The Gut-Lung Immunological Axis

1. Introduction: The Mucosal Immune System as a Unified Network

The human immune system is conventionally conceptualized as a binary construct, distinguishing systemic immunity — anchored in the blood, lymph nodes, and spleen — from mucosal immunity, which is localized at the epithelial surfaces interfacing with the external environment. Yet this dichotomy obscures a critical architectural truth: the mucosal immune system is itself a distributed but interconnected network in which immune induction at one anatomical site can generate effector responses at distant mucosal compartments (Mestecky, Blumberg, Kiyono, & McGhee, 2003). This property, collectively described under the framework of the "common mucosal immune system", has profound implications for vaccine design, immunotherapy, and the clinical deployment of immunomodulatory agents.

Among the body's mucosal surfaces, the gastrointestinal and respiratory tracts are perhaps the most extensively characterized with respect to their immunological interrelationship. The gut-lung axis describes the bidirectional immunological communication between these two organ systems, mediated by shared lymphocyte trafficking programs, cytokine networks, and microbial signals (Esposito et al., 2018). From a clinical standpoint, this axis has been most consequentially exploited in the context of oral bacterial lysate therapies for respiratory infections. This is a pharmacological strategy based on the premise that exposure to intestinal antigens can generate protective immune responses at pulmonary mucosal surfaces (Kearney, Dziekiewicz, & Feleszko, 2015).

Respiratory tract infections represent a global health burden of exceptional magnitude and remain among the leading infectious causes of morbidity and mortality worldwide, with lower respiratory infections placing particular pressure on healthcare systems in both high- and low-income settings (Esposito et al., 2018). The inadequacy of existing prophylactic strategies — including the absence of licensed vaccines against many clinically relevant respiratory pathogens — has sustained decades of interest in non-specific immunomodulatory approaches (Lavelle & Ward, 2022). Oral bacterial lysate preparations, of which OM-85 is the most widely studied paradigmatic example, were developed on the basis that repeated stimulation of the intestinal immune system with inactivated bacterial antigens could prime the respiratory mucosa against pathogenic challenge (De Benedetto & Sevieri, 2013). However, the mechanistic efficiency and clinical reliability of this approach remain subjects of ongoing debate (Esposito et al., 2018).

This article provides a rigorous analytical examination of the gut-lung immunological axis, tracing its anatomical and cellular foundations, evaluating the scientific evidence for gut-to-lung immune communication, and analyzing the capacity of oral bacterial lysate therapy, with particular attention to the clinical record of OM-85, to generate meaningful respiratory mucosal protection. 

2. Anatomical and Cellular Basis of the Gut-Lung Axis

2.1 The Organization of Mucosa-Associated Lymphoid Tissue

The structural foundation of the gut-lung axis resides in the mucosa-associated lymphoid tissue (MALT), a diffuse but organized lymphoid network distributed across all major mucosal surfaces of the body (Cesta, 2006). MALT encompasses several anatomically discrete but functionally related compartments, including the gut-associated lymphoid tissue (GALT) — primarily represented by Peyer's patches and mesenteric lymph nodes — the bronchus-associated lymphoid tissue (BALT), the nasal-associated lymphoid tissue (NALT), and lymphoid tissue associated with the mammary, lacrimal, salivary, and genitourinary glands (Mestecky et al., 2003). These compartments share a similar cellular architecture, including dendritic cell populations, specialized epithelial sampling cells, and IgA-committed B lymphocytes, yet their anatomical distribution across the body's surface enables the common mucosal immune system to mount coordinated responses to pathogens encountered at geographically distinct portals of entry (Mestecky et al., 2003).

The GALT is the largest and best-characterized component of this network. Located predominantly in the small intestine, Peyer's patches (Fig. 1) are discrete lymphoid aggregates overlain by a specialized follicle-associated epithelium containing microfold (M) cells, which are highly specialized epithelial cells whose primary function is the transcytosis of luminal antigens from the intestinal lumen into the subepithelial lymphoid space (Dillon & Lo, 2019). M cell-mediated transcytosis constitutes one of the primary mechanisms by which the mucosal immune system samples the commensal and pathogenic microbial content of the gastrointestinal tract (Jurkiewicz & Zielnik-Jurkiewicz, 2018). The sampling activity of M cells is notably broad in its selectivity; they can internalize particulate material, bacteria, viruses, and protein complexes alike, delivering them to the subepithelial dome where resident dendritic cells, macrophages, and lymphocytes await (Dillon & Lo, 2019).

Figure 1. Scanning electron microscopy of Peyer's patch follicle epithelium, showing a microvillar surface with M cells appearing as depressions (Dillon & Lo, 2019).

