Two CD95 (APO-1/Fas) signaling pathways

We have identified two cell types, each using almost exclusively one of two different CD95 (APO-1/Fas) signaling pathways. In type I cells, caspase-8 was activated within seconds and caspase-3 within 30 min of receptor engagement, whereas in type II cells cleavage of both caspases was delayed for approximately 60 min. However, both type I and type II cells showed similar kinetics of CD95-mediated apoptosis and loss of mitochondrial transmembrane potential (DeltaPsim). Upon CD95 triggering, all mitochondrial apoptogenic activities were blocked by Bcl-2 or Bcl-xL overexpression in both cell types. However, in type II but not type I cells, overexpression of Bcl-2 or Bcl-xL blocked caspase-8 and caspase-3 activation as well as apoptosis. In type I cells, induction of apoptosis was accompanied by activation of large amounts of caspase-8 by the death-inducing signaling complex (DISC), whereas in type II cells DISC formation was strongly reduced and activation of caspase-8 and caspase-3 occurred following the loss of DeltaPsim. Overexpression of caspase-3 in the caspase-3-negative cell line MCF7-Fas, normally resistant to CD95-mediated apoptosis by overexpression of Bcl-xL, converted these cells into true type I cells in which apoptosis was no longer inhibited by Bcl-xL. In summary, in the presence of caspase-3 the amount of active caspase-8 generated at the DISC determines whether a mitochondria-independent apoptosis pathway is used (type I cells) or not (type II cells).

Control of apoptosis and mitotic spindle checkpoint by survivin

Progression of the cell cycle and control of apoptosis (programmed cell death) are thought to be intimately linked processes, acting to preserve homeostasis and developmental morphogenesis. Although proteins that regulate apoptosis have been implicated in restraining cell-cycle entry and controlling ploidy (chromosome number), the effector molecules at the interface between cell proliferation and cell survival have remained elusive. Here we show that a new inhibitor of apoptosis (IAP) protein, survivin, is expressed in the G2/M phase of the cell cycle in a cycle-regulated manner. At the beginning of mitosis, survivin associates with microtubules of the mitotic spindle in a specific and saturable reaction that is regulated by microtubule dynamics. Disruption of survivin-microtubule interactions results in loss of survivin's anti-apoptosis function and increased caspase-3 activity, a mechanism involved in cell death, during mitosis. These results indicate that survivin may counteract a default induction of apoptosis in G2/M phase. The overexpression of survivin in cancer may overcome this apoptotic checkpoint and favour aberrant progression of transformed cells through mitosis.

Mass screening for lung cancer with mobile spiral computed tomography scanner

BACKGROUND

The incidence of and mortality from lung cancer have increased steadily. Most lung cancers are not localised when first detected, but early detection is mandatory to improve prognosis. Since curable early cases are hard to visualise with conventional chest radiography, a new diagnostic means must be found. We assessed whether population-based mass screening with a spiral computed tomography scanner could contribute substantially to detection of smaller cancers, and decrease mortality.

METHODS

In 1996, we screened in a mobile unit 5483 individuals from the general population of Matsumoto, Japan, aged between 40 years and 74 years who had undergone annual chest radiography (miniature fluorophotography) and cytological assessment of sputum. All participants had a low-dose X-ray spiral computed tomography (CT) scan of the thorax; 3967 also underwent miniature fluorophotography. We compared smokers and non-smokers. Further assessments were done for probably benign but suspicious lesions; suspicion of cancer; and indeterminate small nodules by chest radiography and conventional CT, with additional transbronchial biopsy when possible. Thoracotomy was recommended when it was strongly suspected that the patients had lung cancer.

FINDINGS

19 patients were diagnosed as having lung cancer--14 with suspicion of lung cancer, three with benign but suspicious lesions, and two with indeterminate small nodules. 18 cases were surgically confirmed, and one was clinically diagnosed. The mean size of lesions was 17 mm (range 6-47). In four of 19 patients, lung abnormality was seen on CT and miniature fluorophotography. The lung-cancer detection rate with CT was 0.48%, significantly higher than the 0.03-0.05% for standard mass assessments done previously in the same area. CT missed one case that was found solely on a sputum cytology examination.

INTERPRETATION

Our results show that miniature fluorophotography or conventional chest radiography, which have been the main diagnostic techniques for lung cancer, showed few small cancers. CT was more accurate in mass screening for lung cancer and led to early detection and an accurate diagnosis of lung cancer, and should be considered in future health plans.

