Selective up-regulation of macrophage function in granulocyte-macrophage colony-stimulating factor transgenic mice.

Peritoneal and pleural cells from mice transgenic for GM-CSF were studied with regard to their phenotype and functional capacity, and compared with cells from normal littermates. Transgenic mice showed markedly elevated peritoneal and pleural cell counts compared with littermates, and a significantly higher proportion of cells in the transgenic populations were macrophage in phenotype. Transgenic macrophages were larger than the littermate cells, showing abundant foamy cytoplasm and enhanced spreading on plastic. Analysis by flow cytometry showed a more than sixfold increased expression of the macrophage activation markers MAC-2 and MAC-3, but not other markers, on transgenic macrophages. Superoxide production was measured in whole cell populations, both in their basal state and in response to particulate (zymosan) and soluble (PMA) stimuli. Both basal and stimulated superoxide production were markedly elevated in transgenic mice of 12 wk of age, with the largest differences seen in response to PMA. In younger mice, however, only PMA-stimulated superoxide production was significantly greater in transgenic macrophages than in littermate cells and levels of superoxide were generally lower than those seen in 12-wk-old mice. These findings suggest that the enhanced functional capacity of transgenic cells is a maturation-dependent event. In contrast to these findings, drug-dependent cytotoxicity assays performed on cells from 12-wk-old mice revealed no significant differences in killing capacity between the two mouse strains. Taken together these data indicate a selective rather than uniform functional up-regulation in transgenic macrophages compared with their littermates, with a time scale suggestive of a maturational rather than activation process. These findings may provide an indication of the functional macrophage phenotype resulting from long term exposure to GM-CSF in vivo, and help to explain the macrophage-associated pathology seen in GM-CSF-transgenic mice.     

Enhancement of host resistance against Listeria infection by Lactobacillus casei: efficacy of cell wall preparation of Lactobacillus casei

Cell wall, cytoplasm, polysaccharide, and peptidoglycan fractions prepared from Lactobacillus casei, L. plantarum, and L. acidophilus were examined for their efficacies to enhance resistance of host mice against Listeria monocytogenes infection. Intraperitoneal injections of those cellular fractions of L. casei led to elicitation of inflammatory cells in the peritoneal cavity and the efficacy was highest in the case of peptidoglycan. Macrophage ratio in the resultant peritoneal exudate cells was also highest in mice given peptidoglycan. Macrophages induced with cell wall fraction of L. casei showed the most potent phorbol myristate acetate (PMA)-triggered respiratory burst (chemiluminescence and O2- production determined on the basis of nitroblue tetrazolium reduction) followed by those elicited with peptidoglycan. All the macrophages induced with cell wall of L. casei (two strains) and L. acidophilus enhanced O2- production in response to PMA but L. plantarum did not enhance O2(-)-producing ability in such a manner. The L. casei-cell wall also enhanced in vitro listericidal activity of mouse peritoneal macrophages, but such an activity was not noted in the case of L. acidophilus-cell wall. When mice were intravenously given the cellular fractions 7 or 13 days before L. monocytogenes infection, cell wall fractions of L. casei caused the most potent protective activity. A weak protective activity was also found in peptidoglycan of L. casei. Therefore, the protective action of L. casei against L. monocytogenes infection in host mice may be attributed to cell wall compounds and partially to the peptidoglycan moiety.     

Macrophages of athymic nude mice: Fc receptors, C receptors, phagocytic and pinocytic activities

Expression of Fc receptors (FcR) for IgG1, IgG2A, IgG2B, IgM, IgA and IgE, binding of C3 and C5 complement components and phagocytic and pinocytic activities were determined in peritoneal and omental macrophages of nu/nu, nu/+ and +/+ Balb/c mice. nu/nu mice showed a higher proportion of FcR and complement receptor-bearing peritoneal macrophages along with a significantly higher phagocytic activity of peritoneal macrophages both in vitro and in vivo. Tests of pinocytic activity in these cells and phagocytic activity in omental phagocytes yielded similar results. We conclude that athymic mice compensate their immune defects by a higher phagocytic activity of their professional phagocytes and a higher expression of receptors mediating this process.     

