Infection with the cestode Hymenolepis diminuta induces changes in acetylcholine metabolism and muscarinic receptor mRNA expression in the rat jejunum

Total and neuron-specific uptake of [³H] choline into smooth muscle/myenteric plexus (SM/MP) preparations from the jejunum of rats infected with five Hymenolepis diminuta for 30 days compared to uninfected rats was significantly increased, as was choline acetyltransferase activity and acetylcholine biosynthesis. Although acetylcholinesterase and total cholinesterase activity levels in SM/MP preparations from infected rats were not significantly different from uninfected animals, pseudocholinesterase activity was significantly elevated in infected rats. Infection resulted in a significant elevation in the relative expression of muscarinic 2 (M2) receptor mRNA in jejunum compared to uninfected rats. Conversely, in rats infected with 50 worms for 30 days, the relative expression of muscarinic 1 (M1) receptor mRNA in the jejunum was significantly depressed, while the expression of M2 receptor mRNA was not significantly different from that in five worm infections. The relative expression of muscarinic 3 receptor mRNA was unaffected by infection. The present study shows that infection of rats with low numbers of an enteric cestode leads to a significant modulation of the cholinergic components of the myenteric plexus and M2 receptor mRNA, and that large number of worms result in suppression in the relative expression of M1 receptor mRNA.     

Differing effects of NT-3 and GDNF on dissociated enteric ganglion cells exposed to hydrogen peroxide in vitro

Oxidative stress is widely recognized to contribute to neuronal death during various pathological conditions and ageing. In the enteric nervous system (ENS), reactive oxygen species have been implicated in the mechanism of age-associated neuronal loss. The neurotrophic factors, neurotrophin 3 (NT-3) and glial cell line-derived neurotrophic factor (GDNF), are important in the development of enteric neurons and continue to be expressed in the gut throughout life. It has therefore been suggested that they may have a neuroprotective role in the ENS. We investigated the potential of NT-3 and GDNF to prevent the death of enteric ganglion cells in dissociated cell culture after exposure to hydrogen peroxide (H(2)O(2)). H(2)O(2) treatment resulted in a dose-dependent death of enteric neurons and glial cells, as demonstrated by MTS assay, bis-benzimide and propidium iodide staining and immunolabelling. Cultures treated with NT-3 prior to exposure showed reduced cell death compared to untreated control or GDNF-treated cultures. GDNF treatment did not affect neuronal survival in H(2)O(2)-treated cultures. These results suggest that NT-3 is able to enhance the survival of enteric ganglion cells exposed to oxidative stress.     

The cellular site of virus replication in the intestine of chicks infected with an avian adenovirus

Summary The terminal ileum from 10 chicks infected orally with strain 93 avian adenovirus and 5 uninfected chicks were examined by the fluorescent antibody technique and conventional histologic methods. In the infected birds, cells containing viral antigen were scattered, relatively few in number, and present only in the epithelial layer forming the surface of the villi. Both columnar and goblet cells were infected. Neither clusters of infected cells nor contiguous infected cells were observed. On stained sections examined with the light microscope, hypertrophied epithelial cells containing nuclear inclusions were seen to have essentially the same sparse and scattered distribution as cells containing viral antigen as shown by fluorescence techniques. The overall architecture of the epithelium was normal and no lymphoid hyperplasia or inflammatory reaction was present. The fluorescent antibody localization and inclusions implicate the nuclei of epithelial cells as the site of viral replication in the intestine with the particular model employed.This investigation was supported primarily by Grant AI-04294 VR from the National Institute of Allergy and Infectious Diseases, U.S. Public Health Service awarded toD. I. Clemmer. Support ofH. Ichinose by an Advanced Fellowship from the American Cancer Society is also acknowledged.     

Apoptosis of Mycobacterium avium-infected macrophages is mediated by both tumour necrosis factor (TNF) and Fas, and involves the activation of caspases

Mycobacterium avium causes disseminated infection in AIDS patients and several forms of infection in immunocompetent hosts. Recent studies have shown that M. avium infection of macrophages in vitro leads to apoptosis of significant numbers of infected cells. Several strains of M. avium used to infect human macrophages for 5 days (multiplicity of infection of 10) triggered 28-46% higher levels of apoptosis than observed with uninfected macrophages at the same time points. Mycobacterium avium strains unable to replicate intracellularly (rep-) resulted in a 15% rate of apoptosis, while M. smegmatis-infected monolayers showed the same percentage of apoptotic cells as the uninfected macrophage control. The presence of anti-TNF-alpha antibody reduced apoptosis to 17% and the presence of anti-Fas antibody reduced apoptosis to 10%. When both antibodies were used together, the apoptosis level was 5% above the control. Treatment with TGF-beta also reduced the number of apoptotic cells in infected monolayers. If intracellular growth was inhibited, apoptosis of macrophages decreased significantly. It was also shown that apoptosis was associated with IL-1 beta-converting enzyme (ICE) activation and was significantly reduced by a caspase inhibitor. Gaining understanding of the mechanisms of M. avium-associated apoptosis of macrophages will provide important insight into M. avium pathogenesis.     

