Macrophage-melanocyte heterokaryons. I. Preparation and properties

Cell fusion dynamics: mechanisms of multinucleation in osteoclasts and macrophages

Cell-cell fusion is a vital biological process where the membranes of two or more cells merge to form a syncytium. This phenomenon is critical in various physiological and pathological contexts, including embryonic development, tissue repair, immune responses, and the progression of several diseases. Osteoclasts, which are cells from the monocyte/macrophage lineage responsible for bone resorption, have enhanced functionality due to cell fusion. Additionally, other multinucleated giant cells (MGCs) also arise from the fusion of monocytes and macrophages, typically during chronic inflammation and reactions to foreign materials such as prostheses or medical devices. Foreign body giant cells (FBGCs) and Langhans giant cells (LGCs) emerge only under pathological conditions and are involved in phagocytosis, antigen presentation, and the secretion of inflammatory mediators. This review provides a comprehensive overview of the mechanisms underlying the formation of multinucleated cells, with a particular emphasis on macrophages and osteoclasts. Elucidating the intracellular structures, signaling cascades, and fusion-mediating proteins involved in cell-cell fusion enhances our understanding of this fundamental biological process and helps identify potential therapeutic targets for disorders mediated by cell fusion.

Multinucleation resets human macrophages for specialized functions at the expense of their identity

Macrophages undergo plasma membrane fusion and cell multinucleation to form multinucleated giant cells (MGCs) such as osteoclasts in bone, Langhans giant cells (LGCs) as part of granulomas or foreign-body giant cells (FBGCs) in reaction to exogenous material. How multinucleation per se contributes to functional specialization of mature mononuclear macrophages remains poorly understood in humans. Here, we integrate comparative transcriptomics with functional assays in purified mature mononuclear and multinucleated human osteoclasts, LGCs and FBGCs. Strikingly, in all three types of MGCs, multinucleation causes a pronounced downregulation of macrophage identity. We show enhanced lysosome-mediated intracellular iron homeostasis promoting MGC formation. The transition from mononuclear to multinuclear state is accompanied by cell specialization specific to each polykaryon. Enhanced phagocytic and mitochondrial function associate with FBGCs and osteoclasts, respectively. Moreover, human LGCs preferentially express B7-H3 (CD276) and can form granuloma-like clusters in vitro, suggesting that their multinucleation potentiates T cell activation. These findings demonstrate how cell-cell fusion and multinucleation reset human macrophage identity as part of an advanced maturation step that confers MGC-specific functionality.

The historical milestones in the understanding of leukocyte biology initiated by Elie Metchnikoff

Progress in science is made with key discoveries, correct analyses, wrong statements, and disputes within the scientific community. Despite scientific controversies, Elie Metchnikoff has allowed the theory of phagocytes to triumph. Starting his career as a zoologist, Metchnikoff became a pathologist, beautifully defining the role of monocytes, macrophages, and neutrophils during inflammation and innate immunity. The discoveries of immune cells were made by other outstandings scientists, such as Paul Ehrlich, whose key contributions to humoral immunity led him to share the Nobel Prize with Metchnikoff. Ludwig Aschoff grouped certain cells under the term RES, according to their propensity for absorbing and storing vital stains. This classification was not always a source of accurate discoveries, and research on the exact function of RES cells led to some wanderings. This is illustrated by studies about the nature of the antibody-producing cells, which were first thought to belong to the RES, before being identified as plasmocytes and lymphocytes.

Structural properties of the proton translocating complex of the clathrin-coated vesicle

The clathrin-coated vesicle proton pump is a representative member of the new class of endomembrane proton ATPases that share an inhibitor profile which distinguishes them from classic F1F0 and E1E2-type proton pumps. The coated vesicle proton pump is a large (530 kDa) heteroligomer composed of eight polypeptides with molecular masses of 116, 70, 58, 40, 38, 34, 33 and 17 kDa. The 200-fold purified enzyme catalyses ATP-generated proton pumping when reconstituted in liposomes composed of pure lipids. Subunit function has been determined by partial reaction analysis of subunit and subcomplex activities. The isolated 17 kDa subunit, when co-reconstituted with bacteriorhodopsin, forms a dicyclohexylcarbodiimide-inhibitable proton channel. Selective removal of the 116 kDa subunit transforms the proton ATPase from a Mg2+-activatable to a Ca2+-activatable ATPase. Subsequent dissociation and reconstitution of subunits reveals that the 70, 58, 40 and 33 kDa components are required, in composite, to form a functional ATP-hydrolytic core, and that no single subunit or subcomplex deficient in these subunits can catalyse ATP hydrolysis.

Macrophage fusion induced by IL-4 alternative activation is a multistage process involving multiple target molecules

Multinucleated giant cells, characteristic of granulomatous infections, originate from fusion of macrophages, however, little is known about the underlying mechanism. Alternative activation of macrophages by exposure to IL-4 and IL-13 induces macrophage homokaryon formation. We have established a new quantitative bifluorescent system to study IL-4-induced fusion of primary murine macrophages in vitro. Using this assay, we could show that macrophage fusion is not mediated by a single molecule, but involves multiple functional components. Although several murine macrophage populations were not competent to form giant cells, indicating that they fail to display the full fusion machinery, these non-fusogenic macrophages could fuse with fusion-competent macrophages in a heterophilic manner. Since IL-4 induced molecules were needed on both fusion partners, we conclude that at least two functionally distinct molecules mediate macrophage homokaryon formation with each present on one fusion partner. In addition, though IL-4 treatment led to induction of a fusogenic status, macrophages could only fuse efficiently when adherent to a permissive substratum. Based on our findings, we conclude that macrophage fusion is a multistage process involving multiple target molecules. The model we describe will allow analysis of the molecular basis of membrane fusion and possible insight into alternative activation of macrophages.

