Cytological-energy analysis of pleural effusions with predominance of neutrophils.

BACKGROUND: The predominance of neutrophils in pleural effusions of patients with different serious impairments of the pleural cavity organs is often found. The aim of this study was to identify the type of injury using the cytological-energy analysis of pleural effusions. METHODS: We analysed 635 samples of pleural effusions with predominance of neutrophils. We compared the values of the coefficient of energy balance (KEB), lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) catalytic activities in the following subgroups of patients: with transudative effusions, purulent pneumonia, chest empyema and after chest surgery with and without purulent complications. Statistical analysis was performed using the ANOVA Kruskal-Wallis test (p < 0.05 was considered as significant). RESULTS: We found the lowest KEB values in pleural effusions of patients with chest empyema and their gradual increases in patients with purulent pneumonia and with transudative effusions. We observed the highest LDH and AST enzymes activity in patients with chest empyema and their gradual decrease in patients with purulent pneumonia and with transudative effusions. LDH and AST enzymes activity was significantly higher in pleural effusions of patients after chest surgery with purulent complications compared with non-purulent cases. CONCLUSION: The most intensive inflammation and the most extensive tissue destruction in the pleural cavity were found in patients with chest empyema. Significantly better parameters were observed in patients with purulent pneumonia. The absence of serious inflammation and the absence of tissue destruction were typical for patients with transudative effusions. Finally, our results confirmed an anticipated higher tissue destruction in patients after chest surgery. Significantly worse injury was found in surgical patients with purulent complications compared with non-purulent ones. The reviews of this paper are available via the supplemental material section.

Expression of IL-1ß, HMGB1, HO-1, and LDH in malignant and non-malignant pleural effusions.

IL-1ß, HMGB1, HO-1, and LDH in the pleural effusions (PE) of patients with transudative, infectious, and malignant etiologies were determined using ELISA and enzymatic assays. IL-1ß, HMGB1, HO-1, and LDH showed significant differences between the three etiologies. Post-hoc analysis revealed higher levels of HO-1 and HMGB1 in infectious versus transudative effusion. Higher levels of IL-1ß were found in infectious versus transudative or malignant effusion. The comparison of LDH levels showed significant differences. Positive correlations were found between IL-1ß, HMGB1, and LDH in infectious effusions. The samples were then divided into cancerous and non-cancerous groups, and logistic regression revealed that increasing IL-1ß levels were significantly associated with a decrease in cancer risk after adjusting for HMGB1, HO-1, and LDH. Our findings suggest that IL-1ß, HMGB1, HO-1, and LDH are expressed differently, with positive correlations between HMGB1, IL-1ß, and LDH in infectious effusions, and low IL-1ß expression in malignant effusions.

Role of Intrapleural Urokinase in Empyema Thoracis.

OBJECTIVE: To study the role of fibrinolytic therapy in pediatric empyema in relation to duration of hospital stay, need for surgical intervention and survival to discharge. METHODS: Retrospective analysis of case records of children <16 y of age admitted in a tertiary care hospital during January 2013 - December 2017 with diagnosis as empyema thoracis was done. Clinico-laboratory characteristics and the primary and secondary outcomes between the group which received intrapleural urokinase (IPU) and the group which did not (non IPU), were compared. RESULTS: Of the 84 cases, 40 children received IPU. Mean duration of hospital stay in IPU group (17.51 + 4.57 d) was significantly less than non IPU group (24.32 + 10.18 d, CI -10.19 to -3.64, p < 0.001), so was the duration of intercostal drain (ICD) insertion (9.08 + 3.12 d - IPU group vs. 11.20 + 3.95 d - non IPU group, CI -3.68 to -0.50, p < 0.01). No statistically significant difference was found between the groups with regard to need for surgical intervention [IPU - 4 (10%), non IPU - 9 (20.4%), p = 0.23]. There was no mortality or adverse reaction to urokinase in either group. CONCLUSIONS: IPU holds promising results in terms of reduction of hospital stay and duration of ICD insertion. It may be the initial choice of treatment in septated empyema where surgical options are not easily available or cost-effective especially in resource limited settings.

Myocardin Is Involved in Mesothelial-Mesenchymal Transition of Human Pleural Mesothelial Cells.

