Endocan or endothelial cell specific molecule-1 (ESM-1): a potential novel endothelial cell marker and a new target for cancer therapy.

Endocan, previously called endothelial cell specific molecule-1, is a soluble proteoglycan of 50 kDa, constituted of a mature polypeptide of 165 amino acids and a single dermatan sulphate chain covalently linked to the serine residue at position 137. This dermatan sulphate proteoglycan, which is expressed by the vascular endothelium, has been found freely circulating in the bloodstream of healthy subjects. Experimental evidence is accumulating that implicates endocan as a key player in the regulation of major processes such as cell adhesion, in inflammatory disorders and tumor progression. Inflammatory cytokines such as TNF-alpha, and pro-angiogenic growth factors such as VEGF, FGF-2 and HGF/SF, strongly increased the expression, synthesis or the secretion of endocan by human endothelial cells. Endocan is clearly overexpressed in human tumors, with elevated serum levels being observed in late-stage lung cancer patients, as measured by enzyme-linked immunoassay, and with its overexpression in experimental tumors being evident by immunohistochemistry. Recently, the mRNA levels of endocan have also been recognized as being one of the most significant molecular signatures of a bad prognosis in several types of cancer including lung cancer. Overexpression of this dermatan sulphate proteoglycan has also been shown to be directly involved in tumor progression as observed in mouse models of human tumor xenografts. Collectively, these results suggest that endocan could be a biomarker for both inflammatory disorders and tumor progression as well as a validated therapeutic target in cancer. On the basis of the recent successes of immunotherapeutic approaches in cancer, the preclinical data on endocan suggests that an antibody raised against the protein core of endocan could be a promising cancer therapy.

Endocan is a novel chondroitin sulfate/dermatan sulfate proteoglycan that promotes hepatocyte growth factor/scatter factor mitogenic activity.

Proteoglycans that modulate the activities of growth factors, chemokines, and coagulation factors regulate in turn the vascular endothelium with respect to processes such as inflammation, hemostasis, and angiogenesis. Endothelial cell-specific molecule-1 is mainly expressed by endothelial cells and regulated by pro-inflammatory cytokines (Lassalle, P., Molet, S., Janin, A., Heyden, J. V., Tavernier, J., Fiers, W., Devos, R., and Tonnel, A. B. (1996) J. Biol. Chem. 271, 20458-20464). We demonstrate that this molecule is secreted as a soluble dermatan sulfate (DS) proteoglycan. This proteoglycan represents the major form either secreted by cell lines or circulating in the human bloodstream. Because this proteoglycan is specifically secreted by endothelial cells, we propose to name it endocan. The glycosaminoglycan component of endocan consists of a single DS chain covalently attached to serine 137. Endocan dose-dependently increased the hepatocyte growth factor/scatter factor (HGF/SF)-mediated proliferation of human embryonic kidney cells, whereas the nonglycanated form of endocan did not. Moreover, DS chains purified from endocan mimicked the endocan-mediated increase of cell proliferation in the presence of HGF/SF. Overall, our results demonstrate that endocan is a novel soluble dermatan sulfate proteoglycan produced by endothelial cells. Endocan regulates HGF/SF-mediated mitogenic activity and may support the function of HGF/SF not only in embryogenesis and tissue repair after injury but also in tumor progression.

Human endothelial-cell specific molecule-1 binds directly to the integrin CD11a/CD18 (LFA-1) and blocks binding to intercellular adhesion molecule-1.

ICAMs are ligands for LFA-1, a major integrin of mononuclear cells involved in the immune and inflammatory processes. We previously showed that endothelial cell specific molecule-1 (ESM-1) is a proteoglycan secreted by endothelial cells under the control of inflammatory cytokines. Here, we demonstrate that ESM-1 binds directly to LFA-1 onto the cell surface of human blood lymphocytes, monocytes, and Jurkat cells. The binding of ESM-1 was equally dependent on Ca(2+), Mg(2+), or Mn(2+) divalent ions, which are specific, saturable, and sensitive to temperature. An anti-CD11a mAb or PMA induced a transient increase in binding, peaking 5 min after activation. Direct binding of ESM-1 to LFA-1 integrin was demonstrated by specific coimmunoprecipitation by CD11a and CD18 mAbs. A cell-free system using a Biacore biosensor confirmed that ESM-1 and LFA-1 dynamically interacted in real time with high affinity (K(d) = 18.7 nM). ESM-1 consistently inhibited the specific binding of soluble ICAM-1 to Jurkat cells in a dose-dependent manner. These results suggest that ESM-1 and ICAM-1 interact with LFA-1 on binding sites very close to but distinct from the I domain of CD11a. Through this mechanism, ESM-1 could be implicated in the regulation of the LFA-1/ICAM-1 pathway and may therefore influence both the recruitment of circulating lymphocytes to inflammatory sites and LFA-1-dependent leukocyte adhesion and activation.

