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

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.

The effects of azathioprine and 6 MP on immunity

The thiopurines, azathioprine and 6 MP are potent inhibitors of both experimental and clinical immune responses. The primary pharmacological activities are mediated by competitive inhibition of enzymes concerned with de novo purine base synthesis; Immunosuppressive activities appear to result from cytotoxic activities directed against antigen-responsive lymphocytes; this inhibition is maximal when the treatment course coincides with the proliferative expansion phase of the response. By contrast, thiopurines are comparatively ineffective if used during an effector phase of an immune response. Furthermore, administration prior to antigenic challenge does not lead to immune inhibition; in fact, it may lead to augmentation of selected immune responses. Treatment with thiopurines does not result in acute lymphopenia; prolonged courses will cause a moderate decrease in circulating lymphocytes. The drug does not selectively deplete peripheral T or B cells but can acutely reduce K (killer) cells, which are effectors in ADCC responses. In addition, short-lived thymocytes and marrow lymphocytes are rapidly depleted by these anti-metabolites. Many in vitro functions of lymphocytes, from treated animals remain normal. Recent studies indicate that, in vitro, azathioprine is specifically able to bind murine T lymphocytes; this can be shown by their ability to inhibit their capacity to rosette with sheep erythrocytes. Azathioprine is also a potent inhibitor of mixed lymphocyte culture responses and can readily suppress the in vitro generation of cytotoxic T cells. These observations suggest that drugs exert preferential toxicities for murine T cells. B lymphocytes for mice appear to vary in their susceptibility for thiopurines. By contrast, the activity of human B cells can be readily suppressed with this drug whereas T helper function is comparatively resistant. In addition to immunosuppressive properties, thiopurines are capable of exerting anti-inflammatory activities, primarily by inhibiting the replication of hematopoietic precursors.

Substituted 9-benzyladenines: inhibitors of inflammation associated with the delayed hypersensitivity skin reaction

Members of a group of substituted 9-benzyladenines have been identified as effective inhibitors of inflammatory aspects of the delayed hypersensitivity skin reaction in the guinea-pig. Inhibitory activity was demonstrable in the efferent arc of the cell-mediated tuberculin and contact dermatitis (2,4-dinitrochlorobenzene) reactions. One of the compounds, 2-amino-9-benzyladenine, inhibited both turpentine-induced and histamine-induced inflammation in the skin. The toxicity and inhibitory activities of some of these compounds are discussed, and reference is made to the structurally related mercaptopurines.

Structure-activity profile of substituted purines and inflammation in the delayed hypersensitivity skin reaction

Substituted purines were tested for their effectiveness in inhibiting the delayed hypersensitivity skin reaction (DHSR) caused by tuberculin in the guinea-pig. Among the tested purines were naturally occurring derivatives of guanine and adenine, including cyclic AMP. Based on the structure-activity profile, a class of purines was identified, the members of which were very effective inhibitors of inflammatory aspects of the DHSR and are characterized by a benzyl group in position 9, an amino or alkylamino group in position 6, and various substituents in position 2. This class of 2-substituted-9-benzyladenines was more effective in the DHSR than some antimetabolites, particularly the structurally related mercaptopurines.

The effect of azathioprine (Imuran) on the cell cycle of promonocytes and the production of monocytes in the bone marrow

The present communication concerns the effect of azathioprine on the mitotic activity of promonocytes and the production of monocytes. In vitro and in vivo labeling with [3H]thymidine showed that during azathioprine treatment the promonocytes synthesize DNA and that, contrary to expectation, the labeling index increases. Cytospectrophotometric determination of the Feulgen-DNA content of the promonocytes during azathioprine treatment showed an increase in the percentage of tetraploid promonocytes, and determination of the various phases of the cell cycle showed significantly increased DNA synthesis and cell cycle times as compared with the normal steady state. On the basis of these results it can be concluded that azathioprine arrests the cell cycle of the promonocytes late in the DNA synthesis phase or in the postsynthesis (G2) phase and mitosis does not occur. This timing of the effect of azathioprine had not been previously observed. The diminished mitotic activity of the promonocytes during azathioprine treatment depressed monocyte production. During treatment with 3 mg/kg azathioprine the cell cycle time of the promonocytes was on the average 5.5 h longer than in the normal steady state and the rate of monocyte production was reduced by 70%. During an acute inflammatory reaction too, monocyte production is affected by azathioprine. In animals not treated with azathioprine but with an acute inflammation the cell cycle time becomes shorter and the monocyte production increases, but animals treated with (3 mg/kg) azathioprine do not show this effect. The kinetics of the monocyte also changes under the low dosage of azathioprine. As consequence of the diminished production of monocytes, far fewer (about 50%) monocytes enter and leave the circulation than during the normal steady state. During an acute inflammatory reaction the numbers in transit through the circulation are slightly augmented but remain considerably lower than in nonazathioprine-trehat of animals not treated with azathioprine.

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.

Experimental autoimmune thyroiditis in transplanted isologous tissue and attempts to produce parathyroid lesions

Strictly inbred guinea-pigs immunized with isologous thyroid extract in complete Freund's adjuvant consistently developed thyroiditis in intramuscular transplants of isologous thyroid tissue, with reactions at least as severe as those seen in their in situ thyroid glands. Normal isologous thyroid tissue accurately reflected the severity of the systemic thyroiditis within 2 weeks after transplantation into a previously immunized guinea-pig. There was no correlation between the intensity of the histological reaction of experimental autoimmune thyroiditis and the absolute amount of thyroid tissue present in an immunized guinea-pig.

Control animals without antigen–adjuvant immunization failed to demonstrate any thyroiditis after multiple thyroid isografts alone. Parallel immunization procedures with isologous parathyroid tissue extracts failed to produce any parathyroiditis or thyroiditis.

The finding that the cellular reactivity of thyroid autoimmunity is an anatomically generalized phenomenon adds to the similarity of the process to other forms of delayed hypersensitivity such as homotransplantation immunity. Accessible transplants of specific target tissue may be used to monitor the natural course of experimental autoimmune disease or the effect of attempts to modify it.

Selective suppression of delayed hypersensitivity by the induction of immunologic tolerance

Administration of DNP-BGG to newborn guinea pigs resulted, in more than half of the animals, in the specific suppression of delayed hypersensitivity to DNP-BGG and BGG, as shown after immunization with DNP-BGG in complete Freund's adjuvant. In contrast, all animals formed antibodies to DNP-BGG, whether or not delayed hypersensitivity to this antigen was present. No difference in antibody titers was found between pretreated and control animals. All animals had antibodies reacting specifically to the hapten DNP, and most of them to the carrier protein BGG, whether or not delayed hypersensitivity to the carrier protein was present. Furthermore, some animals with and without positive 24 hr skin test to DNP-BGG had antibodies with a combined hapten-carrier protein specificity to this antigen, i.e., a specificity which is similar to that of delayed hypersensitivity. Thus, delayed hypersensitivity and antibody formation to similar antigenic determinant were differently affected by injection of antigen in the neonatal period. The finding that delayed hypersensitivity and antibody formation could be dissociated by the induction of immunologic tolerance supports the assumption that delayed hypersensitivity and antibody formation are different immune processes which are not necessarily linked together.