Pleural fibrosis is characterized by severe inflammation of the pleural space and pleural reorganization. Subsequent thickening of the visceral pleura contributes to lung stiffness and impaired lung function. Pleural mesothelial cells (PMCs) can become myofibroblasts via mesothelial-mesenchymal transition (MesoMT) and contribute to pleural organization, fibrosis, and rind formation. However, the mechanisms that underlie MesoMT remain unclear. Here, we investigated the role of myocardin in the induction of MesoMT. Transforming growth factor ß (TGF-ß) and thrombin induced MesoMT and markedly upregulated the expression of myocardin, but not myocardin-related transcription factor A (MRTF-A) or MRTF-B, in human PMCs (HPMCs). TGF-ß stimulation notably induced the nuclear translocation of myocardin in HPMCs, whereas nuclear translocation of MRTF-A and MRTF-B was not observed. Several genes under the control of myocardin were upregulated in cells undergoing MesoMT, an effect that was accompanied by a dramatic cytoskeletal reorganization of HPMCs consistent with a migratory phenotype. Myocardin gene silencing blocked TGF-ß- and thrombin-induced MesoMT. Although myocardin upregulation was blocked, MRTF-A and MRTF-B were unchanged. Myocardin, α-SMA, calponin, and smooth muscle myosin were notably upregulated in the thickened pleura of carbon black/bleomycin and empyema mouse models of fibrosing pleural injury. Similar results were observed in human nonspecific pleuritis. In a TGF-ß mouse model of pleural fibrosis, PMC-specific knockout of myocardin protected against decrements in lung function. Further, TGF-ß-induced pleural thickening was abolished by PMC-specific myocardin knockout, which was accompanied by a marked reduction of myocardin, calponin, and α-SMA expression compared with floxed-myocardin controls. These novel results show that myocardin participates in the development of MesoMT in HPMCs and contributes to the pathogenesis of pleural organization and fibrosis.

Complement depletion and Coombs positivity in pneumococcal hemolytic uremic syndrome (pnHUS). Case series and plea to revisit an old pathogenetic concept.

Hemolytic uremic syndrome is a rare complication of invasive pneumococcal infection (pnHUS). Its pathogenesis is poorly understood, and treatment remains controversial. The emerging role of complement in various forms of HUS warrants a new look at this "old" disease. We performed a retrospective analysis of clinical and laboratory features of three sequential cases of pnHUS since 2008 associated with pneumonia/pleural empyema, two due to Streptococcus pneumoniae serotype 19 A. Profound depletion of complement C3 (and less of C4) was observed in two patients. One patient was Coombs test positive. Her red blood cells (RBCs) strongly agglutinated with blood group compatible donor serum at 0 °C, but not at 37 °C. All three patients were treated with hemodialysis, concentrated RBCs, and platelets. Patient 2 received frozen plasma for hepatic failure with coagulation factor depletion. Intravenous immunoglobulin infusion, intended to neutralize pneumococcal neuraminidase in patient 3, was associated with rapid normalization of platelets and cessation of hemolysis. Two patients recovered without sequelae or disease recurrence. Patient 2 died within 2½ days of admission due to complicating Pseudomonas aeruginosa sepsis and multiorgan failure. Our observations suggest that pnHUS can be associated with dramatic, transient complement consumption early in the course of the disease, probably via the alternative pathway. A critical review of the literature and the reported cases argue against the postulated pathological role of preformed antibodies against the neuraminidase-exposed Thomsen-Friedenreich neoantigen (T antigen) in pnHUS. The improved understanding of complement regulation and bacterial strategies of complement evasion allows to propose a testable, new pathogenetic model of pnHUS. This model shifts emphasis from the action of natural anti-T antibodies toward impaired Complement Factor H (CFH) binding and function on desialylated membranes. Removal of neuraminic acid residues converts (protected) self to non-self surfaces that supports membrane attack complex (MAC) assembly. Complement activation is potentially exacerbated by decreased CFH availability following tight CFH binding to pneumococcal evasion proteins and/or by the presence of genetic variants of complement regulator proteins. Detailed clinical and experimental investigations are warranted to better understand the role of unregulated complement activation in pnHUS. Instead of avoidance of plasma, a new, integrated model is evolving, which may include short-term therapeutic complement blockade, particularly where genetic or functional APC dysregulation is suspected, in addition to bacterial elimination and, potentially, neuraminidase neutralization.

Streptococcus pneumoniae potently induces cell death in mesothelial cells.

