October - December 2007: 
Volume 20, Issue 4

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The Immunology of the Respiratory System
SUMMARY. The respiratory tract is exposed to a wide variety of exogenous insults as a consequence of the daily inhalation of more than 8,500 litres of air. Recognition and effective host defence against harmful inhaled agents is regulated locally by orchestrated interactions between the humoral and cellular components of innate and adaptive immunity. Recent evidence suggests that the innate immunity has a pivotal role in respiratory antimicrobial defenses by rendering the airway epithelial cells not a passive barrier only, but also an active interface that responds to microbial exposure with the production of a variety of cytokines, chemokines and antimicrobial peptides. The recognition systems of common microbial motifs by airway epithelial cells have recently been described as a diversity of pattern recognition receptors, including the Toll-like receptor family members, which display a certain degree of specificity against a number of pathogen associated molecules. Activation of these receptors regulates not only the killing of microorganisms, but also the recruitment of cells of innate and adaptive immunity, inflammatory and anti-inflammatory responses and, finally, wound repair. The aims of this review are to recapitulate the role of the adaptive immune system in the respiratory tract, highlight the role of innate immunity and describe the new developments in the rapidly evolving area of research into these mechanisms. Pneumon 2007; 20(4):384–394
Full text

The respiratory system consists of the lungs and the system of airways connecting the pulmonary parenchyma to the exterior environment, and is consequently, by its nature, open to the exterior environment. It is estimated that in steady state condition, more than 8,500 litres air are conveyed daily by the airway tracts to ensure the gas exchange. However, inhaled air contains a large quantity of inorganic substances such as smoke and soot, as well as organic particles including pollen, fungi, viruses and, in particular, cocci and bacteria, which all together constitute the extraneous harmful agent or, in the broad sense of the term, the “antigen”. In addition the lung comes into contact with dangerous endogenous factors, such as toxins and immunocomplexes, which reach it via the pulmonary circulation. All these factors may explain the frequency of lung involvement in chronic systematic diseases and also the high incidence of acute lung injury1.

Because of its particular anatomical site and function, the respiratory tract has developed to a significant degree protective mechanisms and defence systems to prevent and respond to the continuous invasion of harmful factors. A matter of vital importance in the immunobiology of the respiratory system is that the locally acting protective and defence systems should operate in ways not affecting adversely its physiology, histology, homeostasis and function2.

As in other systems of the organism, the immunology of respiratory system includes both protective measures and defence systems3,4. In this review, the humoral and cellular components of the respiratory immune system are described in detail, with special focus on the description of structures and mechanisms that mediate inflammatory and anti-inflammatory reactions. Knowledge of the immunobiology of the respiratory system is essential for the comprehensive understanding of the pathogenesis of a spectrum of lung diseases in which the immune system plays a major role.


The respiratory system is structurally and functionally divided into two main compartments: a conducting part that includes the nasal cavity, nasopharynx, larynx, trachea, bronchi and bronchioles, and a respiratory part consisting of the terminal bronchial tree and the alveoli. Harmful agents are prevented from penetrating the mucosa throughout the entire respiratory system by a series of components and functions that constitute its protective measures. Such measures are the structural continuity of the epithelium, the mucosal secretions and the presence of saprophytic microorganisms.

A significant role in the protection of the organism from inhaled injurious factors is played by the pseudostratified columnar ciliated epithelium that lines the major part of the upper respiratory tract and ensures the removal of harmful agents towards pharynx by the coordinated movements of its cilia. The ciliated epithelium, however, apart from providing physical protection also plays an important role as a first line of defence in the immunobiology of the respiratory tree5.

The mucosal secretions are another component of the respiratory protective mechanisms, and these consist of mucus, secreted by caliciform cells, and humoral substances. Mucus is a mixture of macromolecular polysaccharides, which do not constitute nutritious material for microorganisms, but trap foreign particles and when the mucus becomes detached from the mucosal surface, the harmful factor is also removed. An additional principal substance in the respiratory mucosal secretions is lysozyme or muramidase, which is a 14 kD protein consisting of a 129 amino-acid polypeptide chain that folds around itself and is retained in tertiary struture by four disulfide bonds. This functions as a hydrolytic enzyme that breaks the N-acetylglucosamine (GlcNac)- N-acetyl muramic acid (MurNac) bond, which although very frequent in the wall of bacteria, is a bond that is not present in the human organism. Its hydrolysis by muramidase neutralizes the microorganisms, preventing their entry into the tissues.

