A novel point mutation in CD18 causing the expression of dysfunctional CD11/CD18 leucocyte integrins in a patient with leucocyte adhesion deficiency (LAD)

Leucocyte adhesion deficiency type 1 (LAD-1) is characterized by the incapacity of leucocytes to carry out their adhesion functions via their CD11/CD18 antigens, which are also referred to as the leucocyte integrins. The patients generally suffer from poor wound healing and recurrent bacterial and fungal infections. In severe cases, the infections are often systemic and life-threatening. A LAD patient (AW) of moderate phenotype has been identified but, unlike most other cases, the level of CD11/CD18 antigens on her leucocytes are uncharacteristically high for a LAD patient. Molecular analysis revealed that she is a compound heterozygote for CD18 mutations. She has inherited a D231H mutation from her father and a G284S mutation from her mother. By transfection studies, it was established that the G284S mutation does not support CD11/CD18 antigen expression on the cell surface. In contrast, the D231H mutation does not affect CD18 forming integrin heterodimers with the CD11 antigens on the cell surface. However, the expressed integrins with the D231H mutation are not adhesive to ligands.     

Canine leukocyte cell adhesion molecules (LeuCAMs): Characterization of the CD11/CD18 family

The canine LeuCAM (CD11/CD18) family is characterized using a panel of newly developed anti-canine LeuCAM monoclonal antibodies (mAb) as well as a cross-reactive anti-human CD11b mAb. The new anti-canine LeuCAM mAb were developed by immunizing mice with purified canine CD11/CD18 antigen derived from an anti-canine CD18 affinity column. Six mAb with reactivity to the canine LeuCAM family were cloned and characterized. These 6 included a new anti-canine CD18 mAb, 2 anti-canine CD11a mAb, 2 anti-canine CD11c mAb, and a mAb which recognizes a novel canine CD11/CD18-related macrophage antigen, designated LeuCAMmac, which is described in a separate report. The cell and tissue distribution of canine LeuCAMs was found to be similar to that reported in other species with a few notable exceptions. One prominent exception was the finding that canine CD11c, in contrast to human CD11c, was much more widely distributed on dendritic cells than it was on tissue macrophages. The molecular organization of the canine LeuCAM family was also found to be similar to that reported in other species, with the possible exception of the canine CD11b alpha chain, which may exist as several different species. It is anticipated that these anti-canine LeuCAM mAb will prove useful in immunophenotypic studies of canine hemolymphatic system neoplasia.     

Role of the lectin domain of Mac-1/CR3 (CD11b/CD18) in regulating intercellular adhesion

Leukocytediapedesis requires that Mac-1/CR3-dependent adhesion be regulated so that cells can move from one attachment site to another. The high affinity adhesion state of Mac-1/CR3 is generated when it forms alectin-dependent complex with the receptor for urokinase plasminogen activator (uPAR; CD87). The extensively glycosylated uPAR binds to the same C-terminal lectin domain of CD11b that had previously been shown to prime Mac-1/CR3 for cytotoxic degranulation in response to β-glucan uPAR and β-glucan compete for a lectin site that is near to the CBRM1/23 epitope (residues 943–1047) at the C-terminus of CD11b, and thus the lectin domain is critical to both the adhesion and cytotoxic functions of Mac-1/CR3. Adhesion is reversed when the uPA enzyme is captured by its receptor (uPAR), causing uPAR to bind to CD11b at a second site (residues 424–440) that is in between the N-terminal I-domain and the divalent cation binding region.     

Evidence for cell surface association of CD2 and LFA-1 (CD11a/CD18) on T lymphocytes

Previous studies have reported an association of the cell surface adhesion molecule CD2 with the T cell receptor and with CD45 on mouse and human T lymphocytes. In this study the association of CD2 with cell surface molecules was investigated using cell surface biotinylation of T lymphocytes, coupled with immunoprecipitation using two CD2-specific monoclonal antibodies (mAb) (RM2–5 and 12–15) and analysis by SDS-PAGE. Although both CD2 mAb immunoprecipitated CD2 from lysates of murine lymphocytes, it was found that mAb 12–15, but not RM2–5, co-precipitated two other molecules of 95 and 180 kDa. Subsequent studies revealed that the 95- and 180-kDa molecules were associated with a subspecies of CD2 (? 5%) on thymocytes, the antigen-specific T cell line D10, and splenic T cells but not B cells. Two lines of evidence were obtained consistent with the 95- and 180-kDa molecules being the β and α chains of LFA-1. Firstly, an analysis of 12–15 mAb immunoprecipitates on 4–12% gels under reducing and nonreducing conditions shows that the 95- and 180-kDa molecules have a molecular weight and migration pattern identical to LFA-1. Secondly, depletion of LFA-1 from lysates with LFA-1 mAb abolished the ability of CD2 mAb 12–15 to co-precipitate the 95- and 180-kDa molecules, thereby identifying these as the β and α chains of mouse LFA-1, respectively. These results provide evidence for the first time for an association of LFA-1 and CD2 on mouse T lymphocytes, and suggest that the association occurs with an immunologically distinct subspecies of CD2 molecules.     

