Prostaglandin E2–Induced Changes in Alveolar Macrophage Scavenger Receptor Profiles Differentially Alter Phagocytosis of Pseudomonas aeruginosa and Staphylococcus aureus Post–Bone Marrow Transplant

Racquel Domingo-Gonzalez, Samuel Katz, C. Henrique Serezani, Thomas A. Moore, Ann Marie LeVine and Bethany B. Moore
J Immunol June 1, 2013, 190 (11) 5809-5817; DOI: https://doi.org/10.4049/jimmunol.1203274

Abstract

The effectiveness of hematopoietic stem cell transplantation as a therapy for malignant and nonmalignant conditions is complicated by pulmonary infections. Using our syngeneic bone marrow transplant (BMT) mouse model, BMT mice with a reconstituted hematopoietic system displayed increased susceptibility to Pseudomonas aeruginosa and Staphylococcus aureus. BMT alveolar macrophages (AMs) exhibited a defect in P. aeruginosa phagocytosis, whereas S. aureus uptake was surprisingly enhanced. We hypothesized that the difference in phagocytosis was due to an altered scavenger receptor (SR) profile. Interestingly, MARCO expression was decreased, whereas SR-AI/II was increased. To understand how these dysregulated SR profiles might affect macrophage function, CHO cells were transfected with SR-AI/II, and phagocytosis assays revealed that SR-AI/II was important for S. aureus uptake but not for P. aeruginosa. Conversely, AMs treated in vitro with soluble MARCO exhibited similar defects in P. aeruginosa internalization as did BMT AMs. The 3′-untranslated region of SR-AI contains a putative target region for microRNA-155 (miR-155), and miR-155 expression is decreased post-BMT. Anti–miR-155–transfected AMs exhibited an increase in SR-AI/II expression and S. aureus phagocytosis. Elevated PGE2 has been implicated in driving an impaired innate immune response post-BMT. In vitro treatment of AMs with PGE2 increased SR-AI/II and decreased MARCO and miR-155. Despite a difference in phagocytic ability, BMT AMs harbor a killing defect to both P. aeruginosa and S. aureus. Thus, our data suggest that PGE2-driven alterations in SR and miR-155 expression account for the differential phagocytosis of P. aeruginosa and S. aureus, but impaired killing ultimately confers increased susceptibility to pulmonary infection.

Introduction

Hematopoietic stem cell transplantation (HSCT) is a widely used treatment to address a variety of inherited and genetic disorders in patients. Although effective, HSCT patients show increased susceptibility to numerous complications, many involving the lung, in the months to years following treatment (1–3). Infectious and noninfectious complications arise in HSCT recipients despite differences in stem cell source (2, 4). Autopsy reports found 80–89% pulmonary complication rates in both allogeneic and autologous HSCT patients (4, 5), and a recent study found that pulmonary complications manifested in >25% of autologous HSCT recipients, with the majority of the complications being infectious (3).

Bacterial pulmonary complications in HSCT recipients manifest at different times posttransplant and are varied in pathogen species (Gram positive versus Gram negative). Although allogeneic transplant recipients develop pulmonary infectious complications more frequently (potentially due to barrier disruptions secondary to graft-versus-host disease), autologous HSCT patients also experience infectious complications; there is no difference in the types of infections that manifest in autologous and allogeneic HSCT recipients in the first 30 d posttransplant (pre-engraftment phase) (6). Although it is not surprising that infections are common in the pre-engraftment phase (7) or in allogeneic HSCT patients on immunosuppressive therapy, it is less well understood why infections were also reported to occur in autologous HSCT recipients in the late posttransplant phase (8). Both Gram-negative and Gram-positive infections can be problematic in HSCT patients. Prophylactic antibiotics have lowered the incidence of Pseudomonas aeruginosa infection, but invasive Pseudomonas remains a concern in this population (9, 10). Additionally, infections caused by Gram-positive bacteria, particularly Streptococcus and Staphylococcus spp., have risen as predominant infections in HSCT recipients (6).

To better understand the immunological alterations that may characterize innate immune cells in the lung and predispose the host to bacterial infection postengraftment, our laboratory developed a syngeneic (syn) bone marrow transplantation (BMT) animal model. This model allows for an investigation of transplant-related immune alterations that are not caused by immunosuppressive drugs or impaired barrier function secondary to graft-versus-host disease. We demonstrated previously that C57BL/6 mice that received a syn BMT are more susceptible to infection by Gram-negative P. aeruginosa (11). Increased susceptibility to P. aeruginosa was related to overproduction of PGE2 post-BMT, and this increase in PGE2 was regulated by hypomethylation of the COX-2 gene (12, 13). Increases in PGE2 post-BMT impaired alveolar macrophage (AM) phagocytosis and killing of ingested bacteria and limited neutrophil-mediated killing (12). Some of the effects of PGE2 signaling on AMs post-BMT were to upregulate IRAK-M and activation of PTEN (14, 15). These changes limited inflammatory cytokine production and nonopsonized and opsonized phagocytosis of P. aeruginosa, as well as bacterial killing.

