Pseudomonas aeruginosa pulmonary infection results in S100A8/A9-dependent cardiac dysfunction.

Pseudomonas aeruginosa (P.a.) infection accounts for nearly 20% of all cases of hospital acquired pneumonia with mortality rates >30%. P.a. infection induces a robust inflammatory response, which ideally enhances bacterial clearance. Unfortunately, excessive inflammation can also have negative effects, and often leads to cardiac dysfunction with associated morbidity and mortality. However, it remains unclear how P.a. lung infection causes cardiac dysfunction. Using a murine pneumonia model, we found that P.a. infection of the lungs led to severe cardiac left ventricular dysfunction and electrical abnormalities. More specifically, we found that neutrophil recruitment and release of S100A8/A9 in the lungs activates the TLR4/RAGE signaling pathways, which in turn enhance systemic inflammation and subsequent cardiac dysfunction. Paradoxically, global deletion of S100A8/A9 did not improve but aggravated cardiac dysfunction and mortality likely due to uncontrolled bacterial burden in the lungs and heart. Our results indicate that P.a. infection induced release of S100A8/9 is double-edged, providing increased risk for cardiac dysfunction yet limiting P.a. growth.

The Renin-Angiotensin-Aldosterone System Regulates Sarcoidosis Granulomatous Inflammation.

Rationale: Sarcoidosis is a granulomatous disorder of unclear cause notable for abnormal elevation of blood and tissue ACE1 (angiotensin converting enzyme 1) levels and activity. ACE1 regulates the renin-angiotensin-aldosterone system (RAAS), the terminal product of which is aldosterone, which selectively engages mineralocorticoid receptors to promote inflammation. Objectives: We sought to determine whether the RAAS promotes sarcoidosis granuloma formation and related inflammatory responses. Methods: Using an established ex vivo model, we first determined whether aldosterone was produced by sarcoidosis granulomas and verified the presence of CYP11B2, the enzyme required for its production. We then evaluated the effects of selective inhibitors of ACE1 (captopril), angiotensin type 1 receptor (losartan), and mineralocorticoid receptors (spironolactone, eplerenone) on granuloma formation, reflected by computer image analysis-generated granuloma area, and selected cytokines incriminated in sarcoidosis pathogenesis. Measurements and Main Results: Aldosterone was spontaneously produced by sarcoidosis peripheral blood mononuclear cells, and both intra- and extracellular levels steadily increased during granuloma formation. In parallel, peripheral blood mononuclear cells were shown to express more CYP11B2 during granuloma formation. Significant inhibition of sarcoidosis granulomas and related cytokines (TNFα, IL-1ß, IFNγ, IL-10) was observed in response to pretreatments with captopril, losartan, spironolactone, or eplerenone, comparable to that of prednisone. Conclusions: The RAAS is intact in sarcoidosis granulomas and contributes significantly to early granuloma formation and to related inflammatory mediator responses, with important implications for clinical management.

Accelerated Clearance and Degradation of Cell-Free HIV by Neutralizing Antibodies Occurs via FcγRIIb on Liver Sinusoidal Endothelial Cells by Endocytosis.

Neutralizing Abs suppress HIV infection by accelerating viral clearance from blood circulation in addition to neutralization. The elimination mechanism is largely unknown. We determined that human liver sinusoidal endothelial cells (LSEC) express FcγRIIb as the lone Fcγ receptor, and using humanized FcγRIIb mouse, we found that Ab-opsonized HIV pseudoviruses were cleared considerably faster from circulation than HIV by LSEC FcγRIIb. Compared with humanized FcγRIIb-expressing mice, HIV clearance was significantly slower in FcγRIIb knockout mice. Interestingly, a pentamix of neutralizing Abs cleared HIV faster compared with hyperimmune anti-HIV Ig (HIVIG), although the HIV Ab/Ag ratio was higher in immune complexes made of HIVIG and HIV than pentamix and HIV. The effector mechanism of LSEC FcγRIIb was identified to be endocytosis. Once endocytosed, both Ab-opsonized HIV pseudoviruses and HIV localized to lysosomes. This suggests that clearance of HIV, endocytosis, and lysosomal trafficking within LSEC occur sequentially and that the clearance rate may influence downstream events. Most importantly, we have identified LSEC FcγRIIb-mediated endocytosis to be the Fc effector mechanism to eliminate cell-free HIV by Abs, which could inform development of HIV vaccine and Ab therapy.

