Apoptotic cells as sources for biologically active oxidized phospholipids

Acute inflammation is characterized by an accumulation of polymorphonuclear cells (PMNs), generation of reactive oxygen species, subsequent apoptosis of PMNs, and finally phagocytosis of apoptotic cells by macrophages. Recently, it has been demonstrated that during apoptosis oxidation of membrane phospholipids, especially phosphatidylserine, occurs. Moreover, we have shown that membrane vesicles released from apoptotic cells contain biologically active oxidized phospholipids. The involvement of oxidized phospholipids in the development of atherosclerosis, which is described as a chronic inflammatory disease, is increasingly recognized. These oxidized phospholipids were shown to induce several proinflammatory genes, such as monocyte chemoattractant protein 1 or interleukin-8, and it is hypothesized that lipid oxidation products also play a role in other chronic inflammatory disorders. On the other hand, oxidized phospholipids were shown to exert antiendotoxin effects by inhibiting lipopolysaccharide-induced signaling, representing a possible feedback loop during gram-negative infection. Additionally, it has been described that oxidized phospholipids are capable of inducing genes such as heme oxygenase-1 that are important for the resolution of acute inflammation. Moreover, oxidized phospholipids serve as recognition signals on apoptotic cells facilitating phagocytosis. In this review, we discuss the hypothesis that oxidized phospholipids generated in apoptotic cells (a) propagate chronic inflammation and (b) contribute to the resolution of acute inflammation.     

Oxidative response of human neutrophils, monocytes, and alveolar macrophages induced by unopsonized surface-adherent Staphylococcus aureus.

In contrast to results with bacterial suspensions, phagocytosis of unopsonized bacteria readily occurs when bacteria are adhered to glass or plastic surfaces. However, in contrast to neutrophils, alveolar macrophages produced much less DNA denaturation as measured by acridine orange metachromasia of phagocytized Staphylococcus aureus. We have studied the phagocytosis of unopsonized surface-adherent S. aureus and the subsequent production of reactive oxygen species by peripheral blood neutrophils, monocytes, and alveolar macrophages. Phagocyte-free systems were then used to show the relationship of the reactive oxygen species produced by neutrophils and alveolar macrophages and the denaturation of unopsonized S. aureus DNA with acridine orange. Peripheral blood neutrophils, monocytes, and alveolar macrophages from normal human volunteers were added to vials with adherent S. aureus without opsonin. Bacterial uptake and luminol- and lucigenin-dependent chemiluminescence were measured. Neutrophils developed much greater luminol-dependent chemiluminescence than monocytes or alveolar macrophages. Compared with neutrophils and monocytes, alveolar macrophages developed significantly greater concentrations of superoxide, as measured by lucigenin-dependent chemiluminescence and ferricytochrome c reduction. These findings suggested that products of the myeloperoxidase-hydrogen peroxide-halide pathway were generated when peripheral blood neutrophils were stimulated and that alveolar macrophages primarily produced superoxide. When these reactive oxygen species were generated in phagocyte-free systems containing S. aureus, products of the myeloperoxidase-hydrogen peroxide-halide pathway produced denaturation of S. aureus DNA, whereas superoxide did not. Thus, differences in reactive oxygen species produced during phagocytosis may be related to the different capacities of neutrophils and alveolar macrophages to denature unopsonized adherent S. aureus DNA.     

Phagocytosis and killing of bacteria by professional phagocytes and dendritic cells

Dendritic cells (DC) represent a class of professional antigen-presenting cells whose primary function is to alert the immune system, not to clear invading microorganisms. The objective of our study was to compare the abilities of polymorphonuclear neutrophilic leukocytes (PMN), monocytes, monocyte-derived macrophages (MDM), monocyte-derived immature DC (imDC), and mature DC (maDC) to ingest and destroy Staphylococcus aureus and Escherichia coli. Acridine orange staining and fluorescence microscopy demonstrated that MDM, followed by monocytes, imDC, and PMN, internalized bacteria well but that maDC exhibited less pronounced phagocytic activity. PMN, monocytes, and MDM exhibited a much higher capacity to kill ingested bacteria than both imDC and maDC. In summary, these data are in agreement with the generally accepted idea that different types of leukocytes fulfill specialized tasks in antigen presentation and killing of pathogens.     