2.2 Shared Mucosal Homing Receptors and Lymphocyte Trafficking

The common mucosal immune system is represented by shared homing receptors on lymphocytes primed at one mucosal site that enable them to traffic to and colonize distant mucosal surfaces. Following antigen detection in the Peyer's patches and activation in the mesenteric lymph nodes, B and T lymphocytes express specific combinations of homing molecules, most critically the integrin α4β7 — which binds to mucosal addressin cell adhesion molecule-1 (MAdCAM-1) expressed on the high endothelial venules of mucosal tissues — and the chemokine receptor CCR10, which directs migration towards CCL28-expressing mucosal epithelium (Fig. 2) (Brandtzaeg, 2009). The α4β7/MAdCAM-1 interaction is particularly significant because MAdCAM-1 expression is not restricted to the intestinal vasculature but is found on endothelial cells in multiple mucosal compartments, including those of the respiratory tract (Brandtzaeg, 2009). This receptor-ligand interaction thus provides a molecular passport by which gut-primed lymphocytes can gain access to the pulmonary mucosal compartment.

The trafficking of IgA-secreting plasma cells from mesenteric lymph nodes through the systemic circulation to remote mucosal effector sites is a particularly relevant process for oral bacterial lysate administration (Mantis, Rol, & Corthesy, 2011). Following IgA class switching in mesenteric lymph nodes, IgA-committed plasma cell precursors enter the bloodstream and, guided by the α4β7/MAdCAM-1 and CCR10/CCL28 homing axes, disseminate to effector mucosal sites where they complete their differentiation and begin secreting dimeric IgA (Mantis et al., 2011). In the respiratory tract, this dimeric IgA undergoes transcytosis across the epithelial layer via the polymeric immunoglobulin receptor (pIgR), emerging on the luminal surface as secretory IgA (sIgA) — the dominant immunoglobulin of mucosal secretions (Lamm, 1997).

Figure 2. The homing receptor programme of IgA-secreting plasma cell precursors (Pracht, Wittner, Kagerer, Jack, & Schuh, 2023).

2.3 The Common Mucosal Immune System Theory

The common mucosal immune system (CMIS) emerged from several decades of experimental work demonstrating that immunization at one mucosal surface can generate secretory IgA responses at anatomically distant mucosal sites (Mestecky et al., 2003). The CMIS theory proposes that, following antigen priming in inductive sites such as Peyer's patches or NALT, activated lymphocytes disseminate via systemic circulation to seed effector compartments across the mucosa, including the lungs, salivary glands, mammary glands, and genitourinary tract (Davis, 2001). The shared homing receptor program described above constitutes the molecular mechanism through which this dissemination is orchestrated (Brandtzaeg, 2009).

The CMIS concept is well-supported by animal model data demonstrating that oral or intranasal immunization can generate sIgA responses at remote mucosal sites (Hellfritzsch & Scherliess, 2019). However, the efficiency of this inter-compartmental immune trafficking varies considerably depending on the nature of the antigen, the adjuvant system employed, the specific mucosal compartment targeted, and the immunological status of the host (Hellfritzsch & Scherliess, 2019). These sources of variability carry considerable clinical importance when evaluating the value and limitations of oral bacterial lysate therapy as a respiratory immunisation strategy.

3. Evidence for Gut-to-Lung Immune Communication

3.1 Animal Model Studies Demonstrating Pulmonary Protection Following Oral Antigen Administration

The most direct experimental evidence for gut-to-lung immune communication derives from animal model studies in which oral immunization with defined antigens generates detectable antibody responses at the respiratory mucosal surface. Navarro and colleagues demonstrated in a murine model that oral administration of bacterial extracts recruited regulatory T cells to the airways, providing concrete evidence that gut-primed immune cells can adopt residence in pulmonary tissue and modulate local immune responses (Navarro et al., 2011). Studies employing oral live attenuated bacteria or inactivated bacterial antigens have similarly documented induction of antigen-specific sIgA in bronchoalveolar lavage fluid (BALF) and nasal secretions following intestinal antigen delivery, although the titers achieved are characteristically lower than those induced by direct mucosal administration to the respiratory site (Hellfritzsch & Scherliess, 2019). Crucially, the magnitude of pulmonary sIgA responses following oral immunization shows substantial inter-individual variability, likely reflecting differences in the efficiency of mesenteric lymph node priming, the density of α4β7-expressing lymphocytes generated, and the accessibility of MAdCAM-1 on pulmonary high endothelial venules (Brandtzaeg, 2009).

3.2 Microbiome Modulation and Its Secondary Effects on Respiratory Immunity

Beyond the direct trafficking of gut-primed lymphocytes, the gut-lung axis is increasingly recognized to encompass indirect immunomodulatory pathways mediated through the intestinal microbiome. The intestinal commensal microbial community exerts profound and pleiotropic effects on systemic and mucosal immune tone, including the calibration of innate immune responsiveness, the regulation of Th1/Th2/Th17 balance, and the modulation of IgA class-switching efficiency (Esposito et al., 2018). These microbiome-mediated effects extend to the respiratory compartment; perturbation of the intestinal microbiome has been associated with altered susceptibility to respiratory infections in clinical and experimental settings (Esposito et al., 2018). This observation is clinically important in the context of oral bacterial lysate therapy, since BL preparations contain bacterial cell wall components — including lipopolysaccharides, peptidoglycans, and lipoteichoic acids — that can be recognized by pattern recognition receptors expressed by intestinal epithelial cells and resident immune populations, potentially modulating the intestinal microenvironment through mechanisms that extend beyond classical adaptive immunity (Suarez, Ferrara, Rial, Dee, & Chabalgoity, 2020).