Adenylate kinase as a virulence factor of Pseudomonas aeruginosa

Adenylate kinase (AK; ATP:AMP phosphotransferase, EC 2.7.4.3) is a ubiquitous enzyme that contributes to the homeostasis of adenine nucleotides in eukaryotic and prokaryotic cells. AK catalyzes the reversible reaction Mg. ATP + AMP <--> Mg. ADP + ADP. In this study we show that AK secreted by the pathogenic strains of Pseudomonas aeruginosa appears to play an important role in macrophage cell death. We purified and characterized AK from the growth medium of a cystic fibrosis isolate strain of P. aeruginosa 8821 and hyperproduced it as a fusion protein with glutathione S-transferase. We demonstrated enhanced macrophage cell death in the presence of both the secreted and recombinant purified AK and its substrates AMP plus ATP or ADP. These data suggested that AK converts its substrates to a mixture of AMP, ADP, and ATP, which are potentially more cytotoxic than ATP alone. In addition, we observed increased macrophage killing in the presence of AK and ATP alone. Since the presence of ATPase activity on the macrophages was confirmed in the present work, external macrophage-effluxed ATP is converted to ADP, which in turn can be transformed by AK into a cytotoxic mixture of three adenine nucleotides. Evidence is presented in this study that secreted AK was detected in macrophages during infection with P. aeruginosa. Thus, the possible role of secreted AK as a virulence factor is in producing and keeping an intact pool of toxic mixtures of AMP, ADP, and ATP, which allows P. aeruginosa to exert its full virulence.

The macrophage

The macrophage plays an important role in host development and physiology, and in pathogenesis of many infectious, immunologic and degenerative disease processes. It displays marked heterogeneity of phenotype in different tissues, reflecting local interactions with other cell types, and contributes to host homeostasis through a varied repertoire of plasma membrane and secretory molecules. Upon isolation from the body it continues to express special, as well as general, features of cellular organisation and function, which make it a delight to study in cell culture.

Zanvil Alexander Cohn 1926-1993

Zanvil Alexander Cohn, an editor of this Journal since 1973, died suddenly on June 28, 1993. Cohn is best known as the father of the current era of macrophage biology. Many of his scientific accomplishments are recounted here, beginning with seminal studies on the granules of phagocytes that were performed with his close colleague and former editor of this Journal, James Hirsch. Cohn and Hirsch identified the granules as lysosomes that discharged their contents of digestive enzymes into vacuoles containing phagocytosed microbes. These findings were part of the formative era of cell biology and initiated the modern study of endocytosis and cell-mediated resistance to infection. Cohn further explored the endocytic apparatus in pioneering studies of the mouse peritoneal macrophage in culture. He described vesicular inputs from the cell surface and Golgi apparatus and documented the thoroughness of substrate digestion within lysosomal vacuoles that would only permit the egress of monosaccharides and amino acids. These discoveries created a vigorous environment for graduate students, postdoctoral fellows, and junior and visiting faculty. Some of the major findings that emerged from Cohn's collaborations included the radioiodination of the plasma membrane for studies of composition and turnover; membrane recycling during endocytosis; the origin of the mononuclear phagocyte system in situ; the discovery of the dendritic cell system of antigen-presenting cells; the macrophage as a secretory cell, including the release of proteases and large amounts of prostaglandins and leukotrienes; several defined parameters of macrophage activation, especially the ability of T cell-derived lymphokines to enhance killing of tumor cells and intracellular protozoa; the granule discharge mechanism whereby cytotoxic lymphocytes release the pore-forming protein perforin; the signaling of macrophages via myristoylated substrates of protein kinase C; and a tissue culture model in which monocytes emigrate across tight endothelial junctions. In 1983, Cohn turned to a long-standing goal of exploring host resistance directly in humans. He studied leprosy, focusing on the disease site, the parasitized macrophages of the skin. He injected recombinant lymphokines into the skin and found that these molecules elicited several cell-mediated responses. Seeing this potential to enhance host defense in patients, Cohn was extending his clinical studies to AIDS and tuberculosis. Zanvil Cohn was a consummate physician-scientist who nurtured the relationship between cell biology and infectious disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Extracellular ATP stimulates poly(inositol phospholipid) hydrolysis and eicosanoid synthesis in mouse peritoneal macrophages in culture

The effects of extracellular ATP on inositol phospholipid breakdown and synthesis of eicosanoids were studied in mouse peritoneal macrophages. Addition of ATP to intact cells labelled with [3H]inositol stimulated a rapid (within 10 s) formation of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate. In parallel there was also a substantial accumulation of inositol 1,3,4-trisphosphate and the monophosphate and bisphosphate derivatives of inositol. Within 10 s after the addition of 30 microM ATP there was a twofold increase in inositol trisphosphate (InsP3), which declined over 2 min. The ED50 for ATP-stimulated generation of InsP3 was approximately 12 microM. ADP and GTP showed only weak effects on InsP3 formation, while AMP and adenosine were completely ineffective at 30 microM. Furthermore, the rank order of potency of ATP analogues was ATP greater than ATP[S] greater than AdoPP[NH]P = AdoPP[CH2]P greater than AdoP[CH2]PP thus, indicating the presence of a P2y-purinergic receptor. Cells labelled with [3H]arachidonic acid showed a 50% increase of label in 1,2-diacylglycerol after 15 s upon stimulation with ATP. In parallel to the stimulation of inositol phospholipid hydrolysis, ATP also caused a marked synthesis of prostaglandin E2 (PGE2) and leukotriene C4 (LTC4) in mouse peritoneal macrophages. The rank order of potency of ATP analogues was identical with that of InsP3 generation. The effect on eicosanoid synthesis could be mimicked by the calcium ionophore A23187 and the phorbol ester 12-O-tetradecanoylphorbol 13-acetate. These results suggest that ATP-induced activation of P2y-purinergic receptors in mouse peritoneal macrophages triggers inositol phospholipid breakdown and eicosanoid synthesis.

Time course of arrest of immunoglobulin expression in heterokaryons and early hybrids of human lymphoma cells and mouse fibroblasts. A study of transcriptional and translational events

Early events in arrest of immunoglobulin expression were investigated at the levels of both translation and transcription in heterokaryons and early hybrids between human Daudi lymphoma cells and mouse cl. 1D cells. Large populations of 1s: 1s hybrids, isolated by fluorescence-activated cell sorting (FACS) a few hours after fusion, were grown for up to 5 days. A survey at the light-microscopical level of peroxidase-antiperoxidase-immunostained cell populations showed that arrest of expression of IgM heavy chain (mu) occurred in up to 98% of the cells. Furthermore, quantitation of mu chain contents, by using an ELISA technique, suggested that synthesis of IgM was blocked shortly after fusion. The levels of cytoplasmic mRNA specific for mu and kappa chains, respectively, decreased at rates similar to those induced in unfused Daudi cells by treatment with actinomycin D. It is concluded that arrest of immunoglobulin expression in these hybrids occurs immediately or very shortly after fusion by mechanisms that affect the levels of their cytoplasmic mRNAs.