Fas (CD95)-Fas ligand interactions are responsible for monocyte apoptosis occurring as a result of phagocytosis and killing of Staphylococcus aureus

Human peripheral blood monocytes become apoptotic following phagocytosis of Staphylococcus aureus. In this study, we investigated the mechanisms involved in this phenomenon. Cells exposed to bacteria were examined for the surface expression of Fas and Fas ligand (FasL). The level of soluble form of FasL was also measured in the culture supernatants. As Fas-mediated apoptosis involves the activation of caspases, the activities of caspase-8 and caspase-3 were determined. Finally, the involvement of oxidative stress in apoptosis of infected monocytes was investigated. The data indicated that as a consequence of phagocytosis of S. aureus, FasL is released from the monocyte surface and induces apoptosis of phagocytic monocytes and to some extent the bystander cells. The importance of this mechanism was confirmed by demonstrating that blockage of CD95 prevents S. aureus-induced apoptosis of monocytes. Cell death occurring after phagocytosis of S. aureus involves the activation of caspase-3-like proteases, as the specific caspase-3 inhibitor suppressed apoptosis of infected cells. The generation of reactive oxygen intermediates by phagocytic monocytes by itself is not sufficient as a death signal but rather acts in up-regulating FasL shedding and possibly in modulating caspase activity.     

Effect of a factor released by K562 malignant cells in culture on human neutrophil bactericidal activity.

We have previously demonstrated that k562 malignant cells in culture contain and release a low-molecular-mass (8-kDa) factor that inhibits adherence-related functions of neutrophils but does not alter fMet-Leu-Phe- or phorbol ester-induced oxidative burst (M. Amar, N. Amit, T. Pham Huu, S. Chollet-Martin, M.T. Labro, M.A. Gougerot-Pocidalo, and J. Hakim, J. Immunol. 144:4749-4756, 1990). In this study, we investigated the effects of this factor, referred to as inhibitory factor 1 (IF1), on the bactericidal activity of human polymorphonuclear cells (PMNs) on Staphylococcus aureus opsonized in various ways. S. aureus was used either nonopsonized or opsonized with heat-inactivated serum or normal serum containing complement factors. The bactericidal activity of PMNs preincubated with IF1-treated or control medium was examined by counting the surviving bacteria. The ability of IF1-treated PMNs to kill bacteria was diminished when they were opsonized with normal serum. When S. aureus was not opsonized or was opsonized with heat-inactivated serum, the bactericidal activity of IF1-treated PMNs was similar to that of controls. Likewise, the phagocytosis of IF1-treated PMNs was diminished when S. aureus was opsonized with normal serum but was not altered when S. aureus was not opsonized or was opsonized with heat-inactivated serum. These results suggest that the decrease in killing might be due to defective ingestion. The chemiluminescence response of IF1-treated PMNs was inhibited when S. aureus was not opsonized or was opsonized with normal serum. No effect on chemiluminescence was observed when S. aureus was opsonized with heat-inactivated serum. These results suggest that IF1 interferes not only with S. aureus stimulation of PMNs via complement receptors but also with oxygen-dependent bactericidal activity.     

Yersinia pestis type III secretion system-dependent inhibition of human polymorphonuclear leukocyte function

Human polymorphonuclear leukocytes (PMNs, or neutrophils) are the primary innate host defense against invading bacterial pathogens. Neutrophils are rapidly recruited to sites of infection and ingest microorganisms through a process known as phagocytosis. Following phagocytosis by human PMNs, microorganisms are killed by reactive oxygen species (ROS) and microbicidal products contained within granules. Yersinia pestis, the causative agent of plague, is capable of rapid replication and dissemination from sites of infection in the host. Although Y. pestis survives in macrophages, the bacterial fate following interaction with human PMNs is less clear. The ability of Y. pestis to inhibit phagocytosis by human PMNs was assessed by differential fluorescence microscopy and was shown to be dependent on expression of the type III secretion system (TTSS). Previous studies have demonstrated that TTSS expression in enteropathogenic Yersinia spp. also inhibits the respiratory burst in PMNs and macrophages, and we show here that human PMN ROS production is similarly repressed by Y. pestis. However, exclusion of uningested TTSS-expressing Y. pestis with gentamicin revealed that intracellular bacteria are eliminated by human PMNs, similar to bacteria lacking the TTSS. In summary, our results suggest that the Y. pestis TTSS contributes to extracellular survival following interactions with human PMNs and that the intracellular fate is independent of TTSS inhibition of neutrophil ROS production.     

