Sepsis accounts for the majority of fatal casualties in critically ill patients, because extensive research failed to significantly improve appropriate therapy strategies. Thus, understanding molecular mechanisms initiating the septic phenotype is important. Symptoms of septic disease are often associated with monocyte/macrophage desensitization. In this study, we provide evidence that a desensitized cellular phenotype is characterized by an attenuated oxidative burst. Inhibition of the oxidative burst and depletion of protein kinase Cα (PKCα) were correlated in septic patients. To prove that PKCα down-regulation indeed attenuated the oxidative burst, we set up a cell culture model to mimic desensitized monocytes/macrophages. We show that LPS/IFN-γ-treatment of RAW264.7 and U937 cells lowered PKCα expression and went on to confirm these data in primary human monocyte-derived macrophages. To establish a role of PKCα in cellular desensitization, we overexpressed PKCα in RAW264.7 and U937 cells and tested for phorbolester-elicited superoxide formation following LPS/IFN-γ-pretreatment. Inhibition of the oxidative burst, i.e., cellular desensitization, was clearly reversed in cells overexpressing PKCα, pointing to PKCα as the major transmitter in eliciting the oxidative burst in monocytes/macrophages. However, PKCα inactivation by transfecting a catalytically inactive PKCα mutant attenuated superoxide formation. We suggest that depletion of PKCα in monocytes from septic patients contributes to cellular desensitization, giving rise to clinical symptoms of sepsis.

During the pathogenesis of sepsis, monocytes/macrophages are important players due to their ability to produce large amounts of proinflammatory cytokines including, among others, TNF-α, IL-1β, and IFN-γ in response to pathogen contact (1, 2). High doses of these cytokines contribute to severe tissue damage, organ failure, and death (3). During the progression of infection, monocytes can switch to a desensitized phenotype characterized by the expression of anti-inflammatory cytokines and a concomitantly attenuated proinflammatory response (4), giving rise to a hampered immune reaction following secondary infections (5, 6). Therefore, the proper regulation of the balance between pro- vs anti-inflammatory cytokine production has been implicated as a key regulating system in the development of sepsis (2).

Generation of superoxide (O2) by the NADPH oxidase in monocytes/macrophages is one essential component of the innate immune system in response to the first contact with pathogens (7). Following pathogen recognition and phagocytosis, ingested microorganisms are killed by O2-derived reactive oxygen species (ROS). 4 The NADPH oxidase system is a multiprotein complex that exists preformed in resting monocytes/macrophages linked to cellular vesicles and/or the plasma membrane (8). Its activation occurs via a protein kinase C (PKC)-mediated pathway (9). PKC is a family of related enzymes, divided into three classes based on their structure and cofactor requirements (10). Conventional PKC isoforms α, βI, βII, and γ require Ca2+ and diacylglycerol (DAG) for optimal activity. Novel PKC isoforms δ, ε, η, and θ do not contain a Ca2+ binding site, but are activated in response to DAG. Atypical PKC isoforms ζ and ιλ are insensitive to Ca2+ and DAG. Mechanisms leading to the activation of the latter group remain unclear.

Recently, we showed that treatment of monocytes/macrophages with a combination of LPS and IFN-γ to simulate septic conditions inhibits their ability to generate ROS in response to PMA (11). PMA, a DAG-homologue, was used as an established activator of the PKC pathway, provoking NADPH oxidase activity and concomitant ROS production (12). A pathophysiological modulation of the oxidative burst as one characteristic activation marker of monocytes/macrophages in septic patients has not been fully understood. In this regard, signaling mechanisms causing an altered ROS production are so far unknown. Therefore, it was our intention to elucidate the capability of monocytes derived from healthy donors compared with cells obtained from septic patients to generate O2 in response to PMA.

In this study, we show that in monocytes derived from septic patients a correlation between an attenuated oxidative burst in response to PMA and depletion of PKCα exists. In a cell culture model, we mimicked septic conditions to provide evidence that PKCα depletion accounts for a diminished oxidative burst, observed in septic monocytes.

PMA and LPS were purchased from Sigma-Aldrich. Hydroethidine (HE), Escherichia coli, and E. coli BioParticles opsonizing reagent were from Molecular Probes. N-nitro l-arginine methyl ester (L-NAME) was from Alexis. Culture supplements, FCS, and AB-positive human serum were ordered from PAA Laboratories. Murine and human recombinant IFN-γ were from Roche. All other chemicals were of the highest grade of purity and commercially available.

