Binding of plasma fibronectin to the surfaces of BHK cells in suspension at 4 degrees C

Thermoresponsive polymers and their biomedical application in tissue engineering - a review

Thermoresponsive polymers hold great potential in the biomedical field, since they enable the fabrication of cell sheets, in situ drug delivery and 3D-printing under physiological conditions. In this review we provide an overview of several thermoresponsive polymers and their application, with focus on poly(N-isopropylacrylamide)-surfaces for cell sheet engineering. Basic knowledge of important processes like protein adsorption on surfaces and cell adhesion is provided. For different thermoresponsive polymers, namely PNIPAm, Pluronics, elastin-like polypeptides (ELP) and poly(N-vinylcaprolactam) (PNVCL), synthesis and basic chemical and physical properties have been described and the mechanism of their thermoresponsive behavior highlighted. Fabrication methods of thermoresponsive surfaces have been discussed, focusing on PNIPAm, and describing several methods in detail. The latter part of this review is dedicated to the application of the thermoresponsive polymers and with regard to cell sheet engineering, the process of temperature-dependent cell sheet detachment is explained. We provide insight into several applications of PNIPAm surfaces in cell sheet engineering. For Pluronics, ELP and PNVCL we show their application in the field of drug delivery and tissue engineering. We conclude, that research of thermoresponsive polymers has made big progress in recent years, especially for PNIPAm since the 1990s. However, manifold research possibilities, e.g. in surface fabrication and 3D-printing and further translational applications are conceivable in near future.

Human Amniotic Cell Sheet Harvest Using a Novel Temperature-Responsive Culture Surface Coated with Protein-Based Polymer

Human amniotic epithelial (hAE) and mesenchymal (hAM) cells are believed to have the potential to differentiate into various functional cells, such as neurons, hepatocytes, cardiomyocytes, and pancreatic beta cells. However, cell transplantation has been performed by injection of cell suspensions, and thus it is difficult to control shape, size, location, and functions of differentiated cells. To overcome these problems, we developed a novel temperature-responsive culture surface coated with elastic protein-based polymer. By reducing the temperature using a polyvinylidene difluoride (PVDF) membrane, the primary hAE and hAM cell sheet can detach from the coated surface. The recovered cell sheet can be transferred and can re-adhere and re-proliferate on another surface. This represents the first report of harvesting of primary hAE and hAM cell sheets using the novel temperature- responsive polymer. These findings suggest that this new technique of cell sheet detachment from noncytotoxic, highly biocompatible protein-based polymer-coated surfaces may be useful in tissue engineering, as well as in the investigation of hAE and hAM cell sheets for transplantation.

Effect of magnetic field on the fibronectin adsorption, cell attachment and proliferation on titanium surface

OBJECTIVES

We studied the effect of various static magnetic fields (SMFs) on the adsorption of specific recombinant fibronectin (FN) peptide (hFNIII9-10) on the titanium surface. Furthermore, the responses of human osteosarcoma TE-85 cells in the SMF were observed.

MATERIAL AND METHODS

Various magnetic fields--1, 2, 3, 5, 7, 10 mT--were established by controlling the distance from Nd-Fe-B magnet to the disks. For FN adsorption experiment, machined titanium disks were incubated in 1 microM hFNIII9-10 at 37 degrees C overnight under magnetic field. The adsorbed hFNIII9-10 was measured as optical density (OD). For attachment study, TE-85 cells were incubated for 2 h on the hFNIII9-10 coated machined titanium disks and OD values were measured. As for proliferation study, titanium disks were incubated for 48 h after washing unattached cells in 2 h. The amount of proliferated TE-85 cell was also measured as OD value. Attachments of TE-85 cells under various intensities of magnetic field were observed using a scanning electron microscope.

RESULTS

The amount of adsorbed hFNIII9-10 showed no significant difference between control (0 mT) and six experimental groups (1, 2, 3, 5, 7, 10 mT). However, TE-85 cells attached significantly higher in groups of 1, 2, 5, 10 mT than in control group (P=0). Cell attachment in groups of 3, 7 mT showed no significant difference with that of control group. TE-85 cells were observed to attach through filopodia. Especially in 1 mT, flattened cells were predominant. In proliferation assay, 1 mT stimulated TE-85 cells showed significantly higher proliferation than those in 2, 3 and 7 mT (P=0).

