Microbiological monitoring in inbred mouse foundation stocks in Japan

Microbiological monitoring on 128 inbred mouse foundation stocks consisted of common 10 inbred strains and inbred strains originated from outbred dd mice was performed by cooperation of 24 organizations. A total of 881 mice were divided into 647 conventional animals from 95 colonies and 234 barrier-sustained animals from 33 colonies. Three viral, one mycoplasmal, 6 bacterial, one fungal and 3 parasitic agents selected as monitoring microbes according to the proposed selection standards. Among conventional colonies, 84.2% were positive for at least one agent. The highest detection rate was 44.2% for S. obvelata, followed by P. pneumotropica and S. muris, P. aeruginosa, G. muris, Sendai virus, M. pulmonis, MHV and E. coli O115a, c: K (B). Of these agents, only one microbe, P. aeruginosa, was detected in barrier-sustained colonies (36.4%), thus the efficacy of barrier system for the microbiological quality control of the inbred mouse foundation stocks was actually demonstrated. The positive rates of MHV (6.3%) and Sendai v. (16.8%) were significantly low compared with those in experimental mouse colonies. Positivity for parasites was rather high and they were infested together with other pathogens in many cases. Thus parasites including G. muris, S. muris and S. obvelata were regarded as useful indicators to see microbiological contaminations in conventional mice. There observed no strain difference in susceptibility to pathogens except for C57BL/6 and AKR mice which seemed to be high antibody responders to MHV.     

Plasmodium yoelii: Adverse outcome of non-lethal P. yoelii malaria during co-infection with Schistosoma mansoni in BALB/c mouse model

Plasmodium yoelii and Schistosoma mansoni co-infections were studied in female BALB/c mice aged 4-6 weeks to determine the effect of time and stage of concomitant infections on malaria disease outcome. Patent S. mansoni infection in BALB/c mice increased malaria peak parasitemia and caused death from an otherwise non-lethal, self-resolving P. yoelii malaria infection. Exacerbation of malaria parasitemia occurred during both pre-patent and patent S. mansoni infection resulting in a delay of 4-8 days in malaria parasite resolution in co-infected mice. Praziquantel administered to mice with patent schistosome infection protected from fatal outcome during co-infection. However, this treatment did not completely clear the worm infestation, nor did it reduce the peak malaria parasitemia reached, which was nonetheless resolved completely. Hepatosplenomegaly was more marked in schistosome and malaria co-infected mice compared to either infection separately. The results suggest a complex relationship between schistosome co-infection and malaria disease outcome in which the timing of malaria infection in relation to schistosome acquisition is critical to disease outcome and pathology.     

Chronic Giardia muris infection in anti-IgM-treated mice. I. Analysis of immunoglobulin and parasite-specific antibody in normal and immunoglobulin-deficient animals

To investigate the role of B cells and antibody in the immune response of mice to the murine intestinal parasite Giardia muris, we used mice treated from birth with rabbit anti-IgM antisera (aIgM). Such mice developed in serum and in gut secretions extreme Ig deficiency (IgM, IgA, and IgG) relative to control animals. The aIgM-treated mice showed no anti-G. muris antibody in serum or in gut wash material. Infections of G. muris in these mice were chronic, with a high load of parasite present in the small bowel, as reflected by prolonged cyst excretion (greater than 11 wk) and high trophozoite counts. In contrast, normal, untreated mice or NRS-treated animals developed anti-parasite IgA and IgG antibody in serum, demonstrated IgA antibody against the parasite in gut washings, and expelled the parasite within 9 wk. These effects of aIgM treatment on the murine response to primary infection with G. muris were demonstrated in two strains of mice: BALB/c and (C57BL/6 X C3H/He) F1. It was also observed that the response to G. muris infection in untreated animals was characterized by higher than normal total secretion of IgA into the gut and a concomitant increase in the serum polymeric IgA level. Mice treated with aIgM had a marked decrease of both monomeric and polymeric IgA in serum, and little detectable IgA in the intestinal lumen. These experiments provide the first demonstration that anti-IgM treatment suppresses a specific intestinal antibody response to antigen, and provide evidence that B cells and antibody play a role in the development of an effective response to a primary infection with G. muris in mice.     

