The effect of 6-mercaptopurine on primary and secondary immune responses

History of organ transplantation and the development of key immunosuppressants

The history of organ transplantation is limited to the last century. To overcome the barrier of the host immune system, which results in transplant rejection, the pioneers of transplantation achieved their first success by performing a kidney transplantation between identical twins. This achievement led the transplant clinicians to perform transplantations with immunological barriers present. Strategies such as whole-body irradiation combined with steroids yielded success in kidney transplantation between non-identical twins and siblings. However, owing to the toxicity related to irradiation, the paradigm shifted to the use of immunosuppressants. Azathioprine, steroids, and anti-lymphocyte globulin became the first multiple immunosuppressive regimens. With the introduction of cyclosporine, the 1-year survival rate increased by more than 80%. Cyclosporine, azathioprine, and steroids became the new standard maintenance regimens until the introduction of tacrolimus and mycophenolate mofetil, which replaced cyclosporine and azathioprine, respectively. The most recent change in immunosuppressants was the development of monoclonal antibodies with specific binding sites, such as CD20 (rituximab) and CD25 (basiliximab). With the innovation of molecular engineering and new insights into the costimulatory pathways, new molecules are under investigation in the field of transplantation.

Tolerogenic Dendritic Cells in Solid Organ Transplantation: Where Do We Stand?

Over the past century, solid organ transplantation has been improved both at a surgical and postoperative level. However, despite the improvement in efficiency, safety, and survival, we are still far from obtaining full acceptance of all kinds of allograft in the absence of concomitant treatments. Today, transplanted patients are treated with immunosuppressive drugs (IS) to minimize immunological response in order to prevent graft rejection. Nevertheless, the lack of specificity of IS leads to an increase in the risk of cancer and infections. At this point, cell therapies have been shown as a novel promising resource to minimize the use of IS in transplantation. The main strength of cell therapy is the opportunity to generate allograft-specific tolerance, promoting in this way long-term allograft survival. Among several other regulatory cell types, tolerogenic monocyte-derived dendritic cells (Tol-MoDCs) appear to be an interesting candidate for cell therapy due to their ability to perform specific antigen presentation and to polarize immune response to immunotolerance. In this review, we describe the characteristics and the mechanisms of action of both human Tol-MoDCs and rodent tolerogenic bone marrow-derived DCs (Tol-BMDCs). Furthermore, studies performed in transplantation models in rodents and non-human primates corroborate the potential of Tol-BMDCs for immunoregulation. In consequence, Tol-MoDCs have been recently evaluated in sundry clinical trials in autoimmune diseases and shown to be safe. In addition to autoimmune diseases clinical trials, Tol-MoDC is currently used in the first phase I/II clinical trials in transplantation. Translation of Tol-MoDCs to clinical application in transplantation will also be discussed in this review.

EFFECT OF ACTINOMYCIN D ON RNA SYNTHESIS AND ANTIBODY FORMATION IN THE ANAMNESTIC RESPONSE IN VITRO

Antibody synthesis in anamnestic lymphoid cells, measured by incorporation of leucine-C¹⁴ into specific antibody, was inhibited at moderate concentrations of actinomycin D. This was accompanied by marked inhibition of synthesis of RNA as measured by incorporation of H³-cytidine monophosphate. However, at low concentrations of actinomycin D, antibody synthesis was unaffected or even increased while RNA synthesis continued to be inhibited. The results obtained suggest that messenger RNA for antibody synthesis, either because it is relatively stable or present in excess, does not become a limiting factor until its synthesis is maximally inhibited. Puromycin, an inhibitor of amino acid coupling, abolished antibody synthesis in low concentration. 6-Mercaptopurine had no effect on the synthesis of antibody or RNA even at high concentration. The data obtained support the view that antibody synthesis follows pathways similar to those utilized for the formation of other types of proteins.

SUPPRESSION OF ANTIBODY SYNTHESIS AND PROLONGATION OF HOMOGRAFT SURVIVAL BY CHLORAMPHENICOL

Chloramphenicol suppresses primary antibody synthesis in vivo without affecting the ability to develop a normal anamnestic response. Chloramphenicol also prolongs homograft survival in rabbits. The survival of homografts is related to the duration as well as to the amount of chloramphenicol administered. The mechanism of action of chloramphenicol in suppressing immune responses is correlated with its ability to inhibit protein synthesis in proliferating mammalian cells. These observations suggest that the inhibitory effect of chloramphenicol on protein synthesis may be applicable to mammalian cells in vivo as well as to cell-free systems and to intact mammalian cells in vivo.

