TCGF(IL 2)-receptor inducing factor(s). II. Possible role of ATL-derived factor (ADF) on constitutive IL 2 receptor expression of HTLV-I(+) T cell lines

Interleukin 2 (IL 2) receptor (IL 2-R) is constitutively expressed on T cell lines established from the patients with adult T cell leukemia (ATL), which is a human T cell leukemia lymphoma virus (HTLV-1)(+) T4(+)-leukemia endemic in Japan, the United States, and other countries. Many of these cell lines continuously produce an acidic lymphokine, ATL-derived factor (ADF), which preferentially induces the synthesis and expression of IL 2-R on a sensitive HTLV-1(-) non-T cell line (YT). The induced IL 2-R was characterized by the binding of 125I-IL 2 and flow cytometry by using fluoresceinated anti-human IL 2-R monoclonal antibodies (anti-Tac). Scatchard analysis with 125I-IL 2 showed ADF induced high-affinity receptor sites on YT cells. To test the possibility that ADF produced by HTLV-1(+) T cells is involved in the abnormal expression of IL 2-R, we studied the effect of ADF on an HTLV-1(+) IL 2-dependent T cell line (ED) in which the beta-chain gene of the T cell antigen receptor (T beta) was rearranged. Unlike IL 2-independent HTLV-1(+) cell lines that constitutively expressed Il 2-R, the IL 2-R expression on ED cells declined in the absence of crude IL 2 or recombinant IL 2. When either ADF or recombinant IL 2 was added to the culture of ED cells, there was a dose-dependent enhancement of IL 2-R expression in 24 hr. ADF and IL 2 showed a synergism in the IL 2-R induction, and both factors were needed to induce the maximal receptor expression in these T cells. The lack of IL 2 production by ADF-treated YT, as well as ED cell line suggested IL 2 may not be involved in the IL 2-R induction by ADF. Northern blot hybridization with human IL 2-R cDNA probe showed the increase of IL 2-R mRNA in YT cells after ADF-treatment. ADF also enhanced IL 2-R expression of a rat T cell line transformed by HTLV-1(TARS-1), as demonstrated with anti-rat IL 2 receptor monoclonal antibodies (ART-18). An ADF-like IL 2-R-inducing factor was also detected in the conditioned medium of two HTLV-1(+) rat T cell lines (TARL-2 and TART-1), which constitutively expressed a higher number of Il 2-R than TARS-1 cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Modulation of the activity of sea bass (Dicentrarchus labrax) head-kidney macrophages by macrophage activating factor(s) and lipopolysaccharide

The aim of this study was to establish the requirements for macrophage activating factor (MAF) production by sea bass head-kidney leucocytes and the kinetics of macrophage activation when exposed to MAF-containing supernatants and/or lipopolysaccharide (LPS), a known macrophage stimulant. MAF activity was found in culture supernatants of total head-kidney leucocytes pulsed with 5 microg ml(-1)Con A, 5 or 10 ng ml(-1)PMA and 100 ng ml(-1)calcium ionophore, or 10 microg ml(-1)Con A alone, as assessed by the capacity to prime macrophages for enhanced production of reactive oxygen intermediates (ROI). Mixed leucocyte cultures from two or eight fish showed higher MAF activity after stimulation, indicating that a mixed leucocyte reaction was also important for MAF production. MAF-induced activation of macrophage cultures was highest at 18 h of exposure and was lost by 72 h except for MAF induced by Con A-stimulation alone. LPS primed macrophages for increased ROI production at early incubation times and down-regulated ROI production after 24 h. LPS had no effect in further stimulating the MAF-induced priming effect on production of ROI and down-regulated the MAF-priming by 48 h. Sea bass head-kidney macrophages did not show increased nitrite production when exposed to MAF and/or LPS, which may be related to their differentiation status.     

