LPS-binding protein-deficient mice have an impaired defense against Gram-negative but not Gram-positive pneumonia

LPS-binding protein (LBP) can facilitate the transfer of cell wall components of both Gram-negative bacteria (LPS) and Gram-positive bacteria (lipoteichoic acid) to inflammatory cells. Although LBP is predominantly produced in the liver, recent studies have indicated that this protein is also synthesized locally in the lung by epithelial cells. To determine the role of LBP in the immune response to pneumonia, LBP gene-deficient (-/-) and normal wild-type (WT) mice were intra-nasally infected with either Streptococcus pneumoniae or Klebsiella pneumoniae, common Gram-positive and Gram-negative pathogens, respectively. Pneumococcal pneumonia was associated with a 7-fold rise in LBP concentrations in bronchoalveolar lavage fluid of WT mice; LBP-/- mice infected with S. pneumoniae showed a similar survival and a similar bacterial burden in their lungs 48 h post-infection. In Klebsiella pneumonia, however, LBP-/- mice demonstrated a diminished survival together with an enhanced bacterial outgrowth in their lungs. These data suggest that LBP is important for a protective immune response in Klebsiella pneumonia, but does not contribute to an effective host response in pneumococcal pneumonia.     

Central role of toll-like receptor 4 signaling and host defense in experimental pneumonia caused by Gram-negative bacteria

Toll-like receptor 4 (TLR4) has been identified as a receptor for lipopolysaccharide. However, the precise role of TLR4 in regulating gene expression in response to an infection caused by gram-negative bacteria has not been fully elucidated. The role of TLR4 signaling in coordinating gene expression was assessed by gene expression profiling in lung tissue in a mouse model of experimental pneumonia with a low-dose infection of Klebsiella pneumoniae. We analyzed four mouse strains: C57BL/6 mice, which are resistant to bacterial dissemination; 129/SvJ mice, which are susceptible; C3H/HeJ mice, which are susceptible and have defective TLR4 signaling; and their respective control strain, C3H/HeN (intermediate resistance). At 4 h after infection, C57BL/6 and C3H/HeN mice demonstrated the greatest number of genes, with 67 shared induced genes which were TLR4 dependent and highly associated with the resistance phenotype. These genes included cytokine and chemokine genes required for neutrophil activation or recruitment, growth factor receptors, MyD88 (a critical adaptor protein for TLR signaling), and adhesion molecules. TLR4 signaling accounted for over 74% of the gene expression in the C3H background. These data suggest that early TLR4 signaling controls the vast majority of gene expression in the lung in response to an infection caused by gram-negative bacteria and that this subsequent gene expression determines survival of the host.     

Interleukin 18 participates in the early inflammatory response and bacterial clearance during pneumonia caused by nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) is a common gram-negative respiratory pathogen. To determine the role of the proinflammatory cytokine interleukin 18 (IL-18) during NTHi pneumonia, normal wild-type (WT) and IL-18 knockout (KO) mice were intranasally infected with NTHi. IL-18 KO mice displayed a delayed clearance of NTHi from the respiratory tract, resulting in >20-fold higher bacterial loads in their lungs at 24 h after infection, preceded by a strongly attenuated pulmonary innate immune response as determined by cytokine and chemokine induction and histopathology. These data identify IL-18 as part of an adequate innate immune response during NTHi pneumonia.     

Immunostimulation with macrophage-activating lipopeptide-2 increased survival in murine pneumonia

Community-acquired pneumonia (CAP) is associated with high morbidity and mortality, and Streptococcus pneumoniae is the most prevalent causal pathogen identified in CAP. Impaired pulmonary host defense increases susceptibility to pneumococcal pneumonia. S. pneumoniae may up-regulate Toll-like receptor (TLR)-2 expression and activate TLR-2, contributing to pneumococcus-induced immune responses. In the current study, the course of severe murine pneumococcal pneumonia after pulmonary TLR-2-mediated immunostimulation with synthetic macrophage-activating lipopeptide-2 (MALP-2) was examined. Intratracheal MALP-2 application evoked enhanced proinflammatory cytokine and chemokine release, resulting in recruitment of polymorphonuclear neutrophils (PMN), macrophages, and lymphocytes into the alveolar space in WT, but not in TLR-2-deficient mice. In murine lungs as well as in human alveolar epithelial cells (A549), MALP-2 increased TLR-2 expression at both mRNA and protein level. Blood leukocyte numbers and populations remained unchanged. MALP-2 application 24 hours before intranasal pneumococcal infection resulted in increased levels of CCL5 associated with augmented leukocyte recruitment, and decreased levels of anti-inflammatory IL-10 in bronchoalveolar lavage fluid. Clinically, MALP-2-treated as compared with untreated mice showed increased survival, reduced hypothermia, and increased body weight. MALP-2 also reduced bacteremia and improved bacterial clearance in lung parenchyma, as examined by immunohistochemistry. In conclusion, pulmonary immunostimulation with MALP-2 before infection with S. pneumoniae improved local host defense and increased survival in murine pneumococcal pneumonia.     

