Identification of commensal flora-associated antigen as a pathogenetic factor of autoimmune pancreatitis.

OBJECTIVES: Autoimmune pancreatitis (AIP) is a chronic fibro-inflammatory disease of the pancreas constituting, in part, a recently defined nosological entity of IgG4-related systemic sclerosing diseases. The pathogenetic factors of AIP have not been fully elucidated. We previously established a mouse model of AIP using chronic exposure to a commensal bacteria, Escherichia coli. METHODS: To determine the pathogenetically relevant antigen of E. coli, the outer membrane fractions of E. coli were subjected to two-dimensional gel electrophoresis followed by immunoblotting against sera from the AIP model. Immunoreactive spots were determined using MALDI TOF/MS and Mascot search. The recombinant protein of the identified antigen was examined for their ability to induce AIP-like disorder in C57BL/6 mice. Furthermore, the antibody titer against that antigen was determined in AIP patients. RESULTS: One representative spot reacting with sera from E. coli-inoculated mice was identified as FliC from E. coli, based on the results of TOF/MS. The repeated inoculation of recombinant FliC in C57BL/6 mice induced AIP-like pancreatitis and higher titers of anti-lactoferrin and anti-carbonic anhydrase II. Sera from patients with AIP had the highest antibody titer, while those from patients with pancreatic diseases other than AIP had a higher antibody titer against E. coli and FliC, compared with pancreatic disease-free controls. CONCLUSIONS: FliC from E. coli may pathogenetically generate an AIP-like inflammation status. A reconsideration of the importance of commensal bacteria as an environmental factor(s) capable of inducing autoimmunity could provide insight to overcoming AIP.

Clinical profile of primary biliary cirrhosis with features of autoimmune hepatitis: Importance of corticosteroid therapy.

AIM: The aim of this study was to elucidate the clinical and histological features, response to corticosteroid therapy and long-term outcome of primary biliary cirrhosis (PBC) with features of autoimmune hepatitis (AIH). METHODS: Among 280 PBC patients under ursodeoxycholic acid administration, we identified 28 patients with AIH features fulfilling the following criteria: sustained high levels of serum aminotransferases and high immunoglobulin G levels with positivity for antinuclear antibodies or anti-smooth muscle antibodies (ASMA) and/or histological features of moderate to severe interface hepatitis or moderate to severe lobular hepatitis. We analyzed PBC patients with AIH features, focusing mainly on therapeutic responses to corticosteroids. RESULTS: Patients with PBC with AIH features included 26 women (93%). Their median age was 55 years, and the median follow-up period was 7.5 years. Eight of these 28 patients were not actually treated with corticosteroids due to medical conditions. Among the 20 patients receiving corticosteroid therapy, 15 were responders and five were non-responders. A high alkaline phosphatase (ALP) level, negativity for ASMA and positivity for gp210 were identified as risk factors for lack of a response to corticosteroid therapy. Among 28 PBC patients with AIH features, the responders to corticosteroids had an excellent prognosis, while those who could not be treated with corticosteroids and non-responders to corticosteroids had a poor outcome. CONCLUSION: Patients with PBC with AIH features benefit from corticosteroid therapy. Features of PBC such as high ALP level, negativity for ASMA and positivity for gp210 appear to predict a poor response to corticosteroids.

Mouse monoclonal antibody specific for outer membrane protein A of Escherichia coli.

Outer membrane protein A (OmpA) is a major outer membrane protein of Escherichia coli and other Enterobacteriaeae. Although the structural features of OmpA have been well studied, its roles in the pathogenesis of various bacterial infections have not been fully elucidated. Here, we report the generation of mouse monoclonal antibody (MAb) 49.4-15, which specifically recognizes OmpA of E. coli, using immunoblot and confocal microscopic examinations. MAb 49.4-15 might be a useful tool for studying the expression and function of E. coli OmpA.

Involvement of commensal bacteria may lead to dysregulated inflammatory and autoimmune responses in a mouse model for chronic nonsuppurative destructive cholangitis.

