Up-regulation Of LFA-1 Research Articles

Galectin-9 supports primary T cell transendothelial migration in a glycan and integrin dependent manner

Adaptive immunity relies on the efficient recruitment of T cells from the blood into peripheral tissues. However, the current understanding of factor(s) coordinating these events is incomplete. Previous studies on galectin-9 (Gal-9), have proposed a functionally significant role for this lectin in mediating leukocyte adhesion and transmigration. However, very little is known about its function in T cell migration. Here, we have investigated the role of the Gal-9 on the migration behaviour of both human primary CD4+ and CD8+ T cells. Our data indicate that Gal-9 supports both CD4+ and CD8+ T cell adhesion and transmigration in a glycan dependent manner, inducing L-selectin shedding and upregulation of LFA-1 and CXCR4 expression. Additionally, when immobilized, Gal-9 promoted capture and firm adhesion of T cells under flow, in a glycan and integrin-dependent manner. Using an in vivo model, dorsal air pouch, we found that Gal-9 deficient mice display impaired leukocyte trafficking, with a reduction in pro-inflammatory cytokines/chemokines generated locally. Furthermore, we also demonstrate that Gal-9 inhibits the chemotactic function of CXCL12 through direct binding. In conclusion, our study characterises, for the first time, the capture, adhesion, and migration behaviour of CD4+ and CD8+ T cells to immobilised /endothelial presented Gal-9, under static and physiological flow conditions. We also demonstrate the differential binding characteristics of Gal-9 to T cell subtypes, which could be of potential therapeutic significance, particularly in the treatment of inflammatory-based diseases, given Gal-9 ability to promote apoptosis in pathogenic T cell subsets.

Immunosuppressive activity of cyclosporin A in vivo via NFAT-independent inhibition of TCR signaling

Abstract Calcineurin inhibitors (CNIs) such as cyclosporin A (CsA) and FK506 are widely used in treatment of autoimmune diseases and tissue transplant rejection. It is well known that CNIs inhibit NFAT-dependent gene expression, which is thought to be the major mechanism of their therapeutic effectiveness. We recently reported that CNIs inhibit TCR-proximal signaling and the upregulation of LFA-1/ICAM1 binding by preventing calcineurin’s dephosphorylation of Lck Ser-59 (LckS59), an inhibitory modification. This raises the possibility that CsA’s inhibition of TCR signaling may contribute to immunosuppression in clinical settings. Here we find that LckS59A (which cannot accept a phosphate at Ala59) knock-in T cells are resistant to CsA’s inhibition of TCR-proximal signaling. Correspondingly, whereas CsA prevented LckWT T cells from antigen-induced and LFA-1/ICAM-mediated binding to APCs in vitro, LckS59A T cells were resistant. Moreover, CsA was able to reverse existing LckWT T:APC interactions within minutes whereas LckS59A T:DC binding remained stable. To determine if disruption of T:DC binding occurs in vivo, we performed 2-photon imaging of lymph nodes from mice in which labeled antigen-specific T cells and antigen-pulsed DC had been injected. Antigen-specific T:DC formed as early as 3 hr after T cell injection. Notably, CsA markedly reduced the formation of antigen-specific LckWT T:DC but not LckS59A T:DC clusters in lymph nodes. Given that T:DC adhesion is an early and necessary step in the generation of T cell immunity, these data demonstrate a new and perhaps major immunosuppressive mechanism for CNIs immunosuppression.

Lentivirus-Mediated Gene Silencing of Lymphocyte Function-Associated Antigen 1 Inhibits Apoptosis of Hippocampal Neurons in Rats with Acute Cerebral Ischemia After Cerebral Lymphatic Blockage

