Immunoneuroendocrine alterations in patients with progressive forms of chronic Chagas disease.

We studied the features of parallel immunoneuroendocrine responses in patients with different degrees of chronic Chagas myocarditis (indeterminate, mild/moderate or severe). A systemic inflammatory scenario was evident in patients with severe myocarditis compared to healthy subjects. This was paralleled by a disrupted activation of the hypothalamus-pituitary-adrenal axis, characterized by decreased concentrations of dehydroepiandrosterone-sulfate (DHEA-s) and an unbalanced cortisol/DHEA-s ratio, reinforcing the view that severe Chagas disease is devoid of an adequate anti-inflammatory milieu, likely involved in pathology. Our study constitutes the first demonstration of neuroendocrine disturbances, in parallel to a systemic inflammatory profile, during progressive human Chagas disease.

Short treatment with the tumour necrosis factor-alpha blocker infliximab diminishes chronic chagasic myocarditis in rats without evidence of Trypanosoma cruzi reactivation.

Tumour necrosis factor (TNF)-alpha is crucial for resistance to Trypanosoma cruzi acute infection, but there is scant information on its role during the chronic phase. To address this issue, we analysed whether a short treatment with a TNF-alpha blocker affected the course and characteristics of chronic disease in a rat experimental model of T. cruzi infection. An anti-TNF-alpha agent (infliximab) was administered during the chronic phase for a period of 4 weeks (3 mg/kg/week), while control infected rats were inoculated with saline physiological solution. Search for parasites yielded non-successful results in all infected groups, irrespective of treatment. Nevertheless, the presence of T. cruzi kDNA in heart tissue was detected in infected and infected plus treated animals. Because infliximab might induce changes in the anti-parasite cytokine response, circulating levels of interleukin (IL)-10, interferon-gamma and nitric oxide were evaluated. An increase in IL-10 levels was observed only in the infected group treated with the anti-TNF-alpha blocker compared to the remaining groups (P < 0.05). A clear attenuation of histological damage associated with a diminution of cardiac TNF-alpha mRNA expression was observed in the infected and treated animals compared to the infected and non-treated group. Blocking of TNF-alpha during a relatively short period in chronically infected rats did not lead to evident parasite reactivation but reduced myocarditis severity significantly, indicating a role of this cytokine in the pathogenesis of chronic myocardial damage.

Steroidogenic capacity of Trypanosoma cruzi trypomastigotes.

American Trypanosomiasis is caused by the hemoflagellate Trypanosoma cruzi (T. cruzi) and affects millions of persons causing variable degrees of digestive and heart disturbances. As far as we concerned, T. cruzi capacity to synthesize steroid hormones has not been investigated. Therefore, the aim of this work was to investigate the capacity of T. cruzi trypomastigotes to transform tritiated steroid precursors into androgens and estrogens. The T. cruzi Tulahuén strain was obtained from mice blood. The trypomastigotes were cultured for 6 and 24h in Dulbbeco's modified Eagle's medium plus FCS and antibiotics. Tritiated dehydroepiandrosterone or androstendione were added to the culture media and parasites were incubated for 6 or 24h. The cultures were centrifuged and ether extracted. The steroids were analyzed by thin layer chromatography (TLC) in two solvent systems. After incubation with 3H-androstenedione, T. cruzi trypomastigotes synthesized 3H-testosterone (T), 3H-17beta-estradiol (E2) and 3H-estrone (E1). Metabolism of 3H-DHEA by the parasites yielded 3H-androstendione and 3H-androstendiol at 6h of incubation. The recrystallization procedure further demonstrated the 3H-androstendiol and 3H-17beta-estradiol syntheses. Results indicate for the first time that T. cruzi trypomastigotes produce androgens and estrogens when incubated in the presence of steroid precursors and suggest the presence of active parasite steroidogenic enzymes.

Deficient control of Trypanosoma cruzi infection in C57BL/6 mice is related to a delayed specific IgG response and increased macrophage production of pro-inflammatory cytokines.