The saprophytic microorganisms of the respiratory system constitute its “flora”. These are bacteria that do not harm the healthy host but offer protection by competing with and preventing the growth of pathogenic microorganisms


The defence systems of the respiratory tree include humoral factors and cells of non-specific (innate) and specific (adaptive) immunity, which are mobilised to eliminate exogenous biological agents that have invaded the organism6. The term “non-specific” refers to the stereotyped activation of the same defence mechanisms to combat various foreign biological factors, independent of their particular nature. The term “specific” refers to the mobilisation of particular defence mechanisms specific for individual foreign biological agents. The specific mechanisms are not congenital, but have been aquried and evolved through previous contact of the organism with the same harmful factor. In other words, adaptive immunity is characterised by “specificity” and “memory” of the invading biological agent.

1. Non-specific defence systems

1.1. Humoral factors

Non-specific humoral defence systems include a set of biomolecules with bacteriocidal or antiviral properties and other molecules that, when they adhere to the harmful agent, facilitate its recognition (opsonization) and phagocytosis by phagocytes. Central components of the first group are complement, muramidase, interferons (IFNs), antimicrobial peptides and lactoferrin, while the latter group comprises acute phase proteins, complement factors, lipopolysaccharide (LPS)-binding protein (LBP) and collectins. Collectins belong to the family of lectins and were given their name because of a collagenresembling tail in their structure (collectin = collagen + lectin). Among the collectins of the respiratory system are the surfactant proteins (SP), SP-D and SP-A. SP-A binds LPS and induces the phagocytosis of Gram(-) bacteria by alveolar macrophages while SP-D does not induce phagocytosis but acts in the humid phase of airways. The defence mediated by all these factors constitutes the non-specific humoral immunity7.

1.2. Cells

The non-specific cellular defence systems of the respiratory tract include (a) airway epithelial cells, (b) cells with the capacity of phagocytosis (i.e., phagocytes), such as polymorphonuclear neutrophils and eosinophils, monocytes and macrophages, (c) natural killer cells (NKcells), (d) cells critically contributing to the mechanism of inflammation, such as basophils and mast cells, and (e) antigen presenting cells (APCs), such as dendritic cells (DCs). The defence mediated by all these cellular components represents the non-specific cellular immunity7.

Pattern Recognition Receptors

Cells of non-specific immunity recognize pathogenic microorganisms by detecting molecular patterns commonly found in many groups of pathogens, which represent either ontogenetically conserved structures or products of microbe metabolism. These structures, which are known as pathogen-associated molecular patterns (PAMPs), are not characterised by antigenic variability, they are essential for the survival and activity of the microbes and they are not present in the human organism. PAMPs are recognised by receptors called pattern-recognition receptors (PRRs), which are divided into soluble: Mannan-binding lectin (MBL), C-reactive protein (CRP), serum amyloid protein (SAP), LPS-binding protein (LBP); membrane: CD14, macrophage mannose receptor, macrophage scavenger receptor, macrophage receptor with collagenous structure (MARCO), and intracellular: PKR, dsRNA-activated protein kinase. The interaction of PAMPs with the respective PRRs induces either phagocytosis of the microorganisms or activation of intracellular signalling pathways leading to cytokine production and adhesion and co-stimulatory molecule upregulation6,8,9.

Toll-like receptors

Toll-like receptors (TLRs) have recently been described and classified in the category of PRRs9,10. These receptors are transmembrane proteins resembling those described in the phagocytes of the fruit fly Drosophila Toll. They consist of a leucine rich repeat extracellular domain, a short membrane domain and an intracellular domain which shows similarities with that of interleukin (IL)-1beta receptor, and has been designated as “Toll/IL-1-Receptor” (TIR) or simply “TIR domain”11,12.

