MOLECULAR PATHWAYS IN PULMONARY MICROBIAL INFECTIONS

Anogianakis G, Palmieri A, Pellati A, Massi F. Molecular pathways in pulmonary microbial infections. International Journal of Infection. 2023;7(3):68-72

G. Anogianakis^1*, A. Palmieri² , A. Pellati³ and F. Massi⁴

¹ Department of Physiology, Aristotle University of Thessaloniki, Greece;
² Department of Experimental, Diagnostic and Specialty Medicine, Alma Mater Studiorum, University of Bologna, Italy;
³ Department of Translational Medicine, University of Ferrara, Italy;
⁴ Department of Cardiovascular and Thoracic Surgery, Mazzini Hospital, Teramo, Italy.

*Correspondence to:

Prof. George Anogianakis,
Department of Physiology,
Aristotle University of Thessaloniki,
Thessaloniki, Greece.
e-mail: anogian@auth.gr

Received: 21 June, 2023 Accepted: 10 September, 2023

ISSN 1972-6945 [online] Copyright 2023 © by Biolife-publisher This publication and/or article is for individual use only and may not be further reproduced without written permission from the copyright holder. Unauthorized reproduction may result in financial and other penalties. Disclosure: all authors report no conflicts of interest relevant to this article.

ABSTRACT

The alveolar epithelium of the lungs is made up of type I and type II pneumocytes. Type I pneumocytes are specialized cells with thin projections that penetrate between the alveolar septa, while type II are cuboidal cells, which act as progenitor cells for the entire epithelial cell population. In addition, phagocytic macrophages that function as alveolar “scavengers” are a third type. Microbial lung infections activate the immune and inflammatory systems with a response related to the severity of the infection. Alveolar macrophages are the first line of defense located on the respiratory surface and act through the phagocytosis of organic and inorganic particles. Macrophages have both pro-inflammatory and anti-inflammatory functions, they have bactericidal actions using superoxide anion, H₂O₂ and myeloperoxidase, and present antigen to lymphocytes. Toll-like receptors (TLRs) are a family of receptors of innate immunity which can bind a huge number of pathogens and initiate the immune response. TLR-4 binds lipopolysaccharide (LPS) in the presence of the accessory molecules MD-2 and CD14 with activation of signal transduction mechanisms. Nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) are intracellular proteins that play an important role in the innate immune response and recognition of pulmonary microbial infections, where they help detect pathogens and activate immune responses. In the lungs, NLRs recognize viral RNAs, fungal components, and bacterial cell wall peptidoglycans. Activation of NOD1 and NOD2 results in the recruitment of receptor-interacting protein kinase 2 (RIPK2), leading to activation of the NF-κB and mitogen-activated protein kinase (MAPK) pathways, with transcription of inflammatory cytokines.

KEYWORDS: Lung, pulmonary, microbial infection, molecular pathway, immune system

INTRODUCTION

Breathing is a vital process that allows atmospheric oxygen to enter our body and reach the tissues. In collaboration with the cardiovascular system, the respiratory system performs this and other functions thanks to a structure that develops during the embryonic, fetal period, and throughout the first years of life. The life of all mammals and the bioenergetic processes that maintain cellular integrity depend on a continuous supply of oxygen to support aerobic metabolism (1). The transport of oxygen from the air to the mitochondria occurs through a series of processes involving the heart, lungs, and circulation which cooperate in the extraction of O₂ from the atmosphere and together generate a flow of oxygenated blood directed to the tissues (2).

The pulmonary capillary epithelium consists of a monolayer of squamous cells, with very little cytoplasm and some vesicles (3). The alveolar epithelium is a mosaic of two types of cells: type I and type II pneumocytes. The first have some resemblance to endothelial cells and cover 97% of the alveolar surface (4). Type I pneumocytes are specialized cells with thin projections that penetrate between the alveolar septa. This makes these cells fragile and unable to replicate. Type II pneumocytes are cuboidal cells, with a cytoplasm containing the endoplasmic reticulum, the Golgi complex, granules, and lamellar bodies (5).  These organelles cooperate in the production of surfactant and its associated proteins. They also act as progenitor cells for the entire population of epithelial cells, and in the event of damage to the alveolar epithelium, for example, due to microorganisms, they can differentiate into type I pneumocytes, reconstituting the integrity of the alveolar surface. Macrophages are a third type of cell present in the alveolus. These immune cells have cytoplasmic inclusions that allow them to function as alveolar “scavengers” (6).

