P815 Mastocytoma Cells Research Articles

Studies on the Synthesis of Compounds Related to Adenosine 3',5'-Cyclic Phosphate. IX. Synthesis and Cytotoxic Effect of Adenosine 3',5'-Cyclic Alkylphosphoramidates.

A series of adenosine 3',5'-cyclic monophosphoramidates (3, cAMP amidates), including long-chain alkyl amidates, were synthesized from adenosine 3',5'-cyclic monophosphate (1, cAMP) by means of a one-pot reaction. This reaction proceeded by the treatment of cAMP tributylammonium salt (2) with phosphorus pentachloride (PCl5) and alkylamine in N,N-dimethylformamide (DMF). Compounds 3 synthesized were investigated to determine their cytotoxic activities on the growth of mouse mastocytoma P-815 cells, mouse mammary tumor FM3A cells, and human mammary tumor ZR-75 cells in culture. It was found that compounds 3h-m showed significant cytotoxic activities against these cell lines, and that cAMP decylamidate (3j) was the most cytotoxic compound (the concentration required for 50% inhibition of cell growth, ID50 = 6.0, 15.0, 2.2 microM, respectively); the antitumor effect on P-815 cells by a total packed cell volume method showed 81.8% inhibition. The cytotoxic activity of 3 increased with the increase in alkyl chain length up to 10 carbon atoms and decreased in compounds having longer alkyl chain.

CDNA-derived amino acid sequence of L-histidine decarboxylase from mouse mastocytoma P-815 cells

The primary structure of L-histidine decarboxylase (HDC: L-histidine carboxy-lyase, EC 4.1.1.22) from mouse mastocytoma P-815 cells has been determined by parallel analysis of the amino acid sequence of the protein and the nucleotide sequence of the corresponding cDNA. HDC contains 662 amino acid residues with a molecular mass of 74017, which is larger by about 21 000 Da than that of the previously purified HDC subunit (53 kDa), suggesting that HDC might be posttranslationally processed. The HDC cDNA hybridized to a 2.7 kilobase mRNA of mastocytoma cells. Homology was found between the sequences of mouse mastocytoma HDC and fetal rat liver HDC.

Identification of a prostacyclin receptor coupled to the adenylate cyclase system via a stimulatory GTP-binding protein in mouse mastocytoma P-815 cells

A stable analogue of prostacyclin, iloprost, specifically bound to 30,000 x g pellet (the membrane fraction) prepared from mouse mastocytoma P-815 cells. The binding was dependent on time, temperature and pH, and absolutely required a divalent cation. The equilibrium dissociation constant and the maximal concentration of the binding site as determined by Scatchard plot analysis were 10.4 nM and 1.12 pmol/mg of protein, respectively. The Hill coefficient was 1.0, indicating a single entity of binding site and no cooperativity. The binding site was highly specific for iloprost among PGs tested (iloprost > PGE 1 > carbacyclin > PGE 2). In contrast, the membrane fraction had the binding site specific for PGE 2 and PGE 1, which was distinct from the prostacyclin receptor. The dissociation of bound [ 3H]iloprost from the membrane fraction was specifically enhanced by guanine nucleotides. Furthermore, iloprost dose-dependently enhanced the activity of adenylate cyclase in a GTP-dependent manner. These results indicate that a specific prostacyclin receptor is coupled to the adenylate cyclase system via a stimulatory GTP-binding protein in mastocytoma cells.

Transmembrane signaling in P815 mastocytoma cells by transfected IgE receptors.

In order to delineate structural-functional relationships of the mast cell receptor for IgE (Fc epsilon RI) by molecular-genetic analysis, a transfectable cell must be identified which resembles mast cells except for being deficient in receptors. We have found that the well known murine mastocytoma P815 is suitable. These cells express no Fc epsilon RI, lack mRNA for the alpha and beta subunits of the receptor, but contain some mRNA for gamma chains. After transfection with the cDNA for each of the subunits, stable clones could be isolated which expressed several hundred thousand normal Fc epsilon RI and synthesized large amounts of mRNA for alpha, beta, and gamma, the last at 3-fold higher levels than in the untransfected cells. Aggregation of the transfected receptors led to opening of presumptive calcium channels and to activation of phospholipase C, phospholipase A2, and protein kinase C. The kinetics and other characteristics of the signals were similar to those observed after stimulation of the rat tumor mast cells from which the receptor genetic material had been derived but were smaller in magnitude. These weaker signals most likely result from an overall reduced reactivity exhibited by the P815 cells since stimulation by other ligands led to weaker or even no responses. The cells failed to degranulate after either receptor aggregation or reaction with ionophores with or without phorbol ester. Both the transfected and untransfected P815 cells express Fc receptors for IgG (Fc gamma RII) which, interestingly, independently triggered similar responses despite their apparently simpler subunit structure.

