Characterization of an antigen shared by human thymic epithelium and human T cell leukemia virus p19 Gag protein.

The molecular basis for cross-reactive antibody binding to human T cell leukemia virus type I (HTLV-I) p19 core protein and human thymic epithelium has been defined with two monoclonal antibodies (mAbs), 12/1-2 and 13B12, raised to HTLV-I p19. The mAb 12/1-2 has previously been shown to react with HTLV-I p19, HTLV-II p22, and antigens of normal human thymic epithelium, placenta, and foreskin, whereas mAb 13B12 binds only to the carboxyl terminus of HTLV-I p19. In the present study, mAb 12/1-2 bound to a subset of Triton X-100-insoluble intermediate filaments in human thymic epithelium also recognized by antikeratin antibodies AE1 and AE3. The mAb 12/1-2 also reacted in Western blot assays with proteins of 54, 46, and 40 kDa present in extracts of human thymic epithelium and with hexameric peptides containing overlapping sequences of HTLV-I p19 with the amino acids IPP (amino acids 117-119). In contrast, the HTLV-I-specific mAb 13B12 did not bind to human thymic epithelium and reacted with a single hexameric peptide containing the carboxy-terminal HTLV-I p19 sequence IPPPYV (amino acids 117-122). Binding of mAb 12/1-2 to thymic epithelium could be inhibited by adsorption with peptide SP-79 containing a C-terminal sequence (amino acids 112-125) of p19. The crossreactive IPP site is within a region of p19 that has been previously shown to be highly immunogenic in HTLV-I-infected individuals and that is also encoded by genes or mRNA of human cytokeratin 17, keratin 4, epidermal cytokeratin 2, and 50-kDa type I epidermal keratin. Thus, our studies define the sequence of a cross-reactive antigen on HTLV-I p19 that is also associated with keratin intermediate filaments from human thymic epithelium and other normal human tissues and that could serve as a focus of an autoimmune response during HTLV-I infection.

AD2, a human molecule involved in the interaction of T cells with epidermal keratinocytes and thymic epithelial cells.

Interactions between T cells and epithelial cells of the thymus are essential for normal T cell development, and interactions between T cells and skin epidermal keratinocytes occur in the context of inflammatory skin diseases and cutaneous T cell malignancies. On the basis of observations that the T cell ALL cell line, HSB, bound to IFN-gamma activated epidermal keratinocytes (41 +/- 5% of EK with three or more HSB cells bound), whereas the CTCL cell line H9 bound poorly (8 +/- 3%), we have raised mAb 13H12 that identified a 36 kDa molecule, termed AD2, that was highly expressed on HSB but not on H9 T cells. mAb 13H12 inhibited the binding of HSB T cells to IFN-gamma-treated epidermal keratinocytes (43 +/- 5% inhibition, p < 0.01), inhibited the binding of peripheral blood T cells to IFN-gamma-treated EK (62 +/- 3% inhibition, p < 0.001), and inhibited the binding of IFN-gamma-treated human thymic epithelial cells to thymocytes (39 +/- 3% inhibition, p < 0.01). Although AD2 was expressed at a low level on all T cells, AD2 was highly expressed on the CD3-CD4-CD8-, CD3-CD4low+ CD8-, and CD3-CD4+ CD8+ subsets of immature thymocytes in the thymic subcapsular and inner cortex. Taken together, these data suggest a role for the AD2 molecule in interactions of T cells with epithelial cells of skin and thymus.

Human thymic epithelial cells: adhesion molecules and cytokine production.

The ability to culture human thymic epithelial cells has greatly facilitated studies of direct cell-cell interaction between thymic epithelial cells and T lymphocytes in vitro, as well as cytokine production and regulation of cytokine production. In vitro, human thymic epithelial cells bind to T lymphocytes via two adhesion pathways: CD2-lymphocyte function-associated antigen-3 and lymphocyte function-associated antigen-1-intercellular adhesion molecule-1. Cultured human thymic epithelial cells produce interleukins-1 alpha, -1 beta, -3, -6 and -8, granulocyte colony-stimulating factor, macrophage colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, leukemia inhibitory factor and transforming growth factor-alpha. Production of thymic epithelial cell-derived cytokines is regulated by both adhesion molecules (lymphocyte function-associated antigen-3) and soluble factors via both autocrine (interleukin-1 alpha, transforming growth factor-alpha) and paracrine (interleukin-4, interferon-gamma) pathways. Transforming growth factor-alpha and epidermal growth factor regulate various cytokine mRNA at a post-transcriptional level by increasing cytokine mRNA stability.