Characterization of two new CD18 alleles causing severe leukocyte adhesion deficiency

Leukocyte adhesion deficiency (LAD) is an autosomal recessive disease caused by heterogeneous mutations within the gene encoding the common β subunit (CD18) of the three leukocyte integrins LFA-1 (CD11a/CD18), Mac-1 (CD11b/CD18), and pl50,95 (CD11c/CD18). Based on the level of expression of CD18 on patient leukocytes, two phenotypes of LAD have been defined (severe and moderate) which correlate with the severity of the disease. We have investigated the molecular basis of the disease in two unrelated severe patients (HS and ZJO). Both patients share a complete absence of CD18 protein precursor and cell surface expression, but they differ in the level of CD18 mRNA, which is normal in HS and undetectable by Northern blot in ZJO. Determination of the primary structure of the patient HS CD18 mRNA revealed a 10-base pair deletion between nucleotides 190-200 (CD 18 exon 3), which eliminates residues 41–43 and causes a frameshift into a premature termination codon 17 base pairs downstream from the deleted region. The 10-base pair frameshift deletion maps to a region of the CD18 gene where aberrant mRNA processing has been detected in HS and two other unrelated LAD patients. In the ZJO patient, amplification of lymphoblast CD 18 mRNA demonstrated the presence of a non-sense mutation in the third nucleotide of the triplet encoding Cys⁵³⁴ (TGC → TGA), within exon 12. Both genetic abnormalities were also detected at the genomic level, and affect the restriction pattern of their corresponding genes, thus enabling the detection of the mutant alleles among healthy heterozygous alleles in family studies. The identification of two new LAD CD18 alleles, either carrying a non-sense mutation (ZJO) or a partial gene deletion (HS), further illustrates the heterogeneity of the genetic alterations in LAD.     

Increased CD11/CD18 expression on peripheral blood leucocytes of patients with sarcoidosis.

Sarcoidosis is a multisystem disease of unknown etiology characterized by non-caseating granulomata, formed mainly from macrophages surrounded by lymphocytes and plasma cells. Using a novel method for the preparation of blood leucocytes for flow cytometry, we report increased expression of LeuCAMs (CD11/CD18) on peripheral blood leucocytes of 11 Caucasian and 10 Afro-Caribbean patients with sarcoidosis compared with age-, sex- and race-matched controls. Whilst the percentages of the cells expressing CD11/CD18 were no different, the density, expressed as mean fluorescence intensity (MFI), was greater for all leucocytes in sarcoids than in normal individuals. The expression of intercellular adhesion molecule-1 (ICAM-1), a ligand for LFA-1 which is expressed on all leucocytes, was not significantly different from normal, whereas HLA-DR was expressed more intensely on sarcoid monocytes (P less than 0.01) and blood lymphocytes (P less than 0.005) than control cells. Our findings are consistent with leucocyte activation although we were unable to confirm reports of elevated tumour necrosis factor-alpha (TNF-alpha) in the patients' plasma using an ELISA. Increased expression of adhesion molecules on peripheral blood leucocytes may play a role in the cellular extravasation, aggregation, and granuloma formation seen in sarcoidosis.     

A CD44 monoclonal antibody differentially regulates CD11a/CD18 binding to intercellular adhesion molecules CD54, CD102 and CD50

We have made a monoclonal anti-CD44 antibody which is able to activate the leukocyte integrin CD11a/CD18. Activated T cells strongly aggregated, and the aggregation was shown to be intercellular adhesion molecule (ICAM)-1 (CD54) and ICAM-2 (CD102) dependent. Using purified ICAM coated on plastic, only binding to ICAM-1 was increased by the CD44 antibody, whereas activation by phorbol ester increased binding to both ICAM-1 and ICAM-3. The binding to ICAM-2 was not affected by either treatment. These findings show that the CD11a/CD18 integrin can be activated in a ligand-specific manner by engagement of CD44.     