Aberrant activation of the interleukin-2 autocrine loop through the nuclear factor of activated T cells by nonleukemogenic human T-cell leukemia virus type 2 but not by leukemogenic type 1 virus

Human T-cell leukemia virus type 1 (HTLV-1) but not HTLV-2 is associated with adult T-cell leukemia. We found that HTLV-2 Tax2 protein stimulated reporter gene expression regulated by the interleukin (IL)-2 promoter through the nuclear factor of activated T cells (NFAT) in a human T-cell line (Jurkat). However, the activity of HTLV-1 Tax1 was minimal in this system. T-cell lines immortalized by HTLV-2 but not HTLV-1 constitutively exhibited activated NFAT in the nucleus and constitutively expressed IL-2 mRNA. Cyclosporine A, an inhibitor of NFAT activation, abrogated the induction of IL-2 mRNA in HTLV-2-immortalized T-cell lines and concomitantly inhibited cell growth. This growth inhibition was rescued by the addition of IL-2 to the culture. Furthermore, anti-IL-2 receptor antibodies significantly reduced the proliferation of HTLV-2-infected T-cell lines but not that of HTLV-1-infected cells. Our results suggest that Tax2 activates an IL-2 autocrine loop mediated through NFAT that supports the growth of HTLV-2-infected cells under low-IL-2 conditions. This mechanism would be especially important in vivo, where this autocrine mechanism establishes a nonleukemogenic life-long HTLV-2 infection. The results also suggest that differences in long-term cytokine production between HTLV-1 and HTLV-2 infection are another factor for the differences in pathogenesis.     

Increased responsiveness to macrophage-activating factor (MAF) after alteration of macrophage membranes.

Guinea pig macrophages pretreated with the esterase inhibitor, antithrombin III (AT III) show increased responsiveness to macrophage-activating factor (MAF) as demonstrated by their enhanced cytotoxicity for tumor cells. Other proteins that are not esterase inhibitors did not enhance the effect of MAF on the macrophage. Enhancement of MAF activity was also obtained when macrophages were preincubated with the cell surface reactant, diazotized sulfanilic acid (DSA). These studies indicate that the effect of MAF can be enhanced by chemical modifications of the macrophage membrane. They also provide further evidence to support the hypothesis that an esterase on the macrophage membrane modulates this cell's responsiveness to lymphocyte mediators.     

Studies on macrophage-activating factor (MAF) in antitumor immune responses

Summary In the present study we investigated some of the physicochemical properties of macrophage-activating factor(s) (MAF) produced by the tumor-immune Lyt-1⁺2^– T cell subset. Supernatant from mixed culture of spleen and lymph node cells, obtained from C3H/HeN mice immunized with syngeneic MH134 hepatoma or MCH-1-A1 fibrosarcoma, with the corresponding tumor cells exhibited the capability of activating peritoneal exudate macrophages to exert their cytostatic and cytolytic activities on tumor cells. Such MAF production was abolished by treatment of tumor-immune spleen and lymph node cells with anti-Thy-1.2 or anti-Lyt-1.1 antibody plus complement (C) before culturing. Anti-Lyt-2.1 and/or anti-asialo GM1 plus C treatment, however, had only marginal effect on the generation of MAF by these cells, despite the complete disappearance of natural killer (NK) cell activity of spleen and lymph node cells after the treatment with anti-asialo GM1 plus C. Thus, the tumor-specific Lyt-1⁺2^– T cell subset could fulfill a crucial role in generating MAF without the support of NK cells. The MAF activity was heat, acid, and trypsin sensitive. On Sephacryl S-300 column, MAF activity was eluated in a broad single peak around a molecular weight (m.w.) of 70,000 daltons. Antiviral activity was detected in the concentrated pool of MAF-containing fractions from Sephacryl S-300. Gel permeation analysis using HPLC also showed a coincident peak of MAF and antiviral activities at a m.w. of approximately 70,000 daltons. In addition, MAF activity was almost completely neutralized by incubation with rabbit antiserum against recombinant murine -interferon (IFN). Taken together, these results indicate that MAF generated by tumor-immune Lyt-1⁺2^– T cell subset is closely related to IFN.     

Astrocyte-specific expression of human T-cell lymphotropic virus type 1 (HTLV-1) Tax: induction of tumor necrosis factor alpha and susceptibility to lysis by CD8+ HTLV-1-specific cytotoxic T cells.