Nitric oxide exerts distinct effects in local and systemic infections with Streptococcus pneumoniae

Nitric oxide (NO) is known to be involved in the immune response against a range of organisms. Little is known about the effects of nitric oxide in pneumococcal infections. We have now investigated the role of nitric oxide in local and systemic infections caused by Streptococcus pneumoniae in NOS2 deficient mice. Although a deficiency in NO does not affect survival of mice during pneumococcal pneumonia, NO does control pneumococcal viability within the lung airways and tissue. Bronchoalveolar lavage fluid (BALF) from NOS2-deficient mice contained significantly elevated TNF activity, IFNgamma and total protein during mid/late infection. Incubation of S. pneumoniae with the NO donor SNAP revealed a direct anti-pneumococcal effect for NO in vitro. Deficiency in NOS2 did not affect bacteraemia following intranasal infection. In contrast NOS2-deficient mice were significantly less susceptible to intravenous infection with S. pneumoniae than were wild type mice and were able to control pneumococcal viability within the bloodstream. Our results indicate that NO is required within the lungs for anti-bacterial activity during the pneumococcal pneumonia but during Gram-positive bacteraemia NO is associated with increased bacterial loads and reduced survival.     

Leptin improves pulmonary bacterial clearance and survival in ob/ob mice during pneumococcal pneumonia

The adipocyte-derived hormone leptin is an important regulator of appetite and energy expenditure and is now appreciated for its ability to control innate and adaptive immune responses. We have reported previously that the leptin-deficient ob/ob mouse exhibited increased susceptibility to the Gram-negative bacterium Klebsiella pneumoniae. In this report we assessed the impact of chronic leptin deficiency, using ob/ob mice, on pneumococcal pneumonia and examined whether restoring circulating leptin to physiological levels in vivo could improve host defences against this pathogen. We observed that ob/ob mice, compared with wild-type (WT) animals, exhibited enhanced lethality and reduced pulmonary bacterial clearance following Streptococcus pneumoniae challenge. These impairments in host defence in ob/ob mice were associated with elevated levels of lung tumour necrosis factor (TNF)-alpha, macrophage inflammatory peptide (MIP)-2 [correction added after online publication 28 September 2007: definition of MIP corrected], prostaglandin E(2) (PGE(2)), lung neutrophil polymorphonuclear leukocyte (PMN) counts, defective alveolar macrophage (AM) phagocytosis and PMN killing of S. pneumoniae in vitro. Exogenous leptin administration to ob/ob mice in vivo improved survival and greatly improved pulmonary bacterial clearance, reduced bacteraemia, reconstituted AM phagocytosis and PMN H(2)O(2) production and killing of S. pneumoniae in vitro. Our results demonstrate, for the first time, that leptin improves pulmonary bacterial clearance and survival in ob/ob mice during pneumococcal pneumonia. Further investigations are warranted to determine whether there is a potential therapeutic role for this adipokine in immunocompromised patients.     

Toll-like receptor 4 (TLR4) does not confer a resistance advantage on mice against low-dose aerosol infection with virulent type A Francisella tularensis

