Published ahead of print on February 22, 2007, doi:10.1164/rccm.200610-1533OC
© 2007 American Thoracic Society doi: 10.1164/rccm.200610-1533OC
Importance of Phosphoinositide 3-Kinase
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in the Host Defense against Pneumococcal Infection |
ABSTRACT |
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 TOP ABSTRACT AT A GLANCE COMMENTARYMETHODSRESULTSDISCUSSIONREFERENCES |
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Rationale: The pivotal role of phosphoinositide 3-kinase
(PI3K) in leukocyte recruitment makes it an attractive target for immunomodulatory therapy. However, interfering with PI3K signaling might increase the risk of bacterial infections in humans.
Objectives: We hypothesized that deletion or pharmacologic inhibition of PI3K would impair the lung inflammatory response to the prototypic gram-positive bacterial pathogen Streptococcus pneumoniae.
Methods: PI3K knockout (KO) and wild-type mice were infected with S. pneumoniae or challenged with the pneumococcal virulence factor pneumolysin (PLY), and inflammatory leukocyte recruitment, bacterial pathogen elimination, and resolution/repair processes were determined.
Measurements and Main Results: PI3K KO mice challenged with PLY responded with lung edema and neutrophilic alveolitis, but showed a drop in alveolar macrophages and failed to recruit exudate macrophages when compared with wild-type mice. S. pneumoniae–infected PI3K KO mice and wild-type mice pretreated with the pharmacologic inhibitor AS-605240 recruited similar numbers of neutrophils but substantially fewer exudate macrophages into their lungs than control animals. They also displayed a significantly reduced lung pneumococcal clearance and showed an impaired resolution/repair process, leading to progressive pneumococcal pneumonia.
Conclusions: PI3K gene deletion or pharmacologic inhibition of PI3K leads to perturbations of critical innate immune responses of the lung to challenge with S. pneumoniae. These data are of clinical relevance for the treatment of chronic inflammatory diseases where pharmacologic inhibition of PI3K signaling to attenuate effector cell recruitment may have implications for innate immune surveillance of remote organ systems.
Key Words: lung • infection • macrophage
Scientific Knowledge on the Subject No data are currently available addressing the role of phosphoinositide 3-kinase (PI3K) deletion or inhibition on the lung host defense to Streptococcus pneumoniae infection.
What This Study Adds to the Field
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Directional leukocyte recruitment toward inflamed tissues was recently shown to be largely dependent on phosphoinositide 3-kinase isoform
(PI3K) (1–3), which belongs to the family of class I PI3 kinases, making PI3K an attractive target for immunomodulatory therapy (4). All class I PI3Ks are heterodimeric proteins composed of a 110-kD catalytic subunit (class IA: p110
, p110
, and p110
; class IB: p110) and an 85-kD (p85) or a 101-kD (p101) regulatory subunit, respectively. Although the PI3K and PI3K isoforms are ubiquitously expressed, the PI3K and PI3K isoforms are largely confined to leukocytes. PI3K activity is regulated by the p101 regulatory adapter associated with seven-transmembrane G protein–coupled receptors (GPCRs). Activation of PI3K links the chemokine-induced GPCR activation to the generation of phosphatidylinositol 3,4,5-triphosphates (PtdIns [3–5]P3), which play an essential role in downstream signaling to elicit directional cell movement (2, 5). Consequently, PI3K-deficient leukocytes demonstrated impaired migration to various chemoattractant stimuli, which was found to be more confined to the mononuclear phagocytic as compared with the neutrophilic migratory response in a mouse model of septic peritonitis (2, 6, 7). Moreover, PI3K-deficient neutrophils demonstrated reduced respiratory burst formation in response to GPCR-mediated agonist activation in vitro (2). In addition, most recently developed, highly specific, orally active, small-molecule PI3K inhibitors were shown to be effective in suppressing joint inflammation in mouse models of rheumatoid arthritis (8). Streptococcus pneumoniae is the most prevalent gram-positive bacterium causing community-acquired pneumonia, septic meningitis, and otitis media worldwide. Despite its considerable clinical importance, the molecular mechanisms by which S. pneumoniae overcomes innate immune responses are only partially understood. The pathogenicity of S. pneumoniae appears to be largely defined by its ability to generate a variety of virulence factors, among which the pneumococcal virulence factor pneumolysin (PLY) is considered to be of major importance. PLY is an intracellular 53-kD protein and is released during pneumococcal autolysis (9). It belongs to the family of cholesterol-binding cytolysins and is highly cytotoxic toward target cells due to its pore-forming activities. In addition, PLY has been reported to act as a pathogen-associated molecular pattern by signaling via Toll-like receptor 4 (TLR4) and to induce TLR4-dependent apoptosis in macrophages with protective effects in pneumococcal pathogenesis (10, 11).