Microbial lung infections are a broad range of diseases caused by bacteria, viruses, fungi, and other pathogens that target the respiratory system, including the lungs (7). Lung infections are complex and vary depending on the pathogen. Nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) are a class of intracellular pattern recognition receptors (PRRs) that play a key role in the innate immune response. They are responsible for detecting pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) within host cells. Toll-like receptors (TLRs) are PRRs present on immune cells such as dendritic cells and macrophages that recognize PAMPs, conserved components of microbes, such as bacterial lipopolysaccharides (LPS), and viral RNAs.

DISCUSSION

Microbial lung infections are composed of complex molecular reactions involving the host’s immune system (7).  These reactions determine the severity of the infection, which is related to the cellular immune response that can be mild, moderate or severe, and can even lead to death.

In the bronchoalveolar lavage, alveolar macrophages compose 80-90% of the cellular population and constitute the first line of defence at the respiratory surface (8). The sterility of the alveoli and terminal airways in physiological conditions is mainly the result of macrophagic activity. Macrophages, which do not have cilia, engulf microbes, cellular debris, and organic and inorganic particles that have reached the depths of the lung and carry out phagocytosis and bacterial lysis (9). Macrophages are equipped with a rich set of hydrolases and lysosomal proteases, and for this reason they can rapidly digest phagocytosed materials (10). These cells also have bactericidal actions with an oxidative mechanism such as the formation of superoxide anion, H₂O₂, and myeloperoxidase (11).

Macrophages migrate through the alveolar lining of the epithelium, conveying the engulfed substances for their definitive reabsorption via the blood or lymphatic system. In addition to phagocytic activity, they can process and present antigens to lymphocytes and release numerous chemical mediators such as chemotactic factors for polymorphonuclear cells and lymphocytes, cytokines, components of the complement cascade, prostaglandins, leukotrienes, and growth factors for fibroblasts and fibronectin (12).

Overall, alveolar macrophages have both pro-inflammatory and anti-inflammatory functions (13).  In the alveolar space, macrophage function in vivo is modulated by a constant interaction with lipids and proteins of surfactant, a major component of the alveolar microenvironment (14). The latter seems to modulate the expression and function of TLRs, a family of receptors of innate immunity capable of binding a huge number of pathogens, initiating the immune response and acting as a bridge between innate and adaptive immunity. Through these receptors, immune cells, such as monocytes, macrophages, lymphocytes, neutrophils, dendritic cells and mast cells, identify pathogens and activate a series of mechanisms to eliminate them.

The expression and function of TLRs are related not only to the ability to recognize and respond to germs but also to the possibility or otherwise of developing atopy (15).  In this sense, TLRs have a protective role. It has been shown that the nasal mucosa of atopic individuals has much lower levels of TLR-4 than non-atopic individuals (16). This is interesting because the expression of TLR-4 has been associated with the recognition of LPS of bacterial origin and could explain the lack of response to it in atopic individuals (17). TLRs 1, 2, 4, 5, and 6 recognize bacteria, while TLRs 7 and 8 recognize viruses (18). TLR-2 recognizes a broad spectrum of bacterial products, including Gram-positive peptidoglycan, bacterial lipoproteins, mycobacterial lipoarabinomannan, and yeast cell walls (19). TLR-4 binds Gram-negative LPS and was the first receptor of its kind to be characterized.

Upon infection by Gram-negative bacteria, LPS interacts with TLR-4 in the presence of the accessory molecules MD-2 and CD14, which are soluble or present on the cell membrane (20). CD14 is a molecule that binds LPS and facilitates its binding to the TLR-4/MD-2 complex (21). At this point, signal transduction mechanisms are triggered, involving a series of molecules such as IL-1, MyD88, TRAF6, IRAK, mitogen-activated protein kinases (MAPKs), ERK1/2, Junk, and p38 (22) (Table I). All this leads to the expression of genes encoding inflammatory mediators and costimulatory molecules. During phagosome maturation, TLRs induce the production of reactive oxygen species (ROS), and nitric oxide synthase, which are necessary for the elimination of ingested microorganisms.

Table I. Molecular pathways implicated in pulmonary microbial infections.