Earlier work in Trypanosoma cruzi-infected C57BL/6 and BALB/c mice revealed an acute disease, of lethal outcome in the former group and lesser severity in BALB/c mice. Fatal course was not accompanied by an increased parasite load, but by a substantial imbalance between pro- and anti-inflammatory cytokine serum levels. To better characterise the mechanisms allowing the host to restrain the infection, we have now studied the specific IgG production and in vitro behaviour of peritoneal macrophages (PMs) when exposed to T. cruzi. BALC/c mice displayed higher serum levels of specific immunoglobulins in the first weeks of acute infection. In vitro infected PMs showed no between-group differences in the number of intracellular parasites, although TNFalpha levels were significantly higher in culture supernatants from C57BL/6 mice. Because an LPS-based pretreatment (desensitisation protocol followed by a sublethal LPS dose) reduced disease severity of C57BL/6 mice, we next explored the features of the in vitro infection in PMs from mice subjected to such protocol. PMs from LPS-pretreated mice had a decreased production of TNFalpha and IL-1beta, becoming more permissive to parasite replication. It is concluded that deficient control of T. cruzi infection in C57BL/6 mice may also involve a less satisfactory specific IgG response and increased TNFalpha production by PMs. Improved disease outcome in LPS-pretreated mice may be associated with the reduced inflammatory cytokine production by PMs, but the impaired ability of these cells to control parasite growth suggests that compensatory mechanisms are operating in the in vivo situation.

Age-related increase in resistance to acute Trypanosoma cruzi infection in rats is associated with an appropriate antibody response.

Inoculation at weaning with Trypanosoma cruzi in inbred "l" rats resulted in a self-resolving acute infection characterized by marked parasitaemias, whereas challenge to adult rats revealed a mild disease with extremely low parasitaemias. To explore the mechanisms underlying such age-associated differences in disease outcome, we analysed the in vitro replication of T. cruzi, nitric oxide and tumour necrosis factor-alpha (TNF-alpha) production in peritoneal macrophages (PMs), the serum concentrations of the specific immunoglobulins (Igs) IgM and IgG, antibodies exhibiting lytic activity against bloodstream forms of T. cruzi and circulating levels of nitrate, TNF-alpha and interferon-gamma (IFN-gamma). Macrophages from young rats were as effective as their adult counterparts for restraining intracellular parasite replication. When stimulated with IFN-gamma, culture supernatants from young PMs contained higher amounts of nitrite and TNF-alpha. Serum samples from 4 and 7 days post infection revealed easily detectable amounts of nitrate, with values being further augmented by day 7 post infection and significantly higher in the young group. TNF-alpha levels were only detected in the young group by day 7 post infection. Both groups had increased amounts of IFN-gamma in their sera, although in adult rats, this trend was followed by a significant drop at day 7, with young rats showing values still higher by the same time point evaluation. In contrast, young rats presented significantly lower levels of IgM and IgG antibodies during the first week of infection. Increased resistance in adult rats seems to be the result of a more appropriate antibody production.

Trypanocidal drug benznidazole impairs lipopolysaccharide induction of macrophage nitric oxide synthase gene transcription through inhibition of NF-kappaB activation.

In murine macrophages, inducible NO synthase II (NOSII) gene expression is promoted at a transcriptional level by LPS and/or IFN-gamma with benznidazole (BZL), a trypanocidal drug, acting to down-regulate NOSII gene induction and hence inhibiting NO production. By performing transient transfection experiments, we now report that BZL also inhibited the expression of NOSII gene promoter or multimerized NF-kappaB binding site controlled reporter genes. By contrast, no effect was observed on the expression of a reporter gene under the control of the NOSII promoter-derived IFN regulatory factor element. EMSAs demonstrated that BZL inhibited the nuclear availability of NF-kappaB in stimulated macrophages. NF-kappaB is activated in macrophages by phosphorylation, ubiquitination, and subsequent proteolysis of IkappaB. Within this setting, Western blot was also performed to show that BZL blocked IkappaBalpha degradation. Collectively, these results demonstrate that BZL is able to specifically inhibit macrophage NF-kappaB activation after LPS plus IFN-gamma stimulation.

Protected Trypanosoma cruzi infection in rats born to mothers receiving interferon-gamma during gestation is associated with a decreased intramacrophage parasite growth and preferential synthesis of specific IgG2b antibodies.