Cerebral ischemia is considered to be the most common cause of stroke with high mortality. It occurs as a result of the damage of the hippocampal neurons with lymphocyte function-associated antigen (LFA)-1 being emphasized to play a role in the biological functions of hippocampal neurons. This study was conducted in order to investigate the effects of specific knockdown of LFA-1 expression by lentivirus had on the apoptosis of the hippocampal neurons, simulated by rat models of acute cerebral ischemia after cerebral lymphatic blockage. A total of 60 Wistar rats were selected as subjects, among which 50 were used to establish models of the acute cerebral ischemia after cerebral lymphatic blockage, while the remaining 10 rats were treated with the sham operation. The underlying regulatory mechanisms regarding LFA-1 were analyzed with the treatment of si-LFA-1 and LFA-1 vector in the hippocampal CA1 area of brain tissues isolated from the rats with acute cerebral ischemia. The brain water content, electrolyte content, and blood-brain barrier permeability located in ischemic area of rats were measured. TUNEL staining and immunochemistry methods were employed in order to determine the apoptosis rate and positive levels of LFA-1, MMP-9, and Caspase-3. The mRNA and protein levels of related genes were also detected by means of RT-qPCR and western blot assay. The brain water content, Na+ and Ca+ contents, blood-brain barrier permeability, apoptosis rate, positive levels of LFA-1, MMP-9, and Caspase-3 were decreased, and the K+ content was increased in ischemic tissues treated with si-LFA-1. The mRNA and protein levels of LFA-1, MMP-9, Caspase-3, and Bax had all decreased, while the mRNA and protein levels of Bcl-2 were elevated in the hippocampal CA1 area of rat brain tissues treated with si-LFA-1. These situations could be reversed through the up-regulation of LFA-1. In conclusion, LFA-1 gene silencing could improve the acute cerebral ischemia after cerebral lymphatic blockage by inhibiting apoptosis of the hippocampal neurons in rats.

Changes in clinical laboratory parameters and pharmacodynamic markers in response to blinatumomab treatment of patients with relapsed/refractory ALL

BackgroundBlinatumomab has shown a remission rate of 69% in an exploratory single-arm, phase II dose-escalation study in adult patients with relapsed/refractory B-precursor acute lymphoblastic leukemia (ALL). We evaluated changes in laboratory parameters and immunopharmacodynamic markers in patients who received blinatumomab in the exploratory phase II study.MethodsData from 36 adults with relapsed/refractory ALL receiving blinatumomab as 4-week continuous IV infusions in various dose cohorts were analyzed for changes in liver enzymes, first-dose parameters, peripheral blood cell subpopulations, and cytokine/granzyme B release. Associations with clinical response were evaluated.ResultsLiver enzymes and inflammatory parameters transiently increased primarily during the first treatment week without clinical symptoms and reversed to baseline levels thereafter. B and T cells showed expected depletion and redistribution kinetics, respectively. Similarly, thrombocytes and T cells displayed an initial decline in cell counts, whereas neutrophils peaked during the first days after infusion start. T-cell redistribution coincided with upregulation of LFA-1 and CD69. Patients who responded to blinatumomab had more pronounced T-cell expansion, which was associated with proliferation of CD4+ and CD8+ T cells and memory subsets. Release of cytokines and granzyme B primarily occurred during the first week of cycle 1, except for IL-10, which was released in subsequent cycles. Blinatumomab step-dosing was associated with lower cytokine release and lower body temperature.ConclusionsIn this study of relapsed/refractory ALL, blinatumomab-induced changes in laboratory parameters were transient and reversible. The evaluated PD markers demonstrated blinatumomab activity, and the analysis of cytokines supported the rationale for stepwise dosing.(ClinicalTrials.gov Identifier NCT01209286.)

Up-regulation of LFA-1 allows liver-resident memory T cells to patrol and remain in the hepatic sinusoids.

Liver-resident CD8+ T cells are highly motile cells that patrol the vasculature and provide protection against liver pathogens. A key question is: how can these liver CD8+ T cells be simultaneously present in the circulation and tissue-resident? Because liver-resident T cells do not express CD103 - a key integrin for T cell residence in epithelial tissues - we investigated other candidate adhesion molecules. Using intra-vital imaging we found that CD8+ T cell patrolling in the hepatic sinusoids is dependent upon LFA-1-ICAM-1 interactions. Interestingly, liver-resident CD8+ T cells up-regulate LFA-1 compared to effector-memory cells, presumably to facilitate this behavior. Finally, we found that LFA-1 deficient CD8+ T cells failed to form substantial liver-resident memory populations following Plasmodium or LCMV immunization. Collectively, our results demonstrate that it is adhesion through LFA-1 that allows liver-resident memory CD8+ T cells to patrol and remain in the hepatic sinusoids.