Time-course of Giardia muris infection in male and female immunocompetent mice

The time-course of acute Giardia muris infection was compared in male and female immunocompetent BALB/c mice that had not previously been exposed to the parasite. No sex-related difference was observed in the time-course of the infection in these mice. Sexually mature mice of both sexes excreted substantial numbers of G. muris cysts (greater than 10(4)/2 hr) over a longer period than did sexually immature mice.     

Long-term population dynamics of pinworms (Syphacia obvelata and Aspiculuris tetraptera) in mice

The population dynamics of concurrent infections of Syphacia obvelata and Aspiculuris tetraptera (Nematoda) in laboratory mice were investigated under conditions of constant re-exposure over periods of 56 and 115 days. The results indicate that A. tetraptera burdens equilibrate at a higher level than S. obvelata burdens and that both parasites become aggregated in the mouse population. Parasite burdens were higher following short-term (7 day) exposure of uninfected tracer mice to both parasites when compared with parasite burdens attained over long-term exposure, indicating probable development of immunity. A significant positive correlation was detected between numbers of immature S. obvelata and immature A. tetraptera for both experimental and tracer mice.     

Response of Peyer's patch lymphocyte subsets to Giardia muris infection in BALB/c mice. I. T-cell subsets

Initial cellular events in the intestinal immune response occur within Peyer's patches and are subject to complex regulation by T cells. The aim of this study was to analyze the response of murine Peyer's patch T, T-helper (Th), and T-suppressor (Ts) cells to Giardia muris infection. Immunocompetent BALB/c mice were infected with G. muris cysts and, at serial times during the infection, Peyer's patch leukocyte suspensions were incubated with fluorescent monoclonal antibodies that identified murine leukocytes, T, Th, or Ts lymphocytes. These suspensions were examined by flow cytometry to quantify each T-cell subset as a percentage of total leukocytes. Total Peyer's patch leukocytes more than doubled in number during the course of G. muris infection and returned to control levels as the infection was cleared. The percentages of Peyer's patch Th and Ts lymphocytes were 34.1 +/- 0.8% (mean +/- SEM) and 6.2 +/- 0.3%, respectively, in the absence of infection, and did not change significantly during Giardia infection. The Th/Ts ratio in Peyer's patches was 5.6 +/- 0.2 in uninfected BALB/c mice and also did not change significantly during clearance of G. muris. We conclude that Peyer's patch leukocytes double in number in response to G. muris infection in immunocompetent mice, G. muris infection does not lead to altered percentages of Peyer's patch T, Th, or Ts lymphocytes, and clearance of G. muris infection is associated with a Peyer's patch Th/Ts ratio of greater than 5.     

The influence of Mesocestoides corti on subsequent Angiostrongylus cantonensis infections in mice.

The influence of Mesocestoides corti on subsequent Angiostrongylus cantonensis infection in mice (C57BL/6 and BALB/c) was assessed. Both strains of mice infected with M. corti demonstrated a peak blood eosinophilia at around 3 weeks post-infection (p.i.). C57BL/6 and BALB/c mice primarily infected with M. corti were given A. cantonensis infection 18 days later, but pre-existing M. corti infection did not affect the recovery of intracranial worms of A. cantonensis at day 21 p.i. BALB/c mice with mixed parasite infections showed low morbidity and mortality as compared with mice singly infected with A. cantonensis and some mice demonstrated a pulmonary migration of intracranial worms. In C57BL/6 mice, intracranial worms were killed and thus all mice survived. C57BL/6 mice with mixed parasite infections failed to resist A. cantonensis reinfection. The blastogenic responses of spleen cells against A. cantonensis antigen were lower in BALB/c than in C57BL/6 mice and mixed parasite infections also resulted in less blastogenic responses against both concanavalin A and A. cantonensis antigen than monoinfection. The recovery of M. corti biomass was significantly higher in mice with mixed parasite infections than mice with monoinfection with M. corti. These data suggest a distinct difference in response to A. cantonensis infection between C57BL/6 and BALB/c mice, and the induction of immunosuppression in both mouse strains following M. corti infection. Blood eosinophilia provoked by M. corti infection is not directly associated with the killing of worms in subsequent A. cantonensis infection.     