3.3 The Role of Short-Chain Fatty Acids in Gut-Lung Immune Crosstalk

A further indirect mechanism through which the intestinal compartment may influence respiratory immunity involves the production of short-chain fatty acids (SCFAs) by anaerobic fermentation of dietary fiber by commensal bacteria. SCFAs — principally butyrate, propionate, and acetate — exert immunomodulatory effects both locally in the intestine and systemically, including effects on pulmonary immune cell populations (Esposito et al., 2018). Butyrate, in particular, promotes regulatory T cell differentiation and suppresses inflammatory cytokine production, while propionate has been demonstrated to influence dendritic cell and macrophage function through G-protein coupled receptor signalling (Esposito et al., 2018). The relevance of SCFA-mediated gut-lung crosstalk to bacterial lysate therapy has not been directly established at the molecular level, but it is plausible that BL-induced modulation of the intestinal microbial community could secondarily influence SCFA production and thereby contribute to clinical effects that cannot be attributed solely to classical IgA-mediated immunity.

4. How Oral Bacterial Lysates — and OM-85 Specifically — Exploit the Gut-Lung Axis

4.1 M Cell Antigen Sampling in Peyer's Patches

The immunological action of oral bacterial lysates is initiated at the intestinal epithelial surface, where M cells in the follicle-associated epithelium of Peyer's patches sample and internalize BL components from the intestinal lumen. Bacterial lysates are polyvalent preparations derived from the chemical or mechanical disruption of pathogenic bacteria, yielding a mixture of immunostimulatory bacterial cell wall components that function as pathogen-associated molecular patterns (PAMPs) (Suarez et al., 2020). Upon transcytosis by M cells, these PAMPs are delivered to the subepithelial dome, where they are recognized by toll-like receptors (TLRs) expressed on resident dendritic cells, macrophages, and B cells (Dillon & Lo, 2019). TLR engagement triggers innate immune activation, initiating the afferent limb of the adaptive immune response (Braido, Tarantini, Ghiglione, Melioli, & Canonica, 2007).

OM-85 is an orally administered lyophilized extract obtained by alkaline extraction of heat-inactivated bacteria derived from eight pathogenic species consistently implicated in respiratory infections: Haemophilus influenzae, Streptococcus pneumoniae, Streptococcus pyogenes, Streptococcus viridans, Klebsiella pneumoniae subspecies pneumoniae, Klebsiella pneumoniae subspecies ozaenae, Staphylococcus aureus, and Moraxella catarrhalis (De Benedetto & Sevieri, 2013). The preparation consists of a concentrated mixture of acidic proteins, peptides, amino acids, and detoxified lipopolysaccharides, administered as a daily oral capsule (De Benedetto & Sevieri, 2013). This polyvalent composition is designed to present the mucosal immune system with a broad antigenic repertoire reflective of the pathogens most commonly responsible for both upper and lower respiratory infections.

4.2 Dendritic Cell Maturation and Migration to Mesenteric Lymph Nodes

Following PAMP recognition in the Peyer's patch subepithelial dome, immature dendritic cells (iDCs) undergo maturation characterized by upregulation of co-stimulatory molecules (CD80, CD86, CD40), MHC class II antigen-presenting complexes, and the chemokine receptor CCR7, the latter directing migration towards CCL19/CCL21-expressing lymphatics (Banchereau et al., 2000). Mature dendritic cells migrating to mesenteric lymph nodes carry antigen in a form that enables efficient presentation to naïve CD4+ T helper cells, initiating antigen-specific adaptive immune responses (Banchereau et al., 2000). The nature of the DC-derived cytokine milieu at this stage is critical in determining the character of the subsequent immune response; IL-12-rich environments promote Th1 polarization and IgG class switching, while environments enriched in IL-4 and BAFF favor IgA class switching essential for mucosal protection (A. Thakur & Foged, 2020). The BL-derived PAMPs in OM-85, particularly LPS and lipoteichoic acid, predominantly engage TLR4 and TLR2 respectively, driving primarily Th1-biased responses while also facilitating IgA class switching through TGF-β production by regulatory T cells in the mesenteric environment (Braido et al., 2007).

4.3 IgA Class Switching and Trafficking to the Respiratory Mucosa

Within the germinal centers of mesenteric lymph nodes (Fig. 3), B cells undergo IgA class switching under the coordinated influence of T helper cell-derived cytokines, particularly TGF-β and IL-10, and stromal cell-derived BAFF and APRIL (Mantis et al., 2011). IgA-switched plasma cell precursors, now expressing the α4β7 integrin and CCR10, exit via the thoracic duct, enter the systemic circulation, and extravasate into mucosal effector sites including the bronchial mucosa (Brandtzaeg, 2009). Upon reaching the respiratory submucosa, these plasma cells terminally differentiate, secrete dimeric IgA, and this IgA is captured by pIgR on the basolateral surface of respiratory epithelial cells, transcytosed, and released into the airway lumen as sIgA (Lamm, 1997). Secretory IgA performs multiple effector functions at the respiratory mucosal surface, including immune exclusion (for prevention of pathogen attachment to epithelial cells), neutralization of toxins and viral particles, and agglutination of bacteria to prevent colonization (Mantis et al., 2011).