Pleural infection/empyema is common and its incidence continues to rise. Streptococcus pneumoniae is the commonest bacterial cause of empyema in children and among the commonest in adults. The mesothelium represents the first line of defense against invading microorganisms, but mesothelial cell responses to common empyema pathogens, including S. pneumoniae, have seldom been studied. We assessed mesothelial cell viability in vitro following exposure to common empyema pathogens. Clinical isolates of S. pneumoniae from 25 patients with invasive pneumococcal disease and three reference strains were tested. All potently induced death of cultured mesothelial cells (MeT-5A) in a dose- and time-dependent manner (>90% at 107 CFU/mL after 24 hours). No significant mesothelial cell killing was observed when cells were co-cultured with Staphylococcus aureus, Streptococcus sanguinis and Streptococcus milleri group bacteria. S. pneumoniae induced mesothelial cell death via secretory product(s) as cytotoxicity could be: i) reproduced using conditioned media derived from S. pneumoniae and ii) in transwell studies when the bacteria and mesothelial cells were separated. No excess cell death was seen when heat-killed S. pneumoniae were used. Pneumolysin, a cytolytic S. pneumoniae toxin, induced cell death in a time- and dose-dependent manner. S. pneumoniae lacking the pneumolysin gene (D39 ΔPLY strain) failed to kill mesothelial cells compared to wild type (D39) controls, confirming the necessity of pneumolysin in D39-induced mesothelial cell death. However, pneumolysin gene mutation in other S. pneumoniae strains (TIGR4, ST3 and ST23F) only partly abolished their cytotoxic effects, suggesting different strains may induce cell death via different mechanisms.

Rapid developing empyema by group F beta Streptococcus anginosus group.

A 43-year-old male had progressive pleuritic left-sided chest tightness with shortness of breath. He had dental caries and tenderness on palpation of the left lateral chest. Complete blood count showed leucocytosis. CT scan of the chest with pulmonary emboli protocol showed multiple pulmonary nodules and nodular pleural thickening at left posterior lateral pleura. Forty-eight hours post CTPE scan, CT scan of the chest, abdomen and pelvis displayed right lower lobe consolidation and left-sided pleural effusion with superimposed compressive atelectasis. Ceftaroline intravenous was initiated, with CT-guided pigtail chest tube insertion. Pleural fluid later grew group F beta-haemolytic Streptococcus anginosus Patient improved significantly and was discharged 11 days later with intravenous ertapenem. Patients with group F beta-haemolytic streptococci should be managed aggressively with early and accurate diagnosis, antibiotics, drainage and possible surgery.

Modelling staphylococcal pneumonia in a human 3D lung tissue model system delineates toxin-mediated pathology.

Staphylococcus aureus necrotizing pneumonia is recognized as a toxin-mediated disease, yet the tissue-destructive events remain elusive, partly as a result of lack of mechanistic studies in human lung tissue. In this study, a three-dimensional (3D) tissue model composed of human lung epithelial cells and fibroblasts was used to delineate the role of specific staphylococcal exotoxins in tissue pathology associated with severe pneumonia. To this end, the models were exposed to the mixture of exotoxins produced by S. aureus strains isolated from patients with varying severity of lung infection, namely necrotizing pneumonia or lung empyema, or to purified toxins. The necrotizing pneumonia strains secreted high levels of α-toxin and Panton-Valentine leukocidin (PVL), and triggered high cytotoxicity, inflammation, necrosis and loss of E-cadherin from the lung epithelium. In contrast, the lung empyema strain produced moderate levels of PVL, but negligible amounts of α-toxin, and triggered limited tissue damage. α-toxin had a direct damaging effect on the epithelium, as verified using toxin-deficient mutants and pure α-toxin. Moreover, PVL contributed to pathology through the lysis of neutrophils. A combination of α-toxin and PVL resulted in the most severe epithelial injury. In addition, toxin-induced release of pro-inflammatory mediators from lung tissue models resulted in enhanced neutrophil migration. Using a collection of 31 strains from patients with staphylococcal pneumonia revealed that strains producing high levels of α-toxin and PVL were cytotoxic and associated with fatal outcome. Also, the strains that produced the highest toxin levels induced significantly greater epithelial disruption. Of importance, toxin-mediated lung epithelium destruction could be inhibited by polyspecific intravenous immunoglobulin containing antibodies against α-toxin and PVL. This study introduces a novel model system for study of staphylococcal pneumonia in a human setting. The results reveal that the combination and levels of α-toxin and PVL correlate with tissue pathology and clinical outcome associated with pneumonia.

[Contribution of pleural fluid analysis to the diagnosis of pleural effusion]. / Contribución del análisis del líquido pleural al diagnóstico de los derrames pleurales.

