Lymphocyte Surface Antigen L25 Is a Member of the Integrin Receptor Superfamily

Clustering T-cell GM1 lipid rafts increases cellular resistance to shear on fibronectin through changes in integrin affinity and cytoskeletal dynamics

Lipid rafts are small laterally mobile microdomains that are highly enriched in lymphocyte signaling molecules. GM1 gangliosides are a common lipid raft component and have been shown to be important in many T cell functions. The aggregation of specific GM1 lipid rafts can control many T cell activation events, including their novel association with T cell integrins. We found that clustering GM1 lipid rafts can regulate β1 integrin function. This was apparent through increased resistance to shear flow dependent detachment of T cells adherent to the α4β1 and α5β1 integrin ligand fibronectin (FN). Adhesion strengthening as a result of clustering GM1 enriched lipid rafts correlated with increased cellular rigidity and morphology through the localization of cortical F-actin, the resistance to shear induced cell stretching, and an increase in the surface area and symmetry of the contact area between the cell surface and adhesive substrate. Furthermore, clustering GM1 lipid rafts could initiate integrin “inside-out” signaling mechanisms. This was seen through increased integrin-cytoskeleton associations and enhanced soluble binding of FN and VCAM-1 suggesting the induction of high affinity integrin conformations. The activation of these adhesion strengthening characteristics appear to be specific for the aggregation of GM1 lipid rafts as the aggregation of the heterogeneous raft associated molecule CD59 failed to activate these functions. These findings indicate a novel mechanism to signal to β1 integrins and to activate adhesion strengthening processes.

B-1 cells are deficient in Lck: defective B cell receptor signal transduction in B-1 cells occurs in the absence of elevated Lck expression

B-1 cells constitute a unique B cell subset that is primarily responsible for producing nonimmune Ig. This natural Ig acts as a principal line of defense against infection. A key feature of B-1 cells is the failure of BCR-triggered signal transduction. Recently, defective BCR signaling in B-1 cells has been attributed to elevated expression of the canonical T cell src kinase, Lck. In the present study, we re-examined Lck expression in normal B-1 cells. We found that B-1 cells expressed less Lck at both the protein and RNA levels than did B-2 cells. The same B-1 cells manifested defective BCR-mediated induction of IKKbeta phosphorylation, IkappaBalpha degradation, and intracellular Ca(2+) mobilization. Thus, the failure of BCR signaling in B-1 cells does not relate to subset-specific elevation of Lck.

The relative influence of metal ion binding sites in the I-like domain and the interface with the hybrid domain on rolling and firm adhesion by integrin alpha4beta7

We examined the effect of conformational change at the beta(7) I-like/hybrid domain interface on regulating the transition between rolling and firm adhesion by integrin alpha(4)beta(7). An N-glycosylation site was introduced into the I-like/hybrid domain interface to act as a wedge and to stabilize the open conformation of this interface and hence the open conformation of the alpha(4) beta(7) headpiece. Wild-type alpha(4)beta(7) mediates rolling adhesion in Ca(2+) and Ca(2+)/Mg(2+) but firm adhesion in Mg(2+) and Mn(2+). Stabilizing the open headpiece resulted in firm adhesion in all divalent cations. The interaction between metal binding sites in the I-like domain and the interface with the hybrid domain was examined in double mutants. Changes at these two sites can either counterbalance one another or be additive, emphasizing mutuality and the importance of multiple interfaces in integrin regulation. A double mutant with counterbalancing deactivating ligand-induced metal ion binding site (LIMBS) and activating wedge mutations could still be activated by Mn(2+), confirming the importance of the adjacent to metal ion-dependent adhesion site (ADMIDAS) in integrin activation by Mn(2+). Overall, the results demonstrate the importance of headpiece allostery in the conversion of rolling to firm adhesion.