Immunofluorescence analysis of reexpression and activation: the origin of phenotypic diversity of rat hepatoma-mouse fibroblast hybrid colonies

The capacity of young hybrid colonies between 2s rat hepatoma cells and mouse L-fibroblasts to reexpress rat albumin and become activated for mouse albumin production, was examined at the level of individual cells using immunofluorescent staining of intracellular albumin. Most of the colonies that happened to reexpress albumin were observed to present a stable, homogeneous, hepatoma-like morphology, visible long before the first signs of albumin reexpression. These colonies switch from an extinct to an albumin-producing state between one and four weeks after fusion. Colonies that do not express albumin, present an epithelial or fibroblastic morphology. Karyologic analysis of hybrid clones representative of the various morphologic types revealed that their phenotypic diversity is correlated with their rat over mouse chromosome ratio. The results suggest that the potential of hybrid colonies to express albumin is determined at the time of nuclear fusion in the heterokaryons, possibly by the number of parental genomes participating in the formation of the mother hybrid cell. Double immunofluorescent staining of rat and mouse albumin in the same cell has been used to determine whether reexpression and activation are correlated phenomena or appear independently in any single cell. The analysis demonstrates that activation of mouse albumin never takes place without reexpression of the previously expressed rat albumin gene, while the converse is frequent.

Functional and biochemical studies of multinucleated giant cells derived from the culture of human monocytes

We compared phagocytic and metabolic activities of multinucleated giant cells (MGC) and macrophages derived from human monocytes after 9-14 d in culture. Phagocytosis of sheep erythrocytes (E) coated with IgG, of E coated with IgM and complement, and of Candida albicans was comparable in MGC and macrophages. The same percentage of ingested fungi was killed by MGC (24 +/- 4%) and macrophages (21 +/- 5%). Approximately 70% of MGC and macrophages exhibited superoxide-dependent reduction of nitroblue tetrazolium during stimulation. Ia antigen was present on approximately 75% of both cell types. Analysis of cell populations separated by nuclear fluorescence indicated that beta-glucosaminidase, acid phosphatase, and beta-glucuronidase activity per cell was higher in MGC, but specific activity of these enzymes was greater in macrophages. These results suggest that MGC have the capacity to function like macrophages in host defense against infection.

Extinction of muscle-specific properties in somatic cell heterokaryons

In studies of gene regulation using somatic cell fusion techniques, the analysis of heterokaryons circumvents several problematic aspects of the more traditional approach utilizing proliferating hybrid cells. We have analyzed the expression of muscle specific properties in heterokaryons between muscle and nonmuscle cells in order to investigate whether differentiating cells contain regulatory factors that repress the expression of alternative developmental pathways. Heterokaryons and cybrids were derived from polyethylene glycol-mediated fusion of differentiated mononucleate chicken myocytes with mouse melanoma cells, mouse melanoma cytoplasts, chicken fibroblasts, or other chicken myocytes. Our results demonstrate that fusion of a myocyte with a nonmyogenic cell generally results in extinction of muscle-specific properties in the immediate fusion product. Myocyte X melanoma heterokaryons ceased to express the skeletal muscle forms of myosin, desmin and creatine kinase, reinitiated DNA synthesis, and showed a loss of spontaneous fusion competence within 96 hr after their formation. Although chicken myocyte X mouse melanoma heterokaryons showed extinction of muscle specific properties, they continued to synthesize protein and to incorporate [3H]hypoxanthine, presumably due to the continued production of constitutive chicken HPRT. That presence of the melanoma nucleus was required for extinction to be observed was demonstrated by the continued expression of muscle proteins in cybrids between chicken myocytes and melanoma cytoplasts. Significantly, heterokaryons between chicken myocytes and chicken fibroblasts also exhibited extinction of muscle proteins, demonstrating for the first time that extinction is not restricted to fusions in which at least one parental cell type was derived from an established cell line. Our results strongly support the notion that extinction reflects cell-type specific gene regulatory mechanisms operative during development.

The cartilage-resorbing protein catabolin is made by synovial fibroblasts and its production is increased by phorbol myristate acetate

Pig synovial fibroblasts were shown to produce a protein that caused live cartilage to resorb its proteoglycan matrix in vitro. Fibroblasts were obtained either from synovial tissue digest or by allowing them to grow out of explants. The population derived from the digests was homogeneous and free of macrophage-like cells after two passages, but was still producing the cartilage-resorbing protein after seven passages. The active protein was found to have Mr 20,000 on gell filtration, and pI 4.8 on isoelectric focussing in polyacrylamide gel. It was indistinguishable from a protein with the same activity from pig mononuclear leucocytes, which has been called catabolin. Production of the protein was increased if the synovial fibroblasts were cultured with the tumour promoter phorbol 12-myristate 13-acetate. Fibroblasts from other sources (joint capsule and peritoneum) also apparently made the protein. The possibility that catabolin is the same as interleukin-1 is discussed: if they are, then the results suggest that fibroblasts can make an interleukin-1-life protein.