IFITM3 protects the heart during influenza virus infection.

Influenza virus can disseminate from the lungs to the heart in severe infections and can induce cardiac pathology, but this has been difficult to study due to a lack of small animal models. In humans, polymorphisms in the gene encoding the antiviral restriction factor IFN-induced transmembrane protein 3 (IFITM3) are associated with susceptibility to severe influenza, but whether IFITM3 deficiencies contribute to cardiac dysfunction during infection is unclear. We show that IFITM3 deficiency in a new knockout (KO) mouse model increases weight loss and mortality following influenza virus infections. We investigated this enhanced pathogenesis with the A/PR/8/34 (H1N1) (PR8) influenza virus strain, which is lethal in KO mice even at low doses, and observed increased replication of virus in the lungs, spleens, and hearts of KO mice compared with wild-type (WT) mice. Infected IFITM3 KO mice developed aberrant cardiac electrical activity, including decreased heart rate and irregular, arrhythmic RR (interbeat) intervals, whereas WT mice exhibited a mild decrease in heart rate without irregular RR intervals. Cardiac electrical dysfunction in PR8-infected KO mice was accompanied by increased activation of fibrotic pathways and fibrotic lesions in the heart. Infection with a sublethal dose of a less virulent influenza virus strain (A/WSN/33 [H1N1]) resulted in a milder cardiac electrical dysfunction in KO mice that subsided as the mice recovered. Our findings reveal an essential role for IFITM3 in limiting influenza virus replication and pathogenesis in heart tissue and establish IFITM3 KO mice as a powerful model for studying mild and severe influenza virus-induced cardiac dysfunction.

Identification of an Increased Alveolar Macrophage Subpopulation in Old Mice That Displays Unique Inflammatory Characteristics and Is Permissive to Mycobacterium tuberculosis Infection.

The elderly population is more susceptible to pulmonary infections, including tuberculosis. In this article, we characterize the impact of aging on the phenotype of mouse alveolar macrophages (AMs) and their response to Mycobacterium tuberculosis. Uninfected AMs were isolated from bronchoalveolar lavage of young (3 mo) and old (18 mo) C57BL/6 mice. AMs from old mice expressed higher mRNA levels of CCL2, IFN-ß, IL-10, IL-12p40, TNF-α, and MIF than young mice, and old mice contained higher levels of CCL2, IL-1ß, IFN-ß, and MIF in their alveolar lining fluid. We identified two distinct AM subpopulations, a major CD11c+ CD11b- population and a minor CD11c+ CD11b+ population; the latter was significantly increased in old mice (4-fold). Expression of CD206, TLR2, CD16/CD32, MHC class II, and CD86 was higher in CD11c+ CD11b+ AMs, and these cells expressed monocytic markers Ly6C, CX3CR1, and CD115, suggesting monocytic origin. Sorted CD11c+ CD11b+ AMs from old mice expressed higher mRNA levels of CCL2, IL-1ß, and IL-6, whereas CD11c+ CD11b- AMs expressed higher mRNA levels of immune-regulatory cytokines IFN-ß and IL-10. CD11c+ CD11b+ AMs phagocytosed significantly more M. tuberculosis, which expressed higher RNA levels of genes required for M. tuberculosis survival. Our studies identify two distinct AM populations in old mice: a resident population and an increased CD11c+ CD11b+ AM subpopulation expressing monocytic markers, a unique inflammatory signature, and enhanced M. tuberculosis phagocytosis and survival when compared with resident CD11c+ CD11b- AMs, which are more immune regulatory in nature.

