Cyclophosphamide eliminates suppressor T cells in age-associated central regulation of delayed hypersensitivity in mice

Regulation of autoimmune diabetes by interleukin 3-dependent bone marrow-derived cells in NOD mice

Interleukin-3 (IL-3), a multilineage colony stimulating factor, has been shown to augment alloreactive bone marrow-derived suppressor cell activity in vivo and in vitro. The present study examined the effect of IL-3 on autoimmune-mediated diabetes in NOD mice. Administration of IL-3 twice weekly starting at 2-4 weeks of age delayed the onset and reduced the overall incidence of diabetes. Bone marrow cells obtained from IL-3-treated NOD mice protected NOD mice from cyclophosphamide-induced diabetes but failed to prevent adoptively transferred diabetes. In vitro culture of bone marrow cells in medium containing IL-3 produced a Thy-1+CD3epsilonloCD4-CD8-CD25- immature T cell clone which prevented cyclophosphamide-induced diabetes. The cloned cells also effectively delayed the development of diabetes induced by transfer of T cells in adult thymectomized, irradiated, bone marrow-reconstituted NOD mice. These results suggest that IL-3 is capable of regulating extrathymic T cell development from the bone marrow and that these cells mediate strong immunoregulatory function.

Liver injury model induced in mice by a cellular immunologic mechanism--study for use in immunopharmacological evaluations

Various drugs for clinical hepatitis were applied to a new model of liver injury induced in mice by delayed-type hypersensitivity to picryl chloride (PCI-DTH). The hepatoprotective agent, biphenyl dimethyl dicarboxylate showed a remarkable improvement against the elevation of serum transaminase levels as well as the histopathological changes when given during the induction phase but not during the effector phase of DTH reaction. Cyclophosphamide (Cy), an immunosuppressive agent, significantly inhibited the enzymatic elevation given in both induction and effector phases. However, Cy did not affect the sustaining of liver injury 4 weeks after the liver injury eliciting. Moreover, the consecutive administration of prednisolone (Pred), in both induction phase and sustaining process of liver injury, conversely caused a more severe liver damage. Such exacerbation by Pred might be resulted from its toxic action to hepatocytes. As an immunomodulatory and antiinflammatory agent, glycyrrhizin remarkably improved the sustaining process but not the acute phase of the liver injury. Krestin and malotilate also showed an improving effect on the sustaining development of liver injury. These findings that most of above drugs showed an improving action in their respective manner suggest that this model may be useful for the pharmacological evaluation of drugs especially immunomodulating agents for hepatitis.

Chagas' disease: enhancement of systemic inflammatory reaction in cyclophosphamide treated mice

An explanation was sought for the fact that an enhancement of myocarditis occurs when Trypanosoma cruzi infected mice is treated with cyclophosphamide (CY). Several schedules were tested in mice and two polar models were achieved--one presenting parasite exacerbation without inflammatory reaction and the other presenting only cellular infiltrate in function of timing and different dosage of drug. In those receiving the drug after infection, an enhancement of the parasite load was detected and it was associated with an inflammatory reaction when mice were treated every other day with low doses of CY (3 mg/kg), but such inflammation was not present if a single dose of 200 mg/kg was used 5 days after infection. On the other hand, in schedules using 200 mg/kg of CY 2 days before infection an enhancement of myocarditis was observed on day 12 post infection, but this was not associated with parasite proliferation. Finally when employing a single intermediate dose of 40 mg/kg, a parasite load as well as a myocarditis enhancement was observed. A correlation between blood monocytes rebound and the intensity of myocarditis has been observed, and the varying time of drug injection suggested that this cellular infiltrate modulates the parasite load. If the immunosuppressive period of the drug occurred during parasite tissue invasion, parasite load amplification would occur without myocarditis. This was achieved using a single dose (200 mg/kg) of CY five days after infection, since monocytosis occurs 7-8 days after drug injection, allowing for parasite multiplication in tissues. The present data bring a strong suggestion that the modulation of myocarditis and parasite load are directly correlated with the leukocytes rebound occurring after the depression period promoted by CY treatment.

Food antigen activates intraepithelial and lamina propria lymphocytes in food-sensitive enteropathy in mice

Morphologic and immunologic changes in the gut mucosa of food-hypersensitive mice, from a study model generated by feeding ovalbumin (OVA) to female BALB/c mice after intraperitoneal injection of cyclophosphamide (CY), were investigated in an effort to clarify the mechanisms of food-sensitive enteropathy. Villous atrophy, crypt hyperplasia, and increased numbers of intraepithelial lymphocytes (IEL) were confirmed in the antigen-challenged OVA-sensitive mice as seen in food-sensitive enteropathy in humans, whereas no significant morphologic changes were observed in the nontreated control group or groups treated with OVA or CY alone. IEL and lamina propria lymphocytes (LPL) were isolated from the intestinal mucosa before and after the antigen challenge, and surface markers were analyzed by FACScan. After the antigen challenge, the numbers of CD8+ cells increased among the IEL, and the occurrence of both CD4+ and CD8+ cells increased among the LPL. The numbers of Thy-1+ cells and TCR- alpha/beta + cells increased among both the IEL and LPL, and LFA-1 expression was enhanced in both of these lymphocyte populations. The proliferative response of IEL and LPL to OVA increased in a dose-dependent manner after the antigen challenge in the OVA-sensitive mouse model. These results indicate that IEL and LPL, possibly those that have migrated from peripheral blood, are activated by orally administered antigens and cause mucosal damage in the food-sensitive enteropathy.

The regulation of intestinal hypersensitivity reactions to ovalbumin by omega-3 fatty acid enriched diet: studies of IEL and LPL in mucosal damage

In order to clarify the mechanisms of food-sensitive enteropathy, a food hypersensitive model was generated by feeding ovalbumin to female BALB/c mice after intraperitoneal injection of cyclophosphamide and morphological and immunological changes in the gut mucosa were investigated. Villus atrophy, crypt hyperplasia and increased numbers of intra-epithelial lymphocytes (IEL) were confirmed in this model, as seen in food-sensitive enteropathy in humans. Subpopulations of IEL and lamina propria lymphocytes were enumerated by immunohistochemical observation. CD8-positive cells were increased both in epithelium and lamina propria, whereas CD4-positive cells were decreased in lamina propria. We document here that orally administered food antigen actually induces food-sensitive enteropathy and mucosal damage is generated by lymphocytes that infiltrate the intestinal mucosa. We also investigated the effect of feeding an omega-3 fatty acid-enriched diet in this model and found that it was efficient in attenuating mucosal damage.