The mouse monocyte/macrophage cell line RAW264.7 and the human premonocytic cell line U937 were maintained in RPMI 1640 supplemented with 100 U/ml penicillin, 100 μg/ml streptomycin, and 10% heat-inactivated FCS (complete RPMI 1640). All experiments were performed using complete RPMI 1640.

We analyzed human cells from peripheral blood of healthy donors and of patients diagnosed with polymicrobial sepsis as defined (13). Septic patients were recruited from the intensive care unit of the Westpfalz-Klinikum Kaiserslautern. Patients were between 42 and 76 years old. The study has been approved by the ethics-committee of the board of physicians in Rhineland-Palatinate, Germany (processing number 837.141.02/3359).

For monocyte enrichment, we isolated PBMCs from donors using Ficoll-Hypaque gradients (PAA Laboratories). We washed PBMCs twice in PBS, and following T cell depletion (CD3+) using the MACS technology (Miltenyi Biotec), remaining cells were left to adhere on culture dishes (Primaria 3072; Becton Dickinson) for 60 min at 37°C. Nonadherent cells were removed. Afterward, cells were directly used for the experiments (HE assay or Western blotting) or differentiated to macrophages by culturing them in complete RPMI 1640, containing 10% AB-positive human serum (transient transfection experiments). Flow cytometry confirmed that the monocyte-like population was 90–95% pure (CD14+ vs CD14).

To overexpress PKCα in RAW264.7 macrophages, a retroviral vector was constructed to obtain efficient gene transfer, stable integration of the gene, and to allow FACS-sorting for positive cells as well as detection of PKCα translocation. Therefore, the plasmid pPKCα-EGFP (BD Clontech, BD Biociences) was used to clone the PKCα sequence linked to an enhanced GFP (EGFP) as a fluorescent marker into the retroviral pLXIN vector (BD Clontech, BD Biociences) after an EcoRI cut at positions 1471 and 1510. Consequently, primers were designed to generate a DNA fragment, containing the PKCα-EGFP sequence flanked by EcoRI sites. The sequences of the primers were as follows 5′ > 3 5′-CCGGAATTCATGGCTGACGTTTTCCCGGG-3′, 3′ > 5′ 5′-GGCGAATTCCGCGGCCGCTTTACTTGTACAG-3′. After PCR giving rise to a 2794-bp fragment, this was EcoRI cut, run on an agarose-gel, eluted, and ligated into the EcoRI cut pLXIN vector. Correct orientation and sequence were verified by restriction analyses and sequencing.

The retroviral infection of RAW264.7 macrophages and U937 cells was performed essentially as described (14, 15). Briefly, after transiently transfecting the packaging plasmids encoding the vesicular stomatitis virus glycoprotein G (16) and the MLV-gag-pol genes (17) together with the retroviral vectors pLXIN-PKCα-EGFP into 293T cells, target cells were incubated for 24 h with the infectious supernatant containing 8 μg/ml polybrene. Positive selection based on EGFP-expression was done using the sort option of a FACSCalibur flow cytometer (BD Biosciences). Transduced cell populations containing 100% positive EGFP-expressing cells were used for the experiments.

Following a prestimulation regime, 5 × 105 cells were incubated for 30 min with 1 μM PMA or 1 μg/ml opsonized E. coli. Opsonization has been performed by incubating E. coli in E. coli BioParticles opsonizing reagent for 30 min, followed by two PBS washing steps, finally diluting opsonized E. coli to 1 μg/μl in PBS. After cell stimulation, 3 μM HE was added and incubations went on for 30 min. Cells were harvested, washed with PBS, and resuspended in 200 μl of PBS. Flow cytometry analysis was performed using a FACSCalibur flow cytometer (BD Biosciences) and HE was measured through a 630-nm long pass filter (FL3). Data from 10,000 cells were collected to reach significance. All incubations were performed at 37°C.

To follow PKCα translocation, PKCα-EGFP-positive cells were seeded in 6-cm petri dishes. After stimulation 1 μM PMA was added. Translocation of PKCα-EGFP was analyzed using an Axioscope (Carl Zeiss), documented by a CCD camera (Visitron Systems) and the MetaFluor software (Visitron Systems).

