Ma, Q., Jones, D. & Springer, T.A. The chemokine receptor CXCR4 is required for the retention of B lineage and granulocytic precursors within the bone marrow microenvironment. Immunity 10, 4, 463-471 (1999).Abstract

We report that the chemokine receptor CXCR4 is required for the retention of B lineage and granulocytic precursors within fetal liver and bone marrow microenvironment. In CXCR4-deficient embryos, pro-B cells are present in blood but hardly detectable in liver; myeloid cells are elevated in blood and reduced in liver compared to wild-type embryos. Mice reconstituted with CXCR4-deficient fetal liver cells have reduced donor-derived mature B lymphocytes in blood and lymphoid organs. The numbers of pro-B and pre-B cells are reduced in bone marrow and abnormally high in blood. Granulocytic cells are reduced in bone marrow but elevated and less mature in the blood. B lineage and granulocytic precursors are released into the periphery in absence of CXCR4.

Oxvig, C., Lu, C. & Springer, T.A. Conformational changes in tertiary structure near the ligand binding site of an integrin I domain. Proc Natl Acad Sci USA 96, 5, 2215-2220 (1999).Abstract

For efficient ligand binding, integrins must be activated. Specifically, a conformational change has been proposed in a ligand binding domain present within some integrins, the inserted (I) domain [Lee, J., Bankston, L., Arnaout, M. & Liddington, R. C. (1995) Structure (London) 3, 1333-1340]. This proposal remains controversial, however, despite extensive crystal structure studies on the I domain [Lee, J., Bankston, L., Arnaout, M. & Liddington, R. C. (1995) Structure (London) 3, 1333-1340; Liddington, R. & Bankston, L. (1998) Structure (London) 6, 937-938; Qu, A. & Leahy, D. J. (1996) Structure (London) 4, 931-942; and Baldwin, E. T., Sarver, R. W., Bryant, G. L., Jr., Curry, K. A., Fairbanks, M. B., Finzel, B. C. , Garlick, R. L., Heinrikson, R. L., Horton, N. C. & Kelly, L. L. (1998) Structure (London) 6, 923-935]. By defining the residues present in the epitope of a mAb against the human Mac-1 integrin (alphaMbeta2, CD11b/CD18) that binds only the active receptor, we provide biochemical evidence that the I domain itself undergoes a conformational change with activation. This mAb, CBRM1/5, binds the I domain very close to the ligand binding site in a region that is widely exposed regardless of activation as judged by reactivity with other antibodies. The conformation of the epitope differs in two crystal forms of the I domain, previously suggested to represent active and inactive receptor. Our data suggests that conformational differences in the I domain are physiologically relevant and not merely a consequence of different crystal lattice interactions. We also demonstrate that the transition between the two conformational states depends on species-specific residues at the bottom of the I domain, which are proposed to be in an interface with another integrin domain, and that this transition correlates with functional activity.

Bork, P., Doerks, T., Springer, T.A. & Snel, B. Domains in plexins: Links to integrins and transcription factors. Trends Biochem. Sci. 24, 7, 261-263 (1999). Bork_1999_14513.pdf
Springer, T.A. Immunoaffinity chromatography. Current Protocols in Protein Science 9.5.1-9.5.11 (1999). Springer_1999_14618.pdf
Dustin, M.L. & Springer, T.A. Intercellular adhesion molecules (ICAMs). Guidebook to the Extracellular Matrix and Adhesion Proteins 216-220 (1999). Dustin_1999_13747.pdf
Dustin, M.L. & Springer, T.A. Lymphocyte function associated-1 (LFA-1, CD11a/CD18). Guidebook to the Extracellular Matrix and Adhesion Proteins, 228-232 (1999). Dustin_1999_13745.pdf
Casasnovas, J., Pieroni, C. & Springer, T.A. Lymphocyte function-associated antigen-1 binding residues in intercellular adhesion molecule-2 (ICAM-2) and the integrin binding surface in the ICAM subfamily. Proc Natl Acad Sci USA 96, 6, 3017-3022 (1999).Abstract

The crystal structure of intercellular adhesion molecule-2 (ICAM-2) revealed significant differences in the presentation of the critical acidic residue important for integrin binding between I and non-I-domain integrin ligands. Based on this crystal structure, we mutagenized ICAM-2 to localize the binding site for the integrin lymphocyte function-associated antigen-1 (LFA-1). The integrin binding site runs diagonally across the GFC beta-sheet and includes residues on the CD edge of the beta-sandwich. The site is oblong and runs along a flat ridge on the upper half of domain 1, which is proposed to dock to a groove in the I domain of LFA-1, with the critical Glu-37 residue ligating the Mg2+ in the I domain. Previous mutagenesis of ICAM-1 and ICAM-3, interpreted in light of the recently determined ICAM-1 and ICAM-2 structures, suggests similar binding sites. By contrast, major differences are seen with vascular cell adhesion molecule-1 (VCAM-1), which binds alpha4 integrins that lack an I domain. The binding site on VCAM-1 includes the lower portion of domain 1 and the upper part of domain 2, whereas the LFA-1 binding site on ICAM is confined to the upper part of domain 1.