Mast cell tumor destruction by deionized water

SUMMARY In a controlled study, malignant murine P815 mastocytoma cells exposed in vitro to distilled and deionized water died as a result of progressive swelling, degranulation, and membrane rupture. A 90% mean cell death occurred when cells obtained directly from culture were exposed to deionized water for 2 minutes. Of 6 cryopreserved malignant murine cell lines, which included Cloudman S91 melanoma, CMT-93 rectum carcinoma, MMT-06052 mammary carcinoma, and S-180 Sarcoma, only P815 mastocytoma and YAC-1 lymphoma were significantly (P < 0.05) affected by hypotonic shock; Cloudman S91 melanoma cells were the most resistant. Mastocytoma cells were selectively killed by hypotonic solution, and lymphoma cells were also killed by isotonic saline solution. Local mast cell tumor (mct) recurrence and percentage survival were evaluated in 12 cats (21 mct) and 54 dogs (85 mct) subjected to surgery alone or local infiltration of deionized water as an adjunct to surgery. Of all 16 incompletely excised mct in cats, there was no local recurrence following injection. Four mast cell tumors (2 cats) regressed after being injected in situ. In dogs with clinical stage-I mct, local recurrence was detected in 50% (5/10), but with injection after incomplete excision, local mct recurrence was significantly (P < 0.05) less (6.6%, 1/15). Percentage recurrence was significantly (P < 0.05) less and survival significantly greater when incompletely excised grade-II mct were injected. Mean follow-up period after surgery in cats and dogs was 35 and 23.4 months, respectively.

Docosahexaenoic acid, a constituent of rodent fetal serum and fish oil diets, inhibits acquisition of macrophage tumoricidal function.

Macrophage (M phi) activation is deficient in the fetus and neonate, at times when the serum concentration of docosahexaenoic acid (DHA; 22:6n3) is approximately 10-fold higher than in the adult. We tested the effects of highly purified DHA on M phi activation in vitro. M phi were stimulated with rIFN-gamma plus either of two second or "triggering" signals, LPS or heat-killed Listeria monocytogenes. M phi activation was assayed as the lysis of P815 mastocytoma cells, which are resistant to TNF-alpha. DNA inhibited the activation of peritoneal M phi and the M phi line RAW264.7 in a dose-dependent manner at concentrations between 20 and 160 microM. These concentrations are found in fetal and neonatal rodent sera. Another polyunsaturated fatty acid, arachidonic acid (20:4n6), was much less inhibitory. In contrast to its profound effect on tumoricidal activation, DHA did not inhibit phagocytosis and catabolism of 125I-heat-killed Listeria monocytogenes. Increasing the rIFN-gamma or second signals reduced the inhibition of tumoricidal activation by DHA but not M phi incorporation of 14C-DHA. When the rIFN-gamma and second signals were separated in time, DHA was far more inhibitory if delivered with the triggering signal than if delivered with the rIFN-gamma. However, the incorporation of 14C-DHA was the same under these two conditions. In M phi treated with DHA during LPS stimulation, the inhibition was time-dependent, requiring more than 2 h. Although DHA inhibits cyclooxygenase activity, its inhibition of M phi activation was not reversed with the following cyclooxygenase products: PGE2, a stable TXA2 analog (U-46, 619) or a stable PGI2 analog (Iloprost). Although DHA is metabolized by lipoxygenases, the inhibition was not reversed by the lipoxygenase inhibitors 5, 8, 11, 14-eicosatetraynoic acid and nordihydroguaiaretic acid. Altogether, the data indicate that DHA, at concentrations present in fetal and neonatal sera, inhibits M phi activation and may contribute to the previously observed deficits in M phi function in the fetus and neonate.