Effect of Mycobacterium tuberculosis Enhancement of Macrophage P-Glycoprotein Expression and Activity on Intracellular Survival During Antituberculosis Drug Treatment.

BACKGROUND: Tuberculosis is caused by Mycobacterium tuberculosis. Recent emergence of multidrug-resistant (MDR) tuberculosis strains seriously threatens tuberculosis control and prevention. However, the role of macrophage multidrug resistance gene MDR1 on intracellular M. tuberculosis survival during antituberculosis drug treatment is not known. METHODS: We used the human monocyte-derived macrophages to study the role of M. tuberculosis in regulation of MDR1 and drug resistance. RESULTS: We discovered that M. tuberculosis infection increases the expression of macrophage MDR1 to extrude various chemical substances, including tuberculosis drugs, resulting in enhanced survival of intracellular M. tuberculosis. The pathway of regulation involves M. tuberculosis infection of macrophages and suppression of heat shock factor 1, a transcriptional regulator of MDR1 through the up-regulation of miR-431. Notably, nonpathogenic Mycobacterium smegmatis did not increase MDR1 expression, indicating active secretion of virulence factors in pathogenic M. tuberculosis contributing to this phenotype. Finally, inhibition of MDR1 improves antibiotic-mediated killing of M. tuberculosis. CONCLUSION: We report a novel finding that M. tuberculosis up-regulates MDR1 during infection, which limits the exposure of M. tuberculosis to sublethal concentrations of antimicrobials. This condition promotes M. tuberculosis survival and potentially enhances the emergence of resistant variants.

The Lung Mucosa Environment in the Elderly Increases Host Susceptibility to Mycobacterium tuberculosis Infection.

As we age, there is an increased risk for the development of tuberculosis (TB) caused by Mycobacterium tuberculosis (Mtb) infection. Few studies consider that age-associated changes in the alveolar lining fluid (ALF) may increase susceptibility by altering soluble mediators of innate immunity. We assessed the impact of adult or elderly human ALF during Mtb infection in vitro and in vivo. We identified amplification of pro-oxidative and proinflammatory pathways in elderly ALF and decreased binding capability of surfactant-associated surfactant protein A (SP-A) and surfactant protein D (SP-D) to Mtb. Human macrophages infected with elderly ALF-exposed Mtb had reduced control and fewer phagosome-lysosome fusion events, which was reversed when elderly ALF was replenished with functional SP-A/SP-D. In vivo, exposure to elderly ALF exacerbated Mtb infection in young mice. Our studies demonstrate how the pulmonary environment changes as we age and suggest that Mtb may benefit from declining host defenses in the lung mucosa of the elderly.

CD36-Mediated Uptake of Surfactant Lipids by Human Macrophages Promotes Intracellular Growth of Mycobacterium tuberculosis.

Mycobacterium tuberculosis imposes a large global health burden as the airborne agent of tuberculosis. Mycobacterium tuberculosis has been flourishing in human populations for millennia and is therefore highly adapted to the lung environment. Alveolar macrophages, a major host cell niche for M. tuberculosis, are not only phagocytose inhaled microbes and particulate matter but are also crucial in catabolizing lung surfactant, a lipid-protein complex that lines the alveolar spaces. Because macrophage host defense properties can be regulated by surfactant and M. tuberculosis can use host lipids as a carbon source during infection, we sought to determine the receptor(s) involved in surfactant lipid uptake by human macrophages and whether the presence of those lipids within macrophages prior to infection with M. tuberculosis enhances bacterial growth. We show that preformed scavenger receptor CD36 is redistributed to the cell membrane following exposure to surfactant lipids and surfactant protein A. Subsequently, surfactant lipids and/or surfactant protein A enhance CD36 transcript and protein levels. We show that CD36 participates in surfactant lipid uptake by human macrophages, as CD36 knockdown reduces uptake of dipalmitoylphosphatidylcholine, the most prevalent surfactant lipid species. Finally, exposing human macrophages to surfactant lipids prior to infection augments M. tuberculosis growth in a CD36-dependent manner. Thus, we provide evidence that CD36 mediates surfactant lipid uptake by human macrophages and that M. tuberculosis exploits this function for growth.