Regulation of pseudopodia localization in lymphocytes through application of mechanical forces by optical tweezers

T-lymphocytes are responsible for cell-mediated immunity, and recognize antigens on target cells (e.g., tumor cells, virus-infected cells) and antigen presenting cells (e.g., macrophages, dendritic cells). While mechanical forces applied to a cell surface can produce alterations in the cytoskeletal structure, leading to global structural rearrangements and changes in the intracellular biochemistry and gene expression, it remains unknown if local mechanical forces acting at the lymphocyte-antigen interaction site play any role in lymphocyte activation following antigen recognition. In this study we investigate the effect of such forces induced by optical tweezers on the lymphocyte's morphological response. We brought optically trapped polystyrene beads, coated with a specific antibody against a clonotypic epitope of the T-cell receptors (TCRs), in contact with individual lymphocytes and applied mechanical forces at the TCR-antibody interaction site. Although bead size was a factor, simple bead contact tended to induce formation of pseudopodia that appeared randomly over the cell's surface, while application of tangential forces at the interaction site redirected pseudopodia formation toward that site and promoted endocytosis activity. We propose that local forces play a key role in the initial lymphocyte adhesion to antigen-bearing cells, and may be implicated in antigen-specific motility, transendothelial migration, and tissue homing to sites of inflammation.

Alpha 4 integrin increases anoikis of human osteosarcoma cells

Cell motility, growth, and proliferation are regulated by adhesion to the extracellular matrix. Detachment of adherent cells from extracellular matrix results in induction of apoptosis ("anoikis"). Transformed cells often show an anchorage-independent growth that enables them to acquire a motile, invasive phenotype. This phenotype has been associated with the altered expression and function of the integrin family of transmembrane proteins that mediate cell adhesion to the extracellular matrix. Although alpha4 integrin is normally expressed on leukocyte subpopulations, a number of metastatic melanomas and sarcomas express it as well. In this study, we demonstrated the expression of alpha4 integrins on the human osteosarcoma cell line SAOS and on metastatic osteosarcoma lesions from the lung and pericardium. We further demonstrated that alpha4 integrin is coupled to the beta1 subunit by biochemical analysis and by using a mAb directed against a combinatorial epitope unique to the alpha4beta1 molecule. SAOS cells undergo anoikis when adherence is denied. Anoikis involved the activation of caspase 3 and the release of cytochrome c from mitochondria. Treatment of non-adherent SAOS with an anti-alpha4 mAb increased anoikis while anti-beta1 integrin mAbs did not alter anoikis, thus indicating a novel function for the alpha4 subunit in the control of cell death. Since integrins can control cell migration, proliferation, and apoptosis these results demonstrate a potential role for alpha4 integrin during multiple aspects of osteosarcoma metastasis.

Physical association and functional interaction between beta1 integrin and CD98 on human T lymphocytes

CD98 is a cell surface protein previously characterized as a cell activation marker, an amino acid transporter, and has recently been implicated in integrin-related functions. Integrins are cell surface proteins, important for homotypic cell aggregation, cell adhesion, and coactivation of T lymphocytes. We have previously shown that the anti-CD98 mAb 80A10, when coimmobilized with anti-CD3 mAb OKT3, is able to mediate human T cell coactivation that is inhibited by anti-beta1 integrin specific mAb 18D3. These results indicated a functional association of CD98 and beta1 integrin signaling but left open the question of a physical association. We now show the induction of homotypic aggregation through CD98 among human T cells and this aggregation was inhibited by anti-beta1 integrin mAb. Therefore, CD98-dependent lymphocyte proliferation and adhesion may involve integrins. Competitive binding assays and fluorescence colocalization analysis suggested that CD98 and beta1 integrin were physically associated. Differential extraction techniques and immunoprecipitations provided the first evidence that the alpha4beta1 integrin and CD98 are specifically associated on human T lymphocytes.

A novel monoclonal antibody mNI-58A against the alpha-chain of leukocyte function-associated antigen-1 (LFA-1) blocks the homotypic cell aggregation and actively regulates morphological changes in the phorbol myristate acetate (PMA)-activated human monocyte-like cell line, U937