The current studies were undertaken to determine whether syn BMT mice also displayed increased susceptibility to a Gram-positive, S. aureus infection and, if so, whether the mechanisms of susceptibility were related to PGE2 production and impaired phagocytosis mediated by scavenger receptors (SRs). Similar to our previous studies with P. aeruginosa, we find that syn BMT mice are more susceptible to S. aureus, and this is related to overproduction of PGE2. Surprisingly, however, phagocytosis of S. aureus is enhanced, not diminished, post-BMT. Our mechanistic studies identify PGE2-induced changes, including alterations in microRNAs (miRNAs) that alter the functional profile of SRs to differentially regulate phagocytosis. Despite the fact that phagocytosis of S. aureus is improved post-BMT, bacterial killing is impaired, explaining the overall defect in host defense.

Materials and Methods

Animals

Wild type (WT) C57BL/6 (B6) and SRAI/II−/− mice (C57BL/6 background) were obtained from The Jackson Laboratory (Bar Harbor, ME). Mice were housed under specific pathogen–free conditions and monitored daily by veterinary staff. All mice were euthanized by CO2 asphyxiation. All animal experiments were approved by the University of Michigan Committee on the Use and Care of Animals.

Bone marrow transplantation

Bone marrow was harvested from C57BL/6 donor mice and infused by tail vein injection into lethally irradiated C57BL/6 recipients. Ablation of recipient-derived hematopoietic stem cells (HSCs) was achieved by the administration of a fractionated 13-Gy dose of total body irradiation from an x-ray orthovoltage source. Complete immune reconstitution is achieved 5 wk following the infusion of 5 × 106 whole bone marrow cells into total body irradiation recipients (16, 17). The percentage of donor-derived cells was ∼95 ± 1% in the spleen and 82 ± 2% in the lung at this time point, as assessed by transplanting CD45.1+ bone marrow into C57BL/6 CD45.2+ mice (16).

P. aeruginosa (PAO1) and S. aureus (USA300/NRS384) preparation

P. aeruginosa (PAO1) and S. aureus (USA300) stock were grown in tryptic soy broth and nutrient broth (Difco; BD, Sparks, MD), respectively. The culture concentration was determined via absorbance measurements, as previously described (11). For FITC labeling, a culture was centrifuged and washed two times by resuspending the pellet in 1 ml sterile PBS and transferring it into a sterile tube. The bacteria were heat killed by autoclaving for 20 min and resuspended at 109–1010 CFU/ml in 0.1 M NaHCO3 (pH 9.2); 0.2 mg/ml FITC (Sigma, St. Louis, MO) in DMSO was added to heat-killed P. aeruginosa or S. aureus and allowed to incubate in the dark for 1 h on a rocker at room temperature. Following FITC labeling, heat-killed P. aeruginosa or S. aureus were washed three times and resuspended in sterile PBS at 6 × 109 CFU/ml.

Intratracheal infection with P. aeruginosa or S. aureus

A culture of P. aeruginosa or S. aureus was grown as described above, and an inoculum was prepared. Mice were anesthetized and injected intratracheally (i.t.) with 50 μl inoculum to provide either a sublethal dose of 5 × 105 PA01 CFU, as previously described (11, 12), or a sublethal dose of 7 × 107 USA300 CFU.

Quantification of bacterial burden in lung

Mice were euthanized 24 h following i.t. infection with P. aeruginosa or S. aureus. As previously described (12), lungs were collected from each mouse and homogenized, and the bacterial burden of each specimen was assessed by performing a CFU assay. Data are expressed as total CFU/lung.

Harvesting AMs

Resident AMs from mice were obtained via ex vivo lung lavage, using a previously described protocol (12). Briefly, these cells were collected in lavage fluid consisting of complete medium (DMEM, 1% penicillin-streptomycin, 1% l-glutamine, 10% FCS, 0.1% Fungizone) and 5 mM EDTA. The cells were enumerated by counting on a hemocytometer before plating.

In vitro phagocytosis assays

AMs were harvested, as described above, and the ability of the AMs from control and BMT mice to phagocytose via opsonin-independent pathways was examined using a 300:1 ratio of FITC-labeled P. aeruginosa or S. aureus to AMs. Briefly, 2 × 105 AMs were plated on a half-area black 96-well plate and incubated overnight at 37°C. The next day, the medium was changed to serum-free medium, and 10 μl FITC-labeled P. aeruginosa or S. aureus was added to each well. After 2 h at 37°C, trypan blue was added to quench extracellular fluorescence, and phagocytosis was quantified as previously described (18).