Transplantation of anergic histoincompatible bone marrow allografts

BACKGROUND

Successful allogeneic bone marrow transplantation relies on global immunosuppression or elimination of T cells. In contrast, the induction of anergy can inactivate specific sets of alloreactive T cells in the donor marrow. Previous work has shown that anergy can be induced by blocking the interaction of the B7 molecule on the surface of antigen-presenting cells with the CD28 molecule on the surface of T cells, thus preventing key signaling events essential for the activation of T cells. To investigate the feasibility of this approach with respect to transplantation of histoincompatible bone marrow, we undertook a clinical trial of ex vivo induction of anergy in T cells present in donor marrow to recipient alloantigens.

METHODS

Outcomes in 12 transplant recipients were evaluated. The recipients' peripheral-blood lymphocytes were collected before myeloablation and served as alloantigen-presenting cells. To induce alloantigen-specific anergy, bone marrow from a donor mismatched with the recipient for one HLA haplotype was cocultured with irradiated cells from the recipient for 36 hours in the presence of CTLA-4-Ig, an agent that inhibits B7:CD28-mediated costimulation. After conventional myeloablation and immunoprophylaxis, the treated donor cells were transfused into the recipient.

RESULTS

After the induction of anergy, the frequency of T cells capable of recognizing alloantigens of the recipient in donor marrow was sharply reduced (P<0.001), whereas the responsiveness to alloantigens from persons unrelated to the recipient or the donor was unaffected (P=0.51). In the 11 patients who could be evaluated, the haploidentical bone marrow cells engrafted. Of these 11 patients, 3 had acute graft-versus-host disease (GVHD) confined to the gastrointestinal tract. No deaths were attributable to GVHD. Five of the 12 patients were alive and in remission 4.5 to 29 months after transplantation.

CONCLUSIONS

Donor bone marrow treated ex vivo to induce anergy to alloantigens from the recipient can reconstitute hematopoiesis in vivo with a relatively low risk of GVHD.

The specificity of allergic reactions. I. Delayed versus Arthus hypersensitivity

Guinea pigs sensitized with either hen, duck, or goose egg albumin showed delayed hypersensitivity followed by Arthus reactions to the homologous antigen, but tended to have much weaker delayed responses and slower antibody formation to heterologous antigens. Guinea pigs with delayed hypersensitivity to one of the avian antigens had a slower antibody response to a secondary injection of heterologous antigen than to one of the homologous antigen. Sensitization with a protein conjugated with a hapten such as picryl chloride (Pi) or dinitrofluorobenzene (DFB) resulted in delayed hypersensitivity to the homologous conjugate, the homologous protein, and the homologous protein with a heterologous hapten. Circulating antibody and Arthus reactions occurred subsequently to the homologous conjugate, as well as to the homologous hapten attached to a heterologous protein. Delayed hypersensitivity thus seemed associated with the protein moiety, and Arthus responses with the hapten. Anamnestic responses followed injection of an antigen causing delayed hypersensitivity, but not of a hapten not causing delayed reactions. Thus, animals sensitized initially with Pi·HEA, DFB·HEA, or HEA produced antibodies sooner after a secondary injection of Pi·HEA than did unsensitized animals. No anamnestic response resulted when animals sensitized to Pi·BGG were injected with Pi·HEA. Thus, delayed hypersensitivity is indicated to be a preliminary and immature step in the immune process, with specificity directed against broad, more general features of the protein antigen. This intermediate step is followed by production of circulating antibody to any antigen having a similar basic structure, with the specificity of the antibody also directed against smaller immunologically active sites on the antigen molecule.