Fedyk, E.R., Ryan, D.H., Ritterman, I. & Springer, T.A. Maturation decreases responsiveness of human bone marrow B lineage cells to stromal-derived factor 1 (SDF-1). J. Leukoc. Biol. 66, 4, 667-673 (1999).Abstract

We compared the chemotactic responsiveness of different subsets of human B lineage cells to stromal derived factor-1 (SDF-1). High percentages (30-40% of input) of purified bone marrow progenitors including non-B lineage progenitors, pro-B cells, and pre-B cells migrated to SDF-1alpha, demonstrating that SDF-1 is an efficacious chemoattractant of these cells. Pro-B cells responded optimally to a lower concentration of SDF-1 than other subsets, demonstrating that SDF-1 is a more potent chemoattractant of this subset. A lower percentage (10-15% of input) of mature B lymphocytes migrated to SDF-1alpha than pro-B cells, demonstrating that responsiveness of B lineage cells to SDF-1 decreases during differentiation. Inhibition by anti-CXCR4 mAb demonstrated that migration of B lineage cells to SDF-1 was completely dependent on CXC chemokine receptor-4 (CXCR4). Mature B cells expressed higher levels of CXCR4 receptors than uncommitted progenitors and pro-B cells, despite differences in responsiveness to SDF-1. CXCR4 receptors expressed by unresponsive and SDF-1-responsive B cells bound SDF-1alpha with similar affinities (K(D) = 1.7-3.3 x 10(-9) M). Therefore, elements downstream from CXCR4 appear to regulate responsiveness of B cells to SDF-1. We speculate that SDF-1 and CXCR4 direct migration of progenitor cells in microenvironments that promote B lymphopoiesis.

Green, N., et al. Mutational analysis of MAdCAM-1/α4β7 interactions reveals significant binding determinants in both the first and second immunoglobulin domains. Cell Adhes. Comm. 7, 167-181 (1999).Abstract

The selective emigration of blood born leukocytes into tissues is mediated, in part by interactions of Ig-like cell adhesion molecules (IgCAMs) expressed on vascular endothelium and their cognate ligands, the leukocyte integrins. Within mucosal lymphoid tissues and gastrointestinal sites the mucosal vascular addressin. MAdCAM-1 is the predominant IgCAM, mediating specific lymphocyte homing via interactions with its ligand on lymphocytes, the integrin alpha4beta7. Previous studies have shown that an essential binding motif resides in the first Ig domain of all IgCAMs, containing an acidic residue (D or E) preceded by an aliphatic residue (L or I) that resides in strand C or the CD loop. However, domain swap experiments with MAdCAM-1 and VCAM-1 have shown a requirement for both Ig domains 1 and 2 for efficient integrin binding. We describe the use of chimeric MAdCAM-1/VCAM-1 receptors and point mutations in MAdCAM-1 to define other sites that are required for binding to the integrin alpha4beta7. We find that, in addition to critical CD loop residues, other regions in both domain one and two contribute to MAdCAM-1/alpha4beta7 interactions, including a buried arginine residue in the F strand of domain one and several acidic residues in a highly extended DE ribbon in domain 2. These mutations, when placed in the recently solved crystal structure of human MAdCAM-1 give insight into the integrin binding preference of this unique receptor.

Gerwin, N., et al. Prolonged eosinophil accumulation in allergic lung interstitium of ICAM-2 deficient mice results in extended hyperresponsiveness. Immunity 10, 1, 9-19 (1999).Abstract

ICAM-2-deficient mice exhibit prolonged accumulation of eosinophils in lung interstitium concomitant with a delayed increase in eosinophil numbers in the airway lumen during the development of allergic lung inflammation. The ICAM-2-dependent increased and prolonged accumulation of eosinophils in lung interstitium results in prolonged, heightened airway hyperresponsiveness. These findings reveal an essential role for ICAM-2 in the development of the inflammatory and respiratory components of allergic lung disease. This phenotype is caused by the lack of ICAM-2 expression on non-hematopoietic cells. ICAM-2 deficiency on endothelial cells causes reduced eosinophil transmigration in vitro. ICAM-2 is not essential for lymphocyte homing or the development of leukocytes, with the exception of megakaryocyte progenitors, which are significantly reduced.