A monoclonal antibody (mAb), designated mNI-58A, was produced by immunizing mice with the lipopolysaccharide (LPS)-stimulated monocyte-like cell line, U937. The antigen defined by mNI-58A was widely expressed on various lymphoid cells and all cell lines examined except the erythroid cell line, K562. When the reactive patterns between mNI-58A and the mAbs to various human differentiation antigens (CD11a, CD11b, CD11c, CD14, CD16, CD18, CD23, CD28, CD29, CD31, CD43, CD44, CD45RA, CD50, CD54, CD58, CD80, CD102, CD106, HLA-class I and-class II antigen) were compared, that of mNI-58A was found to be similar to those of the leukocyte function-associated antigen-1 (LFA-1) mAbs. Using a competitive immunofluorescence binding assay it was found that the preincubation with one of the CD11a mAbs, 2F12 completely blocked the subsequent binding of mNI-58A. mNI-58A prevented the homotypic cell aggregation of the phorbol myristate acetate (PMA)-activated U937 cells (referred to as PMA-U937) and PMA-activated Epstein-Barr virus (EBV)-transformed B cell lines, B-85 and Mann. mNI-58A markedly induced the spread formation of the PMA-U937 cells following this blocking of the homotypic cell aggregation, whereas 2F12 did not under the same condition. The spread formation induced by mNI-58A was completely blocked by cytochalasin B (CyB), cytochalasin D (CyD), cycloheximide (CHX) or protein kinase C inhibitors, sphingosine and H-7. The U937 cells markedly adhered to the tumor necrosis factor-alpha (TNF-alpha)-stimulated human umbilical vein endothelial cells (HUVECs) and also to the extracellular matrix protein, fibronectin, but mNI-58A did not enhance or block these adhesion process. mNI-58A precipitated two glycoproteins with molecular weight 180 kDa and 95 kDa as determined by SDS-PAGE analysis, which were identical to the LFA-alpha (CD11a) and beta (CD18) chains of leukocyte integrin precipitated by the CD11a mAbs, respectively. Sequential immunoprecipitation studies using the CD11a mAb (2F12) also indicate that mNI-58A recognizes an epitope on the alpha-chain of the LFA-1 molecule. The ability of mNI-58A to block the PMA-U937 cells and to induce the spread formation of these cells suggests that mNI-58A is a novel mAb reacting with an epitope on the alpha-chain of LFA-1 different from those recognized with the existing CD11a mAbs.

Cloning and characterization of the promoter region of the murine alpha-4 integrin subunit

To study the differential expression of the murine VLA-4 (alpha 4 beta 1) integrin, the 5'-flanking region of the gene for the alpha subunit (alpha 4m) was isolated and a cDNA for alpha 4m was obtained with reverse transcriptase polymerase chain reaction (RT-PCR). The cDNA sequence contained a difference in the signal peptide region compared to the previously described cDNA (Neuhaus et al., 1991). As a consequence, another start codon is predicted, resulting in a decrease in size of the signal peptide. This was confirmed by genomic sequencing. The promoter region was delimited by ribonuclease protection assay (RPA) and transfection experiments fusing 5'-upstream fragments to the luciferase gene. A fragment extending from -936 to +221 was capable of controlling the expected cell-type-specific expression. Sequence comparison of the mouse alpha 4m promoter region with the human alpha 4h promoter revealed little homology. Like most integrin subunits, alpha 4m lacks TATA anc CCAAT boxes. Putative recognition sites for DNA-binding nuclear factors (AP1, AP2, Sp1, and PU1) were identified. The characterization of the promoter region and further identification of the transcription regulatory elements should provide insight in the regulation of alpha 4m integrin gene expression.

MAb 18D3 triggering of integrin beta 1 will prevent but not terminate proliferation of human T cells

Triggering of integrins can deliver signals that will regulate T cell activation and proliferation when coupled with TCR/CD3 signaling. While co-activation stimuli can be achieved either with immobilized natural ligands or immobilized monoclonal antibodies specific for various integrin subunits, counterposing effects can be delivered by ligation of the integrin beta 1 chain (CD29) resulting in the downregulation of T cell proliferation. Thus, integrins may play a pivotal role in cell activation and are involved in both positive and negative regulatory pathways. In this report, anti-beta 1 mAb 18D3 was used to investigate the role of beta 1 in the negative regulation of T cell proliferation. T lymphocytes were stimulated to proliferate when activated with immobilized mAb to CD3 in conjunction with all of a panel of immobilized mAb to different alpha 4 (CD49d) and beta 1 epitopes, except the anti-beta 1.1 mAb 18D3. In soluble form, mAb 18D3 inhibited the induction of DNA synthesis dependent on costimulation of CD3 and the integrin alpha 4 subunit by a mechanism independent of anti-adhesive properties. In kinetic experiments, the addition of mAb 18D3 effectively inhibited the ultimate induction of DNA synthesis at all time points until the time coinciding with the onset of T cell proliferation, indicating that triggering the beta 1.1 epitope may only act to quench activation events prior to cellular commitment to synthesize DNA. MAb 18D3 did not induce cell death nor render cells incompetent for restimulation, but appeared to selectively inhibit IL-2 synthesis with little effect on the induction of IL-2 receptor expression.

Glycoconjugate vaccines. What next?