Eleven different types of TLR have been well characterised in humans and each binds a specific PAMP. TLR-1 and TLR-2 form heterodimers that recognize tri-acylated lipopeptides of Gram(+) bacteria while TLR-2/TLR6 heterodimers recognise di-acylated lipopeptides of Gram(+) bacteria. TLR-4 acts as a homodimer that recognizes the LPS of Gram(-) bacteria, while TLR-5 recognizes flagellae of Gram(-) bacteria12. All of the above types of TLR are expressed on the outer cell membrane. Other types of TLRs, such as TLR-3, TLR-7, TLR-8 and TLR-, are expressed on the internal surface of the phagolysosomes, where they recognise structures of the phagocytosed pathogens. TLR-3 recognizes viral double stranded RNA (dsRNA), TLR-7 recognizes imidazol-quinollines, TLR-8 interacts with single stranded RNA (ssRNA) and TLR9 with non-methylated bacterial CpG DNA12,13. The ligands of TLR-10 and TLR-11 have not been elucidated, although it seems that TLR-11 recognizes uropathogenic bacteria14.

TLR-4 has been studied extensively. It is expressed on the outer surface of cells, and upon homodimerization it approaches other molecules to form a complex commonly known as “external assembly”. This assembly consists of TLR, MD-2 membrane protein, the CD14 molecule, LBP, and LPS. MD-2 is a protein secreted by the same cell and functions as a bridge stabilizing the two chains of TLR4/TLR4 homodimer12,13. CD14 is a glycosylphosphatidyl- inositole (GPI) -protein, also produced by the underlying cell, and it functions as a receptor via its leucine rich repeats (LRRs). This molecule adheres to the TLR chain and promotes its activation by the respective PAMP or ligand. The other molecules that are part of this assembly, LBP and LPS, bind CD14 molecules and trigger activation of TLR13,12.

TLRs are expressed by cells of a variety of tissues and organs. In the respiratory system, the cells with increased TLR expression are macrophages, dendritic cells, B-lymphocytes and the airway epithelial cells5. Certain nosological entities of the respiratory system, such as bronchial asthma, are characterized by overexpression of TLRs on factor (M-CSF), and macrophage inhibitory factor (MIF) have been found to affect TLR expression with different mechanisms in the various cellular subtypes13,16,17.

Binding of PAMPs to the respective TLR triggers a series of reactions that ultimately lead to activation of the underlying cell. The recruitment of myeloid differentiation factor 88 (MyD88) and its interaction with IL-1 receptor-associated kinase (IRAK) protein and TNF receptor- associated factor 6 (TRAF6) constitute fundamental steps of TLR signalling. Activation of TRAF6 reults in activation of mitogen-activated protein kinase 3 (MAP3K), which in turn activates the transcription factor nuclear factor-κΒ (NF-κB). Alternatively, activation of TRAF6 may result in activation of two adaptor molecules, which will then activate the transcription factor activator protein-1 (AP-1). Transcription factor NF-κB enters the cell nucleus activating genes that encode for the production of proinflammatory cytokines, namely TNF, IL-1β, ΙL-6, IL-8 and IL-12. Transcription factor AP-1 enters the nucleus and encodes for the synthesis of proteins involved in stress and cellular death. However, TLR activation can trigger alternative pathways which lead to IFNβ production by the underlying cell or to upregulation of adhesion and co-stimulatory molecules18,19 (Figure 1).

Figure 1. Toll-like receptor (TLR ) signal transduction pathway. TLR activation triggers diverse signalling pathways, which induce the expression of genes involved in the defense of the organism. The first molecule involved in TLR signal transduction is MyD88 protein, which interacts with both the TIR domain of TLR and IRAK-4 kinase, which in turn interacts with TRAF6. The complex MyD88-IRAK4-TRAF6 formation generates a series of events that culminate in NF-kB or ΑΡ1 activation. This pathway, which begins with MyD88 recruitment and ends in NF-kB and ΑΡ1 activation can be initiated by all TLR types, apart from TLR3, and is called the “classical signal transduction pathway”. In addition, TLR3, TLR4, TLR7, TLR8 and TLR9 can activate alternative pathways that finally lead to IFNβ production. TLR, Toll-like receptor; MyD88, Myeloid differentiation factor-88; IRAK, IL-1-associated kinase; TRAF6, Tumour necrosis factor receptor-activating factor-6; NF-κB, Nuclear factor-κB; MAP3K,
Mitogen-activated phosphokinase-3; AP-1, Activator protein-1; TRIF, TIR domain-containing adapter inducing IFNβ; RIP: Receptor- interacting protein; IRF: Interferon regulatory factor.