Intradermal inoculations of low doses of Leishmania major and Leishmania amazonensis metacyclic promastigotes induce different immunoparasitic processes and status of protection in BALB/c mice

In order to simulate the natural long term parasitisms which may occur in mammals infected with Leishmania, cutaneous leishmaniases due to Leishmania major or Leishmania amazonensis were induced using a model based on the inoculation of 10-1000 metacyclic promastigotes into the ear dermis of BALB/c mice. The final outcome of these parasitisms was dependent upon the number of inoculated parasites. Only some of the mice inoculated with ten parasites displayed cutaneous lesions, whereas most mice infected with 100 metacyclics and all mice infected with 1000 metacyclics developed progressive lesions. We found, using the latter experimental conditions, that the onset of the pathology was associated with: (a) parasite multiplication in the inoculation site and the draining lymph node correlating with an increase of the lymph node cell number, especially in L. major-infected mice; and (b) the detection of lymph node cells, at least in part CD4(+) T lymphocytes, able to produce high levels of interferon-gamma, interleukin (IL)-4, IL-10 and IL-13. Thereafter, mice infected by L. major harboured few parasites in the ear and had a 100-fold reduction in lymph node parasite load between 23 and 40 weeks post-inoculation. In contrast, the parasite loads of L. amazonensis-infected mice remained stable in the ear and increased in nodes during the same period of time. Only L. major-infected mice that exhibited cutaneous lesions in the primary site were resistant to the re-inoculation of 1000 metacyclic promastigotes, whereas all L. amazonensis-primary infected mice remained susceptible to a second homologous challenge. These results are the first to document that a status of resistance to re-infection, referred to concomitant immunity, is coupled to the development of primary progressive lesions in L. major-infected BALB/c mice. Such a protective status is absent in L. amazonensis-infected BALB/c mice.     

Susceptibility of five strains of mice to Babesia microti of human origin.

One outbred (CF1) and four inbred (BALB/c, C57, CBA and C3H) strains of mice were tested for susceptibility to Babesia microti of human origin. Of these, intact C3H mice developed higher parasitemia than all other intact mice, while BALB/c mice developed the highest parasitemia among splenectomized mice. Susceptibility was not related to H-2 haplotype in any obvious way. Because C3H and BALB/c mice developed relatively high initial peak parasitemias, the parasite was serially passaged in both of these mouse strains in an attempt to increase parasite virulence. After 30 passages in BALB/c and 49 passages in C3H mice over a period of 12 months, maximum parasitemias were 50 times higher than those observed initially. After the peak parasitemias of these two mouse-adapted parasites had stabilized, the relationship between onset and level of maximum parasitemia and number of parasites inoculated was determined. With both C3H- and BALB/c-adapted parasites, as inoculum size increased, the time required to reach maximum parasitemia decreased and the level of maximum parasitemia increased. Studies involving infection of either mouse strain with parasites adapted to the heterologous mouse strain indicated that C3H mice were more susceptible than BALB/c mice to homologous or heterologous parasites. These data suggest that the virulence of B. microti to the mouse can be increased by prolonged passage in this host. Once adaptation to this host species has occurred, virulence appears to be more dependent on the innate susceptibility of the mouse strain than on adaptation of the parasites to a particular strain of mouse.     

Activation of suppressor cells of the delayed hypersensitivity response in BALB/c mice infected or immunized with Leishmania mexicana pifanoi

A T suppressor cell population that specifically shut down delayed hypersensitivity responses (DHR) to the parasite was found in both BALB/c mice chronically infected with Leishmania mexicana pifanoi and in naive mice which had received a single IV supraoptimal dose of killed parasites. At the early phase of infection mice exhibited a transitory state of cell-mediated immunity against the parasite that was abrogated when lesions reached their accelerated phase of growth. Results suggest that in both infected and high-dose immunized mice, the activation of T suppressor cells of DHR is related to antigen overload.     