Effect of ASTA-Z 7575 (INN Maphosphamide) on human lymphokine-activated killer cell induction

Recent studies combining chemotherapeutic agents with various biological response modifiers for the treatment of cancer have shown promising results. Cyclophosphamide (Cy) is the most widely used alkylating agent and a major constituent of combination chemotherapy regimens for many neoplastic diseases. It has been reported that Cy is a cytotoxic drug, which becomes immunosuppressive at higher doses. A synthetic metabolite of Cy, ASTA-Z, has recently been produced. ASTA-Z is more active and stable by itself and does not need to be metabolically converted to an active compound. The combined effect of Cy and interleukin-2 (IL-2) on the induction of lymphokine-activated killer (LAK) cells is not known. Therefore, we decided to investigate the effect of ASTA-Z on the induction and function of LAK. The coculture of peripheral blood mononuclear cells (PBMC) with various concentrations of ASTA-Z (0, 10(-6), 10(-5), 10(-4), and 10(-3) dilution) and IL-2 (50 U/ml) for 4 days produced significant suppression of cytotoxicity and lytic ability of the LAK cells against NK-sensitive (K562) and NK-resistant (M14) tumor cell lines. The lower doses of ASTA-Z did not affect the generation of LAK cells, its cytotoxicity and lytic ability of ASTA-Z against both NK-sensitive and NK-resistant tumor cell lines. Furthermore, the ASTA-Z produced dose-dependent suppression of the proliferative response of LAK cells. The significant therapeutic benefit in the cancer patient may be achieved by the low dose regimen of Cy and IL-2 because it has no deleterious effect on the induction and function of LAK cells.

Novel low immunosuppressive derivatives of the antitumor drug fluoropyrimidine, UK-21 and UK-25: effect on delayed type hypersensitivity and tumor immunity

Previously, we reported that two novel 5-fluoropyrimidine derivatives, 2',3',5'-tris-O-[N-(2-n-propyl-n-pentanoyl)glycyl]-5-fluorouridine (UK-21) and 1-(6-[N-(2-n-propyl-n-pentanoyl)glycyl]amino-n-hexylcarbamoyl)- 5-fluorouracil (UK-25), show potent antitumor activity with low immunotoxicological effects. The purpose of this paper was to evaluate the effect of these drugs on delayed type hypersensitivity (DTH). Not only UK-21 and UK-25 but also tegafur (FT-207) and 5-fluorouracil (5-FU) produced no suppression of picryl chloride (PC)-induced DTH in mice but rather enhanced it. It is known that variation of the sensitizing antigen dose alters the effect of drugs on the immune response. Because it was difficult to control the antigen dose in PC-DTH, the sheep erythrocyte (SRBC)-induced response was used to examine the effect of drugs on delayed type hypersensitivity in the succeeding experiments. Either a therapeutic dose or an over-dose of the respective drug was given to mice sensitized with 5 x 10(5) or 5 x 10(7) SRBC. The suppressive effects of UK-21 and UK-25 on the DTH were lower than those of FT-207 and 5-FU. UK-21 and UK-25 enhanced Meth A tumor-specific DTH in BALB/c mice, but FT-207 and 5-FU did not. UK-21, UK-25 and FT-207 showed a tendency to enhance or restore the Meth A tumor neutralizing activity of spleen cells in mice bearing the tumor, but carmofur (HCFU) did not. These results indicated that the suppressive effects of UK-21 and UK-25 on the tumor immune response were also low.

The effect of cyclophosphamide treatment on lymphocyte subsets in the nonobese diabetic mouse: a comparison of various lymphoid organs

The nonobese diabetic (NOD) mouse is a model for human Type 1 diabetes mellitus. Pancreatic beta-cell destruction in NOD mice is mediated by an autoimmune process which can be accelerated by cyclophosphamide (CP). We studied the phenotype of lymphocytes from central, peripheral and regional lymphoid tissues in prediabetic NOD and C3H mice before and after a single large dose of CP. All lymphoid organs showed a greatly diminished cell number and most alterations appeared early after CP and were transient, but an aggressive insulitis was not seen in NOD mice until 14 d after injection. The pancreatic islets in C3H mice remained intact and were not infiltrated. NOD female mice, which are most prone to spontaneous and CP-induced diabetes, exhibited the most unusual lymphoid kinetics after treatment with CP. Their thymus and spleen showed the least relative drop in total cell number and the most rapid rate of recovery. The thymus of these mice was also found to have an increased proportion of CD3+ thymocytes while CD4/CD8 double positive thymocytes decreased 7 d after CP. At 14 d after CP the number of IL-2R+ thymocytes had surpassed that of normal levels. The most dramatic observation was the rapid recovery and overshoot in the number of pancreatic lymph node cells of female NOD mice which coincided with aggressive insulitis.

Combined effect of interleukin 2 and cyclophosphamide in therapy of mice with transitional cell carcinoma

We investigated the effect of combination therapy with interleukin 2 (IL-2) and cyclophosphamide (CPM) in C3H/HeN mice implanted with mouse bladder tumor cells (MBT2). MBT2-bearing mice were treated with a single intraperitoneal injection of 120 mg/kg CPM on day 10 and/or subcutaneous administration of 5 x 10(4) units IL-2/day from day 11 to day 20. As a result, the growth of tumor in mice treated with IL-2 alone was slightly suppressed. On the other hand, regression was observed in all mice treated with CPM alone, although regrowth of tumor was seen in all of them. However, when IL-2 was administered with CPM, regrowth of tumor was completely suppressed. An immunologic study was done that showed cytotoxicity against YAC-1 and P815 in the spleen cells was suppressed in mice treated with CPM alone, but that both recovered to control levels when IL-2 was administered with CPM. Cytotoxicity against MBT2 in the spleen cells was most elevated in mice treated with both IL-2 and CPM. These findings suggested that clinical evaluation of combination therapy with IL-2 and CPM may be worthwhile.