6-Mercaptopurine pharmacokinetics after use of azathioprine in renal transplant recipients with intermediate or high thiopurine methyl transferase activity phenotype

Prevention of allograft transplant rejection by the immunosuppressive 6-thiopurine drug azathioprine is limited by haematological toxicity (leucopenia or agranulocytosis). This toxicity is particularly apparent in subjects with low thiopurine methyltransferase activity (TPMTase) phenotype (1% in the Caucasian population). The thiopurine derivative 6-mercaptopurine is the active metabolite of azathioprine, and it would be of interest to measure, after validation of plasma measurements, the mean values of the pharmacokinetic parameters in transplant patients with high or intermediate TPMTase phenotypes (85 and 14% of the Caucasian population, respectively). We measured erythrocyte TPMTase activity in 103 kidney transplant recipients of high or intermediate phenotype and calculated, after a test dose of azathioprine, the mean values of the pharmacokinetic parameters for 6-mercaptopurine. We also compared these values with the same parameters from one subject with low TPMTase activity phenotype. The mean observed area under the plasma concentration-time curve (AUC) was 190+/-140 ng mL(-1) h and the elimination rate constant (Kel) was 1.92+/-1. The pharmacokinetic parameters (AUC, Kel, t1/2el (the elimination half-life)) of 6-mercaptopurine in transplant patients are normally distributed and suitable for acceptance as a gold standard value for this population of Caucasian transplant patients. It seems useful to calculate these parameters, representative of the systemic exposure of individual patients to the drug, before prescribing these subjects azathioprine immunosuppressive treatment. In subjects with low TPMTase phenotype these pharmacokinetic measurements could also be an index of dose reduction.

The effect of 6-mercaptopurine on delayed hypersensitivity in guinea pigs

The development of tuberculin hypersensitivity in guinea pigs after BCG stimulation was suppressed by intramuscular administration of 50 mg/kg/day of 6-mercaptopurine started at the time of stimulation. Fasting of guinea pigs after BCG stimulation had no effect on the development of tuberculin hypersensitivity.

STUDIES ON ANTIBODY PRODUCTION. XII. INHIBITION OF PRIMING BY DRUGS

The effect of drugs upon the primary and the secondary antibody response to diphtheria toxoid in mice was studied using an experimental system previously described. Triethylenethiophosphoramide (thio-TEPA), chloramphenicol, 6-mercaptopurine, 8-azaguanine, and versenate were found to inhibit, partially or completely,"priming" for the secondary response. Thio-TEPA, chloramphenicol, and 6-mercaptopurine, in doses exceeding those effective in inhibiting priming, did not cause alteration of the secondary response when given only during the secondary response. However, when chloramphenicol and amethopterin were given for 5 days prior to and at least 5 days after the second antigen injection, slight suppression of peak secondary titers occurred. Therefore, drug dosages effective in suppressing priming had less effect on the secondary response. It thus appears that there is a real difference between "priming" and the induction of antibody synthesis.

THE EFFECT OF 6-MERCAPTOPURINE AND AMINOPTERIN ON EXPERIMENTAL IMMUNE THYROIDITIS IN GUINEA PIGS

The influence of 6-mercaptopurine and aminopterin was studied on immune response and immune thyroiditis in guinea pigs immunized with thyroid extract. 1. Both compounds depressed delayed hypersensitivity to thyroglobulin and immune thyroiditis. 2. Antibody formation to thyroglobulin was strongly depressed by aminopterin but not significantly influenced by 6-mercaptopurine. 3. Immune response and thyroiditis were suppressed as long as the compounds were administered; after discontinuation of treatment, immune response and thyroiditis appeared in the same time intervals as observed in control animals after initiation of immunization. 4. Treatment with 6-mercaptopurine and aminopterin 10 days after immunization lead to suppression of delayed hypersensitivity and thyroiditis. 5. Treatment with 6-mercaptopurine of animals after onset of thyroiditis lead to suppression of delayed hypersensitivity and disappearance or diminution of round cell infiltration in the thyroid. 6. The results are discussed in terms of the pathogenesis of experimental immune thyroiditis, the mode of action of these antimetabolites on this experimental immune disease, and in view of the potential value of these compounds in human diseases.

Cardiac transplantation: a review

Cardiac transplantation is currently a highly successful treatment for selected patients with end-stage cardiac failure. The long-term results are limited by the development of coronary artery vasculopathy, infection and malignancy. The activity of transplantation programmes worldwide is severely limited by the availability of donor organs. Further refinements of immunosuppressive agents are likely to result in improved prevention of both acute and chronic rejection. The donor pool is unlikely to be significantly extended as a result of measures to increase donor organ supply. Alternative methods to allograft transplantation need further investigation to increase the number of therapeutic options available for those patients with end-stage heart failure.