Analysis of pleural fluid can have, on its own, a high diagnostic value. In addition to thoracocentesis, a diagnostic hypothesis based on medical history, physical examination, blood analysis and imaging tests, the diagnostic effectiveness will significantly increase in order to establish a definite or high probable diagnosis in a substantial number of patients. Differentiating transudates from exudates by the classical Light's criteria helps knowing the pathogenic mechanism resulting in pleural effusion, and it is also useful for differential diagnosis purposes. An increased N-terminal pro-brain natriuretic peptide, both in the fluid and in blood, in a due clinical context, is highly suggestive of heart failure. The presence of an increased inflammatory marker, such as C-reactive protein, together with the presence of over 50% of neutrophils is highly suggestive of parapneumonic pleural effusion. If, in these cases, the pH is<7.20, then the likelihood of complicated pleural effusion is high. There remains to be demonstrated the usefulness of other markers to differentiate complicated from uncomplicated effusions. An adenosine deaminase > 45 U/L and>50% lymphocytes is suggestive of tuberculosis. If a malignant effusion is suspected but the cytological result is negative, increased concentrations of some markers in the pleural fluid can yield high specificity values. Increased levels of mesothelin and fibruline-3 are suggestive of mesothelioma. Immunohistochemical studies can be useful to differentiate reactive mesothelial cells, mesothelioma and metastatic adenocarcinoma. An inadequate use of the information provided by the analysis of pleural fluid would results in a high rate of undiagnosed effusions, which is unacceptable in current clinical practice.

Factors predictive of the failure of medical treatment in patients with pleural infection.

BACKGROUND/AIMS: The clinical outcomes of some patients with pleural infection may be favorable with medical treatment alone, but in others, the disease progresses and requires additional surgical treatment. However, little is known about the factors affecting this difference. The aim of this study was to investigate the factors predictive of failure of medical treatment in patients with pleural infection. METHODS: A cohort of 127 consecutive patients who were admitted to the hospital with pleural infection was studied. Clinical manifestations and laboratory findings in patients in whom medical treatment succeeded or failed were reviewed. RESULTS: In univariate analysis, the significant factors associated with medical treatment outcome were age, smoking history, duration of chief complaint, serum albumin level, and pleural fluid glucose and lactate dehydrogenase levels (p < 0.05). Multivariate logistic regression analysis identified age and duration of chief complaint as independent predictive factors for failure of medical treatment, with odds ratios of 0.871 (p = 0.013) and 0.797 (p = 0.026), respectively. Receiver operating characteristic curve analysis determined cutoff values of 50.5 years for age and 4.5 days for duration of chief complaint. CONCLUSIONS: We demonstrated that a younger age < 50.5 years and shorter duration of chief complaint < 4.5 days were independent predictive factors for the failure of medical treatment in patients with pleural infection. This suggests their role as evaluative criteria in setting indications for the optimal treatment in patients with pleural infection. A larger, prospective study is required to confirm these findings.

Utility of measurement of interleukin-1ß and interleukin-8 in the diagnosis of complicated parapneumonic pleural effusions.

OBJECTIVES: Thirty percent of patients with pneumonia develop pleural effusion, and of these, 20% have complicated effusion (CPPE), which may require a chest tube or surgery for resolution. The objective of the study is to compare the diagnostic yield of determining interleukin-1ß and interleukin-8 in pleural fluid (PF) (PFIL-1ß and PFIL-8) with respect to classic criteria (pH <7.2, lactate dehydrogenase [LD] >1,000 IU/mL, and/or glucose <60 mg/dL) in the early diagnosis of CPPE. METHODS: Of the 559 patients studied, 40 had CPPE. All underwent PF analysis: pH, glucose (PFGLUC), LD (PFLD), PFIL-1ß and PFIL-8, and PF/serum ratios (PF/SIL-1ß and PF/SIL-8). RESULTS: The diagnostic criterion that showed the best area under the curve was the combination of PF/SIL-8 and PFIL-1ß (0.906), with a statistically significant difference (P < .05) compared with the classic criterion of pH and PFGLUC or PFLD (0.826). The combination of PF/SIL-8 and PFIL-1ß (cutoffs >5.73 and >9.14 pg/mL, respectively) was significantly more sensitive (72.7%) and more specific (97.9%) (P < .05) than the rest of the parameters used. CONCLUSIONS: Measurement of IL-1ß and IL-8 in pleural fluid may be useful in the early diagnosis of CPPE, although individually, it may not improve the results obtained with the PFLD. Further studies are needed to more firmly establish what role these new parameters can play in the diagnosis of CPPE.