The presence of TLRs at the level of immune cells was previously known. Recently, it has been demonstrated that TLRs are also present at the level of bronchial and alveolar epithelial cells. In fact, TLR-2 and 4 are present at the pulmonary level and their importance in numerous pathologies has been demonstrated (23).  Their presence, together with that of CD14, has been found at the level of alveolar macrophages and monocytes in type II pneumocytes both at the cytoplasmic level and on the cell membrane.

Alveolar epithelial cells constitutively express CD14 and MD2 and can respond to TLR-2 and 4 ligands like macrophages by producing cytokines and participating in the innate immune response (24).  It should be emphasized that alveolar macrophages produce twice as much tumor necrosis factor (TNF) as type II pneumocytes. It has been hypothesised that under basal conditions, alveolar macrophages are the only source of TNF, whereas in the case of bacterial colonzsation, alveolar epithelial cells also release TNF, but in limited quantities (25).  These results support the hypothesis that leukocyte recruitment during bacterial infections is primarily due to a robust response of alveolar epithelial cells rather than alveolar macrophages.

In addition to type II pneumocytes, type I pneumocytes are also sensitive to bacterial LPS, in response to which they produce cytokines and chemokines.  Due to TLRs’ important role in lung defence mechanisms, their dysfunction results in the onset of acute and chronic inflammatory diseases such as asthma (26).  Signals transmitted by TLR4 that are present on structural lung cells induce the production of chemokines and the recruitment of neutrophils, which causes bronchoconstriction and induces the activation of TH2 lymphocytes through the intervention of dendritic cells (27).

NLRs are a family of intracellular molecules that play an important role in the innate immune response (28).  They are involved in recognizing microbial infections and initiating inflammatory responses (29). In pulmonary microbial infections, they help detect pathogens and activate immune responses to fight the infection (30). NLRs are PRRs that detect PAMPs and DAMPs (31).

NLRs in the lung recognize several molecules including viral RNA, fungal components, and bacterial cell wall peptidoglycans (32). Upon activation, NOD1 and NOD2 recruit receptor-interacting protein kinase 2 (RIPK2), leading to the activation of NF-κB and MAPK pathways (33). This results in the transcription of pro-inflammatory cytokines such as IL-1, TNF, IL-6, and IL-8, which recruit immune cells to the site of infection (34).

The NLRP3 inflammasome may be involved in the activation of several stimuli, including microbial infection, which leads to the cleavage of pro-caspase-1 into its active form, caspase-1 (35). Caspase-1 activates pro-IL-1β and pro-IL-18, which are potent inflammatory cytokines. NLRs activate inflammatory responses that are important for detecting and responding to microbial infections in the lungs.

CONCLUSIONS

Understanding the immune response in lung infections and the inflammatory molecular mechanisms is important to generate new therapeutic strategies and specific vaccines. Understanding immune reactions to microorganisms is the key to resolving the pathological state. Therefore, future therapeutic strategies should aim to modulate the action of immune sentinel cells to improve the course of infectious and non-infectious lung diseases. Future studies on these topics will certainly lead to improvements in the health status of the global population.

Conflict of interest

The authors declare that they have no conflict of interest.