Glatiramer acetate attenuates the pro-migratory profile of adhesion molecules on various immune cell subsets in multiple sclerosis

An altered expression pattern of adhesion molecules (AM) on the surface of immune cells is a premise for their extravasation into the central nervous system (CNS) and the formation of acute brain lesions in multiple sclerosis (MS). We evaluated the impact of glatiramer acetate (GA) on cell-bound and soluble AM in the peripheral blood of patients with relapsing-remitting MS (RRMS). Fifteen patients treated de novo with GA were studied on four occasions over a period of 12 months. Surface levels of intracellular cell adhesion molecule (ICAM)-1, ICAM-3, lymphocyte function-associated antigen (LFA)-1 and very late activation antigen (VLA)-4 were assessed in T cells (CD3(+) CD8(+) , CD3(+) CD4(+) ), B cells, natural killer (NK) cells, natural killer T cells (NK T) and monocytes by five-colour flow cytometry. Soluble E-selectin, ICAM-1, ICAM-3, platelet endothelial cell adhesion molecule (PECAM)-1, P-selectin and vascular cell adhesion molecule (VCAM)-1 were determined with a fluorescent bead-based immunoassay. The pro-migratory pattern in RRMS was verified by comparison with healthy controls and was characterized by up-regulation of LFA-1 (CD3(+) CD4(+) T cells, B cells), VLA-4 (CD3(+) CD8(+) T cells, NK cells), ICAM-1 (B cells) and ICAM-3 (NK cells). Effects of GA treatment were most pronounced after 6 months and included attenuated levels of LFA-1 (CD3(+) CD4(+) ) and VLA-4 (CD3(+) CD4(+) , CD3(+) CD8(+) , NK, NK T, monocytes). Further effects included lowering of ICAM-1 and ICAM-3 levels in almost all immune cell subsets. Soluble AM levels in RRMS did not differ from healthy controls and remained unaltered after GA treatment. The deregulated pro-migratory expression profile of cell-bound AM is altered by GA treatment. While this alteration may contribute to the beneficial action of the drug, the protracted development and unselective changes indicate more secondary immune regulatory phenomena related to these effects.

Effect of hyperketonemia on chemokine secretion, adhesion molecule expression, and monocyte adhesion to endothelial cells

Frequent episodes of hyperketonemia are associated with higher incidences of vascular disease. We examined the hypothesis that hyperketonemia increases monocyte adhesion to endothelial cells (EC). Human U937 monocytes or THP‐1 monocytes and HUVECs were cultured with AA (0–10 mM) or BHB (0–10 mM) for 24 hours. Results ( ± SE) show that there was significant increase in IL‐8 and MCP‐1 secretion in AA treated THP‐1 and U937 monocytes and HUVECs, however, BHB had no effect. There was a 10.5% ± 0.04 (ns) increase in protein levels of LFA‐1 in 4 mM AA treated U937 monocytes compared to control and a 13% ± 4.3 (p<0.04) increase in surface expression. There was a 21% ± 3.5 (p<0.01) increase in CD14 surface expression on U937 and a 3.9% ± 1.8 (ns) in THP‐1 treated with 4 mM AA compared to control whereas BHB had no effect. Surface ICAM‐1 increased by 23.8% ± 0.02 (p<0.05) and total protein expression by 40% in HUVECs treated with 4 mM AA compared to the control. Results showed a 33.7% ± 2.3 (p<0.01) increase in U937 adhesion and a 23.5% ± 3.8 (p<0.05) increase in THP‐1 adhesion to HUVEC monolayer's treated with 4 mM AA compared to the control. Monocyte adhesion could be blocked by 14% ± 3.6 (p<0.04) when HUVEC's were pre‐incubated with ICAM‐1 antibody. Thus, hyperketonemia increases monocyte adhesion to EC's by upregulation of LFA‐1, CD‐14 and ICAM‐1, which may contribute to the excess vascular disease in diabetes.

Phenotypic analysis of cells in bronchoalveolar lavage fluid and peripheral blood of maedi visna-infected sheep.

A phenotypic analysis of bronchoalveolar lavage fluid (BALF) and peripheral blood (PB) cells in maedi visna virus (MVV)-infected sheep has been performed. The differential cell count in BALF from MVV-infected animals was characterized by a significant increase (P < 0.05) in lymphocytes and neutrophils. Lymphocyte phenotyping in BALF from MVV-infected sheep showed a significant decrease (P < 0.05) of CD4+ cells, a significant increase (P < 0.05) of CD8+ cells and a significant inversion (P < 0.001) of the CD4+/CD8+ ratio. CD5+ lymphocytes were also significantly decreased (P < 0.05). Gamma delta T cells and B cells did not differ significantly when compared with the controls. No correlation was observed between BALF and PB lymphocyte phenotypes. BALF macrophages from MVV-infected animals showed increased MHC class II expression and BALF lymphocytes from the same animals demonstrated up-regulation of LFA-1 and LFA-3 expression. These findings and their relationship with lentiviral pathogenesis are discussed.