Human T-cell lymphotropic virus type 1 (HTLV-1) is associated with a chronic neurological disease termed HTLV-1-associated myelopathy/tropical spastic paraperesis (HAM/TSP). Although the pathogenesis of this disease remains to be elucidated, the evidence suggests that immunopathological mechanisms are involved. Since HTLV-1 tax mRNA was colocalized with glial acidic fibrillary protein, a marker for astrocytes, we developed an in vitro model to assess whether HTLV-1 infection activates astrocytes to secrete cytokines or present viral immunodominant epitopes to virus-specific T cells. Two human astrocytic glioma cell lines, U251 and U373, were transfected with the 3' portion of the HTLV-1 genome and with the HTLV-1 tax gene under astrocyte-specific promoter control. In this study, we report that Tax-expressing astrocytic glioma transfectants activate the expression of tumor necrosis factor alpha mRNA in vitro. Furthermore, these Tax-expressing glioma transfectants can serve as immunological targets for HTLV-1-specific cytotoxic T lymphocytes (CTL). We propose that these events could contribute to the neuropathology of HAM/TSP, since infected astrocytes can become a source for inflammatory cytokines upon HTLV-1 infection and serve as targets for HTLV-1-specific CTL, resulting in parenchymal damage by direct lysis and/or cytokine release.     

Reconstitution of high cell binding affinity of a marine sponge aggregation factor by cross-linking of small low affinity fragments into a large polyvalent polymer

Species-specific reaggregation of cells from the marine sponge Microciona prolifera is mediated by a proteoglycan-like aggregation factor (MAF) of Mr = 2 X 10(7) which has two functional domains, a cell binding domain and an aggregation factor interaction domain. After extensive trypsin digestion, over 60% of the MAF mass was converted into a glycopeptide fragment of Mr = 10,000 (T-10) which is therefore a representative part of the major portion, but not of the entire MAF molecule. The T-10 fragment has a similar amino acid and carbohydrate composition as the intact MAF and displays species-specific binding. Although T-10 also inhibited MAF association with homotypic cells, its apparent affinity is 3 X 10(6) M-1, i.e. 13,000 times lower than that of native MAF. Reconstitution of binding affinity in the same order of magnitude as native MAF (Ka = 10(10) M-1) was obtained by cross-linking the glycopeptide fragment into polymers of the approximate size of MAF (Mr greater than 1.5 X 10(7) using diepoxybutane and glutaraldehyde, or periodate oxidation and glutaraldehyde. The apparent association constants of intermediate polymers with Mr = 1 X 10(5), 6 X 10(5), 9 X 10(5), 2 X 10(6) and above 1.5 X 10(7) increased proportionally to their size and were in line with association constants of MAF degradation fragments. Since the binding affinity of the T-10 glycopeptide fragment could be reconstituted by cross-linking, and since this fragment accounts for over 60% of MAF, we propose that the specificity and high affinity of the MAF-cell association is based on a highly polyvalent interaction of low affinity cell-binding sites. Such a polyvalency of the cell binding domain is advantageous for efficient cell-cell interactions and thus differs from most known interaction molecules and receptors characterized.     

Characterization and sequencing of prototypic human T-lymphotropic virus type 1 (HTLV-1) from an HTLV-1/2 seroindeterminate patient

Serological screening for human T-lymphotropic virus type 1 (HTLV-1) parallels the standard screening process for human immunodeficiency virus (HIV), in which samples found positive by enzyme-linked immunosorbent assay (ELISA) are confirmed with a modified Western blot procedure. There are a significant number of cases in which HTLV-1/2 ELISA-positive specimens demonstrate an incomplete banding pattern on this Western blot. Individuals providing these atypical antibody responses are categorized as seroindeterminate for HTLV-1/2. Although HTLV-1 genomic sequences are readily detectable in the peripheral blood lymphocytes (PBL) of seropositive individuals, previous studies have repeatedly demonstrated that PBL from the vast majority of HTLV-1/2 seroindeterminate individuals are PCR negative for HTLV-1. As a result, identification of the agent responsible for this indeterminate reactivity has been of interest. We have generated an HTLV-1-positive B-cell line (SI-1 B) from one of these seroindeterminate individuals. Previous screening for HTLV-1 in PBL from this patient had been routinely negative by primary PCR; however, HTLV-1 tax had been periodically detected by nested PCR. DNA sequence data generated with genomic DNA from the SI-1 B cell line and HTLV-1-specific primers demonstrated the presence of a full-length viral genome with >97% homology to the Cosmopolitan form of HTLV-1. A 12-bp deletion was identified in the 3'-gag/5'-prot region, which would predict translation of altered or nonfunctional proteins from these genes. We propose that this HTLV-1/2-seroindeterminate patient is infected with a prototypic form of HTLV-1 at an extremely low viral load and that this finding may explain HTLV-1/2 seroindeterminate reactivity in at least a subset of these individuals.     