BACKGROUND: We previously reported a mouse model of primary biliary cirrhosis (PBC)-like chronic nonsuppurative destructive cholangitis (CNSDC), in which frequent injections of Streptococcus intermedius induced CNSDC and autoantibody production. The present study was performed to verify the model by examining 1) the reappearance of the PBC-like CNSDC after lymphocyte transfer from model to naïve mice, 2) the involvement of autophagy, and 3) the influence of the strain difference. METHODS: Mice were inoculated with S. intermedius weekly for 8 weeks, then sacrificed to obtain samples. Spleen cells obtained from S. intermedius-inoculated mice were transferred to RAG2(-/-) mice. RESULTS: CNSDC and elevated serum level of anti-gp210 titers were observed in S. intermedius-inoculated C57BL/6 mice, similar to the results of our previous report using BALB/c mice. Portal inflammation was induced in the livers of RAG2(-/-) mice by the transfer of spleen cells from S. intermedius-inoculated C57BL/6 mice. Among the inflammatory cells in the RAG2(-/-) mice, CD3-positive cells were predominant. Autophagosome-like structures were detected histologically, in the cytoplasm of infiltrated cells around the bile ducts in the livers of S. intermedius-inoculated both C57BL/6 and BALB/c mice. In S. intermedius-inoculated C3H/HeJ mice, inflammation in the portal area was less extensive than that in the hepatic parenchyma. CONCLUSION: Bacterial component(s) and sequentially upregulated innate and acquired immune responses, accompanied by autophagy, might trigger CNSDC, via autoimmune mechanisms. Throughout the generation of bacteria-triggered PBC-like CNSDC, strain difference may influence the response to S. intermedius-inoculation in the liver.

Commensal Flora, is it an Unwelcomed Companion as a Triggering Factor of Autoimmune Pancreatitis?

The etiopathogenesis of many autoimmune disorders has not been identified. The aim of this paper is to focus on the involvement of bacterial exposure, as an environmental factor, in the pathogenesis of autoimmune pancreatitis (AIP), which is broadly categorized as autoimmune disorders involving pancreatic lesions. Avirulent and/or commensal bacteria, which may have an important role(s) as initiating/progressing factors in the pathogenesis of autoimmune disorder AIP, will be emphasized.

Are dysregulated inflammatory responses to commensal bacteria involved in the pathogenesis of hepatobiliary-pancreatic autoimmune disease? An analysis using mice models of primary biliary cirrhosis and autoimmune pancreatitis.

The etiopathogenesis of many autoimmune disorders has not been identified. The aim of this paper is to focus on the involvement of bacterial exposure in the pathogenesis of primary biliary cirrhosis (PBC) and autoimmune pancreatitis (AIP), both of which are broadly categorized as autoimmune disorders involving hepatobiliary-pancreatic lesions. Avirulent and/or commensal bacteria, which may have important role(s) as initiating factors in the pathogenesis of autoimmune disorders such as PBC and AIP, will be emphasized.

Possible role of LECT2 as an intrinsic regulatory factor in SEA-induced toxicity in d-galactosamine-sensitized mice.

To elucidate whether leukocyte cell-derived chemotaxin 2 (LECT2) controls the progression of staphylococcal enterotoxin A (SEA)-induced toxicity, we examined the role of LECT2 in a mouse model. Almost all the C57BL/6J (B6) mice survived for 72 h after the injection of 0.1 µg of SEA and 20 mg of d-galactosamine (d-GalN). However, the same treatment protocol in LECT2(-/-) mice produced a high lethality (~90%), severe hepatic apoptosis, and massive hepatic and pulmonary hemorrhage, similar to the situation observed in B6 mice treated with 1.0 µg SEA/d-GalN. The plasma LECT2 levels in B6 mice treated with 1.0 µg SEA/d-GalN were inversely correlated with the plasma cytokine levels and were associated with prognosis. LECT2 administration increased the survival of B6 mice and down-regulated TNF-α and IL-6. These results suggest the involvement of LECT2 in the regulation of fatal SEA-induced toxicity in d-GalN-sensitized mice.

A mouse model of autoimmune pancreatitis with salivary gland involvement triggered by innate immunity via persistent exposure to avirulent bacteria.