Purification and Characterization of L-Histidine Decarboxylase from Mouse Mastocytoma P-815 Cells

Histidine decarboxylase was purified from mouse mastocytoma P-815 cells to electrophoretic homogeneity by ammonium sulfate fractionation, dialyses at pH 7.5 and 6.0, chromatographies on DEAE-Sepharose CL-6B, Phenyl-Sepharose CL-4B and Hydroxylapatite, Phenyl-Superose HPLC, Mono Q HPLC, and Diol-200 gel filtration HPLC. Under the assay conditions used, the pure enzyme exhibited a specific activity of 800 nmol/min/mg, which constituted 12,500-fold purification compared to the crude extract, with a 7% yield. The two-step dialysis turned out to be essential for removing the factor(s) which interfered with the enzyme purification. The optimum pH for the enzyme reaction was 6.6 and the isoelectric point of the enzyme was pH 5.4. The molecular mass of the enzyme was found to be approximately 53 kDa on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, 110 kDa on gel filtration, and 115 kDa on polyacrylamide gradient gel electrophoresis in the absence of sodium dodecyl sulfate. The Km value for histidine was estimated to be 0.26 mM at pH 6.8.

Kinetics of Cell-Mediated Cytotoxicity in Mice Fed Diets of Various Fat Contents

We previously reported lower mitogen-induced blastogenic and cytotoxic activity of splenocytes from C3H/OUJ female mice fed 20% soybean oil (SBO) in their diet compared to those fed 5% SBO. The present study examined the kinetics of cell-mediated cytotoxicity using the same animal model and dietary treatments. Kinetic parameters were determined by analyzing the lytic efficiency of splenocytes cultured for various times with several concentrations of radiolabeled P815 mastocytoma cells. The apparent avidity constant (K1/2) of the reaction was not changed by dietary SBO intake (1.0 +/- 0.2 x 10(5) cells for 20% SBO versus 1.3 +/- 0.3 x 10(5) cells for 5% SBO). However, the maximum velocity (Vmax) of the reaction for splenocytes from mice fed 20% SBO was significantly lower than that for splenocytes from mice fed 5% SBO (1.4 +/- 0.2 x 10(4) cells/h for 20% SBO versus 2.3 +/- 0.4 x 10(4) cells/h for 5% SBO, p less than 0.05). The evidence indicates that the rate of target cell lysis, but not the avidity of the lymphocytes for the target cell antigen, was altered by increasing dietary SBO concentration.

Arachidonic acid mobilization among phospholipids in murine mastocytoma P-815 cells: role of ether-linked phospholipids.

The ethanolamine-containing glycerophospholipids, choline-containing glycerophospholipids, and phosphatidylinositol fractions are major sources of arachidonic acid in murine mastocytoma P-815 cloned cells. The choline-linked fraction contained high arachidonic acid contents in 1-O-alkyl-2-acyl- (18%) and 1,2-diacyl-sn-glycero-3-phosphocholine (11%), with smaller amounts in 1-O-alk-1'-enyl-2-acyl species, whereas the arachidonic acid content of the ethanolamine-linked fraction was high in 1-O-alk-1'-enyl-2-acyl (26%) and 1,2-diacyl species (15%) and low in 1-O-alkyl-2-acyl species. The uptake and transfer of [3H]arachidonic acid into the 1,2-diacyl and ether classes of choline-containing glycerophospholipids and ethanolamine-containing glycerophospholipids in mastocytoma cells were examined. There was very rapid incorporation of radioactive arachidonic acid into mastocytoma cells that leveled off after 30 min. By labeling cells with [3H]arachidonic acid for 7.5 min, the radioactivity was recovered in the choline-containing glycerophospholipids (43%), phosphatidylinositol (32%), and ethanolamine-containing glycerophospholipids (20%) with little in other phospholipids, neutral lipid, or free fatty acid fractions. Upon reincubation of the mastocytoma cells in the radiolabel-free medium, the [3H]arachidonate radioactivity was gradually lost from the choline-containing glycerophospholipids fraction and, concomitantly, increased in ethanolamine-containing glycerophospholipids. At the zero time of reincubation, most of the radioactivity was recovered in the 1,2-diacyl species of both choline-containing glycerophospholipids and ethanolamine-containing glycerophospholipids.(ABSTRACT TRUNCATED AT 250 WORDS)