Recent reports from our group demonstrated that intratracheal application of PLY into the lungs of mice not only induced severe lung edema but also provoked a drastic depletion of the resident alveolar macrophage pool within the lungs, implying that PLY not only induces cytotoxicity toward lung epithelial cells (12, 13) but also promotes colonization of the lower respiratory tract by exerting strong cytotoxicity toward sentinel host defense cells of the lung (14). Consequently, to overcome the PLY-induced loss of alveolar macrophage function, the host must rapidly mount an inflammatory leukocyte influx to purge pneumococcal infection from distal airspaces (14).
The blockade of PI3K by small-molecule inhibitors may prove to be a valid approach to modulate excessive effector leukocyte accumulations in inflamed tissues, where leukocyte recruitment is correlated with disease progression, as occurs in rheumatoid arthritis and other diseases (8, 15–17). On the other hand, interfering with inflammatory leukocyte trafficking may adversely affect protective innate immunity of other remote organ systems, such as the lung, thereby increasing the risk of bacterial infections in humans. The current study was performed to test the hypothesis that PI3K gene deletion or pharmacologic inhibition of PI3K signaling would adversely affect the lung innate host defense to challenge with the prototypic gram-positive bacterial pathogen S. pneumoniae.
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METHODS |
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TOPABSTRACTAT A GLANCE COMMENTARYMETHODSRESULTSDISCUSSIONREFERENCES |
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Animals
Wild-type and PI3K
knockout (KO) mice (weight, 20–25 g; age, 8–12 wk) were generated on a 129/Sv background as described previously (2) and maintained under specific pathogen–free conditions. All animal experiments were approved by our local government authorities (LAVES, Oldenburg, Germany). Experiments were performed under blinded conditions.
Reagents
Native PLY was purified from recombinant Escherichia coli as described previously (18). Recombinant murine CCL2 protein (monocyte chemotactic protein-1) was obtained from Peprotech (Offenbach, Germany). The PI3K-specific inhibitor AS-605240 was used as described recently (8). Rat anti-mouse anti-CCR2 monoclonal antibody MC21 was used as described recently (16, 19). For further details, see the online supplement.
Treatment of Animals with PLY or CCL2
Intratracheal application of PLY or CCL2 was performed as described recently (14) and is further outlined in the online supplement.
Bronchoalveolar Lavage
Collection of serum and bronchoalveolar lavage (BAL) fluid for the isolation and quantification of resident alveolar macrophages and alveolar recruited leukocytes was performed as described recently (16, 20) and is outlined in the online supplement.
Isolation of Bone Marrow Cells and Transplantation into Recipient Animals
Bone marrow cells were isolated under sterile conditions from the tibias and femurs of sex-matched, syngeneic PI3K KO mice, and transplanted into lethally irradiated wild-type mice following recently described protocols (16, 21) (see the online supplement).
Preparation of Mouse Bone Marrow–derived Monocytes and In Vitro Chemotaxis
Preparation of mouse bone marrow–derived monocytes (BMDMs) and in vitro chemotaxis assays with murine BMDMs was performed as recently described (5) and is further outlined in the online supplement.