Immunological tolerance to hapten prevents subsequent induction of hapten-immune pulmonary interstitial fibrosis (HIPIF)

Pulmonary interstitial fibrosis (PIF) is a morphological term which in part can be defined as accumulation of collagen in the extracellular matrix. Previously we showed that hamsters sensitized with 2,4,6-trinitro-1-chlorobenzene (TNCB) developed PIF 14 days after an intratracheal challenge with 2,4,6-trinitrobenzene sulfonic acid (TNBS). The participation of delayed-type hypersensitivity (DTH) in lung collagen deposition was clearly demonstrated. In this paper, we use an adaptation of this model to mice and show that the lung collagen deposition observed was related to the genetic ability of the strain to maintain a DTH response to the immunizing hapten (TNP). Specifically, the lung collagen deposition on Day 14 in hapten-sensitized, challenged animals in high responder to TNP (BALB/c, H-2d) was higher than that in low responder mouse (C57BL/6, H-2b). Furthermore, aged C57BL/6 strain (retired breeders) possessed a DTH response to TNP and produced significantly higher accumulation of hydroxyproline than that of TNBS-challenged-only animals. A DTH mechanism for the induction of the fibrosis is consistent with the observation that responder mice that were made tolerant to the antigen were unable to respond to the lung challenge with a specific increase in lung index or collagen deposition. These results suggest that effector T lymphocytes that are important in DTH play a key role in the regulation of lung collagen deposition in hapten-immune pulmonary interstitial fibrosis (HIPIF) in mice.

Conditioned immunopharmacologic effects on cell-mediated immunity

One of the several lines of evidence documenting a relationship between the brain and the immune system is the research on behaviorally induced alterations in immune reactivity. The present review concentrates on the use of immunopharmacologic agents, stress, and antigenic stimuli as unconditioned stimuli (UCSs) in the classically conditioned suppression and enhancement of cell-mediated immune reactions including host-vs-graft and graft-vs-host reactions and delayed type hypersensitivity.

Survival of leukemic cells in a dormant state following cyclophosphamide-induced cure of strongly immunogenic mouse leukemia (DL811)

The B-cell leukemia line, DL811, established in DDD mice, has been characterized by strong immunogenicity leading to occasional spontaneous cure with tumor dormancy. Cyclophosphamide (CY) is an available chemo-immunotherapeutic agent. The effects of CY on the cure and dormancy of DL811 leukemias were investigated. Simultaneous injection of DL811 cells and CY into athymic nude mice revealed that DL811 cells were sensitive to CY doses of 0.5 mg and more per mouse in a dose-related manner. A single dose of 1 mg CY on day 7 after DL811 cell inoculation was most curative of disseminated leukemias (cure rate: 50-80%) as compared with other single CY doses at different times. Cured mice were strongly immune to DL811 leukemias. This therapeutic modality had no curative effect in athymic nude mice. Relapses occurred occasionally in CY-cured mice and most relapsed tumors had the same surface phenotype as that of the DL811 leukemia. In comparison with DL811 leukemias, however, relapsed tumors were less sensitive to rejection by cured mice and less frequently cured by a single dose of 1 mg CY on day 7 of inoculation, implying that they changed in antigenicity and drug sensitivity. Whole-body irradiation of 400 rad gamma-rays to cured mice raised the incidence of relapses from 22.9 to 44.7% and that of deaths from relapses from 8.6 to 26.3%. Thus, leukemic cells may survive against host immune surveillance in a dormant state and relapse after changing their immunologic and chemotherapeutic natures. Lyt-2+, L3T4- T cells appeared to mediate host anti-tumor immunity to eradicate leukemic cells and maintain dormant state.

Lymphocyte electrophoresis as an indicator of modulation of concanavalin A-induced suppressor T-cells in vivo by anti-cancer drugs

We have previously reported that concanavalin A (Con A)-induced suppressor T-cells in vivo are located in the lower electrophoretic mobility (EPM) zone as judged by the ratio of low mobility T-cells to high mobility T-cells (LMT/HMT ratio) in terms of lymphocyte electrophoresis, and that the administration of 5-fluorouracil (5-FU) eliminates their suppressor activity as assayed by Con A-induced lymphocyte blastogenesis in vitro. The current study was undertaken to clarify the relation between changes in EPM and suppressor activity of Con A-induced suppressor T-cells by the administration of antitumor drugs such as cyclophosphamide (CY) and 5-FU when assayed by production of anti-sheep red blood cell antibody in vitro. The pretreatment with 5-FU, but not CY, abrogated the suppressor activity of Con A-induced T-cells followed by restoration of the LMT/HMT ratio of Con A-induced T-cells to that of normal T-cells. These finding suggest that lymphocyte electrophoresis can be applied to an assay for screening immunomodulating agents in Con A-induced suppressor T-cells.

The kinetics of recovery of leukocyte number and lymphocyte function following an injection of a single high dose of cyclophosphamide in C3H/HeJ mice

The short-term effects of the immunosuppressive drug cyclophosphamide (Cy) are well-documented, but the long-term consequences are not as well studied. Here we report on the kinetics of leukocyte number and immune function recovery following a single intraperitoneal injection of 100 mg/kg or 300 mg/kg Cy. The leukocyte number in spleen, lymph node, bone marrow and thymus was unchanged in C3H/HeJ mice injected with the lower dose of Cy, and was severely but transiently depressed in mice injected with 300 mg/kg Cy. Recovery was complete by day 21. Humoral immunity was unchanged with the lower dose of Cy; mice injected with 300 mg/kg recovered normal antibody production after 3-5 weeks. Natural killer cell function was also transiently depressed in animals receiving the higher but not the lower concentration of Cy. Surprisingly, splenic mitogen responses were markedly inhibited in mice injected with either concentration of Cy. The T-cell proliferative response remained depressed even after 5 weeks in mice injected with 300 mg/kg Cy. Lymphocyte subpopulations in spleen were examined by flow cytometry, and, although some deviations were observed, it is unlikely that these changes are responsible for the highly depressed mitogen response. Thus, there is a loss of the proliferative response to mitogens in Cy-treated mice long after recovery of other lymphocyte functions.

Active specific immunotherapy in malignant melanoma

We have recently initiated clinical trials of active specific immunotherapy evaluating a new polyvalent melanoma cell vaccine in patients with high-risk and/or recurrent melanoma. The vaccine has been administered alone, or in combination with low-dose cyclophosphamide, as an immunomodulator of suppressor cells. Cyclophosphamide is effective in lowering suppressor cell activity in some patients undergoing active specific immunotherapy. This is not associated with an enhanced humoral immune response to melanoma-associated antigens, nor is the clinical course of those patients receiving cyclophosphamide favorably influenced. We are hopeful that other immunomodulators, alone or in combination with lower doses of cyclophosphamide, may be effective in some patients, particularly in those patients whose suppressor cell activity remains high. The optimization of the vaccine and the use of immunomodulators will enhance humoral and cellular immune responses to antigens in the allogenic vaccine that cross react with those present in the autologous melanoma, which should more favorably influence the prognosis of melanoma patients.

