Journal of Mucosal Immunology Research
Open Access

Mucosal surfaces; Mucosal epithelium; Mucosal immune system; Immune tolerance; Immunoglobulin A; Mucosal diseases; Mucosal vaccines

Introduction

Mucosal surfaces are specialized epithelial tissues that line the inner surfaces of various organs and body cavities, forming a protective barrier between the body’s internal compartments and the external environment. These surfaces are found in the respiratory, gastrointestinal, genitourinary, and ocular tracts, which are constantly exposed to a diverse array of microorganisms, allergens, toxins, and other environmental factors. The unique structure and immune properties of mucosal surfaces enable them to fulfill crucial functions in maintaining homeostasis, defending against pathogens, and regulating immune responses [1-3]. The mucosal epithelium serves as the primary physical barrier against invading pathogens and harmful substances. It is composed of a layer of tightly interconnected epithelial cells, held together by specialized junctional complexes such as tight junctions and adherens junctions. These junctions regulate the selective permeability of the mucosal barrier, allowing the controlled movement of nutrients, ions, and immune cells while preventing the entry of pathogens and toxins. Beyond their physical barrier function, mucosal surfaces are also actively involved in immune surveillance and immune regulation. The mucosal immune system is a complex network of immune cells, lymphoid tissues, and soluble factors that work together to protect against pathogens and maintain immune homeostasis [4-6]. Mucosaassociated lymphoid tissues (MALT), such as the tonsils, adenoids, Peyer’s patches, and isolated lymphoid follicles, are strategically located throughout the mucosal surfaces and serve as sites for immune cell activation, antigen sampling, and the generation of immune memory. One key feature of the mucosal immune system is the production of immunoglobulin A (IgA), the predominant antibody isotype found in mucosal secretions. IgA plays a critical role in neutralizing pathogens and toxins at mucosal surfaces, preventing their attachment and entry into the body. It acts in conjunction with other immune components, including mucins produced by goblet cells, antimicrobial peptides, and resident immune cells such as dendritic cells, macrophages, and lymphocytes, to maintain immune balance and prevent excessive inflammation. Understanding the intricate mechanisms underlying mucosal surfaces is essential for unraveling the complexities of mucosal immunity, immune tolerance, and the development of effective interventions for mucosal diseases. Dysregulation of mucosal immune responses can lead to chronic inflammatory conditions, autoimmune diseases, and increased susceptibility to infections. Moreover, mucosal surfaces also represent important sites for vaccine delivery, as they offer unique advantages for inducing mucosal and systemic immune responses [7-9]. In this context, this article aims to provide a comprehensive overview of mucosal surfaces, including their structure, function, and immune properties. Furthermore, we will explore the challenges faced by mucosal surfaces, the consequences of mucosal immune dysregulation, and the potential applications of mucosal immunology in the development of therapeutic interventions and vaccine strategies. By enhancing our understanding of mucosal surfaces, we can pave the way for advancements in medical research and interventions that promote the health and well-being of individuals [10].

Materials and Methods

Sample collection: Obtain appropriate ethical approvals and consent for human or animal studies. Collect mucosal samples using non-invasive or minimally invasive techniques, such as swabs, brushings, biopsies, or lavage. Ensure proper handling and storage of samples to maintain sample integrity.

Histological analysis: Fix mucosal samples in appropriate fixatives (e.g., formalin) for a specific duration. Embed samples in paraffin or frozen sections for histological processing. Cut sections of desired thickness using a microtome. Stain sections with appropriate histological stains (e.g., hematoxylin and eosin, periodic acid-Schiff) to visualize tissue morphology. Analyze stained sections using light microscopy to assess epithelial architecture, cellular composition, and inflammatory infiltrates.

Immunohistochemistry and immunofluorescence: Perform antigen retrieval on paraffin-embedded sections using heat-induced or enzymatic methods. Block nonspecific binding sites with appropriate blocking agents. Incubate sections with primary antibodies against specific antigens of interest, targeting epithelial cells, immune cells, or molecular markers. Apply secondary antibodies conjugated to enzymes or fluorophores. Develop colorimetric or fluorescent signals using appropriate detection methods. Visualize and capture images using a light or fluorescence microscope. Quantify staining intensity or cell counts using image analysis software [11-13].

Cellular and molecular techniques: Isolate cells from mucosal samples using enzymatic digestion or mechanical dissociation methods. Culture and propagate primary cells or establish cell lines for in vitro studies. Perform flow cytometry or fluorescence-activated cell sorting (FACS) to analyze immune cell populations, surface markers, or intracellular proteins. Extract RNA or DNA from mucosal samples for gene expression analysis or genomic studies using commercially available kits. Perform quantitative real-time PCR (qPCR), microarray analysis, or next-generation sequencing (NGS) to study gene expression profiles. Conduct proteomic or metabolomic analyses to characterize the protein or metabolic profiles of mucosal surfaces.