REFERENCES

1. Poole DC, Jones AM. Oxygen Uptake Kinetics. Comprehensive Physiology. 2012;2(2). doi:https://doi.org/10.1002/cphy.c100072
2. Roger AJ, Muñoz-Gómez SA, Kamikawa R. The Origin and Diversification of Mitochondria. Current Biology. 2017;27(21):R1177-R1192. doi:https://doi.org/10.1016/j.cub.2017.09.015
3. Townsley MI. Structure and Composition of Pulmonary Arteries, Capillaries, and Veins. Comprehensive Physiology. 2012;2(1). doi:https://doi.org/10.1002/cphy.c100081
4. Simionescu M. Ultrastructural Organization of the Alveolar‐Capillary Unit. Novartis Foundation symposium. 1980;78:11-36. doi:https://doi.org/10.1002/9780470720615.ch2
5. Chignalia AZ, Vogel SM, Reynolds AB, et al. p120-Catenin Expressed in Alveolar Type II Cells Is Essential for the Regulation of Lung Innate Immune Response. The American Journal of Pathology. 2015;185(5):1251-1263. doi:https://doi.org/10.1016/j.ajpath.2015.01.022
6. Giresh A, Parida PK, Chappity P, et al. Prevalence of Skip Metastases to Cervical Lymph-Nodes in Oral cavity Cancer in Eastern India-an observational study. Indian Journal of Otolaryngology and Head & Neck Surgery. 2022;74(S3):6374-6383. doi:https://doi.org/10.1007/s12070-021-03048-z
7. Cribbs SK, Crothers K, Morris A. Pathogenesis of HIV-Related Lung Disease: Immunity, Infection, and Inflammation. Physiological Reviews. 2020;100(2):603-632. doi:https://doi.org/10.1152/physrev.00039.2018
8. Yu YRA, Hotten DF, Malakhau Y, et al. Flow Cytometric Analysis of Myeloid Cells in Human Blood, Bronchoalveolar Lavage, and Lung Tissues. American Journal of Respiratory Cell and Molecular Biology. 2016;54(1):13-24. doi:https://doi.org/10.1165/rcmb.2015-0146oc
9. Cohen SB, Gern BH, Delahaye JL, et al. Alveolar Macrophages Provide an Early Mycobacterium tuberculosis Niche and Initiate Dissemination. Cell Host & Microbe. 2018;24(3):439-446.e4. doi:https://doi.org/10.1016/j.chom.2018.08.001
10. Zheng Z, Deng W, Bai Y, et al. The lysosomal Rag-Ragulator complex licenses RIPK1– and caspase-8–mediated pyroptosis by Yersinia. Science. 2021;372(6549). doi:https://doi.org/10.1126/science.abg0269
11. Meera Gurumurthy, Rao M, Mukherjee T, et al. A novel F ₄₂₀ ‐dependent anti‐oxidant mechanism protects M ycobacterium tuberculosis against oxidative stress and bactericidal agents. Molecular Microbiology. 2013;87(4):744-755. doi:https://doi.org/10.1111/mmi.12127
12. Muntjewerff EM, Meesters LD, van den Bogaart G. Antigen Cross-Presentation by Macrophages. Frontiers in Immunology. 2020;11. doi:https://doi.org/10.3389/fimmu.2020.01276
13. Wculek SK, Heras-Murillo I, Mastrangelo A, et al. Oxidative phosphorylation selectively orchestrates tissue macrophage homeostasis. Immunity. 2023;56(3):516-530.e9. doi:https://doi.org/10.1016/j.immuni.2023.01.011
14. Joshi N, Walter JM, Misharin AV. Alveolar Macrophages. Cellular Immunology. 2018;330(330):86-90. doi:https://doi.org/10.1016/j.cellimm.2018.01.005
15. Papaioannou AI, Spathis A, Kostikas K, Karakitsos P, Papiris S, Rossios C. The role of endosomal toll-like receptors in asthma. European Journal of Pharmacology. 2017;808:14-20. doi:https://doi.org/10.1016/j.ejphar.2016.09.033
16. Dhariwal J, Kitson J, Jones RE, et al. Nasal Lipopolysaccharide Challenge and Cytokine Measurement Reflects Innate Mucosal Immune Responsiveness. Doherty TM, ed. PLOS ONE. 2015;10(9):e0135363. doi:https://doi.org/10.1371/journal.pone.0135363
17. Ciesielska A, Matyjek M, Kwiatkowska K. TLR4 and CD14 trafficking and its influence on LPS-induced pro-inflammatory signaling. Cellular and Molecular Life Sciences. 2020;78(4):1233-1261. doi:https://doi.org/10.1007/s00018-020-03656-y
18. Kadowaki N, Ho S, Antonenko S, et al. Subsets of Human Dendritic Cell Precursors Express Different Toll-like Receptors and Respond to Different Microbial Antigens. Journal of Experimental Medicine. 2001;194(6):863-870. doi:https://doi.org/10.1084/jem.194.6.863