Immunological mechanisms of antitumor activity of some kinds of Chinese herbs: Meth A-induced delayed type hypersensitivity

In the present paper, we confirmed that a delayed type hypersensitivity response can be elicited against Meth A tumor (Meth A-DTH) in BALB/c mice bearing the primary tumor. This response was augmented by lipopolysaccharide. We examined the effects of 4 kinds of Chinese herbs including A. capillaris, S. doederleinii, A. macrocephala and S. subprostrata on the Meth A-DTH, and the results were compared with that of the herbs on picryl chloride-induced delayed type hypersensitivity (PC-DTH). All of the herbs examined augmented the Meth A-DTH 10 days after the primary tumor transplantation, and S. doederleinii, A. macrocephala and S. subprostrata prevented the decay of the response on the 20th day, but A. capillaris did not. On the other hand, none of the herbs affected the PC-DTH. When both DTH responses were caused simultaneously in the same mouse, Meth A-DTH decayed 20 days after the transplantation but PC-DTH did not. In this case, the effects of these 4 herbs on Meth A-DTH and PC-DTH were essentially the same as those seen in the case of separate experiments. The previous and present results suggest that A. capillaris shows antitumor activity mainly through a direct cytotoxicity, although this herb might have certain components to enhance Meth A-DTH, and the other herbs display the activity through the enhancement of T cell-mediated tumor immunity, particularly tumor specific DTH.

Anti-suppressor effect of cyclophosphamide on the development of spontaneous diabetes in NOD mice

In the NOD mouse, an autoimmune process beginning by 5 weeks of age with lymphocyte infiltration and destruction of insulin-secreting beta cells leads to overt diabetes which begins to appear by 11 weeks of age. Although there is a high incidence of insulitis by 10 weeks of age (greater than 80%) in both males and females, by 30 weeks of age diabetic symptoms have occurred in 53-80% of females and in 12-40% of males. Intraperitoneal injection of a high dose (200 mg/kg) of cyclophosphamide (CY) consistently induces the onset of diabetes in male and female NOD mice at an age when spontaneous diabetes rarely occurs. Spleen T cells from CY-induced diabetic mice are capable of transferring the disease into irradiated nondiabetic syngeneic recipients. This indicates that the diabetogenic effect of CY is not mediated by direct toxicity on pancreatic beta cells but is mediated by abrogation of a suppressor mechanism which may prevent activation of T cells responsible for the development of diabetes in the NOD mouse. Additionally, CY is only effective in NOD mice and not in F1 hybrids between NOD and other strains of mice. Thus, the potential beta cell aggressor mechanism is not present in these hybrids as it is in homozygous mice, which indicates that it is not under the control of dominant genes.

Generation of large granular lymphocytes and lymphocyte subset changes linked with cyclophosphamide-induced eosinophilia in rats--and the effects of ciclosporin

Administration of cyclophosphamide (Cy: 150 mg/kg i.p.) to rats 48 h before immunization with a T-dependent antigen (ovalbumin) resulted in a striking absolute eosinophilia in blood, bone marrow, and secondary lymphoid organs after 10 to 14 days. This eosinophilia was preceded by a significant increase in the W3/25+/OX-8+ (T helper/inducer to T cytotoxic/suppressor) ratio in lymph nodes and spleen and accompanied by a pronounced rise in splenic OX-12+ (B cell) numbers. There was also a concomitant increase in cells with the morphology and immunophenotype (OX-8+, OX-19-) of large granular lymphocytes (LGL). It is suggested that the eosinophilia linked with the B lymphocytosis may be due to cell-derived soluble factors, including a possible equivalent of eosinophil differentiation factor (EDF = interleukin 5), which also has B-cell growth factor activity (BCGF II) in mice. Ciclosporin (CsA; 25 mg/kg/day per os) from the time of immunization, did not affect the incidence of W3/25+ cells in spleen or lymph nodes, but abrogated Cy-induced eosinophilia and reduced the extent of B-cell proliferation. In addition, CsA caused a further, marked increase in the incidence of OX-8+, OX-19-LGL within the spleen. The functional role(s) of these latter cells remains to be defined.

Cyclophosphamide-induced eosinophilia in the rat: concomitant changes in T-cell subsets, B cells and large granular lymphocytes within lymphoid tissues

Cyclophosphamide (Cy) treatment (150 mg/kg) of Sprague-Dawley rats 48 hr before immunization with a T-dependent antigen, ovalbumin (OVA), resulted in striking bone marrow, blood and tissue eosinophilia, maximal at 14 days and concurrent with profound lymphopenia. This phenomenon has been tentatively attributed to selective elimination by Cy of T-suppressor cells. In this study, T-cell subsets, B cells and monocytes/macrophages were enumerated following alkaline phosphatase-anti-alkaline phosphatase (APAAP) staining of mononuclear cells isolated from lymphoid tissues of rats exhibiting eosinophilia. In lymph nodes, a significant increase in the A3/25+:OX-8+ ratio compared with normal was maintained from Day 7 to Day 14; in the spleen, however, this effect was no longer apparent by Day 14, due to the emergence of a population of OX-8+, OX-19- large granular lymphocytes. A seven-fold rise in splenic B-cell numbers (OX-12+) between Day 7 and Day 14 coincided with the eosinophilia. These findings are consistent with the potentiated production of TH-cell derived soluble factors affecting eosinophil production and differentiation, including possibly a rat equivalent of eosinophil differentiation factor, which in the mouse has been reported to have B-cell growth factor activity linked with eosinophilia.

Behaviorally conditioned enhancement of delayed-type hypersensitivity in the mouse

Cyclophosphamide (CY), previously used to condition suppression of a number of different immune responses, was used to condition an enhancement of a delayed-type hypersensitivity (DTH) response in mice. Three weeks before sensitization with sheep red blood cells (SRBC), mice were conditioned by pairing their consumption of a saccharin solution (SAC) with injection of CY. Two, three, and four days after sensitization (Day 0), animals were reexposed to SAC and, on Day +3, they were injected with a low dose of CY. This dose suppressed the DTH response to a challenge with SRBC on Day +4 in both conditioned and control animals. Following rechallenge with SRBC on Days +11 and +18, however, DTH responses were enhanced. Conditioned mice that had been reexposed to SAC showed a significantly greater enhancement than either nonconditioned animals or conditioned mice that were not reexposed to SAC. Thus, enhancement as well as suppression of immune responses can be conditioned with CY.