Francisella tularensis, the causative agent of tularemia, is a gram-negative facultative intracellular bacterium. Toll-like receptor (TLR) 4 is considered to be critical for inducing host innate immunity against many gram-negative bacteria including many respiratory pathogens. To determine the role of TLR4 in host defense against airborne F. tularensis infection, TLR4-defective C3H/HeJ (TLR4(d)) or wild-type C3H/HeOuJ (WT) mice were challenged by low-dose aerosol with type A F. tularensis, and the course of the infection and host responses were compared at day 2 and 4 post-inoculation (dpi). At dpi 2, bacterial burdens in the lungs were similar between TLR4(d) and WT mice, but TLR4(d) mice surprisingly harbored approximately 10-fold fewer bacteria in their spleens and livers. However, the bacterial burdens at dpi 4, the mortality and median time to irreversible moribundity were indistinguishable between the two mouse strains. In addition, the inflammatory responses to the infection, as reflected by the cytokine levels and leukocyte influx in the bronchoalveolar lavage fluid and histopathological analysis, were similar between both mouse strains. Additionally, as with C3H mice, we found no difference in either the median time to death or the survival rate between TLR4-deleted C57BL/10ScNJ mice and WT C57BL/10 mice. Combined, these data suggest that TLR4 does not contribute to resistance of mice to airborne type A F. tularensis infection.     

Modulation of the inflammatory response to Streptococcus pneumoniae in a model of acute lung tissue infection

Streptococcus pneumoniae is the leading pathogen of community-acquired pneumonia and is a main cause of infectious deaths. However, little is known about host-pathogen interaction in human lung tissue. We tested the hypothesis that human alveolar macrophages (AMs) and alveolar epithelial cells (AECs) are important for initiating the host response against S. pneumoniae, and we evaluated the role of Toll-like receptor (TLR) 2, TLR4, and p38 mitogen-activated protein kinase (MAPK) signaling in the inflammatory response after pneumococcal infection. We established a novel model of acute S. pneumoniae infection using vital human lung specimens. In situ hybridization analysis showed that S. pneumoniae DNA was detected in 80 to 90% of AMs and 15 to 30% of AECs after in vitro infection accompanied by increased expression of inflammatory cytokines. Enhanced phosphorylation of p38 MAPK and increased TLR2 and 4 mRNA expression were observed in infected lung tissue. Thirty to fifty percent of AMs and 10 to 20% of AECs showed evidence of apoptosis 24 hours after pneumococcal infection. After macrophage deactivation with Clodronate/liposomes, infected lung tissue exhibited a significantly decreased release of inflammatory mediators. Inhibition of p38 MAPK signaling markedly reduced inflammatory cytokine release from human lungs, whereas TLR2 blockade revealed only minor effects. AMs are central resident immune cells during S. pneumoniae infection and are the main source of early proinflammatory cytokine release. p38 MAPK holds a major role in pathogen-induced pulmonary cytokine release and is a potential molecular target to modulate overwhelming lung inflammation.     

The role of CD44 in the acute and resolution phase of the host response during pneumococcal pneumonia

Streptococcus pneumoniae is the most prevalent pathogen causing community-acquired pneumonia. CD44 is a transmembrane adhesion molecule, expressed by a wide variety of cell types, that has several functions in innate and adaptive immune responses. In this study, we tested the hypothesis that CD44 is involved in the host response during pneumococcal pneumonia. On intranasal infection with a lethal dose of S. pneumoniae CD44-knockout (KO) mice showed a prolonged survival when compared with wild-type mice, which was accompanied by a diminished pulmonary bacterial growth and reduced dissemination to distant body sites. Whereas, proinflammatory cytokine responses and lung pathology were not affected, CD44 deficiency resulted in increased early neutrophil influx into the lung. In separate experiments, we confirmed a detrimental role of CD44 in host defense against pneumococci during sublethal pneumonia, as demonstrated by an improved capacity of CD44 KO mice to clear a low infectious dose. In addition, CD44 appeared important for the resolution of lung inflammation during sublethal pneumonia, as shown by histopathology of lung tissue slides. In conclusion, we show here that CD44 facilitates bacterial outgrowth and dissemination during pneumococcal pneumonia, which in lethal infection results in a prolonged survival of CD44 KO mice. Moreover, during sublethal pneumonia CD44 contributes to the resolution of the inflammatory response.     

Tumor necrosis factor mediates lung antibacterial host defense in murine Klebsiella pneumonia.