The pathogenesis of autoimmune pancreatitis (AIP) remains unknown. Here, we investigated the possible involvement of chronic, persistent exposure to avirulent bacteria in the pathogenesis of AIP. C57BL/6 mice were inoculated with heat-killed Escherichia coli weekly for 8 weeks. At 1 week and up to 12 months after the final inoculation, the mice were killed to obtain samples. At 1 week after the final E. coli inoculation, marked cellular infiltration with fibrosis was observed in the exocrine pancreas. Cellular infiltration in the exocrine pancreas was still observed up to 12 months after the completion of E. coli inoculation. At 10 months after the final inoculation, duct-centric fibrosis became obvious. Inflammation around the ducts in the salivary glands was also observed. Furthermore, sera from heat-killed E. coli-inoculated mice possessed anti-carbonic anhydrase, anti-lactoferrin, and antinuclear antibodies. Exposure to E. coli-triggered AIP-like pancreatitis in C57BL/6 mice. We propose a hypothetical mechanism for AIP pathogenesis. During the initiation phase, silently infiltrating pathogen-associated molecular patterns (PAMP) and/or antigen(s) such as avirulent bacteria might trigger and upregulate the innate immune system. Subsequently, the persistence of such PAMP attacks or stimulation by molecular mimicry upregulates the host immune response to the target antigen. These slowly progressive steps may lead to the establishment of AIP and associated extrapancreatic lesions. Our model might be useful for clarifying the pathogenesis of AIP.

Engulfment of gram-positive bacteria by pancreatic stellate cells in pancreatic fibrosis.

OBJECTIVES: We previously reported the finding that pancreatic stellate cells (PSCs) have a phagocytic function. The aim of the present study was to investigate whether engulfment of gram-positive bacteria by PSCs plays any role in the pathogenesis of pancreatic fibrosis. METHODS: Rat PSCs were cultured with lipoteichoic acid (LTA) or bacteria and analyzed for α-smooth muscle actin expression and collagen secretion. Human pancreata were obtained from routine autopsies of 20 cases; a diagnosis of gram-positive sepsis was made in 10 of the cases (sepsis group), but sepsis had not been diagnosed in the other 10 cases (control group). Pancreatic tissue was stained with anti-LTA antibody, and the severity of pancreatic fibrosis was evaluated by histological scoring. RESULTS: Bacteria and LTA were internalized into the cytoplasm of cultured PSCs. Exposure to LTA or bacteria significantly increased α-smooth muscle actin expression and collagen secretion. Blockade of toll-like receptor 2 significantly inhibited the increase in collagen secretion in response to LTA. There was no significant difference in the severity of pancreatic fibrosis between the sepsis group and the control group. CONCLUSIONS: The fibrogenic action of PSCs seems to be more strongly associated with activation of the toll-like receptor-dependent pathway than it is with phagocytosis of bacteria by PSCs.

Long-term bacterial exposure can trigger nonsuppurative destructive cholangitis associated with multifocal epithelial inflammation.

Bacterial infection has become a focus of attention in the pathogenesis of primary biliary cirrhosis (PBC). We earlier reported that the bacterial lipoteichoic acid was detected at the sites of inflammation around damaged bile ducts in the livers of PBC, and PBC patients' sera showed high titers against streptococcal histone-like protein. Here, we investigated whether chronic bacterial exposure could trigger PBC-like epithelial cell damage in normal mouse. BALB/c mice were repeatedly inoculated with various bacteria for 8 weeks. At 1 week (Group 1) and 3, 4, or 20 months (long term; Group 2) after the final inoculation, mice were killed to obtain samples. In the livers of the Streptococcus intermedius (S.i.)-inoculated mice in Group 1, cellular infiltration was predominantly observed around the bile ducts over the hepatic parenchyma. In the S.i.-inoculated mice in Group 2, portal but not parenchymal inflammation was observed in the livers, and periductal cellular infiltrates were detected in the salivary glands. Both S.i.-inoculated Groups 1 and 2 BALB/c mice sera had antibodies against HuCCT1 biliary epithelial cells, anti-nuclear antibodies, and anti-gp210 antibodies, but not anti-mitochondrial antibodies. Immunoreactivity to histone-like DNA-binding protein of S.i. (S.i.-HLP) was detectable around the sites of chronic nonsuppurative destructive cholangitis in the portal area in the livers of both S.i.-inoculated Groups 1 and 2 BALB/c mice. Furthermore, anti-S.i.-HLP antibody bound to synthetic gp210 peptide, as well. Bacteria triggered PBC-like cholangitis, multifocal epithelial inflammation, and autoantibody production. Bacteria are likely involved in the pathogenesis of PBC and of associated multifocal epithelial inflammation.

Treatment of nonalcoholic steatohepatitis with colestimide.

