beta 4) from thymosin fraction 5 (TF5). This purification is based on the use of high-performance preparative/semi-preparative and analytical reversed-phase (Delta-Pak C18) chromatographic columns. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and amino acid compositional analysis have shown that natural, purified T beta 4 is identical to synthetic T beta 4. This procedure can be used to isolate other biologically active peptides from TF5 in sufficient quantity for characterization.

Isolation of thymosin alpha 1 from thymosin fraction 5 of different species by high-performance liquid chromatography

High-performance liquid chromatography (muBondapak C18 column with 0.05% trifluoroacetic acid in acetonitrile as solvent system) was used to isolate thymosin alpha 1 (alpha 1) from thymosin fraction 5 (f5) of various species (calf, pig, sheep and mouse). Each of the f5 preparations gave a protein peak similar in retention time to bovine thymosin alpha 1. This peak coincided with the immunoreactive peak determined by a radioimmunoassay for alpha 1. Chromatographic analysis of fresh thymus tissue extracts using a high-performance liquid chromatographic similar system did not reveal a detectable protein peak or immunoreactive peak at the alpha 1 position. Our results suggest that alpha 1 may be synthesized in a precursor form in animal tissues.

Solid-phase synthesis of thymosin beta 4: chemical and biological characterization of the synthetic peptide

The chemical synthesis of thymosin beta 4 using a solid-phase procedure has been accomplished. The synthetic product was found to be homogeneous on paper electrophoresis at pH 6.5, high-performance liquid chromatography on a reversed-phase column, and isoelectric focusing using polyacrylamide gels. The synthetic material was also shown to be identical with the natural thymosin beta 4 by tryptic peptide mapping, amino acid compositional analyses, and polyacrylamide gel isoelectric focusing. Biologically, synthetic thymosin beta 4 was found to be as active as the natural compound in a terminal deoxynucleotidyltransferase induction assay and in a macrophage migration inhibition assay. The proposed structure of the peptide hormone was thus confirmed by a chemical synthesis.

Modulation of terminal deoxynucleotidyl transferase activity by thymosin

Thymosin fraction 5, a family of acidic polypeptides isolated from bovine thymus, contains several hormonal-like factors which have been shown to influence the maturation, differentiation and functions of T-cells. Some of these peptides have been chemically defined. Two of them, thymosin alpha 1 (M.W. 3108) and thymosin beta 4 (M.W. 4982) have been sequenced. In murine systems, terminal deoxynucleotidyl transferase (TdT) has been shown to be T-cell specific and to be present primarily in the cortisone sensitive immature T-cell populations. The daily injection of thymosin fraction 5 and two of its components, thymosin beta 3 and beta 4, significantly increases TdT activity in immune suppressed mice as compared to control groups. This study indicates that thymosin can act on prothymocytes and influence the early stages of T-cell differentiation. In an in vivo system, thymosin fraction 5 and the purified peptide, thymosin alpha 1, have high activities in decreasing TdT in normal murine thymocytes after a 22-h incubation. This effect suggests that thymosin can also act on thymocytes and regulate the later biochemical processes during T-cell differentiation.

Sympathetic innervation of murine thymus and spleen: a comparative histofluorescence study

Fluorescence microscopy of thymus and spleen from four strains of mice (C3H and ICR controls, AKR spontaneously leukemic and NZB autoimmune) revealed varicose noradrenergic (NE) fibers in perivascular and parenchymal regions of both organs. Thymic innervation was largely perivascular, but isolated islands and strings of free NE fibers were noted among thymic parenchymal cells. A morphological proximity between NE fibers in the thymus and mast cells was noted in all strains studied, but was exceptionally prominent in the NZB thymus. Perivascular plexuses within the splenic white pulp sent single NE fibers between the surrounding lymphocytes. Catecholamines and histamine have been shown to modulate lymphocyte development and activity in vitro. The present study provides morphological evidence that both NE and histamine are available to lymphocytes in thymus and spleen, and thus provides morphological evidence for neural modulation of immune activity in vivo.