Rapid transition of cardiac myocytes from hyperplasia to hypertrophy during postnatal development

The switch from myocyte hyperplasia to hypertrophy occurs during the early postnatal period. The exact temporal sequence when cardiac myocytes cease dividing and become terminally differentiated is not certain, although it is currently believed that the transition takes place gradually over a 1-2-week period. The present investigation has characterized the growth pattern of cardiac myocytes during the early postnatal period. Cardiac myocytes were enzymatically isolated from the hearts of 1, 2, 3, 4, 6, 8, 10, and 12-day-old rats for the measurements of binucleation, cell volume and myocyte number. Almost all myocytes were mononucleated and cell volume remained relatively constant during the first 3 days of age. Increases in cell volume and binucleation of myocytes were first detected at day 4. Myocyte volume increased 2.5-fold from day 3 to day 12 (1416 +/- 320 compared to 3533 +/- 339 microns 3). The percentage of binucleated myocytes began to increase at day 4 and proceeded at a high rate, reaching the adult level of approximately 90% at day 12. Myocyte number increased 68% during the first 3 days (from 13.6 +/- 3.5 x 10(6) at day 1 to 22.9 +/- 5.6 x 10(10) at day 3) and remained constant thereafter. To confirm that no further myocyte division exists after 4 days, bromodeoxyuridine (Brdu) was administered to 4-day-old rats and the fate of DNA-synthesizing myocytes was examined 2 h and 2, 4, 6 and 8 days after Brdu injection. About 12% of myocytes were labeled with Brdu at 2 h and all were mononucleated at that time. Gradually, these Brdu-labeled myocytes became binucleated. However, the percentage of labeled myocytes in all groups was identical, indicating that DNA-synthesizing myocytes were becoming binucleated without further cell division after 4 days of age. Within 8 days after injection, approximately 82% of total labeled myocytes were binucleated, while the others remained mononucleated. Sarcomeric alpha-actinin was fully disassembled in dividing myocytes of 2-day-old rats, while typical alpha-actinin striations were present in dividing myocytes of 4-day-old rats. The results from this study suggest that a rapid switch from myocyte hyperplasia to hypertrophy occurs between postnatal day 3 and 4 in rat hearts.

P-selectin glycoprotein ligand-1 mediates rolling of human neutrophils on P-selectin

Neutrophils roll on P-selectin expressed by activated platelets or endothelial cells under the shear stresses in the microcirculation. P-selectin glycoprotein ligand-1 (PSGL-1) is a high affinity ligand for P-selectin on myeloid cells. However, it has not been demonstrated that PSGL-1 contributes to the rolling of neutrophils on P-selectin. We developed two IgG mAbs, PL1 and PL2, that appear to recognize protein-dependent epitopes on human PSGL-1. The mAbs bound to PSGL-1 on all leukocytes as well as on heterologous cells transfected with PSGL-1 cDNA. PL1, but not PL2, blocked binding of 125-I-PSGL-1 to immobilized P-selectin, binding of fluid-phase P-selectin to myeloid and lymphoid leukocytes, adhesion of neutrophils to immobilized P-selectin under static conditions, and rolling of neutrophils on P-selectin-expressing CHO cells under a range of shear stresses. PSGL-1 was localized to microvilli on neutrophils, a topography that may facilitate its adhesive function. These data indicate that (a) PSGL-1 accounts for the high affinity binding sites for P-selectin on leukocytes, and (b) PSGL-1 must interact with P-selectin in order for neutrophils to roll on P-selectin at physiological shear stresses.

Regulation of apoptosis at cell division by p34cdc2 phosphorylation of survivin

The interface between apoptosis (programmed cell death) and the cell cycle is essential to preserve homeostasis and genomic integrity. Here, we show that survivin, an inhibitor of apoptosis over-expressed in cancer, physically associates with the cyclin-dependent kinase p34(cdc2) on the mitotic apparatus, and is phosphorylated on Thr(34) by p34(cdc2)-cyclin B1, in vitro and in vivo. Loss of phosphorylation on Thr(34) resulted in dissociation of a survivin-caspase-9 complex on the mitotic apparatus, and caspase-9-dependent apoptosis of cells traversing mitosis. These data identify survivin as a mitotic substrate of p34(cdc2)-cyclin B1 and suggest that survivin phosphorylation on Thr(34) may be required to preserve cell viability at cell division. Manipulation of this pathway may facilitate the elimination of cancer cells at mitosis.