We demonstrated that administration of interferon gamma (IFN-gamma) to pregnant rats conferred partial resistance in their offspring to further challenge with Trypanosoma cruzi. Because of the effects of IFN-gamma on macrophage activation and immunoglobulin isotype selection, offspring were now studied to ascertain whether this intervention modifies the in vitro replication of T. cruzi and nitric oxide (NO) production by peritoneal macrophages (PE), together with the anti-T. cruzi IgG isotypes. To evaluate the possibility of a detrimental effect of IFN-gamma, serum levels of anti-sulphatide autoantibodies were also investigated. Offspring were born to mothers undergoing one of the following procedures during gestation: treatment with recombinant rat IFN-gamma, 50,000 IU/rat, five times/week for 3 weeks, which was started on the day of mating; infection with 10(6) trypomastigotes of T. cruzi at 7, 14, and 21 days after mating plus IFN-gamma treatment as given to the former group; the same protocol except that physiological saline was injected instead of IFN-gamma; injection of physiological saline only. Offspring were challenged at weaning with a similar dose of T. cruzi, to constitute four groups of infected young, plus an additional group of age-matched uninfected rats born to control mothers. PE were harvested at day 7 postinfection (pi), exposed to parasites and further investigated for the replication of T. cruzi and NO production, whereas ELISA studies for measuring serum anti-T. cruzi IgG subclasses and anti-sulphatide autoantibodies were performed at day 30 pi. The number of intracellular parasites in PE was markedly decreased in young born to IFN-gamma-treated mothers, this not being accompanied by higher nitrite levels in culture supernatants. Offspring delivered by IFN-gamma-treated mothers showed no higher serum concentrations of anti-sulphatide autoantibodies, but exhibited a preferential synthesis of anti-T. cruzi IgG2b antibodies. This rat isotype is known to fix complement and constitutes the rat counterpart of IgG2a mouse immunoglobulins whose synthesis is favoured by IFN-gamma.

Benznidazole, a drug employed in the treatment of Chagas' disease, down-regulates the synthesis of nitrite and cytokines by murine stimulated macrophages.

Benznidazole (BZL) is a nitroheterocyclic drug employed in the chemotherapy of Chagas' disease, a protozoan disease caused by Trypanosoma cruzi. Because this parasite mostly replicates in macrophages, we investigated whether BZL was likely to modify the synthesis of macrophage mediators such as nitrite, tumour necrosis factor-alpha (TNF-alpha), IL-1beta, IL-6 and IL-10. Control and stimulated murine macrophages (lipopolysaccharide (LPS) and/or interferon-gamma (IFN-gamma)) were treated with BZL and measurements were carried out in culture supernatants collected 24 h later. Synthesis of nitrite, IL-6 and IL-10 was maximal upon combined stimulation with LPS + IFN-gamma, whereas lower amounts of the three mediators were detected when both stimuli were given alone. BZL treatment significantly reduced nitrite, IL-6 and IL-10 production, to undetectable levels in some cases, particularly IL-6 and IL-10. LPS was the most potent stimulus of IL-1beta and TNF-alpha production, followed by LPS + IFN-gamma and IFN-gamma in decreasing order. BZL partly inhibited TNF-alpha synthesis, but this effect was smaller than that observed for nitrite, IL-6 and IL-10. LPS-induced production of IL-1beta was also affected by BZL. Semiquantification of gene expression for inducible nitric oxide synthase (iNOS) showed that BZL completely inhibited iNOS gene induction by IFN-gamma, and resulted in respective inhibitions of 30% and 50% with LPS- and LPS + IFN-gamma-stimulated cells. BZL was not cytotoxic on macrophage cultures, as shown by the lactate dehydrogenase activity. Besides its trypanocidal activity, BZL may also alter the balance between pro- and anti-inflammatory mediators with important consequences for the course of T. cruzi infection.

Administration of interferon-gama to pregnant rats reverses the depressed adjuvant-induced arthritis of their chronically Trypanosoma cruzi-infected offspring