Culture and Quantification of S. pneumoniae and Infection in PI3K KO and Wild-Type Mice
For infection experiments, we used the PLY-producing clinical isolate of S. pneumoniae capsular group 19 strain EF3030 (
4 x 107 cfu/mouse), and for survival experiments, the capsular group 2 strain D39 ( 2 x 103 cfu/mouse), both of which were grown in Todd-Hewitt broth (Difco, Heidelberg, Germany) supplemented with 0.1% yeast extract to midlog phase (22), as outlined in the online supplement in detail.
Lung Histopathology and Lung Permeability Measurements
Lung histopathology and lung permeability were analyzed as described in the online supplement.
Pharmacologic Blockade of PI3K in S. pneumoniae–infected Wild-Type Mice
We evaluated the effect of pharmacologic blockade of PI3K on the pneumococcal clearance capacity of wild-type mice pretreated with the PI3K-specific inhibitor AS-605240. Briefly, AS-605240 compound was dissolved in vehicle (0.5% carboxymethylcellulose/0.25% Tween-20), as recently described (8), and injected intraperitoneally in wild-type mice 2 hours before and at 12-hour intervals subsequent to infection in the mice with S. pneumoniae (EF3030, 2 x 107 cfu/mouse) at a dose of 30 mg/kg body weight.
Determination of Bacterial Loads in the Lungs of PI3K KO and Wild-Type Mice
Bacterial loads in the lungs of S. pneumoniae–infected PI3K KO and wild-type mice were calculated from whole lung washes, as described recently (22), and are outlined in the online supplement.
Statistics
All data are given as mean ± SD. Differences between control animals and respective treatment groups over time were analyzed by analysis of variance followed by post hoc Dunnett test. Significant differences between groups were analyzed by Levene's test for equality of variances followed by Student's t test using SPSS for Windows software package (Version 14.0; SPSS, Inc., Chicago, IL). Survival curves were compared by log-rank test. Statistically significant differences among various treatment groups were assumed when p values were less than 0.05.
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RESULTS |
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TOPABSTRACTAT A GLANCE COMMENTARYMETHODSRESULTSDISCUSSIONREFERENCES |
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Lung Inflammatory Response of Wild-Type and PI3K
KO Mice Challenged with PLYIntratracheal application of PLY induced a rapid, progressive increase in lung permeability in both wild-type and PI3K
KO mice, peaking at 6 hours post-treatment and declining thereafter (Figure 1A). In line with this finding, histopathologic examination of lung tissues collected from PLY-challenged PI3K KO and wild-type mice at 6 and 24 hours post-treatment showed focally accentuated intraalveolar edema in mice of both treatment groups (data not shown). In addition, intratracheal administration of PLY into the lungs of wild-type and PI3K KO mice elicited the alveolar accumulation of neutrophils, with significantly increased numbers noted at 12 hours up until 48 hours post-treatment. However, no significant differences were observed between groups, demonstrating that neutrophils do not require PI3K activity to accumulate within the alveolar airspace of PLY-challenged mice (Figure 1B). Importantly, instillation of PLY into the lungs of wild-type mice and PI3K KO mice induced a strong depletion of resident alveolar macrophages, with a maximal drop observed between 3 and 12 hours post-treatment (Figure 1C). Of note, compensatory alveolar exudate macrophage recruitment to restore alveolar macrophage homeostasis was primarily observed in wild-type mice in response to PLY, and was significantly impaired in PI3K KO mice (Figure 1D). Consequently, restoration of the alveolar macrophage pool after PLY challenge was achieved earlier in wild-type mice as compared with PI3K KO mice (Figures 1C and 1D), and a net expansion of the alveolar macrophage pool was significantly higher than that observed in PI3K KO mice at later time points (120 h post-treatment) (Figure 1C). Of note, control experiments revealed that instillation of vehicle (sterile phosphate-buffered saline/0.1% human serum albumin) into the lungs of mice did not elicit the alveolar accumulation of neutrophils or exudate macrophages into the bronchoalveolar space (24–72 h), thus indicating that neither vehicle per se nor the surgical procedure was responsible for the observed effects (data not shown in detail).