Angiopoietin-1 inhibits endothelial cell apoptosis via the Akt/survivin pathway

A productive angiogenic response must couple to the survival machinery of endothelial cells to preserve the integrity of newly formed vessels. Angiopoietin-1 (Ang-1) is an endothelium-specific ligand essential for embryonic vascular stabilization, branching morphogenesis, and post-natal angiogenesis, but its contribution to endothelial cell survival has not been completely elucidated. Here we show that Ang-1 acting via the Tie 2 receptor induces phosphorylation of the survival serine-threonine kinase, Akt (or protein kinase B). This is associated with up-regulation of the apoptosis inhibitor, survivin, in endothelial cells and protection of endothelium from death-inducing stimuli. Moreover, dominant negative survivin negates the ability of Ang-1 to protect cells from undergoing apoptosis. The activation of anti-apoptotic pathways mediated by Akt and survivin in endothelial cells may contribute to Ang-1 stabilization of vascular structures during angiogenesis, in vivo.

Pleiotropic cell-division defects and apoptosis induced by interference with survivin function

Here we investigate the role of the control of apoptosis in normal cell division. We show that interference with the expression or function of the apoptosis inhibitor survivin causes caspase-dependent cell death in the G2/M phase of the cell cycle, and a cell-division defect characterized by centrosome dysregulation, multipolar mitotic spindles and multinucleated, polyploid cells. Use of a dominant-negative survivin mutant or antisense survivin complementary DNA disrupts a supramolecular assembly of survivin, caspase-3 and the cyclin-dependent-kinase inhibitor p21Waf1/Cip1 within centrosomes, and results in caspase-dependent cleavage of p21. Polyploidy induced by survivin antagonists is accentuated in p21-deficient cells, and corrected by exogenous expression of p21. These findings show that control of apoptosis and preservation of p21 integrity within centrosomes by survivin are required for normal mitotic progression.

Growth rate of small lung cancers detected on mass CT screening

CT has recently been used in mass screening for lung cancer. Small cancers have been identified but the growth characteristics of these lesions are not fully understood. We identified 82 primary cancers in our 3-year mass CT screening programme, of which 61 were examined in the present study. The volume doubling time (VDT) was calculated based on the exponential model using successive annual CT images or follow-up CT images. All cases were also examined in the hospital by high resolution CT (HRCT). Lesions were divided into three types based on HRCT characteristics: type G (n = 19), ground glass opacity (GGO); type GS (n = 19), focal GGO with a solid central component; and type S (n = 23), solid nodule. 18 (95%) lesions of type G, 18 (95%) of type GS and 7 (30%) of type S were invisible on conventional chest radiographs. The mean size of the tumour was 10 mm, 11 mm and 16 mm for type G, type GS and type S, respectively. Most tumours (80%) were adenocarcinomas; 78% of these were GGO (type G and GS). Mean VDT values were 813 days, 457 days and 149 days for type G, type GS and type S, respectively; these are significantly different from each other (p < 0.05). Our results show that annual mass screening CT for 3 successive years resulted in the identification of a large number of slowly growing adenocarcinomas that were not visible on chest radiographs.

PA-457: a potent HIV inhibitor that disrupts core condensation by targeting a late step in Gag processing

New HIV therapies are urgently needed to address the growing problem of drug resistance. In this article, we characterize the anti-HIV drug candidate 3-O-(3',3'-dimethylsuccinyl) betulinic acid (PA-457). We show that PA-457 potently inhibits replication of both WT and drug-resistant HIV-1 isolates and demonstrate that the compound acts by disrupting a late step in Gag processing involving conversion of the capsid precursor (p25) to mature capsid protein (p24). We find that virions from PA-457-treated cultures are noninfectious and exhibit an aberrant particle morphology characterized by a spherical, acentric core and a crescent-shaped, electron-dense shell lying just inside the viral membrane. To identify the determinants of compound activity we selected for PA-457-resistant virus in vitro. Consistent with the effect on Gag processing, we found that mutations conferring resistance to PA-457 map to the p25 to p24 cleavage site. PA-457 represents a unique class of anti-HIV compounds termed maturation inhibitors that exploit a previously unidentified viral target, providing additional opportunities for HIV drug discovery.