Complete amino acid sequence of bovine thymosin beta 4: a thymic hormone that induces terminal deoxynucleotidyl transferase activity in thymocyte populations

The amino acid sequence of thymosin beta 4, a polypeptide isolated from calf thymus, was determined. Thymosin beta 4 is composed of 43 amino acid residues and has a molecular weight of 4982 and an isoelectric point of 5.1. The NH2 terminus of the peptide is blocked by an acetyl group. This molecule induces expression of terminal deoxynucleotidyl transferase (DNA nucleotidylexotransferase, EC 2.7.7.31) in transferase-negative murine thymocytes in vivo and in citro. Thus, it appears that thymosin beta 4 acts on lymphoid stem cells and may control the early stages of the maturation process of thymus-dependent lymphocytes. This peptide is one of several present in thymosin fraction 5 that participates in the regulation, differentiation, and function of thymus-dependent thymocytes.

Effect of long-term treatment with circulating thymic factor on murine lupus

Mice from three different strains (NZB, B/W, and Swan) which spontaneously develop a lupus-like disease and show a premature decline of their secretion of the circulating thymic factor, Facteur Thymique Sérique (FTS), were treated repeatedly with FTS and followed for the evolution of their autoimmune disease. The autoimmune sialoadenoitis (Sjögren's syndrome) appearing in NZB and B/W mice, evaluated here by a scintigraphic method, was completely prevented or even cured by FTS treatment. The increase in anti-erythrocyte autoantibody production was transiently delayed in aged NZB mice. Conversely, antiDNA antibody production either remained unaffected or was accelerated (in B/W mice) by FTS treatment. These results demonstrate that the restoration of the failing thymic secretion does influence autoantibody production, in a manner depending primarily on the autoantigen eliciting the autoimmune response. However, caution is urged in the application of this approach to human lupus without further studies.

Maturational effects of thymic hormones on human helper and suppressor T cells: effects of FTS ('Facteur Thymique Sérique') and thymosin

Human peripheral blood lymphocytes were isolated from healthy donors and pre-incubated with two thymic factors, FTS, a new synthetic polypeptide originally isolated and purified from serum, and thymosin Fraction V. After 24 hr of pre-incubation, the cells were washed and added to an allogeneic MLR and assayed for their ability to induce a suppressor or helper response. It was found that both factots were capable of inducing both helper and suppressor T cells and that the response was unpredictable. In both instances, more than 50% of the subjects studied responded with a helper effect. The addition of Con A augmented these effects. Since both helper and suppressor T cells can be induced by the factors studied herein, one must be aware of the potential effects on T-T and T-B balances of manipulation in clinical situations such as SLE or cellular immune deficiency states.

In vitro effects of thymosin on T-cell subsets in systemic lupus erythematosus

The possible immunomodulatory influence of thymosin on lymphocytes from patients with active systemic lupus erythematosus (SLE) has been evaluated. Such patients have decreased numbers of T-suppressor (T gamma) cells and normal numbers of T-helper (T mu) cells, resulting in an abnormally low T gamma/T mu ratio. In vitro incubation of lymphocytes from active SLE patients with thymosin resulted in a normalization of the T gamma/T mu ratio. This occurred because of a decrease in T mu cells rather than an increase in T gamma cells. The normalization of T gamma/T mu ratios in vitro in the presence of thymosin is compatible with possible in vivo immunomodulatory effects of these peptides.