Tetrazolium dye reduction assay of bacterial killing

The ability of AMs from control and BMT mice to kill P. aeruginosa and S. aureus was quantified using a tetrazolium dye reduction assay, as described elsewhere (19, 20). Briefly, AMs from WT and BMT mice were aliquoted into duplicate 96-well plates: one experimental (37°C) plate and one control (4°C). Cells from both plates were infected with IgG-opsonized P. aeruginosa or S. aureus (2 × 108 CFU/ml; multiplicity of infection 50:1) for 30 min at 37°C. The cells on the experimental plate were washed and then incubated at 37°C for 90 min, whereas the cells on the control plate were washed and then lysed with 0.5% saponin in tryptic soy broth (Sigma, St. Louis, MO) and placed at 4°C. After 90 min, the cells from the experimental plate were lysed with 0.5% saponin in tryptic soy broth. Both plates were then incubated at 37°C for 2.5 h. A total of 5 mg/ml MTT (Sigma) was added to each plate and incubated for 30 min. Solubilization solution was added to dissolve formazan salts, and the absorbance was read at 595 nm. Results were expressed as percentage of survival of ingested bacteria normalized to the percentage of control, where the A595 experiment values were divided by the average of the A595 control values. Survival of ingested bacteria = (A595 experimental plate/A595 control plate) × 100%.

Real-time RT-PCR

Gene-specific primers and probes were designed using Primer Express software (PE Biosystems, Foster City, CA), as described previously (12, 21). Sequences for primers and probes used can be found in Table I. SnoR142 and microRNA-155 (miR-155) expression were measured using TaqMan MicroRNA Assays (Applied Biosystems/Life Technologies, Foster City, CA). Each AM sample was pooled from two or three mice and was run in duplicate. miRNA expression was determined by converting TRIzol-isolated RNA into cDNA using TaqMan Universal PCR Master Mix, No AmpErase UNG (Applied Biosystems/Life Technologies). Real-time RT-PCR was performed on an ABI PRISM 7000 thermocycler (Applied Biosystems, Foster City, CA). Average cycle threshold (CT) was determined for each sample and normalized to β-actin or snoR142. Relative gene expression was calculated as described previously (22).

View this table:
Table I. Primers and probes for semiquantitative real-time RT-PCR

Flow cytometry

A total of 5 × 105 primary AMs from untransplanted control and BMT mice were stained for flow cytometry using a primary Ab (2 μg/ml) against the cell surface receptor MARCO (Hycult Biotech, Plymouth Meeting, PA) after incubation with anti-CD16/CD32 (Fc Block; BD Biosciences, San Jose, CA). Fluorochrome-conjugated secondary Ab, donkey anti-rat PE (Jackson ImmunoResearch, West Grove, PA), was used to detect the primary Ab. Cells were resuspended in PBS and fixed with 4% paraformaldehyde. Data were analyzed using flow cytometry analysis software (FlowJo, version 7.5; TreeStar, Ashland, OR).

In vitro miRNA transfection

Primary AMs were harvested from untransplanted C57BL/6 mice, as described above. A total of 2 × 105 cells was transfected with the antagomir of miR-155 (Dharmacon/Thermo Fisher Scientific, Lafayette, CO) or anti-miR miRNA inhibitor negative control #1 (Ambion/Life Technologies, Grand Island, NY) for 24 h when measuring mRNA expression and for 48 h for phagocytosis assays. Transfections were performed using Lipofectamine RNAiMAX Reagent (Invitrogen/Life Technologies, Grand Island, NY), as described by the manufacturer. Briefly, antagomir or control miR was diluted in Opti-MEM I Reduced Serum Media (Life Technologies, Grand Island, NY) and mixed with Lipofectamine RNAiMAX Reagent prior to transfecting the cells.

Statistical analysis

Statistical significance was analyzed using the GraphPad Prism 5.0 statistical program (La Jolla, CA). Comparisons between two experimental groups were performed using the Student t test, and error bars denote the SEM. Comparisons between three or more values used an ANOVA analysis with a post hoc Bonferroni comparison. A p value < 0.05 was considered statistically significant.

Results

Syn BMT mice are more susceptible to S. aureus

To determine the susceptibility of BMT mice to S. aureus, untransplanted (control) and syn BMT mice were injected i.t. with a methicillin-resistant strain of S. aureus or with the PAO1 strain of P. aeruginosa for comparison. Lungs were harvested 24 h postinfection for CFU analysis. BMT mice were more susceptible to both P. aeruginosa (Fig. 1A) and S. aureus (Fig. 1B) pulmonary infections.

FIGURE 1.
FIGURE 1.

Syn BMT mice are more susceptible to P. aeruginosa and S. aureus. Control (Ctrl) or syn BMT mice received P. aeruginosa (n = 5–6/group) (A) or S. aureus (n = 10/group) (B) i.t., and lungs were collected for CFU analysis 24 h later. *p < 0.05, ***p < .001.

Syn BMT AMs exhibit a defect in phagocytosis of P. aeruginosa but enhanced phagocytosis of S. aureus

Because AMs are the predominant immune cell in the lung during an initial infection (23), we hypothesized that AM function was compromised in a BMT setting. Using AMs harvested from bronchoalveolar lavage of control and syn BMT mice, phagocytosis of P. aeruginosa by AMs was measured. As we reported previously (12), syn BMT AMs exhibited decreased phagocytosis of FITC–P. aeruginosa compared with controls (Fig. 2A). Surprisingly, syn BMT AMs incubated with FITC–S. aureus displayed augmented phagocytosis compared with controls (Fig. 2B).

FIGURE 2.
FIGURE 2.