AIM: To clarify the usefulness of colestimide in patients with nonalcoholic steatohepatitis (NASH) with hyperlipidemia. METHODS: In an open-label randomized controlled trial, 17 NASH patients with hyperlipidemia received colestimide (3 g/day) for 24 weeks. There were 21 control patients. All patients received lifestyle modification therapy. Efficacy was assessed based on metabolic profile, insulin resistance, transaminases, serum hepatic fibrosis markers, adipokine levels, visceral fat on computed tomography (CT), and the fatty liver grade on CT. NASH patients with moderate to severe steatosis by histology were also evaluated separately. RESULTS: Baseline clinical characteristics of the two groups were similar. Both groups experienced a significant decrease of BMI with no difference between them. However, visceral fat decreased significantly more in the colestimide group (P = 0.046). Aspartate aminotransferase (AST) showed a significantly greater decrease in the colestimide group compared with the control group (P = 0.042). In patients with moderate to severe histological steatosis, there were significant differences between the two groups regard to HbA(1c), transaminases, and hyaluronic acid (P = 0.018 for HbA(1c), P = 0.003 for AST, P = 0.042 for alanine aminotransferase, and P = 0.042 for hyaluronic acid). Steatosis significantly improved in patients in the colestimide group who had fatty liver on CT (P = 0.049). In the colestimide group, abdominal distension and/or constipation were seen, but mostly tolerable, no other clinical or laboratory adverse events associated with the use of this medicine were not observed. CONCLUSIONS: Colestimide seems to increase the efficacy of lifestyle modification in NASH patients with hyperlipidemia. Its beneficial effects were more prominent in NASH patients with moderate to severe histological steatosis.

A possible role of histone-like DNA-binding protein of Streptococcus intermedius in the pathogenesis of bile duct damage in primary biliary cirrhosis.

Bacterial infection has become a focus of attention in the pathogenesis of primary biliary cirrhosis (PBC). It was reported that anti-histone autoantibody was detected in PBC, suggesting that bacterial histone-like DNA-binding protein (HLP) may be involved in the pathogenesis of PBC. To identify bacterial species in PBC to confirm this possibility, serum reactivity to bacterial cells was studied by ELISA. The IgM class Streptococcus intermedius titers were significantly higher in PBC than chronic hepatitis due to hepatitis C virus (CH-C) and healthy subjects. Among the streptococci, S. intermedius was selected for further study. The antigenic peptide of S. intermedius of HLP was synthesized to examine the serum reactivity to Si-HLP. IgM class anti-Si-HLP peptide titers were significantly higher in PBC. Immunoreactivity to anti-Si-HLP was detected in the cytoplasm of biliary epithelial cells and inflammatory cells in the portal area in PBC patients' livers. Streptococci, especially S. intermedius, might play a key role in the pathogenesis of PBC, possibly involving HLP.

Systemic multifocal epithelial inflammations associated with PBC-like bile duct damage in chronic colitis harboring TCR alpha -/- x AIM -/- mice: does lipoteichoic acid affect the pathogenesis of epithelial inflammation followed by fibrosis?

Autoimmune disorder and associated multifocal organ inflammations such as dry gland syndrome are occasionally observed; however, their etiologies are not clearly understood. We previously reported that chronic colitis-harboring TCR alpha(-/-) x AIM(-/-) mice show primary biliary cirrhosis (PBC)-like bile duct damage in the liver. Gram-positive bacterial infection is one of the candidates for the pathogenesis of PBC. We also reported that the bacterial cell wall component lipoteichoic acid (LTA) was detected at the sites of inflammation around damaged bile ducts in PBC patients. On the basis of these facts, we hypothesized that LTA might affect the pathogenesis of bile duct damage in the livers of TCR alpha(-/-) x AIM(-/-) mice. LTA was detected not only in the portal area with inflammation in the liver but also throughout the gastrointestinal tract, from the stomach to the colon, and especially in the epithelium at sites of inflammation. In addition, LTA was detected around both pancreatic ducts with inflammation and at the distal renal tubules with inflammation in TCR alpha(-/-) x AIM(-/-) mice. Furthermore, in the liver, pancreas, kidney, and colon, fibrous stroma were detected at the sites of LTA-positive inflammation foci. Bacterial LTA might affect the pathogenesis of epithelial inflammation followed by fibrosis in systemic multifocal epithelial inflammations in chronic colitis-harboring TCR alpha(-/-) x AIM(-/-) mice with PBC-like bile duct damage.

Apoptosis inhibitor expressed by macrophages tempers autoimmune colitis and the risk of colitis-based carcinogenesis in TCRalpha-/- mice.