Airway epithelial cells

Although airway epithelial cells are not classical cells of the immune system, they constitute the first line of defence in the respiratory system20. They are divided into the ciliated epithelial cells of the upper respiratory tracts, Clara cells of the small airways and the type II epithelial cells of the alveoli. They remove the harmful agent by the coordinated movements of their cilia, they recognize pathogenic structures via their PRRs and TLRs, and they release small anionic and cationic antimicrobial peptides and antimicrobial proteins that act as “endogenous antibiotics” to combat the inhaled harmful factors locally. They also release cytokines, chemokines and chemotactic factors that attract cells of specific and non-specific immunity, triggering local inflammatory reactions (Tables 1 and 2)5. In the majority of cases, the local neutralization of inhaled harmful factors (up to 1010 particles/day) by the airway epithelial cells is achieved without causing inflammatory reactions that could potentially harm the host.


Polymorphonuclear neutrophils

The principal biological mission of polymorphonuclear neutrophils is the phagocytosis of every particle they encounter, biological or not, such as cocci, bacteria, fungi, molecular aggregates, carbon, and colloids. The accomplishment of this task is ensured by their four basic properties: plasticity, mobility, potential for phagocytosis and intracellular killing and digestion. After their release from the bone marrow, polymorphonuclear neutrophils circulate in the bloodstream for about 8 hours, after which they finally pass into the tissues via capillaries. They remain in the interstitium for 3-4 days without ever re-entering the bloodstream21.

Because of its rich vasculature, the respiratory system, and particularly the lung, has large numbers of polymorphonuclear neutrophils adhering to the vascular endothelium (marginal pool). The recruitment of neutrophils to the vascular endothelium and their extravasation to tissues is promoted by the expression of adhesion molecules on the endothelial cells such as E-selectin and by the immunoglobulin-structured intercellular adhesion molecule-1 (ICAM-1), as well as by the local release of chemotactic factors and type CXC chemokines, mainly IL-82.

Polymorphonuclear eosinophils

Polymorphonuclear eosinophils display four basic functions: (a) they are involved in the defence against parasites by releasing toxic substances contained in their electron-dense granules, (b) they secrete histaminase to inactivate the potential excess of histamine that has been released by basophils and mast cells, (c) they secrete aryl-sulfatase, an enzyme that inactivates slow reactive substance-A (SRS-A) produced by basophils and mast cells and, (d) they have phagocytic properties, phagocytosing mainly immunocomplexes; the phagocytosis of bacteria and fungi is not among the primary functions of eosinophils. In addition, they respond to specific chemotactic stimuli that are generated by basophils and mast cells and predominantly by activated T-lymphocytes and are known as “eosinophil chemotactic factor of anaphylaxis” (ECF-A), and to type CC chemokines, mainly eotaxin-1 and the “regulated upon activation, normal T cell-expressed and -secreted” (RANTES) cytokine molecule.21

Monocytes and macrophages

Monocytes are produced by bone marrow stem cells and released into the bloodstream from which they finally move into the tissues through the capillaries. Monocytes display biological and functional properties similar to those of neutrophils but generally have the capacity to phagocytose larger particles. However, their mobility and overall response to chemotactic stimuli are far weaker than those of polymorphonuclear neutrophils. Monocyte recruitment is mediated by type CC chemokines, mainly monocyte chemoattractant proteins (MCP), macrophage inflammatory protein (MIP) and RANTES4.