The principle that infants can be protected from invasive diseases caused by encapsulated organisms has been proved with the introduction of Haemophilus influenzae type b conjugate vaccines. The use of glycoconjugates to implement some of the goals of the Children's Vaccine Initiative requires a clear delineation of the chemical and immunological specifications for optimal vaccines.

Analysis of lymphocyte activation and proliferation by video microscopy and digital imaging

Video microscopy and digital imaging were used as a noninvasive method to quantitatively analyze lymphocyte activation and proliferation. This method takes advantage of the fact that upon activation lymphocytes blast and become significantly larger before proliferating. The mean cell sizes of T lymphocytes in an activation kinetics assay were measured by digital image analysis and compared to [3H]-thymidine incorporation of cells under the same treatment. An increase in cell size was observed before [3H]-thymidine incorporation; therefore the digital imaging assay is more sensitive in determining the earliest time-point of activation. Also, the digital imaging assay was comparable to the [3H]-thymidine incorporation assay in providing information about the extent and rates of T lymphocyte proliferation. Cellular DNA was stained with propidium iodide to show that the larger blasting cells in the population of activated T lymphocytes were indeed the cells that accounted for the increase in DNA synthesis and thus an increase in cell size can be correlated with activation.

Homotypic leukocyte aggregation triggered by a monoclonal antibody specific for a novel epitope expressed by the integrin beta 1 subunit: conversion of nonresponsive cells by transfecting human integrin alpha 4 subunit cDNA

The monoclonal antibody 33B6 was found to be specific for the beta 1 integrin subunit. Treatment of leukocytes with this antibody induced a vigorous homotypic aggregation that had similar physiologic conditions as aggregation induced by a monoclonal antibody specific for the alpha 4 subunit. Expression of a beta 1 subunit on the cell surface was not sufficient for mAb 33B6-mediated aggregation to occur, since cells of the K562 erythroleukemia line failed to respond even though they expressed the beta 1 subunit and the 33B6 epitope. However, after transfection with cDNA encoding the alpha 4 subunit, K562 cells acquired the ability to aggregate in response to mAb 33B6 binding. By contrast, mAb 33B6 blocked cell binding to the endothelial surface protein vascular cell adhesion molecule-1 and the extracellular matrix protein fibronectin. These results suggest that the beta 1 epitope defined by mAb 33B6 may play a novel role in regulating leukocyte adhesive interactions.

Cell interaction molecules and cytokines which participate in B lymphopoiesis

A molecular and cellular definition of the bone marrow microenvironment is rapidly contributing to our understanding of lymphohaemopoiesis. While lineage specific genes and their protein products are being identified, information is accumulating about mechanisms which may regulate their expression. Stimulation of B lymphocyte precursor replication, and other discrete functions, are being attributed to cytokines such as interleukin 7 (IL-7). The activity of these factors may be controlled at the level of synthesis, local concentration and interaction with extracellular matrix. Extremely small amounts of IL-7 are made by stromal cells, which are themselves being thoroughly studied as cloned cell lines. This in vitro characterization suggests that stromal cells can make at least 12 cytokines, that they can respond to some of those cytokines themselves, and that they retain differentiation potential. Several molecules have been identified which are probably required for recognition between cells in marrow. It is noteworthy that they belong to several previously described families of adhesion molecules and none is unique to that tissue. VCAM-1 is constitutively expressed on stromal cells in marrow and can be recognized by pre-B cells which bear the integrin VLA-4. The same pair of molecules is probably responsible for extravasation of leukocytes in other tissues during inflammation. Cell adhesion molecules are likely to work in a carefully coordinated and cooperative fashion. Their activity can be controlled by expression or, in some cases, modulated after display on the cell surface. For example, while most haemopoietic cells bear CD44, only certain cells utilize it for recognition of the ligand hyaluronate. The affinity for hyaluronate can be experimentally regulated and depends on the cytoplasmic domain of CD44. This capability for dynamic change may be important for transient interactions between cells, permitting movement of maturing precursors within and from marrow.

