Interleukin‐10 plays a key role in the modulation of neutrophils recruitment and lung inflammation during infection by Streptococcus pneumoniae

Streptococcus pneumoniae is a major aetiological agent of pneumonia worldwide, as well as otitis media, sinusitis, meningitis and sepsis. Recent reports have suggested that inflammation of lungs due to S. pneumoniae infection promotes bacterial dissemination and severe disease. However, the contribution of anti-inflammatory molecules to the pathogenesis of S. pneumoniae remains unknown. To elucidate whether the production of the anti-inflammatory cytokine interleukin-10 (IL-10) is beneficial or detrimental for the host during pneumococcal pneumonia, we performed S. pneumoniae infections in mice lacking IL-10 (IL-10^−/− mice). The IL-10^−/− mice showed increased mortality, higher expression of pro-inflammatory cytokines, and an exacerbated recruitment of neutrophils into the lungs after S. pneumoniae infection. However, IL-10^−/− mice showed significantly lower bacterial loads in lungs, spleen, brain and blood, when compared with mice that produced this cytokine. Our results support the notion that production of IL-10 during S. pneumoniae infection modulates the expression of pro-inflammatory cytokines and the infiltration of neutrophils into the lungs. This feature of IL-10 is important to avoid excessive inflammation of tissues and to improve host survival, even though bacterial dissemination is less efficient in the absence of this cytokine.     

Endoplasmic reticulum chaperone gp96 in macrophages is essential for protective immunity during Gram‐negative pneumonia

Klebsiella pneumoniae is among the most common Gram‐negative bacteria that cause pneumonia. Gp96 is an endoplasmic reticulum chaperone that is essential for the trafficking and function of Toll‐like receptors (TLRs) and integrins. To determine the role of gp96 in myeloid cells in host defence during Klebsiella pneumonia, mice homozygous for the conditional Hsp90b1 allele encoding gp96 were crossed with mice expressing Cre‐recombinase under control of the LysM promoter to generate LysMcre‐Hsp90b1‐flox mice. LysMcre‐Hsp90b1‐flox mice showed absence of gp96 protein in macrophages and partial depletion in monocytes and granulocytes. This was accompanied by almost complete absence of TLR2 and TLR4 on macrophages. Likewise, integrin subunits CD11b and CD18 were not detectable on macrophages, while being only slightly reduced on monocytes and granulocytes. Gp96‐deficient macrophages did not release pro‐inflammatory cytokines in response to Klebsiella and displayed reduced phagocytic capacity independent of CD18. LysMcre‐Hsp90b1‐flox mice were highly vulnerable to lower airway infection induced by K. pneumoniae, as reflected by enhanced bacterial growth and a higher mortality rate. The early inflammatory response in Hsp90b1‐flox mice was characterized by strongly impaired recruitment of granulocytes into the lungs, accompanied by attenuated production of pro‐inflammatory cytokines, while the inflammatory response during late‐stage pneumonia was not dependent on the presence of gp96. Blocking CD18 did not reproduce the impaired host defence of LysMcre‐Hsp90b1‐flox mice during Klebsiella pneumonia. These data indicate that macrophage gp96 is essential for protective immunity during Gram‐negative pneumonia by regulating TLR expression. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

Acute Lung Inflammation in <Emphasis Type="Italic">Klebsiella pneumoniae</Emphasis> B5055-Induced Pneumonia and Sepsis in BALB/c Mice: A Comparative Study

Lungs play an important role in the body's defense against a variety of pathogens, but this network of immune system-mediated defense can be deregulated during acute pulmonary infections. The present study compares acute lung inflammation occurring during Klebsiella pneumoniae B5055-induced pneumonia and sepsis in BALB/c mice. Pneumonia was induced by intranasal instillation of bacteria (10⁴ cfu), while sepsis was developed by placing the fibrin-thrombin clot containing known amount of bacteria (10² cfu) into the peritoneal cavity of animals. Mice with sepsis showed 100% mortality within five post-infection days, whereas all the animals with pneumonia survived. In animals suffering from K. pneumoniae B5055-induced pneumonia, all the inflammatory parameters (TNF-α, IL-1α, MPO, MDA, and NO) were found to be maximum till third post-infection day, after that, a decline was observed, whereas in septic animals, all the above-mentioned markers of inflammation kept on increasing. Histopathological study showed presence of alternatively activated alveolar macrophages (or foam cells) in lungs of mice with pneumonia after third post-infection day, which might have contributed to the induction of resolution of inflammation, but no such observation was made in lungs of septic mice. Hence, during pneumonia, controlled activation of macrophages may lead to resolution of inflammation.     

