In disagreement with this T-cell hypothesis is the observation that mice deficient in the T-cell-expressed inflammatory cytokines gamma interferon and tumor necrosis factor or the T-cell-polarizing cytokine interleukin 12, although more susceptible, nevertheless resolve infection with with identical (tumor necrosis factor and gamma interferon) or only moderately delayed (interleukin 12) kinetics (13, 28). mice. Adoptive transfer of immune B cells, but not na?ve B cells, provided highly variable immunity to recipient MT mice. The results suggest that B-cell-mediated immune responses are central to resolution of a infection but that the mechanism through which this occurs requires further investigation. These data are relevant to understanding immunity to enteric attaching and effacing bacterial pathogens of humans. Many LED209 enteric bacterial pathogens (e.g., serovar Typhi, and (ETEC), enteropathogenic (EPEC), and enterohemorrhagic (EHEC), colonize the lumenal side of the gastrointestinal epithelium. To combat bacterial colonization at this site, the host presumably requires a different set of immune defense mechanisms in order to achieve pathogen eradication. The immunological events associated with eradication of pathogens that employ attaching and effacing (A/E) lesion formation have been partly addressed (8, 13, 14, 28, 31, 32), yet much more remains to be learned, particularly on the role of B- and T-cell interactions and their role in immunity. Human-specific EPEC and EHEC strains are members of a family of noninvasive enteric bacterial pathogens that share conserved virulence mechanisms. A characteristic of members of this family of pathogens is their ability to actively Ccr3 subvert intestinal epithelial cell function to produce a histopathological feature known as the A/E lesion. capable of forming A/E lesions have been recovered from diseased cattle (6), rabbits (15), pigs (1), and dogs and cats (5). In mice, the natural mouse pathogen colonizes large-intestinal enterocytes via A/E lesion formation and causes acute colitis (3). Two bacterium-expressed, surface-associated proteins, intimin and EspA, are, among others, essential for A/E lesion formation (33). Infection of mice with has been used as a surrogate model for studying host responses to pathogens dependent upon A/E lesion formation for colonization of the host (14, 28, 32). Importantly, possesses both established and putative virulence determinants common to human-specific strains of EPEC and EHEC, including a large pathogenicity island (7) and an immunomodulating toxin that is distinct from Shiga-like toxins (17). Furthermore, the A/E lesion induced by is ultrastructurally indistinguishable from those formed by EHEC and EPEC in animals and humans (26). In naturally or experimentally infected mice, infection is associated with colonic crypt hyperplasia, LED209 goblet cell depletion, and mucosal erosion (2, 16). Many of these pathological features also occur in other genetic or immunologically driven mouse models of colitis. In most of these models, CD4+ T cells are critical mediators of the pathological changes that occur in the colonic mucosa (29). The aim of this study was to elucidate the role of T-cell subsets and B cells in the development of immunity to and in the colitis that occurs during infection. The results suggested that CD4+ T cells and B cells are essential for resolution of a primary infection. MATERIALS AND METHODS Mice. Female or male 6- to 8-week-old C57Bl/6J mice were purchased from Harlan Olac (Bichester, United Kingdom) or Bantin & Kingman (Hull, United Kingdom). MT mice (backcrossed to C57Bl/6 LED209 background at least 10 times) were originally purchased from Jackson Laboratories and were maintained by homozygous matings under contract at B&K Universal (Hull, United Kingdom). RAG1?/? mice were originally purchased from Jackson Laboratories and were bred under barrier containment facilities by homozygous mating. All mice used in these studies came from colonies which were specific pathogen free. Sentinel animals were screened for common murine pathogens every 2 months. All animals were housed in individually HEPA-filtered cages with sterile bedding and free access to sterilized food and water. The absence of B cells in MT mice and B and T cells in RAG1?/? mice was verified by flow cytometry using fluorochrome-conjugated antibodies to CD3 and B220 (Becton Dickinson, Cowley, United Kingdom). All mice were between 6 and 12 weeks old when first infected with All animal procedures were approved under the United Kingdom Animal (Scientific) Procedures Act. Bacterial strains and oral infection of mice. The LED209 strain DBS100 used in this study was originally supplied by D. Schauer and has been described previously (27). Bacterial inoculums were prepared by culturing bacteria overnight at 37C in 100 ml of L broth containing nalidixic acid (100 g/ml). Cultures were harvested by centrifugation and resuspended in a 1/10 volume of phosphate-buffered saline (PBS). Mice were orally inoculated, without anesthetic, using.