Factors predictive of the failure of medical treatment in patients with pleural infection

BACKGROUND/AIMS: The clinical outcomes of some patients with pleural infection may be favorable with medical treatment alone, but in others, the disease progresses and requires additional surgical treatment. However, little is known about the factors affecting this difference. The aim of this study was to investigate the factors predictive of failure of medical treatment in patients with pleural infection. METHODS: A cohort of 127 consecutive patients who were admitted to the hospital with pleural infection was studied. Clinical manifestations and laboratory findings in patients in whom medical treatment succeeded or failed were reviewed. RESULTS: In univariate analysis, the significant factors associated with medical treatment outcome were age, smoking history, duration of chief complaint, serum albumin level, and pleural fluid glucose and lactate dehydrogenase levels (p < 0.05). Multivariate logistic regression analysis identified age and duration of chief complaint as independent predictive factors for failure of medical treatment, with odds ratios of 0.871 (p = 0.013) and 0.797 (p = 0.026), respectively. Receiver operating characteristic curve analysis determined cutoff values of 50.5 years for age and 4.5 days for duration of chief complaint. CONCLUSIONS: We demonstrated that a younger age < 50.5 years and shorter duration of chief complaint < 4.5 days were independent predictive factors for the failure of medical treatment in patients with pleural infection. This suggests their role as evaluative criteria in setting indications for the optimal treatment in patients with pleural infection. A larger, prospective study is required to confirm these findings.

Acute onset pyothorax-associated lymphoma (PAL) with inflammatory features.

Pyothorax-associated lymphoma (PAL) is a B-cell non-Hodgkin's lymphoma, and develops after 20-40 years of pyothorax or pleuritis including tuberculosis. An 88-year-old Japanese woman developed fever and right pleural effusion. No mycobacterium was recognized by Ziel-Neelsen stain, culture tests and PCR technique in the effusion. The patient was diagnosed as non-specific pleuritis. No tumor formations were seen in the right pleura by imaging modalities, and she was treated by antibiotics. Eight months later, she complained of severe back pain and fever. Imaging modalities revealed many tumors in the right pleura, and biopsy of the tumors was performed. The biopsy showed severe diffuse proliferation of lymphoid cells. The lymphoid cells consisted of atypical large lymphoid cells and small lymphoid cells. The large atypical cells were positive for CD45 and CD20 but negative for CD15 and CD30, thus confirming B cell neoplasm. Ki-67 labeling was 79%. The small cells were positive for CD45, CD20 and CD3, reflecting a mixture of mature B and T-cells. The small cells appeared non-neoplastic inflammatory cells. CD68-positive macrophages were also scattered. In situ hybridization for EB virus DNA showed positive signals in the large atypical B-cells and, to a lesser degree, in the small lymphocytes. The author thinks that this tumor is PAL with inflammatory reaction. The present case shows that the duration between PAL and pleuritis can be very short, and PAL may be associated with inflammatory reaction at the early neoplastic stage.

Tumor necrosis factor alpha in experimental empyema thoracis.

PURPOSE: To determine serum and pleural concentrations of tumor necrosis factor alpha (TNF-α) in an experimental model of empyema induced by intrapleural inoculation of Staphylococcus aureus or Streptococcus pneumoniae. METHODS: Wistar rats were inoculated with S. aureus (SA group, 17 animals) or S. pneumoniae (SP group, 30 animals). The presence of free fluid or pus in the pleural space was investigated. TNF-α levels >150 pg/ml (minimum detection limit) were determined in pleural fluid and blood. Histopathological examination of pleural tissue was performed to determine the severity of infection. RESULTS: Serum TNF-α was >150 pg/ml in nine SA versus 10 SP rats. In pleural fluid, TNF-α was >150 pg/ml in 11 SA versus 19 SP rats. Pleural and serum TNF-α concentrations were significantly different in the SP group (P = 0.035), but not in the SA group (P = 0.727). Pleural TNF-α was similar in both groups (P = 0.92), but serum TNF-α was significantly higher in SA (P = 0.03). Out of 17 SA animals, 1 (5.8%) did not develop empyema, versus 4 (13.3%) out of 30 SP animals. A mild inflammatory response was predominant in both groups, but the inflammatory process was significantly more severe in SP (P = 0.012). However, TNF-α levels were not associated with severity of the inflammatory response. CONCLUSIONS: We describe a simple and effective rat model of empyema. TNF-α levels above 150 pg/ml in the pleural fluid are useful to confirm empyema, but cannot predict the severity of the inflammatory response. TNF-α levels below 150 pg/ml are useful to rule out empyema.