Studies on macrophage-activating factor (MAF) in antitumor immune responses. I. Tumor-specific Lyt-1+2- T cells are required for producing MAF able to generate cytolytic as well as cytostatic macrophages

In the present study we investigated some of the cellular mechanisms for the generation of macrophage-activating factor(s) (MAF) in immune responses to tumor antigens. C3H/HeN mice were immunized to syngeneic MH134 hepatoma or MCH-1-A1 fibrosarcoma by intradermal inoculation of viable tumor cells, followed by the surgical resection of the tumor. Spleen and lymph node cells from these tumor-immune mice were stimulated in vitro with the corresponding tumor cells, and supernatant from such a culture was tested for an ability to activate macrophages to exert their cytostatic and cytolytic activities as detected on tumor cells unrelated to immunizing tumors. Peritoneal adherent cells as a macrophage source, which were preincubated with supernatant from co-culture of tumor-unimmunized normal spleen and lymph node cells plus tumor cells, failed to exhibit any significant antitumor effect on unrelated X5563 tumor cells, whereas the addition of supernatant from cultures containing immune lymphocytes to adherent cells resulted in appreciably potent cytostatic and cytolytic effects on X5563 tumor cells, indicating the generation of MAF in culture supernatant. The activation of tumor-immune spleen and lymph node cells for MAF generation was tumor-specific, because anti-MH134- and anti-MCH-1-A1-immune lymphocytes produced MAF by the stimulation with the respective but not with the other alternative tumor cells. Such MAF production was abolished by treatment of tumor-immune spleen and lymph node cells with anti-Thy-1.2 or anti-Lyt-1.1 but not with anti-Lyt-2.1 antibody plus complement before culturing. These results indicate that the tumor-specific Lyt-1+2- T cell subset has a crucial role in generating MAF by which an adherent cell population as a source of macrophages acquires the potential for inducing a cytolytic as well as a cytostatic effect on tumor cells.     

Human T-lymphotropic virus type 1 mitochondrion-localizing protein p13(II) is required for viral infectivity in vivo

Human T-lymphotropic virus type 1 (HTLV-1), the etiological agent of adult T-cell leukemia, encodes unique regulatory and accessory proteins in the pX region of the provirus, including the open reading frame II product p13(II). p13(II) localizes to mitochondria, binds farnesyl pyrophosphate synthetase, an enzyme involved in posttranslational farnesylation of Ras, and alters Ras-dependent cell signaling and control of apoptosis. The role of p13(II) in virus infection in vivo remains undetermined. Herein, we analyzed the functional significance of p13(II) in HTLV-1 infection. We compared the infectivity of a human B-cell line that harbors an infectious molecular clone of HTLV-1 with a selective mutation that prevents the translation of p13(II) (729.ACH.p13) to the infectivity of a wild-type HTLV-1-expressing cell line (729.ACH). 729.ACH and 729.ACH.p13 producer lines had comparable infectivities for cultured rabbit peripheral blood mononuclear cells (PBMC), and the fidelity of the start codon mutation in ACH.p13 was maintained after PBMC passage. In contrast, zero of six rabbits inoculated with 729.ACH.p13 cells failed to establish viral infection, whereas six of six rabbits inoculated with wild-type HTLV-1-expressing cells (729.ACH) were infected as measured by antibody responses, proviral load, and HTLV-1 p19 matrix antigen production from ex vivo-cultured PBMC. Our data are the first to indicate that the HTLV-1 mitochondrion-localizing protein p13(II) has an essential biological role during the early phase of virus infection in vivo.     

Macrophage activation factor from EL-4, a murine T-cell line: antigenic characterization by hamster monoclonal antibodies to murine interferon-gamma