Tumor necrosis factor (TNF) is a proinflammatory cytokine which has recently been shown to have beneficial effects in the setting of acquired host immunity. However, the role of TNF in innate immune responses, as in the setting of bacterial pneumonia, has been incompletely characterized. To determine the role of TNF in gram-negative bacterial pneumonia, CBA/J mice were challenged with 10(2) CFU of Klebsiella pneumoniae intratracheally, resulting in the time-dependent expression of TNF MRNA and protein within the lung. Passive immunization of animals with a soluble TNF receptor-immunoglobulin (Ig) construct (sTNFR:Fc) intraperitoneally 2 h prior to K. pneumoniae inoculation resulted in a significant reduction in bronchoalveolar lavage neutrophils, but not macrophages, at 48 h, as compared with animals receiving control IgG1. Furthermore, treatment with sTNFR:Fc resulted in 19.6- and 13.5-fold increases in K. pneumoniae CFU in lung homogenates and plasma, respectively, as compared with animals receiving control IgG1. Finally, treatment of Klebsiella-infected mice with sTNFR:Fc markedly decreased both short- and long-term survival of these animals. In conclusion, our studies indicate that endogenous TNF is a critical component of antibacterial host defense in murine Klebsiella pneumonia.     

Influence of neutropenia on the course of serotype 8 pneumococcal pneumonia in mice

Polymorphoneutrophils (PMNs) are important effector cells in host defense against pneumonia. However, PMNs can also induce inflammation and tissue damage. To investigate the contribution of PMNs to host defense against pneumococcal pneumonia, we determined the effect of the PMN-depleting rat monoclonal antibody RB6-8C5 (RB6) on survival and inflammatory and cellular response in the lungs to a lethal intranasal infection with a serotype 8 pneumococcus in BALB/c mice. Control mice received rat immunoglobulin G (rIgG). Strikingly, the survival of RB6-treated mice was significantly prolonged compared to that of rIgG-treated mice. Although the numbers of CFU in the lungs were statistically similar in both groups 4, 24, and 32 h after infection, rIgG-treated mice developed higher levels of bacteremia, and histopathological examination of the lungs of infected mice revealed marked differences between RB6- and rIgG-treated mice. RB6-treated mice had focal, perivascular lesions without accompanying parenchymal inflammation, and rIgG-treated mice had diffuse, interstitial parenchymal inflammation. Lung homogenates from the rIgG-treated mice had more leukocytes and significantly more total and apoptotic PMNs as determined by fluorescence-activated cell sorter analysis with Annexin V and terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling staining of lung tissue samples. Studies with a pneumolysin-deficient mutant of the serotype 8 strain we used also demonstrated the prolonged survival of RB6- compared to rIgG-treated mice. Taken together, our findings suggest that PMNs enhance the likelihood of early death and alter the pathological response to pneumococcal lung infection in BALB/c mice with serotype 8 pneumonia without significantly affecting bacterial clearance or the cytokine response.     

Prospective study on the etiology of community-acquired pneumonia in children and adults in Spain

The cause of primary pneumonia was diagnosed in 157 of 198 children and 165 of 207 adults seen as inpatients or outpatients in a 12-month period. In children Mycoplasma pneumoniae and pneumococcus were identified in 79 and 29 cases respectively. Twenty-nine of 53 cases of viral infection in children were caused by respiratory syncytial virus, two-thirds of the cases occurring in children under three years of age. No children died of pneumonia. In adults pneumococcus was the most common pathogen, accounting for 81 cases. The overall mortality in adults was 7.7%. A high mortality was found in patients with Haemophilus influenzae and other gram-negative bacilli infections, and in elderly patients with pneumococcal pneumonia. Coagglutination was more sensitive than counterimmunoelectrophoresis for the detection of pneumococcal antigen in respiratory samples (p less than 0.001). Counterimmunoelectrophoresis was the only useful technique for detection of pneumococcal antigen in urine specimens, concentration, overnight storage at 4 degrees C and specific staining significantly increasing positivity (p less than 0.001).     

Alveolar macrophages are required for protective pulmonary defenses in murine Klebsiella pneumonia: elimination of alveolar macrophages increases neutrophil recruitment but decreases bacterial clearance and survival.