Characterization of the secreted form of endothelial-cell-specific molecule 1 by specific monoclonal antibodies.

Endothelial-cell-specific molecule 1 (ESM-1) is a recently identified endothelial cell molecule. As ESM-1 mRNA is preferentially expressed in human lung and kidney tissues, and as ESM-1 mRNA expression is regulated by inflammatory cytokines, ESM-1 is thought to play a role in the vascular contribution to organ-specific inflammation. In order to define its behavior, mouse anti-ESM-1 monoclonal antibodies were developed, and three distinct epitopes were mapped, which allowed development of a specific ELISA assay, immunohistological staining and immunoblot analysis. Here, we demonstrate that ESM-1 is present in cell lysates of human endothelial cells (human umbilical vein endothelial cells) with an apparent molecular weight of 20 kD. In contrast, the secreted form of ESM-1 is shifted to an apparent molecular weight of 50 kD, indicating that the secreted form of ESM-1 is posttranslationally modified. By ELISA, we show that the secretion of ESM-1 is significantly enhanced in the presence of TNFalpha. In contrast, the spontaneous as well as TNFalpha-induced secretion of ESM-1 is strongly inhibited by IFNgamma. Moreover, ESM-1 was detected in the serum of healthy subjects at an average concentration of 1.08 ng/ml, and we demonstrated that the serum level of ESM-1 is dramatically increased in patients presenting a septic shock. Analysis of ESM-1 expression in normal human tissues by immunohistochemistry showed that ESM-1 is localized in the vascular network, but also in the bronchial and renal epithelia. Our results demonstrate that ESM-1 is mainly expressed in the vascular endothelium both in vitro and in vivo, but also by different epithelia. ESM-1 may represent a new marker of endothelial cell activation, and may have a functional role in endothelium-dependent pathological disorders.

Activation of Xenopus eggs by the kinase inhibitor 6-DMAP suggests a differential regulation of cyclin B and p39(mos) proteolysis.

In Xenopus eggs, metaphase II arrest is due to the cytostatic factor that maintains a high level of MPF activity. Kinases are important in this phenomenon since p39(mos) and MAPK play a part in the cytostatic activity whereas p34(cdc2) is the catalytic subunit of MPF. Fertilization induces a rise in intracellular calcium leading to egg activation that can be mimicked by calcium-increasing agents such as calcium ionophore. We have performed on Xenopus eggs a biochemical comparison of the effects of the kinase inhibitor 6-DMAP and the calcium ionophore. Both drugs were able to induce pronucleus formation but the underlying molecular events were different. The inactivation of MAPK occurred earlier in eggs exposed to 6-DMAP. Cyclins B1 and B2 were stable and p39(mos) was proteolysed in 6-DMAP-treated eggs while the three proteins underwent degradation in A23187-treated ones. These results suggest a differential regulation of ubiquitin-dependent proteolysis of cyclin B and p39(mos).

Inhibition of protein tyrosine phosphatases blocks calcium-induced activation of metaphase II-arrested oocytes of Xenopus laevis.

We have studied the effect of a protein tyrosine phosphatases (PTP) inhibitor on calcium-induced activation of Xenopus laevis oocytes arrested at metaphase II. Ammonium molybdate microinjection blocked pronucleus formation following A23187 treatment while cortical granules still underwent exocytosis. Pronuclei still occurred in ammonium molybdate-injected oocytes following 6-DMAP addition. Changes that usually occurred following A23187 exposure were inhibited in the presence of ammonium molybdate in the oocyte: MAPK dephosphorylation, p34(cdc2) rephosphorylation and cyclin B2 and p39(mos) proteolysis. These results suggest that a PTP is involved in the activation of the ubiquitin-dependent degradation machinery.

Release of endothelin in relation to tumor necrosis factor-alpha in porcine Pseudomonas aeruginosa-induced septic shock.