Immunopharmacological actions of an extract isolated from inflamed skin of rabbits inoculated with vaccinia virus (neurotropin); enhancing effect on delayed type hypersensitivity response through the induction of Lyt-1+2- T cells

To clarify the immunopharmacological action of an extract isolated from inflamed skin of rabbits inoculated with vaccinia virus (Neurotropin), its effect on delayed type hypersensitivity (DTH) response to sheep red blood cells (SRBC) in mice was examined. Neurotropin enhanced the DTH response in C57BL/6 mice which were low responders to SRBC, but not in either BALB/c or C3H/He mice (high responders) when administered i.p. for 4 consecutive days prior to sensitization. However, Neurotropin did not affect the formation of plaque-forming cells to SRBC in C57BL/6 mice under the condition where it enhanced the DTH response. We further examined the mechanism by which Neurotropin enhanced the DTH response in C57BL/6 mice by means of cell transfer experiments. Spleen cells from mice administered Neurotropin i.p. for 4 days, but not saline, could enhance the DTH response when transferred i.v. into normal syngeneic mice just before sensitization. However, the treatment of the spleen cells with anti-Thy-1.2 + complement (C) or with anti-Lyt-1.2 + C, but not with anti-Lyt-2.2 + C, abrogated its enhancing effect. The depletion of macrophages from the cells had no effect. On the other hand, the spleen cells from mice administered Neurotropin had no enhancing effect in the effector phase of DTH response, and they showed a helper T cell activity in a DTH helper T cell assay system in which cyclophosphamide-treated mice were used as recipients. These results suggest that Neurotropin enhances the DTH response in low responder mice through the induction of Lyt-1+2- DTH helper T cells.

Long-lasting enhancement of the delayed-type hypersensitivity response to heterologous erythrocytes in mice after a single injection of cyclophosphamide

Previous reports have indicated that cyclophosphamide (CY) treatment can enhance delayed-type hypersensitivity (DTH) reactions by abrogating suppressor T cell functions. Such findings have suggested that cells in the suppressor lineage may be particularly sensitive to this alkylating agent. The experiments reported here demonstrate that a single injection of CY before sensitization can induce a long-lasting state of enhanced DTH responsiveness to sheep red blood cells (SRBC) in mice. This enhancement required concurrent antigenic stimulation and appeared to be antigen-specific. Additionally, CY treatment of sensitized mice before the first antigenic challenge for DTH resulted in suppressed responses to that challenge, followed by enhanced DTH to subsequent challenge with the same antigen. The suppressed response was achieved with a lower dose of CY than the subsequent enhancement and also required concurrent antigenic stimulation. These results indicate that the effects of CY on both effector and suppressor mechanisms are critically dependent upon antigenic stimulation, and suggest that apparent suppressor sensitivity to CY may be a function of differential ability to recover from CY treatment in a context of antigenic stimulation.

Inhibition of drug-induced eosinophilia by cyclosporin A

Administration of cyclophosphamide to Sprague-Dawley rats 2 days before immunization with ovalbumin in complete Freund's adjuvant resulted in a striking blood, bone marrow, and tissue eosinophilia. The number of circulating eosinophils reached a maximum (50-fold increase above normal) 2 weeks after immunization, and eosinophils were also prominent in bone marrow, lymph nodes (paracortical areas), spleen (white and red pulp), and liver. The eosinophilia could be inhibited by daily oral administration of cyclosporin A (CsA), although its effect was dependent both on the duration of treatment and on the dosage of CsA. A similar, inhibitory action of CsA was demonstrated with respect to methotrexate-induced eosinophilia. This experimental model may prove useful in the study of factors regulating eosinophil production and in examining the prospective value of CsA in the treatment of conditions where eosinophils play a central role.

Killed Listeria-induced suppressor T cells involved in suppression of delayed-type hypersensitivity and protection against Listeria infection

Pretreatment of mice by intravenous injection with killed Listeria provided neither delayed-type hypersensitivity to Listeria protoplasm nor protection against Listeria infection. Assuming that this suppression is due to suppressor cells, we attempted to clarify their induction and characterization. Pretreatment with killed BCG instead of killed Listeria suppressed the induction of DTH and protection in subsequent Listeria-immunized mice. Conversely, pretreatment with killed Listeria suppressed subsequent induction of DTH to PPD or protection from tuberculosis. Thus, these suppressions were induced antigen nonspecifically. Transfer of splenic non-adherent cells from killed Listeria-injected mice which had been treated with anti-BA theta serum plus complement, or had been passed through Sephadex G-10 columns, resulted in both afferent and efferent DTH suppression, suggesting that the DTH suppression is closely associated with suppressor T cells. Moreover, the splenic nonadherent cells from killed Listeria-injected mice showed suppression in vitro of listericidal activity of PEC from Listeria-immune mice in the presence of Listeria protoplasm.

Cyclophosphamide-mediated enhancement of antitumor immune potential of immunosuppressed spleen cells from mice bearing a large MOPC-315 tumor

We have utilized 4-hydroperoxycyclophosphamide (4-HP-CY) as a probe for the immunomodulatory activity of the metabolites of cyclophosphamide (CY) since 4-HP-CY hydrolyzes spontaneously in aqueous solution to the same metabolites as those formed after in vivo conversion of CY by microsomal enzymes. Exposure of immunosuppressed MOPC-315 tumor bearer spleens to a low concentration of 4-HP-CY (0.1-3.0 micron) resulted in augmented antitumor immune potential. The level of antitumor immune potential exhibited by 4-HP-CY-treated tumor bearer spleen cells was not further augmented but was actually reduced by depletion of glass-adherent cells, a procedure which is effective in removing the cells known to have immunosuppressive activity (i.e. metastatic tumor cells and macrophages) from the spleen of untreated MOPC-315 tumor-bearing mice. In fac, 4-HP-CY was superior to depletion of glass-adherent cells in augmenting the antitumor immune potential of immunosuppressed tumor bearer spleen cells. When cells from the primary tumor nodule were incubated with a low concentration of 4-HP-CY which only marginally inhibited their proliferation, the drug completely abolished the suppressive activity of the cells for in vitro generation of antitumor cytotoxicity by normal spleen cells. Moreover, a high level of antitumor cytotoxicity developed when normal spleen cells were cultured in vitro with 4-HP-CY-treated tumor cells at a wide range of ratios of spleen cells to tumor cells. Thus, in the MOPC-315 tumor model, metabolites of CY eliminate the inhibitory effectiveness of splenic suppressor cells and induce the appearance of immunopotentiating activity. The results obtained with 4-HP-CY in vitro provide support for the hypothesis that low-dose CY therapy of mice bearing a large MOPC-315 tumor leads to the appearance of augmented antitumor immune potential in their hitherto immunosuppressed spleen cells as a result of the in situ immunomodulatory effect of the drug on cells in the spleen.