Microbiological analysis: Collect mucosal swabs or samples for microbial analysis. Perform microbial culture on selective or nonselective media to isolate and identify microbial species. Use molecular techniques such as polymerase chain reaction (PCR), DNA sequencing, or metagenomic analysis to identify and characterize microbial communities. Quantify microbial load or diversity using appropriate microbiological assays.

Functional assays: Assess barrier function using techniques such as transepithelial electrical resistance (TEER) or paracellular flux assays. Evaluate mucus production and composition using methods such as Alcian Blue staining, Periodic Acid-Schiff staining, or ELISAbased measurements of mucin content. Measure cytokine, chemokine, or immunoglobulin production using enzyme-linked immunosorbent assays (ELISAs) or multiplex assays [14]. Perform functional assays, such as phagocytosis, cytotoxicity, or cytokine release assays, to evaluate immune cell functions.

Animal models: Establish animal models (e.g., mice, rats, pigs) of mucosal diseases or interventions. Administer treatments or interventions (e.g., pathogens, vaccines, therapeutics) via mucosal routes, such as oral gavage, intranasal delivery, vaginal instillation, or ocular application. Collect mucosal samples or perform in vivo imaging techniques to evaluate the impact of interventions on mucosal surfaces [15].

Results

The “Results” section for a study on mucosal surfaces would typically present the findings and data obtained from the experiments and analyses conducted. Here are some possible key findings and results that could be included in this section

Histological analysis: Description and visualization of the mucosal tissue architecture, including the epithelial layers, glandular structures, and cellular composition. Evaluation of tissue integrity, presence of inflammation, and any observed abnormalities or pathologies.

Immunohistochemistry and immunofluorescence: Localization and expression levels of specific antigens or markers on mucosal surfaces, such as epithelial markers (e.g., E-cadherin), immune cell markers (e.g., CD3, CD20), or inflammation markers (e.g., TNF-α, IL- 6). Assessment of immune cell infiltration or activation in the mucosal tissues. Co-localization studies to investigate the interactions between different cell types or the presence of specific molecules.

Cellular and molecular techniques: Gene expression profiles of mucosal tissues, including upregulated or downregulated genes associated with inflammation, immune response, or barrier function. Identification and quantification of immune cell populations using flow cytometry or FACS, including changes in cell subsets, activation markers, or functional characteristics. Analysis of specific signaling pathways or molecular markers involved in mucosal immune responses. Differential expression of cytokines, chemokines, or immunoglobulins in mucosal secretions or tissue lysates.

Microbiological analysis: Identification and characterization of microbial communities present on mucosal surfaces. Analysis of microbial diversity, abundance, or shifts in microbial composition. Correlations between specific microbial taxa and mucosal health or disease states. Functional analysis of microbial factors or metabolites that influence mucosal immunity or barrier function.

Functional assays: Assessment of mucosal barrier function, such as changes in TEER measurements or paracellular flux assays in response to stimuli or interventions. Quantification of mucus production or composition changes associated with mucosal diseases or treatments. Evaluation of immune cell functions, such as phagocytosis, cytotoxicity, or cytokine production in response to mucosal stimuli or interventions.

Animal models: Evaluation of mucosal disease progression or treatment efficacy in animal models. Analysis of histological changes, immune cell infiltration, or microbial alterations in mucosal tissues of animal models. Assessment of immune responses or mucosal immunization outcomes following mucosal vaccine delivery. The “Results” section should present the findings in a clear and organized manner, using appropriate figures, tables, and statistical analyses to support the observed results. It is essential to accurately describe the data obtained and provide interpretations and discussions of the results in relation to the research objectives and existing knowledge in the field.

Discussion

The “Discussion” section of a study on mucosal surfaces provides an opportunity to interpret and contextualize the results obtained in the study, compare them with existing literature, and discuss their implications. Here are some key points that could be included in the discussion of mucosal surfaces

Mucosal barrier function and immune surveillance: Discuss the importance of the mucosal barrier in protecting the body against pathogens, toxins, and allergens. Highlight the role of mucosal epithelial cells, tight junctions, and mucus-producing cells in maintaining the barrier integrity and selective permeability. Address the impact of alterations in barrier function on mucosal diseases, such as increased susceptibility to infections or development of chronic inflammatory conditions.

Mucosal immune responses and immune tolerance: Discuss the role of the mucosal immune system in orchestrating immune responses while maintaining tolerance to harmless antigens. Explore the functions of mucosa-associated lymphoid tissues (MALT) and immune cells present in mucosal tissues, such as dendritic cells, macrophages, and lymphocytes. Highlight the production and functions of immunoglobulin A (IgA) in mucosal secretions and its role in neutralizing pathogens and preventing their attachment to mucosal surfaces. Discuss the mechanisms underlying immune dysregulation in mucosal diseases, including disruptions in immune tolerance or excessive immune activation.

