Uptake Of Latex Beads Research Articles

Generation of large numbers of dendritic cells in a closed system using Cell Factories™

There is a growing interest in using dendritic cells (DC) for vaccine approaches in the treatment of cancer and infectious diseases. This requires a reproducible method for the generation of large numbers of DC in a closed culture system suitable for clinical use and conforming to the current guidelines of good manufacturing practices. We designed a system in which the DC were generated in a closed system from adherent monocytes using Cell Factories™ (DC-CF). Monocytes were enriched from apheresis products by adherence and then cultured in the presence of AB serum or autologous plasma and GM-CSF and IL-4 for 6 days. The DC generated in Cell Factories™ were extensively compared to research-grade DC generated in conventional tissue culture flasks (DC-TCF). At day 6, the immature DC were harvested and the yield, the viability, the immunophenotype and the functional characteristics of the DC were compared. DC-CF and DC-TCF showed similar viability and purity and scored equally when tested for stability, dextran and latex bead uptake, in MLR and in the activation of influenza-specific memory cells after electroporation with influenza matrix protein 1 (IMP1) mRNA. These data indicated that large numbers of functional clinical-grade DC could be generated from adherent cells in a closed system using Cell Factories™.

Membrane recruitment of Rac1 triggers phagocytosis.

Rac1 is a &Rgr;-family GTP-binding protein that controls lamellipodia formation and membrane ruffling in fibroblasts. Recently, Rac1 and Cdc42, another member of the &Rgr;-family, have been shown to regulate Fc receptor-mediated phagocytosis in macrophages by controlling different steps of membrane and actin dynamics leading to particle engulfment. Here, we investigated the function of Rac1 using a membrane recruitment system that mimics phagocytosis. Recruitment of an activated Rac1 protein to the cytoplasmic domain of an engineered membrane receptor by using rapamycin as a bridge induces ingestion of latex beads bound to the receptor. Rac1-mediated bead uptake depends on actin polymerisation since actin filaments accumulate at the bead/membrane binding sites and internalisation is inhibited by cytochalasin D. Internalisation is also abolished upon substitution of Phe37 to Leu in the Rac1 effector region. Our results indicate that by promoting actin polymerisation at particle attachment sites, Rac1 by acting through specific downstream effectors induces plasma membrane remodeling that allows particle internalisation in a membrane-enclosed phagosome.

Internal Defenses of the Snail Biomphalaria glabrata: II. Defense Cells Have Different Phagocytic Responses to Various Injected Foreign Materials

The three populations of hemocytes and the fixed phagocytes of Biomphalaria glabrata show different affinities for the following injected foreign materials: ferritin, horseradish peroxidase, zymosan particles, latex beads, and live bacteria. Ferritin and horseradish peroxidase, which are internalized by micropinocytosis, induce a moderate activation of hemocytes. Particles trigger a strong activation that consists of mobilization of the cytoskeleton, with formation of pseudopodia and filopodia and modifications of the plasma membrane that enable hemocytes to adhere together, as in encapsulations. Phagocytosis of zymosan and live bacteria induces alterations in mitochondria that may reflect changes in the respiratory activity. Phagocytosis of particles follows different processes: fixed phagocytes ingest latex beads of two different sizes according to the “zipper model”, which implies sequential binding between phagocyte receptors and opsonized ligands; zymosan particles are ingested by medium-size hemocytes by a variant of the zipper model in which a limited close contact between cell and particle resembles focal contacts in cell spreading; bacteria or large latex beads are ingested by large hemocytes without apparent close contact following a “trigger model” which resembles macropinocytosis.

Effects of age, sex and physical exercise on the phagocytic process of murine peritoneal macrophages.

Different stages of the phagocytic process, i.e. chemotaxis, ingestion of latex beads and superoxide anion production, were measured in peritoneal macrophages from young (12 +/- 2 weeks old) and old (60 +/- 2 weeks old) male and female BALB/C mice, which were subjected to an acute bout of exercise (swimming until exhaustion) or to a period of training exercise (90 min of swimming each day during 20 days). Sedentary male and female mice as well as young and old animals were used as sex and age controls. The results show that both acute and training exercise stimulate the phagocytic process of murine peritoneal macrophages. The macrophage functions from sedentary controls were lower in the old than in the young animals. The chemotaxis and ingestion capacities of macrophages were higher in the female young and old sedentary mice than in their male counterparts. After exercise, the stimulation of the phagocytic process was higher in old and female mice than in young and male animals. In addition, serum corticosterone levels were measured in order to investigate the relations between stress and macrophage activity. Old and female mice as well as animals subjected to exercise showed, in general, higher corticosterone levels than young, male and sedentary animals.