To study the in vivo role of alveolar macrophages (AM) in gram-negative bacterial pneumonia in mice, AM were eliminated by the intratracheal (i.t.) administration of dichloromethylene diphosphonate encapsulated liposomes. Subsequently, the AM-depleted mice were infected i.t. with 100 CFU of Klebsiella pneumoniae, and the effects of AM depletion on survival, bacterial clearance, and neutrophil (polymorphonuclear leukocyte [PMN]) recruitment were assessed. It was shown that depletion of AM decreases survival dramatically, with 100% lethality at day 3 postinfection, versus 100% long-term survival in the control group. This increased mortality was accompanied by 20- to 27- and 3- to 10-fold increases in the number of K. pneumoniae CFU in lung and plasma, respectively, compared to those in nondepleted animals. This decreased bacterial clearance was not due to an impaired PMN recruitment; on the contrary, the K. pneumoniae-induced PMN recruitment in AM-depleted lungs was sevenfold greater 48 h postinfection than that in control infected lungs. Together with an increased PMN infiltration, 3- and 10-fold increases in lung homogenate tumor necrosis factor alpha (TNF-alpha) and macrophage inflammatory protein 2 (MIP-2) levels, respectively, were measured. Neutralization of TNF-alpha or MIP-2, 2 h before infection, reduced the numbers of infiltrating PMN by 41.6 and 64.2%, respectively, indicating that these cytokines mediate PMN influx in infected lungs, rather then just being produced by the recruited PMN themselves. Our studies demonstrate, for the first time, the relative importance of the AM in the containment and clearance of bacteria in the setting of Klebsiella pneumonia.     

Toll-like receptor 2 does not contribute to host response during postinfluenza pneumococcal pneumonia

Influenza A can be complicated by secondary bacterial pneumonia, which is most frequently caused by Streptococcus pneumoniae and associated with uncontrolled pulmonary inflammation. Evidence points to Toll-like receptor (TLR) 2 as a possible mediator of this exaggerated lung inflammation: (1) TLR2 is the most important "sensor" for gram-positive stimuli, (2) TLR2 contributes to S. pneumoniae-induced inflammation, and (3) influenza A enhances TLR2 expression in various cell types. Therefore, the objective of this study was to determine the role of TLR2 in the host response to postinfluenza pneumococcal pneumonia. TLR2 knockout (KO) and wild-type (WT) mice were infected intranasally with influenza A virus. Fourteen days later they were administered with S. pneumoniae intranasally. Influenza was associated with a similar transient weight loss in TLR2 KO and WT mice. Both mouse strains were fully recovered and had completely cleared the virus at Day 14. Importantly, no differences between TLR2 KO and WT mice were detected during postinfluenza pneumococcal pneumonia with respect to bacterial growth, lung inflammation, or cytokine/chemokine concentrations, with the exception of lower pulmonary levels of cytokine-induced neutrophil chemoattractant in TLR2 KO mice. Toll-like receptor 2 does not contribute to host defense during murine postinfluenza pneumococcal pneumonia.     

Pattern recognition receptors TLR4 and CD14 mediate response to respiratory syncytial virus

The innate immune system contributes to the earliest phase of the host defense against foreign organisms and has both soluble and cellular pattern recognition receptors for microbial products. Two important members of this receptor group, CD14 and the Toll-like receptor (TLR) pattern recognition receptors, are essential for the innate immune response to components of Gram-negative and Gram-positive bacteria, mycobacteria, spirochetes and yeast. We now find that these receptors function in an antiviral response as well. The innate immune response to the fusion protein of an important respiratory pathogen of humans, respiratory syncytial virus (RSV), was mediated by TLR4 and CD14. RSV persisted longer in the lungs of infected TLR4-deficient mice compared to normal mice. Thus, a common receptor activation pathway can initiate innate immune responses to both bacterial and viral pathogens.     

MyD88-dependent responses involving toll-like receptor 2 are important for protection and clearance of Legionella pneumophila in a mouse model of Legionnaires' disease