AUGMENTATION OF CELL-MEDIATED CYTOTOXICITY FOLLOWING 50% PARTIAL HEPATECTOMY

The cytolytic responses of C3H/HeJ mice after 50% hepatectomy (PH) were assessed in a 4-hr 51Cr-release assay. Spleen cells (SC) (50 x 10(6] from normal or PH C3H/HeJ (H-2k) mice were sensitized with equal numbers of irradiated allogeneic DBA/2 (H-2d) spleen cells in a five-day mixed lymphocyte culture. Generated cytolytic activity was measured against 51Cr-labeled P815 mastocytoma (H-2d) and EL4 lymphoma (H-2b) target cells. The wet weight and cell numbers per spleen following 50% partial hepatectomy were 70% and 75% higher than the control values for the first 20 days, and then returned to normal levels by 21 days. The cytolysis by spleen cells from 2-, 14-, and 31-day PH mice were 89.3 +/- 0.7, 86.9 +/- 5.3, and 90.1 +/- 1.3%, respectively, compared with control (sham-operated) values of 56.0 +/- 1.0, 57.0 +/- 2.0, and 49.9 +/- 7.0% (P less than 0.03 at E/T 100:1). This enhanced cytolysis by PH spleen cells remained high for at least 118 days after the liver resection before returning to control levels by 268 days. Cytolytic effector cells in PH SC were generated at least 24 hr earlier than in control SC. When normal and PH cytolysis were compared following primary and secondary in vitro sensitization, the cytolytic levels of primarily-sensitized PH spleen cells were comparable to secondarily sensitized normal spleen cells. Furthermore, the primarily sensitized normal spleen cells did not show crossreactive cytolysis with EL4 target cells (H-2b), while both the primarily sensitized PH spleen cells and the secondarily sensitized normal spleen cells were significantly cross-reactive against the third party EL4 target cells. Adherent PH spleen cells appear to be responsible for this augmented cytolytic capacity since their coculture with normal nonadherent responder spleen cells increased control cytolysis by approximately 30%. These studied demonstrate that, following 50% partial hepatectomy, there is an immediate and sustained increase in the allospecific cytolytic response.

Effect of tunicamycin on functions of PGE 1 receptors from mouse mastocytoma P-815 cells

Prostaglandin E 1 (PGE 1) receptors from mouse mastocytoma P-815 cells were found to bind to a wheat germ agglutinin (WGA)-Agarose column, suggesting that the receptors are glycoproteins. To further elucidate the role of carbohydrate moieties in the PGE 1 receptors for their binding activity to ligand, the P-815 cells were treated with tunicamycin, swainsonine or monensin. Tunicamycin, an inhibitor of N-glycosylation, dose- and time-dependently inhibited the binding of PGE 1 to mastocytoma P-815 cells. Neither swainsonine, an inhibitor of Golgi mannosidase II, nor monensin, an inhibitor of processing beyond the high mannose stage, altered PGE 1 binding properties of the cells. The inhibition of PGE 1 binding by tunicamycin was observed when incorporation of [ 3H]glucosamine into macromolecules was inhibited. The inhibitory effect was not on their affinity but on their number of binding sites. Subcellular distributions of [ 3H]PGE 1-binding activity showed that decreases in the binding activity by tunicamycin were highest in plasma membrane fractions. Treatment of membranes with various endo- and exoglycosidases did not affect PGE 1 binding. PGE 1-stimulated cyclic AMP accumulation in the cells was also inhibited by tunicamycin. These results suggest that PGE 1 receptors of mastocytoma P-815 cells are glycoproteins and that inhibition of N-glycosylation of PGE 1 receptors by tunicamycin results in the arrest of the translocation of newly synthesized receptors to the surface of mastocytoma P-815 cells.

Effect of in vivo administration of T-2 toxin on peritoneal murine macrophages

We investigated the effect of in vivo administration of T-2 toxin, a 12,13-epoxytrichothecene produced by several Fusarium species, on murine macrophage metabolism. Cytoplasmic and lysosomal enzyme levels, generation and release of superoxide anion, phagocytosis and intracellular killing of Salmonella typhi and murine P815 tumour cell lysis were measured under different experimental conditions. When T-2 toxin was administered to mice at sublethal doses (0.50-1.00 mg/kg/24 hr), the levels of lysosomal and cytoplasmic enzyme activity and the generation of superoxide anion were significantly enhanced as compared to controls. This correlated with increased phagocytosis and intracellular killing of S. typhi. Cytotoxic activity against murine P815 mastocytoma cells exhibited by macrophages isolated from mice treated with T-2 toxin was inhibited in a dose-dependent manner. In vivo administration of T-2 toxin may result in the activation of specific metabolic pathways of peritoneal macrophages, while inhibiting other paths.