Engagement of ICAM-1 by major group rhinoviruses activates the LFA-1/ICAM-3 cell adhesion pathway in mononuclear phagocytes

Cell–cell interactions are critical at key points of immune responses and are mediated by a complex array of adhesion receptors. One of the most important adhesion molecules on leukocytes is intercellular adhesion molecule 1 (ICAM-1, CD54). Here we demonstrate that engagement of ICAM-1 with human major group rhinoviruses (HRV) enhances adhesiveness and homotypic aggregation of human monocytes and monocyte-derived dendritic cells (DC). Cluster formation upon engagement of ICAM-1 with HRV14 represents an active process. It is temperature and energy dependent, requires divalent cations, an intact cytoskeleton and protein de novo synthesis. Homotypic interaction between monocytes induced by HRV14 can be inhibited with blocking mAbs against LFA-1 (CD11a/CD18) and ICAM-3 (CD50) as well as with a mAb against the first immunoglobulin (Ig)-domain of PECAM-1 (CD31). Induction of enhanced cytoadhesiveness by HRV14 was not accompanied with an upregulation of LFA-1, ICAM-3 or PECAM-1 expression. Binding studies with recombinant PECAM-1 proteins indicated, however, that monocyte clustering upon engagement of ICAM-1 with HRV was accompanied with increased homophilic PECAM-1 interactions. Taken together the results of our study demonstrate that signalling via ICAM-1 induces adhesiveness of mononuclear phagocytes, which critically involves PECAM-1 and is mediated via LFA-1/ICAM-3.

A small-molecule antagonist of LFA-1 blocks a conformational change important for LFA-1 function

Lymphocyte function-associated antigen (LFA)-1/intercellular adhesion molecule (ICAM)-1 interactions mediate several important steps in the evolution of an immune response. LFA-1 is normally expressed in a quiescent state on the surface of leukocytes and interacts weakly with its ligands ICAM-1, -2, and -3. LFA-1 activity may be regulated by receptor clustering and by increasing the affinity of LFA-1 for its ligands. Affinity modulation of LFA-1 has been shown to occur via a conformational change in the LFA-1 heterodimer that can be detected by using monoclonal antibody 24 (mAb24). We have recently described a small-molecule antagonist of LFA-1, BIRT 377, that demonstrates selective in vitro and in vivo inhibition of LFA-1/ICAM-1-mediated binding events. We now demonstrate that BIRT 377 blocks the induction of the mAb24 reporter epitope on LFA-1 on the surface of SKW-3 cells treated with various agonists known to induce high-affinity LFA-1. These data imply that BIRT 377 exerts its inhibitory effects by preventing up-regulation of LFA-1 to its high-affinity conformation.