We demonstrated that administration of interferon gamma (IFN-gama) to the inbred "l" strain of pregnant rats conferred partial resistance on their offspring to challenge with Trypanosoma cruzi. We now examine if this intervention also modifies the reportedly immunodepressed cellular responses which occur during chronic infection. Offspring were born to mothers undergoing one of the following procedures during gestation: subcutaneous injections of recombinant rat IFN-gama, 50,000 IU/rat, five times/week for 3 weeks, which was started on the day of mating (IFN-Mo); infection with 106 trypomastigotes of T. cruzi at 7, 14, and 21 days after mating plus IFN-gama treatment as given to the former group (TcIFN-Mo); the same protocol except that physiological saline was injected instead of IFN-gama (Tc-Mo); injection of physiological saline only (control-Mo). All offspring groups (N = 8-10/group) were infected at weaning and were assessed 90 days later for their adjuvant-induced arthritic response or levels of major T cell subsets in spleen and lymph nodes. TcIFN-Mo and IFN-Mo offspring showed a reestablished arthritic response, which remained within the range seen in controls. Immunolabeling studies on parallel groups of 90-day-infected offspring showed that the inverse CD4/CD8 cell ratio that is usually seen in lymphoid organs from these chronically infected rats (median 0.61) appeared to have recovered in the TcIFN-Mo and IFN-Mo groups (median 1.66 and 1.78, respectively) and was not different from uninfected controls (1.96). These studies indicate that early stimulation with IFN-gama is able to reverse the immunosuppressive state that is usually present during the chronic period of the experimental infection.

Levels of tumor necrosis factor alpha, gamma interferon, and interleukins 4,6, and 10 as determined in mice infected with virulent or attenuated strains of Trypanosoma cruzi.

Inoculation of BALB/c mice with the virulent Tulahuen (TUL) strain of T. cruzi was shown to lead to progressive and eventually lethal infections, whereas infection with an attenuated strain (TCC) resulted in a hardly noticeable experimental disease producing no tissue damage. To determine whether differences in such infection outcome are associated with a particular pattern of cytokine response, a study was undertaken to investigate the serum levels of TNF-alpha, IFN-gamma, IL-4, IL-6, and IL-10 by using an enzyme-linked immunosorbent assay. Mice from both infected groups were bled at 5, 9, 15, 22, 30 and 48 days post-infection (p.i.), with the same interval being applied for obtention of serum samples in age-matched uninfected mice, a group that yielded negative results in all cases. Infection with the TUL strain of T. cruzi was accompanied by a significant increase of TNF-alpha serum concentrations at day 5 p.i., and detectable amounts of IFN-gamma by day 15 p.i., which were exclusively recorded in this group. Serum IL-4 was mostly present in TCC mice with values at day 15 pi being statistically significant in relation to TUL-infected mice. IL-10 was firstly detected at 3 weeks after infection, and showed higher levels in the TCC group, although comparisons with TUL-infected group were not significant. At our limit of detection, no samples were found to contain IL-6 serum concentrations. Infection with virulent parasites seems to be associated with presence of Th1-type cytokines, whereas challenge with the attenuated TCC strain appears as being related to a Th2-type profile.

Macrophage activity, IL-6 levels, antibody response and heart histology in rats undergoing an attenuated Trypanosoma cruzi acute infection upon treatment with recombinant interferon gamma.

Earlier experiments in Trypanosoma cruzi-infected rats showed that recombinant rat (Rr) interferon (IFN)-gamma given shortly after infection ameliorated acute disease without modifying the serum titers of endogenously synthesized IFN-gamma and tumor necrosis factor. To gain some insight into the processes underlying this protective effect, 21-day old 'I' rats that were infected with T. cruzi and the following day started with a 20-day cycle of RrIFN-gamma injections (20000 IU/rat/day) were investigated for the in vitro replication of T. cruzi and nitric oxide (NO) production by peritoneal macrophages (day 7 post-infection, pi), antibodies with lytic activity against T. cruzi (days 7, 20, and 28 pi), and serum levels of biologically active interleukin (IL)-6 (days 15 and 30 pi). Therapy with RrIFN-gamma rendered cultured peritoneal macrophages less permissive to infection with T. cruzi. Such an effect was not accompanied by higher amounts of NO in macrophage cultured supernatants, compared with those from T. cruzi-infected rats receiving no RrIFN-gamma, which appeared not to be protected from in vitro infection. Acutely T. cruzi-infected rats had significant amounts of IL-6 in their sera - this not being the case in infected rats given RrIFN-gamma, whose levels appeared decreased as in control rats. The presence of complement-mediated anti-T. cruzi lytic antibodies was not modified by RrIFN-gamma. Likewise, heart histology at day 7 pi revealed that treatment with RrIFN-gamma made no differences as to the amount of acute inflammation, but tended to reduce the myocardial parasite load.

Electrocardiographic alterations in chronically Trypanosoma cruzi-infected persons exposed to cardiovascular factors.