Expression of cell-adhesion molecules in the salivary gland microenvironment of Sjögren's syndrome.

OBJECTIVE: The potential role of cell adhesion molecules in the pathogenesis of Sjögren's syndrome (SS) was assessed by examining their expression in salivary gland (SGL) tissue. METHODS: Intercellular adhesion molecule type 1 (ICAM-1), lymphocyte function-associated antigen type 1 (LFA-1), LFA-3, CD2, and CD44 expression were determined using indirect immunofluorescence techniques. RESULTS: In inflamed labial SGL tissue, ICAM-1 expression was evident on infiltrating LFA-1+/CD2+/LFA-3+ mononuclear cells, and to a limited extent on SGL acinar epithelial cells adjacent to sites of intense inflammation. CONCLUSION: In SS, the SGL microenvironment is characterized by only a modest up-regulation of ICAM-1 expression on epithelial cells, despite the presence of T cells bearing an activated phenotype.

Regulation of cytokine production in the human thymus: epidermal growth factor and transforming growth factor alpha regulate mRNA levels of interleukin 1 alpha (IL-1 alpha), IL-1 beta, and IL-6 in human thymic epithelial cells at a post-transcriptional level.

Human thymic epithelial (TE) cells produce interleukin 1 alpha (IL-1 alpha), IL-1 beta, and IL-6, cytokines that are important for thymocyte proliferation. The mRNAs for these cytokines are short-lived and are inducible by multiple stimuli. Thus, the steady-state levels for IL-1 and IL-6 mRNAs are critical in establishing the final cytokine protein levels. In this study we have evaluated the effect of epidermal growth factor (EGF), a growth factor for TE cells, and its homologue transforming growth factor alpha (TGF-alpha), on primary cultures of normal human TE cells for the levels of IL-1 alpha, IL-1 beta, IL-6, and TGF-alpha mRNA. We showed that TE cells expressed EGF receptors (EGF-R) in vitro and in vivo, and that treatment of TE cells with EGF or TGF-alpha increased IL-1 and IL-6 biological activity and mRNA levels for IL-1 alpha, IL-1 beta, and IL-6. Neither EGF nor TGF-alpha increased transcription rates of IL-1 alpha, IL-1 beta, and IL-6 genes, but rather both EGF and TGF-alpha increased cytokine mRNA stability. By indirect immunofluorescence assay, TGF-alpha was localized in medullary TE cells and thymic Hassall's bodies while EGF-R was localized to TE cells throughout the thymus. Thus, TGF-alpha and EGF are critical regulatory molecules for production of TE cell-derived cytokines within the thymus and may function as key modulators of human T cell development in vivo.

Thymic microenvironment induces HIV expression. Physiologic secretion of IL-6 by thymic epithelial cells up-regulates virus expression in chronically infected cells.

The hallmark of infection with HIV-1 is progressive depletion and qualitative dysfunction of the CD4+ Th cell population in infected individuals. Clinical trials of antiretroviral agents have shown that, despite suppression of virus replication, regeneration of the T cell pool does not occur. One proposed explanation for the defective regenerative capacity of the CD4+ T cell pool is infection of early T lymphocyte progenitors or stem cells. An additional explanation could be failure of cells of the intrathymic microenvironment (thymic epithelial (TE) cells) to carry out critical nurturing functions for developing thymocytes, i.e., secretion of thymocyte-trophic cytokines and expression of adhesion molecules. This study examines the effect of HIV on cultured TE cells and determines the role of TE cells in the regulation of viral expression in chronically HIV-infected cells. We found no evidence of infection of TE cells after exposure to HIV-1. However, normal human serum induced secretion of IL-6 by TE cells; induction of TE IL-6 was partially blocked by anti-IFN-gamma antibodies. Moreover, supernatants from TE cells maintained in normal human serum up-regulated HIV replication in chronically HIV-1-infected cells. Because intrathymic T cell precursors can be infected with HIV and T cell precursors come into close contact with TE cells in the thymus, IL-6 secreted by TE cells during normal intrathymic development may induce HIV expression in infected thymocytes in vivo and promote the intrathymic spread of HIV.

Human thymic epithelial cells produce IL-6, granulocyte-monocyte-CSF, and leukemia inhibitory factor.