Altered expression of CD11/CD18 on the peripheral blood phagocytes of patients with tuberculosis.

Tuberculosis (TB), caused by Mycobacterium tuberculosis, is characterized by granulomatous lesions made up of epithelioid cells, giant cells and mononuclear leucocytes. Cell-cell adhesion is important in granuloma formation and in the leucocyte migration which accompanies it. We have recently shown increased expression of the adhesion molecules CD11/CD18 (LeuCAMs, beta 2 integrins) on peripheral blood leucocytes from patients with sarcoidosis (Shakoor & Hamblin, 1992). Here we have studied the expression of CD11/CD18 and CD29 (VLA beta 1 integrin) on the peripheral blood leucocytes of 10 TB patients by flow cytometry. The density (expressed as mean fluorescence intensity) of CD11b on monocytes and polymorphs was increased (P < 0.005), as was CD11c (P < 0.005) and CD18 (P < 0.05) on polymorphs. CD11a expression was significantly reduced on polymorphs (P < 0.05). No differences were found in the expression of CD29, the percentages of cells expressing any molecule and, in contrast to sarcoidosis, the density of any molecule on lymphocytes. Although the cytokine tumour necrosis factor (TNF) has been implicated in the process of up-regulation, an ELISA for TNF failed to detect significant levels in plasma. The results suggest increased peripheral phagocyte CD11/CD18 expression is a feature of TB, which may contribute to the pathological processes involved.     

Anti-CD11b/CD18 antibodies reduce inflammation in acute colitis in rats.

The influx of monocytes and neutrophils into the inflamed tissue could be an important aspect in the pathogenesis of inflammatory bowel disease (IBD). A membrane protein involved in the monocyte/neutrophil adherence to endothelium is CD11b/CD18 or alpha M beta 2 (complement receptor type 3 = CR3). In the present study the role of CD11b/CD18 in experimental IBD was studied by treatment with ED7 and OX42, two MoAbs against CD11b/CD18. Colitis was induced in rats by a single, rectal administration of 30 mg 2,4,6-trinitrobenzene sulfonic acid (TNBS) dissolved in ethanol 30%. Two hours before and 3 days after induction of colitis, the animals were given an i.v. dose of 0.5 mg of either ED7 or OX42 in 1 ml PBS. Controls received PBS or an irrelevant MoAb. Four days after the last treatment with the antibodies, the rats were killed, and macroscopic damage scores of the colon were determined. Macrophages and granulocytes were studied by immunohistochemistry and quantified by Interaktives Bild Analysen System (IBAS), and myeloperoxidase (MPO) activity in colonic tissue was measured. After treatment with ED7 and OX42 the mean damage score of the colon was reduced from 4.2 in IBD animals to 1.0 and 1.3, respectively. Smaller areas of ulcerations and a decrease in the number of ulcerations were observed compared with PBS-treated rats. Furthermore, the amount of infiltrating monocytes and leucocytes in the submucosa was enormously reduced, as well as MPO activity in the colonic tissue. These results show that treatment with MoAbs against CD11b/CD18 reduces clinical signs of experimental IBD in rats by a partial blockade of infiltrating macrophages and granulocytes.     

Modulation of surface CD11/CD18 glycoproteins (Mo1, LFA-1, p150,95) by human mononuclear phagocytes

Mo1, LFA-1, and p150,95 are structurally related glycoproteins of the CD11/CD18 complex that are expressed on the membrane of human leukocytes. In the neutrophil, the surface expression of the CD11/CD18 complex is up-modulated (Mo1 greater than p150,95 much greater than LFA-1) by stimulatory factors that include calcium ionophore A23187, phorbol myristate acetate (PMA), and N-L-formyl-L-leucyl-L-phenylalanine (fMLP). Here, in an immunofluorescence analysis, we have examined CD11/CD18 glycoprotein expression by human monocytes, pulmonary alveolar macrophages (PAM, obtained by bronchoalveolar lavage), and breast milk macrophages (BMM) as compared to neutrophils before and after exposure to A23187 (1 microM), fMLP (0.1 microM), or PMA (0.1 microgram/ml) for 15 min at 37 degrees C. Unstimulated monocytes within unfractionated blood mononuclear cells kept at 4 degrees C (n = 13) expressed all three CD11/CD18 glycoproteins, and exposure to A23187 resulted in significant increases in the surface expression of Mo1 (median of 5.7-fold), LFA-1 (median of 2.1-fold), and p150,95 (median of 7.2-fold). Exposure to fMLP- or PMA-stimulated increases of lesser magnitude. CD11/CD18 expression by PAM (n = 9) was barely detectable and was unaffected by exposure to A23187. In contrast, BMM (n = 11) expressed all three CD11/CD18 glycoproteins (with considerable variability among specimens), but no increase was stimulated by A23187. These results demonstrate that monocytes, like neutrophils, have the capacity to respond to activating factors with an increase in CD11/CD18 glycoprotein expression; macrophage differentiation is accompanied by a loss (PAM) or retention (BMM) of CD11/CD18 expression that is unmodulated in response to activation.     

