The preparation and properties of macrophage-L cell hybrids

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.

NTPDase1 governs P2X7-dependent functions in murine macrophages

P2X7 receptor is an adenosine triphosphate (ATP)-gated ion channel within the multiprotein inflammasome complex. Until now, little is known about regulation of P2X7 effector functions in macrophages. In this study, we show that nucleoside triphosphate diphosphohydrolase 1 (NTPDase1)/CD39 is the dominant ectonucleotidase expressed by murine peritoneal macrophages and that it regulates P2X7-dependent responses in these cells. Macrophages isolated from NTPDase1-null mice (Entpd1(-/-)) were devoid of all ADPase and most ATPase activities when compared with WT macrophages (Entpd1(+/+)). Entpd1(-/-) macrophages exposed to millimolar concentrations of ATP were more susceptible to cell death, released more IL-1beta and IL-18 after TLR2 or TLR4 priming, and incorporated the fluorescent dye Yo-Pro-1 more efficiently (suggestive of increased pore formation) than Entpd1(+/+) cells. Consistent with these observations, NTPDase1 regulated P2X7-associated IL-1beta release after synthesis, and this process occurred independently of, and prior to, cytokine maturation by caspase-1. NTPDase1 also inhibited IL-1beta release in vivo in the air pouch inflammatory model. Exudates of LPS-injected Entpd1(-/-) mice had significantly higher IL-1beta levels when compared with Entpd1(+/+) mice. Altogether, our studies suggest that NTPDase1/CD39 plays a key role in the control of P2X7-dependent macrophage responses.

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)

The generation and characterization of human monocyte hybridomas

Fusion of interferon-gamma activated peripheral blood monocytes to a mutagenized hypoxanthine-guanine phosphoribosyltransferase (HGPRT)-deficient U937 parent line was performed resulting in the generation of a series of unique cloned monocyte hybridomas. These cell lines were proven to be true hybrids by the acquisition of donor class I antigens as well as other donor derived chromosomes. In addition, novel functional characteristics were observed including secretion of specific monokines and the acquisition of phagocytic capabilities. The ability to generate immortalized human monocyte hybridomas will allow for more in depth analysis of monocyte subpopulations and dissection of specific monocyte functions.

Restoration of the LPS responsive phenotype in C3H/HeJ macrophage hybrids: LPS regulation of hepatocyte-stimulating factor production

The fusion of thioglycollate-elicited peritoneal macrophages from lipopolysaccharide (LPS) non-responsive C3H/HeJ mice to an HPRT-negative variant of the murine macrophage cell line P388D1 has resulted in the derivation of a series of macrophage hybrids. Following exposure to LPS, these hybrids now produce the cytokine hepatocyte-stimulating factor (HSF) which induces the synthesis of the acute-phase reactant alpha 2-macroglobulin in primary rat hepatocyte cultures. The concentration of extracellular HSF was dependent upon both the duration and amount of LPS, with optimal HSF being detected after 72 hr incubation with 10 micrograms/ml of LPS. Parallel LPS-stimulated cultures treated with 10(-6)M dexamethasone did not secrete detectable amounts of HSF. Both the molecular weight (29,000 MW), and the fact that HSF activity was not inhibited by an antiserum directed against murine interleukin-1 alpha (IL-1 alpha), suggests that HSF and IL-1 are distinct cytokines. Therefore, macrophage hybrids have been derived which have acquired the LPS-responsive phenotype and which synthesize the cytokine HSF following LPS stimulation. This phenotype appears stable since similar results have been observed with these hybrids after in vitro culture for over 8 months.

Establishment and characterization of murine macrophage hybrids

Proteose peptone-induced peritoneal macrophages from CBA/J (H-2k) mice have been fused to a hypoxanthine phosphoribosyltransferase-negative variant of the P388D1 (H-2d) murine macrophage cell line. Six hybrid clones were isolated following HAT selection and further characterized. Five of the six clones express class I antigens of both parental haplotypes by microelisa and by flow cytometric analysis. Class II antigen expression of both haplotypes was apparent following a 72-hr incubation of the hybrids with concanavalin A-stimulated rat spleen cell supernatant. However, I-Ad was expressed in all hybrids to a greater extent than I-Ak. Three clones with the highest level of I-Ak expression, E5, C2, and C4, were capable of antigen presentation to the I-Ak-restricted T-cell line, D10.G4.1. LPS induction of the hybrids resulted in a 2- to 15-fold increase in the amount of IL-1 produced relative to the P388D1 parent. Finally, in distinction to P388D1, all hybrids demonstrated increased Fc-mediated erythrophagocytosis of chromium-labeled antibody-coated erythrocytes. These murine macrophage hybrids appear stable and should serve as useful models in understanding the regulation of macrophage function.