Syn BMT AMs exhibit defective phagocytosis of P. aeruginosa but not S. aureus. Primary AMs were harvested, and phagocytosis of FITC–P. aeruginosa (A) or FITC–S. aureus (B) was determined (n = 5–10/group). Data are normalized to lactate dehydrogenase levels per well to standardize for cell numbers. Results are presented relative to the average of untransplanted control values set to 100%. ***p < 0.001.

SR profiles are altered post-BMT

Class A SRs are pattern recognition receptors reported to play important roles in the recognition of nonopsonized Gram-positive and Gram-negative bacteria and endogenous ligands (24). Studies showed that MARCO, a class A SR, is important for in vivo host response against Streptococcus pneumoniae, a Gram-positive bacterium (25), and macrophage SR A (MSR-A/SR-A) types I and II were reported to recognize S. aureus (26). However, it remains unclear which SR is important for recognition of P. aeruginosa in the lung. Because MARCO and SRAI/II have been implicated in the recognition and initiation of an immune response within macrophages (24), we wanted to determine whether the difference in phagocytosis between P. aeruginosa and S. aureus could be explained by an alteration in SR expression on AMs. Using RNA isolated from control and BMT AMs, SR profiles were measured utilizing the primers and probes outlined in Table 1. MARCO mRNA expression decreased post-BMT (Fig. 3A, left panel), whereas SR-AI/II mRNA increased (Fig. 3B) compared with control. A decrease in MARCO surface expression was confirmed by flow cytometry (Fig. 3A, right panel), correlating with MARCO mRNA analysis. Commercially available Abs against SRAI/II do not recognize the allele in C57BL/6 mice, precluding our ability to measure protein expression of this receptor by flow cytometry in our model.

FIGURE 3.
FIGURE 3.

MARCO is decreased on syn BMT AMs, whereas SR-AI/II is increased. AMs were harvested from untransplanted control B6 and syn BMT mice, and MARCO (A, left panel) mRNA was detected by real-time RT-PCR. Expression of one control AM sample was set to n = 1 for comparisons (n = 3). (A, right panel) MARCO protein levels were detected by flow cytometry (n = 3 individual mice). (B) SR-AI/II mRNA expression was measured by real-time RT-PCR. Analysis was performed as described with MARCO above (n = 3). **p < 0.01, ***p < 0.001.

MARCO is negatively regulated by PGE2 and is important for P. aeruginosa phagocytosis

To further understand how an altered SR profile on AMs post-BMT contributes to AM function, control AMs were treated with class A SR inhibitors, and AM phagocytic capacity against nonopsonized P. aeruginosa was determined. Interestingly, soluble MARCO (sMARCO) inhibited internalization of P. aeruginosa significantly compared with no treatment (no txt) and treatment with control compounds poly C and chondroitin sulfate (ChSO4) (Fig. 4A). Furthermore, the decrease in AM phagocytosis induced by sMARCO was comparable to the inhibition noted with the pan class A SR inhibitors (fucoidan and polyinosinic acid), suggesting that MARCO may be the primary SR for P. aeruginosa phagocytosis by AMs. Cytochalasin D is an inhibitor of actin polymerization and was included as a negative control for phagocytosis. Although SRCL mRNA expression decreased in syn BMT AMs (data not shown), blocking SRCL did not affect the uptake of nonopsonized P. aeruginosa in control AMs (Fig. 4A), suggesting that this receptor is not used for P. aeruginosa uptake. Soluble mannan did not inhibit P. aeruginosa engulfment either, suggesting that the mannose receptor is not involved in the recognition of P. aeruginosa (data not shown). Finally, Fig. 4B shows that control AMs treated with sMARCO exhibited a similar defect in engulfment of P. aeruginosa as did syn BMT AMs compared with untreated control AMs. This highlights MARCO as the dominant P. aeruginosa–recognizing SR in AMs.

FIGURE 4.
FIGURE 4.

MARCO is necessary for P. aeruginosa phagocytosis, and expression is regulated by PGE2 post-BMT. (A) AMs were harvested and pretreated with pan class A SR inhibitors (fucoidan, polyinosinic acid) or the control compounds poly C and ChSO4, as well as with anti-SRCL or sMARCO, and phagocytosis of P. aeruginosa was measured. Cytochalasin D (cyto D) inhibits phagocytosis by limiting actin polymerization. Values are presented relative to the no treatment (no txt) control set to 100% for one sample (n = 3/group). (B) Phagocytic ability of control, sMARCO-treated control, or syn BMT AMs was measured. Results are presented relative to the average of untransplanted control values set to 100% (n = 3). (C) AMs from control mice were treated for 24 h with 1 μM PGE2 or control DMSO (Veh). MARCO mRNA levels were determined by real-time RT-PCR. Expression of one control AM sample was set to n = 1 for comparisons (n = 3). **p < 0.01, ***p < 0.001.