Critically, any antigen-specific IgA generated through this gut-priming pathway would be expected to recognize epitopes derived from components of the BL preparation and theoretically confer protection against the bacterial species from which those antigens were derived (Jurkiewicz & Zielnik-Jurkiewicz, 2018). For OM-85, this specificity spans the eight species included in its formulation, with IgA trafficking to respiratory mucosal surfaces carrying specificity for antigens from H. influenzae, S. pneumoniae, S. aureus, and the other organisms represented in the preparation (Kearney et al., 2015). This broad polyvalent antigenic coverage, combined with the non-specific innate immune priming conferred by BL-derived PAMPs, provides the mechanistic basis for the clinical utility of OM-85 in reducing the burden of recurrent respiratory infections (Esposito et al., 2018).

Figure 3. Immune responses at inductive and effector sites of the common mucosal immune system (A. Thakur & Foged, 2020)

5. Quantifying the Efficiency of the Oral Route

5.1 Antigen Degradation in the Gastrointestinal Environment

While the immunological logic of oral bacterial lysate therapy is mechanistically coherent, the efficiency with which oral antigen delivery translates into meaningful respiratory mucosal immunity is constrained by several intrinsic limitations. The most fundamental of these is the destructive chemical environment of the gastrointestinal tract, which subjects orally delivered antigens to strongly acidic conditions in the stomach, followed by proteolytic digestion by pancreatic enzymes and intestinal proteases in the small intestine (A. Thakur & Foged, 2020). Protein antigens, which constitute the primary immunostimulatory components of BL preparations, are particularly vulnerable to denaturation and proteolytic degradation in this environment, with the result that the fraction of administered antigen reaching the Peyer's patch M cells in an immunologically intact and recognizable form is substantially less than the administered (A. Thakur & Foged, 2020). For OM-85, this necessity is reflected in its formulation as concentrated lyophilized capsules delivering substantial quantities of multi-species BL material, the majority of which must survive gastrointestinal transit to reach the intestinal immune inductive compartments (De Benedetto & Sevieri, 2013).

5.2 Variable sIgA Induction at Respiratory Sites Following Oral Immunisation

Even when intact antigen successfully reaches the Peyer's patch inductive compartment and initiates an adaptive immune response, the subsequent trafficking of IgA-secreting plasma cells to the respiratory mucosal surface is not guaranteed to be efficient or quantitatively sufficient for protection (Hellfritzsch & Scherliess, 2019). Several factors contribute to this variability. First, the efficiency of M cell-mediated transcytosis varies with the physicochemical properties of the antigen, including size, surface charge, and hydrophobicity (A. Thakur & Foged, 2020). Second, the efficiency of lymphocyte priming in mesenteric lymph nodes depends on the quality of antigen-DC interaction and the cytokine environment at the time of priming (Banchereau et al., 2000). Third, the proportion of primed lymphocytes that ultimately home specifically to the respiratory tract — as opposed to other mucosal compartments such as the intestinal lamina propria, salivary glands, or genitourinary mucosa — is regulated by the specific combination of homing receptors induced during priming and the tissue-specific expression of their counter-receptors (Brandtzaeg, 2009). Collectively, these factors may limit the magnitude, reproducibility, and durability of respiratory sIgA responses following oral immunization compared with direct mucosal delivery to the respiratory tract (Lavelle & Ward, 2022). 

5.3 Dose Requirements and Their Relationship to Mucosal Response Magnitude

The relationship between orally administered antigen dose and the magnitude of respiratory mucosal IgA response is non-linear and subject to substantial inter-individual variability (Hellfritzsch & Scherliess, 2019). Animal model studies have demonstrated that increasing oral antigen doses can enhance mesenteric lymph node priming and, by extension, the quantity of IgA-secreting plasma cells generated; however, the proportion of these plasma cells that successfully home to and colonize the respiratory mucosa does not increase proportionally with dose (Davis, 2001). This dissociation between administered dose and respiratory mucosal protection efficiency represents a fundamental inefficiency of the gut-lung axis as a route for delivering respiratory immunological protection.

The dose-response inefficiency is further complicated by the phenomenon of oral tolerance, whereby repeated oral exposure to high doses of soluble antigen can lead to immune hyporesponsiveness, the induction of antigen-specific regulatory T cells and suppression of effector immune responses (A. Thakur & Foged, 2020). BL preparations, with their complex PAMP content and adjuvant properties, are less susceptible to oral tolerance induction than pure protein antigens; nonetheless, the potential for tolerance to develop with long-term repeated oral dosing, as recommended in clinical protocols for OM-85, cannot be entirely excluded (Esposito et al., 2018). The complexity of these dose-response dynamics underlines the difficulty of achieving reliable and predictable respiratory mucosal protection through the oral route alone.