TREM‐2 Promotes Macrophage‐Mediated Eradication of Pseudomonas aeruginosa via a PI3K/Akt Pathway

Triggering receptor expressed on myeloid cells 2 (TREM‐2) is a cell surface receptor abundantly expressed on myeloid lineage cells such as macrophages and dendritic cells. It is reported that TREM‐2 functions as an inflammatory inhibitor in macrophages and dendritic cells. However, the role of TREM‐2 in bacterial killing remains unclear. This study explored the role of TREM‐2 in bacterial eradication of Pseudomonas aeruginosa (PA), a Gram‐negative bacterium which causes various opportunistic infections. Phagocytosis assay assessed by flow cytometry suggested that TREM‐2 was not involved in the uptake of PA by macrophages, while bacterial plate count data showed that TREM‐2 was required for macrophage‐mediated intracellular killing of PA. Moreover, our results demonstrated that TREM‐2 promoted macrophage killing by enhancing reactive oxygen species (ROS), but not nitric oxygen (NO) production. Treatment with N‐acetylcysteine, a ROS scavenger, diminished the TREM‐2‐mediated intracellular killing of PA. To further investigate the underlined mechanisms of TREM‐2‐promoted bacterial killing, we examined the activation of downstream mitogen‐activated protein kinases and PI3K/Akt pathway. Western blot data showed that silencing of TREM‐2 inhibited phosphorylation of Akt, but not ERK, JNK or P38. In addition, pretreatment with PI3K active product PIP3 DiC16 reversed the elevation of intracellular bacterial load in TREM‐2‐silenced macrophages, while PI3K inhibitor wortmannin restored the decline of bacterial load in TREM‐2‐overexpressed macrophages. These data together suggested that the TREM‐2‐mediated bacterial killing is dependent on the activation of PI3K/Akt signalling, which may provide a better understanding of the host antibacterial immune defence.     

Expression of activation‐induced cytidine deaminase enhances the clearance of pneumococcal pneumonia: evidence of a subpopulation of protective anti‐pneumococcal B1a cells

We describe a protective early acquired immune response to pneumococcal pneumonia that is mediated by a subset of B1a cells. Mice deficient in B1 cells (xid), or activation‐induced cytidine deaminase (AID^?/?), or invariant natural killer T (iNKT) cells (Jα18^?/?), or interleukin‐13 (IL‐13^?/?) had impaired early clearance of pneumococci in the lung, compared with wild‐type mice. In contrast, AID^?/? mice adoptively transferred with AID^+/+ B1a cells, significantly cleared bacteria from the lungs as early as 3 days post infection. We show that this early bacterial clearance corresponds to an allergic contact sensitivity‐like cutaneous response, probably due to a subpopulation of initiating B1a cells. In the pneumonia model, these B1a cells were found to secrete higher affinity antigen‐specific IgM. In addition, as in contact sensitivity, iNKT cells were required for the anti‐pneumococcal B1a cell initiating response, probably through early production of IL‐13, given that IL‐13^?/? mice also failed to clear infection. Our study is the first to demonstrate the importance of AID in generating an appropriate B1a cell response to pathogenic bacteria. Given the antibody affinity and pneumonia resistance data, natural IgM produced by conventional B1a cells are not responsible for pneumonia clearance compared with the AID‐dependent subset.     

Value of intensive diagnostic microbiological investigation in low- and high-risk patients with community-acquired pneumonia

In a prospective study to evaluate the diagnostic yield of different microbiological tests in hospitalised patients with community-acquired pneumonia, material for microbiological investigation was obtained from 262 patients. Clinical samples consisted of the following: sputum for Gram staining, culture, and detection of pneumococcal antigen; blood for culture and serological tests; urine for detection of Legionella pneumophila serogroup 1 antigen and pneumococcal antigen; and specimens obtained by fiberoptic bronchoscopy. A pathogen was identified in 158 (60%) patients, with Streptococcus pneumoniae (n=97) being the most common causative agent of community-acquired pneumonia. In 82% of the 44 patients with an adequate sputum specimen, a positive Gram stain was confirmed by positive sputum culture. S. pneumoniae infections were detected principally when adequate sputum specimens were examined by Gram stain and culture and when adequate and inadequate sputum specimens were tested for the presence of pneumococcal antigen (n=58; 60%). The urinary pneumococcal antigen test was the most valuable single test for detection of S. pneumoniae infections (n=52; 54%) when sputum pneumococcal antigen determination was not performed. Fiberoptic bronchoscopy was of additive diagnostic value in 49% of the patients who did not expectorate sputum and in 52% of those in whom treatment failed. Investigation of sputum by a combination of Gram stain, culture, and detection of pneumococcal antigen was the most useful means of establishing an aetiological diagnosis of community-acquired pneumonia, followed by testing of urine for pneumococcal antigen. Fiberoptic bronchoscopy may be of additional value when treatment failure occurs.This revised version was published online in April 2005 with a correction to the article title.     