Opposite effects of cyclophosphamide pretreatment on tuberculin hypersensitivity during the course of sensitization of mice with Mycobacterium bovis BCG

The influence of cyclophosphamide (CY) pretreatment upon the development of tuberculin hypersensitivity has been studied during the course of infection of mice with BCG. An enhancement of the DTH response to BCG antigens occurred during the induction phase, whereas a depression of this response occurred at the peak and during the decay phase of sensitization. The development of the early DTH-promoting and of the late DTH-depressing effect of CY was favoured by the use of a supra-optimal dose of BCG. Both these effects were cell-dependent since they could be transferred adoptively to syngeneic recipient mice with sensitized lymphoid cells but not with specific immune sera. Pretreatment with CY favoured the emergence of cells capable of responding in vitro to BCG antigens in the draining lymph nodes of BCG-infected mice. No simple association however, exists between this in vitro lymphocyte transformation response and the DTH response.

Antigen-specific augmentation factor involved in murine delayed-type footpad reaction. I. Nature of augmentation factor

A humoral factor capable of augmenting antigen-specific DTH has been found in the culture supernatant of immune spleen cells and erythrocyte antigen. In this study, a similar factor was identified in the sera of mice sensitized and elicited with heterologous erythrocytes, and the nature of this factor was investigated. Elicitation with antigen was essentially required for the production of the augmentation factor in sensitized mice. The factor showed antigen specificity and antigen-binding capacity. The activity was not assigned to immunoglobulins, as demonstrated by an absorption test with rabbit anti-mouse immunoglobulin-conjugated Sepharose. The activity was stable to heating at 56 degrees C for 30 min, to changes of pH from 3 to 10, and to treatment with trypsin or neuraminidase. The molecular weight of this factor was about 200,000 to 450,000.

Immunity to Babesia in mice. I. Adoptive transfer of immunity to Babesia rodhaini with immune spleen cells and the effect of irradiation on the protection of immune mice

Immunisation of Balb/c mice against Babesia rodhaini by an amicarbalide-controlled infection resulted in a solid immunity which lasted for 216 days. With spleen cells of immune mice protection could be transferred both to naive mice pretreated with cyclophosphamide. Treatment of naive mice with cyclophosphamide (300 mg/kg) five days before a lethal B. rodhaini inoculation resulted in over 50% survival. This protective effect of cyclophosphamide is explained by its inhibiting effect on suppressor T-cells. The protection against B. rodhaini challenge infection afforded to immune Balb/c mice was completely resistant to a sublethal irradiation of 400 rad. Since B-lymphocyte function in antibody production is suppressed by this dose, the role of antibodies in the effector phase of the immunity appears to be of minor if any importance. A considerable degree of protection was still preserved after irradiation of immune animals with 875 rad. Sensitivity to this irradiation dose of all immunocompetent cells except macrophages and a small fraction of T-lymphocytes indicates the involvement of these cell types in the effector phase of the specific immunity. Highly radioresistant macrophages are therefore considered to play the major role but T-lymphocytes are also required for complete protection.

The effect of cyclophosphamide on T lymphocytes and T lymphocyte subsets in patients with chronic progressive multiple sclerosis

The lymphocytes in peripheral blood and cerebrospinal fluid of patients with chronic progressive multiple sclerosis (MS) were characterized with monoclonal antibodies to surface antigens of T cells, helper/inducer T cells and suppressor/cytotoxic T cells. The influence of cyclophosphamide treatment on these immune parameters was investigated. Compared to healthy persons, the mononuclear cell fraction of the peripheral blood of patients with chronic progressive MS consisted of normal %s of T cells and helper/inducer T cells, but decreased %s of suppressor/cytotoxic T lymphocytes. Intensive as well as chronic treatment of MS patients with cyclophosphamide resulted in a decline in the %s of T cells and helper/inducer T cells, whereas the %s of suppressor/cytotoxic T cells returned to normal. In cerebrospinal fluid, cyclophosphamide also induced a relative decrease in the % of helper/inducer T cells and an increase in the % of suppressor/cytotoxic T cells compared to untreated MS patients. Intensive as well as chronic therapy with cyclophosphamide both led to a significant decrease in the absolute number of T cells and T cell subsets in the blood of the patients.

Immunoregulation of genetically controlled acquired responses to Leishmania donovani infection in mice: the effects of parasite dose, cyclophosphamide and sublethal irradiation

On a B10 (Lshs) genetic background, the development of acquired T cell mediated immunity to Leishmania donovani infection in mice is under H-2 linked genetic control. Following intravenous inoculation of 10(7) amastigotes three phenotypic patterns of recovery have been described: 'early cure' (H-2r,s), 'cure' (H-2b) and 'non-cure' (H-2d,q,f). In an attempt to determine the immunological basis for this H-2 linked genetic control the effects of varying parasite dose (5 x 10(3) to 5 x 10(7) amastigotes) and of pre-treatments with cyclophosphamide (50 or 200 mg/kg body weight CY) or sublethal irradiation (100 or 550 rad) on the course of infection, and on circulating anti-leishmanial IgG levels, were examined in strains representative of the three phenotypes: B10.D2/n (H-2d), C57BL/10ScSn (H-2b) and B10.RIII (H-2r). It was found that with low parasite doses (5 x 10(3) or 5 x 10(4)) 'non-cure' mice presented a 'cure' profile whilst raising the dose (5 x 10(7)) caused some perturbation of the normal self-curing response in 'cure' (but not 'early cure') mice. The highest dose did not, however, lead to progressive disease in the genetically non-cure strain. For the parasite dose experiments circulating anti-leishmanial IgG levels were higher in the early cure and cure strains than in the H-2d non-cure strain. The higher doses of CY and sublethal irradiation administered prior to infection had a clear prophylactic effect on the non-cure strain with some effect also observed in cure and early cure strains. This was thought to be due to deletion of the precursors of T suppressor (TS) cells suppressing cell-mediated immunity. Resolution of the liver parasite load in pre-treated mice took place despite minimal or undetectable levels of circulating anti-leishmanial IgG. Similarly, the earlier resolution of parasite load in pre-treated cure and early cure mice occurred even though the antibody response was severely reduced. This suggests that the high antibody responses observed in early cure and cure strains do not normally mediate cure and may simply reflect the independent effect of H-2 on T helper function or the humoral response.