Human chagasic serum contains antibodies capable of inhibiting Trypanosoma cruzi egress from tissue culture cells

Previous studies at our laboratory have shown that an antibody (antiegressin) present in the serum of chronically infected mice is capable of inhibiting the egress of Trypanosoma cruzi from infected BALB/c fibroblasts. We have used this in vitro system to evaluate whether human chagasic serum is also capable of inhibiting T. cruzi egress. BALB/c fibroblasts were infected with tissue culture-derived parasites. Five-percent solutions of the individual human serum samples in culture medium were added to the wells, and the number of parasites released was determined at day 5 after infection. The cells cultured with serum from infected individuals released between 37% and 72% fewer parasites than those cultured with control serum. A similar reduction in parasite egress resulted from incubation with the protein-A purified IgG fraction from 3 of these human samples. Immunocytochemical staining employing antineuraminidase antibodies supported the notion that the reduction in parasite levels is due to inhibition at the point of parasite egress. These results indicate that human serum of individuals infected with T. cruzi is capable of inhibiting release of the parasite from infected tissue culture cells and that the phenomenon of egress-inhibition may be relevant during infection of human subjects.     

Isolation of protective T cells from BALB/cJ mice chronically infected with Leishmania donovani

BALB/cJ mice, which are homozygous for Lshs on chromosone 1, are genetically susceptible to Leishmania donovani (S3), but spontaneously reduce their parasite burdens late in the course of infection. Spleens from chronically infected mice (5 to 8 mo) were found to have T cells that responded to leishmanial Ag by proliferating and secreting immune IFN. An IFN-secreting T cell line derived from the spleen of a chronically infected mouse, after boosting with leishmanial Ag and treatment with Pentostam to kill residual parasites, was able to activate macrophages to kill amastigotes in vitro. Furthermore, after adoptive transfer of this cell line, naive BALB/cJ mice challenged with amastigotes demonstrated a 42-fold reduction in parasite burden compared to controls. Four of five clones derived from the protective T cell line secreted IFN and proliferated between days 7 and 14 after Ag stimulation. All clones were Ly1+2-, L3T4+. Another T cell line, which had Ly1+2-, L3T4+ phenotype, was derived from the lymph nodes of s.c. immunized, uninfected mice. It multipled but never produced IFN in response to leishmanial Ag and failed to protect macrophages against L. donovani infection in vitro or in vivo. This nonprotective T cell line and the fifth clone from the protective line, which also did not secrete IFN, proliferated between days 2 and 7 after Ag stimulation. In summary, leishmania-responsive helper/inducer T cells, which produced IFN but were slow to proliferate in response to Ag in vitro and which were able to activate macrophages to reduce amastigotes in vitro and in vivo, are present in chronically infected BALB/cJ mice and may mediate the decrease in parasite burden during chronic infection.     

IL-4 is required to prevent filarial nematode development in resistant but not susceptible strains of mice

The murine Litomosoides sigmodontis model of filarial infection provides the opportunity to elucidate the immunological mechanisms that determine whether these nematode parasites can establish a successful infection or are rejected by the mammalian host. BALB/c mice are fully susceptible to L. sigmodontis infection and can develop patent infection, with the microfilarial stage circulating in the bloodstream. In contrast, mice on the C57BL background are largely resistant to the infection and never produce a patent infection. In this study, we used IL-4 deficient mice on the C57BL/6 background to address the role of IL-4 in the development of L. sigmodontis parasites in a resistant host. Two months after infection, adult worm recovery and the percentage of microfilaraemic mice in infected IL-4 deficient mice were comparable with those of the susceptible BALB/c mice while, as expected, healthy adults were not recovered from wild type C57BL/6 mice. The cytokine and antibody responses reveal that despite similar parasitology the two susceptible strains (BALB/c and IL-4 deficient C57BL/6) have markedly different immune responses: wild type BALB/c mice exhibit a strong Th2 immune response and the IL-4 deficient C57BL/6 mice exhibit a Th1 response. We also excluded a role for antibodies in resistance through infection of B-cell deficient C57BL/6 mice. Our data suggest that the mechanisms that determine parasite clearance in a resistant/non-permissive host are Th2 dependent but that in a susceptible/permissive host, the parasite can develop in the face of a Th2 dominated response.     