CONCLUSION

Magnetic fields under 10 mT did not influence FN adsorption on the titanium surface. However, a significant effect was found on cell attachment and proliferation.

Surface-dependent conformations of human plasma fibronectin adsorbed to silica, mica, and hydrophobic surfaces, studied with use of Atomic Force Microscopy

Human plasma fibronectin (Fn) is a large flexible protein stabilized by intermolecular ionic interactions forming a compact structure. On altering solution conditions, the structure can revert to a more expanded state, thereby exposing previously hidden domains (e.g., cell-binding sites). Electron microscopy images of Fn air-sprayed onto mica surfaces show elongated protein structures, indicating a surface-induced structural change. This makes it interesting to investigate the influence of surface properties on the structure of adsorbed Fn. We have used intermittent-contact Atomic Force Microscopy to investigate the structure of Fn adsorbed onto mica, silica, and methylated silica surfaces. We observed that on silica surfaces, which is hydrophilic, most (70%) of the molecules had an elongated structure with partial intramolecular chain interactions, compare to molecules adsorbed on hydrophobic, methylated surfaces, where a compact structure predominated (70%). On mica surfaces, both compact and elongated protein structures were observed, with a slight preference for the elongated form (53%). Results show that surface physical properties influence the molecular structure of fibronectin on adsorption, which could provide useful information in understanding surface-induced in vivo responses.

A 49-residue peptide from adhesin F1 of Streptococcus pyogenes inhibits fibronectin matrix assembly

F1 is an adhesin of Streptococcus pyogenes which binds the N-terminal 70-kDa region of fibronectin with high affinity. The fibronectin binding region of F1 is comprised of a 43-residue upstream domain and a repeat domain comprised of five tandem 37-residue sequences. We investigated the effects of these domains on the assembly of fibronectin matrix by human dermal fibroblasts, MG63 osteosarcoma cells, or fibroblasts derived from fibronectin-null stem cells. Subequimolar or equimolar concentrations of recombinant proteins containing both the upstream and repeat domains or just the repeat domain enhanced binding of fibronectin or its N-terminal 70-kDa fragment to cell layers; higher concentrations of these recombinant proteins inhibited binding. The enhanced binding did not result in greater matrix assembly and was caused by increased ligand binding to substratum. In contrast, recombinant or synthetic protein containing the 43 residues of the upstream domain and the first 6 residues from the repeat domain exhibited monophasic inhibition with an IC(50) of approximately 10 nm. Truncation of the 49-residue sequence at its N or C terminus caused loss of inhibitory activity. The 49-residue upstream sequence blocked incorporation of both endogenous cellular fibronectin and exogenous plasma fibronectin into extracellular matrix and inhibited binding of 70-kDa fragment to fibronectin-null cells in a fibronectin-free system. Inhibition of matrix assembly by the 49-mer had no effect on cell adhesion to substratum, cell growth, formation of focal contacts, or formation of stress fibers. These results indicate that the 49-residue upstream sequence of F1 binds in an inhibitory mode to N-terminal parts of exogenous and endogenous fibronectin which are critical for fibronectin fibrillogenesis.

Decrease in tumor-cell attachment and in a 140-kDa fibronectin receptor correlate with greater expression of multiple 34-kDa surface proteins and cytoplasmic 54-kDa components

B16 melanoma cells attach to matrix-bound fibronectin but fail to adhere to albumin-coated surfaces supplemented with soluble fibronectin. Attachment to substratum is also decreased in the presence of an adhesion-disrupting antibody, or when cells are seeded on substrates poorly adhesive for these cells, such as collagen gels. We have now investigated some of the more general adhesion-related alterations that occur between flattened and poorly attached cells. Immune blots of octylglucoside extracts with the adhesion-disrupting IgG revealed a 140-kDa component in flattened cells, in contrast to the increased detection of a 54-kDa species in a comparable assay with rounded cells. Surface iodination also showed a decreased external exposure of a 140-kDa fibronectin binding species and an increased labelling in multiple 34-kDa protein species, in cells with decreased attachment to substratum. Analysis of 35S-methionine-labelled cell aggregates cultured on collagen gels also revealed a decrease in the 140-kDa region and a greater labelling of multiple 54-kDa components, compared to the same cells flattened on fibronectin. A change in 54- and 34-kDa species was also seen in matrix-associated components of rounded cells that failed to attach with soluble fibronectin. Since the 34-kDa species increase in poorly adherent cells is mainly detected by iodination, and the 54-kDa species increase in the same cells is partly associated with the corresponding detergent-insoluble matrices, we propose that these 2 novel proteins may relate to cell rounding, through a transmembrane modulation involving both surface membrane and cytoskeletal structures.