Issekutz, A.C., Rowter, D. & Springer, T.A. Role of ICAM-1 and ICAM-2 and alternate CD11/CD18 ligands in neutrophil transendothelial migration. J. Leukoc. Biol. 65, 117-126 (1999).Abstract

We evaluated the relative contribution of ICAM-1 and ICAM-2, known ligands on endothelium for LFA-1 and Mac-1, in spontaneous neutrophil (PMN) transendothelial migration (TEM) across IL-1-activated HUVEC monolayers or TEM induced by C5a or IL-8 across unstimulated HUVEC grown on polycarbonate filters. Adhesion blocking mAb to ICAM-1 [R6.5 F(ab)2] or ICAM-2 [CBR IC2/2 F(ab)2] tended to inhibit TEM under each condition but, in general, inhibition was significant only with both ICAM-1 and ICAM-2 blockade. mAb to LFA-1 partially inhibited migration to C5a or IL-8 across unstimulated HUVEC and inhibition was not altered by additional treatment of HUVEC with mAbs to ICAM-1 and -2. In contrast, with IL-1 HUVEC, mAb to ICAM-1 significantly inhibited this LFA-1-independent TEM. mAb to Mac-1 alone partially inhibited TEM and, when combined with mAb to LFA-1, migration was almost completely blocked with all TEM conditions tested. The contribution of alternate ligands for Mac-1 in mediating Mac-1-dependent but ICAM-1/-2-independent C5a-induced TEM was examined using anti-LFA-1-treated PMN and anti-ICAM-treated resting HUVEC. Addition of RGD peptides, fibronectin, fibrinogen, heparins, collagens alone or in combination, even to heparinase-treated HUVEC, did not inhibit this Mac-1-mediated PMN TEM. The results indicate that: (1) LFA-1 mediates PMN TEM primarily by interaction with ICAM-1 and ICAM-2; (2) ICAM-2 may function in concert with ICAM-1 in this role, especially on unstimulated endothelium, and (3) Mac-1 on PMN also plays a major role in TEM and can utilize yet to be identified ligands distinct from ICAM-1 or -2, especially on unstimulated endothelium.

Bleul, C.C., Schultze, J.L. & Springer, T.A. B lymphocyte chemotaxis regulated in association with microanatomic localization, differentiation state, and B cell receptor engagement. J. Exp. Med. 187, 5, 753-762 (1998).Abstract

Migration of mature B lymphocytes within secondary lymphoid organs and recirculation between these sites are thought to allow B cells to obtain T cell help, to undergo somatic hypermutation, to differentiate into effector cells, and to home to sites of antibody production. The mechanisms that direct migration of B lymphocytes are unknown, but there is evidence that G protein-coupled receptors, and possibly chemokine receptors, may be involved. Stromal cell- derived factor (SDF)-1alpha is a CXC chemokine previously characterized as an efficacious chemoattractant for T lymphocytes and monocytes in peripheral blood. Here we show with purified tonsillar B cells that SDF-1alpha also attracts naive and memory, but not germinal center (GC) B lymphocytes. Furthermore, GC B cells could be converted to respond to SDF-1alpha by in vitro differentiation into memory B lymphocytes. Conversely, the migratory response in naive and memory B cells was significantly reduced after B cell receptor engagement and CD40 signaling. The receptor for SDF-1, CXC chemokine receptor 4 (CXCR4), was found to be expressed on responsive as well as unresponsive B cell subsets, but was more rapidly downregulated on responsive cells by ligand. Finally, messenger RNA for SDF-1 was detected by in situ hybridization in a layer of cells surrounding the GC. These findings show that responsiveness to the chemoattractant SDF-1alpha is regulated during B lymphocyte activation, and correlates with positioning of B lymphocytes within a secondary lymphoid organ.