Our laboratory showed previously that the defect in AMs post-BMT is mediated, in part, by upregulated COX-2 expression and overproduction of PGE2. Treatment with indomethacin, a COX inhibitor, rescues host defense in BMT mice in vivo and restores the ability of BMT AMs to phagocytose P. aeruginosa (12). However, it remains unknown whether decreased MARCO post-BMT is driven by PGE2. Therefore, untransplanted AMs were treated with 1 μM PGE2 or vehicle DMSO in vitro. After 24 h, RNA was harvested, and real-time RT-PCR analysis of MARCO mRNA expression indicated that decreased MARCO expression is driven by PGE2 (Fig. 4C).

SR-AI/II mediates phagocytosis of S. aureus, but not P. aeruginosa, by AMs

Syn BMT AMs exhibited increased S. aureus phagocytosis compared with control AMs (Fig. 2B), and SR-AI/II mRNA expression was increased post-BMT (Fig. 3B). To determine whether the observed AM phagocytosis of S. aureus may be due to increased SR-AI/II on macrophages following transplant, CHO cells (which do not express class A SRs) were transfected with SR-AI. Transfected and untransfected CHO cells were measured for internalization of S. aureus. SR-AI–transfected CHO cells were more efficient at phagocytosing S. aureus than were untransfected CHO cells (Fig. 5A, left panel), whereas P. aeruginosa internalization was unaffected by SR-AI transfection in CHO cells (Fig. 5A, right panel). These data verify that SR-AI is used for the recognition of S. aureus and explain why increases in SR-A do not influence P. aeruginosa uptake. To confirm this result post-BMT, AMs harvested from BMT mice that received HSCs from SR-AI/II−/− mice (SR-AI/II−/− BMT) were unable to phagocytose S. aureus compared with WT BMT AMs (Fig. 5B).

FIGURE 5.
FIGURE 5.

SR-AI/II is important for AM phagocytosis of S. aureus. (A) Phagocytosis of FITC–S. aureus (left panel) or FITC–P. aeruginosa (right panel) by control CHO or SR-A-I–transfected CHO cells was measured. Results are presented relative to the average of control CHO values set to 100% (n = 12/group). (B) Phagocytosis of FITC–S. aureus by WT BMT (n = 4) or SR-AI/II−/− BMT (n = 3) AMs was measured. Data are normalized to lactate dehydrogenase levels per well. Results are presented relative to the average of WT BMT values set to 100%. **p < 0.01, ***p < 0.001.

SR-AI/II negatively regulates MARCO expression

The fact that the phagocytic ability of AMs from SR-AI/II−/− BMT mice was not completely abolished (Fig. 5B) suggested that there may be some compensatory upregulation of MARCO expression in these mice that allowed phagocytosis of S. aureus. MARCO was shown to mediate S. aureus uptake (27, 28). To determine whether this were true, we compared the percentage of MARCO-expressing AMs in untransplanted WT and SR-AI/II−/− mice (Fig. 6A). In fact, SR-AI/II exerts a negative influence on MARCO expression. To study the effect of this in vivo, we created all combinations of chimeric BMT mice. In Fig. 6B, untransplanted control C57BL/6 mice or SR-AI/II−/− mice were compared with BMT mice created with the following donor→host combinations: WT→WT, SR-AI/II−/−→SR-AI/II−/−, or SR-AI/II−/−→WT. With regard to in vivo host defense against S. aureus, untransplanted SR-AI/II−/− mice had similar lung CFU as control C57BL/6 mice, presumably reflecting adequate MARCO expression in each strain at baseline. All BMT combinations were more susceptible than untransplanted mice, but there was no difference between the BMT groups. This suggests that uptake of S. aureus is mediated by either SR-AI/II or MARCO post-BMT as long as either receptor is available. This uptake was sufficient to maintain the infection in the lung, because no dissemination of S. aureus to the blood was noted (data not shown). In contrast, with regard to in vivo responses to P. aeruginosa, a bacterium that can be recognized by MARCO but not SR-A, SR-AI/II−/−→WT mice (which likely have a compensatory MARCO upregulation) showed decreased lung and blood CFU burden compared with WT→WT BMT mice. There was no difference between WT→WT and WT→SR-AI/II−/− BMT mice.

FIGURE 6.
FIGURE 6.

SR-AI/II negatively regulates MARCO expression. (A) MARCO protein expression was determined on AMs from untransplanted SR-AI/II−/− mice by flow cytometry (n = 3 individual mice). Untransplanted B6 (Ctrl), untransplanted SR-AI/II−/−, or chimeric BMT mice were challenged in vivo with S. aureus (n = 3–4) (B) or P. aeruginosa (n = 4–5) (C) via i.t. injection 24 h prior to lung and blood harvest for CFU analysis. *p < 0.05, **p < 0.01, ***p < 0.001.