Biologic response modifiers: the future of cancer therapy?

The major shift today has been away from nonspecific compounds acting on immune mechanisms to using biologics which have specific, defined roles in acting on the immune response. The field of biologic response modification is progressing very rapidly. New peptides are being identified, as are receptors for these peptides, autocrines, lymphokines, cytokines, growth factors, differentiation factors, hormones, and so on-all of which will control body function, cell populations, and cell to cell interactions. This rapidly advancing area of research in cancer biology and cancer therapy may hold the key to the future of successful therapy.

Long-term influence of adjuvant therapy on natural killer cell activity in breast cancer

Unstimulated IFN- and IL2-stimulated (NK) cell activities were investigated in patients with breast cancer who had received either local radiotherapy alone or adjuvant chemotherapeutic treatment with CMF combined with radiotherapy 12 to 18 months previously. When tested against the primarily NK-sensitive K562 cell line, patients who had received adjuvant chemotherapeutic treatment with CMF were shown to have a significantly decreased unstimulated and IFN-stimulated NK cell activity, as compared to both patients after radiotherapy only (P less than 0.002) and P less than 0.005, respectively) and healthy control persons (P less than 0.05). The former group of patients also had a significantly decreased IFN-stimulated NK cell activity, when tested against the primarily NK-insensitive Chang hepatoma cell line, as compared to patients after radiotherapy only (P less than 0.005) and healthy controls (P less than 0.05). Moreover, patients after radiotherapy only proved to have a significantly increased unstimulated (P less than 0.01) and IFN-stimulated NK cell activity (K562: P less than 0.05; Chang hepatoma cell line: P less than 0.05), as compared to healthy control individuals. In contrast, no difference in IL2-stimulated NK cell activity was detected. The investigation for the expression of CD3 and/or Leu 19 antigens as phenotypic markers of cells with non-MHC restricted cytotoxicity showed a significantly lower percentage of cells with the CD3+ phenotype in patients with breast cancer, irrespective of the chosen post-operative treatment, as compared to healthy controls (P less than 0.01). Finally, patients with breast cancer who had received radiotherapy only had a significant trend towards an increased percentage of CD3+/Leu 19+ PMNC, as compared to both patients after CMF treatment (P less than 0.05) and healthy controls (P less than 0.025). We conclude that patients with breast cancer vary on a long-term basis in their NK activity and in the phenotype of their PMNC depending on their post-operative adjuvant management.

Impaired primary, but not secondary, immune response in breast cancer patients under adjuvant chemotherapy

The antibody response after vaccination against tick-borne encephalitis (TBE) was studied in patients with breast cancer. Although sex- and age-matched control persons produced high titers of anti-TBE antibodies 2 to 4 weeks after the second of two consecutive vaccinations, patients with breast cancer who were first vaccinated after the start of adjuvant chemotherapy consisting of cyclophosphamide, methotrexate and 5-fluorouracil (CMF) failed to do so. The lack of anti-TBE antibody production was found not only in patients under CMF chemotherapy, but also in those primarily vaccinated 6 to 12 months after the termination of CMF treatment. Patients with breast cancer who had been vaccinated either before or after the onset of disease, but before the initiation of chemotherapy, were shown to have developed significant anti-TBE antibody titers which persisted throughout the course of adjuvant treatment and could be boostered by revaccination during the course of CMF administration. The authors conclude that patients with breast cancer undergoing adjuvant chemotherapy experience a serious and prolonged defect in primary antibody production, whereas secondary immune responses remain unimpaired.

Differential inhibition by azathioprine and 6-mercaptopurine of specific suppressor T cell generation in mice

Doses of 100 mg/kg/day and 55 mg/kg/day of azathioprine (AZ) and 6-mercaptopurine (6-MP), respectively, significantly suppressed anti sheep red blood cell (SRBC) responses in CBA mice, assayed by the complement-dependent cellular cytotoxicity (CDCC) test and by serum titration. Four injections of similar doses of the agents were given to donors of carrier-specific suppressor T cells, generated by two immunizations with SRBC, and transferred to syngeneic recipients sensitized with the TNP hapten on SRBC carrier. Anti-TNP response of the recipients was assayed by the CDCC, using TNP-coated EL4 target cells. Whereas 6-MP, given after or before the second immunization of the donors with carrier SRBC, caused abrogation of suppressor cell activity, equimolar doses of AZ were less inhibitory to suppressor cell generation.