The development of conditions for culturing normal human thymic epithelial (TE) cells free from contaminating stromal cells has allowed us to characterize a number of cytokines produced by TE cells. Using cDNA probes for human IL-6, granulocyte-monocyte-CSF, and leukemia inhibitory factor (LIF), we identified mRNA for these cytokines by RNA blot analysis of total RNA preparations derived from TE cells. We demonstrated that TE cells produced IL-6 transcripts and that TE cell culture supernatants contained IL-6 biologic activity, as determined by the ability to support proliferation of the T1165 plasmacytoma line. The 1.0-kilobase (kb) transcript of granulocyte-monocyte-CSF was also detected in TE cell-derived total RNA. TE cell culture supernatants contained LIF activity, as determined by proliferation of the murine cell line DA-1a, and a 4.0-kb LIF transcript was detected in TE cell-derived total RNA preparations. The 4.0-kb LIF transcript from TE cell-derived total RNA corresponded in size to the LIF transcripts in PMA-activated T lymphocytes. Thus, using biologic assays and RNA blot analysis, we demonstrated that cultured normal human TE cells produced both immunoregulatory cytokines and cytokines that drive various differentiation stages of human hematopoiesis. Our findings support the hypothesis that TE cells may play a role in providing cytokines that are important for the proliferation and differentiation of hematopoietic precursor cells that migrate to the thymus during fetal and postnatal human thymic development.

Interactions between epithelial cells and T lymphocytes: role of adhesion molecules.

Cell-cell interaction is critical for normal T cell development and function. A number of adhesion molecules important in T cell interactions with other cell types have been defined. This paper reviews the role of two adhesion pathways, CD2/LFA-3 and LFA-1/ICAM-1, in T cell interactions with epithelial cells of the thymus and skin. While thymic epithelium-T cell interactions are mediated by both the LFA-1/ICAM-1 pathway and the CD2/LFA-3 pathway, epidermal-T cell interactions are mediated primarily by the LFA-1/ICAM-1 pathway. Although ICAM-1 is not expressed in vivo on epidermal keratinocytes in normal skin, ICAM-1 is expressed by epidermal keratinocytes at the site of T cell infiltration in inflammatory dermatitis. ICAM-1 is expressed in vivo on thymic epithelium. These antigen independent adhesion molecules play an important role in the cell-cell interactions associated with T cell differentiation and function.

Ligand binding to the LFA-3 cell adhesion molecule induces IL-1 production by human thymic epithelial cells.

We have shown that human thymic epithelial (TE) cells produce IL-1 alpha, IL-1 beta, and TE cells bind to thymocytes by CD2 and LFA-1 molecules on thymocytes and LFA-3, ICAM-1 on TE cells. We investigated whether ligand binding to LFA-3 on human TE cells can modulate TE cell IL-1 production. First, we investigated the ability of human thymocytes to regulate IL-1 release by TE cells. Both autologous and allogenic emetine-treated thymocytes when cultured with TE cells augmented IL-1 release by TE cells. The augmentation of IL-1 release was cell density dependent. Inasmuch as the interaction between thymocytes and TE cells is mediated in part by CD2 molecules on thymocytes and LFA-3 molecules on TE cells we next determined the effect on IL-1 release of ligand binding (anti-LFA-3 mAb TS2/9) to TE cell surface LFA-3. Purified anti-LFA-3 mAb augmented IL-1 release in a concentration-dependent fashion. The anti-LFA-3-mediated augmentation of IL-1 release required both new protein and RNA synthesis as shown by the ability of cycloheximide and actinomycin-D to inhibit augmentation of IL-1 production by TE cells, and by direct quantitation of IL-1 alpha and IL-1 beta mRNA by Northern blot analysis. Both F(ab)'2 and Fab' fragments of anti-LFA-3 mAb augmented IL-1 alpha and IL-1 beta mRNA production, indicating that monovalent binding to cell surface LFA-3 was sufficient to provide the inducing signal. The identification of LFA-3, the cell surface ligand for thymocyte CD2 molecules, as a molecule via which TE cell-derived cytokine production may be regulated suggests a mechanism at the cell surface by which direct TE cell-thymocyte interaction might result in the triggering of local IL-1 release within the human thymic microenvironment.

The role of adhesion molecules in epithelial-T-cell interactions in thymus and skin.