Apoptotic killing and phagocytosis of host cells by the parasite Entamoeba histolytica

The ability of Entamoeba histolytica to kill and phagocytose host cells correlates with parasite virulence. This study addressed the role of apoptotic cell killing and host cell phosphatidylserine exposure in the subsequent phagocytosis of Jurkat T cells by E. histolytica. Ingested host cells were apoptotic, as evidenced by the activation of caspase 3 in 88% +/- 3% (mean and standard deviation [SD] of the mean) of Jurkat cells engulfed by E. histolytica; ingested cells without detectable active caspase 3 were already disrupted and partially digested. That apoptotic cell killing preceded phagocytosis was supported by the demonstration that a higher percentage of amebae ingested apoptotic cells than ingested healthy cells (62% +/- 7% versus 30% +/- 9%, respectively [mean and SD]) (P = 0.008). E. histolytica also ingested apoptotic Jurkat cells more rapidly than necrotic control cells (8.5% +/- 0.4% versus 3.5% +/- 0.7%, respectively [mean and SD]) (P < 0.001). The inhibition of amebic cytotoxicity with D-galactose (which blocks the amebic Gal/GalNAc lectin) blocked the phagocytosis of healthy cells by greater than 80%, providing further evidence that apoptosis preceded engulfment. In contrast, D-galactose blocked the phagocytosis of already apoptotic cells by only 40%, implicating an additional host ligand (besides D-galactose) in amebic engulfment of apoptotic cells. The most characteristic surface change on apoptotic cells is phosphatidylserine exposure. Consistent with a role for host cell phosphatidylserine exposure in amebic ingestion of killed cells, Jurkat cell phosphatidylserine was exposed during incubation with E. histolytica (27% +/- 1% [mean and SD] specific increase at 30 min) (the P value versus the control was 0.0003). Approximately 50% more amebae ingested viable Jurkat cells expressing phosphatidylserine on the outer leaflet of the plasma membrane than ingested control cells (30.3% +/- 2.2% versus 19.8% +/- 1.9%, respectively [mean and SD]) (P = 0.003). By analogy with phagocytic clearance during apoptosis in metazoans, amebic apoptotic host cell killing followed by phagocytosis may limit inflammation and enable amebae to evade the host immune response.     

Caspase-8 activation precedes alterations of mitochondrial membrane potential during monocyte apoptosis induced by phagocytosis and killing of Staphylococcus aureus

Human peripheral blood monocytes become apoptotic following phagocytosis and killing of Staphylococcus aureus. Although this type of monocyte apoptosis is known to be initiated by Fas-Fas ligand (FasL) interactions, the downstream signaling pathway has not been determined. In this work the involvement of mitochondria and the kinetics of caspase-8 and caspase-3 activation after phagocytosis of S. aureus were studied. Caspase-8 activity was measured in cell lysates by using the fluorogenic substrate Ac-IETD-AFC. Active caspase-3 levels and mitochondrial membrane potential (Deltapsi(m)) were measured in whole cells by flow cytometry using monoclonal antibodies reacting with activated caspase-3 and chloromethyl-X-rosamine, respectively. The results show that caspase-8 was activated shortly after phagocytosis of bacteria. Caspase-8 activation was followed by progressive disruption of Deltapsi(m), which is associated with the production of reactive oxygen intermediates. The irreversible caspase-8 inhibitor zIETD-FMK prevented the disruption of Deltapsi(m) and the release of cytochrome c from S. aureus-exposed monocytes. Caspase-3 activation occurred following disruption of Deltapsi(m). These results strongly suggest that apoptosis of monocytes that have phagocytosed and killed S. aureus is driven by the Fas-FasL-initiated pathway, which is typical for type II cells.     

Apoptosis of monocytes and prolonged survival of granulocytes as a result of phagocytosis of bacteria.

Monocytes and granulocytes were incubated with suspensions of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, or Salmonella enteritidis and, after being washed free of bacteria, cultured for up to 48 h. Every few hours, samples of cultured cells were taken for DNA isolation. Monocytes which phagocytosed bacteria showed features of apoptotic cells, as determined by light microscopy and DNA fragmentation detected by gel electrophoresis. The phenomenon was observed 2 to 4 h after phagocytosis, in contrast, control monocytes did not show signs of apoptosis until 48 h of culture. Intact control granulocytes spontaneously became apoptotic after 12 h of culture. In contrast, degradation of DNA in cells exposed to bacteria was delayed by 12 to 24 h. In conclusion, our observation suggests that granulocytes and monocytes react differently to phagocytosis of bacteria.     