Current status of thymosin research: evidence for the existence of a family of thymic factors that control T-cell maturation

Thymosin fraction 5 contains several distinct hormonal-like factors which are effective in partially or fully inducing and maintaining immune function. Several of the peptide components of fraction 5 have been purified, sequenced and studied in assay systems designed to measure T-cell differentiation and function. These studied indicate that a number of the purified peptides act on different subpopulations of T-cells (see Figure 1). Thymosin beta 3 and beta 4 peptides act on terminal deoxynucleotidyl transferase (TdT) negative precursor T-cells to induce TdT positive cells. Thymosin alpha 1 induces the formation of functional helper cells and conversion of Lyt- cells to Lyt 1+, 2+, 3+ cells. Thymosin alpha 7 induces the formation of functional suppressor T-cells and also converts Lyt- cells to Lyt 1+, 2+, 3+ cells. These studies have provided further evidence that the thymus secretes a family of distinct peptides which act at various sites of the maturation sequence of T-cells to induce and maintain immune function. Phase I and Phase II clinical studied with thymosin in the treatment of primary immunodeficiency diseases, autoimmune diseases, and cancer point to a major role of the endocrine thymus in the maintenance of immune balance and in the treatment of diseases characterized by thymic malfunction. It is becoming increasingly clear that immunological maturation is a process involving a complex number of steps and that a single factor initiating a single cellular event might not be reflected in any meaningful immune reconstitution unless it is the only peptide lacking. Given the complexity of the maturation sequence of T-cells and the increasing numbers of T-cell subpopulations that are being identified, it would be surprising if a single thymic factor could control all of the steps and populations involved. Rather, it would appear that the control of T-cell maturation and function involves a complex number of thymic-specific factors and other molecules that rigidly control the intermediary steps in the differentiation process.

Loss of suppressor T cells in the pathogenesis of the autoimmunity of NZB/W mice

Loss of suppressor T cells was demonstrated in NZB/W mice, an animal model of autoimmunity. As NZB/W mice matured they lost splenic T cells that could be activated by concanavalin A (Con A) to become suppressor cells and lost the ability to produce the regulator of humoral immune responses, soluble immune response suppressor (SIRS). However, NZB/W spleen cells retained the capacity to respond to suppressor signals from Con A pulsed normal spleen cells. Thrice weekly administration of SIRS containing supernatants of Con A pulsed normal spleen cells to young NZB/W mice lead to a striking reduction in the manifestations of autoimmunity.

Effect of altered lymphocyte function on immunologic disorders in NZB/NZW mice. III. Acceleration of disease by thymosin

Administration of thymosin fraction V to NZB/NZW F1 mice, an animal model for human SLE, accelerated the appearance of proteinuria and anti-nDNA antibodies, increased deposition of immunoglobulins in kidneys, and significantly shortened survivals. Although the addition of thymosin to in vitro cultures of spleen and lymph node cells from thymosin-treated mice increased DNA synthesis in response to stimulation with Con A, in vivo treatment with thymosin did not affect the Con A response. There was no effect on in vitro responses to PHA or LPS, or on IgM antibody formation to SRBC (T cell dependent) or SSS III (T cell independent) immunizations. Antibodies to thymosin or contamination of our thymosin preparations with nucleic acids could not be demonstrated. The acceleration of autoimmune disease produced by thymosin treatment could not be explained by alteration of the T and B cell functions studied.

Comparison of T cell-mediated immune responsiveness of NZB, (NZB x &NZW)F1 hybrid and other murine strains

The age-dependent capacity of NZB and (NZB x NZW)F1 hybrid, BALB/c, DBA/2, C57BL/6 and C3H mice to generate T cell-mediated immune responses was assessed qualitatively and quantitatively by measuring the following effector functions: (a) the time course of alloreactive cytotoxic T-cell activity triggered in vitro was comparable for NZ and other mouse strains; cell reactivity generated in vivo against EL4 tumour cells was low in young (NZB x NZW)F1 mice and in DBA/2 mice but was comparable for older (NZB x NZW)F1, NZB and other mouse strains; (b) the time-dependent, vaccinia virus-specific, cytotoxic T-cell activity after systemic infection was similar for all mouse strains; (c) the T cell-dependent primary footpad swelling after local injection with lymphocytic choriomeningitis virus was within the same range for all mouse strains tested with respect to size and kinetics of the reaction; (d) the cell-mediated immune protection against Listeria monocytogenes after systemic infection revealed that NZ mice are, independent of age, more susceptible than C3H or C57BL/6 mice and comparable to A strain mice. Therefore, these responses in young, or clinically relatively normal older, NZB or (NZB x NZW)F1 strains that are affected by a lupus-like autoimmune disease did not differ markedly from the range of responses of other mouse strains of 2-14 months of age, which are not known to be similarly diseased. Thus, overall cell-mediated immunity of NZ mice as assessed quantitatively and kinetically in these functional models is within normal ranges. Possible T-cell defects may therefore be selective and either do not occur or were not detected in these models.