NGF和GDNF基因重组逆转录病毒体外感染神经干细胞及其鉴定

目的 探讨神经干细胞（NSC）直接作为基因靶细胞能否被重组逆转录病毒感染以及感染后目的基因的表达。方法 分别用NGF和GDNF基因重组逆转录病毒上清液感染NSC2d;G418筛选后加入bFGF扩增培养;取感染后的NSC培养液（NGF／GDNF条件培养液）培养PC12细胞和中脑腹侧神经元;用免疫组织化学染色方法检测中脑腹侧多巴胺神经元的形态改变以及感染后NSC对目的基因的表达。结果 经G418筛选后,约50％感染NGF和GDNF基因重组逆转录病毒的NSC呈G418抗性;这些感染后的NSC开始分化,分裂球向四周长出放射状突起,部分细胞沿突起向外迁移生长;感染NGF基因的NSC呈星形,胞体较大,突起较粗,感染GDNF基因的NSC呈梭形,胞体较小,突起较长。经NGF条件培养液培养的PC12细胞,数量增多,突起明显变长。经GDNF条件培养液培养的中脑多巴胺神经元（TH阳性细胞）其胞体亦增大,突起伸长。大部分G418抗性的NSC出现NGF和GDNF免疫染色。结论 神经干细胞可直接作为基因靶细胞,能被NGF和GDNF基因重组逆转录病毒有效感染而表达和分泌有生物学活性的NGF和GDNF。     

Polyclonal B cell activation during the course of Angiostrongylus cantonensis infection

The numbers of IgM antibody forming cells against various heterologous antigens: SRBC, HRBC, TNP and FITC, were found to be elevated in the spleens of rats after infection with Angiostrongylus cantonensis. In these rats, the number of splenic B cells and total IgG and IgM levels in serum also increased spontaneously about 2- to 3-fold of uninfected rats during the course of the infection. These results suggested that A. cantonensis infection may activate B cells polyclonally in the spleens of the infected rats. While, it was clearly observed that the antibody response to SRBC injection was significantly depressed in these rats, compared to those of uninfected rats. The relation between the polyclonal B cell activation and the immunodepression was discussed.     

Porcine reproductive and respiratory syndrome virus-induced cell death exhibits features consistent with a nontypical form of apoptosis

Porcine reproductive and respiratory syndrome virus (PRRSV), an arterivirus, belongs to a group of RNA viruses that are cytopathic for macrophages and establish persistent infections. Apoptosis is the presumed mechanism of cell death in monkey kidney cell lines and porcine alveolar macrophages after infection with European PRRSV isolates. However, evidence from in vivo and in vitro studies using North American strains have failed to identify apoptosis in cells supporting virus replication and suggest that apoptosis is present in only uninfected bystander cells. The purpose of this study was to evaluate the mechanism of cell death following the infection of MARC-145 cells with wild-type (P6) and a cell culture-adapted (P136) strains derived from the North American isolate SDSU-23983. At 2 days after infection with P136, cytoplasmic blebbing and nuclear condensation were absent in monolayers containing almost 90% infected cells. By day 3, these infected cells detached and showed evidence of apoptosis, including nuclear condensation and inter-nucleosomal DNA fragmentation. Apoptosis in single infected floating cells was confirmed by the co-localization of FITC-anti-digoxigenin antibody, used to detect uridine-digoxigenin-labled nuclear DNA in a terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end-labeling assay, and Texas red-labeled PRRSV antibody. A majority of infected floating cells were also positive for the uptake of trypan blue, an indicator of necrotic cell death. These results demonstrate that apoptosis does occur in PRRSV infected cells, but is a late event during PRRSV replication and rapidly culminates in a necrotic-like death.     