IFN‐γ protects from apoptotic neutrophil‐mediated tissue injury during acute Listeria monocytogenes infection

Listeria monocytogenes (LM) is a foodborne Gram‐positive intracellular pathogen that can cause listeriosis in humans and animals. Although phagocytes are known to be involved in the response to this infection, the role of neutrophils is not entirely clear. Here, we have demonstrated that soon after LM infection, a large number of IFN‐γ‐producing neutrophils quickly accumulated in the spleen, blood, and peritoneal cavity. Both in vivo and in vitro experiments demonstrated that neutrophils were an important source of IFN‐γ. IFN‐γ played a critical protective role against acute LM infection, as demonstrated by the poor survival of Ifng^?/? mice. Moreover, IFN‐γ promoted bacterial clearance by the neutrophils, thereby inhibiting LM‐induced neutrophil apoptosis and spleen damage. In addition to this, IFN‐γ could effectively drive macrophage‐mediated phagocytosis of apoptotic neutrophils, which was accompanied with TGF‐β secretion and was involved in protection against tissue injury. Importantly, by phagocytizing apoptotic neutrophils, macrophages obtained myeloperoxidase, an important bactericidal molecule only produced by neutrophils, which further promoted the antibacterial activity of macrophages. These findings demonstrate that neutrophils are an important source of IFN‐γ at the early stage of LM infection, which is characterized by both LM elimination and tissue‐protective effects.     

Pneumonia‐induced sepsis in mice: temporal study of inflammatory and cardiovascular parameters

The aim of the present work is to provide a better comprehension of the pneumonia‐induced sepsis model through temporal evaluation of several parameters, and thus identify the main factors that determine mortality in this model. Klebsiella pneumoniae was inoculated intratracheally in anesthetized Swiss male mice. Inflammatory and cardiovascular parameters were evaluated 6, 24 and 48 h after the insult. The results show that severity of infection and the mortality correlated with the amount of bacteria. Six, 24 and 48 h after inoculation, animals presented pathological changes in lungs, increase in cell number in the bronchoalveolar lavage, leukopenia, increase in TNF‐α and IL‐1β levels, hypotension and hyporesponsiveness to vasoconstrictors, the two latter characteristics of severe sepsis and septic shock. Significant numbers of bacteria in spleen and heart homogenates indicated infection spreading. Interestingly, NOS‐2 expression appeared late after bacteria inoculation, whereas levels of NOS‐1 and NOS‐3 were unchanged. The high NOS‐2 expression coincided with an exacerbated NO production in the infection focus and in plasma, as judging by nitrate + nitrite levels. This study shows that K. pneumoniae inoculation induces a systemic inflammatory response and cardiovascular alterations, which endures at least until 48 h. K. pneumoniae‐induced lung infection is a clinically relevant animal model of sepsis and a better understanding of this model may help to increase the knowledge about sepsis pathophysiology.     

Alveolar macrophages in pulmonary host defence – the unrecognized role of apoptosis as a mechanism of intracellular bacterial killing

Alveolar macrophages play an essential role in clearing bacteria from the lower airway, as the resident phagocyte alveolar macrophages must both phagocytose and kill bacteria, and if unable to do this completely must co‐ordinate an inflammatory response. The decision to escalate the inflammatory response represents the transition between subclinical infection and the development of pneumonia. Alveolar macrophages are well equipped to phagocytose bacteria and have a large phagolysosomal capacity in which ingested bacteria are killed. The rate‐limiting step in control of extracellular bacteria, such as Streptococcus pneumoniae, is the capacity of alveolar macrophages to kill ingested bacteria. Therefore, alveolar macrophages complement canonical microbicidal strategies with an additional level of apoptosis‐associated killing to help kill ingested bacteria.     