6. Clinical Correlates: What OM-85 Studies Tell Us

6.1 Documented Immune Responses in Respiratory Secretions Following Oral OM-85 Administration

Clinical studies evaluating the immunological effects of oral OM-85 (OM-85 BV) administration have documented a range of responses in both pediatric and adult populations, providing empirical evidence for the mechanistic pathway described above. Increases in serum IgA, salivary IgA, and nasal wash sIgA have been reported following oral OM-85 treatment, consistent with gut-primed IgA plasma cells trafficking to mucosal effector sites including the respiratory tract (Braido et al., 2007). Activation of innate immune pathways, including the activation of monocytes and macrophages and modulation of circulating dendritic cell populations, has also been documented, suggesting that BL-derived PAMPs in OM-85 exert immunostimulatory effects extending beyond the intestinal inductive compartment (Kearney et al., 2015).

A pivotal multicenter double-blind placebo-controlled study by Schaad and colleagues demonstrated that two courses of OM-85 over a school year significantly reduced both the frequency of upper respiratory tract infections and the number of days with respiratory symptoms in treated children compared with placebo (Schaad, 2010). Gutierrez-Tarango and Berber similarly documented that two courses of OM-85 BV administered to children over twelve months were associated with a significant reduction in the incidence of respiratory tract infections and improved quality-of-life measures, with an acceptable safety and tolerability profile (Gutierrez-Tarango & Berber, 2001). The comprehensive systematic review by Schaad further synthesized evidence from multiple randomized controlled trials, concluding that OM-85 reduces the recurrence of upper and lower respiratory infections in pediatric populations, with clinical benefits attributed to its capacity to activate both innate and adaptive mucosal immune pathways (Schaad, 2010).

6.2 Cases Where OM-85 Demonstrated Measurable but Modest Protection

Despite the clinical evidence supporting some degree of efficacy for OM-85, the breadth and consistency of the protection it confers must be interpreted with caution. The immunological evidence is not uniformly positive; the magnitude of respiratory sIgA responses documented in clinical studies is typically modest, and the relationship between measured sIgA titers and clinical protection against specific respiratory pathogens has not been rigorously quantified (De Benedetto & Sevieri, 2013). Furthermore, considerable heterogeneity in trial methodology, patient populations, outcome definitions, and duration of follow-up complicates meta-analytic interpretation of the OM-85 evidence base (Esposito et al., 2018).

Reflecting these concerns, the European Medicines Agency (EMA), following a systematic review of the evidence for bacterial lysate medicines including OM-85, concluded that such preparations should be used exclusively to prevent recurrent respiratory infections and should not be employed for the treatment of ongoing infections or the prevention of pneumonia (EMA, 2019). The EMA further noted that the quality of clinical trial evidence was variable, with concerns regarding methodological rigor and the absence of standardized immunological endpoints, and called for additional data on safety and effectiveness from new clinical studies (EMA, 2019). This regulatory assessment encapsulates the central tension inherent in oral BL therapy: demonstrable but inconsistent clinical benefit attributable to the mechanistic inefficiencies of the gut-lung immune trafficking pathway.

7. The Case for Direct Pulmonary Targeting

7.1 Bypassing the Gut-Lung Axis Inefficiency Through Inhalation

The limitations of oral OM-85 therapy logically suggest an alternative immunization strategy: the direct delivery of bacterial antigens to the respiratory mucosal surface through inhalation. By circumventing the gastrointestinal route entirely, inhalation-based antigen delivery eliminates the immunological inefficiencies inherent in the gut-lung axis. This includes antigen degradation in the acid-proteolytic gastrointestinal environment, incomplete M cell uptake, variable mesenteric lymph node priming, and imprecise homing of gut-primed lymphocytes to the respiratory compartment. Instead, pulmonary administration delivers antigen directly to the inductive sites of the respiratory mucosal immune system (Lavelle & Ward, 2022). These inductive sites — including the BALT and the inducible BALT (iBALT) that forms in the lung under conditions of sustained antigenic stimulation (Fig. 5) — contain all the cellular machinery required for local antigen sampling, dendritic cell activation, T cell priming, and IgA class switching (Moyron-Quiroz et al., 2004).

Figure 4. Inducible bronchus-associated lymphoid tissue (BALT) development schematic (Hwang, Randall, & Silva-Sanchez, 2016)

The potential immunological advantages of direct pulmonary antigen delivery are substantial. Antigen delivered by inhalation to the mid- and lower airways encounters a rich population of alveolar macrophages, dendritic cells, and M cells in the bronchial epithelium, all capable of antigen sampling and presentation (Hellfritzsch & Scherliess, 2019). The local generation of sIgA following inhalational immunization directly targets the respiratory mucosa, producing immune exclusion precisely at the site of pathogen entry. Furthermore, pulmonary antigen delivery activates both innate and adaptive immune responses locally, and under conditions of sustained or repeated antigen exposure can induce iBALT formation, which further amplifies and concentrates the immune response in the lung parenchyma (Randall, 2010). The BALT tissue, while not an innately permanent structure in healthy adults, forms in response to microbial exposure or inflammatory stimuli, and the presence of iBALT is associated with accelerated and concentrated immune responses capable of controlling infection and limiting pathological tissue damage (Moyron-Quiroz et al., 2004).