Analysis of muscle protein expression in polyethylene glycol-induced chicken: rat myoblast heterokaryons

Heterokaryons derived from polyethylene glycol-mediated fusion of myoblasts at different stages of development were used to investigate the transition of cells in the skeletal muscle lineage from the determined to the differentiated state. Heterokaryons were analyzed by immunofluorescence, using rabbit antibodies against the skeletal muscle isoforms of chicken creatine kinase and myosin, and a mouse monoclonal antibody that cross-reacts with chicken and rat skeletal muscle myosin. When cytochalasin B-treated rat L8(E63) myocytes (Konieczny S.F., J. McKay, and J. R. Coleman, 1982, Dev. Biol., 91:11-26) served as the differentiated parental component and chicken limb myoblasts from stage 23-26 or 10-12-d embryos were used as the determined, undifferentiated parental cell, heterokaryons exhibited a progressive extinction of rat skeletal muscle myosin during a 4-6-d culture period, and no precocious expression of chicken differentiated gene products was detected. In the reciprocal experiment, 85-97% of rat myoblast X chicken myocyte heterokaryons ceased expression of chicken skeletal muscle myosin and the M subunit of chicken creatine kinase within 7 d of culture. Extinction was not observed in heterokaryons produced by fusion of differentiated chicken and differentiated rat myocytes and thus is not due to species incompatibility or to the polyethylene glycol treatment itself. The results suggest that, when confronted in a common cytoplasm, the regulatory factors that maintain myoblasts in a proliferating, undifferentiated state are dominant over those that govern expression of differentiated gene products.

Onset of DNA replication in nuclei of proliferating and resting NIH 3T3 fibroblasts following fusion

NIH 3T3 mouse fibroblasts arrested in medium containing 0.5% serum were fused with stimulated cells taken at 2-h intervals after replacing the medium with one containing 10% serum, and DNA synthesis was studied in mono-, homo- and heterokaryons using radioautography with double-labelling technique. The presence of a resting nucleus in a common cytoplasm with a stimulated nucleus from the prereplicative period has an inhibitory effect on the entry of the stimulated nucleus into the S period in medium containing either 0.5 or 10% serum, but ongoing DNA synthesis continues. After a 24-h stay in a common cytoplasm with resting nuclei the stimulated nuclei return into the state of rest. When resting cells are stimulated by 10% serum, their inhibitory effect on stimulated nuclei in heterokaryons still persists, at least for 2 h following stimulation. Preincubation of resting cells with cycloheximide for 4 h abolishes their ability to suppress DNA synthesis in stimulated nuclei. The data suggest that resting cells produce an endogenous inhibitor of cell proliferation, whose formation depends upon the synthesis of protein. When stimulated, the cells can proliferate only after decreasing the level of this inhibitor. The results obtained are consistent with the idea of a negative control of cell proliferation.

Macrophage polykaryon formation in vitro by peritoneal cells from mice given injections of sodium periodate

Peritoneal macrophages from mice that have received two separate intraperitoneal injections of the sterile, soluble oxidant NaIO4 form macrophage polykaryons (MPs) in vitro, but peritoneal macrophage from untreated, peptone-treated, or mice infected with bacille Calmette-Guerin (BCG) do not. The polykaryons are noted after 18-24 hours of culture and continue to form over a 60-72-hour period. The MPs do not form if the macrophage density is less than 4 x 10(3)/sq mm. The polykaryons appear in vitro only in cultures with less than or equal to 1-5 ng/ml lipopolysaccharide (LPS) (amounts of LPS that commonly contaminate culture medium and serum). Hydrocortisone hemisuccinate (2.6 x 10(-9) M) inhibits MP formation in vitro. Lymphocytes do not influence the polykaryon formation, and supernatants from MP cultures do not cause fusion of other macrophages. Microcinephotography demonstrates fusion of the macrophages to form the large polykaryons, which are less motile than uninuclear macrophages. The polykaryons assume different forms; generally, the nuclei (mean, 16.8 nuclei/MP; range, 2-137 nuclei/MP) are centrally located, and the nuclear chromatin of all nuclei appears similar. The MPs phagocytize polystyrene spheres and glutaraldehyde-treated erythrocytes to the same degree as do uninuclear macrophages when determined as particles per nucleus (phagocytic index), but their phagocytic index of IgG-coated erythrocytes is decreased. Peritoneal macrophages from mice given double injections of NaIO4 are nontumoricidal in the absence of LPS, but LPS, in amounts sufficient to inhibit polykaryon formation, renders the macrophages tumoricidal. Populations of macrophages containing MPs formed over 3 days of cultures also respond to LPS or macrophage activating factor (MAF) to demonstrate enhanced tumoricidal activity.

Inflammatory giant cells

Giant cells are commonly seen in granulomas produced by a wide variety of known and unknown agents. It is widely accepted that giant cell formation results from fusion of mononuclear phagocytes. Both experimental and circumstantial evidence suggests that fusion takes place following the attachment of more than one macrophage to the same endocytic material. This view is in keeping with the consistent observation of giant cell formation in granulomas, where macrophages are actively ingesting material in close apposition to other macrophages. The experimental evidence for this view derives from the finding that if more than one macrophage is attached simultaneously to the same endocytic material in vitro, fusion results. Study of the phagocytic capacity of giant cells formed in vivo suggests that the same mechanism of fusion operates in granulomatous inflammation. Giant cell formation in granulomas thus appears to be the incidental result of macrophages ingesting material in close apposition to other macrophages. The material responsible for fusion may either be the visible particulate cause of granuloma formation or the denatured macromolecules formed by the inflammatory process.

Immunofluorescence analysis of the time-course of extinction, reexpression, and activation of albumin production in rat hepatoma-mouse fibroblast heterokaryons and hybrids

We have used a combination of a sensitive immunocytochemical stain for intracellular albumin, and Hoechst 33258 dye for identification of parental nuclei to investigate the time-course of extinction, reexpression, and activation of albumin production in fusion products of 1s (hyperdiploid) or 2s (hypertetradiploid) rat hepatoma cells with mouse fibroblasts (L cells or embryonic cells). In all combinations, the initial event is extinction of albumin production. Extinction occurs immediately after fusion when the mouse fibroblast is a normal embryonic (senescent?) cell. In the case of an L cell, rat albumin is synthesized and secreted during the first 12 h after fusion; no production of mouse albumin occurs. Thereafter, albumin production ceases. 8-12 d after fusion, young hybrid colonies are found to resume the synthesis of rat albumin (reexpression), and several days later the production of mouse albumin begins (activation). The patterns of reexpression and activation indicate (a) that chromosome loss is not necessary for either event to occur and (b) that the cells active in the synthesis of mouse albumin are a subpopulation of those cells already engaged in the production of rat albumin. We conclude that (a) extinction is mediated by diffusible factor(s) from the L-cell parent that act in the hepatoma nucleus to prevent the formation of new albumin messenger RNA; (b) reexpression and activation are gene dosage-dependent but extinction is not; and (c) previously active genes are more rapidly expressed than previously silent ones.