Blood-Borne Lipopolysaccharide Is Rapidly Eliminated by Liver Sinusoidal Endothelial Cells via High-Density Lipoprotein.

During Gram-negative bacterial infections, excessive LPS induces inflammation and sepsis via action on immune cells. However, the bulk of LPS can be cleared from circulation by the liver. Liver clearance is thought to be a slow process mediated exclusively by phagocytic resident macrophages, Kupffer cells (KC). However, we discovered that LPS disappears rapidly from the circulation, with a half-life of 2-4 min in mice, and liver eliminates about three quarters of LPS from blood circulation. Using microscopic techniques, we found that ∼75% of fluor-tagged LPS in liver became associated with liver sinusoidal endothelial cells (LSEC) and only ∼25% with KC. Notably, the ratio of LSEC-KC-associated LPS remained unchanged 45 min after infusion, indicating that LSEC independently processes the LPS. Most interestingly, results of kinetic analysis of LPS bioactivity, using modified limulus amebocyte lysate assay, suggest that recombinant factor C, an LPS binding protein, competitively inhibits high-density lipoprotein (HDL)-mediated LPS association with LSEC early in the process. Supporting the previous notion, 3 min postinfusion, 75% of infused fluorescently tagged LPS-HDL complex associates with LSEC, suggesting that HDL facilitates LPS clearance. These results lead us to propose a new paradigm of LSEC and HDL in clearing LPS with a potential to avoid inflammation during sepsis.

Macrophage immunoregulatory pathways in tuberculosis.

Macrophages, the major host cells harboring Mycobacterium tuberculosis (M.tb), are a heterogeneous cell type depending on their tissue of origin and host they are derived from. Significant discord in macrophage responses to M.tb exists due to differences in M.tb strains and the various types of macrophages used to study tuberculosis (TB). This review will summarize current concepts regarding macrophage responses to M.tb infection, while pointing out relevant differences in experimental outcomes due to the use of divergent model systems. A brief description of the lung environment is included since there is increasing evidence that the alveolar macrophage (AM) has immunoregulatory properties that can delay optimal protective host immune responses. In this context, this review focuses on selected macrophage immunoregulatory pattern recognition receptors (PRRs), cytokines, negative regulators of inflammation, lipid mediators and microRNAs (miRNAs).

γ-Tilmanocept, a New Radiopharmaceutical Tracer for Cancer Sentinel Lymph Nodes, Binds to the Mannose Receptor (CD206).

γ-Tilmanocept ((99m)Tc-labeled-tilmanocept or [(99m)Tc]-tilmanocept) is the first mannose-containing, receptor-directed, radiolabeled tracer for the highly sensitive imaging of sentinel lymph nodes in solid tumor staging. To elucidate the mannose-binding receptor that retains tilmanocept in this microenvironment, human macrophages were used that have high expression of the C-type lectin mannose receptor (MR; CD206). Cy3-labeled tilmanocept exhibited high specificity binding to macrophages that was nearly abolished in competitive inhibition experiments. Furthermore, Cy3-tilmanocept binding was markedly reduced on macrophages deficient in the MR by small interfering RNA treatment and was increased on MR-transfected HEK 293 cells. Finally, confocal microscopy revealed colocalization of Cy3-tilmanocept with the macrophage membrane MR and binding of labeled tilmanocept to MR(+) cells (macrophages and/or dendritic cells) in human sentinel lymph node tissues. Together these data provide strong evidence that CD206 is a major binding receptor for γ-tilmanocept. Identification of CD206 as the γ-tilmanocept-binding receptor enables opportunities for designing receptor-targeted advanced imaging agents and therapeutics for cancer and other diseases.