Role of intestinal mucus on the uptake of latex beads by Peyer's patches and on their transport to mesenteric lymph nodes in rats.

The effects of N-acetylcysteine (NAC) as a mucolytic agent on the uptake of fluorescent polystyrene microparticles by Peyer's patches, on intestinal permeability, and on subsequent transport to mesenteric lymph nodes (MLNs) were investigated to establish the role of mucus gel layer in this process. Twenty rats were divided into two groups: control (n = 10) and NAC (n = 10). Fluorescent polystyrene latex beads of 3.2+/-0.2 microm in diameter were used as a probe for measuring the previously mentioned parameters. The solution of latex beads (0.1 mL) was injected into a 2-cm length of ileal loop containing Peyer's patches, with 0.1 mL of saline (control group) or with 0.1 mL of NAC solution (NAC group) within 10 cm proximal from the ileocaecal valve. Intestinal loops, portal blood, and neighboring MLNs were taken within 1 hour of injection. Intestinal sections were stained by periodic acid-Schiff reagent. Peyer's patches and MLNs were analyzed for the count of particles by image analysis using a confocal laser scanning microscope. Morphologically, periodic acid-Schiff positive uniform mucus gel was present in front of Peyer's patches of the control group, and mucus gel layer was disrupted and noncontinuous in the NAC group. The number of particles within Peyer's patches and MLNs in the NAC group was significantly higher than that in the control group (p<.001). Intestinal permeability of latex beads in the NAC group was significantly higher than that in the control group (p<.001). These data suggest that the mucus gel layer located in front of Peyer's patches is one of the important factors for the uptake of noxious macromolecules, and this in turn plays a major role on small intestinal permeability and subsequent translocation to MLNs.

Nonopsonic and Opsonic Association ofMycobacterium tuberculosiswith Resident Alveolar Macrophages Is Inefficient

AbstractThe association of Mycobacterium tuberculosis with alveolar macrophages (Mφ) in a serum-free environment is a crucial first step in the pathogenesis of this facultative intracellular pathogen. We present data demonstrating that freshly explanted alveolar Mφ do not efficiently bind M. tuberculosis in a serum-free system, although a small subpopulation of these Mφ (10–15%) can bind mycobacteria. In contrast, almost 100% of a peritoneal Mφ population bind mycobacteria under the same conditions. The poor binding of mycobacteria by alveolar Mφ does not reflect a general inability to associate with particles; binding and ingestion of latex beads and zymosan particles were comparable with that seen with peritoneal Mφ. Resident alveolar Mφ did not efficiently bind mycobacteria in the presence of serum and expressed poorly several Mφ surface receptors, including CR3. Furthermore, we demonstrate that bovine surfactant protein A does not enhance the association of M. tuberculosis with alveolar Mφ. Differentiation of alveolar Mφ in vitro resulted in increased expression of Mφ surface receptors and an increased capacity to bind mycobacteria in the presence and absence of serum. Evidence is presented that opsonic binding of M. tuberculosis by differentiated alveolar Mφ is mediated by complement and CR3, and that the poor binding by resident alveolar Mφ is due to their poor expression of CR3. The receptor mediating nonopsonic binding of M. tuberculosis to differentiated alveolar Mφ was not unequivocally identified in this study, but could also be CR3.

Ethanol Induced Changes in Superoxide Anion and Nitric Oxide in Cultured Microglia

Induction of oxidative stress has been implicated as a causative factor in fetal alcohol syndrome although the source of reactive oxygen species is not clear. One potential source is the microglia, the CNS macrophage, which generate superoxide anion as part of their normal immune function. Our data indicate that chronic exposure to ethanol alters the function of cultured neonatal hamster microglia by inducing superoxide anion production in resting (nonstimulated) cells. An increase in superoxide anion was seen at 24 or 48 hr of ethanol treatment but was not seen during acute exposures of up to 3 hr. This effect was dose dependent and was maximal at 20 mM ethanol. Treatment with ethanol did not directly activate the respiratory burst seen in microglia and did not act as a priming agent to enhance phorbol-ester-stimulated superoxide anion production. Lipopolysaccharide-mediated priming of microglial superoxide anion production was also not affected by exposure to 20 mM ethanol for 24 hr. Ethanol treatment, however, did depress nitric oxide (NO) levels in hamster microglia which had been stimulated to produce NO by polyinosinic:polycytidylic acid (poly I:C). Uptake of latex beads was increased by 24 hr of exposure to ethanol. The overall action of ethanol on neonatal hamster microglia is to shift the balance between the production of superoxide anion and NO. Because NO is protective to mammalian cells, these changes predict that oxidative stress in the CNS would be enhanced.