Reactivation of rat peritoneal leukocyte and mast cell nuclei in heterokaryons with actively growing mouse cells

Leukocytes and mast cells of rat peritoneal exudate (PE) were fused in vitro with actively growing mouse cells. Segmented ring-shaped nuclei of granulocytes undergo drastic changes which result in dispersion of tightly condensed chromatin and gradual disappearance of the opening in the centre of the nucleus. These changes are paralleled by a resumption of RNA and DNA synthesis, as shown by autoradiography with [3H]uridine and [3H]thymidine. Solid inactive nuclei of mast cells, lymphocytes, monocytes and macrophages also resume DNA replication and high level of RNA synthesis. Fusion of thymidine kinase-deficient 3T3-4E cells with PE cells results in the incorporation of [3H]thymidine into the nuclei of heterokaryons. This may be considered evidence of the phenotypic expression of rat thymidine kinase gene in heterokaryons. A similar way in which segmented and non-segmented dormant nuclei undergo reactivation suggests that the reversibility of nuclear inactivation is a common feature of differentiated somatic cells.

Short-lived cytoplasmic regulators of gene expression in cell cybrids

Somatic cell hybridization is a valuable tool for investigation the control of gene expression in eukaryotic cells. Studies of hybrid cells, heterokaryons, reconstructed cells and cybrids (cytoplasmic hybrids) have suggested that cytoplasmic factors may be involved in this regulatory process. Unfortunately, studies of this kind usually require that hybrid or modified cells be maintained for some time in a selective environment during which chromosomal losses or other changes may modify the genetic functions of the cells and thus vitiate conclusions about the mechanism of gene regulation. We report here the preparation of cybrids between enucleated mouse fibroblasts (Cl-1-D) and differentiated rat hepatoma cells (Fao) and the use of a combination of histological techniques to identify these modified cells early after fusion without the use of selective media. We found that albumin production in most cybrids was suppressed (extinguished) at 12-20 h after fusion but was restored by 48 h. These results suggest that there is a cytoplasmic factor in the fibroblast which exerts negative control over expression of the albumin gene, but which in the absence of the fibroblast nucleus, is not renewed and therefore short-lived.

Replicating, differentiated macrophages can serve as in vitro targets for transformation by avian myeloblastosis virus

Pure cultures of chicken macrophages were characterized functionally and transformed by avian myeloblastosis virus. Transformed cells exhibited an altered function. The efficiency of transformation was limited by the mitotic activity of the macrophages.

Control of serum protein production in hepatocyte hybridomas: immortalization and expression of normal hepatocyte genes

"Hepatocyte hybridomas" have been isolated after fusion of adult hepatocytes and alpha-fetoprotein (AFP)-producing mouse hepatoma cells. The yield of viable hybrid clones was low but could be increased by culturing the cells in the presence of insulin. On the basis of their chromosome constitution, the hybrids were classified into two groups characterized by either a single or a double set of mouse (hepatoma) chromosomes. The hybrids segregated rat chromosomes and thus constitute an excellent material for gene mapping studies in the rat. Most of the hepatocyte hybridomas retained the production of one or more rat serum proteins, indicating that the corresponding structural genes, contributed by the normal hepatocyte parent, have been immortalized and maintained in the active state after fusion. However, these hybrids do not produce rat AFP, although mouse AFP synthesis is maintained. This result strongly suggests that silent rat (hepatocyte) AFP genes coexist in hepatocyte hybridoma nuclei with active mouse (hepatoma) AFP genes. Finally, on the basis of certain properties of these hepatocyte-hepatoma hybrids, we suggest that the nondividing state of the hepatocytes is actively controlled by a regulatory mechanism which prevents DNA synthesis or entry into mitosis or both.

Reinitiation of DNA synthesis in postmitotic nuclei of myotubes by virus-mediated fusion with embryonic fibroblasts

Myotubes, whose nuclei have stopped DNA synthesis were fused with replicative embryonic fibroblasts. In heterokaryons the postmitotic muscle nuclei resumed DNA synthesis. Incorporation of radioactive thymidine into muscle, and also into fibroblast nuclei was dependent upon the time elapsed between virus-mediated fusion and administration of radioactive thymidine. Whereas incorporation into fibroblast nuclei diminished with time, there was an early increase of labelling into muscle nuclei followed by a decrease of incorporation of 3H thymidine. DNA synthesis was also dependent upon the ratio of noncycling (muscle) to cycling (fibroblast) nuclei. There was a greater incorporation of 3H thymidine into muscle and fibroblast nuclei in myotubes containing larger numbers of fibroblast nuclei. A model is discussed for the control of DNA synthesis in polykaryocytes derived from fusion of cycling and noncycling cells.

Macrophage polykarya

Multinucleated giant cells are commonly found in a wide variety of inflammatory reactions. They are formed at sites of tissue injury by fusion of freshly exuded monocytes, the rate of fusion being dependent on a range of extracellular and intracellular factors. Electron miscroscopy shows that the pooled components of the fused monocytes are not randomly dispersed in the syncytium, but are highly reorganized into a functioning unit. In addition, histochemical and biochemical profiles of cell populations containing these polykarya display a range of metabolic activities, including DNA synthesis, which, on occasions, is followed by successful mitotic division and the formation of polyploid daughter cells. Fusion results in the loss of some surface receptors which in turn interferes with the phagocytic performance of polykarya, which is generally less pronounced than their mononuclear precurses. In addition, polykarya are not as actively motile as macrophages although phenomena of contact inhibition are less obvious. On the other hand, the multinucleate giant cells display prominent exocytosis which may aid in the degradation of extracellular material. The properties of macrophage polykarya contrast with macrophage homokarya produced in vitro. The latter are actively phagocytic, do not synthesize DNA, and have a longer half-life than the syncytia produced in chronic inflammatory reactions. It may well be that the polykarya in such reactions are not true homokarya.