Workshop E: Cell Traffic, Adhesion, Chemokines, Abstract E1 bis E25

SummaryE. 1 Induction of apoptosis coincides with expression of the chemoattractant MIP-2 in mast cellsE. 2 Stage-dependent expression of interleukin-16 and lymphotactin in distinct clinical stages of mycosis fungoidesE. 3 Differences in the fate of effector T cells in vivo: CD4+ T cells mainly proliferate within the lymphoid tissue while CD8+ T cells preferentially migrate throughout the bodyE. 4 Activation of NF-κB via the IκB kinase complex is both essential and sufficient for proinflammatory gene expression and adhesion of monocytes to primary endothelial cellsE. 5 Intradermal application of bone marrow derived dendritic cells «instructs» CD8+ T cells for skin homingE. 6 Interleukin-3 induces downregulation of CC chemokine receptor 3 protein and mRNA in human eosinophilsE. 7 Junctional adhesion molecule interacts with the tight junction protein ASIP/PAR-3E. 8 Upregulation of LFA-1 by CD44 variant isoform expressionE. 9 MCP-1 mRNA expression in bronchoalveolar cells of patients with sarcoidosisE. 10 Chemokine receptor expression on γδ T cells versus αβ T cellsE. 11 Expression of adhesion molecules LFA-1 and L-selectin on neutrophils of patients with lung cancerE. 12 Independent and cooperative signals induced by platelet factor 4 in human neutrophilsE. 13 Antigen-dependent migration of naïve CD4+ T cells into the liverE. 14 E-cadherin-mediated interactions in the human thymusE. 15 Fractalkine is expressed by smooth muscle cells in response to IFN-γ and TNF-α and modulated by metalloproteinase activityE. 16 Identification of bone marrow-derived neutrophils as carriers of deposited MIP-2E. 17 The HCMV encoded chemokine receptor US28 is constitutively phosphorylated and mediates activation of multiple MAP kinasesE. 18 Th1 and Th2 cells, differentiated both in vitro and in vivo, exhibit a distinct chemokine secretion patternE. 19 FTY720, a novel immunosuppressant, rescues lymphocyte homing in plt and CCR7-deficient miceE. 20 Leukocyte migration during pediatric cardiac surgery with cardiopulmonary bypass: in vitro measurement by flow and laser scanning cytometryE. 21 Influenza A virus infection of monocytes abrogates the functional responsiveness to chemokinesE. 22 Platelet-derived CXC-chemokines PF-4 and CTAP-III modulate transendothelial migration of neutrophilsE. 23 Cleavage of the CXC chemokine I-TAC by dipeptidyl peptidase IV (CD26) expressed on activated T cells inactivates the chemotactic but not the desensitizing potential of the chemokineE. 24 The CXC-chemokine NAP-2 mediates differential heterologous desensitization of neutrophil effector functions elicited by platelet activating factorE. 25 Epithelial IL-8 release in response to Th1/Th2 cytokines

Requirement for Nuclear Factor-κB Activation by a Distinct Subset of CD40-Mediated Effector Functions in B Lymphocytes

Abstract CD40 stimulation, which is crucial for generating an effective T-dependent humoral response, leads to the activation of transcription factors NF-AT (nuclear factor of activated T cells), AP-1 (activator protein-1), and NF-κB (nuclear factor-κB). However, which CD40-mediated B cell functions actually require activation of specific transcription factors is unknown. We examined the causal relationship between NF-κB activation and CD40 effector functions by evaluating CD40 functions in the presence of an inducible mutant inhibitory κBα (IκBα) superrepressor. IκBαAA inhibited nuclear translocation of multiple NF-κB dimers without the complicating effect of depriving cells of NF-κB during development. This approach complements studies that use mice genetically deficient in single or multiple NF-κB subunits. Interestingly, only a subset of CD40 effector functions was found to require NF-κB activation. Both CD40-induced Ab secretion and B7-1 up-regulation were completely abrogated by expression of IκBαAA. Surprisingly, up-regulation of Fas, CD23, and ICAM-1 was partially independent, and up-regulation of LFA-1 was completely independent, of CD40-induced NF-κB activation. For the first time, it is clear that distinct transcription factors are required for the dynamic regulation of CD40 functions.

Expression of beta-2-integrins and L-selectin by leukocytes and changes in acute-phase reactants in total hip replacement surgery.

The aim of this study was to characterize the changes in the quantitative expression of beta 2-integrins and L-selectin detected by means of fluorochrome-conjugated monoclonal antibodies and flow cytometry on leukocytes in the systemic circulation after a major musculoskeletal trauma, i.e. hip replacement surgery, and to relate these changes to parameters of the acute-phase response [plasma acute-phase reactants (C-reactive protein, CRP, and interleukin-6, IL-6) and parameters of coagulation activation (thrombin-antithrombin III complexes, TAT)]. Eight patients with either primary or secondary osteoarthritis of the hip received uncemented total hip prostheses. LFA-1 (CD11a/CD18) was upregulated on granulocytes during the operation. MAC-1 (CD11b/CD18) expression on monocytes increased to peak levels 20 h after surgery, whereas the L-selectin (CD62L) expression on monocytes and granulocytes reached peak values at the end of surgery. The changes in expression of LFA-1 on monocytes, MAC-1 on granulocytes and p150,95 (CD11c/CD18) on monocytes and granulocytes during and after the operation did not reach statistical significance. TAT and IL-6 increased during surgery and reached peak values at the end of the operation and 20 h after surgery, respectively. In contrast, CPR concentrations increased after surgery with peak levels 44 h postoperatively. Significant upregulation of LFA-1 on granulocytes and L-selectin on monocytes and granulocytes preceded the increase in IL-6 which again preceded the increase in CRP. However, the up- or downregulation of leukocyte beta 2-integrins and L-selectin during and after surgery was not significantly correlated with the increase in IL-6. The increases in TAT correlated well with the upregulation of L-selectin on monocytes, but not with the beta 2-integrins known to participate in the coagulation process in vitro. The rise in CRP was inversely correlated with the maximal increase in expression of MAC-1 on monocytes. In conclusion, the changes in leukocyte adhesion molecules during and after surgery indicate changes in critical leukocyte functions. The lack of correlation between quantitative up- and downregulation of leukocyte beta 2-integrins and parameters of the acute phase response suggests that these processes are regulated through independent pathways or that functional up- and downregulation of adhesion molecules, shedding, leukocyte-endothelial adhesion and mobilization of new unactivated cells may result in a net estimate of leukocyte activation not suspected to be positively correlated to acute-phase reactants.