A single mRNA encodes the alpha 150 and alpha 80/70 forms of the alpha subunit of VLA4

VLA4 is a cell surface heterodimer (alpha 4 beta 1, CD49d/CD29) which belongs to the integrin family and is involved in cell-extracellular matrix interactions, as well as in intercellular adhesion. Unlike other integrin alpha subunits, the alpha subunit of VLA4 (alpha 4) can appear as a 150-kDa polypeptide (alpha 150), or cleaved into two non-covalently associated polypeptides (alpha 70-80). The relative proportion of each form is highly variable among different cells and is dependent on the state of cellular activation. The alpha 4 cleavage site has recently been shown to occur between residues Arg558-Ser559. We report the isolation of genomic clones encoding the alpha 4 subunit, the location of the cleavage site-encoding nucleotides to a specific exon and the determination of the exon-intron organization around the cleavage site. Comparison with the gpIIb (CD41) and the alpha x (p150,95 alpha, CD11c) genes revealed a similar genomic structure in this region, with exons of similar length separated by introns of identical phase. The structure of the alpha 4 mRNA in cells expressing the alternative forms of alpha 4 has been analyzed by means of reverse transcription-polymerase chain reaction. Our results indicate that the exon encoding the cleavage site is present in alpha 4 mRNA molecules from cells expressing either the alpha 150 or the alpha 70-80 form on the cell surface. Moreover, the structure of the alpha 4 mRNA around the cleavage site does not change during the switch towards the alpha 70-80 form that takes place upon lymphocyte activation. Therefore, both forms of alpha 4 arise from a common mRNA, the alpha 150 form contains the cleavage sequence and the alpha 70-80 form must be generated by a post-translational proteolytic event.

Lymphocyte activation and regulation of three adhesion molecules with supposed function in homing: LECAM-1 (MEL-14 antigen), LPAM-1/2 (alpha 4-integrin) and CD44 (Pgp-1)

Directed migration of lymphocytes from blood into lymph nodes and gut-associated lymphatic tissue, also referred to as homing, is subject to change following activation. Lymphocyte migration into lymphoid organs in vivo and binding to high endothelial venules in vitro is largely suppressed after short-term stimulation with phorbol esters. The observed functional alterations were correlated with changes in the expression of three putative homing receptors, LECAM-1 (MEL-14 antigen), LPAM-1/2 (alpha 4-integrin) and the murine CD44 (Pgp-1, H-CAM, Hermes-antigen equivalent) upon different modes of cellular activation. Expression of LECAM-1 (gp90 MEL-14), a lymphocyte adhesion molecule implicated in targeting extravasation into lymph nodes, was found to be lost almost completely within minutes after protein kinase C activation. LECAM-1 re-expression occurred within less than 24 h. Rapid loss of LECAM-1 was also observed after calcium ionophores whereas anti-CD3 or concanavalin A elicited a gradual and heterogeneous loss of LECAM-1 becoming detectable after several hours only. A number of cytokines tested were not able to induce alterations in LECAM-1 expression. In contrast, expression of LPAM-1/2 (alpha 4-integrin) and CD44 (Pgp-1, H-CAM), two adhesion molecules supposed to direct extravasation into Peyer's patches, remained stable for hours after every stimulus tested; CD44 expression gradually increased 24 h after mitogenic activation, whereas a small reduction only was observed for the expression of the alpha 4-chain under certain conditions. Thus, reduced extravasation of lymphocytes into Peyer's patches after activation is not due to a decline in the surface density of LPAM-1/2 alpha-chain or CD44 whereas alterations in migration into lymph nodes parallel the expression of LECAM-1.

Structure-function analysis of the human integrin VLA-4 (alpha 4/beta 1). Correlation of proteolytic alpha 4 peptides with alpha 4 epitopes and sites of ligand interaction

The structure-function relationship of the human integrin VLA-4 (alpha 4/beta 1; CD49d/CD29), has been studied in the human B-cell line Ramos by immunochemical and functional analysis. Ramos cells expressed the 150-kDa non-proteolyzed form of the alpha 4 chain, which could be digested upon mild trypsin treatment to generate the 80- and 65-kDa proteolyzed forms, as well as alpha 4 polypeptides of 55 and 50 kDa. In addition, treatment of Ramos cells with high doses of pronase predominantly yielded the 55- and 50-kDa alpha 4 peptides. The trypsin-generated 80- and 65-kDa alpha 4 polypeptides, but not the 55- and 50-kDa fragments, were able to associate with the beta 1 chain. Distinct anti-VLA-4 mAb against four different alpha 4 epitopes, referred to as epitopes A, B1, B2, and C, recognized the 150-kDa alpha 4 chain both associated or non-associated with the beta 1 chain. The alpha 4 proteolytic forms of 80, 65 and 50 kDa were precipitated by the anti-alpha 4 mAb directed against the four different alpha 4 epitopes. On the other hand, the 55-kDa alpha 4 peptide was present in precipitates from anti-alpha 4 mAb specific for epitopes A, B1 and C, but absent in precipitates from the anti-alpha 4 mAb specific for epitope B2. The different adhesive capacities of the VLA-4 integrin, namely the interaction with a 38-kDa fibronectin fragment containing the CS-1 region of plasma fibronectin (Fn-38), the binding to the vascular cell adhesion molecule-1 (VCAM-1), or the ability to mediate the anti-alpha 4-induced cell aggregation, were not altered on VLA-4 from cells upon mild trypsin treatment, when compared to non-treated cells. However, the 55- and 50-kDa alpha 4 forms generated by high-dose pronase cell treatment, failed to mediate cell interaction with Fn-38 or VCAM-1 ligands, and cell aggregation could not be triggered through VLA-4 under these conditions.