NK cells exacerbate the pathology of influenza virus infection in mice

NK cells offer a first line of defense against viruses and are considered beneficial to the host during infection. Nevertheless, little is understood regarding the phenotype and function of NK cells in the lung during influenza virus infection. We found that the frequency of NK cells in mouse lung increased during influenza infection, with the majority of a mature phenotype. Cell surface CD107a and intracellular IFN‐γ were detected in cells expressing multiple NK‐cell receptors in infected lung, suggesting that NK cells were activated during infection. The activating receptor NKp46 was predominantly negative on such cells, possibly as a result of encountering influenza HA. Depletion of NK cells in vivo with anti‐asialo GM1 or anti‐NK1.1 reduced mortality from influenza infection and surviving mice recovered their body weight. Pathology induced by NK cells was only observed with high, not medium or low‐dose influenza infection, indicating that the severity of infection influences NK‐cell‐mediated pathology. Furthermore, adoptive transfer of NK cells from influenza‐infected lung, but not uninfected lung, resulted in more rapid weight loss and increased mortality of influenza‐infected mice. Our results indicate that during severe influenza infection of the lung, NK cells have a deleterious impact on the host, promoting mortality.     

Homeostasis and function of goblet cells during rotavirus infection in mice

Rotaviruses are the leading cause of severe viral gastroenteritis in young children. To gain insight in goblet cell homeostasis and intestinal mucin expression during rotavirus infection, 6-day-old mice were inoculated with murine rotavirus. To determine epithelial cell migration, mice were injected with BrdU just before inoculation. Small intestines were isolated at different days postinfection (dpi) and evaluated for rotavirus and goblet cell-specific gene expression. Small intestinal mucins of control and infected animals at 1, 2, and 4 dpi were isolated and tested for their capability to neutralize rotavirus infection in vitro. After inoculation, two peaks of viral replication were observed at 1 and 4 dpi. During infection, the number of goblet cells in infected mice was decreased in duodenum and jejunum, but was unaffected in the ileum. Goblet cells in infected animals accumulated at the tips of the villi. Muc2 mRNA levels were increased during the peak of viral replication at 1 dpi, whereas at other time points Muc2 and Tff3 mRNA levels were maintained at control levels. Muc2 protein levels in the tissue were also maintained, however Tff3 protein levels were strongly decreased. The number of goblet cells containing sulfated mucins was reduced during the two peaks of infection. Mucins isolated at 1 and 2 dpi from control and infected mice efficiently neutralized rotavirus infection in vitro. Moreover, mucins isolated from infected mice at 4 dpi were more potent in inhibiting rotavirus infection than mucins from control mice at 4 dpi. In conclusion, these data show that during rotavirus infection, goblet cells, in contrast to enterocytes, are relatively spared from apoptosis especially in the ileum. Goblet cell-specific Muc2 expression is increased and mucin structure is modified in the course of infection. This suggests that goblet cells and mucins play a role in the active defense against rotavirus infection and that age-dependent differences in mucin quantities, composition, and/or structure alter the anti-viral capabilities of small intestinal mucins.     

Evaluation of a flow cytometric model for monitoring HIV antigen expression in vitro.

Using flow cytometry, monoclonal antibodies to the HIV proteins p24, gp41 and p17 were evaluated for their ability to detect HIV antigens associated with HIV-infected T cells. Mixtures containing varying ratios of HIV-infected and uninfected cells were subjected to analysis with these monoclonal antibodies. In most cases, the monoclonal antibodies identified the correct ratio of HIV-infected cells to uninfected cells in the mixtures tested. An HIV anti-p24 monoclonal antibody was selected for further studies. Flow cytometric analysis was performed on various populations of cells including uninfected, acutely infected and chronically infected cells. Based on cell population fluorescence intensity three distinct regions were identified. In the first region were cells having low level fluorescence that were considered negative for HIV antigens, a profile detected in uninfected cells, and in the majority of cells in the first days following acute HIV infection. In the second region were those cells exhibiting strong fluorescence such as chronically infected cells or acutely infected cells several days after infection. A third region was identified containing cells that were intermediate in fluorescence intensity. Cells exhibiting intermediate intensity fluorescence appeared to have low concentrations of HIV p24 antigen associated with them either through viral adsorption and uptake or through low level virus expression. These intermediate region cells appeared in the early stages following acute infection, and also when chronically infected cells and uninfected cells were permeabilized together, suggesting a 'leaching' of HIV proteins from highly infected cells to uninfected cells. This leaching type of phenomenon could present problems in determining gating parameters for positive cells since uninfected cells that have associated HIV antigens exhibit higher fluorescence intensity than uninfected cells.     