Phagocytosis and killing of staphylococci by human polymorphonuclear and mononuclear leucocytes.

The phagocytosis and killing of 3H-thymidine-labelled Staphylococcus aureus by polymorphonuclear leucocytes (PMNs) and monocytes (MNs) obtained from 50 health donors were evaluated. In addition, extracellular factors that might influence phagocytosis and killing were studied. The method described gave highly reproducible results. No significant difference was observed in the phagocytic and killing functions of a single donor's PMNs and MNs when studied several times in one day and longitudinally over a period of 1-12 weeks for six donors tested. Likewise, no signigicant difference in uptake and killing was observed when bacteria were opsonised with sera from 11 different normal donors. When Staph. aureus opsonised with normal serum was added to the leucocytes in a ratio of 10 bacteria: 1 leucocyte, the uptake by PMNs and MNs from 50 donors after 20 minutes' incubation was 85% +/- 7 standard deviation (SD) (range 75-98%) and 69% +/- 11 SD (range 54-90%), respectively. The rate of uptake by MNs in the first three minutes of the assay period was only 60% of that by PMNs.     

Neutrophil apoptosis and the resolution of infection

Polymorphonuclear leukocytes (PMNs) are the most abundant white cell in humans and an essential component of the innate immune system. PMNs are typically the first type of leukocyte recruited to sites of infection or areas of inflammation. Ingestion of microorganisms triggers production of reactive oxygen species and fusion of cytoplasmic granules with forming phagosomes, leading to effective killing of ingested microbes. Phagocytosis of bacteria typically accelerates neutrophil apoptosis, which ultimately promotes the resolution of infection. However, some bacterial pathogens alter PMN apoptosis to survive and thereby cause disease. Herein, we review PMN apoptosis and the ability of microorganisms to alter this important process.     

Rapid whole-blood microassay using flow cytometry for measuring neutrophil phagocytosis.

A simple flow cytometric method (FCM) for measuring phagocytosis of Staphylococcus aureus by human neutrophils (polymorphonuclear leukocytes [PMNs]) is described. This assay utilizes 100 microliters of EDTA-anticoagulated whole blood and a simplified method of fluorescently labeling bacteria. A commercially available whole-blood lysing reagent allows for the removal of erythrocytes and the exclusion of external free or adherent bacteria. Phagocytized bacteria are unaffected by this reagent, so PMNs containing internalized bacteria can be easily identified by FCM. Advantages of this method include the following: (i) small sample size, (ii) no requirement for PMN separation, (iii) rapid reliable method of labeling the bacteria, (iv) ability to distinguish between adherent bacteria and those which are actually internalized, (v) avoidance of vital dyes as quenching agents, and (vi) ability to fix cells and store for future FCM analysis.     

The expression and roles of Toll-like receptors in the biology of the human neutrophil

Neutrophils are amongst the first immune cells to arrive at sites of infection, where they initiate antimicrobial and proinflammatory functions, which serve to contain infection. Sensing and defeating microbial infections are daunting tasks as a result of their molecular heterogeneity; however, Toll-like receptors (TLRs) have emerged as key components of the innate-immune system, activating multiple steps in the inflammatory reaction, eliminating invading pathogens, and coordinating systemic defenses. Activated neutrophils limit infection via the phagocytosis of pathogens and by releasing antimicrobial peptides and proinflammatory cytokines and generating reactive oxygen intermediates. Through the production of chemokines, they additionally recruit and activate other immune cells to aid the clearance of the microbes and infected cells and ultimately, mount an adaptive immune response. In acute inflammation, influx of neutrophils from the circulation leads to extremely high cell numbers within tissues, which is exacerbated by their delayed, constitutive apoptosis caused by local inflammatory mediators, potentially including TLR agonists. Neutrophil apoptosis and safe removal by phagocytic cells limit tissue damage caused by release of neutrophil cytotoxic granule contents. This review addresses what is currently known about the function of TLRs in the biology of the human neutrophil, including the regulation of TLR expression, their roles in cellular recruitment and activation, and their ability to delay apoptotic cell death.     