Animal virus replication and RNAi-mediated antiviral silencing in Caenorhabditis elegans

The worm Caenorhabditis elegans is a model system for studying many aspects of biology, including host responses to bacterial pathogens, but it is not known to support replication of any virus. Plants and insects encode multiple Dicer enzymes that recognize distinct precursors of small RNAs and may act cooperatively. However, it is not known whether the single Dicer of worms and mammals is able to initiate the small RNA-guided RNA interference (RNAi) antiviral immunity as occurs in plants and insects. Here we show complete replication of the Flock house virus (FHV) bipartite, plus-strand RNA genome in C. elegans. We show that FHV replication in C. elegans triggers potent antiviral silencing that requires RDE-1, an Argonaute protein essential for RNAi mediated by small interfering RNAs (siRNAs) but not by microRNAs. This immunity system is capable of rapid virus clearance in the absence of FHV B2 protein, which acts as a broad-spectrum RNAi inhibitor upstream of rde-1 by targeting the siRNA precursor. This work establishes a C. elegans model for genetic studies of animal virus-host interactions and indicates that mammals might use a siRNA pathway as an antiviral response.

Results of three-year mass screening programme for lung cancer using mobile low-dose spiral computed tomography scanner

The aim of this study was to evaluate the usefulness of annual screening for lung cancer by low-dose computed tomography (CT) and the characteristics of identified lung cancers. Subjects consisted of 5483 general population aged 40-74 years, who received initial CT scans in 1996, followed by repeat annual scans for most subjects in 1997 and 1998, with a total of 13 786 scans taken during 1996-1998. Work-up examinations for patients with suspicious lesions were conducted using diagnostic CTs. The initial screening in 1996 detected suspicious nodules in 279 (5.1%) of 5483 subjects, and 22 (8%) were confirmed surgically to have lung cancer. Corresponding figures in 1997 and 1998 screening studies were 173 (3.9%) of 4425 and 25 (14%) of 173, and 136 (3.5%) of 3878 and 9 (7%) of 136, respectively. The sensitivity and specificity of detecting surgically confirmed lung cancer were 55% (22/40) and 95% (4960/5199) in 1996 and 83% (25/30) and 97% (4113/4252) in 1997 screening, respectively. 88% (55/60) of lung cancers identified on screening and surgically confirmed were AJCC stage IA. Our trial allowed detection of nearly 11 times the expected annual number of early lung cancers. Repeat CT allowed the detection of more aggressive, rapidly growing lung cancers, compared to those in the initial screening.

Apoptotic cell death and tissue remodelling during mouse mammary gland involution

During post-lactational mammary gland involution, the bulk of mammary epithelium dies and is reabsorbed. This massive cell death and tissue restructuring was found to be accompanied by a specific pattern of gene expression. Northern blot analysis showed that weaning resulted in a dramatic drop in ODC, a gene involved in synthesis of a component of milk, and the nearly simultaneous induction of SGP-2, a gene associated with apoptotic cell death. These changes were followed by decreases in expression of milk protein genes to basal levels and expression of genes associated with regulation of cell proliferation and differentiation, p53, c-myc and TGF-beta 1. Subsequently, additional genes implicated in stress response, tissue remodelling, and apoptotic cell death were transiently expressed, expression peaking at about 6 days post-weaning. A non-random degradation of DNA yielding the oligonucleosomal length fragmentation pattern typical of apoptotic cell death (Wyllie, 1980; Wyllie et al., 1980) was detected in association with morphological changes and gene expression. The correlations between: (a) changes in morphology, (b) pattern of gene expression and (c) changes in DNA integrity suggest that complementary programs for cell death and tissue remodelling direct post-lactational mammary gland involution.

Cell-specific induction of apoptosis by microinjection of cytochrome c. Bcl-xL has activity independent of cytochrome c release