Iris Pigment Epithelial Cells of Long Evans Rats Demonstrate Phagocytic Activity

The phagocytic activities of iris pigment epithelial (IPE) cells and retinal pigment epithelial (RPE) cells of Long Evans rats towards latex beads and rod outer segments (ROS) were compared in vitro. IPE and RPE cells of Long Evans rats were isolated and pure cultures obtained. The cultures were incubated with latex beads, fixed, and analysed computer morphometrically, IPE and RPE cell cultures were also incubated with isolated ROS and examined using transmission electron microscopy. IPE cells were able to ingest latex beads. There was no significant difference between the number of latex particles phagocytized by IPE and RPE cells. After incubation with isolated ROS, IPE cells also recognized and ingested the ROS particles. However, the specific phagocytic capacity of IPE cells was 76% of that of RPE cells.The autologous IPE cells might have the potential to be used as an alternative to RPE cells for transplantation in the subretinal space.

Leishmania major:Promastigotes Induce Expression of a Subset of Chemokine Genes in Murine Macrophages

Recent studies suggest thatLeishmania majorpromastigotes infect cultured macrophages in a stealthy fashion, activating little or no host gene expression and often interfering with the host's ability to respond to further stimulation. Here we examined macrophage transcription at early times following infection, when virulent parasites must execute steps required for survival. Stationary-phase promastigotes induced rapid and transient expression of transcripts of the chemokines JE (human MCAF/MCP-1) and KC (human GRO) in bone marrow-derived macrophages from BALB/c mice. JE and KC expression rose four- to sixfold shortly after infection and returned to uninduced levels by 4–24 hr. In contrast, chemokines MIP-1α, C10, and RANTES were not induced, nor were TGF-β, IL-10, IL-12, or i-NOS. Chemokine induction did not occur following ingestion of latex beads, implicating a parasite-specific stimulus. Elevated expression of a subset of chemokines is the earliest known transcriptional response of macrophages toL. majorinfection and potentially may provide a signal for the initiation of downstream immunological responses which occurin vivo,such as cytokine induction and chemotaxis of monocytes and macrophages. Thus,Leishmaniahas a remarkable ability to take an active role in either inducing or preventing the expression of distinct sets of host genes during macrophage invasion and successful intracellular parasitism.

Melatonin and the phagocytic process of heterophils from the ring dove (Streptopelia risoria).

A functional connection between the pineal gland and the immune system in mammals and birds has been established. This study investigates the effect of melatonin upon the non-specific immunity of heterophils isolated from the ring dove. The different stages of the phagocytic process: adherence to nylon fiber, spontaneous and induced mobility, ingestion of latex beads and digestion were evaluated for heterophils incubated in the presence of 5, 25, 50, 75, or 100 microM of melatonin. In addition, the chemoattractant power of the hormone for heterophils was studied. The 100 microM melatonin dose possessed a significant chemoattractant ability for heterophils whilst ingestion of latex particles was enhanced at all doses studied. The superoxide anion level, as measured by the free radicals produced during the metabolic burst, is decreased after incubation with 100 microM of melatonin.

Total parenteral nutrition increases uptake of latex beads by Peyer's patches.

Dysfunction of the intestinal barrier, as evidenced by increased intestinal permeability and bacterial translocation, has been reported under total parenteral nutrition (TPN). However, the role of Peyer's patches on the intestinal barrier in TPN is not well understood. We investigated whether TPN alters the uptake of microparticles by the follicle-associated epithelium of Peyer's patches. Twenty rats were divided into two groups, a control group and a TPN group. Fluorescent polystyrene latex beads, 3.2 +/- 0.2 microns in diameter, were used as a probe for measuring the uptake by Peyer's patches. After 1 week of consuming either the control or TPN diet, rats were killed. On the day of killing, 0.1 mL of latex beads solution was injected into a 1-cm length of ileal loop, within 10 cm of the ileocecal valve. Samples were taken after 30 minutes of injection, sectioned by cryostat, and then viewed under a fluorescent microscope. Follicle-associated epithelial length and particles were counted using a confocal laser scanning microscope. The number of particles within each compartment was standardized per unit length of epithelium of Peyer's patches. Particle numbers within Peyer's patch dome of the TPN group were significantly increased compared with those of the control group (p < .01). These data suggest that dysfunction of the intestinal barrier in TPN might be associated with a change of uptake by Peyer's patches.