A cloned variant of the EL-4 murine T-cell line treated with phorbol myristate acetate (PMA) releases a factor that activates macrophages for nonspecific tumor cytotoxicity. This macrophage activation factor (MAF) is both physicochemically (Mr 25,000; pH 2 stable) and biologically different from interferon-gamma (IFN-gamma). However, EL-4 MAF may represent a breakdown product or otherwise altered fragment of IFN-gamma. We examined this possibility with a unique pair of hamster monoclonal antibodies against different epitopes of murine IFN-gamma. Both antibodies inhibited IFN-gamma-induced fibroblast antiviral activity; H21 but not H1 antibody also inhibited lymphokine (LK)-induced macrophage-mediated tumor cytotoxicity. Neither antibody, however, had any effect on the EL-4 MAF throughout a broad dose response. Moreover, passage through a H21 immunoaffinity chromatography column or addition of staphylococcal protein A and antibody completely inhibited LK-induced macrophage tumoricidal activity but did not affect the activity in EL-4 MAF. Identical effects in both fluid and solid phase were observed with polyclonal rabbit antisera to murine IFN-gamma. Results with all of these antibodies strongly suggest that the EL-4 MAF and murine IFN-gamma are antigenically distinct.     

Involvement of carbohydrates as multiple low affinity interaction sites in the self-association of the aggregation factor from the marine sponge Microciona prolifera

Cell aggregation in the marine sponge Microciona prolifera is mediated by a multimillion molecular weight aggregation factor (MAF) and is based on two functional properties, a Ca2+-independent cell binding activity and a Ca2+-dependent factor-factor self-interaction. Monoclonal antibodies were prepared against purified MAF, and one clone was characterized which selectively inhibited the MAF-MAF association activity. Binding of the blocking antibody (Block 1) to MAF demonstrated that this epitope structure was present in 1100 copies per one MAF molecule of Mr = 2 X 10(7). Such blocking antibodies precipitated a small molecular weight protein-free glycan fraction prepared from MAF by Pronase digestion, thus indicating that the highly repetitive epitope is located in the carbohydrate portion of the molecule. Since the inhibitory activity of the Block 1 antibody could only be achieved when most of the sites were occupied by Fab fragments of this antibody, the self-association of MAF seemed to be based on the polyvalency of the carbohydrate determinants. The affinity of the protein-free glycans was very low as shown by the fact that they did not display any measurable self or MAT binding activity in their monomeric form. After cross-linking them with glutaraldehyde into polymers of the size of MAF, however, the self-interaction could be reconstituted. Thus, the MAF-MAF association activity, which is needed for cell aggregation in sponges, seems to be based on multiple low affinity carbohydrate-carbohydrate interactions, which is different from most interactions mediated by adhesion molecules characterized so far.     

Macrophage-activating factor produced by a T cell hybridoma: physiochemical and biosynthetic resemblance to gamma-interferon

Biochemical and biosynthetic evidence has been obtained which indicates that the macrophage activating factor (MAF) produced by the murine T cell hybridoma clone 24/G1, which primes macrophages for nonspecific tumoricidal activity, is a form of gamma-interferon (IFN gamma). MAF and antiviral activities were generated by the clone in proportional amounts under a variety of culture conditions. Production of both activities showed identical dependence on cell density even when production precipitously ceased at cell concentrations greater than 1.2 X 10(6) cells/ml. MAF and antiviral activities displayed identical sensitivities to pH and temperature and were indistinguishable on the basis of binding to insolubilized polynucleotides. Dye-ligand chromatography of a stimulated hybridoma supernatant on a column of Matrex Gel Red A resulted in the 1500-fold purification and 100% recovery of the MAF activity. A qualitatively identical elution profile was also obtained for the antiviral activity; however, only 32% of the original activity was recovered. When subjected to gel filtration on a high performance liquid chromatography system (HPLC), the MAF and antiviral activities present in the Matrex Red pool displayed identical elution profiles and exhibited an apparent m.w. of 50,000. This technique resulted in another 10-fold purification and 50% recovery of the two activities. On HPLC chromatofocusing the MAF activity in the Matrex Red pool could be resolved into seven chromatographically distinct species yet could not be resolved from the antiviral activity on either a qualitative or quantitative basis. These results thus provide quantitative molecular evidence to support the concept that IFN gamma can act as a MAF.     

p21(Waf1/Cip1/Sdi1) prevents apoptosis as well as stimulates growth in cells transformed or immortalized by human T-cell leukemia virus type 1-encoded tax