A survey of Neospora caninum and Toxoplasma gondii infection in urban rodents from Brazil

Toxoplasma gondii is a protozoan parasite that infects humans and other warm-blooded animals; it uses feral and domestic cats as the definitive hosts. Neospora caninum is a protozoan parasite of animals whose life cycle is very similar to T. gondii but uses canids as definitive hosts. Small rodents play an important role in the life cycle of T. gondii , and a few findings indicated that they may be natural intermediate hosts for N. caninum . The present study was aimed at identifying infections by T. gondii and N. caninum in urban rodents. Infections by T. gondii were quantified using isolation of the parasite by bioassay in mice; molecular methods were also used for both parasites. Overall, 217 rodents were captured. Brain and heart tissues of all rodents were bioassayed in mice for the detection of T. gondii infection. Brain and heart tissues of 121 rodents had the DNA extracted for molecular analysis. Toxoplasma gondii was isolated by bioassay from a single rodent. From the 121 rodents tested for the presence of T. gondii DNA, 2 animals were positive. In contrast, DNA of N. caninum was not detected in any of the samples. In conclusion, the surveys of N. caninum and T. gondii infection in Rattus rattus , Rattus norvegicus , and Mus musculus captured in urban areas of S?o Paulo reveal a striking low frequency of occurrence of these infections.     

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.     

Taenia crassiceps cysticercosis: Variations in its parasite growth permissiveness that encounter with local immune features in BALB/c substrains

This study describes the first days of Taenia crassiceps infection in BALB/c substrains, BALB/cAnN and BALB/cJ, using two stocks of the same strains which were kept in different animal facilities, conventional and pathogen-free conditions, respectively. This study shows that parasite growth restriction shown by conventional BALB/cJ mice changed to parasite growth permissiveness when pathogen-free BALB/cJ mice were used. In addition, the higher number of macrophages, NK cells and intraperitoneal level of IFN-γ found in the conventional restrictive BALB/cJ substrain vanished when the permissiveness to the parasite growth increased. No differences were found in DNA sequences of parasites collected before and after the change in the permissiveness to parasite growth which favors the possibility that the observed modifications could be due to changes in the murine strains and/or their maintenance conditions.     

Genetic diversity and recruitment pattern of Schistosoma mansoni in a Biomphalaria glabrata snail population: a field study using random-amplified polymorphic DNA markers.

Random-amplified polymorphic DNA markers have been used to assess the amount and the distribution of the genetic diversity of Schistosoma mansoni within a natural population of Biomphalaria glabrata at a transmission site of the murine schistosomiasis focus of Guadeloupe. Despite high infection rate and heavy schistosome load within the definitive hosts (Ratus rattus), prevalences within intermediate snails ranged from 0.2 to 4.8%. Whatever the transmission season may be (rainy vs. dry), most of the infected snails were spatially aggregated and 88.4% of them harbored a single parasite genotype indicative of a monomiracidial infection; 4.7% had dual sex infections and a parasite intensity not exceeding 3 miracidia per snail. A substantial resistance level toward the parasite and recruitment regulatory process within snails may explain in part the observed low parasite prevalences and intensities. Considering such a distribution pattern of larval S. mansoni genetic diversity among B. glabrata, mobility of the definitive hosts, or rapid turnover of infected snails, or both, are required to maintain genetic heterogeneity within adult schistosome populations.     

Foxp3+ Regulatory T Cells Delay Expulsion of Intestinal Nematodes by Suppression of IL-9-Driven Mast Cell Activation in BALB/c but Not in C57BL/6 Mice