SR-AI/II expression is regulated by PGE2 and miR-155 post-BMT

Fig. 5 suggests that SR-A plays a prominent role in the recognition and phagocytosis of S. aureus. However, it is unclear why SR-AI/II is upregulated post-BMT. To determine whether PGE2 signaling could upregulate SR-AI/II, control AMs were treated with PGE2 in vitro, and SR-AI/II mRNA expression was analyzed by real-time RT-PCR. Fig. 7A demonstrates that PGE2 signaling increases SR-AI/II mRNA expression. Increases in SR-AI/II mRNA in AMs post-BMT (Fig. 3B) [which have a high basal PGE2 production (12)] and in exogenous PGE2-treated macrophages (Fig. 7A) suggest that changes in the transcriptional machinery or actions of key miRNAs indeed play a role in PGE2-enhanced SR-AI/II expression. To our knowledge, this receptor does not have CREB (the main transcription factor induced by cAMP) secondary to PGE2 signaling within the promoter region. Rather, SR-A type I contains a predicted target sequence in its 3′ untranslated region for miR-155 (TargetScan). Thus, we hypothesized that miRNAs are involved in SR regulation post-BMT. Because SR-AI/II mRNA expression increased post-BMT and PGE2 treatment, the data suggested the loss of a regulatory miRNA.

FIGURE 7.
FIGURE 7.

miR-155 mRNA expression is decreased in syn BMT AMs. (A) SR-AI/II expression was measured in AMs treated with 10 μM of PGE2 or DMSO (Veh) for 24 h (n = 3). Expression of one control AM sample was set to n = 1 for comparisons. Similar results were also noted with treatment with 1 μM PGE2 (data not shown). (B) miR-155 expression was measured by real-time RT-PCR in syn BMT AMs (n = 2 samples pooled from multiple mice). Expression was normalized to snoR142, and one control AM sample was set to n = 1 for comparison. (C) Primary AMs were treated with 10 μM PGE2 or control DMSO (Veh) for 24 h prior to RNA isolation and cDNA conversion. miR-155 expression was compared with snoR142 (n = 2 samples pooled from multiple mice). *p < 0.05, ***p < 0.001.

Analysis of miRNA expression profiles with a focused miRNA array (SABiosciences) revealed miR-155 as the only miRNA significantly downregulated (∼7-fold post-syn BMT, Supplemental Fig. 1). To confirm that miR-155 was in fact decreased after BMT, miR-155 expression was determined by real-time RT-PCR using miR-155–specific primers. Our data show that there was a 6-fold decrease in miR-155 expression in syn BMT (Fig. 7B), which correlated well with the PCR array data. To determine whether the decrease in miR-155 expression was related to overproduction of PGE2 in the lung after BMT, control AMs were harvested and treated with 10 μM PGE2 for 24 h. miR-155 mRNA expression decreased upon treatment with PGE2 compared with AMs treated with DMSO control (Fig. 7C).

miR-155 regulates SR-AI/II expression and S. aureus phagocytosis

Because miR-155 is decreased post-BMT, regulation of SR-AI/II by miR-155 was further explored. An antagomir of miR-155 (anti–miR-155) was transfected into control AMs to inhibit miR-155 endogenous expression. SR-AI/II mRNA expression was increased upon transfection of anti–miR-155 compared with cells transfected with a control antisense oligomer (anti-miR) (Fig. 8A). Because SR-AI/II was shown in Fig. 5 to be important for S. aureus internalization, we were interested in understanding whether a decrease in miR-155 would have a functional effect on S. aureus internalization. AMs transfected with anti–miR-155 at increasing concentrations were measured for their ability to phagocytose S. aureus. Fig. 8B shows that S. aureus internalization increased dose-dependently with anti–miR-155. Transfection with anti–miR-155 had no effect on MARCO expression within a 24 h time point (data not shown).

FIGURE 8.
FIGURE 8.

Anti–miR-155–transfected AMs exhibit increased expression of SR-AI/II and increased phagocytosis of FITC-SA. (A) Primary AMs were transfected with either control anti-miR (10 nM) or anti–miR-155 (2 or 10 nM) 24 h prior to harvesting RNA. SR-AI/II mRNA expression was determined by real-time RT-PCR, and the expression of one control AM sample was set to n = 1 for comparison (n = 3–4 from three combined experiments). (B) Primary AMs were transfected with either control anti-miR (10 nM) or anti–miR-155 (2 or 10 nM) 48 h prior to the 2-h incubation with FITC-SA for phagocytosis measurement. Values are normalized to lactate dehydrogenase levels per well. Results are presented relative to the average of control values set to 100% (n = 3–4). *p < 0.05, **p < 0.01, ***p < 0.001.

Syn BMT AMs are compromised in intracellular killing of P. aeruginosa and S. aureus

Bacterial clearance by AMs relies on phagocytosis of the pathogen, as well as on intracellular killing of the organism. Although a defect in phagocytosis, as shown in Fig. 2A, would increase susceptibility to P. aeruginosa, AMs collected from syn BMTs are also unable to kill P. aeruginosa (Fig. 9A), as we reported previously (12). Because the BMT mice are more susceptible to S. aureus in vivo (Fig. 1B), yet display enhanced phagocytosis of this Gram-positive bacteria (Fig. 2B), we examined the ability of BMT AMs to kill ingested S. aureus; we found that killing of this pathogen was impaired in syn BMT AMs compared with control AMs (Fig. 9B). Thus, killing defects likely account for the in vivo susceptibility to infection, despite enhanced phagocytosis. Taken together, it is interesting to speculate that, because both phagocytosis and killing are defective for P. aeruginosa, this could explain the ∼4-log increase in bacterial susceptibility to this pathogen in BMT mice in vivo, whereas the improved phagocytosis of S. aureus moderates the killing defect to account for an ∼1-log difference in susceptibility to this pathogen in BMT mice in vivo.