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Significant (p < 0.01) increase/decrease compared with the respective 0 h control values; Single, double stacked, and triple stacked asterisks indicate significant difference between wild-type and PI3KTo elucidate the role of PI3K
activity in circulating monocytes as opposed to sessile lung cells on alveolar exudate macrophage recruitment, chimeric wild-type mice, generated as described in the online supplement, were challenged with PLY (16, 21). Notably, chimeric wild-type mice lacking PI3K activity in circulating leukocytes but not resident lung cells did not recruit exudate macrophages into the alveolar airspace after challenge with intratracheal PLY (Figure 1E), whereas alveolar neutrophil recruitment was not impaired in these mice (data not shown). In contrast, wild-type mice with a fully intact PI3K system demonstrated a well-coordinated influx of exudate macrophages into the alveolar compartment in response to PLY (Figure 1E). These data demonstrate that inflammatory macrophage recruitment into the alveolar compartment of lungs challenged with PLY alone is critically dependent on PI3K activity in the circulating monocytes.
Effect of PI3K Deletion on Alveolar Exudate Macrophage Recruitment in Response to Intratracheal CCL2
Because PI3K acts downstream of G protein–coupled chemokine receptors, we hypothesized that deletion of PI3K would render circulating monocytes unresponsive to CCL2, the primary monocyte chemoattractant binding to and signaling via CCR2. Unlike wild-type mice, PI3K KO mice failed to elicit monocyte/macrophage recruitment into their lungs upon intratracheal application of CCL2. These data conclusively demonstrate that PI3K is required for CCL2-dependent inflammatory mononuclear phagocyte trafficking in vivo (Figure 2A) (16, 23).
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To further delineate the role of PI3K
in CCL2-dependent monocyte trafficking in vitro, BMDMs from wild-type mice or PI3K KO mice were allowed to transmigrate toward optimal concentrations of recombinant CCL2 in the presence of either LY294002, a pan-PI3K inhibitor exerting most of its activity on PI3K with no selectivity for any class I PI3K, or a PI3K-specific inhibitor, AS-605240. As shown in Figure 2B, BMDMs exhibited a strong chemotactic response to CCL2 that was only weakly blocked by pretreatment with LY294002, whereas AS-605240 pretreated BMDMs (10 µM) showed markedly reduced chemotaxis to CCL2, and similar results were obtained with correspondingly pretreated, purified peripheral blood monocytes (data not shown). Collectively, these findings underline the importance of PI3K activity in CCL2-dependent monocyte chemotaxis in vitro and in vivo.
Effect of PI3K Deficiency on the Lung Inflammatory Response to S. pneumoniae Infection
Histopathologic examination of lung sections from S. pneumoniae–infected PI3K KO and wild-type mice demonstrated a similar degree of developing neutrophilic alveolitis by Day 2 postinfection (Figure 3A). Up until Day 4 of infection, PI3K KO mice developed progressive histopathologic manifestations similar to wild-type mice, as reflected by a strong neutrophilic alveolitis and alveolar fibrinous exudate formation within the lungs, with overall inflamed lung tissue ranging between 40 and 50%. However, marked differences were observed at Day 7 postinfection, at which time virtually all PI3K KO mice demonstrated severe pneumococcal pneumonia affecting whole lung lobes (50–70% inflamed lung tissue), characterized by established purulent pleuritis, fibrinous exudate formation, and neutrophilic necrosis (see arrows in Figure 3A, left). At this time point, most of the wild-type mice demonstrated resolving pulmonary inflammation with only occasional, focally restricted fibrinous exudates and strongly increased numbers of alveoli-accumulating mononuclear phagocytes, with an overall inflamed lung tissue ranging between 5 and 10% (see arrows in Figure 3A, right). Notably, severe pneumococcal pneumonia developing in the lungs of PI3K KO mice was also evident from the macroscopic appearance of infected lungs as compared with wild-type control lungs (Figure 3B). Moreover, PI3K KO mice demonstrated an approximately 20% loss of body weight at Day 2 of pneumococcal infection compared with wild-type mice, which only showed a body weight loss of approximately 10% (Figure 3C); wild-type mice largely regained their body weight during the 7-day observation period, whereas S. pneumoniae–infected PI3K KO mice were unable to recover from loss of body weight within this period, again reflecting the severe disease progression in these mice.