The plasmid, encoding for a catalytically inactive PKCα protein (pEF-PKCα K368R), kindly provided by G. Baier (Institute of Medical Biology and Human Genetics, University of Innsbruck, Innsbruck, Austria) (18), was transfected into primary human macrophages using jetPEI (Biomol) according to the manufacturer’s instructions. Transfection of the empty pEF-vector was used as a control. Selection of positive clones was performed by 1-wk treatment with the antibiotic G418. Surviving cells were used for the HE assay.

Cell lysis was achieved with lysis buffer (50 mM Tris, 5 mM EDTA, 150 mM NaCl, 0.5% Nonidet-40, 1 mM PMSF (pH 8.0)) and sonication (Branson sonifier; 20 s, duty cycle 100%, output control 60%). Whole cell lysates were cleared by centrifugation (10,000 × g, 5 min), and protein concentration was determined with the Lowry method. Protein (50 μg) was resolved on 10% polyacrylamide gels and blotted onto nitrocellulose sheets, basically following standard methodology. Equal loading and correct protein transfer to nitrocellulose was routinely quantitated by Ponceau S staining. Filters were incubated with the anti-PKCα, -PKCδ, -PKCε, and -PKCλ Abs (1:500; Transduction Laboratories, BD Biosciences) or anti-actin Ab (1:2000; Amersham Biosciences) overnight at 4°C. HRP-conjugated polyclonal Abs (1:5000; Amersham Biosciences) were used for ECL detection.

Due to small sample volumes some experiments were only performed twice. Otherwise each experiment was performed at least three times. Statistical analysis was performed using the paired t test. We considered p ≤ 0.05 as significant. Otherwise representative data are shown.

The respiratory burst is an early activation marker in response to pathogen contact and constitutes a pivotal role in killing microorganisms (19). Therefore, we first established a test system to follow oxygen radical formation of primary monocytes derived from septic patients compared with healthy donors in response to PMA as described (11). In this study, we provide evidence that in monocytes of 7 of 12 preparations from septic patients the ability to generate superoxide radicals in response to PMA was abolished (Fig. 1,A, left panel). Activation of all 13 control samples evoked ROS production (Fig. 1,A, right panel). Statistics of flow cytometric results revealed no significant induction of superoxide radical formation in monocytes derived from septic patients compared with a significant increase in healthy donors (Fig. 1 B).

FIGURE 1.

Oxygen radical production in monocytes derived from septic patients. A, ROS production in response to 1 μM PMA was analyzed by flow cytometry using 3 μM HE as the redox-sensitive dye. Experiments were performed in duplicate. B, Statistical evaluation of flow cytometric data (∗, p ≤ 0.05). For details see Materials and Methods.

FIGURE 1.

Oxygen radical production in monocytes derived from septic patients. A, ROS production in response to 1 μM PMA was analyzed by flow cytometry using 3 μM HE as the redox-sensitive dye. Experiments were performed in duplicate. B, Statistical evaluation of flow cytometric data (∗, p ≤ 0.05). For details see Materials and Methods.

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Because it is known that monocytes of sepsis patients often show a desensitized phenotype (6, 20), we elucidated the molecular mechanisms attenuating the oxidative burst in septic monocytes. We took into consideration that PMA activates the NADPH oxidase by phosphorylating p47 phagocyte oxidase, consequently causing its translocation to the phagosome, and in turn allowing the assembly of the functional O2 producing NADPH oxidase complex via PKCα (9). Therefore, we asked whether expression of this PKC family member is modulated during sepsis.

To correlate diminished ROS formation in monocytes of septic patients in response to PMA with the expression of PKCα, Western blot analyses were performed. Protein expression revealed a constitutive PKCα expression in monocytes from healthy individuals (n = 11), and in line with our hypothesis, ROS formation in response to PMA as shown for four samples in Fig. 2 (upper panel, ROS formation; lower panel, PKCα expression). Interestingly, we observed no PKCα expression in monocytes derived from septic patients where ROS formation was inhibited (n = 7). Four individual preparations are depicted in Fig. 2. These data support the assumption that changes in PKCα expression might cause septic desensitization of monocytes. To elucidate whether down-regulation of PKCα indeed causes an attenuated oxidative response, we setup a cell culture model to mimic septic monocytes/macrophages.

FIGURE 2.