Zymosan but not phorbol myristate acetate induces an oxidative burst in rat bone marrow-derived macrophages

We found that rat bone marrow-derived macrophages responded to opsonized zymosan by releasing superoxide anion. However, these cells were defective in the response to the potent oxidative burst activator phorbol myristate acetate (PMA). This result was observed whatever the concentration of agonist used and with different concentrations of cells. Since it is strongly suspected that protein kinase C (PKC) is involved in the transductional pathway induced by PMA in numerous cell types, and particularly in phagocytes, we studied PKC and we observed that it was functional in rat bone marrow-derived macrophages, but only present at a low level. Thus, we suggest that our results are consistent with the possibility that zymosan-induced respiratory burst may be independent of PKC and that these cells may not possess the minimal level of PKC required for responding to PMA.     

The macrophage adherence phenomenon: Its relationship to prostaglandin E2 and superoxide anion production and changes in transmembrane potential

Mononuclear phagocytes are undoubtedly the sine qua non of chronic inflammatory reactions. This is demonstrated by their unique ability to function as phagocytic, secretory, or effector cells during the course of an immune event. Although macrophages can perform a variety of immune tasks, their ability to function appropriately is dependent upon the mode of elicitation, the stimulus under investigation, the source of the macrophages (peritoneal, alveolar, etc.), and whether the macrophages are monolayers or in suspension. We have examined the relationship between adherent and non-adherent elicited peritoneal macrophages in terms of prostaglandin E₂ (PGE₂) and superoxide anion (O₂) production; in addition, we hae studied these elicited macrophages in suspension for their ability to undergo transmembrane potential changes in response to several stimuli. Non-adherent, elicited peritoneal macrophages demonstrated an increase in basal PGE₂ production, and were refractory to particulate stimulus. After monolayer formation, basal PGE₂ levels dropped and the cell could respond to both soluble and particulate stimuli. Only adherent macrophages could respond to a specific challenge and synthesize O₂. Both O₂ production and depolarization of the transmembrane potential were suppressed in cell in suspension. Furthermore, both exogenous PGE₂ and supernatant from macrophages in suspension could modulate O₂ production by PMA challenged macrophages monolayers. These studies indicate that PGE₂ may modulate macrophage function and dictate activity as macrophages go from the non-adherent to adherent state.     

Effect of angiotensin II and losartan on the phagocytic activity of peritoneal macrophages from Balb/C mice

Angiotensin II (AII), a product of rennin-angiotensin system, exerts an important role on the function of immune system cells. In this study, the effect of AII on the phagocytic activity of mouse peritoneal macrophages was assessed. Mice peritoneal macrophages were cultured for 48 h and the influence of different concentrations of AII (10(-14) to 10(-7) M) and/or losartan, 10(-16) to 10(-6) M), an AT1 angiotensin receptor antagonist, on phagocytic activity and superoxide anion production was determined. Dimethylthiazoldiphenyltetrazolium bromide reduction and the nucleic acid content were used to assess the cvtotoxicity of losartan. A stimulatory effect on phagocytic activity (P < 0.05) was observed with 10(-13) M and 10(-12 M) AII concentrations. The addition of losartan (up to10(-14) M) to the cell cultures blocked (P < 0.001) the phagocytosis indicating the involvement of AT1 receptors. In contrast, superoxide anion production was not affected by AII or losartan. The existence of AT1 and AT2 receptors in peritoneal macrophages was demonstrated by immunofluorescence microscopy. These results support the hypothesis that AII receptors can modulate murine macrophage activity and phagocytosis, and suggest that AII may have a therapeutic role as an immunomodulatory agent in modifying the host resistance to infection.     

Modulation of murine macrophage function by N-acetylcysteine in a model of endotoxic shock

In previous studies we have observed changes in several functions of peritoneal macrophages from BALB/c mice with irreversible endotoxic shock caused by intraperitoneal injection of E. coli lipopolysaccharide (LPS) (100 mg/kg), which were associated with a high production of superoxide anion. Since antioxidants, such as N-acetylcysteine (NAC), are free radical scavengers that improve the immune response, in the present work we have studied different functions of peritoneal macrophages from BALB/c mice suffering the endotoxic shock above indicated and administered N-acetylcysteine (150 mg/kg i.p.) at 30 minutes after LPS injection. In the peritoneal macrophages obtained at 2, 4, 12 and 24 h after LPS injection, the following functions were studied: adherence to substrate, mobility, ingestion of particles, and production of superoxide anion and tumour necrosis factor (TNF alpha). The increase in adherence, ingestion and superoxide anion and TNF alpha production shown by macrophages from animals with endotoxic shock was counteracted by NAC injection. Moreover, the survival time of mice with endotoxic shock was increased in the presence of NAC. These data suggest that NAC, administered intraperitoneally, may be useful for the treatment of irreversible endotoxic shock by modulation of the function of macrophages with decreased superoxide anion and TNF alpha production and concomitant increase of survival time.     