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Effect of Deletion or Pharmacologic Inhibition of PI3K
on Lung Neutrophil and Mononuclear Phagocyte Recruitment and Bacterial Clearance Capacity in Pneumococcal PneumoniaWe next determined the lung pneumococcal clearance capacity in PI3K
KO and wild-type mice either in the absence or presence of the PI3K-specific inhibitor AS-605240. Both PI3K KO and wild-type mice pretreated with AS-605240 compound demonstrated a strongly reduced lung pneumococcal clearance capacity compared with wild-type mice, resulting in greater than 10-fold higher bacterial loads in AS-605240–pretreated, S. pneumoniae–infected mice as compared with control mice by 48 hours postinfection (Figures 4A and 4B). Importantly, this reduced pneumococcal clearance was not attributable to reduced alveolar neutrophil recruitment either in null or PI3K inhibitor–pretreated wild-type mice (Figures 4C and 4D).
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Effect of PI3K
Deficiency on Alveolar Exudate Macrophage Recruitment and Bacterial Clearance Capacity in Pneumococcal PneumoniaWe also determined the role of PI3K
in alveolar exudate macrophage recruitment in response to S. pneumoniae infection. Exudate macrophages were distinguished from resident alveolar macrophages using their differential 2 integrin CD11b and CD11c cell surface antigen expression profile (Figure 5A). Gating of BAL fluid macrophages according to their forward scatter (FSC) versus side scatter (SSC) and FSC versus F4/80 antigen expression (Figure 5A, left and right dot plots) allowed us to identify resident alveolar macrophages from untreated and S. pneumoniae–infected (96 h) mice (P1), according to their CD11c+/CD11b– cell surface antigen expression (P1 in Figure 5A, left and right dot plots), whereas monocyte-derived exudate macrophages were found to be CD11c positive and strongly CD11b positive (P2 in Figure 5A, right dot plot), thereby allowing their flow cytometric differentiation from resident alveolar macrophages (P1 in Figure 5A, right dot plot). Both resident alveolar and exudate macrophages lacked major histocompatibility class II and CD86 antigen expression, thus confirming their macrophage phenotype (histograms P1 [left side] and P1 and P2 [right side] of Figure 5A).
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Both PI3K
KO mice and wild-type mice pretreated with the PI3K-specific inhibitor AS-605240 demonstrated significantly impaired alveolar macrophage exudate recruitment in response to pneumococcal infection (Figures 5B and 5C), which was accompanied by a strong depletion of resident alveolar macrophages at 12 hours up to 48 hours postinfection (Figure 5C), thus strongly supporting the concept that PI3K activity is an essential component of lung mononuclear phagocyte trafficking during pneumococcal pneumonia. To further assess the role of elicited exudate macrophages in the early onset of pneumococcal pathogen elimination, wild-type mice were infected with S. pneumoniae either in the absence or presence of the function-blocking anti-CCR2 antibody MC21. Interestingly, although MC21 application nearly completely inhibited the inflammatory exudate macrophage influx into the alveolar airspace of infected mice, bacterial loads determined in lung washes of infected wild-type mice did not show any significant changes compared with control mice (Figures 6A and 6B).
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Survival of PI3K
KO versus Wild-Type Mice Subsequent to S. pneumoniae InfectionFinally, we analyzed the effect of PI3K
deletion on the survival of mice infected with S. pneumoniae D39. Importantly, we found that wild-type mice demonstrated a significantly prolonged survival upon infection with 2 x 103 cfu D39 as compared with PI3K KO mice within the 7-day observation period (Figure 7).
