Sialoadhesin is a macrophage‐restricted adhesion molecule of 185 kDa that mediates sialic acid‐dependent binding to cells. It is expressed strongly by macrophages in lymphoid and haemopoietic tissues where it is likely to mediate cell‐cell interactions. Here we report the molecular cloning of murine sialoadhesin and show that it is a new member of the immunoglobulin (Ig) superfamily with 17 Ig‐like domains. COS cells transfected with a cDNA encoding full‐length sialoadhesin bound mouse bone marrow cells in a sialic acid‐dependent manner. Alternatively spliced cDNAs, predicting soluble forms of sialoadhesin containing the first three or 16 Ig‐like domains of sialoadhesin, were expressed in COS cells and the respective proteins purified. When immobilized on plastic, the 16‐domain form bound cells in a sialic acid‐dependent manner, suggesting that sialoadhesin can function in both secreted and membrane‐bound forms. The most similar proteins in the database were CD22, myelin‐associated glycoprotein, Schwann cell myelin protein and CD33. Like sialoadhesin, CD22 mediates sialic acid‐dependent cell adhesion. The sequence similarity of sialoadhesin to CD22 and related members of the Ig superfamily indicates the existence of a novel family of sialic acid binding proteins involved in cell‐cell interactions.

Resident macrophages (MO)' are widely distributed throughout tissues of adult animals, even in the absence of overt inflammation or infection. At portals of entry they may play a role in innate resistance to infection, but in other sites, particularly hematopoietic and lymphoid tissues where stromal MO are relatively abundant (1-3), their precise functions are less clear. It is possible, however, that in addition to scavenging worn-out and damaged cells and ingesting erythrocyte nuclei, stromal Mo in these tissues may play a regulatory role in cellular proliferation and differentiation. Support for this possibility has come from studies on bone marrow and thymus (2-5). In both cases, stromal M(k establish intimate physical associations in situ with immature hematopoietic cells or thymocytes (2, 3). After enzymatic dispersion of the tissues, it is possible to obtain clusters that contain Mo and attached cells (4, 5). To date, there have been few attempts to determine the functional importance of cluster formation in these tissues or to characterize the receptors and ligands involved . To carry out such studies, it is necessary to isolate the relevant M0, rather than to use a more easily obtained population such as peritonea[ MO, which may differ substantially in function and properties (5) .We recently described (5) a method for isolating resident MO from murine bone marrow (RBMM), in reasonable yield and purity, which allowed us to study the properties of RBMM (5). When their phenotype was compared with that of resident peritoneal MO (RPM), we found both quantitative and qualitative differences in expression of various antigens and receptors. The most striking difference was the ability of RBMM, but not RPM, to bind unopsonized sheep E without ingestion. In view of the possibility that this novel E receptor (SER) could be involved in Mo-binding to hematopoietic cells in murine bone marrow, it seemed important to characterize it further. In the present paper we show that SER on RBMM is a lectin-like agglutinin with specificity for sialylated component(s) on E. Our studies show that SER is also expressed on stromal Mo isolated

CD33 is a member of the Ig superfamily that is restricted to cells of the myelomonocytic lineage but whose functions and binding properties are unknown. It shares sequence similarity with sialoadhesin, CD22, and the myelin-associated glycoprotein, which constitute the Sialoadhesin family of sialic acid-dependent cell adhesion molecules. In the present study, we show that CD33 is a fourth member of this family. As a model for sialic acid-dependent binding, human erythrocytes were derivatized with N-acetylneuraminic acid (NeuAc) in different linkages. A recombinant soluble form of CD33, Fc-CD33, bound red blood cells with a specificity similar to that of sialoadhesin, preferring NeuAc alpha 2,3Gal in N- and O-glycans over NeuAc alpha 2,6Gal in N-glycans. Fc-CD33 also bound selectively to the myeloid cell lines HL-60 and U937. However, CD33 was unable to mediate cell binding after transient expression in COS cells, despite high levels of surface expression. Pretreatment of the CD33-transfected cells with sialidase rendered them capable of mediating sialic acid-dependent binding. These results show that CD33 can function as a sialic acid-dependent cell adhesion molecule and that binding can be modulated by endogenous sialoglycoconjugates when CD33 is expressed in a plasma membrane.

The CD4 antigen is a plasma membrane glycoprotein of^-55 kD that is expressed on most thymocytes and on Th cells in all mammalian species examined, including human, rat, mouse, sheep, and pig (1) . The antigen is also present on rat macrophages (MO) and human monocytes in a similar molecular form (2, 3) . Studies with mAbs (4) have demonstrated that CD4 on Th cells is an important accessory molecule involved in recognition of antigen plus MHC class II molecules during immune responses . In humans, a more recently defined property of the CD4 antigen is to act as the receptor for the AIDS virus (human immunodeficiency virus or HIV) (5) . Although nothing is known concerning the the regulation and functional significance of CD4 expression on MO, its presence is likely to be at least one reason why these cells become infected by HIV (6, 7) . Here we demonstrate that, unlike human and rat M0, mouse Mo do not express the CD4 (L3T4) antigen . This species disparity indicates, firstly, that CD4 may not be essential for Mo function, and secondly, that MO regulate CD4 expression differently from Th cells . Volume 166 August 1987 613-618 Materials and Methods BriefDefinitive ReportAnimals. Specific pathogen-free C57BL/6 mice and AO rats were bred at the Sir William Dunn School of Pathology and both sexes were used between 6 and 12 wk of age .Antibodies. The following rat mAbs to mouse CD4 (L3T4) were obtained as shown and used as tissue culture supernatants at saturation : H129 .19 (IgG2a) (8), RL 172 .4 (IgM) (9), and GK1 .5 (IgG2b) (10), provided by Dr . H . R . MacDonald, Ludwig Institute for Cancer Research, Lausanne, Switzerland; and YTS 191 .1 (IgG2b) and YTA .3 (IgG2b) (11), provided by Dr . S . P . Cobbold, Department of Pathology, Cambridge University, Cambridge, United Kingdom . The noncompeting mouse anti-rat CD4 (W3/25) mAbs, W3/25 (IgGI) and MRC OX-35 (IgG2a) (2), were from the MRC Cellular Immunology Research Unit, University of Oxford, Oxford, United Kingdom . Other mAbs used were MRC OX-21 (IgGI), a mouse mAb to human C3b inactivator (2), and F4/80 (IgG2b), a rat mAb specific for mature mouse Mo (12) .Labeling of Cells with Antibodies . Mouse and rat resident peritoneal MO were purified by adherence for 1 h to bacteriologic petri dishes followed by extensive washing . The adherent cells (>90% Mo by morphology) were detached by a 15-min incubation in calcium-and magnesium-free PBS at 37°C followed by gentle pipetting . Thymocytes