Wang, C.Y., et al. Cardiac graft intercellular adhesion molecule-1 (ICAM-1) and interleukin-1 expression mediate primary isograft failure and induction of ICAM-1 in organs remote from the site of transplantation. Circ. Res. 82, 7, 762-772 (1998).Abstract

During the first few hours after heart transplantation, the occurrence of graft failure is unpredictable and devastating. An explosive cascade of inflammatory events within the reperfused graft vasculature is likely to be mediated, at least in part, by the local expression of the leukocyte adhesion receptor intercellular adhesion molecule-1 (ICAM-1, CD54). Furthermore, although proinflammatory cytokines such as interleukin-1 (IL-1) are known to autoinduce their own (and ICAM-1) expression in vitro, there are no data to identify their functional in vivo cross talk in the setting of isograft transplantation. To determine the role of ICAM-1 in primary graft failure, we used an isogeneic vascularized model of heterotopic cardiac transplantation. ICAM-1 mRNA and protein increased in grafts during the early posttransplant period and were predominantly localized in the endothelium. The functional significance of this was established using donor hearts obtained from either ICAM-1-deficient (ICAM-1 -/-) or control (ICAM-1 +/+) mice. ICAM-1 +/+ grafts exhibited increased neutrophil infiltration, reduced left ventricular compliance, and poorer survival than did ICAM-1 -/- grafts. Increased ICAM-1 expression was not limited to ICAM-1 +/+ grafts but also occurred in unmanipulated recipient organs located remote from the site of surgery (but only after transplantation of ICAM-1 +/+, not ICAM-1 -/-, cardiac grafts). This expression of ICAM-1 in remote organs appeared to be triggered by IL-1alpha released from the graft, because (1) in situ hybridization revealed increased IL-1 mRNA within cells of the reperfused graft, including myocytes and endothelial cells; (2) ICAM-1 expression in remote organs coincided with a significant increase in serum levels of IL-1alpha after transplantation of ICAM-1 +/+ grafts; both remote organ ICAM-1 expression and IL-1alpha levels were blunted by implantation of ICAM-1 -/- grafts; and (3) remote organ ICAM-1 expression and neutrophil infiltration and IL-1 levels could be blocked by the administration of an IL-1 receptor antagonist. These data demonstrate an apparent positive-feedback loop in which local ICAM-1 and IL-1 expression leads to a mutual amplification of each other's expression within the reperfused graft, promulgating inflammatory events that are likely to be an important cause of primary cardiac graft failure. Because IL-1 receptor blockade reduces the IL-1-mediated autoinduction of IL-1, reduces the expression of ICAM-1 in both the graft and remote organs, and improves graft survival, it may provide a new and effective strategy to prevent the occurrence of primary cardiac graft failure.

Kitayama, J., Mackay, C.R., Ponath, P.D. & Springer, T.A. The C-C chemokine receptor CCR3 participates in stimulation of eosinophil arrest on inflammatory endothelium in shear flow. J. Clin. Invest. 101, 9, 2017-2024 (1998).Abstract

Chemokines are widely hypothesized to stimulate firm adhesion of leukocytes on endothelium in shear flow. Thus far, this has been demonstrated experimentally for exogenously added chemoattractants, but not for those released by endothelium. We found that human umbilical cord endothelial cells (HUVEC) stimulated with TNF-alpha and IFN-gamma secreted eosinophil chemoattractants into the culture supernatant. This material induced transendothelial chemotaxis, stimulated eosinophil binding to purified intercellular adhesion molecule 1, and augmented binding to purified vascular cell adhesion molecule 1 in a 3-min static assay. Chemotaxis and stimulation of adhesion were abrogated completely by the pretreatment of eosinophils with an mAb to the C-C chemokine receptor 3 (CCR3). Eosinophils accumulated efficiently on HUVEC stimulated with TNF-alpha and IFN-gamma in shear flow at 1.5 dyn/cm2. CCR3 mAb slightly but significantly reduced eosinophil arrest and accumulation, by preventing development of firm adhesion by some of the tethered eosinophils, so that they detached within 30 s after the initial tethering. In the presence of mAb to the alpha4 integrin subunit, the effect of CCR3 mAb was more prominent, and approximately half of eosinophil arrest and accumulation was abolished. Inhibition by CCR3 mAb in the presence of beta2 integrin mAb was similar to that in control eosinophils. This is the first evidence that endothelial cell-derived chemokines can activate firm adhesion through alpha4 and beta2 integrins even in the presence of shear flow.