Interaction of T lymphocytes with other cell types is important for normal T-cell development and function. Recently, a number of adhesion molecules important in T-cell interactions with other cell types have been defined. In this paper we review the role of two adhesion pathways, CD2/LFA-3 and LFA-1/ICAM-1, in T-cell interactions with epithelial cells of the thymus and skin. While thymic epithelium-T-cell interactions were mediated by both the LFA-1/ICAM-1 pathway and the CD2/LFA-3 pathway, epidermal-T-cell interactions were mediated primarily by the LFA-1/ICAM-1 pathway. Although ICAM-1 was not expressed in vivo on epidermal keratinocytes in normal skin, ICAM-1 was expressed by epidermal keratinocytes at the site of T-cell infiltration in inflammatory dermatitis. ICAM-1 was expressed in vivo on thymic epithelium. Both LFA-3 and ICAM-1 were expressed on epithelial cells of thymus and skin early on in fetal ontogeny. These antigen-independent adhesion molecules play an important role in the cell-cell interactions associated with T-cell differentiation and function.

Thymocyte LFA-1 and thymic epithelial cell ICAM-1 molecules mediate binding of activated human thymocytes to thymic epithelial cells.

We have investigated the binding in vitro of activated thymocytes to thymic epithelial (TE) cells, and studied the effect of up-regulation of TE cell surface intracellular adhesion molecule 1 (ICAM-1) and HLA-DR by IFN-gamma on the ability of TE cells to bind to both resting and activated human thymocytes. TE cell binding to activated and resting thymocytes was studied by using our previously described suspension assay of TE-thymocyte conjugate formation. We found that activated mature and immature thymocytes bound maximally at 37 degrees C to IFN-gamma-treated ICAM-1+ and HLA-DR+ TE cells and this TE-activated thymocyte binding was inhibited by antibodies to LFA-1 alpha-chain (CD11a) (68.1 +/- 5.6% inhibition, p less than 0.01) and ICAM-1 (73.9 +/- 7.7% inhibition, p less than 0.05). Neither anti-HLA-DR antibody L243 nor anti-MHC class I antibody 3F10 inhibited IFN-gamma-treated TE binding to activated thymocytes. As with antibodies to LFA-3 and CD2, antibodies to LFA-1 and ICAM-1 also inhibited PHA-induced mature thymocyte activation when accessory signals were provided by TE cells in vitro. Finally, LFA-1 and ICAM-1 were expressed early on in human thymic fetal ontogeny in patterns similar to those seen in postnatal thymus. Taken together, these data suggest that resting mature and immature thymocytes bind to TE cells via the CD2/LFA-3 ligand pair, whereas activated thymocytes bind via both CD2/LFA-3 and LFA-1/ICAM-1 ligand systems. We postulate that IFN-gamma produced intrathymically may regulate TE expression of ICAM-1 and therefore potentially may regulate TE cell binding to activated thymocytes beginning in the earliest stages of human thymic development.

Human intrathymic T cell differentiation.

The human thymus develops early on in fetal gestation with morphologic maturity reached by the beginning of the second trimester. Endodermal epithelial tissue from the third pharyngeal pouch gives rise to TE3+ cortical thymic epithelium while ectodermal epithelial tissue from the third pharyngeal cleft invaginates and splits during development to give rise to A2B5/TE4+ medullary and subcapsular cortical thymic epithelium. Fetal liver CD7+ T cell precursors begin to colonize the thymus between 7 and 8 weeks of fetal gestation, followed by rapid expression on thymocytes of other T lineage surface molecules. Human thymic epithelial cells grown in vitro bind to mature and immature thymocytes via CD2 and CD11a/CD18 (LFA-1) molecules on thymocytes and by CD58 (LFA-3) and CD54 (ICAM-1) molecules on thymic epithelial cells. Thymic epithelial cells produce numerous cytokines including IL1, IL6, G-CSF, M-CSF, and GM-CSF--molecules that likely are important in various stages of thymocyte activation and differentiation. Thymocytes can be activated via several cell surface molecules including CD2, CD3/TCR, and CD28 molecules. Finally, CD7+ CD4-CD8- CD3- thymocytes give rise to T cells of both the TCRab+ and TCR gd+ lineages.

Antibodies against the CD44 p80, lymphocyte homing receptor molecule augment human peripheral blood T cell activation.