BACKGROUND: Crohn's disease and ulcerative colitis (UC) are the two main entities involved in human inflammatory bowel disease (IBD). However, their precise etiologies remain unclear. To study the development of mucosal inflammation, and chronic inflammation-based dysplasia and carcinoma formation, we examined possible roles of the apoptosis inhibitor expressed by macrophages (AIM) in an experimental IBD model. METHODS: In this study, we used T cell receptor alpha deficient (TCRalpha(-/-)) mice, a known UC-like colitis model. We generated TCRalpha(-/-) x AIM(-/-) double knockout mice by crossbreeding TCRalpha(-/-) with AIM(-/-) mice. At 24 weeks of age, mice were killed to obtain colon tissues for pathological examinations. TCRalpha(-/-) x AIM(+/-) mice, heterozygous littermates of TCRalpha(-/-) x AIM(-/-) mice, were used as controls. RESULTS: Severe colitis was observed in TCRalpha(-/-) x AIM(-/-) mice, when compared with TCRalpha(-/-) x AIM(+/-) mice. Dysplasia was detected in TCRalpha(-/-) x AIM(-/-) mice, but not in TCRalpha(-/-) x AIM(+/-) mice. Adenocarcinoma formation was observed from dysplasia only in TCRalpha(-/-) x AIM(-/-) mice. CONCLUSION: Not only a high incidence of severe colitis but also dysplasia and adenocarcinoma formation were observed in TCRalpha(-/-) x AIM(-/-) mice only. AIM have some regulatory roles in inflammation and progression of dysplasia to carcinoma in TCRalpha(-/-) mice.

Lipoteichoic acid may affect the pathogenesis of PBC-like bile duct damage and might be involved in systemic multifocal epithelial inflammations in chronic colitis-harboring TCRalpha-/-xAIM-/- mice.

BACKGROUND: Chronic colitis-harboring TCRalpha(- / - ) x AIM(- / - ) mice showed PBC-like bile duct damage in the liver. Bacterial infection is one of the candidates for the pathogenesis of PBC. We demonstrated that the bacterial cell wall component lipotheicoic acid (LTA) was detected at sites of inflammation around damaged bile ducts in PBC patients. The aim of this study was to investigate the pathophysiology of the liver and other organs in TCRalpha(- / - ) x AIM(- / - ) mice. METHODS: Thirteen female TCRalpha(- / - ) x AIM(- / - ) mice were sacrificed at 24 weeks of age. The liver, stomach, small intestine, colon, pancreas, kidney and spleen were studied for pathological examination. Using anti-LTA antibody as the primary antibody, an immunohistochemical study was carried out. RESULTS: In the liver, LTA was mainly detected in the portal area with inflammation, and some of the cytoplasm of hepatocytes. Inflammations were also observed in the stomach, intestine, pancreas and kidney. Throughout the gastrointestinal tract, from the stomach to the colon, LTA was detected in the epithelium at sites of inflammation. Furthermore, LTA was detected around both pancreatic ducts with inflammation and distal renal tubules with inflammation. CONCLUSIONS: The development of inflammations in the liver as well as extensive organs, strongly suggests a close relationship between bile duct damage and systemic multifocal epithelial inflammations, perhaps involving bacterial LTA, in TCRalpha(- / - ) x AIM(- / - ) mice.

Spur cell anemia associated with a cirrhotic non-alcoholic steatohepatitis patient.

Spur cell anemia is an acquired hemolytic anemia, which may occur in patients with severe liver dysfunction, especially in those with alcoholic cirrhosis. Recently, non-alcoholic steatohepatitis (NASH) has been highlighted as one of the causes of chronic liver diseases, which could progress to cirrhosis. Histological features of NASH are indistinguishable from those of alcoholic hepatitis. We report on a cirrhotic NASH patient who developed progressive indirect dominant jaundice and intractable hemolytic anemia during his follow-up. This report shows the first case of a cirrhotic NASH patient who died as a result of spur cell anemia.

Lipoteichoic acid may affect the pathogenesis of bile duct damage in primary biliary cirrhosis.