Human T-cell leukemia virus type 1 (HTLV-1) Tax regulates the expression of virally encoded genes, as well as various endogenous host genes in trans. Tax-mediated regulation of gene expression is important for the immortalization of normal human T lymphocytes and the transformation of fibroblast cells, such as Rat-1 cells. Tax has the ability to transactivate p21(Waf1/Cip1/Sdi1), resulting in high expression levels in HTLV-1-immortalized cells. Since p21 expression is suppressed due to methylation of the promoter region in Rat-l cell line, p21 may not be critical for the transformation of this cell line by Tax. To further understand the role of p21 for the proliferation of Tax-transformed Rat-1 cells, we examined the effect of ectopic expression of p21 in these cells. Here, we observed that p21 expression enhanced the transformation of this cell line via at least two mechanisms: (i) the enhancement of NF-kappaB activation and/or CREB signaling and (ii) the excitation of antiapoptotic machinery. To analyze the role of p21 that is overexpressed in HTLV-1-immortalized lymphocytes, p21 expression was suppressed by using an antisense oligonucleotide specific for p21 mRNA; these cells then became sensitive to apoptotic induction. These results suggest that p21 plays an important role in the proliferation of Tax-expressing cells through the regulation of at least two independent mechanisms.     

Activation of human monocyte cytotoxicity by natural and recombinant immune interferon

Human T cell hybridomas were established by fusion of SH9 cells, the 6-thioguanine-resistant mutant line of human T lymphoma Hut 102-B2, with concanavalin A-stimulated human peripheral blood lymphocytes. Hybridoma line L38 produced a macrophage activating factor (MAF) with the ability to activate human peripheral blood monocytes to show enhanced cytotoxicity against human colon adenocarcinoma HT-29 cells in a 72-hr 125iododeoxyuridine-release assay. The L38 line was then cloned by the limiting dilution technique and two sublines, L38B and L38D, were found to produce high levels of MAF constitutively. Interferon activity was also detected in L38B and L38D supernatants. When interferon activity was neutralized with specific antiserum to purified human immune interferon (IFN-gamma), MAF activity was abrogated. To confirm that the MAF activity is indeed due to IFN-gamma, IFN-gamma was purified from the culture supernatant of another human T cell hybridoma, L265K2, a cell line known to produce high levels of IFN-gamma. Two highly purified IFN-gamma fractions with m.w. of 20,000 and 25,000, respectively, were obtained by NaDodSO4/polyacrylamide gel electrophoresis (SDS-PAGE). Similar fractions were obtained from IFN-gamma derived from human peripheral blood lymphocyte (PBL) cultures induced with 12-0-tetradecanoylphorbol-13-acetate (TPA) and phytohemagglutinin (PHA). In comparison, Escherichia coli-derived recombinant human IFN-gamma separated by SDS-PAGE yielded two major active fractions with m.w. of 17,000 and 34,000. With all three types of preparations, a close correlation was found between the presence of IFN-gamma activity demonstrable in an antiviral assay and MAF activity in individual fractions. Substantial quantitative differences were observed in the ability of various human IFN to activate monocytes. Although no MAF activity was detected with IFN-alpha and IFN-beta at concentrations up to 200 U/ml, both natural and recombinant IFN-gamma showed marked MAF activity at concentrations as low as 0.3 to 1 U/ml.     

Human T-cell lymphotropic virus type 1 (HTLV-1)-induced syncytium formation mediated by vascular cell adhesion molecule-1: evidence for involvement of cell adhesion molecules in HTLV-1 biology.

While studying the potential role of vascular cell adhesion molecule-1 (VCAM-1) in infection of endothelial cells by human immunodeficiency virus (HIV), we found that VCAM-1 can mediate human T-cell lymphotropic virus type 1 (HTLV-1)-induced syncytium formation. Both expression-vector-encoded and endogenously expressed VCAM-1 supported fusion of uninfected cells with HTLV-1-infected cells. Fusion was obtained with cell lines carrying the HTLV-1 genome and expressing viral proteins but not with an HTLV-1-transformed cell line that does not express viral proteins. In clones of VCAM-1-transfected cells, the degree of syncytium formation observed directly reflected the level of VCAM-1 expression. Syncytium formation between HTLV-1-expressing cells and VCAM-1+ cells could be blocked with antiserum against HTLV-1 gp46 and with a monoclonal antibody (MAb) against VCAM-1. Fusion was not blocked by antiserum against HIV or a MAb against VLA-4, the physiological counter-receptor for VCAM-1. The results indicate that VCAM-1 can serve as an accessory molecule or potential coreceptor for HTLV-1-induced cell fusion and provide direct evidence of a role for cell adhesion molecules in the biology of HTLV-1.