Evidence for involvement of more than one class of glycoprotein in cell interactions with fibronectin

The receptor mechanism by which cells attach to fibronectin has been investigated by a combined immunologic and electrophoretic approach. One particular antiserum directed against 3T3 cell plasma membranes was found to contain antibodies that blocked spreading of these murine cells on fibronectin but not on laminin or serum spreading factor (vitronectin). Proteolysis experiments confirmed that this cell line has calcium-protected polypeptides necessary for cell spreading on fibronectin. Consequently, protein antigens were fractionated according to size by SDS gel electrophoresis, and antigens that could block the inhibitory activity of the polyclonal antibody were identified. One class of blocking antigen appeared to correspond to the 140,000-dalton complexes favored by several laboratories as fibronectin receptor candidates, but a second class of 45,000 daltons was also apparent. This 45,000-dalton antigen was a major absorbing activity from 3T3 cell membranes and the predominant activity from L929 membranes. By isoelectric focusing and two-dimensional gel electrophoresis, it was found to exist as a set of isoelectric point variants with pK = 5.3 to 6.2. Our results indicate that current models postulating a simple, unimolecular receptor mechanism for fibronectin may be oversimplified and that fibronectin may instead interact with more than one protein receptor component on the fibroblast cell surface.

Characterization of a 140-kD avian cell surface antigen as a fibronectin-binding molecule

A 140,000-D protein cell surface antigen (140k) complex has been implicated in fibronectin-mediated cell-substratum attachment. We have used three different experimental systems to evaluate the hypothesis that this 140k complex can function as a fibronectin receptor. A monoclonal antibody that binds to the 140k complex specifically inhibits the direct binding of 3H-labeled 75,000-D fibronectin cell-binding fragment (f75k) to chicken embryo fibroblasts in suspension. The 140k complex is retarded in its passage through an affinity column consisting of immobilized f75k, and this interaction is specifically inhibited by a synthetic peptide that contains the fibronectin cell-recognition signal sequence. Finally, exogenous purified 140k complex inhibits the attachment and spreading of chicken embryo fibroblasts on fibronectin-coated substrates. Thus, our results indicate that the 140k complex can bind directly to fibronectin and is likely to be a fibronectin receptor for chicken cells.

Characteristics of a BHK cell variant defective in the cell-substratum contact process

In this paper we document the phenotypic characteristics of a novel BHK cell adhesion variant designated FN-2. Unlike parental cells, FN-2 cells did not attach to fibronectin (pFN)-coated dishes, even after 4-hr incubations on dishes treated with 100 micrograms/ml of pFN. Mixing experiments with the variant and parental cells revealed that the parental cells attached normally in the presence of a ninefold excess of variant cells and the variant cells failed to attach in the presence of a ninefold excess of parental cells. Therefore, the defect in FN-2 cells could not be explained by secretion of a factor inhibiting attachment or lack of secretion of a factor required for attachment. Also, the inability of FN-2 cells to attach to pFN-coated dishes could not be explained by an absence of cell pFN receptors since the variant cells bound normal numbers of small (ca. 0.8 micron) pFN-coated latex beads, although they phagocytosed the beads poorly compared to parental cells. Also, the variant cells were not able to bind large (5.7 or 16.8 microns) pFN-coated beads. When tested on dishes coated with ligands that, unlike fibronectin, have a high affinity for cell surface receptors, e.g., lectins and anti-BHK antibodies, FN-2 cells were observed to attach at a rate similar to that of parental cells but spread much more slowly. The phenotypic characteristics of FN-2 cells suggest that they are deficient in what previously has been called the "cell contact" process in cell adhesion. It is proposed that the cell contact process is the initial formation by an individual cell of a sufficient number of cell-substratum bonds to resist the shear forces operationally used to define "attachment," and that more cell-substratum bonds are necessary for cell attachment to large substrata (dishes or large beads) than for attachment to small substrata (small beads). The molecular defect in FN-2 cells was studied by electroblotting analysis. A high molecular weight (ca. 370 kd) glycoprotein detected by blotting with anti-BHK antibodies and ConA that was present in parental cell membranes was reduced or absent in the variant cells.