Piali, L., et al. The chemokine receptor CXCR3 mediates rapid and shear-resistant adhesion-induction of effector T lymphocytes by the chemokines IP10 and Mig. Eur. J. Immunol. 28, 3, 961-972 (1998).Abstract

Integrin-mediated adhesion to the vascular endothelium is an essential step in leukocyte diapedesis. We show that the chemokines 10-kDa inflammatory protein (IP10) and monokine induced by IFN (Mig) induce rapid and transient adhesion of human IL-2-stimulated T lymphocytes (IL-2 T cells) to immobilized integrin ligands through their receptor CXCR3, which is selectively expressed on activated T cells. Induction of adhesion by IP10 and Mig was already observed at subnanomolar concentrations and was maximal at 5-10 nM, resulting in three- to sixfold increase in adhesion of IL-2 T cells over background. No effect was seen with resting naive/memory T cells which lack CXCR3 and migration responses to IP10 and Mig. Both chemokines are produced in human umbilical vein endothelial cells (HUVEC) upon stimulation with IFN-gamma and TNF-alpha. These chemokines induce IL-2 T cell adhesion also when captured on the surface of endothelial cells. Under conditions of flow, IL-2 T cells roll and rapidly adhere to IP10/Mig-expressing HUVEC, and anti-CXCR3 mAb treatment reduces arrest and firm adhesion. This is the first study that shows chemokine-induced adhesion in activated memory/effector T cells which represent the fraction of T cells that are selectively mobilized in inflammation. The critical role of IFN-gamma as inducer of IP10/Mig production in HUVEC indicates that these chemokines are essential mediators of effector T cell recruitment to IFN-gamma-dependent pathologies.

Petruzzelli, L., Maduzia, L. & Springer, T.A. Differential requirements for LFA-1 binding to ICAM-1 and LFA-1-mediated cell aggregation. J. Immunol. 160, 9, 4208-4216 (1998).Abstract

Cellular adhesion through the beta2 integrin lymphocyte function-associated Ag (LFA)-1 is a complex event involving activation, ligand binding, and cell shape changes that ultimately result in enhanced adhesion. In this report we define requirements for ligand binding and post receptor signaling by comparing two mechanisms of activation of LFA-1: 1) inside-out signaling and 2) direct activation by the beta2 Ab, CBR LFA-1/2. Our results demonstrate that activation of LFA-1 binding to ICAM-1 by CBR LFA-1/2, in contrast to inside-out signaling mechanisms, does not require protein kinase C activation or protein phosphatase 2A activity nor is it affected by agents that interfere with reorganization of the cytoskeleton. Inhibition of protein tyrosine kinase activity does not affect ICAM- binding by either mechanism of activation. However, activation by either mode does require the presence of the beta cytoplasmic domain; deletion of the C-terminal phenylalanine or the five amino acid stretch between 756-762 abolished activation of LFA-1. This, combined with the observation that intracellular energy pools must be preserved, implicates the beta cytoplasmic domain in a key energy-dependent conformational change in LFA-1 that is required to achieve enhanced ligand binding. Post ligand binding events induced by both PMA and Ab stimulation, as measured by homotypic aggregation, require protein tyrosine kinase, phosphatase, and RhoA activities. By examining both ligand binding and aggregation, we have been able to dissect the signaling components critical in the multistep process of LFA-1-mediated cellular adhesion.

Casasnovas, J.M., Stehle, T., Liu, J.-H., Wang, J.-H. & Springer, T.A. A dimeric crystal structure for the N-terminal two domains of ICAM-1. Proc Natl Acad Sci USA 95, 8, 4134-4139 (1998).Abstract

The 3.0-A structure of a 190-residue fragment of intercellular adhesion molecule-1 (ICAM-1, CD54) reveals two tandem Ig-superfamily (IgSF) domains. Each of two independent molecules dimerizes identically with a symmetry-related molecule over a hydrophobic interface on the BED sheet of domain 1, in agreement with dimerization of ICAM-1 on the cell surface. The residues that bind to the integrin LFA-1 are well oriented for bivalent binding in the dimer, with the critical Glu-34 residues pointing away from each other on the periphery. Residues that bind to rhinovirus are in the flexible BC and FG loops at the tip of domain 1, and these and the upper half of domain 1 are well exposed in the dimer for docking to virus. By contrast, a residue important for binding to Plasmodium falciparum-infected erythrocytes is in the dimer interface. The presence of A' strands in both domains 1 and 2, conserved hydrogen bonds at domain junctions, and elaborate hydrogen bond networks around the key integrin binding residues in domain 1 make these domains suited to resist tensile forces during adhesive interactions. A subdivision of the intermediate (I) set of IgSF domains is proposed in which domain 1 of ICAM-1 and previously described I set domains belong to the I1 set and domain 2 of ICAM-1, ICAM-2, and vascular cell adhesion molecule-1 belong to the I2 set.