Induction of delayed type hypersensitivity against ultrasonicated Mycobacterium lepraemurium bacilli without simultaneous local reactivity against live bacilli or protective immunity

Delayed type hypersensitivity (DTH) was induced in C3H mice by subcutaneous immunization with Mycobacterium lepraemurium (MLM) antigens in Freund's complete (FCA) or Freund's incomplete (FIA) adjuvant. The total ultrasonicate (MLMSon-P) of MLM bacilli as well as the water soluble fraction (MLMSon-S) of this ultrasonicate was found effective. MLMSon-S was used as the test antigen. Specific DTH also developed after immunization with heat-killed MLM bacilli in FIA, but not with heat-killed bacilli in saline. Some mice were pre-treated with cyclophosphamide (CY) or splenectomized to augment the effect of immunization. In no instance was DTH to MLMSon-S accompanied by detectable local reactivity to live MLM bacilli measured as swelling of the infected footpad or by reduced multiplication or dissemination of the bacilli during the first 11 weeks after inoculation. As determined by testing in the infected footpad 8 weeks after inoculation, MLM infection did not induce DTH to MLMSon-S in non-immunized mice, and MLM infection was found to neither augment nor suppress established DTH to MLMSon-S. The experiments thus demonstrated a clear dissociation between DTH to MLMSon-S and local reactivity to live MLM bacilli, as well as between DTH to MLMSon-S and protective immunity to MLM infection.

Aging and the mucosal-associated lymphoid system

Several lines of evidence support the notion that the mucosal-associated lymphoid system differs from the systemic. It is possible that the mucosal lymphoid system may also differ from the systemic system with regard to immune competence with age. Our findings in these studies indicate a lack of age-associated immune dysfunction in the mucosal-associated lymph nodes: that is, mesenteric and mediastinal lymph node PFC responses of old mice that revealed no decline in magnitude were found to be highly heterogeneous with regard to antibody affinity and revealed no appreciable anti-idiotype-blocked, hapten-augmentable plaque-forming cells. By contrast, the number of splenic and, as well, draining peripheral lymph node IgM, IgG, and IgA anti-TNP PFC responses to TNP-BGG was found to decrease with age with a preferential loss of high affinity plaque-forming cells. This decline in immune activity in the systemic tissues coincided with the increased appearance of anti-idiotype-blocked, hapten-augmentable plaque-forming cells. This differential effect of aging on immune responses in vivo of mucosal and systemic lymphoid tissues imply a site preference for an age-related decline in immune function, and a division of the immune system into regulatory compartments during the normal immune response to antigen in old mice. The present demonstration of a differential effect of aging on immune function in vivo raises an important issue with regard to age-related host defense mechanisms. For example, it would seem reasonable to predict that if immune function totally wanes with age, old individuals would be greatly susceptible to disease and infection. On the contrary, host defense mechanisms relevant to infection show minimal alterations in healthy aged individuals. Thus, we believe that mucosal immunity may play a very important role in host defenses in elderly individuals.

Influence of dietary protein restriction on the delayed-type hypersensitivity response to sheep red blood cells in mice

Delayed-type hypersensitivity responses to sheep erythrocytes were studied in inbred C57BL/6 and outbred NMRI mice fed either protein-deficient diets containing 8% and 4% casein or a normal diet with 27% casein. Following sensitization with optimal doses of antigen, the magnitude of the response was similar in mice fed the 8% protein and the normal diet. Large numbers of sheep red blood cells which suppressed the delayed hypersensitivity response in normal mice, failed to inhibit this response in animals fed the 8% casein diet. However, the titres of serum haemagglutinins were similar in mice of either dietary group immunized with high doses of antigen. Sensitized spleen cells from deficient mice kept on the 8% casein diet, had lower suppressor capacity than those from normal mice upon transfer into syngeneic hosts. Delayed-type hypersensitivity was significantly depressed in mice fed the 4% protein diet whereas the titres of serum antibodies to sheep erythrocytes were not diminished.

Development of immunity against Listeria monocytogenes in athymic nude versus neonatally thymectomized mice

The thymus requirement for the development of immunological responsiveness was determined by estimation of immune responses raised to Listeria monocytogenes in athymic nude, neonatally thymectomized, and sham-operated mice at 6 weeks of age. Not only sham-operated mice, but also neonatally thymectomized mice could completely eliminate the bacteria from the spleen and liver, while athymic nude mice could not eliminate them and showed a persistent form of infection. A strong delayed footpad reaction and acquired cellular resistance could be raised in neonatally thymectomized mice just as well as in sham-operated mice, but not in athymic nude mice. The delayed footpad reaction could be induced in neonatally thymectomized mice without an accompanying ability to inhibit macrophage migration. These results suggest that T cells responsible for immunity against listerial infection require the presence of the thymus for only a very short period in their development.

Comparison of 2'-fluoro-arabinosyl pyrimidine nucleosides and 1-beta-D-arabinofuranosylcytosine on immunological parameters in vitro

1-beta-D-arabinofuranosylcytosine (ara C)3, 2'-fluoro-5-iodo-arabinofuranosyl-cytosine (FIAC) and 2'-fluoro-5-methyl-arabinofuranosyluracil (FMAU) were analyzed for immunosuppressive activity in vitro. In assay systems quantifying both humoral and cellular immune reactivity, FMAU and FIAC were less immunosuppressive than ara C by several orders of magnitude.

Experimental paracoccidioidomycosis in immunosuppressed mice

Mice were immunosuppressed by means of wholebody irradiation or cyclophosphamide, in order to investigate the influence on the initial phase of infection induced by a strain of the fungus, Paracoccidioides brasiliensis, in the yeast phase and inoculated intraperitoneally. A group of mice was irradiated with 600 rad (cobalt gamma-irradiation) 24 h before infection. Two groups were treated with cyclophosphamide (200 mg/kg intravenously), one two days before, and the other, one day after infection. A control group received the fungus, but no radiation or cyclophosphamide. All animals developed lesions at the site of inoculation. Metastatic lesions were observed in 100% of the animals in the irradiated group, 67% in each of the cyclophosphamide-treated groups and 33% in the control group. These lesions were found both in the liver and lungs, being more numerous in the irradiated group, followed by the cyclophosphamide-treated group in which the drug was given after the infection; they were slight in both viscera in the other cyclophosphamide-treated group and also slight in the liver and absent in the lungs of the controls.