Delayed Hypersensitivity to Toxoplasma and Unrelated Antigens in Toxoplasma-Infected Mice: Induction and Elicitation of Delayed-Type Hypersensitivity by Antigen-Pulsed Macrophages

Delayed-type hypersensitivity (DTH) to Toxoplasma and unrelated antigens in Toxoplasma-infected BALB/c mice was investigated by the radioisotopic uptake method of Vadas et al. (Int. Arch. Allergy Appl. Immunol. 49: 670-692, 1975). DTH became positive on day 30 of infection and remained positive during chronic infection. The expression of DTH in mice infected with the relatively avirulent C37 strain of the parasite paralleled the Toxoplasma antibody response as detected by the Sabin-Feldman dye test. Mice sensitized with Toxoplasma, keyhole limpet hemocyanin, or sheep erythrocytes during the acute or chronic phase of Toxoplasma infection showed a DTH reaction similar to that of uninfected sensitized controls. No parasite antigens could be detected by immunofluorescence techniques on the surface of Toxoplasma-infected cells. When killed organisms were added to the cell cultures, specks of fluorescence appeared on cells containing intracellular parasites as well as on cells without intracellular organisms. That the antigens may be present in or on macrophages in a form readily recognizable by T cells is suggested by experiments in which we demonstrated that injection of uninfected normal macrophages pulsed with Toxoplasma-soluble antigens into the ears of chronically infected mice elicited a DTH reaction comparable to that observed when 10⁶ Formalin-fixed tachyzoites were used as the test antigen. When macrophages pulsed with Toxoplasma antigen were used in attempts to induce DTH in naive uninfected mice, the intensity of the reaction was similar to that observed in infected mice.     

Infection with Trypanosoma cruzi selectively upregulates B7-2 molecules on macrophages and enhances their costimulatory activity.

T-cell-mediated immune responses are essential for protection against infection with the protozoan Trypanosoma cruzi. In this study, we investigated the influence of infection of murine macrophages with T. cruzi on costimulatory signals for T lymphocytes provided by these cells. We demonstrate that bone marrow-derived macrophages (BMMph) selectively and strongly upregulate expression of B7-2 molecules after infection, while the expression of other costimulatory molecules such as B7-1, intercellular adhesion molecule 1, lymphocyte function-associated antigen 3, and heat-stable antigen is not significantly affected. Infection by live trypanosomes was required. As a consequence of the strong B7-2 upregulation, the infected macrophages are able to induce proliferation of splenic CD4+ T cells in the presence of anti-CD3 antibodies with much higher efficiency than uninfected macrophages. Costimulation could be inhibited by an antibody to B7-2. Furthermore, costimulatory activity for established T-cell clones of Th1 and Th2 phenotype was also strongly enhanced in BMMph by infection with T. cruzi. Th1 cells stimulated either via anti-CD3 antibodies or via specific antigen proliferated with higher efficiency in the presence of infected macrophages than in the presence of uninfected cells. BMMph stimulated with gamma interferon (IFN-gamma), expressing elevated levels of B7-2 molecules, are also able to enhance Th1 cell proliferation, which was highest, using macrophages which were infected and in parallel were stimulated with IFN-gamma. Noteworthy, for cloned Th2 cells, the mechanism of costimulation differed, because costimulation of Th2 cells was not dependent on B7-2 upregulation but was due to secretion of interleukin-1alpha. These findings demonstrate that infection of macrophages with T. cruzi transforms the macrophage into a potent costimulatory cell based on the induction of two different costimulatory activities.     

Intercellular spread of Shigella flexneri through a monolayer mediated by membranous protrusions and associated with reorganization of the cytoskeletal protein vinculin.

The spread of Shigella flexneri in a monolayer of infected Henle and HeLa cells was studied by using immunofluorescence and electron microscopy. Infected cells produced numerous bacterium-containing membranous protrusions up to 18 microns in length that penetrated adjacent cells and were subsequently phagocytosed. Fluorescence staining of actin and vinculin in infected cells with phalloidin and monoclonal antibody to vinculin, respectively, demonstrated that the protrusions containing the bacteria consisted of these cytoskeletal proteins. Actin accumulated predominantly at the poles of bacteria distal to the tip of protrusions and appeared as trails extending back towards the host cell cytoplasm. Vinculin, however, was distributed uniformly around the bacteria and throughout the protrusion. A profound rearrangement of vinculin occurred in Henle and HeLa cells following infection with shigellae: whereas in uninfected cells it was distributed mainly around the cell periphery, in infected cells it concentrated mainly around clusters of bacteria in the cytoplasm. This suggests a possible involvement of the vinculin cytoskeletal protein in the intercellular spread of shigellae during an infection.     