In vitro superoxide activity in the haemolymph of the West Indian leaf cockroach,Blaberus discoidalis

The respiratory burst is an NADPH oxidase-driven reduction of molecular oxygen to superoxide, which can occur in phagocytic cells as part of an antimicrobial defence, and is well documented among the vertebrates. This paper describes a process resembling the respiratory burst, which occurs in the haemolymph and haemocytes of the cockroach, Blaberus discoidalis. The in vitro reduction of nitroblue tetrazolium by superoxide to formazan was measured spectrophotometrically in B. discoidalis haemolymph in response to various immune elicitors. Nitroblue tetrazolium reduction was partly impeded in the presence of superoxide dismutase, a specific antioxidant which converts superoxide to hydrogen peroxide, as well as by chemicals known to inhibit the respiratory burst in vertebrates (trifluoperazine, diphenylene iodonium, and N-ethylmaleimide). This suggests the generation of superoxide anions by haemolymph as part of an immune response. Furthermore, formazan staining of elicitor-treated haemocytes was observed microscopically, with less intense staining in the presence of superoxide dismutase. Finally, respiratory burst inhibitors and superoxide dismutase enhanced the growth of E. coli incubated in whole haemolymph, implying a role for haemolymph-derived superoxide in antibacterial defence.     

Cyclosporin A inhibits phorbol ester-induced activation of superoxide production in resident mouse peritoneal macrophages.

Peritoneal resident macrophages from mice are sensitive to inhibition by cyclosporin A (CsA) of phorbol 12-myristate 13-acetate (PMA)-stimulated oxidative burst. Inhibition was assessed in terms of superoxide anion (O2.-) and H2O2 production. Key findings were as follows. (a) CsA inhibited in a dose-dependent manner the production of O2.- when cells were stimulated with PMA. CsA did not alter the respiratory burst induced by other stimuli (zymosan, concanavalin A and fMet-Leu-Phe). It was verified that CsA itself had no scavenger effect. (b) A concomitant decrease in H2O2 liberation following CsA exposure was found. This inhibition was observed both in the initial rate of synthesis and in the accumulation after 15 min of incubation. (c) NADPH oxidase activity in the crude supernatant was unaffected by the previous incubation of macrophages with CsA. CsA does not inhibit glucose transport measured as 14CO2 production. (d) The production of O2.- was strongly dependent on the glucose concentration. Sodium oleate also stimulated O2.- production in resident macrophages. These data might be correlated with the inhibitory effect of CsA upon other functions of macrophages.     

Changes in several functions of murine peritoneal macrophages by N-acetylcysteine and thioproline ingestion. Comparative effect between two strains of mice

The administration of the thiol compounds, N-acetylcysteine (NAC) and in particular thioproline (thiazolidine-4-carboxylic acid) at 0.1% w/w concentration in the diet, improves lymphocyte functions in old female Swiss mice, as has been shown in our previous studies. In the present work, adult mice from two different strains, namely BALB/c (an inbred strain) and OF1-Swiss (noninbred strain), were fed a diet supplemented with the above dose of each thiol compound jointly for five weeks. At 28 weeks of age, peritoneal cell suspensions were obtained and different steps of the phagocytic process, the most representative activity of macrophages, as well as interleukin-1beta (IL-1beta) production, were studied. Thus, adherence to substrate, mobility directed to a chemoattractant gradient (chemotaxis), ingestion of inert particles and superoxide anion production were analysed. The results show that diet supplementation with NAC plus thioproline increased all macrophage functions studied with the exception of superoxide anion production, which was decreased. These effects were more evident in macrophages from Swiss mice, whereas in BALB/c mice the stimulation of phagocytosis and IL-1beta production was lower and no differences were seen after treatment in adherence and superoxide anion production. These data suggest that immune function can be improved in adult mice by administration of the above thiol compounds, especially in the noninbred strain of OF1-Swiss mice.     