Heat-Shocked Monocytes Are Resistant to Staphylococcus aureus-Induced Apoptotic DNA Fragmentation due to Expression of HSP72

Human peripheral blood monocytes became apoptotic following phagocytosis of Staphylococcus aureus. The consequences of heat stress for monocytes were studied with regard to the effect on S. aureus-induced apoptosis. Exposure of monocytes to 41.5 degrees C for 1 h resulted in HSP72 expression and had no influence on phagocytosis of bacteria; moreover, phagocytosis of S. aureus immediately or shortly after heat shock had no effect on the S. aureus-induced monocyte apoptosis, as evidenced by DNA fragmentation assay. In contrast, cells which recovered from heat shock for 18 to 24 h, although active as phagocytes, were resistant to the S. aureus-induced apoptosis. The observed protective effect was related to the induction of HSP72, since blocking of HSP72 synthesis by an antisense oligomer abolished the protective effect of heat shock on bacterium-induced monocyte apoptosis.     

In vivo administration of GM-CSF promotes the clearance of apoptotic cells: effects on monocytes and polymorphonuclear leukocytes

The clearance of apoptotic cells is crucial to avoid chronic inflammation and autoimmunity. Little is known about the factors that regulate it in vivo. We show that granulocyte-macrophage colony-stimulating factor (GM-CSF) administration to carcinoma patients confers to their leukocytes a significantly higher ability to phagocytose apoptotic cells than before (P < 0.005). GM-CSF increased the concentration of monocytes and polymorphonuclear leukocytes in the peripheral blood and activated circulating polymorphonuclear leukocytes. Both effects abated early after treatment, whereas phagocytosis of apoptotic cells was still significantly higher after 18 days compared with basal values (P < 0.005 and P < 0.025 for monocytes and polymorphonuclear leukocytes, respectively). On in vitro phagocytosis of apoptotic cells monocytes, but not polymorphonuclear leukocytes, up-regulated MHC class II membrane expression. These findings are consistent with the possibility that GM-CSF endows both scavenger and antigen-presenting leukocytes with the ability to internalize apoptotic tumor cells.     

Altered antigen-presenting capacity of human monocytes after phagocytosis of bacteria.

The antigen-presenting and accessory functions of monocytes were studied after phagocytosis of bacteria. Peripheral blood monocytes isolated from mononuclear cells by counterflow elutriation were incubated with suspensions of opsonized bacteria (Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, or Salmonella enteritidis) under conditions in which at least 80% of the monocytes engulfed microorganisms. Either the cells were pulsed with antigen (purified derivative of tuberculin or tetanus toxoid) and used as antigen-presenting cells for autologous T lymphocytes or the accessory function of the cells was examined in pokeweed mitogen-activated cultures of T cells. It has been found that phagocytosis of bacteria by monocytes reduces their ability to trigger antigen- and mitogen-induced proliferation. The reduced proliferative response of T lymphocytes was not due to a change of the kinetics of the response or triggering of suppressor mechanisms. Furthermore, antigen processing was not affected much after phagocytosis of bacteria since antigen-pulsed and paraformaldehyde-fixed cells containing bacteria were comparable to control cells in their antigen-presenting capacity. This phenomenon was observed after phagocytosis of both living and dead bacteria and was not correlated to the viability of monocytes, which were more affected after phagocytosis of living bacteria than of dead ones. As a result of phagocytosis of bacteria, reduced expression of CD54, CD14, and HLA-DQ, variable reduction of HLA-DP, CD58, and CD64, and reduced viability of monocytes were observed. In conclusion, phagocytosis of bacteria by monocytes affects their antigen-presenting and accessory functions presumably because of changes in the expression of molecules essential for monocyte-T-cell interactions and reduction of their viability.     

Roles of the ribosomal protein S19 dimer and the C5a receptor in pathophysiological functions of phagocytic leukocytes