An outbreak of pneumonia associated with S. pneumoniae at a military training facility in Finland in 2006

Streptococcus pneumoniae is a well‐known cause of community‐acquired bacterial pneumonia. The purpose of this study was to assess the cause and extent of the outbreak of pneumonia which occurred among military recruits following a 1‐week hard encampment in Finland. We also assessed the carriage rate and molecular characteristics of the S. pneumoniae isolates. All pneumococcal isolates were studied for antibiotic susceptibility, serotyped, genotyped by multilocus sequence typing (MLST), and the presence of pneumococcal rlrA pilus islet was detected. The genotype results defined by MLST corresponded with the serotype results. S. pneumoniae serotype 7F, ST2331, seemed to be associated with an outbreak of pneumonia and nasopharyngeal carriage among 43 military recruits. Of the 43 military recruits, five (12%) were hospitalized with pneumonia and two (40%) of them were positive for S. pneumoniae serotype 7F, ST2331 by blood culture. Eighteen (42%) of the 43 men were found to be positive for S. pneumoniae by nasopharyngeal culture, and nine (50%) of them carried pneumococcal serotype 7F, ST2331. The outbreak strain covered 55% of all the pneumococcal findings. Outbreaks of invasive pneumococcal disease seem to occur in a crowded environment such as a military training facility even among previously healthy young men.     

Caspase‐11 non‐canonical inflammasome: a critical sensor of intracellular lipopolysaccharide in macrophage‐mediated inflammatory responses

Inflammatory responses mediated by macrophages are part of the innate immune system, whose role is to protect against invading pathogens. Lipopolysaccharide (LPS) found in the outer membrane of Gram‐negative bacteria stimulates an inflammatory response by macrophages. During the inflammatory response, extracellular LPS is recognized by Toll‐like receptor 4, one of the pattern recognition receptors that activates inflammatory signalling pathways and leads to the production of inflammatory mediators. The innate immune response is also triggered by intracellular inflammasomes, and inflammasome activation induces pyroptosis and the secretion of pro‐inflammatory cytokines such as interleukin‐1β (IL‐1β) and IL‐18 by macrophages. Cysteine‐aspartic protease (caspase)‐11 and the human orthologues caspase‐4/caspase‐5 were recently identified as components of the ‘non‐canonical inflammasome’ that senses intracellular LPS derived from Gram‐negative bacteria during macrophage‐mediated inflammatory responses. Direct recognition of intracellular LPS facilitates the rapid oligomerization of caspase‐11/4/5, which results in pyroptosis and the secretion of IL‐1β and IL‐18. LPS is released into the cytoplasm from Gram‐negative bacterium‐containing vacuoles by small interferon‐inducible guanylate‐binding proteins encoded on chromosome 3 (GBP^chr3)‐mediated lysis of the vacuoles. In vivo studies have clearly shown that caspase‐11^−/− mice are more resistant to endotoxic septic shock by excessive LPS challenge. Given the evidence, activation of caspase‐11 non‐canonical inflammasomes by intracellular LPS is distinct from canonical inflammasome activation and provides a new paradigm in macrophage‐mediated inflammatory responses.     

MYD88‐dependent and ‐independent activation of TREM‐1 via specific TLR ligands

Triggering receptor expressed on myeloid cells (TREM)‐1 plays an important role in myeloid cell‐activated inflammatory responses. Although TLR ligands such as LPS and lipoteichoic acid have been shown to upregulate TREM‐1 expression in macrophage and neutrophils, the role of specific TLR in inducing the expression of TREM‐1 remains unclear. In this study, we investigated whether the presence of TLR is necessary for the expression of TREM‐1. We show that BM‐derived macrophages from TLR4 and TLR2 KO mice failed to induce expression of TREM‐1 message and protein in response to their specific ligands. Interestingly, the expression of TREM‐1 in response to LPS is not altered in myeloid differentiation factor 88 (MyD88) KO macrophages, suggesting that downstream of TLR a MyD88‐independent pathway induces the expression of TREM‐1. Inhibiting toll/IL‐1R domain‐containing adaptor‐inducing IFN‐β (TRIF) expression by siRNA decreased TREM‐1 expression in response to LPS, suggesting that the expression of TREM‐1 in response to LPS was mediated by the TRIF signaling pathway. On the other hand, the expression of TREM‐1 in response to lipoteichoic acid is dependent on MyD88 expression. These data indicate that the expression of TREM‐1 in response to TLR ligands occurs secondary to downstream signaling events and that the presence of TLR is necessary for the expression of TREM‐1 in response to their specific ligands. However, the downstream signaling required for the expression of TREM‐1 is dependent on the stimulus and the surface receptor through which the signaling is initiated.     