Interaction between 6/94 virus, a parainfluenza type 1 strain, and mouse macrophages

The 6/94 virus, a type 1 parainfluenza virus recovered from multiple sclerosis brain cells after lysolecithin-induced fusion of these cells with African green monkey kidney cells (CV-1), has been found to grow in splenic and peritoneal macrophages obtained from outbred and different strains of inbred mice. Macrophages from C57BL animals were least susceptible to infection, a resistance apparently partially age related. The virus also has been found to replicate in IC21 cells, a line of simian virus 40 virus-transformed mouse macrophages. Viral growth was detected by development of hemadsorption in infected cells, followed by the appearance of infectious virus. The growth of 6/94 virus had different kinetics in mouse macrophages, in the standard continuous cell lines L, 3T3, and CV-1, and in primary mouse kidney and mouse embryo cells. The virus produced in macrophages could be passed in series to other macrophage cultures. In addition, once infected, the cultures continued to produce virus, and permanently infected cell lines were thus obtained. Macrophages from immunized mice with high titers of humoral neutralizing antibodies were found variably able to support virus growth.

Macrophage fusion factor elicited from BGG-sensitized lymphocytes

Lymphocytes obtained from rabbit lymph nodes sensitized to bovine gamma globulin produce in vitro the lymphokine macrophage fusion factor (MFF) which mediates the fusion of approximately 100% of normal alveolar and oil-induced peritoneal macrophages. Giant cells (GC) of Langhans and foreign body type form large syncytia containing as many as several hundred nuclei per cell. Nuclei of GC appear more spherical and larger than those of the normal mononucleated macrophages, and they possess several prominent nucleoli. Giant cells of peritoneal macrophage origin show enhanced intracytoplasmic vacuolization. Normal macrophages cultured as a monolayer in MFF-rich supernatants form cell clusters which progressively fuse during the 24-hour incubation period. A signoid dose-response curve was obtained for cell fusion with MFF-rich supernatants possessing high titers, ie, the latter supernatants undiluted partially inhibited macrophage fusion. MIF-like activity was detected in MFF-rich supernatants as well as a factor(s) which inhibited 3H-thymidine uptake by giant cells.

The effects of asbestos on macrophages

The exact role of the alveolar macrophage in the pathogenesis of asbestosis is not known. Most studies of the effect of asbestos on macrophages have been concerned with the in vitro biochemical or cytotoxic properties of the dust and have made use of peritoneal macrophages. In general, chrysotile had a toxic effect on the macrophages, whereas amphibole varieties did not. Most forms of absetos, however, are actively fibrogenic in man and animals, and there is no clear correlation between in vitro cytotoxicity of various forms of asbestos and their fibrogenicity. Recent experiments in which animals are exposed to asbestos in vivo provide evidence of alteration of macrophage activity, as demonstrated by changes in surface morphology and IgG receptor sites, as well as released of various secretory products. Deposition of complement components found on the surface of alveolar marcophages from animals exposed to asbestos could be a manifestation of a humoral immune response directed against an altered cell. The capacity of macrophages to participate in inflammation, tissue repair, and immunity suggests an immunopathogenic concept for the development of asbestosis.

Studies on the phagocytic capacity of macrophage polykaryons

Following the demonstration by Chambers (1977a,b and c) that simultaneous attachment of macrophages to endocytogenic material in vitro leads to fusion, as analysis of the phagocytic potential of giant cells has provided evidence that the same mechanism is operating in macrophage polykaryon formation in vovo. Unlike previously proposed models for macrophage fusion, fusion by simultaneous attempted endocytosis is associated with the interiorisation of surface membrane during fusion. This has been shown to have occurred during glass coverslip-induced fusion in vivo. Evidence has also been presented that it is the endocytogenic material produced in the granulomatous environment which leads to fusion, not necessarily the simulaneous attempted endocytosis of the implanted foreign material.

[On the endogenous peroxidase in the spleen of swine (author's transl)]

We studied the distribution of endogenous peroxidase in the spleen of swine by modifications of the Graham and Karnovsky diaminobenzidine procedure. There is a peroxidatic activity in the majority of the ellipsoid cells (cells of the sheathed capillaries of Schweigger-Seidel), which is localized in the endoplasmic reticulum and the perinuclear cisterna. This staining is inhibited completely by aminotriazole and is rapidly destroyed even by low concentratoins of glutaraldehyde. Furthermore, the reaction is abolished after boiling of tissue sections or in the absence of H2O2. The macrophages of the red pulp and a minority of the ellipsoid cells are peroxidase negative. Our results are discussed in respect to some recent studies on the system of mononuclear phagocytes. It is suggested, that the enzyme active ellipsoid cells represent a special form of macrophages, enzyme histochemically related to Kupffer cells and resident peritoneal macrophages. The enzyme negative cells of the ellipsoids are probably fibroblasts.

A biochemical profile of glass-adherent cell populations containing multinucleated foreign body giant cells

Some biochemical characteristics of peritoneal macrophages, subcutaneous macrophages and subcutaneous cell populations containing multinucleate giant cells were compared. Subcutaneous macrophages possessed higher concentrations of succinate dehydrogenase, acid phosphatase, aryl hydroxylase, free RNase II, lecithin and free fatty acids than peritoneal macrophages, while the latter had higher concentrations of 5' -nucleotidase esterified cholesterol. These differences may be due to environmental variations depending on their anatomical position or more likely to their degree of activation. As significant numbers of multinucleate giant cells appear in the subcutaneous population the concentration aryl hydroxylase, 5' -nucleotidase lactate dehydrogenase, acid phosphatase, free ribonuclease II and esterified cholesterol falls. The concentration of succinate dehydrogenase decreases but then rises while the concentration of glucose-6-phosphate dehydrogenase increases. These highlight the differences between cell populations containing multinucleated giant cells and those composed from their precursor mononuclear phagocytes only.