Expression of CD11a and CD45R isoforms defines distinct subsets of CD8+ TCR alpha beta and TCR gamma delta CTL in vivo.

Intraperitoneal immunization of rats with a syngeneic lymphoma and allogeneic leukocytes induced enrichment of antigen-selective TCR alpha beta+ and TCR gamma delta+ CTL. The peritoneal cavity seems to be a suitable site for enrichment of antigen-selective CTL, since the induced effector cells executed strong cytotoxicity without any requirement for in vitro reactivation. Tumor-selective CTL expressed high cell surface levels of CD45RC, allogeneic CTL expressed a variable level of CD45RC, while SAg-reactive CTL had low CD45RC expression. CTL with tumor and allogeneic selectivity as well as SAg-induced CTL all expressed the LFA-1high phenotype, suggesting that upregulation of LFA-1 is a hallmark for in vivo activated CTL. RT-PCR analyses showed that all CD8+TCR alpha beta+ CTL lost expression of CD45R exon 4 mRNA, which is compatible with the view that effector/memory T cells are CD45RA-. In contrast, TCR gamma delta+ CTL retained the CD45RA phenotype but showed a down-regulation of CD45R exon 8 mRNA. Since isoforms of the CD45 tyrosine phosphatase have been reported to differentially affect T-cell activation, the unique CD45R splice pattern observed in TCR alpha beta+ and TCR gamma delta+ CTL implies that CD45R may influence the TCR signal transduction distinctly in various effector CTL subsets.

Cytokine‐induced generation of multinucleated giant cells in vitro requires interferon‐γ and expression of LFA‐1

Multinucleated giant cells (MGC), which are a common feature of various pathologic states, were generated in vitro by cytokine-stimulation of human peripheral blood monocytes. As expected, conditioned medium, i.e. the supernatant of concanavalin A-stimulated peripheral blood mononuclear cells, readily caused generation of MGC. Addition of a monoclonal antibody (mAb) against interferon-gamma (IFN-gamma) completely abrogated this effect. IFN-gamma alone, however, had a much smaller effect than the conditioned medium. All other cytokines tested [including interleukin (IL)2, IL4 and tumor necrosis factor-alpha, which are known to activate monocytes] did not induce MGC nor did they enhance the effect of IFN-gamma. Formation of MGC could almost entirely be inhibited by mAb to the alpha or beta chain of LFA-1 and to a lesser extent by relatively high concentrations of a mAb against ICAM-1, one of the ligands of LFA-1. In contrast to the anti-IFN-gamma mAb that had no significant effect on the formation of monocyte clusters, mAb against LFA-1 inhibited clustering very efficiently. Antibodies directed to a number of different antigens present on the surface of monocytes (alpha chains of CR3 and CR4, HLA class I and II molecules, CD14 and CD16 antigens) had little or no effect on the generation of MGC. IFN-gamma, but not the concanavalin A-induced supernatant clearly enhanced expression of LFA-1 and ICAM-1 on monocytes. The results indicate that cytokine-induced generation of MGC is not possible without IFN-gamma, but most probably additional factor(s) enhance this effect. The mechanism(s) by which IFN-gamma promotes monocyte fusion apparently includes, among others, up-regulation of LFA-1 whose expression seems to be necessary but not sufficient for fusion.