The GDNF-induced neurite outgrowth and neuronal survival in dissociated myenteric plexus cultures of the rat small intestine decreases postnatally

 Glial cell-line-derived neurotrophic factor (GDNF), a member of the transforming growth-factor- (TGF-) β-family, is an essential factor for the development of the enteric nervous system (ENS) during embryogenesis. In the present study, the effects of GDNF on postnatal ENS development were investigated using cultures of myenteric plexus from the small intestine of newborn albino rats of different developmental phases (P1, P7, P14). Myenteric plexus was dissociated and cultivated as mixed cultures of enteric neurons and glial cells. After seeding, the cultures were kept for 24 h or 7 days in serum-free medium containing various doses (1, 10, 100 ng/ml) of GDNF. The effect of the neurotrophic factor was evaluated using parameters such as cell size, neuronal survival, or neurite elongation. While neither glial-cell nor neuronal size was influenced by GDNF, there was an observable effect upon neuronal survival and neurite elongation. The cultures treated with GDNF displayed increased neurite outgrowth. The promoting effect was dose- and age-dependent, decreasing clearly during the early postnatal period. Already after 24 h, neuronal survival was increased in P1 and P7, but not in P14 cultures. In long-term cultures, a marked tendency to form cell aggregates and dense fiber networks was observed when treated with GDNF. These observations suggest that GDNF plays an important role not only in pre-, but also in postnatal development of the enteric nervous system. Received: 29 May 1998 / Accepted: 10 December 1998     

Role of nitric oxide during rotavirus infection

The pathophysiological mechanisms behind rotavirus-induced diarrhoea still remain incomplete. Current views suggest that the non-structural protein 4 (NSP4) of rotavirus and the enteric nervous system (ENS) participate in water secretion and diarrhoea. In the present work the role of nitric oxide (NO) in rotavirus infection and disease has been studied in vitro, mice and humans. Incubation of human intestinal epithelial cells (HT-29) with purified NSP4 but not with infectious virus produced NO2/NO3 accumulation in the incubation media. The NSP4-induced release of NO metabolites occurred within the first minutes after the addition of the toxin. Mice infected with murine rotavirus (strain EDIM) accumulated NO2/NO3 in the urine at the onset for diarrhoea. Following rotavirus infection, inducible nitric oxide synthetase (iNOS) mRNA was upregulated in ileum, but not in duodenum or jejunum of newborn pups within 5 days post-infection. A prospective clinical study including 46 children with acute rotavirus infection and age-matched controls concluded that rotavirus infection stimulates NO production during the course of the disease (P < 0.001). These observations identify NO as an important mediator of host responses during rotavirus infection.     

海洛因依赖大鼠空、回肠嗜银细胞定位和表达

目的探讨海洛因依赖对大鼠空、回肠嗜银细胞的影响。方法成年SD大鼠66只,按配对原则随机分为空白组,盐水组和海洛因组,建立海洛因依赖动物模型。分别于10、17、24、31、38 d取空、回肠组织,应用镀银染色法及图像分析法观察空、回肠嗜银细胞的数量及染色强度变化。结果与空白组及盐水组比较,海洛因组大鼠空、回肠嗜银细胞的数量增加,染色明显加深。图像分析结果表明海洛因组大鼠空、回肠嗜银细胞平均灰度值显著降低(P<0.01);空肠各时间点、回肠第17天和第24天时间点嗜银细胞数量明显增多(P<0.05)。结论海洛因依赖期间,大鼠空、回肠嗜银细胞数量和表达明显增加。     

Mycobacterium avium infection of macrophages results in progressive suppression of interleukin-12 production in vitro and in vivo.