A case of Aspergillus empyema successfully treated with combination therapy of voriconazole and micafungin: excellent penetration of voriconazole and micafungin into pleural fluid.

A 62-year-old man with empyema caused by Aspergillus fumigatus was successfully treated with a combination of voriconazole (VRCZ) and micafungin (MCFG). Data regarding the penetration of antifungal agents into pleural fluid are limited. Thus, we measured the concentration of VRCZ and MCFG in his plasma and pleural fluid. Penetration of VRCZ and MCFG into the pleural fluid was excellent. Therefore, the combination therapy using VRCZ and MCFG may contribute to successful management of Aspergillus empyema.

Pharmacokinetics of liposomal amphotericin B in pleural fluid.

We report the penetration of liposomal amphotericin B into the pleural fluid of a patient with pulmonary zygomycosis and empyema. The ratio of area under the concentration-versus-time curve in pleural fluid (AUC(pleural fluid)) to that in serum (AUC(serum)) for liposomal amphotericin B over 24 h was 9.4%, with pleural fluid concentrations of 2.12 to 4.91 microg/ml. Given the relatively low level of intrapleural penetration of liposomal amphotericin B, chest tube drainage may be warranted for successful treatment of zygomycotic empyema.

Penetration of azithromycin in experimental pleural empyema fluid.

There were no data about the extent of azithromycin penetration into the empyemic pleural fluid in humans and in experimental animals. An empyema was created via the intrapleural injection of an Escherichia coli solution into the pleural space of New Zealand white rabbits. After an empyema was verified by thoracocentesis, 24h post inoculation, azithromycin (15 mg/kg) was administered intravenously. Antibiotic levels were determined in samples of pleural fluid and blood serum, collected serially at 2, 8, 24, 48 and 72 h, after administration. Azithromycin levels were estimated using an HPLC analytical method with fluorimetric detection. Azithromycin penetrated well into the empyemic pleural fluid, exhibiting a slower onset and decline compared to the corresponding blood serum levels. Equilibration between pleural fluid and blood serum compartments seemed to occur at 2h, with peak pleural fluid levels (C(maxpf) of 0.48 microg/ml) occurring 24h post administration and decreasing thereafter. Azithromycin peak serum concentration (C(maxserum) of 0.24 microg/ml) was observed 2h after administration and, thereafter, serum antibiotic levels remained lower than the corresponding pleural fluid ones. The area under the concentration versus time curve (AUC) and terminal half-life (T(1/2)) of azithromycin was three- to six fold and twofold higher, respectively, in the pleural fluid compared to the blood serum compartment. After intravenous administration, azithromycin penetrated well into the empyemic pleural fluid, exhibiting pleural fluid levels that are inhibitory for most erythromycin-sensitive pathogens causing empyema.

Bacterial induction of early response genes and activation of proapoptotic factors in pleural mesothelial cells.

In bacterial empyema the pleural mesothelium is constantly exposed to microorganisms. Staphylococcus aureus (S. aureus) is one of the most frequent pathogens associated with empyema. In an earlier study we demonstrated that S. aureus induced barrier dysfunction in pleural mesothelial cell monolayers. In the present study we report that S. aureus activates the early response genes c-fos and c-jun and activator protein-1 (AP-1), and induces proapoptosis genes Bad and Bak in primary mouse pleural mesothelial cells (PMCs). Our data indicate that in PMCs S. aureus induces apoptosis in a time- and multiplicity of infection (MOI)-dependent manner. Staphylococcus aureus induced Bcl (2), Bcl-X (L), c-fos, c-jun, and AP-1 expression in PMCs during the initial phase of infection. In S. aureus-infected PMCs, Bad and Bak gene expression was increased and correlated with DNA fragmentation and cytochrome-c release. Bcl (2) and Bcl-X (L) gene expression was significantly lower in S. aureus-infected PMCs than in uninfected PMCs 12 h postinfection. We conclude that at the initial stage of infection S. aureus modulates expression of early response genes c-fos and c-jun, and in the late phase of infection S. aureus induces expression of proapoptotic genes Bak and Bad in PMCs. Silencing AP-1 significantly inhibited S. aureus-induced Bak and Bad expression in PMCs. The upregulation of early response genes during the early phase of infection may contribute to the activation of proapoptotic genes Bak and Bad and release of cytochrome-c, caspase-3 thereby resulting in apoptosis in PMCs.