Triggering of the proteinase dipeptidyl peptidase IV (CD26) amplifies human T lymphocyte proliferation

CD26 (Ta1, dipeptidyl peptidase IV) is a Mr 105,000 protein expressed at high levels on activated T lymphocytes and is a potential marker of memory T cells. Reciprocal immunodepletion and solid phase double determinant binding studies showed that mAb AC7 and the CD26-specific mAb anti-Ta1 reacted with spatially distinct sites on the same molecule. The proteinase dipeptidyl peptidase IV (DPP IV) was immunoprecipitated with mAb AC7 and its enzymatic activity directly assayed using an enzyme overlay membrane system. High levels of DPP IV activity were detected on the T cell tumor line CCRF-HSB-2 and on PBMC stimulated by a variety of methods. By itself, soluble mAb AC7 was not mitogenic for T cells but enhanced T cell proliferation that resulted from treatment with phorbol myristic acetate (PMA) in the presence of accessory cells. T cell proliferation was also induced by co-immobilized mAb AC7 and mAb OKT3 (anti-CD3). Cultures of T cells growing in the presence of IL-2 responded with accelerated growth when exposed to a combination of immobilized mAb AC7 and soluble mAb OKT3, a result not seen with freshly isolated T cells.

A novel thymic epithelial adhesion molecule

Interactions between thymocytes and thymic stromal cells are responsible for positive and negative selection of T cell, their differentiation, maturation and proliferation. The signals required for these events to occur often necessitate close contact, and indeed adhesion, between the cell types involved. The identification of specific adhesion molecules in this context, is, therefore, a vital first step in determining the nature of the signal they mediate or facilitate at a given stage of differentiation. In the present work we identify, isolate and partially characterize a ligand present on thymic medullary epithelial cells which selectively binds CD4+ CD8+ thymocytes found primarily in the thymic cortex. This adhesion molecule is composed of two non-covalently associated glycoproteins of 23 kDa and 45 kDa, respectively, both of which are needed to bind to thymocytes. The importance of the finding is that the ligand, in isolated immobilized form, binds the same thymocyte subset as the original epithelial cell line from which it was isolated. The CD4+ CD8+ thymocyte subset is the precursor of single-positive mature T cells; hence the putative biological activity of the ligand in question takes place at a pivotal stage of T cell differentiation.

Haptotactic activity of fibronectin on lymphocyte migration in vitro

In addition to mediating cell adhesion, fibronectin (FN) also affects the migration of different cell types. However, the role of FN in lymphocyte migration is unclear. In this study, we examined the effects of FN on the in vitro migration of lymphocytes. Using the checkerboard analysis in a blind-well microchemotaxis assay, soluble FN was determined to have neither a chemotactic nor chemokinetic effect on spleen or thymus lymphocytes. However, when the nitrocellulose filter was coated unidirectionally with FN, the migration of both spleen and thymus lymphocytes into the filter was enhanced, indicating that FN is haptotactic for lymphocytes. When the filter was coated bidirectionally, no enhancement in migration was observed, indicating that FN is not haptokinetic for lymphocytes. When the FN cell-binding domain and the heparin-binding domain were tested, the cell-binding domain was haptotactic for both spleen and thymus lymphocytes, whereas the heparin-binding domain was only haptotactic for spleen lymphocytes. Because the heparin-binding domain can mediate strong adhesion of thymus lymphocytes, the lack of haptotactic activity is likely to be the result of excessive binding that prevents cell motility.