Bcl-xL, an antiapoptotic member of the Bcl-2 family, inhibits programmed cell death in a broad variety of cell types. Recent reports have demonstrated that cytochrome c is released from mitochondria during apoptosis and have suggested that this release may be a critical step in the activation of proapoptotic caspases and subsequent cell death. Furthermore, it has been demonstrated that Bcl-2 can prevent the release of cytochrome c from mitochondria in cells triggered to undergo apoptosis. This has led to the hypothesis that the antiapoptotic effects of Bcl-2 family members are due specifically to their ability to prevent cytochrome c release thus preventing subsequent cytochrome c-dependent caspase activation. In the present report, we use microinjection techniques to investigate the relationship between cytochrome c release, induction of apoptosis, and Bcl-xL activity in intact cells. We demonstrate that microinjection of cytochrome c into the cytosol of human kidney 293 cells results in a dose-dependent induction of apoptosis. In contrast, MCF7 breast carcinoma cells (stably transfected to express the Fas antigen CD95, and denoted MCF7F) that lack detectable levels of caspase 3 (CPP32), are totally resistant to microinjection of cytochrome c. However, transfection of MCF7F cells with an expression plasmid coding for pro-caspase 3, but not other pro-caspases, restores cytochrome c sensitivity. Although MCF7F cells are insensitive to cytochrome c microinjection, they rapidly undergo apoptosis in a caspase-dependent manner in response to either tumor necrosis factor or anti-Fas plus cycloheximide, and these deaths are strongly inhibited by Bcl-xL expression. Furthermore, microinjection of cytochrome c does not overcome these antiapoptotic effects of Bcl-xL. Our results support the concept that the release of cytochrome c into the cytoplasm can promote the apoptotic process in cells expressing pro-caspase 3 but that cytochrome c release is not sufficient to induce death in all cells. Importantly, the ability of Bcl-xL to inhibit cell death in the cytochrome c-insensitive MCF7F cells cannot be due solely to inhibition of cytochrome c release from mitochondria.

Expression and targeting of the apoptosis inhibitor, survivin, in human melanoma

The newly described apoptosis inhibitor survivin is expressed in many human cancers and appears to play a critical part in both apoptosis regulation and cell cycle progression. Its potential role in malignant melanoma is unknown. In a panel of 30 malignant melanomas, survivin was strongly expressed in all cases (15 of 15) of metastatic malignant melanomas and 13 of 15 cases of invasive malignant melanomas by immunohistochemistry. In invasive malignant melanomas, survivin was also expressed in the in-situ component of the lesion. Survivin expression was found in all cases (11 of 11) of nevi, but not in melanocytes in sections of normal skin. The apoptosis inhibitor bcl-2 was expressed in 26 of 30 cases, but generally at lower levels than that of infiltrating lymphocytes. The mitotic index, as assessed by MIB-1 staining, was consistently higher in metastatic than invasive malignant melanomas. Assessment of apoptotic index by in situ end-labeling revealed extremely low rates of apoptosis in most malignant melanomas. Survivin expression by western blotting was detected in four human metastatic malignant melanoma cell lines but not in cultured normal human melanocytes. Transfection of both YUSAC-2 and LOX malignant melanoma cells with green fluorescence protein-conjugated survivin anti-sense or green fluorescence protein-conjugated survivin dominant negative mutant (Cys84Ala) [corrected] resulted in increased apoptosis in the absence of other genotoxic stimuli. Two-color flow cytometry confirmed that YUSAC-2 cells transfected with survivin anti-sense expressed less endogenous survivin and exhibited an increased fraction of cells with sub-G1 DNA content. These data demonstrate that apoptosis inhibition by survivin may participate in the onset and progression of malignant melanomas, and suggest that therapeutic targeting of survivin may be beneficial in patients with recurrent or metastatic disease.

Reprogramming of the tumour microenvironment by stromal PTEN-regulated miR-320

Phosphatase and tensin homolog deleted on chromosome ten (Pten) in stromal fibroblasts suppresses epithelial mammary tumors, but the underlying molecular mechanisms remain unknown. Using proteomic and expression profiling, we show that Pten loss from mammary stromal fibroblasts activates an oncogenic secretome that orchestrates the transcriptional reprogramming of other cell types in the microenvironment. Downregulation of miR-320 and upregulation of one of its direct targets, ETS2, are critical events in Pten-deleted stromal fibroblasts responsible for inducing this oncogenic secretome, which in turn promotes tumor angiogenesis and tumor cell invasion. Expression of the Pten-miR-320-Ets2 regulated secretome distinguished human normal breast stroma from tumor stroma and robustly correlated with recurrence in breast cancer patients. This work reveals miR-320 as a critical component of the Pten tumor suppressor axis that acts in stromal fibroblasts to reprogram the tumor microenvironment and curtail tumor progression.