AIM: Intrahepatic bile ducts are the targets for inflammation in primary biliary cirrhosis (PBC), but their pathogenesis is not known. Gram-positive bacterial DNA was detected recently in gallbladder bile of PBC patients. In the present study, we assessed the possible pathological role of lipoteichoic acid (LTA), the gram-positive bacterial cell wall component, in PBC. METHODS: Liver samples, obtained from 20 patients with PBC (stage 1-2 with CNSDC: stage 3-4 with loss of bile ducts = 10:10) and from 13 patients with chronic hepatitis due to hepatitis C virus (CH-C) with lymphocytic cholangitis, were subjected to immunohistochemical staining with polyclonal rabbit anti-LTA as the primary antibody. Serum reactivities to LTA were studied by ELISA. After 1 microg of purified LTA was placed in a 96-well microplate as an antigen, an antibody capture assay was carried out using serum samples from PBC (n = 20), CH-C (n = 13) and healthy subjects (n = 11). RESULTS: LTA was localized around the sites of chronic non-suppurative destructive cholangitis (CNSDC) in the portal area in stage 1-2 PBC but was not detected in the portal area in CH-C. In stage 3-4 PBC, LTA was localized around sites of ductular proliferation at the periphery of portal tracts. IgM class anti-LTA serum titers were significantly higher in PBC than in CH-C. IgA class anti-LTA serum titers were significantly higher in PBC than in healthy subjects. CONCLUSIONS: In the PBC livers, the profile of immunoreactivity to LTA changed markedly as the disease progressed. Sera from PBC showed higher levels of anti-LTA titers than CH-C (IgM) or from healthy subjects (IgA). The LTA-mediated immune system might affect the initiation and/or progression of PBC.

Intrahepatic biliary epithelial cell damage and inflammation in portal tract in association with chronic colitis-harboring TCRalpha(-/-) mice.

BACKGROUND: Intrahepatic bile ducts are the target for inflammation in both primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC). The mechanisms of biliary epithelial cell damage in both diseases are not clearly understood. Ulcerative colitis (UC) is one of the known complications in PSC. In this study, we assessed the possible influence of apoptosis inhibitor expressed by macrophages (AIM) on intrahepatic bile ducts in the chronic colitis-harboring condition by generating T cell receptor alpha-deficient (TCRalpha(-/-))xAIM-deficient (AIM(-/-)) double-knockout mice. METHODS: TCRalpha(-/-)xAIM(-/-) mice were generated by crossbreeding TCRalpha(-/-) mice with AIM(-/-) mice. At 24 weeks of age, mice were sacrificed to obtain liver tissues for pathological examinations, and blood was collected to study the serum levels of IgG, IgM and IgA. RESULTS: In female TCRalpha(-/-)xAIM(-/-) mouse livers, mixed cellular infiltration in the portal area and epithelial cell damage in bile ducts were observed, when compared with female TCRalpha(-/-)xAIM(+/-) and male TCRalpha(-/-)xAIM(-/-) mice. Inflammation in hepatic parenchyma was mild to none in all mice. In the female mouse group, the serum IgA level was relatively increased in TCRalpha(-/-)xAIM(-/-) mice compared to TCRalpha(-/-)xAIM(+/-) mice. CONCLUSION: The defect of AIM might be involved not only in colonic mucosal inflammation but also in portal inflammation, as well as in biliary epithelial cell damage in the livers of female TCRalpha(-/-)xAIM(-/-) mice.

Characterization of a conformationally sensitive murine monoclonal antibody directed to the metal ion-dependent adhesion site face of integrin CD11b.

Integrin binding to physiologic ligands requires divalent cations and an inside-out-driven switch of the integrin to a high-affinity state. Divalent cations at the metal ion-dependent adhesion site (MIDAS) face of the alpha subunit-derived A domain provide a direct bridge between ligands and the integrin, and it has been proposed that activation dependency is caused by reorientation of the surrounding residues relative to the metal ion, forming an optimal binding interface. To gain more insight into the functional significance of the protein movements on the MIDAS face, we raised and characterized a murine mAb 107 directed against the MIDAS face of the A domain from integrin CD11b. We find that mAb 107 behaves as a ligand mimic. It binds in a divalent-cation-dependent manner to solvent-exposed residues on the MIDAS face of CD11b, blocks interaction of 11bA or the holoreceptor with ligands, and inhibits spreading and phagocytosis by human neutrophils. However, in contrast to physiologic ligands, mAb 107 preferentially binds to the inactive low-affinity form of the integrin, suggesting that its antagonistic effects are exerted in part by stabilizing the receptor in the low-affinity state. These data support a functional relevance of the protein movements on the MIDAS face and suggest that stabilizing the A domain in the low-affinity state may have therapeutic benefit.