Prevention of autoimmunity in experimental lupus erythematosus by soluble immune response suppressor

Young NZB/W mice, treated with injections of soluble immune response suppressor (SIRS)(supernatant from mouse spleen cells exposed to concanavalin A), showed decreased immunoglobulin levels, less antibody to cell nuclei, less proteinuria, and less renal pathology as compared with NZB/W mice receiving a control preparation. Thus, SIRS administration beginning at an early age appears to be an effective therapy of the autoimmune disease in NZB/W mice.

Relationship of antibody affinity to onset of immune complex disease in New Zealand mice

Affinity (KR) of antibody to human serum transferrin in New Zealand mice was measured by a globulin precipitation technique. KR was low in mice immunized early in life, increased with age of immunization up to 16-26 weeks, and then fell to low levels. KR in young and old NZB mice was increased by immunization with antigen in Freund's complete adjuvant. The correlation between low affinity antibody production and susceptibility to immune complex disease is discussed.

Thymosin alpha1: isolation and sequence analysis of an immunologically active thymic polypeptide

The amino acid sequence of a biologically active polypeptide isolated from calf thymus, termed thymosin alpha1, has been determined. Thymosin alpha1 is a heat stable, highly acidic molecule composed of 28 amino acid residues. This peptide is one of several present in thymosin fraction 5 that may participate in the regulation, differentiation and function of thymus-dependent lymphocytes (T cells). A nomenclature for the family of polypeptides present in thymosin fraction 5 is suggested.

Effect of altered lymphocyte function on immunologic disorders in NZB/NZW mice

NZB/NZW F1 female mice were treated with the immunosuppresive enzyme L-asparaginar antibodies, diminished deposition of gamma-globulins in kidneys, significantly delayed the onset of proteinuria, and reduced deaths from nephritis. These effects were associated with reduction of cellular IgM antibody synthesis to both T-dependent and T-independent antigens, but the graft-versus-host reaction was not affected. After several weeks of therapy, antibodies against Asnase appeared in the circulation, the effect on antibody synthesis was lost, ANA and anti-DNA appeared, followed by proteinuria and deaths from nephritis. Therefore Asnase proved to be an effective therapy in NZB/NZW mice, but its usefulness was limited by the appearance of inactivating antibodies.

Adult thymectomy prevention of the appearance of suppressor T cells which depress contact sensitivity to picryl chloride and reversal of adult thymectomy effect by thymus extract

Suppressor cells, which depress the passive transfer of contact sensitivity appear in the lymph nodes and spleen of mice injected with picryl sulfonic acid (PSA). These cells produce a soluble suppressor T cell product (s-TCP), and immune lymph node cells incubated in s-TCP fail to transfer contact sensitivity. This paper shows that the appearance of suppressor T cells following the injection of PSA was prevented by adult thymectomy (ATx). ATx also limited the production of s-TCP. However, ATx had no effect on the DNA synthesis which occurs in the lymph nodes of mice injected with PSA. The adverse effect of ATx on suppressor cells was completely reversed by a neonatal thymus graft placed under the renal capsule and partially reversed by grafts given 600 r in vitro and to a limited extent by grafts given 1000 r. The injection of thymus extract also reversed the effect of ATx whereas splenic extract was inactive. It is suggested that the suppressor T cell which depresses contact sensitivity is dependent on the presence of the thymus because it requires a thymus hormone, and not primarily because it belongs to a short-lived population which is rapidly renewed by cells coming from the thymus.