The effect of monensin on cell aggregation of normal and dystrophic human skin fibroblasts

Measurements of aggregation kinetics using couette viscometry show that freshly trypsinized skin fibroblasts from patients with Duchenne muscular dystrophy have values of intercellular adhesiveness approx. 40% those of normal cells. If cells are allowed to recover from the effects of trypsinization (by incubation for 2 h at 37 degrees C in serum-containing medium) the intercellular adhesiveness of both cell types increases, and normal and Duchenne cells aggregate to the same extent. Exposure to the ionophore monensin during the recovery phase leads to suppression of recovery in both cell types, and this effect of the drug is greater in Duchenne fibroblasts. These results are discussed in relation to other data on the reported differential effects of trypsin and monensin on normal and Duchenne fibroblasts.

Two distinct mechanisms for the interaction of cells with fibronectin substrata

Fibronectin (Fn) was adsorbed onto neutral, sulfonated, imine-conjugated or gelatin coated polystyrene latex beads. In all cases, the Fn coated beads bound effectively to Chinese hamster ovary (CHO) cells in suspension. However, the binding of Fn coated neutral or positively charged imine conjugated bead was inhibited by low concentrations of heparin or heparan sulfate or by treatment of the cells with Flavobacterium heparanase. By contrast, binding of Fn coated sulfonated or gelatin beads was insensitive to inhibition by heparin and to heparanase treatment of cells. Adhesion of CHO cells to Fn coated tissue culture plastic was not sensitive to heparin, whereas adhesion of CHO cells to Fn-coated imine-conjugated plastic was sensitive to heparin. These observations imply that the functional status of Fn can be modulated by the nature of the surface to which the Fn is adsorbed. They further imply that, under some circumstances, the heparin/heparan sulfate binding domains of Fn can play a role in the attachment of Fn to the cell membrane via membrane proteoglycans. Under other circumstances, the interaction of Fn with the cell may primarily involve other receptors for Fn, presumably cell surface glycoproteins.

Formation of stress fibres and focal adhesion sites in monensin-exposed cultured human fibroblasts in response to exogenously added cellular fibronectin

Human fibroblasts, plated and cultured in serum-free conditions in the presence of monensin, a carboxylic ionophore, attach and attain a flattened morphology, although fibronectin deposition does not take place. They fail, however, to develop organized stress fibers, vinculin plaques, and focal adhesion sites. In the present study, we demonstrate that the actomyosin-vinculin system of the monensin-exposed cells can be restored by plating the cells on the culture substratum coated with cellular fibronectin. Direct overlay of the electrophoretically separated polypeptides of the cultured fibroblasts with iodinated fibronectin revealed several polypeptides which could represent putative cell-surface receptors for fibronectin.

Secondary structure of human plasma fibronectin: conformational change induced by calf alveolar heparan sulfates

The quantitative analysis of circular dichroic spectra of native human plasma fibronectin according to the method of Provencher and Glöckner [Provencher, S. W., & Glöckner, J. (1981) Biochemistry 20, 33-37] indicated the presence of beta-sheet (79%), beta-turn (21%), but no alpha-helix or random coil in the secondary structure. The calf alveolar heparan sulfates induced a change in the conformation of fibronectin: the magnitude of the change depended on the molecular properties of the particular heparan sulfate preparations.

Fibronectin, as well as other extracellular matrix proteins, mediate human keratinocyte adherence

During the reepithelialization of cutaneous wounds, the migratory epidermis transits over a provisional matrix of fibronectin and fibrin in the absence of laminin and type IV collagen as well as ultrastructurally identifiable basement membrane. Since significant quantities of fibronectin occur at these sites of reepithelialization, we surmised that fibronectin is a suitable substrate for keratinocyte adherence and therefore undertook the in vitro investigations reported here. Purified human plasma fibronectin precoated on bacteriologic microtiter wells was demonstrated to mediate human keratinocyte adherence when concentrations greater than 10 micrograms/ml fibronectin were used. Maximal keratinocyte adherence was obtained in wells precoated with 100 micrograms/ml fibronectin and when cells were incubated with substrate for 60 min or longer at 37 degrees C. Both primary and second-passaged human keratinocytes adhered as well or better to fibronectin than to types I and III collagen, laminin, or type IV collagen under both high- and low-Ca++ culture conditions. However maximal adherence to all substrates occurred when second-passaged keratinocytes were assayed in low-Ca++ medium. Under these latter culture conditions, keratinocyte phenotype resembles the phenotype of cells in the migrating epidermis. To determine specificity of these adherence reactions, antifibronectin antibodies were shown to block keratinocyte adherence to fibronectin but not to laminin substrates. Conversely, antilaminin antibodies blocked adherence to laminin but not fibronectin substrates. Thus, human keratinocytes demonstrate specific adherence to fibronectin in a time- and dose-dependent fashion and this adherence relies on de novo protein synthesis. We take these observations to support our hypothesis that the provisional fibronectin matrix observed beneath the migrating epithelium during tissue repair plays a functional role in the reepithelialization process.