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.

Establishment of a cell line retaining Langerhans cell characteristics

A discontinuous gradient of polyethylene glycol (PEG) with dimethyl sulfoxide (DMSO) was used to induced fusion between guinea pig epidermal cells (GPEC) enriched in Langerhans cells (LC) and cells of a human fibrosarcoma line (T-1080) deficient in hypoxanthine phosphoribosyl transferase (HPRT). The rapidly proliferating carrier T-1080 cells lack plasma membrane ATPase activity, Fc receptors, C3b receptors, guinea pig Ia-antigens, and nonspecific esterase activity, all characteristic of LC, and absent on other guinea pig epidermal cells. GPE/T-1080 hybrid cells were selected in hypoxanthine/aminopterin/thymidine (HAT) medium and by C3b and Fc rosette formation. A resultant continuous cell line was characterized karyotypically, histochemically, ultrastructurally and immunologically.

The characterization and functional significance of plasma membrane Fc Receptors

The importance of various components of the plasma membrane in the generation of the immune response has long been recognized. The identification and characterization of functional membrane receptors are essential for an understanding of the molecular basis of the immune response. The Fc receptor can be operationally defined as a site on the plasma membrane which is capable of binding the Fc portion of IgG. The Fc receptor has been reported on both lymphoid and nonlymphoid cell types. The functional significance of the receptor on these cell populations, as well as the biochemistry of the receptor, is not well established and is currently an important area of investigation. In this review, the Fc receptor will be discussed, primarily in human and murine lymphoid and macrophage cell populations, as a model of membrane immunoreceptors. The review will be divided into four sections. In the first section, consideration will be given to the binding assays used to study Fc receptor-ligand interactions and the class and subclass specificity of the receptor. The possibility of receptor-receptor interaction and quantitation of receptor sites will be discussed. The second phase of the review will be concerned with the fractionation and biochemistry of this receptor system, emphasizing the differences reported in receptor composition from different cell sources. In the third section of the review, the biologic significance of the receptor will be examined. The importance of the Fc receptor in opsonization and phagocytosis by mononuclear phagogytes, B-cell activation, antibody-dependent cell cytotoxicity, and the possible relationship of Fc receptors to the histocompatibility-linked immune response genes will be discussed. Finally, in the fourth section, the possible mechanisms of receptor regulation will be considered. Cell fusion experiments which have been performed and the expression of receptor activity on the hybrid cell clones will be discussed. Receptor regulation at the physiologic level will include the effects of cyclic nucleotide levels and cell cycle on receptor expression. It is the intent of this review to provide a detailed analysis of the Fc receptor and to emphasize its importance in microbiology, cell biology, and immunology.

Responsiveness to insulin is a dominant characteristic in somatic cell hybrids

The mouse melanoma cell line PG19 has been found to be unresponsive to the growth-stimulatory action of insulin, although it responds well to other growth factors present in serum. Insulin stimulates DNA synthesis in mouse embryo fibroblasts, and responsiveness to insulin has been found to be a dominant characteristic in mouse fibroblast x PG19 hybrids. To examine the possibility that the unresponsiveness to insulin of the melanoma cells is attributable to a lack of insulin receptors, we have measured the binding of 125I-labeled insulin to the fibroblasts, melanoma cells, and fibroblast x melanoma hybrids. Insulin binds to the surface of the melanoma cells; however, the binding affinity appears to be lower than that observed for binding to diploid fibroblasts. In addition, the dissociation of insulin from the melanoma cells is not accelerated by excess unbound insulin, a kinetic effect observed in the dissociation of insulin from the fibroblasts and fibroblast x melanoma hybrids. This suggests that the class of insulin receptors characterized by this effect is absent on the PG19 cells, and present on the fibroblasts and fibroblast x PG19 hybrids.

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.