The macrophages of the respiratory system are derived, as those of other tissues, from blood mononuclear precursors that have migrated into the tissues. They are divided into two types, mobile, that move around in the interstitium, and fixed, that are immobile. The latter are further subdivided into macrophages of the parenchyma, macrophages in the endothelium and macrophages of serous cavities. A significant role in the immunology of respiratory system is played by the alveolar macrophages that are located among the epithelial cells lining the alveoli, but can pass through the alveolar wall into the lymphatic vessels, the connective substratum of the lung or even the pleura. Macrophages often undergo apoptosis and are then found in the serous fluid of the pleura and in tracheobronchial secretions. Alveolar macrophages constitute roughly 95% of airway white blood cells

In terms of function, macrophages perform four basic tasks: (a) phagocytosis and degradation of extrinsic particles or dead and aberrant host material following recognition by their TLRs and other PRRs, (b) release of oxidant radicals in order to kill the bacteria in their immediate environment, (c) degradation of extraneous biological agents and presentation of fragments of them to their major histocompatibility complex (MHC) class II surface molecules, so that they may be recognized by Thelper lymphocytes (TH), and (d) secretion of inflammatory mediators into their environment, contributing to the initiation of the “inflammatory reaction”21. It is of interest that alveolar macrophages appear to have evolved mainly the function of phagocytosis compared to that of antigenpresentation, in order to limit the local inflammatory reactions that could cause tissue damage22.

Natural killer cells

NK-cells constitute an independent type of lymphocyte, characterized by the expression of CD56 surface antigen and the absence of expression of CD3, which defines T-lymphocytes. 85% of CD56+ NK-cells co-express CD16 antigen, while 45%-50% co-express CD57 antigen. They possess azurophilic granules in their cytoplasm, which contain perforine and granzyme. The principal biological mission of NK-cells is the recognition and lysis of neoplastic and virus-infected cells by release of the content of their granules. They also exercise antibody-dependent cellular cytotoxicity (ADCC) against target cells via the CD16 molecule, receptor of the Fc fraction of antibodies (FcγRIII), and have the capacity to produce immunoregulatory cytokines. It has been demonstrated that CD56dim NK-cells show high cytotoxicity, both natural and antibody dependent, while CD56bright NK-cells play an immunomodulatory role following treatment with IL-223.

NK-cells express killer inhibitory receptor (KIR) membrane receptors and CD94/NKG2A and recognize MHC class I antigens expressed by all normal nucleated normal cells, blockading their lysis. Cancer and virus-infected cells, however, have decreased expression of MHC I molecules and consequently are not recognized by these NK-cell inhibitory receptors, which results in their lysis. The principal factors involved in the recruitment of NK-cells to the respiratory system are type CC (MCP1-5, MIP-1α, β, RANTES) and CXXXC (fractalkine) chemokines.

Polymorphonuclear basophils and mast cells

Polymorphonuclear basophils and mast cells are well known for their role in allergic, anaphylactic and inflammatory reactions24. Basophils are derived from the myeloid progenitor cell in the bone marrow, released into the circulation and finally pass into the tissues. Certain immature forms, the mast cell progenitors (MCPs) expressing the receptor/tyrosine kinase c-kit, pass into tissues and give rise to tissue mast cells under the stimulation of stem cell factor (SCF) and other cytokines such as IL-625. Tissue mast cells are further divided in mucosal mast cells (MMCs) and connective tissue mast cells (CTMCs). MMCs mature in the presence of factors secreted by T-helper lymphocytes, particularly during TH2 immune responses such as allergic reactions. It appears that T-lymphocyte derived inducing factors are not mandatory for the generation of CTMCs.

Basophils and mast cells express membrane receptors for the Fc fraction of IgE immunoglobulins (FcεRI), which, via their Fab fraction, constitute the receptors of foreign biological agents. The cross-linking of these receptors leads to degranulation of the cells and the release of tryptase and histamine-like mediators. These substances cause contraction of the post-capillary venules and dilation of the capillaries in order to restrain the spread of the invading agents and facilitate the passage of innate and adaptive immunity factors of the host from the bloodstream to the site of foreign agent entry. It has recently been reported that mast cells express corticotrophin-releasing hormone (CRH) receptors, enabling them to secrete mediators such as vascular endothelial growth factor (VEGF) in conditions of stress without a degranulation process26.

A marked increase of bronchial tree mast cells is observed in patients with allergic bronchial asthma. In these cases the extravasation of MCPs into the tissues is induced by the interaction of their surface α4β7 and α4β1 integrins with the immunoglobulin-structured vascular cell adhesion molecule-1 (VCAM-1), which is expressed on bronchial epithelial cells as a result of the local inflammation. Administration of neutralizing antibodies against α4, β7, α4β1 and VICAM-1 reduces the recruitment of MCPs to the bronchial tree in experimental models of allergic asthma. Finally, an important role in the migration of basophils into the tissues is played by the presence of type CC chemokines (MCP1-5, eotaxin-1, eotaxin-2, RANTES)27,28.