Accumulating evidence suggests that IL-9-mediated immunity plays a fundamental role in control of intestinal nematode infection. Here we report a different impact of Foxp3⁺ regulatory T cells (Treg) in nematode-induced evasion of IL-9-mediated immunity in BALB/c and C57BL/6 mice. Infection with Strongyloides ratti induced Treg expansion with similar kinetics and phenotype in both strains. Strikingly, Treg depletion reduced parasite burden selectively in BALB/c but not in C57BL/6 mice. Treg function was apparent in both strains as Treg depletion increased nematode-specific humoral and cellular Th2 response in BALB/c and C57BL/6 mice to the same extent. Improved resistance in Treg-depleted BALB/c mice was accompanied by increased production of IL-9 and accelerated degranulation of mast cells. In contrast, IL-9 production was not significantly elevated and kinetics of mast cell degranulation were unaffected by Treg depletion in C57BL/6 mice. By in vivo neutralization, we demonstrate that increased IL-9 production during the first days of infection caused accelerated mast cell degranulation and rapid expulsion of S. ratti adults from the small intestine of Treg-depleted BALB/c mice. In genetically mast cell-deficient (Cpa3-Cre) BALB/c mice, Treg depletion still resulted in increased IL-9 production but resistance to S. ratti infection was lost, suggesting that IL-9-driven mast cell activation mediated accelerated expulsion of S. ratti in Treg-depleted BALB/c mice. This IL-9-driven mast cell degranulation is a central mechanism of S. ratti expulsion in both, BALB/c and C57BL/6 mice, because IL-9 injection reduced and IL-9 neutralization increased parasite burden in the presence of Treg in both strains. Therefore our results suggest that Foxp3⁺ Treg suppress sufficient IL-9 production for subsequent mast cell degranulation during S. ratti infection in a non-redundant manner in BALB/c mice, whereas additional regulatory pathways are functional in Treg-depleted C57BL/6 mice.     

Response of Peyer's patch lymphocyte subsets to Giardia muris infection in BALB/c mice. II. B-cell subsets: enteric antigen exposure is associated with immunoglobulin isotype switching by Peyer's patch B cells

The aim of this study was to quantify the response of Peyer's patch B cells, surface IgA-bearing (sIgA) B cells, and surface IgM-bearing (sIgM) B cells to Giardia muris infection. Following infection of a cohort of immunocompetent BALB/c mice with G. muris cysts, Peyer's patch cell suspensions were prepared at serial time points during the infection, incubated with fluorescein-conjugated monoclonal antibodies directed against murine leukocytes, B cells, sIgA B cells, sIgM B cells, or T cells, and analyzed by flow cytometry. Of total Peyer's patch leukocytes, the percentages of B cells, sIgA B cells, and sIgM B cells in uninfected BALB/c mice were 64.7 +/- 2.0% (mean +/- SEM), 30.3 +/- 1.5%, and 52.5 +/- 2.4%, respectively. The total number of Peyer's patch leukocytes increased significantly (1.8 X) during G. muris infection, and returned to control levels as the infection was cleared. The percentages of Peyer's patch T and total B cells did not change significantly during Giardia infection. However, sequential changes were observed in the percentages and numbers of sIgM and sIgA B cells during the infection. Peyer's patch sIgM B cells rapidly increased in percentage and number, reaching maximum levels 1 week after cyst inoculation. After remaining constant the first week, the number of Peyer's patch sIgA B cells increased during the second week of G. muris infection, reaching a maximum level 11-14 days after cyst inoculation. The data support the hypothesis that immunoglobulin isotype switching in Peyer's patches is induced by antigen exposure.     

Leishmania donovani Infection of a Susceptible Host Results in Apoptosis of Th1-Like Cells: Rescue of Anti-Leishmanial CMI by Providing Th1-Specific Bystander Costimulation

A protective immune response against Leishmania donovani infection is mediated by T-helper type 1 (Th1) cells. Th1 induced cell-mediated immunity (CMI), as assessed by anti-leishmanial DTH response, is lost in a susceptible host such as BALB/c mice. Although the impaired Th1 function eventuates in unhindered parasite growth and in manifestation of the susceptible phenotype, the mechanism of down-regulation of the Th1 function is yet to be elucidated. Here, we provide evidence that the parasite downregulates the expression of a Th1-specific costimulatory molecule, M150, on the surface of infected BALB/c mice-derived macrophages. Th cells are rendered unresponsive to anti-CD3 Ab-mediated stimulation after interaction with infected macrophages. The anergized T cells produce much less IL-2, IL-4 and IFN-γ compared to those T cells which were costimulated using normal macrophages. The defect in proliferation, anti-CD3 Ab induced unresponsiveness and IFN-γ but not IL-4 production can be restored by providing bystander costimulation through M150. These results not only unfold a novel immune evasion strategy used by the parasite but also clarify the mechanism of Th1 cell debilitation during the disease. Recovery of Th1 cytokine production by bystander costimulation through M150 may help in formulating a new strategy for the elimination of intracellular parasites.