Attachment of tumor cells to endothelial monolayers: Detection of surface molecules involved in cell-cell binding

Trypsinization of P815 mastocytoma cells interferes with their ability to bind to endothelial monolayers in vitro, suggesting the involvement of proteins in cell-cell binding. Binding is also dependent upon divalent cations. Incubation of tumor cells with tunicamycin, which blocks protein glycosylation, or the monosaccharide N-acetyl- d-glucosamine could also inhibit binding. In addition, antibodies prepared against whole P815 cells had the ability to interfere with tumor cell binding. The protein fragments released from P815 cells by trypsin interfered with the binding of fresh P815 cells to endothelium. Moreover, those fragments as well as CHAPS extracts of tumor cell membranes were able to neutralize the inhibitory effect of an anti-P815 antibody on tumor cell-endothelial cell binding. These observations, taken together, strongly suggest the presence of adhesion molecules on the tumor cell. The anti-P815 antibody preparation could also inhibit endothelial binding of an unrelated tumor, Ehrlich ascites (EA). In addition, membrane preparations of EA could neutralize the antibody's effect on intact P815, and membrane preparations of P815 could neutralize the antibody's effect on EA. These observations suggest that there are common epitopes on these tumor cells and raise the possibility that a variety of tumors may share common mechanisms for attachment to endothelium. This, in turn, raises the possibility that monospecific antibodies against such epitopes might be useful agents for interfering with metastasis in vivo.

Cyclic AMP, calcium and the growth of mastocytoma cells

Arresting P815 mastocytoma cell growth with N 6, O 2′-dibutyryladenosine 3′:5′ cyclic monophosphate (db cAMP) and theophylline increased 45Ca 2+ uptake and efflux by the cells (i.e., Ca 2+ cycling) without altering cytoplasmic free Ca 2+ concentrations or the amount or distribution of protein kinase C in the cells. Attempts to identify the Ca 2+ channels involved using a wide variety of drugs were unsuccessful. However, the inhibitory effect of db cAMP on growth was greatly increased in medium containing low Ca 2+ concentrations, confirming that interactions between Ca 2+ and cyclic AMP can affect mastocytoma cell growth.

Alteration of in vitro murine peritoneal macrophage function by dietary enrichment with eicosapentaenoic and docosahexaenoic acids in menhaden fish oil

The effects of diets containing menhaden fish oil (MFO), compared with those of diets containing safflower oil (SAF) or an essential fatty acid deficient hydrogenated coconut oil (HCO), on in vitro activation of tumoricidal capacity by murine macrophages were assessed. Mice fed the experimental diets for 4 weeks were injected intraperitoneally with sterile thioglycollate broth 3 days before use. There was no difference between any of the groups with respect to total peritoneal exudate cells or the percentage of macrophages, although the fatty acid profile of purified adherent macrophages closely paralleled that of the diets. Macrophages from mice fed MFO killed fewer P815 mastocytoma cells upon activation with recombinant interferon γ (IFNγ) and lipopolysaccharide. Macrophages from all diets were equally competent for tumoricidal capacity when activated pharmacologically with calcium ionophore, phorbol 12-myristate 13-acetate, and lipopolysaccharide (LPS), suggesting that MFO diet macrophages were hyporesponsive to IFNγ. Priming with higher concentrations of IFNγ restored the partial defect in activation of MFO macrophages. When activated for 24 hr with high levels of LPS, macrophages from mice fed SAF displayed little cytolytic capacity; addition of indomethacin (1 μ M) resulted in enhanced levels of P815 kill. In contrast, MFO and HCO diet macrophages were highly cytolytic with similar LPS treatment with or without indomethacin. Macrophages from mice fed SAF produced threefold more prostaglandin E in response to LPS than did MFO and HCO diet macrophages. These results suggest that dietary manipulation of fatty acids can alter activation of tumoricidal capacity of macrophages, possibly both dependent and independent of changes in eicosanoid synthesis.