Dendritic cells

DCs are APCs and by presenting antigen they constitute the link between specific and non-specific immunity29,30. They are derived from CD34+ haematopoietic progenitors in the bone marrow. Upon differentiation, they are released into bloodstream and into the tissues, to sites such as the skin, gut and lungs, where they have a high probability of contacting harmful agents, and where they play a vital role in the processing and presentation of the antigen. This is mainly achieved by the high expression of a variety of PRRs. During this process and under the influence of locally released cytokines, they mature, differentiate and finally migrate to lymphatic tissues, such as the lymph nodes, where they activate the antigen-specific T-lymphocytes as well as B-, NK- and NKT-cells. Furthermore, they can direct the immune response towards type TH1 or TH2 depending on their degree of maturation and their environment31. Two types of DCs have been recognised so far, myeloid and plasmacytoid DCs.

Myeloid DCs (DC1) express CD11c antigen but they lack CD123 (IL-3 receptor) expression. They are generated from CD34+ myeloid progenitor cells under the influence of GM-CSF, IL-4 and TNFα and constitute 0.3%-0.7% of peripheral blood mononuclear cells. These are the Langerhans DCs of the skin, the interstitial DCs of tissues and the DCs of monocyte origin. The latter can be generated from blood monocytes in the presence of GM-CSF and IL-4 in vitro. As in other peripheral tissues, interstitial DCs are present in the respiratory system, where they are located among mucosal epithelial cells as well as in the connective tissue around the blood vessels. Certain intraepithelial DCs are characterized by the presence of Birbeck granules and are called Langerhans cells by analogy with the respective skin cells32. As all myeloid DC types, respiratory DCs are initially found as immature cells and they undergo maturation upon activation of their TLRs and other PRRs by the respective ligands (L); these can be microbial structures but also inhaled particles or fragments of apoptotic airway epithelial cells. Under conditions of local inflammation, however, respiratory DCs are greatly increased in number; this increase is probably mediated by recruitment of monocytoid precursors in the area of inflammation under the influence of certain chemotactic factors (antimicrobial peptides, C5a factor of complement) and chemokines (MCP1-5, MIP-1α, β, RANTES). These precursors differentiate and finally maturate into DCs from the effect of locally released pro-inflammatory cytokines (GM-CSF, TNFa). Mature DCs are characterized by vigorous expression of CD40, CD80, and CD86 costimulatory molecules and upon their migration to the lymph nodes they produce large amounts of IL-12 as well as the specific for naive T-lymphocyte chemokines (MIP-3β), triggering a type TH1 immune response. This response is characterized by IFNγ and TNFα production by the CD4+ lymphocytes, leading to macrophage and CD8+ lymphocyte activation.

Plasmacytoid dendritic cells (DC2, pDCs) do not express CD11c antigen but are characterized by CD123 antigen expression. Their origin has not been definitively elucidated, but it is considered that they are derived from a CD34+ bone marrow lymphatic progenitor cell in the presence of IL-3. They constitute 0.5% of peripheral blood mononuclear cells and are located in the lymph nodes. They express CD40, CD80, and CD86 co-stimulatory molecules, CD4 antigen, CD36 and CD68 as well as BDCA-2 and BDCA-4 specific indices, and they also express diverse TLRs. These cells display a certain immunoregulatory potential. In the presence of viruses they produce IFNα and trigger type TH1 immune responses, and on stimulation with IL-3 they induce a type TH2 immune response which is characterised by IL-4, IL-5, IL-6, IL-9, IL-10 and IL-13 cytokine production and IgE type antibody production.

2. Specific defence systems

2.1. Humoral factors

Antibodies constitute the humoral factors of specific immunity. They are immunoglobulins that locally neutralize invading biological agents and this function is mainly achieved through stereochemical interactions with antigenic epitopes resulting in activation of complement, opsonization and/or the induction of ADCC.