ROS production and PKCα expression in monocytes of septic patients. ROS production in monocytes is shown as mean HE fluorescence (log FL3) of four individual sepsis patients and four healthy donors untreated and in response to 1 μM PMA (upper panel). The corresponding PKCα expression was detected by Western blot analysis as described in Materials and Methods (lower panel). Individual experiments were performed twice.

FIGURE 2.

ROS production and PKCα expression in monocytes of septic patients. ROS production in monocytes is shown as mean HE fluorescence (log FL3) of four individual sepsis patients and four healthy donors untreated and in response to 1 μM PMA (upper panel). The corresponding PKCα expression was detected by Western blot analysis as described in Materials and Methods (lower panel). Individual experiments were performed twice.

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We treated monocytes/macrophages for 15 h with a combination of LPS and IFN-γ, two established mediators of the septic disease to obtain a desensitized phenotype (21, 22). Because these agents are known activators of inducible NO synthase, at least in murine cells (23), we routinely used L-NAME to suppress NO formation and thereby exclude any possible interference of reactive nitrogen species during ROS determination. Subsequently, we performed Western blot experiments to follow expression of classical, novel, and atypical PKC isoforms to examine their contribution on monocyte/macrophage desensitization. Resting RAW264.7 macrophages and U937 monocytes express PKCα, δ, ε, and λ (Fig. 3, A and B). Expression of the PKCα, δ, and ε are down-regulated in response to increasing concentrations of PMA added for 15 h. This can be explained based on their structural organization, containing a DAG-binding site. In contrast, the expression of PKCλ, an atypical PKC family member, did not change after PMA addition, again consistent with the absence of a DAG-responsive site. Treatment of the cells for 15 h with a combination of LPS/IFN-γ/L-NAME suppressed expression of PKCα only. To verify the significance of this effect for primary cells, we treated primary macrophages derived from peripheral blood monocytes, with LPS/IFN-γ/L-NAME for different times. PKCα expression declined 1 h after LPS/IFN-γ/L-NAME treatment. As shown in Fig. 3 C, depletion of PKCα was not as complete, although clearly visible.

FIGURE 3.

PKC expression in monocytes/macrophages under the influence of LPS/IFN-γ/L-NAME. RAW264.7 macrophages (A) and U937 cells (B) were stimulated for 15 h with the indicated concentrations of PMA, with a combination of LPS (10 μg/ml), IFN-γ (100 U/ml), and L-NAME (1 mM), or remained as controls. PKCα, δ, ε, and λ expression were detected by Western blot analysis as described in Materials and Methods. Experiments were performed three times. C, Primary monocyte-derived macrophages were treated for 0.5, 1, and 4 h with a combination of LPS (10 μg/ml), IFN-γ (100 U/ml), and L-NAME (1 mM), or remained as controls. PKCα expression was detected by Western blot analysis as described in Materials and Methods. Experiments were performed three times.

FIGURE 3.

PKC expression in monocytes/macrophages under the influence of LPS/IFN-γ/L-NAME. RAW264.7 macrophages (A) and U937 cells (B) were stimulated for 15 h with the indicated concentrations of PMA, with a combination of LPS (10 μg/ml), IFN-γ (100 U/ml), and L-NAME (1 mM), or remained as controls. PKCα, δ, ε, and λ expression were detected by Western blot analysis as described in Materials and Methods. Experiments were performed three times. C, Primary monocyte-derived macrophages were treated for 0.5, 1, and 4 h with a combination of LPS (10 μg/ml), IFN-γ (100 U/ml), and L-NAME (1 mM), or remained as controls. PKCα expression was detected by Western blot analysis as described in Materials and Methods. Experiments were performed three times.

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Previously, we showed that this activation procedure inhibits the PMA-initiated oxidative burst (11). To provide unequivocal evidence for the role of PKCα in ROS generation in response to PMA, we transiently transfected primary macrophages with a catalytically inactive PKCα mutant (pEF-PKCα K368R) (18). Transfected cells were selected and treated with PMA to assess ROS formation by flow cytometry. As expected, ROS generation was significantly inhibited in cells transfected with the inactive PKCα mutant (Fig. 4, right panel), pointing to the involvement of PKCα in superoxide radical formation in response to PMA. In extending experiments, we hypothesized that increasing the level of PKCα by PKCα overexpression might abolish, at least in part, activation-dependent depletion, and thus counteract inhibition of O2 production. Therefore, a retroviral vector was constructed containing the sequence for a PKCα-EGFP hybrid protein to follow transfection efficiency as well as allowing FACS sorting.

