The macrophage scavenger receptor SR-AI/II and lung defense against pneumococci and particles

Abstract

The class A macrophage scavenger receptor SR-AI/II is implicated as a pattern recognition receptor for innate immunity, but its functional role in lung defense has not been studied. We used mice genetically deficient in SR-AI/II and their wild-type C57BL/6 counterparts to investigate the contribution of this receptor to defense against pneumococcal infection and inhaled particles. SR-AI/II deficiency caused impaired phagocytosis of fluorescent bacteria in vivo, diminished clearance of live bacteria from the lungs, and substantially increased pneumonic inflammation. Survival studies also showed increased mortality in SR-AI/II-deficient mice with pneumococcal lung infection. Similarly, after challenge of the airways with TiO(2) particles, SR-AI/II-deficient mice showed increased proinflammatory cytokine levels in lung lavage fluid and a more pronounced neutrophilic inflammation. The data indicate that the lung macrophage class A scavenger receptor SR-AI/II contributes to innate defense against bacteria and inhaled particles.

Figure 1.

Absence of SR-AI/II increases mortality in S. pneumoniae–infected mice. SR-AI/II^+/+ and SR-AI/II^−/− mice were intranasally inoculated with 4.5 × 10⁵ CFU of S. pneumoniae serotype 3, and survival was monitored over 13 d. Data are expressed as the percentage of mice surviving at each time point. Kaplan-Meier survival plots are shown for 10 animals per group. Mantel-Cox logrank test showed a P value of 0.0416.

Figure 2.

Diminished bacterial clearance from the airways of SR-AI/II^−/− after infection with S. pneumoniae. SR-AI/II^+/+ and SR-AI/II^−/− (n > 6) were intranasally infected with ∼ 10⁵ CFU of S. pneumoniae. Lung bacterial load was determined at 4 h (A) and 24 h (B) postinfection. Data shown are representative of at least three separate experiments and are presented as mean ± SEM.

Figure 3.

SR-AI/II^−/− mice with pneumococcal pneumonia develop a stronger inflammatory response in the airways. BALF was harvested from wild-type (n = 10) and deficient mice (n = 8) 4 h postinfection, and the total number of leukocytes and the percentage of PMNs were determined (A). MIP-2 and TNF-α contents in the BALF were determined using ELISA (B). Data shown are representative of at least three separate experiments and are presented as mean ± SEM.

Figure 4.

Decreased phagocytic capacity in SR-AI/II^−/− AMs. Mice were intranasally given 10⁷ FITC-S. pneumoniae under slight anesthesia. Lungs were lavaged with PBS 3 h later, and the amount of bacteria associated with AMs was quantified by flow cytometry. Data shown are pooled from two separate experiments representing a total of seven mice per group.

Figure 5.

Elevated lung inflammation in SR-AI/II–deficient mice in response to inhaled TiO₂ particles. (A) PMN recruitment into BALF of PBS and TiO₂–treated mice (n = 3–35 mice/group). (B) BALF TNF-α after instillation with PBS or two different doses of TiO₂, as measured by ELISA (n = 3–9 mice/group). (C) Analysis of lung gene expression in the lungs of PBS or TiO₂-treated mice by RNase protection assay shows increased MIP-2 response in SRA-I/II–deficient mice. (D) Fold-increase of MIP-2 mRNA expression in TiO₂-treated mice over PBS-treated mice in SR-AI/II^+/+ (n = 3) and SR-AI/II^−/− mice (n = 4).