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DISCUSSION |
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TOPABSTRACTAT A GLANCE COMMENTARYMETHODSRESULTSDISCUSSIONREFERENCES |
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Using both gene KO and pharmacologic approaches, we investigated the role of PI3K
in lung inflammation developing in response to both PLY (one of the most important pneumococcal virulence factors) and S. pneumoniae infection in mice. PI3K KO and wild-type mice pretreated with the PI3K-specific inhibitor AS-605240 responded with a neutrophilic alveolitis, but showed an impaired exudate macrophage recruitment in response to PLY and S. pneumoniae infection. Both null and AS-605240–inhibitor pretreated wild-type mice displayed a strongly reduced pneumococcal clearance and showed a reduced resolution/repair process, most probably due to defects in mounting an appropriate mononuclear phagocyte recruitment to S. pneumoniae challenge. Collectively, the data show that either deletion or blockade of PI3K impairs central innate host defense mechanisms of the lung upon challenge with the prototypic gram-positive bacterial pathogen S. pneumoniae.
Alveolar mononuclear phagocyte recruitment in response to intratracheal PLY challenge was recently found to critically depend on the CCL2–CCR2 axis (14), and for the first time, the data presented in the current study specify a role of PI3K in the PLY-triggered CCL2–CCR2 downstream signaling cascade evoked in monocytes in vivo. This is based on the findings that PI3K null exudate macrophages were unable to accumulate within the alveolar airspace of CCL2-treated mice, just as they were unable to accumulate within the alveolar airspace of chimeric wild-type mice challenged with PLY, which again highlights the particular importance of PI3K activity in circulating leukocytes (2, 8, 24) but not sessile endothelial cells (25) for the PLY-induced monocyte extravasation process in vivo. In addition, for the first time, we have shown that both BMDMs and highly purified primary blood monocytes did not migrate toward CCL2 chemokine gradients in the presence of the PI3K-specific inhibitor AS-605240 in vitro, whereas inhibition of P13K by LY294002 did not affect monocyte chemotaxis, thus further highlighting the particular importance of the isoform of PI3K in the CCL2-induced downstream signaling cascade.
PLY challenge of wild-type mice and PI3K KO mice induced a strong depletion of resident alveolar macrophages. This PLY effect on alveolar macrophages was recently found to be attributable to its pore-forming, cytotoxic activities, because its noncytotoxic derivative, PdB (PLY with a Trp433-Phe mutation), lacking the pore-forming activity of PLY, was not able to deplete macrophages in vivo (14, 18). PI3K KO as opposed to wild-type mice also demonstrated a significantly delayed repopulation of the depleted alveolar macrophage pool, and although our experiments with anti-CCR2 antibodies to block inflammatory monocyte recruitment did not support a direct contribution of recruited monocytes/macrophages to the early bacterial clearance, such a perturbation of alveolar macrophage homeostasis may at least indirectly promote the bacterial spread within distal airspaces by attenuating resolution/repair processes after pneumococcal infection. In line with this, previous publications have shown that liposomal clodronate–mediated alveolar macrophage depletion significantly affected survival rates in various bacterial lung infection models in mice (26–28). Very similar to the effects seen with the purified pathogen-associated molecular pattern PLY, infection of the mice with PLY-producing S. pneumoniae also caused a transient depletion of alveolar macrophages in both PI3K KO and wild-type mice pretreated with AS-605240. Macrophages from PI3K KO or inhibitor-pretreated wild-type mice appear not to be more sensitive to depletion by S. pneumoniae than wild-type macrophages, because S. pneumoniae–infected wild-type mice pretreated with the anti-CCR2 antibody MC 21 also responded with a strongly depleted alveolar macrophage pool upon pneumococcal challenge (unpublished observations). Interestingly, this transient drop in alveolar macrophages noted in the current study had no effect on the neutrophilic alveolitis that developed in the two experimental groups. Despite this, both null and inhibitor-pretreated wild-type mice had a nearly 10-fold lower pneumococcal clearance in their lungs compared with control animals, most probably due to killing defects in the recruited PI3K-null neutrophils (2), which is supported by our recent observations in which PI3K-deficient neutrophils were found to be defective in respiratory burst and reactive oxygen species production triggered by bacterial peptides in vitro (2). Such defective respiratory burst machinery also very likely underlies the defective pneumococcal killing observed in the current study in vivo.