Effects of endotoxic shock in several functions of murine peritoneal macrophages

Gram negative sepsis and septic shock continue to be a major medical problem, with a complex physiopathology and it is associated with high mortality. Although secretion of cytokines such as tumor necrosis factor-alpha by macrophages is the principal host mediator of septic shock, other characteristic functions of macrophages implicated in their phagocytic capacity have not been studied in the process of endotoxic shock. In the present study we have used an intraperitoneal injection of E. coli lipopolysaccharide (LPS) (100 mg/kg) in order to obtain an endotoxic shock model in adult female BALB/c mice. Peritoneal cell suspensions were obtained at several times (2, 4, 12 or 24 h) after injection and the following functions were studied on the peritoneal macrophages: adherence to substrate, mobility (spontaneous and directed or chemotaxis), ingestion of particles and superoxide anion production. The results showed a stimulation of adherence, ingestion and superoxide production as well as a decrease of chemotaxis in the animals injected with LPS. These effects changed with time after LPS injection. Thus, the increase of adherence and the decrease of mobility were higher during the first hours, whereas the increase in ingestion and superoxide production turned larger with time.     

Chemiluminescence response of phagocytes in E. coli induced experimental ascending pyelonephritis.

The mechanism of tissue injury at the cellular level by following the chemiluminescence response of various phagocytes in E. coli induced experimental pyelonephritis in mice was investigated. There was a marked increase in the capacity of various phagocytic cells viz; renal neutrophils and macrophages peritoneal macrophages, blood monocytes and neutrophils to produce reactive oxygens species through the respiratory burst activity as monitored by the chemiluminescence response. The chemiluminescence response was observed to be increased significantly (p less than 0.001) with increasing days post infection in all phagocytic cells. However, the quantity of total reactive oxygen species produced per million cells was much more in the renal and peritoneal macrophages as compared to blood monocytes and neutrophils. The peak chemiluminescence response time was observed to be decreased from 4 to 2 minutes with the progression of the diseases. The implications of these findings have been discussed.     

Phorbol ester-stimulated superoxide production by murine bone marrow-derived macrophages requires preexposure to cytokines

Murine resident peritoneal macrophages (RPM) generate superoxide (O2-) in response to stimulation with PMA or zymosan. Murine bone marrow-derived macrophages (BMM) generate O2- in response to zymosan but not PMA. However, the ability to generate O2- in response to PMA could be induced in BMM by pre-exposing the cells to certain cytokines, including granulocyte-macrophage CSF (GM-CSF), tumor necrosis factor-alpha (TNF-alpha), IFN-gamma, and, to a lesser extent, IL-1 alpha. Bacterial LPS also induced the ability to respond to PMA. These same agents were also shown to prime RPM for enhanced PMA-induced respiratory burst. In contrast to GM-CSF, CSF-1 did not enhance the ability of BMM or RPM to generate O2- in response to PMA. Pretreatment with GM-CSF or TNF-alpha did not significantly affect the zymosan-induced release of O2- by BMM. These results suggest that unprimed BMM have a deficiency in the PMA-dependent signaling pathway that is corrected by exposure to selected cytokines. The results also raise the possibility that the basal ability of tissue macrophages to generate a respiratory burst in response to PMA may be a reflection of in vivo exposure to cytokines.     

Flow cytometric analysis of group B streptococci phagocytosis and oxidative burst in human neutrophils and monocytes

Group B streptococci (GBS) are a major cause of meningitis and septicemia in neonates and numerous invasive diseases in adults. Host defense against GBS infections relies upon phagocytosis and killing by phagocytic cells. To better understand the importance of this defense mechanism a flow cytometric assay was developed to study phagocytosis and oxidative burst of leukocytes stimulated by bacteria. GBS labeled with fluorescein isothiocyanate were used for phagocytosis experiments and the extracellular fluorescence was quenched by ethidium bromide to differentiate intracellular from extracellular bacteria. The intracellular oxidative burst was determined by using 2',7'-dichlorofluorescein diacetate to measure hydrogen peroxide production and hydroethidine for superoxide anion production. We found that for GBS serotypes Ia, Ib/c, II, and III phagocytosis was greater in neutrophils than monocytes. Hydrogen peroxide production and superoxide anion production were also greater for neutrophils than monocytes in all serotypes tested. A comparison of seven type III strains revealed greater phagocytosis and superoxide anion production by neutrophils than monocytes but no difference in hydrogen peroxide production. Therefore, monocytes react similarly as neutrophils in response to GBS but at a reduced level. This methodology of measuring both phagocytosis of GBS and oxidative burst simultaneously in neutrophils and monocytes should be very useful in further studies on the importance of factors such as complement and IgG receptors for the killing of bacteria.     