FIGURE 9.
FIGURE 9.

Syn BMT mice have defective killing of P. aeruginosa and S. aureus. Primary AMs from untransplanted control B6 and syn BMT mice were assessed for killing of serum-opsonized P. aeruginosa (n = 8–10/group) (A) and S. aureus (n = 5–6/group) (B). Results are shown relative to the average of control values set to 100%. **p < 0.01.

Discussion

Syn BMT mice show increased in vivo susceptibility to both S. aureus and P. aeruginosa, with the latter observations confirming previously published data (11, 12). Although this impaired host response can be explained by an inability of BMT AMs to engulf P. aeruginosa, the same is not true for S. aureus. In fact, BMT AMs exhibit enhanced phagocytosis of S. aureus. This difference suggested that receptors mediating phagocytosis may have been altered following transplantation. Although MARCO and SR-AI/II were both reported to have the ability to recognize Gram-positive and Gram-negative bacteria (25–27, 29), and SR-AI/II was linked to recognition of S. aureus (26), it was unclear which receptor was important for P. aeruginosa in control cells, and nothing was known about the regulation of these SRs post-BMT. In this study, we show that MARCO is significantly downregulated at both the mRNA and protein levels, whereas SR-AI/II mRNA expression is increased post-BMT. These data indicate that MARCO is important for P. aeruginosa internalization, because elevated levels of SR-AI/II were unable to rescue phagocytosis of this Gram-negative pathogen in BMT AMs or CHO cells. This observation was further supported by the finding that control AMs pretreated with sMARCO were unable to phagocytose P. aeruginosa as efficiently.

A surprising result was the enhanced phagocytosis of S. aureus given that BMT mice were more susceptible to infection in vivo. Because SR-AI/II can regulate S. aureus phagocytosis (26), increases in SR-AI/II may explain this result. We show that CHO cells expressing SR-AI only were able to phagocytose S. aureus but not P. aeruginosa. Furthermore, recipient mice reconstituted with SR-AI/II−/− donor marrow showed a reduction in S. aureus phagocytosis compared with mice transplanted with WT (C57BL/6) marrow (Fig. 5B). These data support the importance of SR-AI/II for S. aureus and MARCO for P. aeruginosa phagocytosis. Interestingly, the fact that phagocytosis of S. aureus in SR-AI/II−/− BMT mice was not completely abolished suggested that there may be some compensatory upregulation of MARCO in these mice, because MARCO was shown to recognize S. aureus (27, 28). These data highlight an interesting inhibitory role for SR-AI/II on MARCO expression that we verified in Fig. 6A. SR-AI/II−/− untransplanted mice have a higher percentage of MARCO+ cells in the absence of infection. Therefore, it is also possible that the overexpression of SR-AI/II observed post-BMT may contribute to the decreased expression of MARCO in addition to the PGE2 signaling. We believe that the inhibitory effects of SR-AI/II on MARCO may not be immediate, because transfection of the anti–miR-155, which upregulated SR-AI/II expression within 24 h (Fig. 8A), did not yet alter MARCO expression, whereas PGE2 stimulation reduced MARCO in 24 h (Fig. 4C). Ultimately, SR-AI/II−/− BMT mice may upregulate MARCO to improve their host defense against both pathogens, which is consistent with the observations in Fig. 6. Obviously, the coregulation of these receptors is complex, but on the whole our in vivo results are consistent with the conclusion that MARCO is uniquely responsible for P. aeruginosa uptake, whereas S. aureus can be recognized by either MARCO or SRAI/II. Our findings of SR-AI/II inhibition of MARCO expression are also consistent with previously published data that suggest SR-AI/II may have an inhibitory role on inflammatory responses (30, 31).

miRNA analysis of BMT AMs indicated that altered miRNA expression may be responsible for the upregulation of SRAI/II post-BMT. Specifically, miR-155 expression was decreased 6-fold, and SR-AI contains a putative target sequence for miR-155 in its 3′ untranslated region. Because miRNAs generally function to destabilize or inhibit protein translation of the targets that they regulate, we hypothesized that decreased expression of miR-155 would be responsible for the increase in SR-AI/II observed post-BMT. Anti–mir-155–transfected AMs showed a significant increase in SR-AI/II expression. To determine whether this correlated with the functional phenotype observed after transplant, anti–mir-155–transfected AMs measured for phagocytosis of S. aureus exhibited a dose-dependent increase in S. aureus internalization. This provided convincing evidence that, after transplant, the increase in SR-AI/II was regulated by decreased miR-155 expression. Our data also demonstrate that this altered miRNA expression profile is secondary to PGE2 signaling. PGE2 also decreases MARCO, but the mechanism does not seem to involve miR-155. This finding provides new insight into intracellular regulation of SRs by miRNA. A recent study suggested miR-155 as a novel therapeutic target for improving graft-versus-host disease (32). This is also likely to cause an inhibition of SR-AI/II, which may impair phagocytosis of some pathogens, and the long-term effects of this treatment on MARCO expression would need to be evaluated.