Septic shock is characterized by surges of tumor necrosis factor-alpha (TNF-alpha) along with myocardial dysfunction and systemic hypotension. TNF-alpha promotes the release of immunoreactive endothelin (ET). Because TNF-alpha is elevated in septic shock, we hypothesized that elevated levels of endothelin can contribute to cardiac dysfunction and hypotension. We infused live Pseudomonas aeruginosa into anesthetized, hemodynamically monitored young swine and measured ET and TNF-alpha. Septic swine developed systemic arterial hypotension and had significantly elevated TNF-alpha (4.15 +/- .41 U/ml at 1 h versus .40 +/- .13 U/ml at time zero) compared to control animals. ET levels were significantly elevated at 4 h (52.38 +/- 12.88 pg/ml vs. 10.45 +/- 1.82 pg/ml at time zero) and correlated negatively with the decline in cardiac output. We then passively immunized swine using anti TNF-alpha prior to the induction of sepsis to examine if TNF played a central role in the release ET. The anti TNF-alpha effectively removed circulating TNF-alpha bioactivity in septic animals. Anti-TNF-alpha-treated animals did not develop significant systemic arterial hypotension and had significant attenuation in endothelin (19.01 +/- 4.18 pg/ml at 4 h compared to 52.38 +/- 12.88 pg/ml in septic animals at 4 h) which correlated with preservation of cardiac output. TNF-alpha may cause cardiac dysfunction in sepsis syndrome through increased release of ET.

Anti-CD18 antibody attenuates neutropenia and alveolar capillary-membrane injury during gram-negative sepsis.

Activated polymorphonuclear leukocytes (PMNs) are implicated in the pathogenesis of acute lung injury (ALI) associated with sepsis. Adhesion of activated PMNs to endothelial monolayers is mediated by the CD18 adhesion-receptor complex on the PMN cell surface. Monoclonal antibody 60.3 (MoAb 60.3) blocks CD18-dependent PMN-endothelial adhesion in vitro and in vivo. This study was designed to determine the role of CD18-dependent PMN adhesion in ALI associated with gram-negative sepsis. Anesthetized, ventilated (FiO2 0.5, positive end-expiratory pressure 5 cm H2O) pigs received sterile saline (control, n = 8) or live Pseudomonas aeruginosa, 5 x 10(8) colony-forming units/ml at 0.3 ml/20 kg/min (septic, n = 9) for 1 hour. A third group (n = 7) received MoAb 60.3, 2 mg/kg intravenously, 15 minutes before Pseudomonas infusion. Animals were studied for 300 minutes. MoAb 60.3 significantly (p less than 0.05) attenuated the neutropenia seen in sepsis (15 +/- 1 vs 6 +/- 1 x 10(3) PMNs/mm3 at 300 min). Alveolar-capillary membrane injury was assessed by bronchoalveolar-lavage protein content and extravascular lung water determination. MoAb 60.3 significantly (p less than 0.05) reduced BAL protein at 300 minutes (388 +/- 75 vs 1059 +/- 216 micrograms/ml in septic animals) and attenuated the increase in extravascular lung water to 240 minutes (7.1 +/- 2 vs 14.2 +/- 1.2 ml/kg in septic animals). Systemic hypotension, decreased cardiac index, pulmonary hypertension, and relative hypoxemia, all characteristic of this model, were not altered by MoAb 60.3. These data suggest that, in this model of septic ALI, neutropenia is, in part, CD18 dependent and that blocking CD18-dependent PMN adhesion protects the alveolar-capillary membrane independently of altered hemodynamic status.

In situ pulmonary vascular perfusion for improved recovery of pulmonary intravascular macrophages.

The microcirculation contains mononuclear phagocytes, with features characteristic of macrophages, adhered to luminal capillary surfaces by intercellular adhesion plaques. These pulmonary intravascular macrophages may play an important role in regulating lung vascular tone and capillary permeability, and may modulate capillary endothelial cell growth and replication by the secretion of soluble mediators (i.e., arachidonate metabolites, cytokines). This study describes a technique which utilizes in situ lung perfusion to remove intravascular macrophages in large numbers from the microcirculation of porcine lung (n = 26). This technique yielded 3.8 +/- 0.5 x 10(8) (mean +/- SEM) mononuclear cells which were highly phagocytic toward particulate carbon (phagocytic index, 80 +/- 6%). Harvested mononuclear phagocytes reestablished intercellular adhesion plaques when placed on small vessel porcine pulmonary artery endothelial cell monolayers and exhibited histochemical characteristics typical of monocyte/macrophage lineage cells. Mononuclear cells obtained from lung microcirculation displayed size heterogeneity varying from 10.4 to 16.5 microns in diameter. Both large and small cell populations phagocytosed particulate carbon. Morphometric studies performed on collagenase-treated lung demonstrated that in situ perfusion removed significant numbers of intravascular macrophages in lung capillaries. The technique described permits the rapid removal of anchored mononuclear phagocytes from lung capillaries with minimal postmortem delay.