Loss of suppressor T cells in adult NZB/NZW mice

We have investigated suppressor T-cell activity in female NZB/NZW F1 mice using PWM-driven IgM biosynthesis in vitro as an indicator system. In initial we studied we observed that spleen cells from normal mice (BALB/c, C57BL/6), as well as from young (4 wk) and adult (18 wk) NZB/NZW mice, cultured in the presence of PWM synthesize 860 +/- 120 ng IgM/10(6) cells/7 days. However, when Con A (at 2 mug/ml) was added directly to the cultures (along with PWM), cells obtained from adult normal mice and young NZB/NZW mice showed a 94% suppression of IgM synthesis, whereas cells obtained from adult NZB/NZW mice were suppressed significantly less. To analyze these findings we studied the effect of Con A-induced suppressor cells (cells cultured with Con A for 24 h and washed free of Con A) on PWM-driven IgM biosynthesis. Spleen cells obtained from normal mice cultured in the presence of Con A-pulsed cells obtained from normal mice and young NZB/NZW mice showed an 83-88% suppression of PWM-driven IgM synthesis. Similarly, supernates obtained from Con A-pulsed cells of normal mice or of young NZB/NZW mice suppressed PWM-driven IgM synthesis. This suppression by Con A-pulsed cells and their supernates required T cells since T-cell fractions but not B-cell fractions eluted from anti-Fab Sephadex columns mediated suppression of co-cultured normal cells; in addition, Con A-pulsed cells treated with anti-theta and complement do not mediate suppression. These studies of Con A-induced suppressor cell activity in normal mice and young NZB/NZW mice contrast with studies of Con A-induced suppressor cell activity in adult NZB/NZW mice. We found that adult NZB/NZW Con A-pulsed cells and supernates obtained from the Con A-pulse cells had vastly decreased suppressor potential; in this case the Con A-pulse cells and supernatant fluids derived from such cells did not suppress PWM-driven IgM synthesis by normal cells. Finally, whereas spleen cells from young and adult NZB/NZW mice differ in their suppressor cell potential, cells from both sources could respond equally to suppressor signals in that Con A-pulsed normal cells or supernates derived from such cells caused equivalent suppression of PWM-driven IgM synthesis by young and adult NZB/NZW cells. These observations allow us to conclude that NZB/NZW mice lose suppressor T-cell activity as they age.

Study of thymic factors. II. Failure of thymosin to alter the natural history of NZB and NZB/NZW mice

The effects of daily injections of thymosin, bovine fraction V, on the natural history of NZB and NZB/NZW F1 mice were investigated. With the use of several dose schedules, no significant differences were discovered in treated versus control groups when survival, autoantibodies, and mitogen responsiveness were compared. These results provide further evidence that thymosin may have little or no role in the treatment of the autoimmune disease of New Zealand mice. More encouraging research in thymic extracts and their measurement is necessary before clinical trials in SLE are considered.

Immunobiology of aging

Normal immune functions can begin to decline shortly after an individual reaches sexual maturity. Although changes in cellular environment are partially responsible, the decline is due primarily to changes within the cells, especially the T cells and to some extent the stem cells. This is reflected in their inability to proliferate and differentiate efficiently. What needs to be resolved is whether the altered properties of T cells and stem cells are permanent or reversible and, if permanent, whether they are due to a stochastic or genetically programmed event. The decline with age in certain normal immune functions is associated with an increase in the frequency of autoimmune and immune complex diseases, certain types of cancer, and viral and fungal infections. These diseases, compromise normal immune functions in short-lived strains of mice. In long-lived mice and in humans, however, the decline in immune functions to threshold levels seems to predispose in individuals to illness.