Expression of immunoglobulin synthesis in human myeloma x non-lymphoid cell heterokaryons: evidence for negative control

Heterokaryons formed between human myeloma cells and various types of mouse and human non-lymphoid cells loose their cytoplasmic content of lambda light chains, a component of IgE produced by the myeloma parent. This loss of immunoglobulin content was observed regardless of the species origin (mouse or human) of the non-lymphoid partner cell, suggesting that the factors responsible for extinction of this differentiated function are not specific for a species. The kinetics of the loss of immunoglobulin content was essentially identical in the different experiments, since all myeloma X non-lymphoid cell heterokaryons were scored as negative after immunofluorescence staining for lambda chains 4-6 hr after infusion. Myeloma cells treated with inhibitors of protein synthesis (puromycin and cycloheximide) also lost their cytoplasmic content of immunoglobulin after 4 hr. These results indicate that the fusion of myeloma cells with non-lymphoid cells results in an immediate inhibition of immunoglobulin synthesis.

Production of collagenase and prostaglandins by isolated adherent rheumatoid synovial cells

We have studied cells dispersed with proteolytic enzymes from rheumatoid arthritic synovectomy specimens to determine the cell type(s) responsible for joint destruction. Initially 10-50% of these cells adhered to culture dishes within 24 hr and were of two main types: small, round cells and larger, stellate cells. During 1-4 days of culture, 5-25% had Fc receptors and 25-50% showed brisk phagocytosis. Daily producition, per 10(6) cells of collagenase (EC 3.4.24.3) (after trypsin pretreatment) was up to 70 mug of collagen fibrils lysed per min at 37 degrees (70 units), of prostaglandin (PGE2), up to about 1200 ng, and of lysozyme, up to about 100 mug. Under identical conditions of assay, fibroblasts grown from explants of synovium produced no detectable collagenase or lysozyme, and PGE2 was only 2-4 ng. With the dispersed cell preparations, macrophage markers (Fc receptors and lysozyme) were undetectable after 4 days and PGE2 decreased rapidly after about 7 more days. However, collagenase production continued for 3-25 weeks, and in some cultures, after cell passage. At these later stages, large, slow-growing stellate cells were predominant and could phagocytose carbon particles if incubated for greater than 6-8 hr. Indomethacin (14 muM) inhibited PGE2 but stimulated collagenase production whereas dexamethasone (10 nM) inhibited both. Production of PGE2 and collagenase in large amounts in vitro by these cells suggests that they may be involved in joint destruction in vivo. The precise origin of these synovial cells and the mechanisms responsible for the sustained production of collagenase at a high rate remain unidentified.

Autoradiographic studies of nicotinic acid utilization in human-mouse heterokaryons and inhibition of utilization in newly-formed hybrid cells

Although most mammalian cell lines can utilize either nicotinic acid or nicotinamide for the biosynthesis of nicotinamide adenine dinucleotide (NAD), thymidine kinase-deficient, mouse 3T3-4F cells are unable to utilize nicotinic acid. When 3T3-4E cells were fused with human D98/AH2 cells, autoradiography showed that the resultant heterokaryons synthesized NAD from nicotinic acid at rates comparable to the human parental cell. The rate of nicotinic acid utilization in heterokaryons remained unchanged over the four-day period of study following cell fusion. In contrast to the results observed with heterokaryons, nicotinic acid utilization was markedly reduced in hybrid cells. Of 100 hybrid clones examined at four or five days following cell fusion, 60 utilized nicotinic acid at rates less than one tenth that of the parental human cell. Similar results were observed in hybrid clones at nine or ten days following fusion. Uniformly high rates of NAD biosynthesis were observed in hybrid clones with nicotinamide as the precursor. This excludes the possibility that the reduction in nicotinic acid utilization in hybrid cells is due to a general metabolic dysfunction. The biochemical mechanism by which nicotinic acid utilization is markedly reduced has not been determined with certainty, however, several observations suggest genetic suppression.

Heterokaryons in the analysis of genes and gene regulation

Cytological and chemical analysis of heterokaryons, the immediate product of cell fusion, offer new possibilities for studying the factors responsible for genetic regulation in eukaryotic cells. In comparison with proliferating cell hybrids the heterokaryon state offers the important advantage that a heterokaryon contains two complete genomes since chromosome loss does not occur, but since segregation and recombination are absent, heterokaryons cannot be used for gene mapping in the same way as proliferating cell hybrids. However, if two cell types carrying different genetic defects are fused the analysis can be used for studies of gene complementation. The biological information obtained with heterokaryons has emphasized the role of the cytoplasm in the control of nuclear activity. When a G1 nucleus is brought into contact with the cytoplasm of an S phase cell the G1 nucleus is stimulated to synthesize DNA. If the nucleus is brought into a mitotic cell, the chromatin of the G1 nucleus is forced to condense into prematurely condensed chromosomes. Inactive nuclei such as the dormant chick erythrocyte nucleus will be stimulated to initiate RNA and DNA synthesis when brought into contact with an active cytoplasm by cell fusion. Specific nuclear proteins have been shown to be responsible for this process of reactivation. Other inactive nuclei such as the nuclei of macrophages and spermatozoa have likewise been shown to be reactivated by fusion with active cells. The degree of activation in all of these cases appears to be determined by the state of the active cell. Inactive nuclei are activated to the same level as the active nucleus but seldom beyond this level. If differentiated cells are fused with undifferentiated cells, usually the differentiated character is lost rapidly after fusion. This observation is in agreement with several studies on proliferating cell hybrids indicating some type of negative control of differentiated properties. In heterokaryons obtained by fusion of cells of a similar type of histotypic differentiation usually coexpression of the differentiated markers is observed.