Control of apoptosis during angiogenesis by survivin expression in endothelial cells

Mechanisms controlling endothelial cell survival during angiogenesis were investigated. Stimulation of quiescent endothelial cells with mitogens, including vascular endothelial growth factor and basic fibroblast growth factor, induced up to approximately 16-fold up-regulation of the cell cycle-regulated apoptosis inhibitor survivin. Mitogen stimulation rapidly increased survivin RNA expression in endothelial cells, which peaked after 6 to 10 hours in culture and decreased by 24 hours. Inflammatory cytokines, tumor necrosis factor alpha, and interleukin-1 did not induce survivin expression in endothelial cells. Formation of three-dimensional vascular tubes in vitro was associated with strong induction of survivin in endothelial cells, as compared with two-dimensional cultures. By immunohistochemistry, survivin was minimally expressed in endothelium of nonproliferating capillaries of normal skin, whereas it became massively up-regulated in newly formed blood vessels of granulation tissue in vivo. Recombinant expression of green fluorescent protein survivin in endothelial cells reduced caspase-3 activity and counteracted apoptosis induced by tumor necrosis factor alpha/cycloheximide. These findings identify survivin as a novel growth factor-inducible protective gene expressed by endothelial cells during angiogenesis. Therapeutic manipulation of survivin expression and function in endothelium may influence compensatory or pathological (tumor) angiogenesis.

Prolonged production of NADPH oxidase-corrected granulocytes after gene therapy of chronic granulomatous disease

Little is known about the potential for engraftment of autologous hematopoietic stem cells in human adults not subjected to myeloablative conditioning regimens. Five adult patients with the p47(phox) deficiency form of chronic granulomatous disease received intravenous infusions of autologous CD34(+) peripheral blood stem cells (PBSCs) that had been transduced ex vivo with a recombinant retrovirus encoding normal p47(phox). Although marrow conditioning was not given, functionally corrected granulocytes were detectable in peripheral blood of all five patients. Peak correction occurred 3-6 weeks after infusion and ranged from 0.004 to 0.05% of total peripheral blood granulocytes. Corrected cells were detectable for as long as 6 months after infusion in some individuals. Thus, prolonged engraftment of autologous PBSCs and continued expression of the transduced gene can occur in adults without conditioning. This trial also piloted the use of animal protein-free medium and a blood-bank-compatible closed system of gas-permeable plastic containers for culture and transduction of the PBSCs. These features enhance the safety of PBSCs directed gene therapy.

A human-driven decline in global burned area

Fire is an essential Earth system process that alters ecosystem and atmospheric composition. Here we assessed long-term fire trends using multiple satellite data sets. We found that global burned area declined by 24.3 ± 8.8% over the past 18 years. The estimated decrease in burned area remained robust after adjusting for precipitation variability and was largest in savannas. Agricultural expansion and intensification were primary drivers of declining fire activity. Fewer and smaller fires reduced aerosol concentrations, modified vegetation structure, and increased the magnitude of the terrestrial carbon sink. Fire models were unable to reproduce the pattern and magnitude of observed declines, suggesting that they may overestimate fire emissions in future projections. Using economic and demographic variables, we developed a conceptual model for predicting fire in human-dominated landscapes.

Different subsets of enteric bacteria induce and perpetuate experimental colitis in rats and mice

Resident bacteria are incriminated in the pathogenesis of experimental colitis and inflammatory bowel diseases. We investigated the relative roles of various enteric bacteria populations in the induction and perpetuation of experimental colitis. HLA-B27 transgenic rats received antibiotics (ciprofloxacin, metronidazole, or vancomycin-imipenem) in drinking water or water alone in either prevention or treatment protocols. Mice were treated similarly with metronidazole or vancomycin-imipenem before or after receiving 5% dextran sodium sulfate (DSS). Germfree transgenic rats were colonized with specific-pathogen-free enteric bacteria grown overnight either in anaerobic or aerobic atmospheres. Nontransgenic rats colonized with anaerobic bacteria served as negative controls. Although preventive metronidazole significantly attenuated colitis in transgenic rats and DSS-treated mice, it had no therapeutic benefit once colitis was established. Ciprofloxacin also partially prevented but did not treat colitis in B27 transgenic rats. In both animal models vancomycin-imipenem most effectively prevented and treated colitis. Germfree transgenic rats reconstituted with enteric bacteria grown under anaerobic conditions had more aggressive colitis than those associated with aerobic bacteria. These results suggest that a subset of resident luminal bacteria induces colitis, but that a complex interaction of commensal aerobic and anaerobic bacteria provides the constant antigenic drive for chronic immune-mediated colonic inflammation.