Interleukin-12 (IL-12) has been shown to have an important role in the host defense against Mycobacterium avium. We sought to determine if human monocyte-derived macrophages produce IL-12 upon M. avium infection. Although IL-12 can be measured in supernatants of M. avium-infected macrophages at 24, 48, and 72 h following infection, intracellular staining showed that 24 to 48 h after infection, IL-12 was synthesized chiefly by uninfected macrophages in the monolayer, suggesting that M. avium infection inhibits IL-12 production. In addition, the data also suggest that the longer macrophage monolayers were infected, the less IL-12 they were able to produce. Stimulation of macrophages with IFN-gamma prior to infection with M. avium resulted in greater production of IL-12 compared with unstimulated macrophages. Culture supernatant of M. avium-infected macrophage monolayers, but not control macrophages, partially inhibited IL-12 production by IFN-gamma-stimulated macrophages. This partial inhibition was not reversed by anti-interleukin-10 (anti-IL-10) and anti-transforming growth factor beta 1 (anti-TGF beta 1)-neutralizing antibodies. M. avium infection of macrophages in vitro also suppressed IL-12 synthesis induced by Listeria monocytogenes infection. Immunohistochemistry staining of spleen of infected mice showed that IL-12 production by splenic macrophages was more pronounced in the beginning of the infection but decreased later. Our data indicate that M. avium infection of macrophages suppresses IL-12 production by infected cells and that the suppression was not a result of the presence of IL-10 and TGF beta 1 in the culture supernatant.     

Trypanosoma cruzi-infected macrophages are defective in major histocompatibility complex class II antigen presentation

Trypanosoma cruzi, the intracellular protozoan parasite that causes Chagas' disease, interferes with the host immune response to establish a persistent infection. In this report, we demonstrate that macrophages infected with T. cruzi are unable to effectively present antigens to CD4 T cells. The interference is due to defective antigen-presenting cell (APC) function, as antigen-independent stimulation of the T cell in the presence of infected macrophages is not affected. The defect is distal to antigen processing and is not due to decreased major histocompatibility complex (MHC) class II expression, decreased viability, defective peptide loading in the infected macrophages, nor absence of CD28 co-stimulation. There was a role for gp39:CD40 co-stimulation during antigen presentation to the T cells we studied, but the expression of CD40 on T. cruzi-infected macrophages was not decreased. Antigen-specific adhesion between macrophages and T cells was reduced by infection. Equivalent levels of the adhesion molecules lymphocyte function-associated antigen-1, intercellular adhesion molecule-1, vascular cell adhesion molecule-1 or very late antigen-4 are found on infected and uninfected APC, suggesting that reduced expression of these adhesion molecules was not responsible for the defect in antigen-specific adhesion. The defective T cell:macrophage adhesion may be due to the reduced expression of other adhesion molecules or other changes in the cell induced by infection. Interfering with MHC class II antigen presentation in infected macrophages may help T. cruzi to blunt the immune response by the host.     

Down-regulation of MHC class II molecules and inability to up-regulate class I molecules in murine macrophages after infection with Toxoplasma gondii

Toxoplasma gondii is able to invade phagocytic cells of the monocyte-macrophage lineage and replicates within a parasitophorous vacuole. Since macrophages may activate specific T lymphocytes by presenting pathogen-derived antigens in association with molecules of the MHC, we investigated the in vitro expression of host cell molecules involved in antigen processing and presentation before and during infection of murine bone marrow-derived macrophages (BMM) with T. gondii. Fifty-one hours after addition of T. gondii tachyzoites at different parasite-to-host ratios, up-regulation of total MHC class II molecules by interferon-gamma (IFN-γ) was dose-dependently abrogated in up to 50% of macrophages compared with uninfected control cultures. Quantitative analyses by flow cytometry revealed that the IFN-γ-induced surface expression of class II antigens as well as the IFN-γ-induced up-regulation of class I molecules was significantly decreased in T. gondii-infected macrophage cultures compared with uninfected controls. However, the constitutive expression of MHC class I antigens was not altered after parasitic infection, and infected BMM remained clearly positive for these molecules. After infection of macrophages preactivated with IFN-γ for 48 h, T. gondii also actively down-regulated an already established expression of MHC class II molecules. Furthermore, kinetic analysis revealed that the reduction in intracellular and plasma membrane-bound class II molecules started ≈ 20 h after infection. While MHC class II antigens were most prominently reduced in parasite-positive host cells, culture supernatant from T. gondii-infected BMM cultures also significantly inhibited expression of these molecules in uninfected macrophages. However, down-regulation of MHC class II molecules was not mediated by an increased production of prostaglandin E₂, IL-10, transforming growth factor-beta or nitric oxide by infected BMM compared with uninfected controls. Our data indicate that intracellular T. gondii interferes with the MHC class I and class II antigen presentation pathway of murine macrophages and this may be an important strategy for evasion from the host's immune response and for intracellular survival of the parasite.