Evidence for the involvement of the NADPH oxidase enzyme complex in the optimal accumulation of Platelet-activating factor in the human cell line PLB-985.

Platelet-activating factor (PAF) is an early product of the inflammatory environment, influencing development and resolution of inflammation. Its production is greater in neutrophils and macrophages, which predominantly synthesize 1-alkyl sn-2 acetyl glycerophosphocholine (GPC) than in nongranulocytes (B cells and endothelial cells), which lack a respiratory burst and synthesize 1-acyl sn-2 acetyl GPC as their major PAF species. This study investigated whether the respiratory burst was responsible for the quantitative and qualitative differences in sn-2 acetyl GPC species generation by neutrophils and macrophages versus those cells lacking the NADPH oxidase complex. The myeloid cell line PLB-985 (capable of differentiation into neutrophils) was used to test this hypothesis, since these cells had previously been generated with a non-functional respiratory burst (X-CGD PLB-985). Differentiated PLB-985 cells underwent a large respiratory burst in response to PMA (phorbol ester), and smaller respiratory bursts in response to A23187 (calcium ionophore), and the bacterial polypeptide fMLP (receptor mediated activation). Concurrently, treated cells were assessed for production of 1-hexadecyl and 1-palmitoyl sn-2 acetyl GPC species by gas chromatography/mass spectrometry. Neither cell type generated these lipid species in response to PMA, but both cell types generated equal levels of sn-2 acetyl GPC in response to A23187, with five times more 1-hexadecyl than 1-palmitoyl species. Upon fMLP activation, X-CGD PLB-985 cells produced significantly less 1-hexadecyl and 1-palmitoyl sn-2 acetyl GPC in comparison to the wild-type PLB-985 cells. These findings suggest phagocytic oxidant production by NADPH oxidase is not essential for sn-2 acetyl GPC generation, but appears important for optimal production of PAF in response to some stimuli.     

Regulation of the growth and differentiation of a human monocytic cell line by lymphokines. I. Induction of superoxide anion production and chemiluminescence

The U937 human monocytic cell line was studied to determine its ability to generate a respiratory burst after stimulation with phorbol myristate acetate (PMA) or opsonized zymosan. U937 cells cultured in normal medium produced virtually no superoxide anion or chemiluminescence in response to either stimulus. In contrast, U937 cells cultured in medium containing soluble factors from activated lymphocytes produced significant O2- and chemiluminescence when stimulated with PMA or opsonized zymosan. The chemiluminescence in response to PMA was maximal in U937 cells precultured with these soluble factors for 3 days, whereas maximal responsiveness to opsonized zymosan was not observed until 5 to 6 days of lymphokine exposure. Although this ability to generate a respiratory burst persisted for a number of days in U937 cells that were subsequently recultured in normal medium, this responsiveness was gradually lost in the continued absence of these factors. The data indicate that the U937 monocytic cell line can be activated or induced to differentiate by soluble factors released by activated lymphocytes. In the process, these cells acquire the ability to generate a respiratory burst. The U937 cell line may serve as a useful model for the study of the ontogeny and regulation of the respiratory burst during human monocytic differentiation.     

Activation of the respiratory burst in macrophages. Phosphorylation specifically associated with Fc receptor-mediated stimulation

Inflammatory macrophages elicited from the peritoneal cavity of mice injected with endotoxin can avidly ingest E opsonized with IgG antibody (EIgG) or with IgM antibody and C (EIgMC). However, only ingestion of EIgG is associated with activation of the respiratory burst and release of superoxide anion. We compared the endogenous phosphorylation of proteins from macrophages stimulated by interaction with EIgG or EIgMC on the premise that proteins phosphorylated after stimulation by EIgG but not EIgMC could play a role in activating the enzyme (oxidase) responsible for the respiratory burst. Proteins were separated by one-dimensional and two-dimensional electrophoresis in polyacrylamide gels. We found that proteins with approximate Mr of 20 kDa, 23 kDa, 46 kDa, 48 kDa (three proteins), 67 kDa, and 130 kDa were more heavily phosphorylated after EIgG stimulation than after EIgMC stimulation. Exposure to PMA, which activates the respiratory burst oxidase, induced phosphorylation of the 23-kDa, 48-kDa group, and 130-kDa proteins that were phosphorylated after stimulation by EIgG. Activity of protein kinase C was found to be significantly increased in the particulate fraction of macrophages stimulated by EIgG but not in the particulate fraction of EIgMC-stimulated cells. These data are compatible with the hypotheses that phosphorylation of specific cellular proteins, especially with a Mr of approximately 48 kDa, is involved in activation of the respiratory burst oxidase, and that function of protein kinase C also plays a part in this activation process.     