7.2 Comparative Mucosal IgA Induction: The Immunological Case for Respiratory Route Delivery

The immunological principle that local antigen delivery generates superior local immunity to delivery at an anatomically remote site is both theoretically well-grounded and empirically well-supported across the mucosal vaccine literature. Mucosal vaccines delivered directly to the respiratory tract have consistently demonstrated superior capacity to induce sIgA at the pulmonary mucosal surface compared with parenteral or oral routes of immunization (Lavelle & Ward, 2022). This superiority is mechanistically explicable: antigen introduced directly into the airway lumen is immediately encountered by the dense and diverse populations of antigen-presenting cells (APC) resident in the respiratory epithelium — including alveolar macrophages, dendritic cells, and M cells distributed across the bronchial mucosa — without the cumulative efficiency losses incurred during gastrointestinal transit, incomplete Peyer's patch M cell uptake, and the serial steps of mesenteric lymph node priming and cross-compartmental lymphocyte trafficking (Lu & Hickey, 2007). The inductive-to-effector pathway for respiratory sIgA production is substantially shortened, localized, and mechanistically direct when the inhalation route is employed, bypassing each of the sequential efficiency losses that characterize the gut-lung axis.

Contemporary evidence from inhaled vaccine development programs provides compelling empirical support for this principle across multiple respiratory pathogens. Investigations into inhaled formulations for SARS-CoV-2 — a respiratory pathogen for which mucosal immunity has been recognized as a critical but persistently underachieved objective of conventional parenteral vaccination — have demonstrated that aerosolized antigen delivery can generate robust sIgA responses in respiratory secretions alongside systemic antibody titers, producing protection at the site of pathogen entry that parenteral strategies largely fail to achieve (Ye et al., 2023). Inhaled single-dose dry powder aerosol formulations have been shown in non-human primate studies to elicit neutralizing mucosal antibodies in both the upper and lower respiratory tracts, providing immunological coverage across the full anatomical range of pathogen challenge (Ye et al., 2023). The strategic advantage of mucosal boosting following primary systemic immunization has been further demonstrated in primate challenge models, where an inhalational boost elicited substantially enhanced mucosal IgA responses and improved protection against respiratory pathogen challenge compared with a matched systemic boost administered at the same antigen dose (McMahan et al., 2023). These findings across distinct respiratory pathogens and vaccine platforms converge on a consistent conclusion: antigen delivery to the respiratory mucosa preferentially induces immunity at that surface, producing sIgA titers in airway secretions that substantially exceed those achievable through the oral or systemic routes (Masjedi, Montahaei, Sharafi, & Jalali, 2022).

A critical qualitative advantage of direct pulmonary antigen delivery, beyond the magnitude of sIgA induction, is its capacity to establish persistent local immune architecture within the lung itself. Repeated or sustained inhalational antigen exposure promotes the formation of iBALT — organized lymphoid structures that develop within the lung parenchyma under conditions of microbial or inflammatory stimulation — which function as dedicated local inductive sites (Moyron-Quiroz et al., 2004). iBALT concentrates antigen-presenting cells, T and B lymphocytes, and follicular dendritic cells in proximity to the respiratory epithelium, enabling rapid and locally amplified immune responses upon subsequent antigen encounter and sustaining germinal center reactions that high-titer local sIgA secretion depends upon (Randall, 2010). This self-reinforcing local immune architecture is qualitatively unattainable through oral OM-85 administration, which engages the intestinal rather than the pulmonary inductive compartment and therefore does not drive antigen-induced lymphoid organogenesis within the lung (Correa, Portilho, & De Gaspari, 2022). Furthermore, inhaled antigens processed by respiratory dendritic cells and alveolar macrophages are trafficked to lung-draining mediastinal lymph nodes, initiating systemic adaptive immune responses in parallel with local mucosal activation (Hellfritzsch & Scherliess, 2019). This simultaneous induction of mucosal sIgA at the respiratory surface and systemic IgG in circulation provides more comprehensive immunological defense against respiratory pathogens than the predominantly systemic response generated by parenteral vaccination, or the modest and geographically imprecise mucosal response generated by oral BL administration through the gut-lung axis (Dotiwala & Upadhyay, 2023).