FIGURE 4.

Overexpression of a PKCα kinase-inactive mutant attenuated ROS production. ROS production in response to 1 μM PMA in primary monocyte-derived macrophages, transfected with a kinase inactive PKCα mutant (pEF-PKCα K368R) or an empty control vector (pEF-control), was analyzed by flow cytometry using 3 μM HE as the redox-sensitive dye. Experiments were performed three times.

FIGURE 4.

Overexpression of a PKCα kinase-inactive mutant attenuated ROS production. ROS production in response to 1 μM PMA in primary monocyte-derived macrophages, transfected with a kinase inactive PKCα mutant (pEF-PKCα K368R) or an empty control vector (pEF-control), was analyzed by flow cytometry using 3 μM HE as the redox-sensitive dye. Experiments were performed three times.

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After retroviral infection with the pLXIN-PKCα-EGFP construct, RAW264.7 expressed endogenous PKCα and an ∼30 kDa larger PKCα form, which corresponds to its EGFP tag as determined by Western blot analysis (Fig. 5,A, left panel). Similar results were obtained in U937 cells (Fig. 5,A, right panel). Functionality of the PKCα-EGFP protein was verified by analyzing its membrane translocation in response to PMA treatment by fluorescence microscopy (data not shown). Subsequently, cells were treated for 15 h with LPS/IFN-γ/L-NAME and, finally, PKCα was analyzed in cell lysates. Only endogenous PKCα is down-regulated, whereas the level of exogenous EGFP-linked PKCα remained unchanged (Fig. 5 B). Based on these Western blot results, we performed a final set of experiments to characterize the influence of PKCα overexpression in monocyte/macrophage desensitization.

FIGURE 5.

Overexpression of PKCα attenuated LPS/IFN-γ/L-NAME-mediated macrophage desensitization. A, Overexpression of PKCα-EGFP was detected in RAW264.7 macrophages and U937 cells by Western blot analysis as described in Materials and Methods. B, PKCα-overexpressing RAW264.7 macrophages and U937 cells were stimulated for 15 h with a combination of LPS (10 μg/ml), IFN-γ (100 U/ml), and L-NAME (1 mM), or remained as controls. PKCα expression was detected by Western blot analysis as described in Materials and Methods. C, PKCα-overexpressing RAW264.7 macrophages and U937 cells were pretreated for 15 h with a combination of LPS, IFN-γ, and L-NAME. PKCα translocation in response to 1 μM PMA was analyzed by fluorescence microscopy. For details, see Materials and Methods. D, PKCα-overexpressing RAW264.7 macrophages and control RAW264.7 cells were stimulated for 15 h with a combination of LPS (10 μg/ml), IFN-γ (100 U/ml), and L-NAME (1 mM), or remained as controls. ROS production in response to 1 μM PMA was analyzed by flow cytometry using 3 μM HE as the redox-sensitive dye. E, Statistical evaluation of flow cytometric data for RAW264.7 macrophages (left panel) and U937 cells (right panel) (∗, p ≤ 0.05). For details, see Materials and Methods.

FIGURE 5.

Overexpression of PKCα attenuated LPS/IFN-γ/L-NAME-mediated macrophage desensitization. A, Overexpression of PKCα-EGFP was detected in RAW264.7 macrophages and U937 cells by Western blot analysis as described in Materials and Methods. B, PKCα-overexpressing RAW264.7 macrophages and U937 cells were stimulated for 15 h with a combination of LPS (10 μg/ml), IFN-γ (100 U/ml), and L-NAME (1 mM), or remained as controls. PKCα expression was detected by Western blot analysis as described in Materials and Methods. C, PKCα-overexpressing RAW264.7 macrophages and U937 cells were pretreated for 15 h with a combination of LPS, IFN-γ, and L-NAME. PKCα translocation in response to 1 μM PMA was analyzed by fluorescence microscopy. For details, see Materials and Methods. D, PKCα-overexpressing RAW264.7 macrophages and control RAW264.7 cells were stimulated for 15 h with a combination of LPS (10 μg/ml), IFN-γ (100 U/ml), and L-NAME (1 mM), or remained as controls. ROS production in response to 1 μM PMA was analyzed by flow cytometry using 3 μM HE as the redox-sensitive dye. E, Statistical evaluation of flow cytometric data for RAW264.7 macrophages (left panel) and U937 cells (right panel) (∗, p ≤ 0.05). For details, see Materials and Methods.