CD18 adhesion receptors, tumor necrosis factor, and neutropenia during septic lung injury.

Sequestration of neutrophils (PMNs) in the pulmonary microvasculature and associated neutropenia are characteristic features of experimental models of septic lung injury. The etiology of altered PMN kinetics during septic lung injury is uncertain, but may be partially due to increased adhesiveness of activated PMNs to pulmonary endothelium. This study examines the relationship between the expression of PMN CD18 adhesion receptors, the evolving neutropenia, and plasma tumor necrosis factor (TNF) activity in a porcine model of septic lung injury. Acute lung injury was induced by infusion of live Pseudomonas aeruginosa (5 x 10(8) CFU/ml at 0.3 ml/20 kg/min) for 60 min (Group Ps, n = 6). Control animals (Group C, n = 3) received a 60-min infusion of sterile 0.9% saline. CD18 expression of circulating PMNs was measured by quantitative immunofluorescent flow cytometry. Plasma TNF activity was measured by L929 fibroblast cytolytic assay. Group Ps developed a significant neutropenia by 30 min (14.9 +/- 2.5 vs 23.4 +/- 3.3 x 10(3) cells/microliter at baseline, P less than 0.05, ANOVA) with circulating neutrophils exhibiting significantly increased CD18 expression by 60 min (6.34 +/- 0.72 vs 5.01 +/- 0.52 equivalent soluble fluorescence molecules (ESFM) x 10(3) at baseline, P less than 0.05, ANOVA). Group Ps demonstrated a significant increase in plasma TNF activity by 30 min (2.5 +/- 0.9 vs 0.7 +/- 0.3 U/ml at baseline). There was no significant change in PMN count, PMN CD18 expression, or plasma TNF activity in Group C. In complimentary in vitro studies, porcine PMNs stimulated with recombinant human TNF-alpha (n = 5) demonstrated a time- and dose-dependent increase in CD18 expression.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo interleukin-2 activated sheep lung lymph lymphocytes increase ovine vascular endothelial permeability by non-lytic mechanisms.

Therapeutic doses of recombinant interleukin-2 (rIL-2) often result in systemic toxicity consistent with increased vascular permeability. rIL-2 activated lymphocytes (IALs) may produce endothelial dysfunction and have cytolytic potential. However, much of the data on IAL cytotoxicity comes from the use of in vitro activated IALs. Alternatively, rIL-2 may enhance permeability directly or via release of various cytokines by host effector cells. The cytotoxicity of in vivo activated lung lymph lymphocytes has been studied in an ovine model of rIL-2 toxicity. The in vivo IALs had no significant endothelial cytolysis at effector to target ratios of 100:1. However, the in vivo IALs increased endothelial monolayer permeability to albumin, dependent on the concentration of IALs. rIL-2 induced no endothelial cytolysis or permeability alterations at doses of 10(5) and 2 x 10(5) U/ml, respectively. These findings suggest that the acute endothelial dysfunction characteristic of the vascular leak syndrome is not due to rIL-2 directly, but is mediated by in vivo IALs via non-cytolytic mechanisms and/or the release of secondary cytokines in response to rIL-2.

Nonspecific cytotoxicity of recombinant interleukin-2 activated lymphocytes.

The administration of interleukin-2 (IL-2) and lymphokine activated killer (LAK) cells to patients with advanced metastatic cancer has yielded encouraging results. The purported ability of LAK cells to be discriminatively tumoricidal, thus sparing normal host tissue, represents a major advance over conventional chemotherapy. However, IL-2 adoptive immunotherapy results in dose-limiting toxicity characterized by weight gain, dyspnea, ascites, and peripheral-pulmonary edema suggestive of a vascular leak syndrome. It is unclear whether the observed toxicity is directly related to IL-2 and/or LAK cells. The authors examined the cytolytic nature of human LAK cells against human endothelial, epithelial, and fibroblast cell lines. Bovine endothelial cells also were studied. Using a 51Cr release assay, the cytolytic potential, time course, and effect of reactive oxygen intermediate inhibitors were studied. LAK cells were uniformly toxic against all cell lines, in contrast to high dose rIL-2 and excipient. Significant cytolysis was observed within 30 minutes and increased over the first 2 hours of LAK cells coming in contact with target cells. Reactive oxygen intermediate inhibitors did not reduce cytolytic activity. The authors thus found human LAK cells to be rapidly cytolytic against a variety of human and bovine cell lines. This cytolysis was independent of reactive oxygen intermediates.