Mechanisms in the suppression of delayed hypersensitivity in the guinea pig by 6-mercaptopurine. II: Kinetic and morphologic studies on the monocyte-macrophage component

The effect of 6-mercaptopurine on the development and expression of delayed hypersensitivity was studied in the guinea pig. Results indicated that 6-MP produced its suppressive effects primarily by action on cells of the monocyte-macrophage series. Suppression could occur under conditions of both developing and pre-established delayed hypersensitivity. The defect primarily involved newly synthesized, bone marrow-derived monocytes. Marked alterations in monocyte macrophage generation and distribution, especially the T1/2 of circulating monocytes were demonstrated. Suppressive effects were associated with the appearance of a unique morphologic microscopy. Finally, the in vivo expression of delayed hypersensitivity correlated better with a variety of parameters relating to qualitative macrophage function and distribution rather than those relating to quantitative macrophage levels.

Immunosuppressive therapy of idiopathic thrombocytopenic purpura

The autoimmune nature of idiopathic thrombocytopenic purpura, as currently defined, is well established. Manipulations of the immune apparatus aimed at abating this deviant immunologic state may be one mode of approach to the therapy of this disease. Several cytotoxic compounds are capable of inhibiting the primary and secondary immune response to experimentally injected antigens in animals and man. Their beneficial role in the treatment of an autoimmune lupus-like syndrome in NZB mice24 is well documented. In human autoimmune disease, efficacy of the drugs is still to be established. The mechanisms by which immunosuppressive agents effect therapeutic response, and, in particular, whether this action is linked to suppression of immune reactivity needs clarification. Although preliminary analysis of the efficacy of immunosuppressive drugs in idiopathic thrombocytopenic purpura is encouraging, their therapeutic role has not superseded that of conventional management with steroids and splenectomy. Carefully controlled randomized clinical trials are now necessary so that more rational use of these agents can be recommended in future reports.

Comparison of the immunosuppressive efficacy of 6-mercaptopurine, azathioprine, cyclophosphamide and 036.5122 (Asta) on the primary and secondary immune response of mice to sheep erythrocytes

Two alkylating (cyclophosphamide and 036.5122 Asta) and two antiproliferative agents (6-mercaptopurine and azathioprine) have been compared for their immuno-suppressive potency on the primary and secondary humoral immune response of mice. If equitoxic dosages of the respective drugs are compared, the alkylating agents proved to be of much higher immunosuppressive potency than the antiproliferative agents. In non toxic dosages alkylating agents were able to completely inhibit a primary or secondary immune response, whilst a similar effect with antiproliferative drugs could not be obtained even within toxic dose ranges. Induction of immunological tolerance was possible only by use of the alkylating agents. The significance of these comparative studies is discussed in view of the frequent use of the tested drugs in clinical immunosuppression.

Mechanisms in the suppression of delayed hypersensitivity in the guinea pig by 6-mercaptopurine

The mechanism of suppression, of delayed hypersensitivity to tuberculoprotein by 6-mercaptopurine (6-MP) was studied in guinea pigs. Under the conditions of the protocol, suppression of tuberculin delayed skin test reactivity was not associated with a significantly altered end-organ response to mediators of permeability. No significant alteration of in vivo lymphoid activity, as measured by reconstitution studies, was found. In addition, lymphoid cells from 6-MP-treated animals reacted in a fashion similar to those of placebo-treated animals with respect to (a) antigen-induced lymphocyte proliferation, (b) antigen-induced liberation of macrophage inhibitory factor activity, (c) direct inhibition by antigen of peritoneal exudate cell migration. Conversely, suppression was seen in levels of blood monocytes and in vitro function of macrophages from 6-MP-treated animals in several respects: (a) adherence to glass, (b) migratory rate, (c) phagocytic capacity. Therefore, it would appear that a ma]or mechanism of 6-MP-induced suppression of delayed hypersensitivity is through its action on effector cells.