Close modal

To clarify whether pretreated cells still overexpress functional PKCα, we first incubated cells for 15 h with LPS/IFN-γ/L-NAME. Then, we analyzed PKCα-EGFP translocation in response to 1 μM PMA by fluorescence microscopy. PKCα-EGFP completely translocated from the cytosol to the cell membranes in response to PMA within 5 min (Fig. 5,C, left panel). This response is similar to nonprestimulated cells (data not shown). Similar results were obtained in U937 cells (Fig. 5,C, right panel). We conclude that LPS/IFN-γ/L-NAME stimulation did not alter the ability of exogenous PKCα to translocate in response to PMA activation. Consequently, we determined the effect of PKCα overexpression on superoxide radical production. Overexpression of PKCα restores the ability of LPS/IFN-γ/L-NAME prestimulated RAW264.7 cells to generate O2 in response to PMA (Fig. 5,D). In U937 cells, we received similar findings (data not shown). Statistical evaluation indicated inhibition of superoxide radical formation in RAW264.7 and U937 cells pretreated with LPS/IFN-γ/L-NAME in response to PMA compared with a significant increase in PMA-elicited ROS production in PKCα-overexpressing RAW264.7 and U937 cells, despite their being prestimulated with LPS/IFN-γ/L-NAME (Fig. 5 E).

To exclude the possibility that our data are restricted to the use of PMA we performed a final set of experiments using opsonized E. coli to provoke ROS formation in U937 cells. As shown in Fig. 6, LPS/IFN-γ/L-NAME pretreatment of U937 cells inhibited the E. coli-initiated oxidative burst, while overexpression of PKCα-EGFP restored ROS formation. These data point to PKCα as one important component in the signaling cascade leading to ROS production in response to pathogen contact. Its depletion observed in monocytes derived from septic patients might therefore be responsible for cellular desensitization.

FIGURE 6.

LPS/IFN-γ/L-NAME-prestimulation attenuated ROS production in response to opsonized E. coli. PKCα-overexpressing U937 cells and control U937 cells were stimulated for 15 h with a combination of LPS (10 μg/ml), IFN-γ (100 U/ml), and L-NAME (1 mM), or remained as controls. ROS production in response to 1 μg/ml opsonized E. coli was analyzed by flow cytometry using 3 μM HE as the redox-sensitive dye. Experiments were performed three times. Statistical evaluation of flow cytometric data (∗, p ≤ 0.05). For details, see Materials and Methods.

FIGURE 6.

LPS/IFN-γ/L-NAME-prestimulation attenuated ROS production in response to opsonized E. coli. PKCα-overexpressing U937 cells and control U937 cells were stimulated for 15 h with a combination of LPS (10 μg/ml), IFN-γ (100 U/ml), and L-NAME (1 mM), or remained as controls. ROS production in response to 1 μg/ml opsonized E. coli was analyzed by flow cytometry using 3 μM HE as the redox-sensitive dye. Experiments were performed three times. Statistical evaluation of flow cytometric data (∗, p ≤ 0.05). For details, see Materials and Methods.

Close modal

The balance between the pro- vs anti-inflammatory immune response plays a prominent role in regulating innate immunity. In sepsis, overproduction of proinflammatory cytokines provokes a hyperinflammatory immune response often causing severe tissue damage, organ failure, and death (2). Depending on the patient’s coexisting conditions, e.g., age, nutritional status, and polymorphisms in cytokine genes, this period is followed by a hypoimmune state, characterized by the loss of B and CD4+ T cells. Moreover, in a subgroup of septic patients, a desensitized phenotype of monocytes/macrophages is observed, accompanied by down-regulation of HLA-DR expression and TNF-α release (5, 6). Molecular mechanisms provoking this anti-inflammatory appearance remain unclear.