セファランチン含有リポソームのマクロファージどん食能に及ぼす影響

Cepharanthine (CEP) is reported to be effective in prevention of immune suppression and leukopenia induced by administration of antitumor agents. In this study, we examined relationship between the phagocytotic activity of peritoneal macrophages and the immuno-modulating action of CEP-incorporating liposomes composed of egg phosphatidylcholine and cholesterol in a molar ratio of 7 : 2. Multilamellar liposomes (MLV) were prepared by vortexing dried lipid films, and small unilamellar liposomes (SUV) by ultrasonication of MLV. Antigen (pneumococcal polysaccharides) and CEP-containing liposomes were injected intraperitoneally to BALB/c mice, and peritoneal macrophages were collected at designated times. The phagocytotic activity of the peritoneal macrophages was assessed with regard to the uptake of latex beads by the cells. The time-course of phagocytotic activiy was shown to be correlated with the antigen-specific IgM antibody production. The phagocytotic activity was dependent on the dose of CEP, but was not influenced by peritoneal exudate cell number or the presence of antigen. The CEP-containing MLV increased the phagocytotic activity of peritoneal macrophages to a greater extent than SUV. In conclusion, it was demonstrated that the immuno-modulating activity of peritoneally injected CEP is closely related to the phagocytotic activity of peritioneal macrophages, that CEP-containing MLV exhibit greater stimulatory effect on the phagocytotic activity of macrophages than CEP-containing SUV, and that the immuno-modulating action of CEP is enhaced by incorporation into liposomes.

Effects of a long-term training program of increasing intensity on the immune function of indoor Olympic cyclists.

We have studied, on blood samples, the level of immunocompetence (concentration of immune cells, phagocytic process of polymorphonuclear neutrophils, proliferative response of lymphocytes to mitogens), the ascorbic acid content of such immunocompetent cells and the "stress hormone" status (cortisol, ACTH and beta-endorphin) of 10 cyclists, members of the Spanish Indoor Olympic Team and participants in the Olympic Games of Barcelona '92. The study was performed twice during their training for such an event: during the third year of the program (February, 1991) and immediately before the Games (June, 1992). As regards the phagocytic process of neutrophils, we studied the different steps of this process: adherence to endothelium, directed mobility or chemotaxis, ingestion of latex beads and superoxide anion production measured by the nitroblue tetrazolium (NBT) reduction test. We observed a statistically significant increase in chemotaxis and NBT reduction activity just before the Games as compared to the third year of the program, whereas variations were not found in the other parameters. The values of the proliferative capacity of lymphocytes were slightly higher in June '92 than in February '91, but no statistically significant differences were found. The ascorbic acid content decreased strikingly (especially in lymphocytes) immediately before the Games. Regarding the stress hormones and neuropeptides (cortisol, ACTH and beta-endorphin), we observed an increase in serum ACTH and beta-endorphin levels in the last determination (June '92) in comparison to the first one (February '91). These results suggest that, at the end of a long-term training program. no immunosuppression occurs, although an important increase in the concentration of stress hormones (ACTH and beta-endorphin) is found. This is probably caused by the psychological stress associated to the participation in such an important event as the Olympic Games.

The Effect of Dexamethasone on Functional Properties of HL60 Cells During All- trans Retinoic Acid Induced Differentiation. Are There Implications for the Retinoic Acid Syndrome?

