The role of surfactant-associated protein A in pulmonary host defense

Targeted RNA-Seq Reveals the M. tuberculosis Transcriptome from an In Vivo Infection Model

Simple Summary

High-throughput sequencing techniques such as RNA-seq allow a more detailed characterization of the gene expression profile during in vivo infections. However, using this strategy for intracellular pathogens such as Mycobacterium tuberculosis (Mtb) entails technical limitations. Some authors have resorted to flow cytometers to separate infected cells or significantly increase sequencing depth to obtain pathogens’ gene expression. However, these options carry additional expenses in specialized equipment. We propose an experimental protocol based on differential cell lysis and a probe-based ribosomal depletion to determine the gene expression of Mtb and its host during in vivo infection. This method allowed us to increase the number of observed expressed genes from 13 using a traditional RNA-seq approach to 702. In addition, we observed the expression of genes essential for establishing the infection, codifying proteins such as PE-PGRS, lipoproteins lppN and LpqH, and three ncRNAs (small RNA MTS2823, transfer-messenger RNA RF00023, and ribozyme RF00010). We believe our method represents a valuable alternative to current RNA-seq approaches to study host–pathogen interactions and will help explore host–pathogen mechanisms in tuberculosis and other similar models of intracellular infections.

Abstract

The study of host-pathogen interactions using in vivo models with intracellular pathogens like Mycobacterium tuberculosis (Mtb) entails technical limitations, such as: (i) Selecting an efficient differential lysis system to enrich the pathogen cells; (ii) obtaining sufficient high-quality RNA; and (iii) achieving an efficient rRNA depletion. Thus, some authors had used flow cytometers to separate infected cells or significantly increase the sequencing depth of host–pathogen RNA libraries to observe the pathogens’ gene expression. However, these options carry additional expenses in specialized equipment typically not available for all laboratories. Here, we propose an experimental protocol involving differential cell lysis and a probe-based ribosomal depletion to determine the gene expression of Mtb and its host during in vivo infection. This method increased the number of observed pathogen-expressed genes from 13 using the traditional RNA-seq approach to 702. After eliminating rRNA reads, we observed that 61.59% of Mtb sequences represented 702 genes, while 38.41% represented intergenic regions. Some of the most expressed genes codified for IS1081 (Rv2512c) transposase and eight PE-PGRS members, such as PGRS49 and PGRS50. As expected, a critical percent of the expressed genes codified for secreted proteins essential for infection, such as PE68, lppN, and LpqH. Moreover, three Mtb ncRNAs were highly expressed (small RNA MTS2823, transfer-messenger RNA RF00023, and ribozyme RF00010). Many of the host-expressed genes were related to the inflammation process and the expression of surfactant proteins such as the Sftpa and Sftpc, known to bind Mtb to alveolar macrophages and mi638, a microRNA with no previous associations with pulmonary diseases. The main objective of this study is to present the method, and a general catalog of the Mtb expressed genes at one point of the in vivo infection. We believe our method represents a different approach to the existing ones to study host–pathogen interactions in tuberculosis and other similar intracellular infections, without the necessity of specialized equipment.

Mitogen-activated protein kinases and NFkappaB are involved in SP-A-enhanced responses of macrophages to mycobacteria

Background

Surfactant protein A (SP-A) is a C-type lectin involved in surfactant homeostasis as well as host defense in the lung. We have recently demonstrated that SP-A enhances the killing of bacillus Calmette-Guerin (BCG) by rat macrophages through a nitric oxide-dependent pathway. In the current study we have investigated the role of tyrosine kinases and the downstream mitogen-activated protein kinase (MAPK) family, and the transcription factor NFκB in mediating the enhanced signaling in response to BCG in the presence of SP-A.

Methods

Human SP-A was prepared from alveolar proteinosis fluid, and primary macrophages were obtained by maturation of cells from whole rat bone marrow. BCG-SP-A complexes were routinely prepared by incubation of a ratio of 20 μg of SP-A to 5 × 10⁵ BCG for 30 min at 37°C. Cells were incubated with PBS, SP-A, BCG, or SP-A-BCG complexes for the times indicated. BCG killing was assessed using a 3H-uracil incorporation assay. Phosphorylated protein levels, enzyme assays, and secreted mediator assays were conducted using standard immunoblot and biochemical methods as outlined.

Results

Involvement of tyrosine kinases was demonstrated by herbimycin A-mediated inhibition of the SP-A-enhanced nitric oxide production and BCG killing. Following infection of macrophages with BCG, the MAPK family members ERK1 and ERK2 were activated as evidence by increased tyrosine phosphorylation and enzymatic activity, and this activation was enhanced when the BCG were opsonized with SP-A. An inhibitor of upstream kinases required for ERK activation inhibited BCG- and SP-A-BCG-enhanced production of nitric oxide by approximately 35%. Macrophages isolated from transgenic mice expressing a NFκB-responsive luciferase gene showed increased luciferase activity following infection with BCG, and this activity was enhanced two-fold in the presence of SP-A. Finally, lactacystin, an inhibitor of IκB degradation, reduced BCG- and SP-A-BCG-induced nitric oxide production by 60% and 80% respectively.