Casasnovas, J.M., Bickford, J.K. & Springer, T.A. The domain structure of ICAM-1 and the kinetics of binding to rhinovirus. J. Virol. 72, 7, 6244-6246 (1998).Abstract

Fragments of intercellular adhesion molecule 1 (ICAM- 1) containing only the two most N terminal of its five immunoglobulin SF domains bind to rhinovirus 3 with the same affinity and kinetics as a fragment with the entire extracellular domain. The fully active two-domain fragments contain 5 or 14 more residues than a previously described fragment that is only partially active. Comparison of X-ray crystal structures show differences at the bottom of domain 2. Four different glycoforms of ICAM- 1 bind with identical kinetics.

Nagashima, M., et al. Effects of a monoclonal antibody to P-selectin on recovery of neonatal lamb hearts after cold cardioplegic ischemia. Circulation 98, II391-II398 (1998).Abstract

BACKGROUND: The interaction between endothelium and leukocytes plays a crucial role in ischemia-reperfusion injury. P-selectin, which is expressed on activated endothelium, mediates the first step in leukocyte adherence to the endothelium. This study examined the effects of a monoclonal antibody (mAb) against P-selectin on the recovery of cardiac function and myocardial neutrophil infiltration after ischemia.
METHODS AND RESULTS: Thirteen blood-perfused, isolated neonatal lamb hearts underwent 2 hours of hypothermic cardioplegic arrest and 2 hours of reperfusion. Immediately before reperfusion, mAb to P-selectin was administered to the perfusate (15 micrograms/mL) in 6 hearts (group P-sel). In control (n = 7), the same volume of saline was added. Isovolumic left ventricular function and coronary blood flow were measured. At 2 hours after reperfusion, myocardial myeloperoxidase activity, an index of neutrophil accumulation, was assayed. At 30 minutes of reperfusion, hearts treated with mAb to P-selectin achieved significantly greater recovery of maximum developed pressure (70 +/- 4% in control versus 77 +/- 2% in group P-sel, P < 0.01), maximum positive first derivative of pressure (dP/dt) (64 +/- 7% in control versus 73 +/- 5% in group P-sel, P < 0.05), and maximum negative dP/dt (61 +/- 6% in control versus 70 +/- 6% in group P-sel, P < 0.05) compared with control. Percent baseline of coronary blood flow was also significantly increased in group P-sel (135 +/- 40% in control versus 205 +/- 43% in group P-sel, P < 0.05). Myocardial myeloperoxidase activity was significantly lower (P < 0.05) in group P-sel (4.7 +/- 3.2) versus control (16.0 +/- 10.1). (Units are change in absorbance/min/g tissue.)
CONCLUSIONS: The functional blockade of P-selectin resulted in better recovery of cardiac function and attenuated neutrophil accumulation during early reperfusion. Strategies to block P-selectin mediated neutrophil adherence may have clinical application in improving myocardial function at early reperfusion.

Oxvig, C. & Springer, T.A. Experimental support for a β-propeller domain in integrin α-subunits and a calcium binding site on its lower surface. Proc Natl Acad Sci USA 95, 9, 4870-4875 (1998).Abstract

Integrins are large, heterodimeric surface molecules of wide importance in cell adhesion. The N-terminal half of all integrin alpha-subunits contains seven weak sequence repeats of approximately 60 amino acids that are important in ligand binding and have been predicted to fold cooperatively into a single beta-propeller domain with seven beta-sheets. We provide evidence supporting this model with a mouse mAb to human Mac-1 (alphaM beta2, CD11b/CD18). This antibody, CBRM1/20, binds to amino acid residues that are in different repeats and are 94 residues apart in the primary structure in the loop between strands 1 and 2 of beta-sheet 5 and in the loop between strands 3 and 4 of beta-sheet 6. The 1-2 loops of beta-sheets 5-7 in integrins have EF hand-like Ca2+-binding motifs. CBRM1/20 binds to Mac-1 in the presence of Ca2+ or Sr2+ with an EC50 of 0.2 mM. Mg2+ or Mn2+ cannot substitute. Antibodies to other epitopes on the Mac-1 beta-propeller domain bind in the absence of calcium. mAb CBRM1/20 does not block ligand binding. Thus, the region on the lower surface of the beta-propeller domain to which mAb CBRM1/20 binds does not bind ligand and, furthermore, cannot bind other integrin domains, such as those of the beta-subunit.