We provide evidence that in monocytes of 7 of 12 preparations from septic patients the ability to generate superoxide radicals in response to PMA was abolished. Because it is known that monocytes of sepsis patients often show a desensitized phenotype (6, 20), we elucidated the underlying mechanism. An attenuated respiratory burst in monocytes/macrophages has been described in response to IL-4, causing LPS-receptor (CD14) and TLR-4 down-regulation (24). These transcriptional mechanisms prevent signal transmission provoking cellular activation following LPS binding. Inhibition of superoxide radical formation in chronic granulomatous disease has also been observed as a consequence of the deletion or attenuation of expression of a component of the NADPH oxidase system, not allowing functional assembly of this multienzyme complex (25). Furthermore, inhibition of PKCα, an established activator of the O2 generating system (26), has been identified to reduce the oxidative burst (27). Considering that the first event in PMA-mediated signaling causing superoxide production is activation of PKCα (26), we analyzed its expression in monocytes of septic donors. We found a correlation between PKCα depletion and inhibition of the oxidative burst, suggesting a role of PKCα in monocyte desensitization in sepsis. In the animal model, a decrease of PKCα expression during sepsis has been described also for other cell systems. In hepatocytes PKCα depletion causes hepatic failure in sepsis (28) and in lymphocytes PKCα expression declines during progression of sepsis in rats (29).

To establish a cause-effect relation between down-regulation of PKCα in monocytes derived from septic patients and an attenuated oxidative response, we established a cell culture model to mimic desensitized monocytes/macrophages. Therefore, immunological activation of macrophages is achieved by the TH1 cytokine IFN-γ and LPS, the outer membrane-constituent of Gram-negative bacteria (30). Using this system, we verified the role of PKCα in ROS production, illustrating the LPS/IFN-γ/L-NAME pretreatment-dependent depletion of PKCα and concomitant inhibited O2 formation. An attenuated oxidative burst in response to LPS/IFN-γ/L-NAME has been shown previously (11). However, the role of PKCα was not analyzed. Considering our new results, suggesting PKCα depletion to cause inhibition of ROS formation, it was attractive to speculate that a catalytically inactive PKCα mutant may inhibit ROS generation. As predicted in humans, primary monocyte-derived macrophages transfected of a PKCα inactive mutant attenuated ROS production in response to PMA, thus substantiating the role of PKCα in our system.

To finally verify the impact of PKCα in ROS formation we overexpressed PKCα to restore the desensitized cellular phenotype. To achieve high gene transfer efficiency and the possibility to sort positive cells we used a retroviral system to transduce RAW264.7 and U937 cells, giving rise to 100% positive clones for PKCα-EGFP. A similar fluorescent vector system was used by Maasch et al. (31) to elucidate the role of intracellular free calcium in targeting PKCα in vascular smooth muscle cells. In our experiments, PKCα-EGFP overexpression was functional and, in contrast to the endogenous form of PKCα, was not altered in response to LPS/IFN-γ/L-NAME prestimulation. This is in some analogy to the work of Gschwend et al. (32), when in the human prostate cancer cell line LNCaP, bryostatin 1 led to a time-dependent depletion of endogenous PKCα. This depletion was attenuated in cells overexpressing exogenous PKCα. In our system, overexpression of PKCα is controlled by a Moloney murine sarcoma virus long terminal repeat, provoking a constitutive level of PKCα expression, which explains high PKCα-EGFP expression in response to LPS/IFN-γ/L-NAME prestimulation. Consequently, incorporating these cells into our experimental approach, we tested their capability to generate superoxide radicals after the LPS/IFN-γ/L-NAME pretreatment regime in response to PMA. As expected, the PMA-initiated oxidative burst after LPS/IFN-γ/L-NAME prestimulation is restored in monocytes/macrophages overexpressing PKCα. We noticed similar effects in PKCα-overexpressing cells in response to opsonized E. coli demonstrating the pathophysiological significance of our data.

Our results support a role of PKCα in ROS production, suggesting a likely involvement of PKCα expression regulation in desensitizing monocytes in sepsis. It will be challenging to define molecular mechanisms leading to PKCα depletion in sepsis, thus attenuating the oxidative burst in one part of septic patients while not modulating PKCα expression and causing ROS formation in the other part.

The authors have no financial conflict of interest.

We thank J. Zimmerman and N. Wallner for expert technical assistance, and Drs. T. Huber and R. Wendler for providing the septic material and supporting the project.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1

This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Br 999).

4

Abbreviations used in this paper: ROS, reactive oxygen species; PKC, protein kinase C; DAG, diacylglycerol; HE, hydroethidine; L-NAME, N-nitro l-arginine methyl ester; EGFP, enhanced GFP.

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