COX-2 and PGE2 are elevated following transplantation (12, 33), and inhibition of COX-2 by indomethacin treatment rescues host defense (12). PGE2 production is not influenced by SR expression, because PGE2 levels were similarly elevated in all chimeric BMT mice in this study (data not shown); however, PGE2 can dramatically alter SR profiles and inhibit bacterial killing. We show that in vitro treatment with PGE2 decreases miR-155 and MARCO while increasing SR-AI/II expression. Although increased SR-AI/II expression enhances S. aureus phagocytosis, syn BMT mice remain susceptible to bacteria in vivo. Further investigation showed that effective killing of both P. aeruginosa and S. aureus was impaired in syn BMT mice compared with their untransplanted controls. Therefore, a better understanding of how killing is impaired remains an area of future investigation. We showed previously that BMT AMs express an immature phenotype (low CD11a and CD11c, high CD11b) (11) and that the bronchoalveolar lavage fluid of BMT mice contains aberrant mediator expression (elevated IL-6, GM-CSF, and PGE2; decreased TNFα, IFN-γ, H2O2, and leukotrienes) (11, 12, 22, 34). Overall, BMT AMs seem impaired in their ability to become activated, which is likely important for intracellular bacterial killing. Phagocytosis and killing function are rescued upon treatment with indomethacin (12), indicating that the impairment is due to overexpression of the cyclooxygenase pathway. Microarray analysis also revealed increased expression of miR-27b (8-fold) and miR-29b (17-fold) (Supplemental Fig 1). However, overexpression of these miRNA on primary AMs did not induce a change in expression of either SR-AI/II or MARCO, indicating that these miRNAs are not involved in regulating SR profiles (data not shown). However, NF-κB suppresses miR-29b expression (35), and miR-27b was found to inhibit NF-κB translocation into the nucleus (36), supporting the idea that these miRNA function in an anti-inflammatory, and perhaps functionally suppressive manner, and that their expression pattern is influenced by BMT. Interestingly, miR-29b was shown to directly regulate DNA methyltransferases and affect COX-2 expression in lung epithelial cells (37). Thus, upregulation of miR-29b may be promoting COX-2 overexpression in BMT AMs, because the COX-2 promoter is significantly hypomethylated following transplantation (13). We show that PGE2 produced by the COX pathway can decrease miR-155 and affect SR-AI/II expression. Furthermore, miR-155 was shown to regulate TNF-α by mRNA stabilization. TNF-α is important for macrophage activation and immune function, and it is decreased post-BMT (22). Our results suggest that miR-155 loss post-BMT may also play a role in destabilizing TNF-α. These differences related to dysregulated miRNA expression may be responsible, in part, for the defective killing post-BMT.

Taken together, our results show that AMs undergo significant functional alterations in the setting of syn BMT. We showed previously that DNA hypomethylation accounts for the increased production of PGE2 in AMs post-BMT (13). Our current studies highlight the fact that elevated PGE2 leads to alterations in the SR profile (decreases MARCO and increases SR-AI/II via downregulation of miR-155). These epigenetic changes differentially affect phagocytosis of P. aeruginosa and S. aureus; however, bacterial killing of both pathogens is impaired post-BMT. It is important to note that these changes observed in AMs post-BMT are only seen in the setting of myeloablative conditioning. Tarling et al. (38) suggested that AMs derive from a lung resident stem cell that is naturally radioresistant. At lower doses of radiation, AMs likely repopulate from these radioresistant host stem cells and only upon myeloablative conditioning (8-13 Gy, or using high-dose chemotherapy combinations) did AM reconstitution derive from the donor bone marrow, resulting in the PGE2-induced impaired function (16). Thus, interventions to limit PGE2 production post-HSCT or to reverse these epigenetic alterations may improve outcomes for patients developing bacterial infections.

Disclosures

The authors have no financial conflicts of interest.

Acknowledgments

We thank Michelle Reed for early help with the class A SR inhibitors and Carol Wilke for assistance with BMT protocols.

Footnotes

  • This work was supported by National Institutes of Health Grants R56AI065543 (to B.B.M.), HL-103777-03 (to C.H.S.), T32AI007413 (to R.D.-G.), and R25HL108842 (to S.K.) and an award from the American Heart Association (to R.D.-G.).

  • The online version of this article contains supplemental material.

  • Abbreviations used in this article:

    AM
    alveolar macrophage
    B6
    C57BL/6
    BMT
    bone marrow transplantation
    HSC
    hematopoietic stem cell
    HSCT
    hematopoietic stem cell transplantation
    i.t.
    intratracheal(ly)
    miR-155
    microRNA-155
    miRNA
    microRNA
    sMARCO
    soluble MARCO
    SR
    scavenger receptor
    syn
    syngeneic
    WT
    wild type.

  • Received November 28, 2012.
  • Accepted March 29, 2013.

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