Fluorescent gangliosides as probes for the retention and organization of fibronectin by ganglioside-deficient mouse cells

Ganglioside-deficient transformed mouse fibroblasts (NCTC 2071A cells), which grow in serum-free medium, synthesize fibronectin but do not retain it on the cell surface. When fluorescent derivatives of gangliosides, containing either rhodamine or Lucifer yellow CH attached to the sialic acid residues, were added to the culture medium, the cells incorporated the derivatives and their surfaces became highly fluorescent. When the cells were stained with anti-fibronectin antibodies and a fluorescent second antibody, fibrillar strands of fibronectin were observed to be attached to the cell surface, with partial coincidence of the patterns of direct ganglioside fluorescence and indirect fibronectin immunofluorescence at the cell surface. When the cells were exposed to bacterial neuraminidase during the time of ganglioside insertion, similar patterns of fluorescence were observed. Because the fluorescent gangliosides are resistant to the enzyme, these results suggest that neuraminidase-sensitive endogenous glycoconjugates were not involved in the ganglioside-mediated retention and organization of endogenous fibronectin. After cells were exposed to exogenous chicken fibronectin, most of the fibronectin was attached to the substratum and only a few fibrils were attached to the cells. When exogenous gangliosides were included in the incubation, there was a striking increase in cell-associated exogenous fibronectin, which was highly organized into a fibrillar network. Conversely, cells incubated for 18 h with exogenous unmodified gangliosides exhibited a highly organized network of endogenously derived fibronectin. Upon further incubation of the cells for 2 h with fluorescent gangliosides, there was considerable co-distribution of the fluorescent gangliosides with the fibronectin network as revealed by immunofluorescence. Our results support the concept that gangliosides can mediate the attachment of fibronectin to the cell surface and its organization into a fibrillar network.

The cell attachment determinant in fibronectin

Fibronectin possesses a domain that interacts with cell surfaces. The ability of fibronectin to promote cell attachment can be duplicated with a short amino acid sequence, glycyl-L-arginyl-glycyl-L-aspartyl-L-serine, taken from that domain. The tripeptide Arg-Gly-Asp appears to be irreplaceable for maintenance of the activity of this peptide, whereas the serine residue can be replaced with some, but apparently not all, possible residues. This recognition sequence, or a closely related sequence, is present in a number of proteins other than fibronectin that interact with cells. These proteins include collagens, fibrinogen, thrombin, a bacterial surface protein, and two viral proteins, as well as discoidin-I, a protein implicated in the aggregation of Dictyostelium discoideum. A similar sequence is also repeated in some, but not all, fibronectin molecules, making it possible that some fibronectin molecules have more than a single cell attachment site. Synthetic peptides constructed from sequences taken from several of these other proteins have also been shown to promote cell attachment. The tripeptide sequence may, therefore, constitute an ancient cellular recognition mechanism common to many proteins.

Cell attachment activity of fibronectin can be duplicated by small synthetic fragments of the molecule

The ability of fibronectin to bind cells can be accounted for by the tetrapeptide L-arginyl-glycyl-L-aspartyl-L-serine, a sequence which is part of the cell attachment domain of fibronectin and present in at least five other proteins. This tetrapeptide may constitute a cellular recognition determinant common to several proteins.