Immunoregulation in senescence: increased inducibility of antigen-specific suppressor T cells and loss of cell sensitivity to immunosuppression in aging mice

Azobenzenearsonate (ABA)-specific T cell-mediated suppression has been studied in aging mice. ABA-specific suppressor T cells were induced in young and old mice by injection of ABA conjugated to syngeneic spleen cells (ABA-SC). These suppressor cells were tested for their ability to suppress the in vitro anti-trinitrophenyl (TNP) antibody response of lymph node cells obtained from ABA-keyhole limpet hemocyanin (KLH)-primed young or old mice and cultured with TNP-ABA-KLH. Suppressor T cells were found to be more easily induced in old than in young mice but to suppress less efficiently the antibody response of cells from old than from young mice. The increased inducibility of antigen-specific suppressor T cells in old mice is compatible with the age-dependent decline of immune responsiveness to exogenous antigens. The loss of cell sensitivity to antigen-specific immunosuppression as well as the lack of evidence for increased nonspecific suppression in old mice is consistent with the age-related increase in autoimmune disorders. These findings provide a unifying explanation for the most relevant immunological phenomena of senescence.

Effect of cyclophosphamide on development of reticulum cell sarcoma in SJL/J mice

Studies to determine the effect of cyclophosphamide (CY) on the development of reticulum cell sarcoma (RCS) in SJL/J mice indicated a dependence on the duration of the test period. Age also appeared a factor of importance. Thus, a comparison of tumor incidences at 52 weeks of age showed maximal inhibition when CY was administered at 40 weeks, minimal inhibition when the drug was given at 30 weeks, and intermediate inhibition when treatment was initiated at 10 and 20 weeks. Consistent with these findings, long-term treatment of 40-week-old SJL/J mice with low doses of CY resulted in an increase in the mean survival time and in a reduction in the incidence of RCS.

Modification of immune response by cinnarizine

Effects of cinnarizine on immune response in mice were investigated. Mice were orally administered with cinnarizine and were immunized with sheep red blood cells (SRBC) intravenously. Numbers of plaque forming cells (PFC) to SRBC in spleen of these mice were assayed and delayed-type hypersensitivity (DTH) response to SRBC was measured. 1) PFC response in immunization with 5 X 10(6) cells/mouse of SRBC was enhanced by administration of 25 mg/kg of cinnarizine, while the response in immunization with 5 X 10(8) cells/mouse was suppressed by 25 to 200 mg/kg of cinnarizine. 2) From study on timing of administration, suppression of PFC response by 6.25 to 200 mg/kg of cinnarizine was observed at 24 hr. after the immunization. 3) 12.5 to 200 mg/kg of cinnarizine suppressed polyclonal B cell activation induced by lipopolysaccharide (LPS). 4) Colchicine induced suppressor T cell inactivation was prevented by administration of 50 mg/kg of cinnarizine and it was suggested that cinnarizine may induce suppressor T cells from the study of adoptive cell transfer system. 5) 50 mg/kg of cinnarizine showed the suppression of DTH response in expression phase, but not in induction phase. It was concluded that immune responses in mice were modified by cinnarizine.

Immunologic monitoring in breast cancer patients receiving postoperative adjuvant chemotherapy

A group of 17 patients, having undergone modified radical mastectomy for breast cancer, received 12 cycles of chemotherapy with methotrexate, 5-fluorouracil, and chlorambucil during 17 months. The number of circulating T and non T lymphocytes, as defined by E, EAC, and ME rosette formation, were reduced during treatment. The Non-T lymphocytes, however, were reduced to the highest relative extent. Relative phytohemagglutinin and mixed lymphocyte culture responses of the cells decreased, whereas purified protein derivative responses were unchanged. Serum concentrations of IgM were reduced, but IgA and IgG concentrations were unchanged or slightly increased. Antibody titres to morbilli and herpes simplex were not changed, whereas the antibody activity against cytomegalovirus (CMV) increased in several seropositive patients. None of these patients, however, developed signs of a CMV infection.

Enhancement of delayed hypersensitivity reaction with varieties of anti-cancer drugs. A common biological phenomenon

Delayed hypersensitivity reaction in mice was commonly enhanced with various anti-cancer agents administered as single or intermittent high doses but not consecutive divided doses. The effect of anti-cancer agents on the delayed hypersensitivity reaction was thought to be due to elimination of suppressor T cell activity.

Enhanced elimination of Listeria monocytogenes at the site of delayed footpad reaction

The protective activity against a challenge infection with Listeria monocytogenes was investigated at the site of a delayed footpad reaction in mice immunized with viable or killed listeria. Delayed footpad reactivity was induced only in mice immunized with viable bacteria. Rapid and marked elimination of challenge bacteria was observed only at the site of reaction in mice immunized with viable bacteria but not in mice immunized with killed bacteria. Macrophage migration inhibitory activity was observed equally in both groups of mice. These results suggest that the delayed footpad reaction contributes directly to the elimination of bacteria irrespective of macrophage migration inhibitory activity.

Immunological regulation of experimental cutaneous leishmaniasis. III. Nature and significance of specific suppression of cell-mediated immunity in mice highly susceptible to Leishmania tropica

BALB/c mice have been an exceptional susceptibility to Leishmania tropica infection such that cutaneous lesions grow without restraint in all cases leading to fatal metastasis and visceralization in normal and x-irradiated, bone-marrow reconstituted (XBM) animals. Adult thymectomized, x-irradiated, bone marrow-reconstituted (ATxXBM) BALB/c mice, however, show pronounced retardation of lesion growth leading to some survival and even cures. A similar trend was also found in moderately susceptible (BALB/c X C57BL/6)F1 mice, in contrast with the "resistant" CBA strain, in which, as previously known, ATxXBM animals showed impairment of normal, spontaneous self-healing. These convere effects are paralleled by respective leishmania-specific delayed-type hypersensitivity (DTH) reactivities, prior thymectomy leading to diminution in CBA and augmentation in BALB/c and (BALB/c X C57BL/6)F1. Anti-leishmanial DTH responses, amplfiable by cyclophosphamide pretreatment, can be detected in BALB/c mice within 10 d of infection with 2 X 10(7) promastigotes, but becomes near-totally suppressed by day 25-35. No such suppressin is found in CBA, C57BL/6, or (BALB/c X C57BL/6)F1 mice together with varying degrees of immune control of lesion development or regression. Suppression of DTH in BALB/c mice is leishmania specific and does not extent to 2,4-dinitrofluorobenzene (DNFB) or sheep erythrocytes specificities. Spleen cells from suppressed L. tropica-infected mice when transferred to normal BALB/c mice impaired the induction of DTH to leishmanial antigen. This property resided in the T cell-enriched fraction and not in the T cell-depleted fraction. It is concluded that a major component of the striking inability of BALB/c mice to control L. tropica infection involves profound impairment of a potentially curative cell-mediated immune response by suppressor T cell generation. The possibility is discussed that this may be secondary to rapid amastigote (antigen) accumulation in macrophages expressing the primary genetic "defect."