The CD44 inhibitor Lutheran [In(Lu)]-related p80 molecule has recently been shown to be identical to the Hermes-1 lymphocyte homing receptor and to the human Pgp-1 molecule. We have determined the effect of addition of CD44 antibodies to in vitro activation assays of PBMC. CD44 antibodies did not induce PBMC proliferation alone, but markedly enhanced PBMC proliferation induced by a mitogenic CD2 antibody pair or by CD3 antibody. CD44 antibody addition had no effect upon PBMC activation induced by PHA or tetanus toxoid. CD44 antibody enhancement of CD2 antibody-induced T cell activation was specific for mature T cells as thymocytes could not be activated in the presence of combinations of CD2 and CD44 antibodies. CD44 antibody enhancement of CD2-mediated T cell triggering occurred if CD44 antibody was placed either on monocytes or on T cells. In experiments with purified monocyte and T cell suspensions, CD44 antibodies A3D8 and A1G3 augmented CD2-mediated T cell activation by three mechanisms. First, CD44 antibody binding to monocytes induced monocyte IL-1 release, second, CD44 antibodies enhanced the adhesion of T cells and monocytes in CD2 antibody-stimulated cultures, and third, CD44 antibodies augmented T cell IL-2 production in response to CD2 antibodies. Thus, ligand binding to CD44 molecules on T cells and monocytes may regulate numerous events on both cell types that are important for T cell activation. Given that recent data suggest that the CD44 molecule may bind to specific ligands on endothelial cells (vascular addressin) and within the extracellular matrix (collagen, fibronectin), these data raise the possibility that binding of T cells to endothelial cells or extracellular matrix proteins may induce or up-regulate T cell activation in inflammatory sites.

CD44 antibody against In(Lu)-related p80, lymphocyte-homing receptor molecule inhibits the binding of human erythrocytes to T cells.

The cluster of differentiation 44 (CD44) Ag system, originally described in brain and mature T cells, has been subsequently shown to be identical with the human lymphocyte homing-receptor defined by the Hermes-1 antibody and to be involved in T cell/endothelial cell interactions in synovium, mucosa, and lymph node. CD44 is also present on human E. On E, CD44 has been shown to be regulated by the In(Lu) dominant inhibitor gene and to express the Ina and Inb blood group Ag. Because human E have been shown to interact with human T cells via CD2 on T cells and LFA-3 on human E, we have studied the ability of human E and T lymphocyte CD44 Ag to participate in CD2/LFA-3 interactions between human E and T cells. In this study, we demonstrate that a mAb (A3D8) against the CD44, In(Lu)-related p80, lymphocyte homing-receptor molecule inhibited the binding of human E to human T cells. Whereas whole CD44 antibody molecules inhibited human E binding to T cells, saturating amounts of CD44 Fab fragments did not inhibit human E to T cell binding. Our data demonstrated that anti-CD44 antibody A3D8 acted at the level of the E to inhibit CD2/LFA-3 interactions between human E and T cells.

Immature human thymocytes can be driven to differentiate into nonlymphoid lineages by cytokines from thymic epithelial cells.

The signals and cellular interactions required for hematopoietic stem-cell commitment to the T lineage are unknown, yet are central to understanding the early stages of normal T-cell development. To study the differentiative capacity of T-cell precursors, we isolated CD4-, CD8-, surface(s) CD3- thymocytes from postnatal human thymuses and determined their capacity to differentiate into lymphoid and nonlymphoid lineages in vitro. We found that CD4-, CD8-, sCD3- thymocytes, which differentiated in the presence of T-cell conditioned medium plus interleukin 2 into T cells expressing the gamma delta receptor for antigen, were capable of differentiating into myeloid or erythroid lineages in the presence of either 5637 bladder carcinoma cell line conditioned medium plus recombinant human erythropoietin or human thymic epithelial cell conditioned medium. Thymic epithelial cell conditioned medium was as effective as 5637 supernatant plus erythropoietin in inducing myeloerythroid differentiation in the CD4-, CD8-, sCD3- thymocytes. Sixty-eight +/- 14% of CD4-, CD8-, sCD3- thymocytes underwent nonlymphoid differentiation within 4 days in culture with 5637 supernatant plus erythropoietin. Twenty-six +/- 4% of freshly isolated CD4-, CD8-, sCD3- cells were CD34+, and clonal granulocyte/macrophage, granulocyte/erythrocyte/monocyte/megakaryocyte, and T-cell progenitors were found in both CD34+ and CD34- subsets of CD4-, CD8-, sCD3- thymocytes. Thus, cells within the human CD4-, CD8-, sCD3- thymocyte subset can give rise to gamma delta+ T cells as well as to cells of myeloerythroid lineages. Moreover, CD34+, CD4-, CD8-, sCD3- cells can give rise to clonal T-cell progenitors as well as to clonal myeloid progenitors.