Conclusion

These results demonstrate that BCG and SP-A-BCG ingestion by macrophages is accompanied by activation of signaling pathways involving the MAP kinase pathway and NFκB.

Modulatory effects of ozone on THP-1 cells in response to SP-A stimulation

Ozone (O(3)), a major component of air pollution and a strong oxidizing agent, can lead to lung injury associated with edema, inflammation, and epithelial cell damage. The effects of O(3) on pulmonary immune cells have been studied in various in vivo and in vitro systems. We have shown previously that O(3) exposure of surfactant protein (SP)-A decreases its ability to modulate proinflammatory cytokine production by cells of monocyte/macrophage lineage (THP-1 cells). In this report, we exposed THP-1 cells and/or native SP-A obtained from bronchoalveolar lavage of patients with alveolar proteinosis to O(3) and studied cytokine production and NF-kappaB signaling. The results showed 1) exposure of THP-1 cells to O(3) significantly decreased their ability to express TNF-alpha in response to SP-A; TNF-alpha production, under these conditions, was still significantly higher than basal (unstimulated) levels in filtered air-exposed THP-1 cells; 2) exposure of both THP-1 cells and SP-A to O(3) did not result in any significant differences in TNF-alpha expression compared with basal levels; 3) O(3) exposure of SP-A resulted in a decreased ability of SP-A to activate the NF-kappaB pathway, as assessed by the lack of significant increase and decrease of the nuclear p65 subunit of NF-kappaB and cytoplasmic IkappaBalpha, respectively; and 4) O(3) exposure of THP-1 cells resulted in a decrease in SP-A-mediated THP-1 cell responsiveness, which did not seem to be mediated via the classic NF-kappaB pathway. These findings indicate that O(3) exposure may mediate its effect on macrophage function both directly and indirectly (via SP-A oxidation) and by involving different mechanisms.

Bench-to-bedside review: beta2-Agonists and the acute respiratory distress syndrome

The acute respiratory distress syndrome (ARDS) is a devastating constellation of clinical, radiological and pathological signs characterized by failure of gas exchange and refractory hypoxia. Despite nearly 30 years of research, no specific pharmacological therapy has yet proven to be efficacious in manipulating the pathophysiological processes that underlie this condition. Several in vitro and in vivo animal or human studies suggest a potential role for β₂-agonists in the treatment of ARDS. These agents have been shown to reduce pulmonary neutrophil sequestration and activation, accelerate alveolar fluid clearance, enhance surfactant secretion, and modulate the inflammatory and coagulation cascades. They are also used widely in clinical practice and are well tolerated in critically ill patients. The present review examines the evidence supporting a role for β₂-agonists as a specific pharmacological intervention in patients with ARDS.

Surfactant protein A increases matrix metalloproteinase-9 production by THP-1 cells

Matrix metalloproteinase (MMP)-9 from alveolar macrophages is a major source of elastolytic activity in the lung. It is increased in the bronchoalveolar lavage fluid of patients with emphysema. Although the importance of macrophage-derived elastolytic activity in the pathogenesis of emphysema is well established, questions remain about MMP-9 regulation and activity. Because surfactant protein A (SP-A) is capable of modulating other functions of human monocytic cells, we hypothesized that SP-A may regulate MMP-9 expression. Vitamin D3-differentiated THP-1 cells and peripheral blood mononuclear cells were stimulated in vitro with several concentrations of SP-A for different incubation times. MMP-9 mRNA expression was measured by dot-blot analysis, gelatinolytic activity in the medium was determined by gel zymography, protein expression was determined by ELISA, and a specific MMP-9 activity assay was used to measure the state of activation of this enzyme in the cell supernatants. SP-A induced the expression of MMP-9 in both cell types, the effect was time and dose dependent, and MMP-9 was released in its zymogen form. On the basis of results of neutralizing antibody studies, we believe that SP-A action is mediated through Toll-like receptor-2. Even though the biological meaning of these findings remains to be elucidated, these observations suggest the presence of a novel, locally controlled mechanism by which MMP-9 levels may be regulated in alveolar macrophages. We speculate that SP-A may influence the protease/antiprotease balance in the lungs of patients with quantitative and/or qualitative changes in surfactant constituents favoring an abnormal breakdown of extracellular matrix components.

Cutting edge: the immunostimulatory activity of the lung surfactant protein-A involves Toll-like receptor 4

The collectin surfactant protein-A (SP-A) is involved in the innate host defense and the regulation of inflammatory processes in the lung. In this work we investigated the molecular mechanisms related to the immunostimulatory activity of SP-A using macrophages from C3H/HeJ mice, which carry an inactivating mutation in the Toll-like receptor (TLR)4 gene, and TLR4-transfected Chinese hamster ovary cells. We demonstrate that SP-A-induced activation of the NF-kappaB signaling pathway and up-regulation of cytokine synthesis such as TNF-alpha and IL-10 are critically dependent on the TLR4 functional complex. These findings support the concept that TLR4 is a pattern recognition receptor that signals in response to both foreign pathogens and endogenous host mediators.