ASC and NLRP3 impair host defense during lethal pneumonia caused by serotype 3 Streptococcus pneumoniae in mice

Streptococcus (S.) pneumoniae is the most common cause of community‐acquired pneumonia. The Nod‐like receptor family pyrin domain containing 3 (NLRP3) inflammasome, consisting of NLRP3, ASC (the adaptor apoptosis‐associated speck‐like protein containing a CARD) and caspase‐1, has been implicated in protective immunity during pneumonia induced by high doses of S. pneumoniae serotype 2. Here we investigated the role of the NLRP3 inflammasome in the host response during lethal airway infection with a low dose of serotype 3 S. pneumoniae. Mice were euthanized at predefined endpoints for analysis or observed in survival studies. In additional studies, Tlr2^?/?/Tlr4^?/? mice and Myd88^?/? mice incapable of Toll‐like receptor signaling were studied. In stark contrast with existing literature, both Nlrp3^?/? and Asc^?/? mice showed a strongly improved host defense, as reflected by a markedly reduced mortality rate accompanied by diminished bacterial growth and dissemination. Host defense was unaltered in Tlr2^?/?/Tlr4^?/? mice and Myd88^?/? mice. These results show that the NLRP3 inflammasome impairs host defense during lethal pneumonia caused by serotype 3 S. pneumoniae. Our findings challenge the current paradigm that proximal innate detection systems are indispensable for an adequate host immune response against bacteria.     

IL‐1β activation in response to Staphylococcus aureus lung infection requires inflammasome‐dependent and independent mechanisms

Maintaining balanced levels of IL‐1β is extremely important to avoid host tissue damage during infection. Our goal was to understand the mechanisms behind the reduced pathology and decreased bacterial burdens in Ifnlr1^?/? mice during lung infection with Staphylococcus aureus. Intranasal infection of Ifnlr1^?/? mice with S. aureus led to significantly improved bacterial clearance, survival and decrease of proinflammatory cytokines in the airway including IL‐1β. Ifnlr1^?/? mice treated with recombinant IL‐1β displayed increased bacterial burdens in the airway and lung. IL‐1β levels in neutrophils from Ifnlr1^?/? infected mice lungs were decreased when compared to neutrophils from WT mice. Mice lacking NLRP3 and caspase‐1 had reduced IL‐1β levels 4 h after infection, due to reductions or absence of active caspase‐1 respectively, but levels at 24 h were comparable to WT infected mice. Ifnlr1^?/? infected mice had decreases in both active caspase‐1 and neutrophil elastase indicating an important role for the neutrophil serine protease in IL‐1β processing. By inhibiting neutrophil elastase, we were able to decrease IL‐1β levels by 39% in Nlrp3^?/? infected mice when compared to WT mice. These results highlight the crucial role of both proteases in IL‐1β processing, via inflammasome‐dependent and ‐independent mechanisms.     

Pneumococcal serology in children’s respiratory infections

The role of Streptococcus pneumoniae in the etiology of respiratory tract infections has been studied serologically using microbe-specific antibody and immune complex assays. Serological methods are sensitive in the bacteremic pneumococcal pneumonia of adults. In children, however, pneumococcal pneumonia is seldom bacteremic, and, thus, in the absence of a gold standard for the detection of pneumococcal infection, serological methods are still insufficiently validated. We report here indirect evidence for the sensitivity and specificity of pneumococcal serology in children. Serological evidence of pneumococcal infection has been found in 27% to 38% of children with radiologically confirmed pneumonia, in 7% to 8% of children with viral wheezy bronchitis, and in <1% to 5% of children and young adults with viral upper respiratory infection. Serological findings for pneumococcal infection have been dependent on the study venue, whether in hospital or ambulatory subjects, and on the test panel used. Where both antibody and immune complex assays have been available, the proportion of children with pneumococcal infection has been 32% to 37% in inpatients and 27% to 28% in outpatients. The respective rates have been 10% to 18% by antibody assays alone. Pneumococcal acute otitis media, when present with pneumonia, may confound findings in pneumococcal serology, but pure nasopharyngeal carriage of S. pneumoniae has little effect. In contrast, carriage acquisition of a new serotype may induce significant antibody production. Thus, understandably, significant rises between paired sera in antibodies to pneumococcal capsular polysaccharides and pneumococcal pneumolysin have been found in <1% to 3% of non-symptomatic children and young adults. Findings from the last 20 years indirectly suggest that pneumococcal antibody and immune complex assays are sensitive and specific enough for the detection of pneumococcal infection in children. However, the methods are too complex for routine clinical practice, and, so far, serological methods for S. pneumoniae infections have only been used for research purposes.     