7.3 Particle Engineering as an Enabler of Pulmonary Immune Targeting

The translation of inhaled antigen delivery into clinically effective immunization is critically dependent on the design of particulate carrier systems that simultaneously satisfy several engineering requirements: aerodynamic suitability for mid- to lower-airway deposition, physical stability as dry powders, biological compatibility with respiratory tissues, protection of encapsulated antigens from degradation during manufacture and storage, and active engagement with the antigen-presenting cell populations populating the pulmonary inductive compartment (Hellfritzsch & Scherliess, 2019). Soluble antigen preparations are poorly suited to meet these demands; antigen in soluble form is rapidly cleared from the respiratory surface by mucociliary action and is poorly sampled by APCs relative to the same antigen presented in a particulate format (De Temmerman et al., 2011). Particulate antigen delivery systems confer fundamental advantages: antigens associated with particles are more efficiently internalized, processed, and presented by APCs than equivalent soluble preparations, while the particulate architecture simultaneously enables control of antigen release kinetics, depot formation at the deposition site, and cell-type-specific targeting through modulation of particle size, surface chemistry, and material composition (De Temmerman et al., 2011). These properties represent qualitative shifts in how the respiratory immune system encounters and processes antigenic material, with direct consequences for the magnitude, durability, and character of the induced immune response.

Particle size is among the most consequential engineering parameters governing pulmonary immune cell engagement (Fig. 4). Particles in the 1–5 μm aerodynamic diameter range are preferentially deposited in the mid- to lower airways (bronchiolar and alveolar regions where BALT can develop) and are efficiently internalized by these cells through phagocytosis, a mechanism that mimics microbial uptake and promotes humoral immune responses including the IgA class switching essential for respiratory mucosal protection (Baranov, Kumar, Sacanna, Thutupalli, & van den Bogaart, 2020). Particles exceeding 5 μm in aerodynamic diameter deposit preferentially in the upper airways and oropharynx, reducing access to the deep pulmonary immune compartment, while particles smaller than 1 μm are largely exhaled before deposition occurs (Patel, Gupta, & Ahsan, 2015). Engineering aerodynamic particle size to fall within the therapeutically optimal 1–5 μm window is therefore a prerequisite for effective pulmonary immune targeting, requiring careful co-optimization of particle density, morphology, and surface energy during formulation development to achieve a mass median aerodynamic diameter (MMAD) and fine particle fraction (FPF) consistent with mid-airway deposition (Pilcer & Amighi, 2010). These aerodynamic parameters constitute the primary physical determinants distinguishing inhaled vaccine formulations capable of reaching the pulmonary immune inductive compartment from those that deposit inefficiently in the upper airway and are subsequently swallowed.

Figure 5. Respiratory tract showing particle deposition by aerodynamic size (A. K. Thakur, Kaundle, & Singh, 2020)

Beyond aerodynamic sizing, the surface chemistry of inhaled particulate systems can be exploited to direct antigen toward specific immune cell populations through receptor-mediated mechanisms, amplifying both the efficiency and the immunological quality of antigen uptake. Carrier materials presenting mannose or galactose residues serve as ligands for C-type lectin receptors and the mannose receptor (CD206) constitutively expressed on alveolar macrophages and immature dendritic cells, enabling receptor-targeted endocytosis that substantially enhances antigen uptake efficiency and the quality of downstream MHC-II-restricted antigen presentation compared with receptor-independent particle internalization (Rodrigues & Grenha, 2015). This targeted approach is of particular immunological relevance because CD206-mediated uptake specifically engages professional antigen-presenting cells, the cellular population responsible for initiating adaptive immune responses and IgA class switching within the respiratory inductive compartment, rather than distributing antigen indiscriminately across cell types (Rodrigues & Grenha, 2015). Natural polysaccharide-based carrier materials are especially attractive candidates for achieving this receptor-targeted functionality, offering intrinsic biocompatibility, susceptibility to enzymatic degradation by enzymes present in pulmonary tissue, and structural flexibility to incorporate receptor-targeting moieties alongside mucoadhesive functional groups within a single carrier matrix (Rodrigues & Grenha, 2015). Synthetic polymer systems, including poly(lactic-co-glycolic acid) (PLGA) microparticles, have also been extensively evaluated for particulate subunit vaccine delivery, with compelling evidence that particle-associated antigens elicit significantly stronger and more durable humoral immune responses than equivalent soluble antigens at matched doses (Silva, Soema, Slutter, Ossendorp, & Jiskoot, 2016). These converging lines of evidence from both natural and synthetic polymer carriers establish that the particulate format itself, independent of any specific material, confers fundamental immunological advantages for inhaled antigen delivery.