缺血预适应阻抑犬冠状窦血中性粒细胞CD11b/CD18的表达

本文旨在探讨缺血预适应（ＩＰ）心肌保护效应是否与其防止中性粒细胞（ＰＭＮ）ＣＤ１１ｂ／ＣＤ１８分子的表达有关。采用犬衣降支动脉（ＬＡＤ）建立ＩＰ模型,在长缺血前各５ｍｉｎ血和再灌注,反复４次,于基础、缺血前、缺血１ｈ、再灌注０．５和２ｈ分别采集冠状窦血作式细胞仪分析ＰＭＮＣＶＤ１１ＣＤ１８的表达,并一单纯ＩＲ组比较。ＩＲ组在心肌缺因与再灌注各时点ＰＭＮＣＤ１１ｂ／ＣＤ１８表达均显著增加,而ＩＰ组除     

Expression of the H52 epitope on the β2 subunit is dependent on its interaction with the α subunits of the leukocyte integrins LFA-1, Mac-1 and p150,95 and the presence of Ca2+

Integrin-mediated adhesion is a divalent cation-dependent process. Whether divalent cations directly participate in ligand binding or exert their effects indirectly by affecting the overall structure of the integrin heterodimers is not known. In this study we describe the epi tope of the mAb H52 which has been mapped to a predicted disulfide-bonded loop (C386 and C400) in the β2 integrin subunit. In the presence of Ca²⁺ and Mg²⁺ , the H52 epitope is expressed on the monomeric β2 subunit, the LFA-1 and Mac-1 heterodimers but not on p150,95, thus implying that this epitope is masked in p150,95. However, expression of the H52 epitope on Mac-1, but not on LFA-1, or the monomeric β2 subunit, is dependent on the presence of Ca²⁺ , thus suggesting that the chelation of Ca²⁺ causes a conformational change in Mac-1 which results in the loss of the epitope. These results suggest that expression of the H52 epitope on the β2 subunit is dependent on its interaction with the different α sub units. Since the epitope itself is not required for heterodimer formation nor for ligand binding, occupancy of a Ca²⁺ binding site(s) must therefore affect the α β subunit interactions, and thus the overall conformation of Mac-1.     

The role of alpha(4) and LFA-1 integrins in selectin-independent monocyte and neutrophil migration to joints of rats with adjuvant arthritis

Monocytes and neutrophils are chronically recruited to joints in rheumatoid arthritis. In the joints of rats with adjuvant arthritis, this is mediated, in part, by selectin-dependent and selectin-independent mechanisms. To define the selectin-independent mechanisms, (51)Cr-labeled blood monocytes, (111)In-labeled neutrophils and function blocking mAb to the selectins and integrins were utilized. Integrins contributed to the selectin-independent monocyte migration to arthritic joints with 58-70% inhibition of this recruitment by anti-alpha(4) or anti-LFA-1 mAb, relative to selectin blockade alone. alpha(4) plus P-selectin blockade was as effective as combined blockade of alpha(4), P-, E- and L-selectin, mediating approximately 83% of the overall monocyte migration to the joints. In contrast, LFA-1 was the predominant selectin-independent mechanism for neutrophil recruitment to the joints. LFA-1 together with P-selectin had essential roles in the talar joint. In dermal inflammation in the arthritic rats, LFA-1 accounted for most (69%) of the selectin-independent monocyte migration to the chemoattractant C5a(desArg) (zymosan-activated serum), whereas LFA-1 and Mac-1 both contributed to selectin-independent neutrophil recruitment to C5a(desArg). alpha(4) integrin and P-selectin in concert mediated monocyte recruitment to lipopolysaccharide and IFN-gamma lesions (81%). Thus: (1) either alpha(4) or LFA-1 can mediate monocyte migration to arthritic joints in the absence of selectin function and alpha(4) together with P-selectin is particularly important; (2) LFA-1 is the predominant mechanism of selectin-independent migration of neutrophils to inflamed joints; and (3) in arthritic rats, selectin-independent migration of monocytes and neutrophils to dermal inflammation is mediated by alpha(4) or LFA-1 or both LFA-1 and Mac-1, depending on the leukocyte type, and inflammatory stimulus.     