Identification and characterization of the monoblast in mononuclear phagocyte colonies grown in vitro

A liquid culture technique for growing mononuclear phagocyte colonies on a glass surface is described. This useful and reliable technique made it possible to study immature mononuclear phagocytes. In the mononuclear phagocyte colonies the cells grow separate from each other in a single layer. Three types of cells are recognized in these colonies, namely nondividing macrophages, and proliferating promonocytes and monoblasts. The macrophage and the promonocyte exhibit the typical characteristics previously demonstrated by the other methods, whereas the monoblast could only be fully characterized by the present liquid culture method. This proliferating cell (labeling index with [3H]thymidine, 92-96%) is almost round (diameters, 10 X 10 mum), has only a small rim of strongly basophilic cytoplasm, almost devoid of granules, and shows a certain degree of ruffling of the cell surface. The monoblast is positive for esterase with alpha-naphthyl butyrate as substrate (91%), for peroxidase (78% in the peroxidase-positive colonies), and lysozyme (43%). The monoblast is able to pinocytize dextran sulphate (15-20%) and to phagocytize opsonized bacteria (20-30%), latex particles (47%), and IgG-coated red cells (96%). IgG receptors (94%) and complement receptors (16%) are present at the cell surface. In these respects the monoblast has the typical characteristics of the mononuclear phagocytes, but its properties show it to be a more immature cell type than the promonocyte. On the basis of these criteria and the sequence of appearance of the different cell types during incubation and during the development of the individual mononuclear phagocyte colony, monoblasts being present before promonocytes appear in the colony, it is concluded that the monoblast is the precursor of the promonocyte. In these cultures granulocyte colonies are also formed, consisting of myeloblasts, (pro)myelocytes, stabs, and polymorphonuclear neutrophils. Besides the typically tight structure of this kind of colony, the granulocytic cells themselves are quite distinct from the mononuclear phagocytes by their morphology, cytochemical characteristics (e.g. all negative for esterase with alpha-naphthyl butyrate, but 96% positive with N-acetyl DL-alanyl 1-naphthylester), functional characteristics (pinocytic index 13-21%; phagocytic index; for opsonized bacteria 15-36%, for latex particles 10%, and for IgG-coated red cells 0%), and their very small number of IgG receptors and lack of complement receptors. On the basis of these criteria, these granulocytic cells are easily distinguished from the immature cells of the mononuclear phagocyte colonies. The present study confirms the conclusion that the mononuclear phagocytes are a separate cell line, quite distinct from the granulocytic series, since even the most immature cells so far identified--the monoblast and the myeloblast--have quite different characteristics.

Effects of concanavalin A on mouse peritoneal macrophages. I. Stimulation of endocytic activity and inhibition of phago-lysosome formation

Concanavalin A (Con A) binds to saccharide residues on the mouse peritoneal macrophage plasma membrane and stimulates extensive pinocytic interiorization of the membrane. The overall pinocytic rate is increased 3.5-4.5 times by the addition of Con A, and the surface marker enzyme adenosine triphosphatase can be identified histochemically in association with the cytoplasmic vesicles generated after exposure of the cells to Con A. Once formed, these pinocytic vesicles may persist for several days and fail to show morphologic evidence of fusion with primary or preformed secondary lysosomes. There is no apparent effect on the capacity of the macrophage to ingest either latex particles or IgG-coated SRBC administered either simultaneously with or subsequent to the Con A.

In vitro synthesis and secretion of lysozyme by mononuclear phagocytes

Pure cultures of three types of mononuclear phagocytes-mouse peritoneal macrophages, unstimulated or after thioglycollate stimulation, and human monocytes-synthesize and secrete large amounts of lysozyme in vitro. The macrophage lysozyme is indistinguishable from authentic lysozyme in its ability to lyse M. lysodeikticus, inhibition by specific antisera, a similar size of 14,000 and cationic charge. Lysozyme secretion in culture is characterized by a large net increase in total lysozyme, 4-20-fold in 3 h, 75-95% of which is in the medium, and its continued extracellular accumulation over at least 2 wk in culture. Lysozyme is the major (14)C-labeled protein secreted into the medium by both unstimulated and thioglycollate-stimulated macrophages and the 0.75-1 microg produced per 1 x 10(6) cells/day represents 0.5-2.5% of the total cell protein. Lysozyme is a cell-specific marker for mononuclear phagocytes and the PMN, which contains preformed enzyme, since it is absent in lymphoid cells and a variety of fibroblast and epithelioid cell lines. Lysozyme production is also a useful measure of mononuclear phagocyte cell number. The rate of lysozyme production and secretion is remarkably constant for all cell types under a variety of culture conditions. Production by the mouse macrophage increases threefold on the 2nd day in culture and then remains linear with time. Production is optimal at a relatively low serum concentration, but can be maintained, in the absence of serum, in lactalbumin hydrolysate or, at a reduced level in basal media. The production and secretion of lysozyme are independent of the production of macrophage acid hydrolases. Net increase and secretion of lysozyme occur under conditions where acid hydrolases like N-acetyl beta-glucosaminidase, beta-glucuronidase, beta-galactosidase, and cathepsin D are neither accumulated nor secreted. Massive phagocytosis of latex particles has no effect on lysozyme production and secretion. Lysozyme production can be rapidly inhibited by treatment with cycloheximide (0.4 microg/ml) whereas inhibition of its production by colchicine (10(-6) M) occurs only after a lag period of more than 8 h, and is probably due to a secondary effect. These results show that mouse macrophages provide a simple in vitro system to measure lysozyme secretion and its control. These studies also indicate the possible importance of mononuclear phagocytes in the secretion of a variety of biologically active products and in the modification of their environment.

Heterocytolysis by macrophages activated by bacillus Calmette-Guérin: lysosome exocytosis into tumor cells

The cytotoxic activity of activated macrophages against tumorigenic target cells appears to be mediated by lysosomal enzymes of activated macrophage origin. Lysosomes of activated macrophages are secreted directly into the cytoplasm of susceptible target cells, which subsequently undergo heterolysis. This reaction can be inhibited by agents which prevent the exocytosis of macrophage lysosomes (hydrocortisone) or which interfere with the action of lysosomal enzymes (trypan blue).

Tumor-host cell hybrids in radiochimeras

F(1) hybrid mice syngeneic or semiallogeneic with respect to the relevant tumor were lethally irradiated and then reconstituted with hemopoietic cells from strain CBAT6T6 mice. After chimerism had been established, the animals were inoculated with solid or ascites tumors. Tumor-host cell hybrids were selected from enzyme-deficient solid tumors by explanting the tumor cell suspension into hypoxanthine-amethopterin-thymidine containing medium. The selection of hybrid cells from ascites tumors was achieved by exploiting the difference between the ascites tumor cells and hybrid cells in their ability to adhere to the surface of culture vessels. T6T6 chromosomal and H-2 antigenic markers served to distinguish between the hemopoietic cells derived from the donor graft and the cells of the host. All solid tumors tested fused with cells of the irradiated host, whereas ascites tumors fused with repopulating cells of hemopoietic origin.