The mucoadhesive properties of particulate carriers represent a further engineering dimension of immunological consequence. Mucoadhesive polymers strengthen the interaction between inhaled particles and the respiratory tract mucosa through hydrogen bonding, electrostatic attraction, and physical entanglement with mucin glycoproteins, prolonging the residence time of deposited antigen at the mucosal surface beyond what non-adhesive formulations achieve (Guo et al., 2021). Extended mucosal residence translates directly into increased duration of antigen contact with tissue-resident APCs, providing a sustained stimulatory signal that supports germinal center formation, antibody affinity maturation, and the development of immunological memory, which are parameters of critical importance for durable respiratory protection (Vasquez-Martinez, Guillen, Moreno-Mendieta, Sanchez, & Rodriguez-Sanoja, 2023). For inhaled BL preparations designed to exploit the respiratory inductive compartment, mucoadhesion-mediated retention maximizes the probability of antigen encounter by resident APCs before mucociliary clearance displaces deposited material, effectively lowering the administered dose threshold required for immunologically meaningful stimulation (Guo et al., 2021). Finally, the stability advantages of dry powder formulations, which avoid the aqueous degradation kinetics and cold chain requirements that constrain liquid antigen preparations, are of substantial practical significance for BL preparations containing complex protein and cell wall component mixtures, supporting the manufacturing feasibility and global distribution of inhaled BL vaccines in settings where refrigeration infrastructure is limited (Masjedi et al., 2022). Engineering principles such as aerodynamic sizing and controlled antigen release for sustained immune stimulation define the design space for inhalable particulate antigen carriers capable of systematically addressing the immunological limitations inherent in the oral OM-85 approach.

8. Conclusion: Axis Exploitation Versus Direct Mucosal Targeting

The gut-lung immunological axis is a genuine and consequential biological phenomenon, underpinned by the shared homing receptor programs, mesenteric lymph node trafficking pathways, and IgA secretion mechanisms that constitute the common mucosal immune system. Its clinical exploitation through oral bacterial lysate therapy, exemplified most extensively by OM-85, represents a logical extension of this biology, and the empirical evidence from both preclinical and clinical studies confirms that the gut-lung axis can be therapeutically leveraged to generate some degree of respiratory immune protection (Kearney et al., 2015; Schaad, 2010). The accumulated clinical record of OM-85 in particular provides proof that gut-primed immune responses do reach the respiratory mucosa with sufficient frequency and magnitude to reduce the burden of recurrent respiratory infections in a meaningful proportion of treated patients (Gutierrez-Tarango & Berber, 2001; Schaad, Mutterlein, Goffin, & Group, 2002).

However, the inherent inefficiencies of this approach, such as antigen degradation in the gastrointestinal environment and imprecise lymphocyte homing to the respiratory compartment, constitute fundamental constraints on the efficacy and consistency of oral OM-85 therapy (Hellfritzsch & Scherliess, 2019; Lavelle & Ward, 2022). These limitations are not incidental or correctable through formulation optimization alone; they are structural features of the gut-to-lung immune trafficking pathway. The multiple sequential steps required for oral antigen to generate a respiratory mucosal immune response each impose an efficiency cost, and the cumulative effect substantially diminishes the respiratory immune output achievable per unit of orally administered antigen.

Direct pulmonary antigen delivery by inhalation offers a mechanistically superior alternative, delivering antigen to the precise anatomical location where respiratory immunity is needed, engaging a richer complement of antigen-presenting cells, and generating stronger local sIgA responses at reduced antigen doses. The development of inhalable polysaccharide microparticulate systems and galactomannan-based platforms such as LBG in particular as engineered carriers for bacterial lysate antigens represents a convergence of receptor-targeted immune cell recognition, precision aerosol engineering, and mucosal immunology that holds meaningful potential for the transformation of respiratory infection prophylaxis (Hellfritzsch & Scherliess, 2019; Rodrigues & Grenha, 2015). In this framing, the gut-lung axis is replaced by a more direct, more efficient, and more immunologically precise approach to mucosal protection of the respiratory tract.

The analysis presented in this article underscores a broader principle in mucosal immunology: that the common mucosal immune system, while enabling remote protection through inter-compartmental immune trafficking, imposes an intrinsic loss of efficiency with each step of lymphocyte transit from inductive to effector site. Inhalation strategies that minimize this transit by co-localizing immune induction and effector function at the respiratory mucosa stand to offer superior prophylactic efficacy for respiratory tract infections, a clinical need of considerable and enduring global magnitude (Esposito et al., 2018). Future research should focus on optimizing antigen dose, particulate carrier architecture, delivery device, and administration schedule for inhaled BL preparations, and on establishing the immunological correlates of protection necessary to support regulatory approval of this approach as a novel mucosal immunization strategy.

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Visual Sources

Cover Image: Monstera Production (2020). Creative illustration of microorganisms inside the human body showing diversity and microbiology concept. Pexels. https://www.pexels.com/photo/cutout-paper-illustration-of-person-with-bacilli-in-body-5842119/

Figure 1. Scanning electron microscopy of Peyer's patch follicle epithelium, showing a microvillar surface with M cells appearing as depressions (Dillon & Lo, 2019).

Figure 2. The homing receptor programme of IgA-secreting plasma cell precursors (Pracht, Wittner, Kagerer, Jack, & Schuh, 2023).

Figure 3. Immune responses at inductive and effector sites of the common mucosal immune system (A. Thakur & Foged, 2020).

Figure 4. Inducible bronchus-associated lymphoid tissue (BALT) development schematic (Hwang, Randall, & Silva-Sanchez, 2016).

Figure 5. Respiratory tract showing particle deposition by aerodynamic size (A. K. Thakur, Kaundle, & Singh, 2020).