METHODS: We analyzed the potential influence that associated risk factors (ARF), such as smoking, alcoholism, overweight, and hypertension, could have on the establishment of chronic chagasic cardiomyopathy (CC). The sample was comprised of 124 individuals, 69 males and 55 females (mean age +/- SD, 41 +/- 9.5 years), who were born in en demic areas of Northern Argentina and migrated further to Rosario City, an area where autochthonous cases of Chagas' disease have never been registered. Assessments included the following: clinical examination to discard previous cardiomyopathies; search for the presence of ARF according to standard criteria; specific serology; frontal chest X-ray, and 12-lead resting electrocardiogram (ECG). Subjects were classified on the basis of their serological status and presence of ARF into four groups: Tc+ARF+ T. cruzi-infected persons with ARF (n = 41); Tc-ARF+ seronegativity in presence of ARF (n = 27); Tc+ARF- individuals showing positive serology that lacked ARF (n = 27), and Tc-ARF- seronegative individuals having no ARF (n = 29). RESULTS: Except for a higher female/male ratio in groups presenting no ARF (p < 0.02), no statistical differences as to age, length of residence in endemicity areas (LR), and ARF distribution were recorded among groups. Forty-one persons presented abnormal ECG tracings, distributed thus: Tc+ARF+, 18/41; Tc-ARF+, 14/27, Tc+ARF-, 14/27, and Tc-ARF, 4/29 (p < 0.01, in relation to the latter group). Subjects from the Tc+ARF+, Tc-ARF+, and Tc+ARF- groups had 4.89-, 6.7-, and 6.7-fold increases, respectively, if having an abnormal ECG when compared with Tc-ARF- individuals. Comparisons on the frequency of abnormal ECG between seropositives carrying ARF or not yielded a non-significant odds ratio, be it estimated as crude, or after adjusting for sex, age, and LR in multivariate analysis. CONCLUSIONS: Presence of ARF was not associated with an increasing risk of cardiac affectation in chronically T. cruzi-infected persons, but resulted in chagasic-compatible ECG abnormalities in those seronegative individuals.

Reversal of doxorubicin resistance by the amiloride analogue EIPA in multidrug resistant human colon carcinoma cells.

Although multidrug resistance (mdr) may arise through a variety of mechanisms, the most widely studied and accepted form is associated with an increased concentration of P-glycoprotein (P-gp), a 170 kd protein found in the membrane fraction of a number of mammalian cells. Since mdr seems to be related to the ability of resistant cells to extrude drugs and the circumvention of mdr is supposed to be due to the restored ability to accumulate drugs, membrane has been regarded as the crucial site for such a regulation and an important role for membrane ion exchangers has been suggested. The aim of this work was to elucidate whether the Na+/H+ antiporter is involved in the mechanism of regulation and circumvention of mdr and if 5-(N-ethyl-N-isopropyl) amiloride (EIPA), a selective inhibitor of the Na+/H+ exchanger, can modulate the functional expression of the mdr phenotype. The effect of EIPA on doxorubicin (DX) resistant cells (LoVo/DX) obtained from a human colon adenocarcinoma cell line (LoVo) was studied. EIPA at concentrations ranging from 10 to 50 mu M was able to increase the antibiotic cytotoxicity in the resistant Lovo/DX cells. The reversal of DX resistance paralleled an increase of the ability of the cells to accumulate the drug. Both drug loading and sensitivity to the inhibitory effect of DX on cell proliferation were restored by EIPA in a dose-dependent way. These results suggest a new mechanism of mdr reversal and indicate that amiloride and its derivatives may be useful in reversing DX resistance and in enhancing the clinical effectiveness of chemotherapeutics.

Acute and chronic experimental Trypanosoma cruzi infection in the rat. Response to systemic treatment with recombinant rat interferon-gamma.