Microbial interactions and mucosal health: Discuss the complex interplay between the mucosal surfaces and the resident microbial communities, known as the microbiota. Address the mutualistic relationship between the microbiota and the host immune system, influencing immune development, barrier function, and overall mucosal health. Highlight the potential role of dysbiosis, characterized by alterations in microbial composition or diversity, in the development of mucosal diseases or immune disorders. Discuss the potential therapeutic strategies targeting the microbiota to restore mucosal homeostasis or treat mucosal diseases.

Clinical implications and future directions: Discuss the clinical relevance of the study findings and how they contribute to our understanding of mucosal diseases or interventions. Address the potential implications of the results for developing diagnostic markers, therapeutic targets, or preventive strategies for mucosal diseases. Identify the limitations of the study and suggest future research directions or areas that require further investigation. Discuss the potential translational applications, such as the development of mucosal vaccines or targeted therapies for mucosal diseases.

Conclusion

Summarize the key findings and their significance in the context of mucosal surfaces and immune responses. Emphasize the importance of continued research on mucosal surfaces to advance our understanding of mucosal diseases, immune regulation, and therapeutic interventions. Provide a closing statement that highlights the broader implications of the study and the potential impact on human health. The discussion should be comprehensive, well-structured, and supported by the study’s results as well as relevant literature. It should provide a critical analysis of the findings and contribute to the broader understanding of mucosal surfaces and their significance in health and disease.

1. Hurst JH, McCumber AW, Aquino JN, Rodriguez J, Heston SM, et al. (2022) Age-related changes in the nasopharyngeal microbiome are associated with SARS-CoV-2 infection and symptoms among children, adolescents, and young adults. Clinical Infectious Diseases 25-96.

Indexed at, Google Scholar, Crossref

2. Imai Y, Kuba K, Rao S, Huan Y, Guo F, et al. (2005) Angiotensin-converting enzyme 2 protects from severe acute lung failure. Nature 436: 112-116.

Indexed at, Google Scholar, Crossref

3. Janssen WJ, Stefanski AL, Bochner BS, Evans CM (2016) Control of lung defence by mucins and macrophages: ancient defence mechanisms with modern functions. Eur. Respir J 48: 1201-1214.

Indexed at, Google Scholar, Crossref

4. Karki R, Kanneganti TD (2021) The ‘cytokine storm’: molecular mechanisms and therapeutic prospects. Trends Immunology 42: 681-705.

Indexed at, Google Scholar, Crossref

5. Huang C, Wang Y, Li X, Ren L, Zhao J, et al. (2020) Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China. The Lancet 395: 497-506.

      Indexed at, Google Scholar, Crossref

6. Humphries DC, O Connor RA, Larocque D, Chabaud Riou M, Dhaliwal K, et al. (2021) Pulmonary-resident memory lymphocytes: pivotal Orchestrators of local immunity against respiratory infections. Front Immunol 12: 3817-3819.

Indexed at, Google Scholar, Crossref

7. Geitani R, Moubareck CA, Xu Z, Karam Sarkis D, Touqui L,et al. (2020) Expression and Roles of Antimicrobial Peptides in Innate Defense of Airway Mucosa: Potential Implication in Cystic Fibrosis. Front Immunol 11: 1198-1204.

Indexed at, Google Scholar, Crossref

8. Gersuk GM, Underhill DM, Zhu L, Marr KA (2006)  Dectin-1 and TLRs Permit macrophages to distinguish between different Aspergillus fumigatus cellular states. J Immunol 176: 3717-3724.

Indexed at, Google Scholar, Crossref

9. Campbell DJ, Butcher EC (2002) Rapid acquisition of tissue-specific homing phenotypes by CD4+ T cells activated in cutaneous or mucosal lymphoid tissues. J Exp Med 195: 135-141. 

Indexed at, Google Scholar, Crossref

10. Iwata M, Hirakiyama A, Eshima Y, Kagechika H, Kato C, et al. (2004) Retinoic acid imprints gut-homing specificity on T cells. Immunity 21: 527-538.

Indexed at, Google Scholar, Crossref

11. Izcue A, Coombes JL, Powrie F (2009) Regulatory lymphocytes and intestinal inflammation. Annu Rev Immunol 27: 313-338.

Indexed at, Google Scholar, Crossref

12. Brandtzaeg P (2007) Induction of secretory immunity and memory at mucosal surfaces. Vaccine 25: 5467-5484.

Indexed at, Google Scholar, Crossref

13. Brandtzaeg P (2010) Function of mucosa-associated lymphoid tissue in antibody formation. Immunol Invest 39: 303-355.

Indexed at, Google Scholar, Crossref

14. Ley RE, Peterson DA, Gordon JI (2006) Ecological and evolutionary forces shaping microbial diversity in the human intestine. Cell 124: 837-848. 

Indexed at, Google Scholar, Crossref

15. Mazmanian SK, Liu CH, Tzianabos AO, Kasper DL (2005) An immunomodulatory molecule of symbiotic bacteria directs maturation of the host immune system. Cell 122: 107-118. 

Indexed at, Google Scholar, Crossref