Substrate adhesion of rat hepatocytes: on the mechanism of attachment to fibronectin

We examined the mechanisms of cell attachment to fibronectin-coated substrates. Inhibition of cell attachment was obtained by species-specific antifibronectin antibodies, which presumably recognize a distinct antigenic structure in the protein located at, or in the immediate vicinity of, the cell-binding site. The inhibiting antibodies could be adsorbed on a column of Sepharose substituted with plasma fibronectin. The initial phase of cell attachment was also inhibited by addition of soluble fibronectin to the incubation medium in a reaction that exhibited specificity and concentration dependence. These data suggest that cell-binding sites are available in an active form on the surface of soluble fibronectin. However, the inhibitory effect of fibronectin was greatly enhanced by adding the protein together with heparin, heparan sulfate, collagen, or a fibronectin-binding collagen peptide (CB-7), which is consistent with an "activation" of fibronectin on binding to these matrix components. A similar activation of fibronectin was obtained by cleaving the protein with trypsin. We discuss these findings in relation to conformational rearrangements in the fibronectin molecule. Data is presented supporting a mechanism of cell attachment to fibronectin involving multiple weak interactions between cellular receptors and substrate molecules, although some steps in the attachment process appear to disobey the requirements for such a mechanism.

Fibronectin in cell adhesion and invasion

Fibronectin plays a major role in the adhesion of many cell types. The extent of cell adhesion in vitro is related not only to the ability of the cells to interact with matrix-bound fibronectin, when it is present, but also to the synthesis or lack of synthesis of fibronectin by the cells, and to the lack of deposition of synthesized fibronectin into an insoluble matrix surrounding the cells. Many malignant cells, regardless of whether they synthesize subnormal or normal amounts of fibronectin, fail to deposit that fibronectin into a surrounding insoluble matrix. The lack of fibronectin around such cells appears to reflect a general absence of extracellular matrix since other matrix components, such as collagen, laminin, and heparan sulfate proteoglycan, are concomitantly missing. Cells that lack their own cell surface fibronectin due either to lack of deposition or to lack of synthesis can nevertheless adhere to insoluble fibronectin matrices elaborated by other cells. These cellular characteristics appear to be associated with cell migration in vivo during embryogenesis, and the same characteristics may enhance the invasive potential of malignant cells. The remarkable effects that fibronectin has on cellular adhesion and the association of lack of extracellular matrix components with poorly differentiated and highly metastatic tumors in vivo mandates that more be learned about the molecular and cellular details of the interactions of cells with their surrounding matrix. Important information concerning tumor invasion will parallel such an understanding and may eventually become the basis for therapeutic approaches.

Fibronectin-mediated binding and phagocytosis of polystyrene latex beads by baby hamster kidney cells

The binding and phagocytosis of fibronectin (pFN)-coated latex beads by baby hamster kidney (BHK) cells was studied as a function of fibronectin concentration and bead diameter. Cells were incubated with radioactive pFN-coated beads, and total bead binding (cell surface or ingested) was measured as total radioactivity associated with the cells. Of the bound beads, those that also were phagocytosed were distinguished by their insensitivity to release from the cells by trypsin treatment. In continuous incubations, binding of pFN-coated beads to cells occurred at 4 degrees C or 37 degrees C, but phagocytosis was observed only at 37 degrees C. In addition, degradation of 3H-pFN from ingested beads occurred at 37 degrees C, as shown by the release of trichloroacetic acid-soluble radioactivity into the incubation medium. When the fibronectin density on the beads was varied, binding at 4 degrees C and ingestion at 37 degrees C were found to have the same dose-response dependencies, which indicated that pFN densities that permitted bead binding were sufficient for phagocytosis to occur. The fibronectin density for maximal binding of ingestion was approximately 250 ng pFN/cm2. When various sized beads (0.085-1.091 micron), coated with similar densities of pFN, were incubated with cells at 4 degrees C, no variation in binding as a function of bead size was observed. Under these conditions, the absolute amount of pFN ranged from less than 100 molecules on the 0.085-micron beads to greater than 15,000 molecules on the 1.091-micron beads. Based upon these results it can be concluded that the critical parameter controlling fibronectin-mediated binding of latex beads by BHK cells is the spacing of the pFN molecules on the beads. Correspondingly, it can be suggested that the spacing between pFN receptors on the cell surface that is optimal for multivalent interactions to occur is approximately 18 nM. When phagocytosis of various sized beads was compared, it was found that the largest beads were phagocytosed slightly better (two fold) than the smallest beads. This occurred both in continuous incubations of cells with beads and when the beads were prebound to the cells. Finally, the kinetic constants for the binding of 0.085 microM pFN-coated beads to the cells were analyzed. There appeared to be approximately 62,000 binding sites and the KD was 4.03 X 10(-9) M. Assuming a bivalent interaction, it was calculated that BHK cells have approximately 120,000 pFN receptors/cell and the binding affinity between pFN and its receptor is approximately 6 X 10(-5) M.