We examined the effects of recombinant rat interferon-gamma (IFN-gamma) injections on the parasitologic, serologic, immunologic and histopathologic features of acute and chronic experimental Trypanosoma cruzi (T. cruzi) infections in "l" rats. Upon infection at weaning, two rat groups were allocated to receive a 20-day cycle of IFN-gamma injections, 20,000 IU/rat each, which started at 1, and 7 days post-infection (pi). Treatment with IFN-gamma, initiated at either 1 or 7 days pi, resulted in comparatively lower peak parasitemias (P < 0.02) but in similar levels of anti-T. cruzi circulating antibodies and serum IFN-gamma activities. The latter appeared significantly increased during acute infection whereas biologically active tumor necrosis factor was virtually undetectable in serum from infected rats regardless of whether they had been given IFN-gamma or not. The prevalence of chronic focal myocarditis in IFN-gamma-treated infected rats showed no differences with respect to the one recorded in control-infected counterparts. The inverse CD4/CD8 ratio of spleen and lymph node T cells that usually accompanies chronic infection was reversed by IFN-gamma. Mononuclear cells carrying class II I-A and I-E molecules, that were found to have increased at both compartments, appeared also modified upon IFN-gamma treatment with an overincrease of I-A-positive cells, and a normalization of I-E-bearing cells.

Depressed adjuvant arthritis in chronically Trypanosoma cruzi infected rats: reversal by cyclophosphamide.

Chronically Trypanosoma cruzi infected "I" rats and syngeneic naive recipients, transferred with a T cell enriched spleen cell population from infected donors, develop an attenuated arthritis when challenged with complete Freund's adjuvant. We report that cyclophosphamide, 40 mg/kg body weight, given 48 h before induction, was able to reestablish or exacerbate adjuvant arthritis in infected and control rats, respectively. Although the T cell enriched spleen cells from infected donors continued to down regulate adjuvant arthritis in syngeneic recipients given cyclophosphamide 48 h before cell transfer, treatment of infected donors with cyclophosphamide, 48 h before cell collection, prevented these cells from exerting such effect when transferred to healthy recipients receiving no cyclophosphamide. It is suggested that cyclophosphamide may primarily affect a suppressor cell population, present in the infected host, with regulatory activity on adjuvant arthritis.

Efecto de la reinfeccion sobre la evalucion de ratas infectadas con Trypanosoma cruzi / Effect of reinfection on the evolution of rats infected with Trypanosoma cruzi

El objetivo de este trabajo fue comprobar si una de las variables medio-ambientales, la reinfeccion, puede modificar el comportamiento observado en un modelo de rata a nivel de parasitemia, anticuerpos sericos, manifestaciones electrocardiograficas y/o lesion miocardica. Los grupos experimentales fueron: GI: ratas infectadas al destete com 1 x "10 POT 6" T. cruzi; GR: igual a GI mas reinfecciones cada 30 dias hasta los 150 dias post-infeccion inicial (p.i.i.); "GI IND 1". Los xenodiagnosticos fueron negativos en los tres grupos. Los anticuerpos sericos no se modificaron significativamente en GR respecto de GI, salvo en los anticuerpos 7S, pues los del GR presentaron titulos superiores en algunos de los dias estudiados. Los ECG basales no mostraron cambios distintivos en las ratas infectadas. La pruieba de ajmalina mostro una disminucion de la FC independiente del tratamiento; el PR, QaT y QRS se prolongaron significativamente en todos los grupos respecto del basal (p < 0.05), salvo el QaT en el GT; ademas el cambio de PR y QaT fue mayor en los infectados (p < 0.05). En los grupos infectados hubo tambien una amplia variacion en la orientacion del eje electrico respecto del valor basal, acompanado de cambios morfologicos mas manifiestos emGR. La proporcion de lesion cardiaca detectada histologicamente en los grupos

Depressed adjuvant arthritis in rats transferred with spleen cells from Trypanosoma cruzi-infected syngeneic donors

In the present we investigated whether the attenuating effect of chronica Trypanosoma cruzi (Tc) infection on adjuvant arthritis (AA) in the rat could be transferred to naive recipients. Transfer of whole spleen cells, but not of serum, from Tc-infected rats reduced AA (x ñ SEM: 11 ñ 0.5) in recipient animals (control values, x ñ SEM: 19 ñ 0.7). Transfer of a T-cell enriched subpopulation from spleen cells of Tc-infected rats (obtained by filtration through a nylon column) resulted in a similar attenuationb of AA (x ñ SEM: 7.5 ñ 2.2). The arthritic response of rats intraperitoneally inoculated with 2 x 10**5 Tc 48h before induction did not differ from that observed in controls. Neither parasites nor specific antibodies were observed in suckling mice inoculated with serum or cell suspensions employed in transfer experiments. Consequently, the depressive effect on AA could not be directly attributed to Tc per se. We hypothesize that a homeostatic immunosuppressor mechanism may be responsible for this phenomenon