The respiratory antibodies are mainly type IgA and IgG immunoglobulins. Tracheobronchial excretions, in particular, contain high concentrations of IgA immunoglobulins, in the form of dimers and sometimes trimers consisting of α2κ2 or α2λ2 units, which are structurally joined together with the conjunctive protein J, as well as with a non-immunoglobulin protein, the transport piece or secretory piece. The secretory piece is part of a receptor that respiratory epithelial cells have on their membrane in order to bind and transfer into their cytoplasm free IgA monomers produced by the proximate plasma cells. The bound IgA molecules are endocytosed along with the receptor. In the cytoplasm, a peptide that lies between the Fc-fractions of two IgA molecules, dissociates from the receptor and constitutes the secretory piece that stabilizes the IgA dimer. This process enables IgA immunoglobulins to pass through the cytoplasm of epithelial cells and be released on mucosal surfaces without undergoing proteolysis during their passing. The primary biological role of IgA antibodies is the neutralization of viruses and bacteria that reach and try to penetrate the mucosal surfaces to pass into the tissues. IgA molecules are used to bind the invaders and consequently to facilitate either their removal by mucociliary clearance or their phagocytosis by the macrophages of respiratory tree21.

2.2. Cells

B- and T-lymphocytes comprise the specific cellular defence component of the respiratory tree. They are found either as non-organised lymphatic tissue in the base membrane and among epithelial cells of the airway mucosae, or as well-organised lymphatic tissue; these are the lymphocytes of the lymph nodes and the lymphocyte accumulations with similar structure and function to that of lymph nodes, such as nasal associated lymphoid tissue (NALT) and bronchus associated lymphoid tissue (BALT), which are part of the mucosal associated lymphoid tissue (MALT)33,34.


B-lymphocytes are abundant in the organised and non-organised lymphatic tissue of the respiratory tree, but they are rarely found in bronchoalveolar lavage. Their principal role is antibody production and involvement in immune responses, both T- and non T-cell-mediated, mainly during respiratory viral infections2,35,36.


The respiratory system contains a large number of CD4 + helper and CD8 + cytotoxic T-lymphocytes. These cells recognize MHC class II and class I glycoproteins, respectively, on the antigen-presenting cells and react with antigenic epitopes via their T-cell receptor (TcR) which is classically comprised of an α and a β chain (TcRαβ). This two-site connection, that is further promoted bilaterally by other membrane molecules, results in: (a) stimulation of CD4+ to produce cytokines, promoting the differentiation of B-cells into antibody-producing plasma cells and the production of T-cytolytic and T-helper cells carrying an antigen-specific TcR on their surface, and (b) activation of CD8+ and the subsequent induction of apoptosis in target cells. A particular characteristic of the respiratory T system is the raised proportion of TcRγδ lymphocytes compared to other tissues and the relatively high proportion of CD4-/CD8- TcRαβ lymphocytes, which are rarely found in other tissues2,37. The role of these lymphocyte subpopulations in the respiratory system has not been yet elucidated.

The circulation of lymphocytes in the respiratory system

Unlike phagocytes, which when extravasated into tissues never return into the bloodstream, T- and Blymphocytes continuously move between the bloodstream and lymphatic organs, on the alert to protect the organism from the entry of an exogenous insult.

The recruitment of lymphocytes in sites of organised lymphatic tissue is ensured by the interaction of adhesion molecules expressed by both lymphocytes and high endothelial venules (HEV) of lymphatic tissue. The selective expression of specific adhesion molecules by both sides enables the selective migration of specific lymphocyte subpopulations to specific sites of lymphatic tissue (homing). It has been shown that in the respiratory system, the initial contact (tethering and rolling) of circulating lymphocytes with endothelial cells in sites of BALT is ensured by the interaction of lymphocyte L-selectin with the carbohydrate rich molecule/ligand PNAd of HEV38. The firm adhesion of lymphocytes to endothelium then occurs via the interaction of the α4β1 integrine of lymphocytes with the immunoglobulin structured VCAM-1 molecule of HEV that is more vigorously expressed in sites of BALT than in other areas of organised lymphatic tissue. In addition, a significant role is played by the interaction of lymphocyte LFA-1 (leukocyte function antigen-1, CD11a/CD18) and PSGL-1 (P-selectin glycoprotein ligand-1) molecules with the immunoglobulin structured ICAM-1 molecule and P-selectin of HEV, as well as the α1β1 integrine, very late antigen-1 (VLA-1) of lymphocytes with airway collagen38,39. Unlike for other MALT sites, the interaction of α4β7 integrin, expressed by certain lymphocyte subpopulations, with the molecule/ligand MAdCAM, has no role in the selective migration of lymphocytes to airway lymphatic tissues. The next step is the extravasation of lymphocytes, which depends on the presence of specific chemokines. For example, the exit of T-lymphocytes is ensured by RANTES chemokine, while CXCL9, CXCL10 and CXCL11 chemokines ensure the extravasation of CD8+, and CXCL16 that of CD4+ lymphocytes38,40,41.