Legionella pneumophila is a gram-negative facultative intracellular parasite of macrophages. Although L. pneumophila is the causative agent of a severe pneumonia known as Legionnaires' disease, it is likely that most infections caused by this organism are cleared by the host innate immune system. It is predicted that host pattern recognition proteins belonging to the Toll-like receptor (TLR) family are involved in the protective innate immune responses. We examined the role of TLR-mediated responses in L. pneumophila detection and clearance using genetically altered mouse hosts in which the macrophages are permissive for L. pneumophila intracellular replication. Our data demonstrate that cytokine production by bone marrow-derived macrophages (BMMs) in response to L. pneumophila infection requires the TLR adapter protein MyD88 and is reduced in the absence of TLR2 but not in the absence of TLR4. Bacterial growth ex vivo in BMMs from MyD88-deficient mice was not enhanced compared to bacterial growth ex vivo in BMMs from heterozygous littermate controls. Wild-type mice were able to clear L. pneumophila from the lung, whereas respiratory infection of MyD88-deficient mice caused death that resulted from robust bacterial replication and dissemination. In contrast to an infection with virulent L. pneumophila, MyD88-deficient mice were able to clear infections with L. pneumophila dotA mutants, indicating that MyD88-independent responses in the lung are sufficient to clear bacteria that are unable to replicate intracellularly. In vivo growth of L. pneumophila was enhanced in the lungs of TLR2-deficient mice, which resulted in a delay in bacterial clearance. No significant differences were observed in the growth and clearance of L. pneumophila in the lungs of TLR4-deficient mice and heterozygous littermate control mice. Our data indicate that MyD88 is crucial for eliciting a protective innate immune response against virulent L. pneumophila and that TLR2 is one of the pattern recognition receptors involved in initiating this MyD88-dependent response.     

Beta2-microglobulin-dependent bacterial clearance and survival during murine Klebsiella pneumoniae bacteremia

Klebsiella pneumoniae is a leading cause of both community-acquired and nosocomial gram-negative bacterial pneumonia. A significant clinical complication of Klebsiella pulmonary infections is peripheral blood dissemination, resulting in a systemic infection concurrent with the localized pulmonary infection. We report here on the critical importance of beta(2)-microglobulin expression during murine K. pneumoniae bacteremia. Beta(2)-microglobulin knockout mice displayed significantly increased mortality upon intravenous inoculation that correlated with increased bacterial burden in the blood, liver, and spleen. As beta(2)-microglobulin knockout mice lack both CD8(+) T cells and invariant NK T cells, mouse models specifically deficient in either cell population were examined to see if this would account for the increased mortality noted in beta(2)-microglobulin knockout mice. Surprisingly, neither CD8 T-cell-deficient (TAP-1 knockout; in vivo anti-CD8 antibody treatment) nor invariant NK (iNK) T-cell-deficient (CD1d knockout, J alpha281 knockout) mice were more susceptible to K. pneumoniae bacteremia. Combined, these studies clearly indicate the importance of a beta(2)-microglobulin-dependent but CD8 T-cell- and iNK T-cell-independent mechanism critical for survival during K. pneumoniae bacteremia.     

青壮年与中老年社区获得性肺炎病原体的对比分析

目的 通过研究对青壮年及中老年人社区获得性肺炎（CAP）的常见呼吸道病原体分布,为CAP的合理用药治疗提供参考依据。方法 收集2017年6月~2018年6月于我院住院治疗的青壮年及中老年CAP各120例,通过痰细菌学培养、痰液病原体核酸检测、血清呼吸道病原体IgM抗体检测等方法检测病原体。结果 120例青壮年CAP中,通过以上方式检测到病原体感染有52例,病原体最多的为肺炎支原体（31.67%）,其次为流感嗜血杆菌（7.50%）、肺炎链球菌（3.33%）;中老年CAP中,检测到的病原体感染患者有62例,最多的病原体为肺炎支原体（17.50%）,其次为肺炎克雷伯菌（13.33%）、流感嗜血杆菌（12.50%）。非典型病原体感染在青壮年CAP中比例多于中老年人,而细菌感染在中老年人CAP中多于青壮年（P＜0.05）。结论 非典型病原体、细菌感染均是青壮年、中老年人CAP的常见病原体,在青少年CAP中以非典型病原体感染为主,在中老年CAP中以细菌感染为主。     

Innate immune defense against pneumococcal pneumonia requires pulmonary complement component C3

Complement is known to be involved in protection against systemic infection with Streptococcus pneumoniae. However, less is known about effects of complement within the lungs during pneumococcal pneumonia. By intranasally infecting transgenic mice unable to express complement C3, we investigated the role of complement in pulmonary defenses against S. pneumoniae. It was demonstrated that within the lungs, there is a requirement for C3 during the initial hours of infection. It was found that within 1 h of infection, bacterial loads decreased within lung airways of control mice as C3 protein increased. The lack of C3 resulted in the inability to control growth of wild-type or attenuated pneumococci within the lungs and bloodstream, resulting in an overwhelming inflammatory response and shorter survival times. Our results show that during the initial hours of infection with S. pneumoniae, C3 is protective within the lungs and subsequently plays an important role systemically.