ABSTRACT Differentiation therapy for acute promyelocytic leukemia (APL) using all- trans-retinoic acid (ATRA) has improved the prognosis of the disease. ATRA therapy also causes a newly recognized clinical syndrome, the "retinoic acid syndrome" (RAS), which can be successfully managed with dexamethasone. Because aberrant function of maturing leukemic granulocytes may cause this syndrome, and because dexamethasone is useful clinically, we studied functional properties of maturing HL60 cells cultured in the presence and absence of dexamethasone. HL60 cells were cultured for 4 days with ATRA and studied daily to determine acquisition of mature neutrophil-like properties including phagocytosis, NBT reduction, actin polymerization, chemotaxis and adhesion molecule expression. Undifferentated HL60 cells could not polymerize actin or reduce NBT, and exhibited only a minimal abilty to undergo chemotaxis or ingest latex beads. Following 4 days of maturation with ATRA, the cells would increase F-actin content in response to interleukin-8, ingest latex beads, migrate in a chemotaxis chamber, reduce NBT, and express CD11b. When dexamethasone was added to the cells in culture, there was no major enhancement or suppression of these properties. We also studied the effect of dexamethasone on functional properties of normal neutrophils and found minimal if any effect on their function. Overall, these studies suggest that in vitro, dexamethasone has little effect on the function of leukemic and normal granulocytes. To further investigate the pathophysiology of the retinoic acid syndrome, future studies may need to use endothelial cells, cytokines, or granulocytes obtained from APL patients.

Modification of nitrite production and phagocytosis of thioglycollate‐elicited mouse peritoneal macrophages by Bovine Casein Digests

The effect of bovine milk casein digests on the ability of thioglycollate‐elicited mouse peritoneal macrophages to produce nitrite and to ingest latex beads was investigated. Intact α s1‐casein and κ‐casein significantly inhibited nitrite production by the macrophages. The inhibitory activity of the latter was little influenced by trypsin or chymotrypsin digestion, while that of the former decreased with increasing proteinase digestion time. In contrast, pepsin digestion changed the inhibitory activity of a s1‐casein and κ‐casein into an enhancing activity. Intact ß‐casein, however, had an enhancing effect on nitrite production, and its activity increased noticeably when the protein was digested with pepsin. The enhancing activity of pepsin digests of each casein component reduced when the digest was treated with carboxypeptidase A. Moreover, a trypsin or chymotrypsin digest of α s1‐casein and κ‐casein inhibited ingestion of latex beads by the macrophages, while a pepsin digest of each casein component enhanced it. These results suggest that the neonate ingestion of infant formula made with bovine milk might reduce resistance to infection via suppression of macrophage functions, such as antimicrobial nitrogen intermediate production and phagocytosis in their gut mucosa, since it is known that gastric secretion and pepsin activity are weak in neonates and the major portion of the caseins ingested by neonates leaves the stomach after minimal digestion.

Modulation of murine peritoneal macrophage functions by gastrin.

The effect in vitro of gastrin-17 and gastrin-34 was studied at concentrations from 10(-12) to 10(-6) M on several functions of resting peritoneal macrophages from BALB/c mice: adherence to substrate, mobility (spontaneous and directed by chemical gradient or chemotaxis), and ingestion of inert particles (latex beads) or cells (Candida albicans). Both gastrins, at concentrations from 10(-10) to 10(-8) M, inhibited significantly all functions studied with the exception of adherence, which was increased. A dose-response relationship was observed, with a maximum inhibition of macrophage functions found at 10(-9) M. These peptides induced in murine macrophages a significant increase of cAMP levels at 60 and 120 s. Adenosine, an adenylate cyclase inhibitor, significantly increased the ingestion of latex beads, whereas the combined presence of adenosine and either G-17 or G-34 produced similar values to those of control samples without adenosine or gastrin. These results suggest that gastrin is a negative modulator of several macrophage functions, and that the inhibition of these activities is carried out through an increase of intracellular cAMP levels.

Postmortem delay effects on neuroglial cells and brain macrophages from Lewis rats with acute experimental allergic encephalomyelitis: an immunohistochemical and cytochemical study