From the above description it is illustrated that certain interactions between lymphocyte subpopulations and HEV endothelium, as well as certain locally released humoral factors determine selective migration of lymphocytes to the respiratory system and their return into the bloodstream, in order to contribute to specific immune responses and defend against harmful agents.

Figure 2. A synopsis of the immune response in the respiratory system. Invasion of the respiratory system by harmful factors induces the release of antimicrobial peptides, pro-inflammatory cytokines and chemokines by airway epithelial cells, resulting in dendritic cell (DC) recruitment. DCs travel via the lymph vessels towards the T-area of lymph nodes where they present the antigen to T-cells that have already moved to the area via HEV. These interactions result in IL-2 and IL-12 production by T-lymphocytes and DCs respectively. IL-2 causes the proliferation of antigen specific CD4+ and CD8+ lymphocytes that pass into the bloodstream and tissues. IL-12 production by DCs promotes TH1 immune response. Some of the antigen specific CD4+ lymphocytes migrate to the follicles where they interact with DCs of the blastic centre and induce the production of antigen specific antibodies and memory Bcells. LPS, Lipopolysaccharide; HEV, High endothelial venules; Ig, Immunoglobulins. TH, T-helper; LPS, Lipopolysaccharide; CpGDNA, bacterial DNA containing non methylated CpG dinucleotides; dsRNA, double stranded RNA; ssRNA, single stranded RNA.

3. A synopsis of immune responses in the respiratory system

Invasion of the respiratory system by exogenous factors induces the release by the airway epithelial cells of antimicrobial peptides, pro-inflammatory cytokines and chemokines, resulting in recruitment of specific and nonspecific immune cells to the area32. At this stage a vital role is played by the recruitment of immature DCs, which recognize, phagocytose and process (antigen processing) antigenic elements of the “insult” by expressing a variety of PRRs at high concentration. DCs travel via the lymph vessels to the T-area of lymph nodes, presenting antigens of the harmful factor on their MHC surface molecules to T-lymphocytes that have migrated to the area via HEV and which also recognize the antigens with their TcR. For a successful immune response, it is essential that an interaction between the adherent molecules of the DCs (Dc-specific ICAM-3 grabbing nonintegrin, DC-SIGN ICAM-1) and the T-lymphocytes (LFA-3, LFA-1), as well as the bilateral interaction of co-stimulatory molecules (CD80 and CD86 with CD28, respectively) are established42. These interactions result in the production of IL-2 by Tlymphocytes and the induction of surface CD40L (TNF family molecule), which, on ligation with the respective receptor CD40 (TNF receptor family, TNFR) of DCs, gives rise to the production of IL-12 and further expression of CD80 and CD86 co-stimulatory molecules43,44. A prerequisite is the expression of other molecules of the TNF/TNFR family by DCs and T-lymphocytes, such as TNF-related activation induced cytokine (TRANCE), TRANCE-R, OX40/ OX40L, respectively, which further induce the differentiation and survival of the cellular populations described above45,46. IL-2 production by T-lymphocytes, along with the simultaneous expression of IL-2 receptors, causes the proliferation of antigen specific (clonal expansion) CD4+ and CD8+ lymphocytes that leave the lymph nodes for the bloodstream and tissues. Some of the antigen specific CD4+ lymphocytes migrate to the follicles where they interact with DCs of the blastic centre and induce, via CD40L, the production of antigen specific antibodies by B-cells30. All of these steps are depicted in Figure 2.


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