Mycobacterium tuberculosis decreases human macrophage IFN-γ responsiveness through miR-132 and miR-26a.

IFN-γ-activated macrophages play an essential role in controlling intracellular pathogens; however, macrophages also serve as the cellular home for the intracellular pathogen Mycobacterium tuberculosis. Based on previous evidence that M. tuberculosis can modulate host microRNA (miRNA) expression, we examined the miRNA expression profile of M. tuberculosis-infected primary human macrophages. We identified 31 differentially expressed miRNAs in primary human macrophages during M. tuberculosis infection by NanoString and confirmed our findings by quantitative real-time RT-PCR. In addition, we determined a role for two miRNAs upregulated upon M. tuberculosis infection, miR-132 and miR-26a, as negative regulators of transcriptional coactivator p300, a component of the IFN-γ signaling cascade. Knockdown expression of miR-132 and miR-26a increased p300 protein levels and improved transcriptional, translational, and functional responses to IFN-γ in human macrophages. Collectively, these data validate p300 as a target of miR-132 and miR-26a, and demonstrate a mechanism by which M. tuberculosis can limit macrophage responses to IFN-γ by altering host miRNA expression.

Fine tuning inflammation at the front door: macrophage complement receptor 3-mediates phagocytosis and immune suppression for Francisella tularensis.

Complement receptor 3 (CR3, CD11b/CD18) is a major macrophage phagocytic receptor. The biochemical pathways through which CR3 regulates immunologic responses have not been fully characterized. Francisella tularensis is a remarkably infectious, facultative intracellular pathogen of macrophages that causes tularemia. Early evasion of the host immune response contributes to the virulence of F. tularensis and CR3 is an important receptor for its phagocytosis. Here we confirm that efficient attachment and uptake of the highly virulent Type A F. tularensis spp. tularensis strain Schu S4 by human monocyte-derived macrophages (hMDMs) requires complement C3 opsonization and CR3. However, despite a>40-fold increase in uptake following C3 opsonization, Schu S4 induces limited pro-inflammatory cytokine production compared with non-opsonized Schu S4 and the low virulent F. novicida. This suggests that engagement of CR3 by opsonized Schu S4 contributes specifically to the immune suppression during and shortly following phagocytosis which we demonstrate by CD11b siRNA knockdown in hMDMs. This immune suppression is concomitant with early inhibition of ERK1/2, p38 MAPK and NF-κB activation. Furthermore, TLR2 siRNA knockdown shows that pro-inflammatory cytokine production and MAPK activation in response to non-opsonized Schu S4 depends on TLR2 signaling providing evidence that CR3-TLR2 crosstalk mediates immune suppression for opsonized Schu S4. Deletion of the CD11b cytoplasmic tail reverses the CR3-mediated decrease in ERK and p38 activation during opsonized Schu-S4 infection. The CR3-mediated signaling pathway involved in this immune suppression includes Lyn kinase and Akt activation, and increased MKP-1, which limits TLR2-mediated pro-inflammatory responses. These data indicate that while the highly virulent F. tularensis uses CR3 for efficient uptake, optimal engagement of this receptor down-regulates TLR2-dependent pro-inflammatory responses by inhibiting MAPK activation through outside-in signaling. CR3-linked immune suppression is an important mechanism involved in the pathogenesis of F. tularensis infection.

N-glycolylated peptidoglycan contributes to the immunogenicity but not pathogenicity of Mycobacterium tuberculosis.

Mycobacteria produce an unusual, glycolylated form of muramyl dipeptide (MDP) that is more potent and efficacious at inducing NOD2-mediated host responses. We tested the importance of this modified form of MDP in Mycobacterium tuberculosis by disrupting the gene, namH, responsible for this modification. In vitro, the namH mutant did not produce N-glycolylated muropeptides, but there was no alteration in colony morphology, growth kinetics, cellular morphology, or mycolic acid profile. Ex vivo, the namH mutant survived and replicated normally in murine and human macrophages, yet induced diminished production of tumor necrosis factor α. In vivo, namH disruption did not affect the bacterial burden during infection of C57BL/6 mice or cellular recruitment to the lungs but modestly prolonged survival after infection in Rag1(-/-) mice. These results indicate that the modified MDP is an important contributor to the unusual immunogenicity of mycobacteria but has a limited role in the pathogenesis of M. tuberculosis infection.