Increased adhesion to vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 of eosinophils from patients with asthma

Adhesion of peripheral blood eosinophil and neutrophil granulocytes to the endothelial cell adherence receptors E-selectin, vascular cell adhesion molecule-1, and intercellular adhesion molecule-1 has been measured. The study included patients with allergic rhinitis, patients with mild allergic and nonallergic asthma, and healthy individuals; 10 persons were in each group. In addition, assay of eosinophil and neutrophil cell surface expression of the receptor complex CD11b/CD18 was performed. Increased eosinophil adhesion to vascular cell adhesion molecule-1 (p < 0.05) and intercellular adhesion molecule-1 (p < 0.05) was demonstrated in the patients with a more labile asthma, that is, a peak expiratory flow rate variability of more than 10%, suggesting a relationship to the degree of ongoing inflammation in the airways of the patients. The increased eosinophil adhesion was most probably due to a functional upregulation of the CD11b/CD18 and very late activation antigen-4 receptors, because the number of receptors measured as cell surface expression was unaltered. The increased eosinophil adhesion in the patients with high peak expiratory flow rate variability appeared independent of atopy. The increased adhesion was not entirely specific to the eosinophils, because neutrophils from patients with a peak expiratory flow rate variability of more than 10% also demonstrated increased adhesion to intercellular adhesion molecule-1 (p < 0.05) when compared with neutrophils from the patients with low peak expiratory flow rate variability. In conclusion, the demonstrated priming of eosinophil adhesion to vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 might be one contributing mechanism behind the selective accumulation of eosinophils in the lung tissue of patients with asthma. (J ALLERGY CLIN IMMUNOL 1995;96:941-50.)     

Increased expression of adhesion molecules (ICAM-1 and LFA-1) on alveolar macrophages from asthmatic patients

In the airways inflammation observed in asthma, activated macrophages are present in increased numbers. Adhesion molecules are required for the cell: cell contacts between leukocytes and endothelial cells or other leukocytes, and they are induced by inflammatory stimuli. We studied the expression of two adhesion molecules (ICAM-1 and LFA-1) on alveolar macrophages recovered by bronchoalveolar lavage from 11 normal subjects and 13 asthmatic patients by using immunocytochemistry. Two specific monoclonal antibodies were used, and the reaction was revealed by the alkaline phosphatase-antialkaline phosphatase (APAAP) method. The percentage of cells expressing ICAM-1 or LFA-1 was significantly increased in asthmatic patients, as compared with normal subjects ( P < 0.001 and P < 0.002, respectively; Mann-Whitney U test), and there was a significant correlation with the percentage of cells expressing both markers in asthma ( P < 0.03, Spearman rank test). This study highlights the importance of macrophages in the inflammation of asthma and suggests that macrophage interactions with other cells play a role in this inflammation.     

Granulocyte and monocyte CD11b expression during plasma separation is dependent on complement factor 5 (C5) – an ex vivo study with blood from a C5‐deficient individual

The aim of the study was to investigate the role of complement factor 5 (C5) in reactions elicited by plasma separation using blood from a C5‐deficient (C5D) individual, comparing it to C5‐deficient blood reconstituted with C5 (C5DR) and blood from healthy donors. Blood was circulated through an ex vivo plasma separation model. Leukocyte CD11b expression and leukocyte–platelet conjugates were measured by flow cytometry during a 30‐min period. Other markers were assessed during a 240‐min period. Granulocyte and monocyte CD11b expression did not increase in C5D blood during plasma separation. In C5DR samples granulocytes CD11b expression, measured by mean fluorescence intensity (MFI), increased from 10481 ± 6022 (SD) to 62703 ± 4936, and monocytes CD11b expression changed from 13837 ± 7047 to 40063 ± 713. Granulocyte–platelet conjugates showed a 2.5‐fold increase in the C5DR sample compared to the C5D sample. Monocyte–platelet conjugates increased independently of C5. In the C5D samples, platelet count decreased from 210 × 10⁹/L (201–219) (median and range) to 51 × 10⁹/L (50–51), and C3bc increased from 14 CAU/mL (21–7) to 198 CAU/mL (127–269), whereas TCC formation was blocked during plasma separation. In conclusion, up‐regulation of granulocyte and monocyte CD11b during plasma separation was C5‐dependent. The results also indicate C5 dependency in granulocyte–platelet conjugates formation.     