19. Palaniyar N. Pulmonary Innate Immune Proteins and Receptors that Interact with Gram-positive Bacterial Ligands. Immunobiology. 2002;205(4-5):575-594. doi:https://doi.org/10.1078/0171-2985-00156
20. Fu Y, Xu B, Huang S, et al. Baicalin prevents LPS-induced activation of TLR4/NF-κB p65 pathway and inflammation in mice via inhibiting the expression of CD14. Acta Pharmacologica Sinica. 2020;42(1):88-96. doi:https://doi.org/10.1038/s41401-020-0411-9
21. Kuzmich N, Sivak K, Chubarev V, Porozov Y, Savateeva-Lyubimova T, Peri F. TLR4 Signaling Pathway Modulators as Potential Therapeutics in Inflammation and Sepsis. Vaccines. 2017;5(4):34. doi:https://doi.org/10.3390/vaccines5040034
22. Huang Q, Yang J, Lin Y, et al. Differential regulation of interleukin 1 receptor and Toll-like receptor signaling by MEKK3. Nature Immunology. 2003;5(1):98-103. doi:https://doi.org/10.1038/ni1014
23. Go H, Koh J, Kim HS, Jeon YK, Chung DH. Expression of toll-like receptor 2 and 4 is increased in the respiratory epithelial cells of chronic idiopathic interstitial pneumonia patients. Respiratory Medicine. 2014;108(5):783-792. doi:https://doi.org/10.1016/j.rmed.2013.12.007
24. Thorley AJ, Grandolfo D, Lim E, Goldstraw P, Young A, Tetley TD. Innate Immune Responses to Bacterial Ligands in the Peripheral Human Lung – Role of Alveolar Epithelial TLR Expression and Signalling. Rojas M, ed. PLoS ONE. 2011;6(7):e21827. doi:https://doi.org/10.1371/journal.pone.0021827
25. Hu Q, Lyon CJ, Fletcher JK, Tang W, Wan M, Hu TY. Extracellular vesicle activities regulating macrophage- and tissue-mediated injury and repair responses. Acta Pharmaceutica Sinica B. 2021;11(6):1493-1512. doi:https://doi.org/10.1016/j.apsb.2020.12.014
26. Hadjigol S, Netto KG, Maltby S, et al. Lipopolysaccharide induces steroid‐resistant exacerbations in a mouse model of allergic airway disease collectively through IL‐13 and pulmonary macrophage activation. Clinical & Experimental Allergy. 2019;50(1):82-94. doi:https://doi.org/10.1111/cea.13505
27. Belchamber KBR, Donnelly LE. Macrophage Dysfunction in Respiratory Disease. Results and Problems in Cell Differentiation. 2017;62:299-313. doi:https://doi.org/10.1007/978-3-319-54090-0_12
28. Banoth B, Cassel SL. Mitochondria in innate immune signaling. Translational Research. 2018;202:52-68. doi:https://doi.org/10.1016/j.trsl.2018.07.014
29. Groeger S, Meyle J. Oral Mucosal Epithelial Cells. Frontiers in Immunology. 2019;10(208). doi:https://doi.org/10.3389/fimmu.2019.00208
30. Leissinger M, Kulkarni R, Zemans RL, Downey GP, Samithamby Jeyaseelan. Investigating the Role of Nucleotide-Binding Oligomerization Domain–Like Receptors in Bacterial Lung Infection. American Journal of Respiratory and Critical Care Medicine. 2014;189(12):1461-1468. doi:https://doi.org/10.1164/rccm.201311-2103pp
31. Ohto U. Activation and regulation mechanisms of NOD-like receptors based on structural biology. Frontiers in Immunology. 2022;13. doi:https://doi.org/10.3389/fimmu.2022.953530
32. Mortaz E, Adcock IM, Tabarsi P, et al. Pattern recognitions receptors in immunodeficiency disorders. European Journal of Pharmacology. 2017;808:49-56. doi:https://doi.org/10.1016/j.ejphar.2017.01.014
33. You J, Y. Alan Wang, Chen H, Jin F. RIPK2: a promising target for cancer treatment. Frontiers in Pharmacology. 2023;14. doi:https://doi.org/10.3389/fphar.2023.1192970
34. Hwang HS, Lee MH, Choi MH, Kim HA. NOD2 signaling pathway is involved in fibronectin fragment-induced pro-catabolic factor expressions in human articular chondrocytes. BMB Reports. 2019;52(6):373-378. doi:https://doi.org/10.5483/bmbrep.2019.52.6.165
35. Kelley N, Jeltema D, Duan Y, He Y. The NLRP3 Inflammasome: An Overview of Mechanisms of Activation and Regulation. International Journal of Molecular Sciences. 2019;20(13):3328. doi:https://doi.org/10.3390/ijms20133328