Estrogen modulation of endosome-associated toll-like receptor 8: an IFNα-independent mechanism of sex-bias in systemic lupus erythematosus.

Females of child-bearing age are more resistant to infectious disease and have an increased risk of systemic lupus erythematosus (SLE). We hypothesized that estrogen-induced gene expression could establish an immunoactivated state which would render enhanced defense against infection, but may be deleterious in autoimmune development. Using peripheral blood mononuclear cells (PBMCs), we demonstrate enhanced responses with immunogen stimulation in the presence of 17ß-estradiol (E2) and gene array analyses reveal toll-like receptor 8 (TLR8) as an E2-responsive candidate gene. TLR8 expression levels are up-regulated in SLE and PBMCs stimulated with TLR8 agonist display a female sex-biased, E2-sensitive response. Moreover, we identify a putative ERα-binding region near the TLR8 locus and blocking ERα expression significantly decreases E2-mediated TLR8 induction. Our findings characterize TLR8 as a novel estrogen target gene that can lower the inflammatory threshold and implicate an IFNα-independent inflammatory mechanism that could contribute to higher SLE incidence in women.

Mycobacterium tuberculosis lipomannan blocks TNF biosynthesis by regulating macrophage MAPK-activated protein kinase 2 (MK2) and microRNA miR-125b.

Contact of Mycobacterium tuberculosis (M.tb) with the immune system requires interactions between microbial surface molecules and host pattern recognition receptors. Major M.tb-exposed cell envelope molecules, such as lipomannan (LM), contain subtle structural variations that affect the nature of the immune response. Here we show that LM from virulent M.tb (TB-LM), but not from avirulent Myocobacterium smegmatis (SmegLM), is a potent inhibitor of TNF biosynthesis in human macrophages. This difference in response is not because of variation in Toll-like receptor 2-dependent activation of the signaling kinase MAPK p38. Rather, TB-LM stimulation leads to destabilization of TNF mRNA transcripts and subsequent failure to produce TNF protein. In contrast, SmegLM enhances MAPK-activated protein kinase 2 phosphorylation, which is critical for maintaining TNF mRNA stability in part by contributing microRNAs (miRNAs). In this context, human miRNA miR-125b binds to the 3' UTR region of TNF mRNA and destabilizes the transcript, whereas miR-155 enhances TNF production by increasing TNF mRNA half-life and limiting expression of SHIP1, a negative regulator of the PI3K/Akt pathway. We show that macrophages incubated with TB-LM and live M.tb induce high miR-125b expression and low miR-155 expression with correspondingly low TNF production. In contrast, SmegLM and live M. smegmatis induce high miR-155 expression and low miR-125b expression with high TNF production. Thus, we identify a unique cellular mechanism underlying the ability of a major M.tb cell wall component, TB-LM, to block TNF biosynthesis in human macrophages, thereby allowing M.tb to subvert host immunity and potentially increase its virulence.

The Many Hosts of Mycobacteria 9 (MHM9): A conference report.