Effects of glycyrrhizin and glycyrrhetinic acid on dexamethasone-induced changes in histamine synthesis of mouse mastocytoma P-815 cells and in histamine release from rat peritoneal mast cells

The effects of glycyrrhizin and its aglycone, glycyrrhetinic acid, on dexamethasone-induced changes in the histamine synthesis of mastocytoma P-815 cells and in the histamine release from antigen-stimulated rat peritoneal mast cells were investigated. Glycyrrhetinic acid but not glycyrrhizin, at concentrations from 20 to 35 μM, almost completely inhibited the dexamethasone-induced increases in both the histamine content and histidine decarboxylase activity of cultured mastocytoma P-815 cells. Glycyrrhetinic acid, however, showed practically no inhibition of [ 3H]dexamethasone binding to the cytoplasmic receptor. On the other hand, glycyrrhetinic acid but not glycyrrhizin markedly inhibited the release of histamine from antigen-stimulated rat mast cells, and intensified the inhibitory activity induced by dexamethasone. Glycyrrhetinic acid inhibited the antigen-induced release and incorporation of [ 3H]arachidonic acid in immunized rat mast cells. The administration of glycyrrhizin into rats, in contrast to the in vitro treatment of the cells with glycyrrhizin, markedly inhibited histamine release from antigen-stimulated rat mast cells. These results suggest that glycyrrhetinic acid inhibited dexamethasone-induced changes in the histamine synthesis of mastocytoma P-815 cells, and in the histamine release from rat mast cells. On the other hand, glycyrrhizin may exert its effect after conversion to glycyrrhetinic acid in vivo.

Humoral mechanism of the membrane-toxic effect of mastocytoma P815 and leukemia EL4 cells

The membranotoxicity activity of P815 and EL4 tumor cells and their humoral factors containing in cultural supernatants and ascitic fluids was estimated. Tumor cells and their humoral factors exert dose-dependent membranotoxicity effect on murine splenocytes and lymphoblasts. Normal murine splenocytes and renal cells have no membranotoxicity properties. Thus tumor cells membranotoxicity effect is carried out by humoral mechanism.

Action of humoral factors of mastocytoma P815 cells on formation of allospecific killer cells in mixed lymphocyte culture and on their cytotoxic activity

Action of humoral factors of mastocytoma P815 cells on formation of allospecific killer cells in mixed lymphocyte culture and on their cytotoxic activity

Effect of immune lymph node factors on immunologic memory formation

It was shown previously that peripheral lymph node cells secrete an antigen-specific factor after short-term contact with antigens in vivo [2, 3]. Similar mediators were found in the serum of animals 3-6 h after immunization or in a cell culture in vitro by other workers [7, 8]. Injection of immune lymph node factors (ILNF) into intact animals led to changes similar to those found after injection of the antigens themselves. Just like antigen, ILNF induced a decrease in antigen-specific stimulating activity and an increase in antigen-nonspecific suppressor activity in bone marrow cells [2, 3]. Meanwhile injection of ILNF and antigens (sheep's red blood cells or P-815 mastocytoma cells) led to a three to sixfold increase in the intensity of humoral and cellular reactions [i].

Binding of Activated Macrophages to Tumor Cells through a Macrophage Lectin and Its Role in Macrophage Tumoricidal Activity1

A 45-60 kDa Gal/GalNAc-specific macrophage lectin was found to participate in the interaction between tumor cells and tumoricidal macrophages activated by an antitumor streptococcal preparation, OK-432, and in the tumoricidal activity of the activated macrophages. The binding between OK-432-elicited activated macrophages and murine mastocytoma P-815 cells was inhibited on preincubation of the macrophages with a neoglycoprotein (Gal-BSA) or a complex-type glycopeptide (unit B) which was a specific inhibitor of the macrophage lectin. This binding of the macrophages to P-815 cells was also inhibited on the addition of anti-macrophage lectin antiserum. Contrary to the case of OK-432-elicited macrophages, the binding of thioglycolate-elicited (responsive) macrophages to P-815 cells was inhibited only a little by Gal-BSA and unit B, and not inhibited by the antiserum. Furthermore, the tumoricidal activity of the activated macrophages was inhibited by the addition of the anti-macrophage lectin antiserum. These results suggest that the binding of activated macrophages to tumor cells through the Gal/GalNAc-specific macrophage lectin is an important part of the tumor cell killing mechanism.