Macrophage Rac2 Is Required to Reduce the Severity of Cigarette Smoke-induced Pneumonia

RATIONALE

Cigarette smoking is prevalent in the United States and is the leading cause of preventable diseases. A prominent complication of smoking is an increase in lower respiratory tract infections (LRTIs). Although LRTIs are known to be increased in subjects that smoke, the mechanism(s) by which this occurs is poorly understood.

OBJECTIVES

Determine how cigarette smoke (CS) reduces reactive oxygen species (ROS) production by the phagocytic NOX2 (NADPH oxidase 2), which is essential for innate immunity in lung macrophages.

METHODS

NOX2-derived ROS and Rac2 (Ras-related C3 botulinum toxin substrate 2) activity were determined in BAL cells from wild-type and Rac2-/- mice exposed to CS or cadmium and in BAL cells from subjects that smoke. Host defense to respiratory pathogens was analyzed in mice infected with Streptococcus pneumoniae.

MEASUREMENTS AND MAIN RESULTS

NOX2-derived ROS in BAL cells was reduced in mice exposed to CS via inhibition of the small GTPase Rac2. These mice had greater bacterial burden and increased mortality compared with air-exposed mice. BAL fluid from CS-exposed mice had increased levels of cadmium, which mediated the effect on Rac2. Similar observations were seen in human subjects that smoke. To support the importance of Rac2 in the macrophage immune response, overexpression of constitutively active Rac2 by lentiviral administration increased NOX2-derived ROS, decreased bacterial burden in lung tissue, and increased survival compared with CS-exposed control mice.

CONCLUSIONS

These observations suggest that therapies to maintain Rac2 activity in lung macrophages restore host defense against respiratory pathogens and diminish the prevalence of LRTIs in subjects that smoke.

Allergic airway sensitization impairs antibacterial IgG antibody responses during bacterial respiratory tract infections

BACKGROUND

Mycoplasma pneumoniae, an atypical human pathogen, has been associated with asthma initiation and exacerbation. Asthmatic patients have been reported to have higher carriage rates of M pneumoniae compared with nonasthmatic subjects and are at greater risk for invasive respiratory infections.

OBJECTIVE

We sought to study whether prior allergen sensitization affects the host response to chronic bacterial infection.

METHODS

BALB/cJ and IL-4 receptor α^-/- mice were sensitized with ovalbumin (OVA) and then infected with M pneumoniae or Streptococcus pneumoniae. Immune parameters were analyzed at 30 days postinfection and included cellular profiles in bronchoalveolar lavage fluid (BALF) and serum IgG and IgE antibody levels to whole bacterial lysate, recombinant P1 adhesin, and OVA. Total lung RNA was examined for transcript levels, and BALF was examined for cytokine protein profiles.

RESULTS

Anti-M pneumoniae antibody responses were decreased in allergen-sensitized, M pneumoniae-infected animals compared with control animals, but OVA-specific IgG responses were unaffected. Similar decreases in anti-S pneumoniae antibody levels were found in OVA-sensitized animals. However, M pneumoniae, but not S pneumoniae, infection augmented anti-OVA IgE antibody responses. Loss of IL-4 receptor signaling partially restored anti-M pneumoniae antibody responses in IgG_2a and IgG_2b subclasses. Inflammatory cytokine levels in BALF from OVA-sensitized, M pneumoniae-infected or S pneumoniae-infected animals were reduced compared with those in uninfected OVA-sensitized control animals. Unexpectedly, airway hyperreactivity to methacholine was essentially ablated in M pneumoniae-infected, OVA-sensitized animals.

CONCLUSIONS

An established type 2-biased host immune response impairs the host immune response to respiratory bacterial infection in a largely pathogen-independent manner. Some pathogens, such as M pneumoniae, can augment ongoing allergic responses and inhibit pulmonary type 2 cytokine responses and allergic airway hyperreactivity.

PcpA Promotes Higher Levels of Infection and Modulates Recruitment of Myeloid-Derived Suppressor Cells during Pneumococcal Pneumonia

We used two different infection models to investigate the kinetics of the PcpA-dependent pneumococcal disease in mice. In a bacteremic pneumonia model, we observed a PcpA-dependent increase in bacterial burden in the lungs, blood, liver, bronchoalveolar lavage, and spleens of mice at 24 h postinfection. This PcpA-dependent effect on bacterial burden appeared earlier (within 12 h) in the focal pneumonia model, which lacks bacteremia or sepsis. Histological changes show that the ability of pneumococci to make PcpA was associated with unresolved inflammation in both models of infection. Using our bacteremic pneumonia model we further investigated the effects of PcpA on recruitment of innate immune regulatory cells. The presence of PcpA was associated with increased IL-6 levels, suppressed production of TRAIL, and reduced infiltration of polymorphonuclear cells. The ability of pneumococci to make PcpA negatively modulated both the infiltration and apoptosis of macrophages and the recruitment of myeloid-derived suppressor-like cells. The latter have been shown to facilitate the clearance and control of bacterial pneumonia. Taken together, the ability to make PcpA was strongly associated with increased bacterial burden, inflammation, and negative regulation of innate immune cell recruitment to the lung tissue during bacteremic pneumonia.

The Effects of CFTR and Mucoid Phenotype on Susceptibility and Innate Immune Responses in a Mouse Model of Pneumococcal Lung Disease

Recent studies have reported the isolation of highly mucoid serotype 3 Streptococcus pneumoniae (Sp) from the respiratory tracts of children with cystic fibrosis (CF). Whether these highly mucoid Sp contribute to, or are associated with, respiratory failure among patients with CF remains unknown. Other mucoid bacteria, predominately Pseudomonas aeruginosa, are associated with CF respiratory decline. We used a mouse model of CF to study pneumococcal pneumonia with highly mucoid serotype 3 and non-mucoid serotype 19A Sp isolates. We investigated susceptibility to infection, survival, and bacterial counts from bronchoaviolar lavage samples and lung homogenates, as well as associated inflammatory cytokines at the site of infection, and lung pathology. Congenic CFTR-/- mice and wild-type (WT)-mice were infected intranasally with CHB756, CHB1126, and WU2 (highly mucoid capsular serotype 3, intermediately mucoid serotype 3, and less mucoid serotype 3, respectively), or CHB1058 (non-mucoid serotype 19A). BAL, lung homogenates, and blood were collected from mice 5 days post-infection. Higher CFU recovery and shorter survival were observed following infection of CFTR-/- mice with CHB756 compared to infection with CHB1126, WU2, or CHB1058 (P≤0.001). Additionally, CFTR-/- mice infected with CHB756 and CHB1126 were more susceptible to infection than WT-mice (P≤0.05). Between CFTR-/- mice and WT-mice, no significant differences in TNF-α, CXCL1/KC concentrations, or lung histopathology were observed. Our results indicate that highly mucoid type 3 Sp causes more severe lung disease than non-mucoid Sp, and does so more readily in the lungs of CFTR-/- than WT-mice.