Control antibodies of the same isotypes did not exert any effect. live mycobacteria. BCG tradition supernatant, BCG lysate, or inert particles in combination with T-SN did not induce MGC formation. Experiments using transwell plates comprising a Gramine semipermeable membrane exposed that induction of the fusion process is dependent on direct contact of monocytes and mycobacteria. MGC formation induced by BCG plus T-SN could be inhibited by addition of monoclonal antibodies to gamma interferon (but not tumor necrosis element alpha) as well as to the chain (CD18) of 2-integrins. These results demonstrate that contact with mycobacteria in combination with cytokine-containing supernatants is able to induce human being monocytes to form Gramine MGC and that membrane-bound molecules of mycobacteria and monocytes are involved in the fusion process. Multinucleated huge cell (MGC) formation is definitely a common histopathologic feature of various granulomatous diseases (including tuberculosis, leprosy, schistosomiasis, and sarcoidosis) and of foreign body reactions. The presence of MGC within the tuberculous granuloma was first described in detail by Langhans in 1868 (27). MGC originate from fusion of monocytes, but Gramine the exact mechanism of their formation and the contribution of these cells to the pathogenesis of tuberculosis are still poorly recognized. MGC can be generated in vitro in quite different ways by stimulating cells of the monocyte/macrophage lineage with cytokines (13C16, 30, 31, 36, 37, 62), lectins (6, 57), conditioned press (1, 26, 39, 47, 52), or monoclonal antibodies (MAbs) (29, 43, 55). It is not clear which of these in vitro models reflects most precisely the generation of MGC in vivo. In particular, it is not known whether mycobacteria contribute directly to MGC formation of human being monocytes during a mycobacterial illness. Several studies with cells from different varieties reported an indirect effect of mycobacteria, i.e., induction of a soluble lymphocyte-derived fusion element following activation by mycobacteria or mycobacterial products (20, 46, 47, 61). In mice with pneumonia, however, MGC formation can occur independently of lymphocytes and their soluble products (22). As far as induction of MGC formation by mycobacteria is concerned, it was shown recently that swine microglia infected with or form MGC in vitro (45). To our knowledge, direct induction of MGC formation by mycobacteria in the human system has not been reported. It is remarkable that many authors who investigated the interactions of mycobacteria and human monocytes/macrophages do not DNAJC15 mention the occurrence of MGC. In most studies on MGC formation, macrophages (from humans or other species) were used. However, there is evidence that monocytes newly arriving at the site of contamination play a key role in MGC formation (5, 21, 35, 53). Furthermore, recent investigations by our group have shown that this in vitro fusion capacity of human monocytes following stimulation with cytokine-containing supernatants is usually gradually lost during monocyte-to-macrophage maturation (40). For this reason, we used human peripheral blood monocytes for our studies on the role of mycobacteria in MGC formation. The effects of cytokines and anti-cytokine MAbs on MGC formation have been investigated in many studies. Both in vivo and in vitro experiments suggest a role for gamma interferon (IFN-) and tumor necrosis factor alpha (TNF-) in MGC and granuloma formation, although results have been somewhat conflicting. Among cytokines inducing fusion, IFN- appears to play a central role. IFN- has been reported to induce MGC formation directly (16, 42, 62) and to enhance fusion rates induced by other stimuli (1, 15, 37, 57). Antibodies against IFN- inhibit MGC formation in vitro (17, 39) as well as in vivo (3, 9). In several other studies, anti-IFN- antibodies had no effect on MGC formation (1, 30, 36, 37, 57), and even prevention of fusion by IFN- was reported (56, 60). Peterson et al. found that TNF- contributes to mycobacterium-induced fusion of swine microglia (45). In contrast, TNF- did not induce MGC formation with murine (60) or human (15, 37, 39, 57) monocytes/macrophages. Antibodies to TNF- have been reported to inhibit the formation of MGC (23, 57) and of granulomas (10, 23, 25). However, in another study, anti-TNF- MAb had no effect on cell fusion (37). Since contact between cells is usually a prerequisite for fusion, surface molecules and especially adhesion molecules of cells undergoing fusion must be important for MGC formation. Inhibition of cell aggregation and/or fusion by antibodies against the and/or chain of leukocyte function-associated antigen 1 was found in various systems (16, 24, 30, 32, 39, 45, 55). Furthermore, Gramine it was demonstrated that this.