Differential function of LFA-1 family molecules (CD11 and CD18) in adhesion of human monocytes to melanoma and endothelial cells.

Human peripheral blood monocytes from normal, healthy donors express the leucocyte function-associated antigen (LFA)-1, CR3 and p150,95. These heterodimeric antigens are members of a glycoprotein family sharing a common beta subunit but endowed with distinct alpha chains. They have been shown to play an important role in cell-cell interactions. In the present study we have investigated the role of these molecules in the interaction of monocytes with endothelial cells and melanoma (tumour) cells. Heterotypic cell-cell interactions were studied in single cell conjugate assays and by adhesion of monocytes to monolayers of cells. The results demonstrate that monoclonal antibodies directed against LFA-1 alpha, CR3 alpha, p150,95 alpha and the common beta chain strongly reduce the number of conjugates (71, 50, 60 and 89% inhibition, respectively), formed between monocytes and melanoma or endothelial cells in a single cell assay. In contrast, adhesion of monocytes to monolayers of the same cells seems only to depend on p150,95, since only antibodies directed to the alpha chain of this molecule and to the common beta chain inhibited adhesion. Interestingly, the number of conjugates formed with melanoma cells in single cell assays was at least twice the number of conjugates formed between monocytes and endothelial cells, whereas no differences were observed in the adhesion of monocytes to monolayers of these cells. However, the basis for this phenomenon is not yet clear. These results indicate that not only LFA-1 but also CR3 and p150,95 can mediate adhesion to target cells in suspension, but that monocyte adhesion to monolayers is caused by a different mechanism in which the p150,95 molecule seems to play a prominent role.     

Leishmania major-human macrophage interactions: cooperation between Mac-1 (CD11b/CD18) and complement receptor type 1 (CD35) in promastigote adhesion.

It has been suggested that the developmental maturation of Leishmania major promastigotes can affect their interaction with human complement receptors. To study this, we measured the adhesion of metacyclic and logarithmic-phase L. major promastigotes to complement receptors expressed on primary macrophages, to recombinant receptors expressed on transfected cells, or to purified complement receptors in a cell-free system. We demonstrate that complement-opsonized promastigotes can bind to both Mac-1 and complement receptor type 1 (CR1) and that the transition of promastigotes from the noninfectious logarithmic phase of growth to the infectious metacyclic stage does not affect this interaction. Furthermore, we show that Mac-1 and CR1 can cooperate to mediate the efficient adhesion of complement-opsonized metacyclic promastigotes to cells expressing both receptors. On human monocyte-derived macrophages, Mac-1 appears to make a quantitatively greater contribution to this adhesion than does CR1, since blocking macrophage Mac-1 diminishes metacyclic promastigote adhesion to a greater extent than does blocking CR1. In addition, bovine monocytes lacking Mac-1 exhibit a dramatic decrease in complement-dependent promastigote adhesion, relative to normal monocytes. The predominance of Mac-1 in these interactions is due, at least in part, to the factor I cofactor activity of CR1, which facilitates the conversion of C3b to iC3b. The stable adhesion of complement-opsonized metacyclic promastigotes to Mac-1 is a prerequisite for phagocytosis by human monocyte-derived macrophages. Blocking Mac-1 on macrophages abrogates the majority of the complement-dependent phagocytosis of promastigotes, whereas blocking CR1 has no detectable effect on phagocytosis. In addition, bovine monocytes lacking Mac-1 exhibit a dramatic reduction in promastigote phagocytosis relative to normal bovine monocytes. We conclude, therefore, that the two complement receptors, Mac-1 and CR1, can cooperate to mediate the initial complement-dependent adhesion of metacyclic promastigotes to human monocyte-derived macrophages and that Mac-1 is the predominant complement receptor responsible for the phagocytosis of complement-opsonized metacyclic promastigotes.