The Many Hosts of Mycobacteria (MHM) meeting series brings together basic scientists, clinicians and veterinarians to promote robust discussion and dissemination of recent advances in our knowledge of numerous mycobacterial diseases, including human and bovine tuberculosis (TB), nontuberculous mycobacteria (NTM) infection, Hansen's disease (leprosy), Buruli ulcer and Johne's disease. The 9th MHM conference (MHM9) was held in July 2022 at The Ohio State University (OSU) and centered around the theme of "Confounders of Mycobacterial Disease." Confounders can and often do drive the transmission of mycobacterial diseases, as well as impact surveillance and treatment outcomes. Various confounders were presented and discussed at MHM9 including those that originate from the host (comorbidities and coinfections) as well as those arising from the environment (e.g., zoonotic exposures), economic inequality (e.g. healthcare disparities), stigma (a confounder of leprosy and TB for millennia), and historical neglect (a confounder in Native American Nations). This conference report summarizes select talks given at MHM9 highlighting recent research advances, as well as talks regarding the historic and ongoing impact of TB and other infectious diseases on Native American Nations, including those in Southwestern Alaska where the regional TB incidence rate is among the highest in the Western hemisphere.

Structural differences in lipomannans from pathogenic and nonpathogenic mycobacteria that impact CD1b-restricted T cell responses.

Mannosylated molecules on the Mycobacterium tuberculosis surface are important determinants in the immunopathogenesis of tuberculosis. To date, much attention has been paid to mannose-capped lipoarabinomannan, which mediates phagocytosis and intracellular trafficking of M. tuberculosis by engaging the macrophage mannose receptor and subsequently binds to intracellular CD1b molecules for presentation to T cells. Another important mannosylated lipoglycan on the M. tuberculosis surface is lipomannan (LM). Comparative structural detail of the LMs from virulent and avirulent strains is limited as is knowledge regarding their differential capacity to be recognized by the adaptive immune response. Here, we purified LM from the avirulent M. smegmatis and the virulent M. tuberculosis H(37)R(v), performed a comparative structural biochemical analysis, and addressed their ability to stimulate CD1b-restricted T cell clones. We found that M. tuberculosis H(37)R(v) produces a large neutral LM (TB-LM); in contrast, M. smegmatis produces a smaller linear acidic LM (SmegLM) with a high succinate content. Correspondingly, TB-LM was not as efficiently presented to CD1b-restricted T cells as SmegLM. Thus, here we correlate the structure-function relationships for LMs with CD1b-restricted T cell responses and provide evidence that the structural features of TB-LM contribute to its diminished T cell responsiveness.

Pulmonary surfactant protein A and surfactant lipids upregulate IRAK-M, a negative regulator of TLR-mediated inflammation in human macrophages.

Alveolar macrophages (AMs) are exposed to frequent challenges from inhaled particulates and microbes and function as a first line of defense with a highly regulated immune response because of their unique biology as prototypic alternatively activated macrophages. Lung collectins, particularly surfactant protein A (SP-A), contribute to this activation state by fine-tuning the macrophage inflammatory response. During short-term (10 min-2 h) exposure, SP-A's regulation of human macrophage responses occurs through decreased activity of kinases required for proinflammatory cytokine production. However, AMs are continuously exposed to surfactant, and the biochemical pathways underlying long-term reduction of proinflammatory cytokine activity are not known. We investigated the molecular mechanism(s) underlying SP-A- and surfactant lipid-mediated suppression of proinflammatory cytokine production in response to Toll-like receptor (TLR) 4 (TLR4) activation over longer time periods. We found that exposure of human macrophages to SP-A for 6-24 h upregulates expression of IL-1 receptor-associated kinase M (IRAK-M), a negative regulator of TLR-mediated NF-κB activation. Exposure to Survanta, a natural bovine lung extract lacking SP-A, also enhances IRAK-M expression, but at lower magnitude and for a shorter duration than SP-A. Surfactant-mediated upregulation of IRAK-M in macrophages suppresses TLR4-mediated TNF-α and IL-6 production in response to LPS, and IRAK-M knockdown by small interfering RNA reverses this suppression. In contrast to TNF-α and IL-6, the surfactant components upregulate LPS-mediated immunoregulatory IL-10 production, an effect reversed by IRAK-M knockdown. In conclusion, these data identify an important signaling regulator in human macrophages that is used by surfactant to control the long-term alveolar inflammatory response, i.e., enhanced IRAK-M activity.