Monocytes and neutrophils, the major phagocytic leukocytes, migrate to inflammatory sites by sensing chemoattractants such as anaphylatoxin C5a with membrane receptors such as C5a receptor. Upon stimulation, the leukocytes increase cytoplasmic Ca(2+) concentration and generate radical oxygen species. These leukocytes have different functions in inflammation. Neutrophils migrate more rapidly and induce vascular plasma leakage upon infiltration. Monocytes infiltrate tissue more slowly but have superior capacities of phagocytosis and antigen presentation. There must be mechanisms to separately recruit the leukocyte species at an inflammatory site. Ribosomal protein S19 (RP S19) is a component of ribosome. During apoptosis, RP S19 is dimerized and obtains a ligand capacity to C5a receptor. The RP S19 dimer attracts monocytes to phagocytically clear the apoptotic cells that released the dimer molecules. The phagocytic monocytes/macrophages then translocate to regional lymph nodes and present apoptotic cell-derived antigens. Oppositely, the RP S19 dimer inhibits C5a-induced neutrophil migration and promotes apoptosis of neutrophils via the C5a receptor. The RP S19 dimer seems to prevent excessive tissue destruction induced by neutrophils. Skp is a molecular chaperon of Gram-negative bacteria. Skp also attracts monocytes and neutrophils as a ligand of C5a receptor. However, it promotes neither cytoplasmic Ca(2+) enhancement nor radical oxygen generation.     

Production of interleukin 1-like factor from human peripheral blood monocytes and polymorphonuclear leukocytes by superoxide anion: the role of interleukin 1 and reactive oxygen species in inflamed sites

In the present study, we investigated the possibility that oxidative stress to human peripheral blood monocytes and polymorphonuclear leukocytes (PMNs) can induce interleukin 1 (IL-1)-like activity. Oxidative stress that we used was superoxide anion (O2-) and hydrogen peroxide (H2O2). O2-, but not H2O2, could induce an IL-1-like factor(s) from monocytes and PMNs. IL-1-like activity from monocytes and PMNs induced by O2- was due to de novo synthesis because no IL-1-like activity was found in culture supernatants and in the lysate of unstimulated cells. We next examined the effects of radical scavengers on production of an IL-1-like factor(s). Generation of IL-1-like activity from monocytes was amplified by preincubation with catalase (H2O2 scavenger), although it was suppressed by preincubation with either superoxide dismutase (O2- scavenger) or vitamin E (antioxidant analogs). These results suggest that production of an IL-1-like factor(s) from monocytes and PMNs was due to O2- stimulation. Our data that production of an IL-1-like factor(s) from inflammatory cells by stimulation with O2- imply a model of the up-regulation mechanism of inflammation mediated by enhanced IL-1-like factor production stimulated with reactive oxygen species.     

Ultrastructural characterization of leucocytes in the pronephros of carp (Cyprinus carpio, L.)

In the pronephros of carp (Cyprinus carpio, L.) the following cells were found and ultrastructurally characterized: erythrocytes; lymphocytes and plasma cells; thrombocytes; neutrophilic, eosinophilic and basophilic granulocytes; phagocytic reticular cells and monocytes and non-phagocytic reticular cells. The cells could be separated on a Percoll continuous density gradient and relatively pure fractions could be obtained. In vitro phagocytosis of bacteria (Bacillus megaterium) was found in monocytes and neutrophilic granulocytes, while basophilic and eosinophilic granulocytes engulfed bacterial cells without actual endocytotic uptake.     

Group A streptococcal peptidoglycan-polysaccharide inhibits phagocytic activity of human polymorphonuclear leukocytes.

Injection of sterile aqueous preparations of the peptidoglycan-polysaccharide of group A streptococci (PG-APS) produces chronic inflammation in several animal models. Chronic bacterial infection may be involved in some aspects of the pathogenesis of inflammation associated with the accumulation of PG-APS. Accordingly, the effect of PG-APS on human neutrophil (polymorphonuclear leukocyte [PMN]) bactericidal activity was studied with the supposition that this interaction may contribute to the inflammation observed. Concentrations of PG-APS greater than 10 micrograms/ml inhibited the ability of PMNs to kill Staphylococcus aureus. This inhibition was not due to a cytotoxic effect of PG-APS on PMNs, nor did PG-APS inhibit PMN metabolism required for the formation of microbicidal oxygen reduction products. PG-APS concentrations of 10 micrograms/ml or greater in the presence of 10% normal serum inhibited the attachment of bacteria to PMNs by 49% as compared with control cell populations. The concentrations of PG-APS required to inhibit uptake of Staphylococcus aureus were identical to those required for inhibition of PMN bactericidal activity. This inhibition did not occur in the presence of serum-free medium or medium with sera that had been heated to inactivate complement. These results show that PG-APS interacts with serum to inhibit PMN-mediated killing of S. aureus, most probably by interfering with bacterial uptake.