Cardiorespiratory and cellular changes with interleukin 2 infusion in sheep.

Recombinant interleukin 2 (rIL-2) administration, a new form of therapy for patients with far-advanced cancer, is associated with a "third space" syndrome, i.e., pulmonary edema, respiratory distress, and hypoxemia, which limits the dose and duration of treatment. To extend our knowledge regarding this toxicity, we established a sheep chronic lung lymph fistula model and measured hemodynamics, arterial blood gases, caudal mediastinal (lung) lymph flow (QL), and blood and lung lymph cellular changes before, during, and after (recovery) a 3-day continuous rIL-2 infusion (9 x 10(5) U/kg). Moderate systemic hypotension, mild pulmonary hypertension, and an increase in alveolar-arterial PO2 gradient was present on day 3 of rIL-2 infusion. QL increased from a base line of 1.9 +/- 0.2 to a maximum of 4.3 +/- 1.1 ml/15 min on day 3 of rIL-2 infusion. At no time was there a change in lymph-to-plasma protein ratio. The leukocyte count increased significantly to 16.1 +/- 4.5 x 10(3) cells/mm3 at recovery day 1. The percentage of blood lymphocytes decreased significantly by day 1 of rIL-2 infusion, returned to base-line levels on day 3, and significantly increased on day 2 of recovery. Lung lymph lymphocytes decreased significantly on days 1 and 2 of rIL-2 infusion. There was a shift in their size; i.e., their area increased from 32 +/- 7 to 57 +/- 19 micron 2 (P less than 0.05) by day 2 of rIL-2 infusion. By day 1 of recovery, lung lymph lymphocyte counts increased significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiopulmonary toxicity of adoptive immunotherapy.

Adoptive immunotherapy, the administration of interleukin-2 (IL-2) and interleukin-2 activated cells, leads to tumor regression in some patients with advanced cancer. Although this new therapeutic modality offers hope for the future, at present, a multitude of toxicities limit the total dose and duration of therapy. Among the toxic side effects a purported third space or vascular leak syndrome is the most serious. In this review, we detail the evidence for a third space syndrome (peripheral edema, ascites, oliguria, elevated serum creatinine levels) and cardiopulmonary dysfunction (hypotension, respiratory distress, pulmonary edema, hypoxemia) with adoptive immunotherapy in human and animal studies. We conclude that IL-2 administration is associated with increased pulmonary microvascular permeability, infiltration of the lung parenchyma with large esterase negative lymphoid cells, hypoxemia, systemic hypotension, positive fluid balance and, in animals, transient pulmonary hypertension. These abnormalities do not seem to be caused by IL-2 directly; the causes may be mediated by IL-2 activated lymphocytes or other IL-2 activated cellular mediators.

Macrophage spreading disparity: alveolar vs peritoneal.

Macrophage spreading over glass surfaces is a recognized in vitro manifestation of activation. We examined macrophage spreading using a simplified assay. Hartley guinea pigs (n = 15) were anesthetized with pentobarbital (0.4 mg/gm) IM. Macrophages were obtained by lavaging the peritoneal (PM) and alveolar (AM) spaces with sterile 0.9% NaCl (NS). AM or PM (150,000 cells) were placed into chambers of Lab-Tek microtiter slides +/- phorbol myristate acetate (PMA). Slides were incubated /37 degrees C in 5% CO2 for 20 minutes. Macrophages with a diameter greater than 2 x control cells were considered spread. Compared to PM, resident AM show increased spontaneous spreading (16 +/- 2% vs 79 +/- 2%) respectively. AM demonstrated no significant concentration-dependent response to PMA stimulation. The PM has served as the basis for much of the morphological and functional observations attributed to macrophages in general. The above spreading data support the concept of disparity of macrophage function dependent upon various factors, including site of origin. Our observations suggest that extrapolation of macrophage characteristics to cells from discordant sources may not be possible.