In contrast to the strongly reduced exudate macrophage recruitment, deletion or pharmacologic blockade of PI3K did not affect the development of neutrophilic alveolitis, both in PLY-challenged or S. pneumoniae–infected mice. Another study also demonstrated lack of suppressed lung tissue sequestration of PI3K-deficient neutrophils compared with wild-type neutrophils upon peritoneal E. coli sepsis, possibly due to increased CD47 (integrin-associated protein) and 3 integrin association of PI3K-null neutrophils with extracellular vitronectin (29). On the other hand, lack of PI3K activity was found to impair neutrophil trafficking to LPS-inflamed mouse lungs (25). Thus, the different inflammatory stimuli and experimental models used (acute vs. chronic; E. coli vs. S. pneumoniae) may well explain the differential contribution of PI3K in inflammatory neutrophil trafficking in vivo. In addition, it is conceivable that alternative PI3K signaling pathways such as that mediated by PI3K may explain the observed lack of effect of PI3K inhibition on the neutrophil recruitment process to S. pneumoniae–induced lung infection.
Histopathologic examinations of lung tissue sections from S. pneumoniae–infected PI3K KO mice showed a substantial degree of lung neutrophil necrosis within distal airspaces. Resident lung mononuclear phagocytes are central to the resolution/repair phase in bacterial lung infection, including the elimination of necrotic material from distal airspaces (28, 30). Perturbations of the mononuclear phagocyte system observed in the current study in S. pneumoniae–infected PI3K KO mice and wild-type mice pretreated with AS-605240 most probably contributed to the inefficient resolution/repair phase subsequent to pneumococcal infection, ultimately leading to progressive pneumonia. This concept may be supported by two observations. First, in a recent study, liposomal clodronate application to deplete alveolar macrophages was reported to elicit an aggravated lung inflammatory response to S. pneumoniae infection, partially due to the reduced resolution/repair mechanisms in macrophage-depleted S. pneumoniae–infected lungs (28, 31). Second, ongoing experiments in S. pneumoniae–infected wild-type mice pretreated with anti-CCR2 antibody MC21 for 7 days revealed a strongly reduced inflammatory mononuclear phagocyte mobilization and a heavily attenuated resolution/repair phase, ultimately resulting in progressive lobar pneumonia (32). In this context, experiments with PI3K KO mice infected with the serotype 2 pneumococcal strain D39, which is known to progress into septic pneumococcal disease, illustrates the possible consequences for the infected host.
Targeting PI3K signaling is a promising approach in the treatment of chronic inflammatory diseases. However, in view of a clinical application of small-molecule PI3K inhibitors, target validation will be an important future aspect to discriminate between specific effects of the drug and potential side effects (33). Although intracellular signaling pathways are considered to be redundant, the current report shows that alveolar macrophage homeostasis subsequent to pneumococcal challenge is significantly reduced in both null and inhibitor-pretreated mice, supporting the view that PI3K signaling represents a central component in inflammatory mononuclear phagocyte mobilization after pneumococcal infection.
In summary, in view of a clinical application of PI3K inhibitors for the treatment of chronically ill patients, the current data point to potentially evolving side effects of PI3K inhibition that may render the lung innate host defense system less effective to adequately respond to lower respiratory tract infections.
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FOOTNOTES |
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Supported by grants SFB 587 and SFB 547, German Research Council.
This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org
Originally Published in Press as DOI: 10.1164/rccm.200610-1533OC on February 22, 2007
Conflict of Interest Statement: U.A.M. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. M.B. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. C.W. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. M.S. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. M.K.S. is employed by Merck Serono. T.R. is employed by Merck Serono. J.C.P. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. D.B. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. M.M. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. T.W. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. R.M. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. R.M.B. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. W.S. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. C.R. is employed by Merck Serono. E.H. has been reimbursed as a consultant to Serono. J.L. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. K.T.P. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript.
Received in original form October 24, 2006; accepted in final form February 20, 2007
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