Antigen stimulation of prostaglandin synthesis and control of immune responses

Within 2 min following the intravenous injection of sheep erythrocytes (sRBC) there occurs 20 to 80-fold increase in prostaglandin (PG) F2alpha in the spleen. This burst of synthesis is followed by a slow decline to control levels over the next 1-4 hr. No increase in splenic PGF2alpha levels is observed between 24 and 72 hr after injection. Injection of colloidal carbon results in a small increase, approximately 20% of the increase in PGF2alpha observed with sRBC. The early increase in splenic PGF2alpha levels stimulated by sRBC is also dependent upon thymus-derived (T) cells, since the increase is small or nonexistent in athymic mice and NZB mice. Also, the elevation of splenic PGF2alpha levels is blocked by the administration of indomethacin or Ro 20-5720, both of which block the synthesis of prostglandin. A small increase (2-fold) in PGF2alpha levels occurs in the thymus. A soluble antigen, bovine gamma globulin, stimulated a bimodal increase in splenic PGF2alpha levels, the early peak occurring at 2 hr and the later increase occurring at 48 hr. Using inhibitors of prostaglandin synthesis, it is possible to enhance the appearance of cells forming 19S antibody against sRBC, both in vivo and in vitro. Furthermore, inhibition of prostaglandin synthesis enhances DNA synthesis induced in a two-way mixed-lymphocyte reaction only in whole spleen cell cultures and not in cultures of spleen cells purified by passage over glass wool. Based on this evidence, it is proposed that the prostaglandins represent a major soluble mediator in the control of T cell-T cell interactions and also play an important part in T-B (bone-marrow derived) cell interactions.

Regulation of immune balance by thymosin: potential role in the development of suppressor T-cells

Studies in a variety of animal and human models indicate that thymosin plays a role in the differentiation of a number of T-cell subpopulations. The hypothesis presented is that a normal immune balance depends heavily upon the presence of thymosin-activated suppressor or regulator T-cells. A major thrust in our present research program is to determine whether or not the various disorders discussed here are causally related to abnormal thymosin production by the thymus gland. We are also assessing in animal models the potential value of thymsin in the treatment of specific autoimmune diseases. This information may yield new insights for the management of autoimmune type disorders such as SLE. Results from clinical trials to date suggest that thymosin will have a role in boosting the immune responses of patients with specific thymic malfunctions and may indeed exert an influence via the production of suppressor or regulator T-cells.

T- and B-cell interactions in autoimmune syndromes

We have reviewed briefly some of the individual capabilities of T and B cells and how these can be modified by interactions between the two cell lines. Evidence that T cells have their own distinct receptors for antigens, yet under certain circumstances may bind IgM or IgG produced by B cells, has been particularly emphasized. The probability that B cells, in turn, may bind the antigen receptor of T-cell origin reflects the balanced nature of an intricate communication system in which interactions between antigens, antigen receptors, and binding sites for these receptors all serve to modulate the integrated functioning of T and B cells. It is suggested that this communication system is disturbed in patients with rheumatoid arthritis; specifically, it is proposed that defective feedback activity of IgG-antigen complexes on activated T cells may exist in some patients and could result in unchecked and harmful T-cell activity in the joint. Therapeutic implications of this idea are mentioned.

Regulation of the immune response by subclasses of T lymphocytes. II. The effect of adult thymectomy upon humoral and cellular responses in mice

The immunologic consequences resulting from thymectomy in adult life were investigated. Primary humoral responses were not diminished shortly after adult thymectomy, as judged by responses of intact thymectomized mice as well as by the ability of spleen cells from such mice to transfer primary responses. However, secondary humoral responses were substantially reduced in irradiated recipients of spleen cells from primed adult-thymectomized mice, suggesting that the thymus is required in adult life to maintain a population of cells important in the generation of immunologic helper memory. By contrast, small doses of anti-thymocyte serum, which primarily affect recirulating T cells, abrogated the primary humoral response, but allowed the subsequent development of immune memory. Using a technique permitting in vitro sensitization of purified T cells to alloantigens, it was shown that adult thymectomy increases the ability of T cells to generate primary cytotoxic responses, but had little effect upon the development of cytotoxic T memory activity. These experiments suggest that in adult life the thymus maintains a regulatory population of T cells in peripheral tissues which suppress early T cell differentiation to cytotoxic effector cells and potentiates the development of immune memory.