A comparison of the cardiopulmonary effects of continuous versus bolus infusion of recombinant interleukin-2 in sheep.

The systemic administration of recombinant interleukin-2 (rIL-2) is used for the treatment of patients with far advanced cancer. However, treatment may be limited by a so-called "third space" syndrome. Whether these side effects are due to the total dose used or the method of administration is unclear. To define whether the continuous (Group 2) or bolus (Group 3) i.v. infusion of 9 x 10(5) units/kg rIL-2 over 72 h is associated with similar toxicities, we established a chronic sheep model and monitored changes in systemic and pulmonary vascular pressures, cardiac function, and gas exchange. At 72 h lung lymph flow, lymph/plasma protein ratios, lung histology, and extravascular lung water/dry lung weight were obtained. In both groups the infusion of rIL-2 resulted in an increase in high protein lung lymph flow, an increase in cardiac output, and a decrease in systemic vascular resistance. Large lymphoid cells were found by histology to be infiltrating the lung interstitium. In Group 2, in addition, there were mild pulmonary hypertension [pulmonary artery pressures increased from 14 +/- 5 to 22 +/- 6 mmHg (P less than 0.05)], systemic hypotension [81 +/- 7 compared to a baseline of 95 +/- 9 mmHg (P less than 0.01)], and worsening gas exchange. We conclude that a 72-h continuous or bolus infusion of equivalent doses of rIL-2 are associated with cardiopulmonary toxicity; however, pulmonary hypertension, systemic hypotension, and gas exchange are worse in animals receiving the continuous infusion.

Cardiopulmonary effects of recombinant interleukin-2 infusion in sheep.

The systemic administration of recombinant interleukin-2 (rIL-2) with or without lymphokine-activated killer (LAK) cells, a new treatment for patients with advanced cancer, is associated with a presumed "third-space" syndrome. To further define the extent and time course of this toxicity, we established a chronic sheep model and monitored changes in systemic and central vascular pressures, cardiac function, and gas exchange during a 72-h continuous intravenous infusion of rIL-2 at a total dose of 5 (group 3) or 9 x 10(5) U/kg (group 4). At 72 h, caudal mediastinal lymph flow, histology, and extravascular lung water-to-dry lung weight ratio (EVLW/DLW) were obtained. During the rIL-2 infusion there was a dose-dependent significant decrease in systemic blood pressure and arterial Po2 and an increase in core temperature. In group 4, pulmonary arterial pressure increased from a base line of 13 +/- 5 to 21 +/- 6 mmHg (P less than 0.05). Lung lymph flow was significantly increased in groups 3 and 4 compared with animals receiving 0.9% NaCl or excipient infusions (groups 1 and 2). EVLW/DLW values were elevated in groups 3 and 4 (P less than 0.01). In animals receiving rIL-2, histological evaluation revealed a dose-dependent infiltration of lung tissue by lymphoblastoid cells that stained esterase negative. We conclude that rIL-2 infusion in doses comparable to those given to humans results in alterations in systemic and central hemodynamics, gas exchange, high-protein lung lymph flow, and infiltration of lymphoblastoid cells into the lung parenchyma.

Responsiveness of guinea pig alveolar cells.

Guinea pig alveolar cells were obtained in situ via bronchoalveolar lavage. The cells were 86% macrophages (GPAM), (greater than 97% viability) with the remainder of the population comprised of lymphocytes and eosinophils. The following battery of functional assays were studied in GPAM: chemotaxis was stimulated by N-formyl-methionyl-leucine-phenylalanine (FMLP) and by phorbol-12-myristate-13-acetate (PMA) in a concentration-related manner; cytotoxicity as measured by 51Cr release from target cells +/- PMA was induced in P815 mastocytoma cells and less strongly in 3T3 normal mouse fibroblasts; release of N-acetyl-beta-D-glucosaminidase (NAGA) was stimulated by the calcium ionophore A23187, but not by PMA or the combination of PMA + A23187; superoxide anion production as measured by the reduction of ferricytochrome C was stimulated 25-fold by PMA; phagocytosis of opsonized 51Cr sheep red blood cells occurred in a time-related manner and reached its maximum after 120 min; and cell spreading, which exhibited a high rate of spontaneous spreading (76%), was only minimally stimulatable by PMA. The responsiveness of the GPAM, the ease of retrieval, and the large numbers of cells available make the guinea pig an ideal system for future study.