Treatment with broadly neutralizing influenza antibodies reduces severity of secondary pneumococcal pneumonia in mice

Secondary bacterial pneumonia is a frequent complication of influenza, associated with high morbidity and mortality. We hypothesized that treatment with neutralizing influenza A antibody AT10_002 protects against severe secondary pneumococcal infection in a mouse model of influenza A infection. Influenza A (H3N2) virus–infected male C57Bl6 mice were treated intravenously with either AT10_002 or a control 2 days postinfection. Seven days later, both groups were infected with Streptococcus pneumoniae and killed 18 hours later. Mice receiving AT10_002 showed less loss of bodyweight compared with controls (+1% vs ?12%, P < .001), lower viral loads in bronchoalveolar lavage fluids (BALFs) (7 vs 194 RNA copies per µL; P < .001), and reduced bacterial outgrowth in lung homogenates (3.3 × 10¹ vs 2.5 × 10⁵ colony‐forming units per mg; P < .001). The treatment group showed lower pulmonary wet weights, lower cell counts, and lower protein levels in BALF compared with controls. Treatment with AT10_002 was associated with lower levels of tumor necrosis factor‐α, interleukin (IL)‐6, cytokine‐induced neutrophil chemoattractant (KC), and interferon‐γ in BALF and lower IL‐6 and KC in lung homogenates. Treatment with anti‐influenza antibody AT10_002 is associated with reduced weight loss, viral load, bacterial outgrowth, and lung injury in a murine model of secondary pneumococcal pneumonia following influenza infection.     

Caseinolytic protease: a protein vaccine which could elicit serotype‐independent protection against invasive pneumococcal infection

Invasive pneumococcal diseases incur significant mortality, morbidity and economic costs. The most effective strategy currently available to reduce the burden of these diseases is vaccination. In this study, we evaluated the protective efficacy of immunizing mice with caseinolytic protease (ClpP) protein antigen whose gene sequences were shown to be highly conserved in different strains of Streptococcus pneumoniae in an invasive‐disease model (intraperitoneal infection model), and protection against invasive challenge with 12 different serotypes of S. pneumoniae was assessed in two murine strains. Our findings demonstrated that active immunization with ClpP and passive immunization with antibodies specific for ClpP could elicit serotype‐independent protection effectively against invasive pneumococcal infection. Therefore, to our knowledge, this study is the first report that immunization with single pneumococcal ClpP protein antigen could protect against such broad‐range pneumococal strains, which thus supports the development of ClpP as a human penumococcal vaccine.     

Type I Interferon Signaling Regulates Activation of the Absent in Melanoma 2 Inflammasome during Streptococcus pneumoniae Infection

Streptococcus pneumoniae, a Gram-positive bacterial pathogen, causes pneumonia, meningitis, and septicemia. Innate immune responses are critical for the control and pathology of pneumococcal infections. It has been demonstrated that S. pneumoniae induces the production of type I interferons (IFNs) by host cells and that type I IFNs regulate resistance and chemokine responses to S. pneumoniae infection in an autocrine/paracrine manner. In this study, we examined the effects of type I IFNs on macrophage proinflammatory cytokine production in response to S. pneumoniae. The production of interleukin-18 (IL-18), but not other cytokines tested, was significantly decreased by the absence or blockade of the IFN-α/β receptor, suggesting that type I IFN signaling is necessary for IL-18 production. Type I IFN signaling was also required for S. pneumoniae-induced activation of caspase-1, a cysteine protease that plays a central role in maturation and secretion of IL-18. Earlier studies proposed that the AIM2 and NLRP3 inflammasomes mediate caspase-1 activation in response to S. pneumoniae. From our results, the AIM2 inflammasome rather than the NLRP3 inflammasome seemed to require type I IFN signaling for its optimal activation. Consistently, AIM2, but not NLRP3, was upregulated in S. pneumoniae-infected macrophages in a manner dependent on the IFN-α/β receptor. Furthermore, type I IFN signaling was found to contribute to IL-18 production in pneumococcal pneumonia in vivo. Taken together, these results suggest that type I IFNs regulate S. pneumoniae-induced activation of the AIM2 inflammasome by upregulating AIM2 expression. This study revealed a novel role for type I IFNs in innate responses to S. pneumoniae.