Epidermal keratinocytes express the adhesion molecule intercellular adhesion molecule-1 in inflammatory dermatoses.

Using indirect immunofluorescence assays on frozen tissue sections of skin from healthy subjects and subjects with inflammatory skin diseases, we found that intercellular adhesion molecule-1 (ICAM-1) was expressed in a cell surface pattern on epidermal keratinocytes at the site of lymphoid infiltration in cutaneous dermatoses. ICAM-1 was not expressed on epidermal keratinocytes in noninflamed skin. Its expression was not related solely to epidermal hyperproliferation, as hyperproliferative, tape-stripped epidermis did not express ICAM-1. We have reported previously that ICAM-1 expression on epidermal keratinocytes was upregulated by treatment with interferon gamma and that activated T lymphocytes bound to cultured epidermal keratinocytes in vitro by lymphocyte function associated-1 (LFA-1) molecules on T cells and ICAM-1 on epidermal keratinocytes. Taken together, these data suggest that upregulation of expression of ICAM-1 is an important feature of cutaneous inflammation.

Ontogeny of T-cell precursors: a model for the initial stages of human T-cell development.

Although most investigators agree that the CD4- CD8- CD3- thymocyte subset represents the most immature intrathymic T cell capable of repopulating the thymus in vivo, little is known of the earliest stages of human T-cell development. Using mAbs to hematopoietic and T-cell lineage molecules in quantitative immunofluorescence studies, new insight has been gained regarding the phenotype of human T-cell precursors before and after colonization of human thymic rudiment. In this article, Barton Haynes and colleagues discuss the sequential expression of CD7, CD4, CD8, CD3, CD2, CD1, CD45, TCR gamma delta and TCR alpha beta, and propose a model defining the stages of T-cell precursors during fetal ontogeny.

Removal of fibroblasts from human epithelial cell cultures with use of a complement fixing monoclonal antibody reactive with human fibroblasts and monocytes/macrophages.

A complement fixing IgM monoclonal antibody (1B10) that reacts with surface membrane molecules of human fibroblasts, tissue macrophages, and peripheral monocytes was produced. In Western blot analysis of detergent extracts of cultured human foreskin fibroblasts, antibody 1B10 detected protein bands of Mr 43,000 and 72-80,000. We used the 1B10 antibody with complement to eliminate most 1B10 positive nonepithelial cells from thymic epithelial (TE) cell cultures, thereby allowing us to grow highly enriched populations of human TE cells.

Immunohistologic analysis of the distribution of cell adhesion molecules within the inflammatory synovial microenvironment.

Antigen-independent binding of T lymphocytes to a variety of cell types has been shown to be mediated by receptor-ligand pairs of adhesion molecules. In forms of inflammatory synovitis (including rheumatoid arthritis), T cells home to synovium, become activated, and participate in the generation of chronic synovitis. Using indirect immunofluorescence assays on synovial frozen tissue sections and on synovial fibroblast cell lines, we studied the distribution of cell adhesion molecules on components of the synovial microenvironment in inflammatory synovitis. We reasoned that analysis of the cell types within synovium that express adhesion molecules might provide clues to lymphocyte-stromal interactions that occur in inflammatory synovitis. We found that antibodies against the lymphocyte function-associated antigen 3 (LFA-3) molecule and the intercellular adhesion molecule 1 (ICAM-1) both reacted with macrophage-like type A synovial cells and synovial fibroblasts, as well as with tissue macrophages and vessel endothelium. Using flow cytometry, we found that anti-LFA-3 and anti-ICAM-1 (but not antibodies against their ligands CD2 and LFA-1) reacted with synovial fibroblast cells cultured in vitro. Thus, these data demonstrate that the ligands for lymphocyte LFA-1 molecules (ICAM-1) and for T cell CD2 molecules (LFA-3) are widely distributed among cell types of the synovial microenvironment and provide numerous cell types with which lymphocytes can interact via these 2 adhesion pathways during the course of inflammatory synovitis.