Superoxide production by phagocytes in myeloid Graffi tumor-bearing hamsters

A progressive suppression of the phagocytic ability of peritoneal macrophages and polymorphonuclears (PMNs) in hamsters with transplanted myeloid tumors was previously established. The i.p. application of Cu/Zn SOD, isolated from the fungal strain Humicola lutea (HLSOD) (2 injections before and 5 injections after tumor transplantation) induced the mean survival time of the animals as well as a temporally stimulating action on the macrophage and PMNs phagocyting indices. In the present work, the superoxide production of peritoneal macrophages and PMNs during 30 days of tumor progression was followed. Effects of the application of HLSOD in an optimal protective dose on the superoxide production in peritoneal macrophages and blood PMNs were examined. The spontaneous and phorbol-myristate acetate (PMA)-inducible O2- production in both types of phagocytes was 4-5-fold increased in tumor-bearing hamsters (TBH), as compared to the controls, at day 14 after tumor transplantation (the day of tumor appearance in transplanted animals). Furthermore, O2- production was also similar to the control values for the following days of observation. HLSOD treatment of TBH induced a normalization of superoxide production in macrophages and PMNs. Therefore, the established decrease of superoxide anions in phagocyting cells of TBH indicates possible effects of HLSOD on the host antioxidant defense.     

Mycobacterium lepraemurium activates macrophages but fails to trigger release of superoxide anion

Mycobacterium lepraemurium failed to stimulate a normal respiratory burst when presented to mouse peritoneal or bone marrow macrophages. By comparison, Mycobacterium bovis (strain Bacillus Calmette-Guerin) or Saccharomyces cerevisiae, as expected, stimulated macrophages to release a large amount of superoxide anion (O2-). M. lepraemurium did not interfere with the response to yeast when both microbes were added together to macrophages. The low release of O2- induced by M. lepraemurium was not due to failure of M. lepraemurium to activate or prime macrophages, because exposure of macrophages to M. lepraemurium caused the expected enhancement of O2- release when the macrophages were stimulated by PMA. Similarly, macrophages taken from mice infected with M. lepraemurium were activated, as indicated by high PMA-stimulated O2- release. Macrophages primed in vitro by exposure to Escherichia coli LPS for 24 h did show a moderate O2- response when stimulated by M. lepraemurium, but macrophages primed by exposure to IFN-gamma muramyl dipeptide, or M. lepraemurium showed a weak response when subsequently challenged with M. lepraemurium. The priming effect of M. lepraemurium or LPS decreased substantially after macrophages were cultured in fresh medium for 24 h. Heat killing or opsonization of M. lepraemurium caused the M. lepraemurium to stimulate a high amount of O2- release from LPS-primed macrophages, but heat killing or opsonization of M. lepraemurium had no effect on release of O2- from unprimed macrophages. The results suggest that M. lepraemurium is taken into macrophages by a mechanism that bypasses the FcR and other receptors that are capable of triggering the production of O2-.     

Inflammatory and Phagocytic Response to Experimental Campylobacter jejuni Infection in Mice

After intraperitoneal inoculation with Campylobacter jejuni BALB/c, Swiss and DBA mice show a peritoneal inflammatory response of different intensity. Only BALB/c mice have a strong peritoneal response. Simultaneous intraperitoneal inoculation of C. jejuni plus FeCl₃ increase both inflammatory response and phagocytic activity in Swiss mice, without production of diarrhea. Some thermostable compounds of C. jejuni have a very strong chemotactic activity against peritoneal cells of mice, whereas a diffusible, thermolabile and glutaraldehyde-resistant factor has an inhibitory effect over murine peritoneal cell phagocytosis. Bactericidal activity of peritoneal cells increased after in vitro re-challenge with C. jejuni. Bacteremia is present in all the mice strains tested, but the clearance is quick in DBA and slow in BALB/c and Swiss mice. These experiments confirm that in mice, peritoneal non-specific mechanisms of defense, such as macrophages, play an important role in order to control C. jejuni infection.