The effects of increasing postmortem delay (PMD) times on morphological, immunological and functional characteristics of various brain cells both in situ and in vitro were studied in postmortem brain tissue derived from rats with acute experimental allergic encephalomyelitis (EAE). A decline of the brain tissue structure was first noted after a PMD of 6 h. Radial glia in the cerebellum were frequently interrupted and retractions artifacts appeared around brain cells. However, even after the longest PMD interval of 18 h the quality of the cell and tissue structure was still good enough for immunohistochemical characterization. Immunohistochemical staining of frozen and fixed rat brain tissue sections resulted in an enhancement of the immunoreactivity after a PMD of 4 h, using a panel of mono and polyclonal antibodies directed against glial fibrillary acidic protein (GFAP), basement membranes (laminin), brain macrophage antigens (EDI and ED2), and various immunologically important surface molecules, such as major histocompatibility complex (MHC) class II (Ia) antigen (OX6), CR3 complement receptor (ED8), and leukocyte common antigen (OX1). No increase in staining intensities with the EDI, ED8 and OX6 mAbs specific for macrophage antigens could be detected on brain macrophages that were isolated from brain tissue of rats with EAE obtained after various PMD intervals. Irrespective of the PMD interval, viable astrocyte cell cultures were obtained with comparable staining intensities for GFAP. These cultured astrocytes were capable of ingesting Latex beads and were highly proliferative as measured by BrdU uptake, at all investigated PMDs. Thus, even after long PMD intervals, brain material can be used successfully. Other data suggest that the situation is similar to human brain material, even though the PMD times may be somewhat different.

Effects Of Acute Exposure To Phosgene On Pulmonary Host Defenses And Resistance To Infection

AbstractPhosgene is a toxic gas widely used in industrial processes. The most sensitive endpoint for phosgene toxicity in mice is decreased resistance to challenge with bacterial infection or tumor cells. These effects were attributed to impaired alveolar macrophage (AM) and pulmonary natural killer cell (NK) function. The purpose of this study was to investigate whether similar effects occurred in Fischer 344 rats. Intrapulmonary killing of bacteria was impaired and the inflammatory response enhanced in rats infected by aerosol with Streptococcus zooepidemicus immediately after a 6-/1 exposure to 0.1 or 0.2 ppm phosgene as compared to air controls. Also, ingestion of latex beads in vitro by AM obtained from bronchoalveolar lavage (BAD fluid of rats exposed to 0.2 ppm phosgene was significantly less than controls. If infection or BAL were delayed until 18 h after exposure, there was no difference between phosgene and air exposed rats. In uninfected rats polymorphonuclear leukocytes were increased in BAL f...

Effect of pinealectomy upon the nonspecific immune response of the ring‐dove (Streptopelia risoria)

The different stages of the phagocytic process by granulocytes of pinealectomized or sham-pinealectomized ring doves (Streptopelia risoria) as well as hematological parameters (total white blood cells, smear, and total protein) and serum hormone levels (triiodothyronine (T3), thyroxine (T4), and corticosterone) were studied. A number of immunological parameters of the phagocytosis process, including adherence capacity, mobility rate, the phagocytosis capacity for inert particles and the digestion capacity of ingested material, were studied. Adult male ring doves were injected intravenously with either 0.1 ml of normal sheep serum (NSS) or saline (SS). Blood samples were collected before injection, and 1 hr, 3 hr, 24 hr, and 4 days afterwards. The results indicate that pinealectomy produces a significant increase in the number of total white blood cells and total protein concentration in plasma in addition to altering different stages of the phagocytic process. During the immunization study, a decrease in the percentage of leukocytes and lymphocytes and an increase in the percentage of heterophils accompanied by an increase in the concentration of serum corticosterone were observed 3 hr following treatment. For the immunological parameters, adherence capacity and latex bead ingestion were increased 3 hr after NSS injection and the NBT reduction test 3 and 24 hr after NSS treatment. In addition, the administration of NSS produced a significant increase in serum T3 and T4 concentrations 4 days following injection. These results show that pinealectomy has a marked effect on both the number and function of immune cells.

Leishmania donovani infection enhances macrophage viability in the absence of exogenous growth factor

Bone marrow-derived macrophages rapidly die in the absence of macrophage growth factor (M-CSF). However, as demonstrated here, bone marrow-derived macrophages infected with Leishmania donovani exhibit increased viability in the absence of exogenous growth factor. Forty-eight hours after inoculation with promastigotes or amastigotes, infected cell cultures contained 180 and 95% more cells, respectively, than control cultures. This effect was specific to Leishmania infection, as uptake of latex beads or avirulent promastigotes by macrophages did not enhance cell viability. L. donovani-infected macrophages also displayed increased phagocytic capacity, as compared with control macrophages and macrophages grown continuously in M-CSF-containing medium. Supernatants collected from infected cells elaborated a factor(s) that enhanced macrophage viability but did not stimulate macrophage DNA synthesis. This activity of L. donovani-infected cell-conditioned medium could be abrogated by preincubation of macrophages with cycloheximide before inoculation with the parasite, implying that macrophage protein synthesis is required for the elaboration of this factor(s).