Cutting edge: organogenesis of nasal-associated lymphoid tissue (NALT) occurs independently of lymphotoxin-alpha (LT alpha) and retinoic acid receptor-related orphan receptor-gamma, but the organization of NALT is LT alpha dependent

A Novel Organized Nasopharynx-Associated Lymphoid Tissue in Teleosts That Expresses Molecular Markers Characteristic of Mammalian Germinal Centers

Nasal immunity is an ancient and conserved arm of the mucosal immune system in vertebrates. In teleost fish, we previously reported the presence of a nasopharynx-associated lymphoid tissue (NALT) characterized by scattered immune cells located in the trout olfactory lamellae. This diffuse NALT mounts innate and adaptive immune responses to nasal infection or vaccination. In mammals, lymphoid structures such as adenoids and tonsils support affinity maturation of the adaptive immune response in the nasopharyngeal cavity. These structures, known as organized NALT (O-NALT), have not been identified in teleost fish to date, but their evolutionary forerunners exist in sarcopterygian fish. In this study, we report that the rainbow trout nasal cavity is lined with a lymphoepithelium that extends from the most dorsal opening of the nares to the ventral nasal cavity. Within the nasal lymphoepithelium we found lymphocyte aggregates called O-NALT in this study that are composed of ∼ 56% CD4+, 24% IgM+, 16% CD8α+, and 4% IgT+ lymphocytes and that have high constitutive aicda mRNA expression. Intranasal (i.n.) vaccination with live attenuated infectious hematopoietic necrosis virus triggers expansions of B and T cells and aicda expression in response to primary i.n. vaccination. IgM+ B cells undergo proliferation and apoptosis within O-NALT upon prime but not boost i.n. vaccination. Our results suggest that novel mucosal microenvironments such as O-NALT may be involved in the affinity maturation of the adaptive immune response in early vertebrates.

Immunofibroblasts regulate LTα3 expression in tertiary lymphoid structures in a pathway dependent on ICOS/ICOSL interaction

Immunofibroblasts have been described within tertiary lymphoid structures (TLS) that regulate lymphocyte aggregation at sites of chronic inflammation. Here we report, for the first time, an immunoregulatory property of this population, dependent on inducible T-cell co-stimulator ligand and its ligand (ICOS/ICOS-L). During inflammation, immunofibroblasts, alongside other antigen presenting cells, like dendritic cells (DCs), upregulate ICOSL, binding incoming ICOS + T cells and inducing LTα3 production that, in turn, drives the chemokine production required for TLS assembly via TNFRI/II engagement. Pharmacological or genetic blocking of ICOS/ICOS-L interaction results in defective LTα expression, abrogating both lymphoid chemokine production and TLS formation. These data provide evidence of a previously unknown function for ICOSL-ICOS interaction, unveil a novel immunomodulatory function for immunofibroblasts, and reveal a key regulatory function of LTα3, both as biomarker of TLS establishment and as first driver of TLS formation and maintenance in mice and humans.

Skin immunity: dissecting the complex biology of our body's outer barrier

Our skin contributes critically to health via its role as a barrier tissue, carefully regulating passage of key substrates while also providing defense against exogenous threats. Immunological processes are integral to almost every skin function and paramount to our ability to live symbiotically with skin commensal microbes and other environmental stimuli. While many parallels can be drawn to immunobiology at other mucosal sites, skin immunity demonstrates unique features that relate to its distinct topography, chemical composition and microbial ecology. Here we provide an overview of skin as an immune organ, with reference to the broader context of mucosal immunology. We review paradigms of innate as well as adaptive immune function and highlight how skin-specific structures such as hair follicles and sebaceous glands interact and contribute to these processes. Finally, we highlight for the mucosal immunology community a few emerging areas of interest for the skin immunity field moving forward.

Putative Immunological Functions of Inducible Skin-Associated Lymphoid Tissue in the Context of Mucosa-Associated Lymphoid Tissue

Acquired immunity is orchestrated in various lymphoid organs, including bone marrow, thymus, spleen, and lymph nodes in humans. However, mucosa-associated lymphoid tissue (MALT) is evolutionally known to be emerged in the oldest vertebrates as an immunological tissue for acquired immunity, much earlier than the advent of lymph nodes which appeared in endotherms. Furthermore, the lymphocytes which developed in MALT are known to circulate within the limited anatomical areas. Thus, MALT is comprehended as not the structure but the immune network dedicated to local immunity. As for the skin, skin-associated lymphoid tissue (SALT) was previously postulated; however, its existence has not been proven. Our group recently showed that aggregations of dendritic cells, M2 macrophages, and high endothelial venules (HEVs) are essential components to activate effector T cells in the murine contact hypersensitivity model and termed it as inducible SALT (iSALT) since it was a transient entity that serves for acquired immunity of the skin. Furthermore, in various human skin diseases, we reported that the ectopic formation of lymphoid follicles that immunohistochemically analogous to MALT and regarded them as human counterparts of iSALT. These data raised the possibility that SALT can exist as an inducible form, namely iSALT, which shares the biological significance of MALT. In this article, we revisit the evolution of immunological organs and the related components among vertebrates to discuss the conserved functions of MALT. Furthermore, we also discuss the putative characteristics and functions of iSALT in the context of the MALT concept.

Skin-Associated B Cells in the Pathogenesis of Cutaneous Autoimmune Diseases-Implications for Therapeutic Approaches

B lymphocytes are crucial mediators of systemic immune responses and are known to be substantial in the pathogenesis of autoimmune diseases with cutaneous manifestations. Amongst them are lupus erythematosus, dermatomyositis, systemic sclerosis and psoriasis, and particularly those driven by autoantibodies such as pemphigus and pemphigoid. However, the concept of autoreactive skin-associated B cells, which may reside in the skin and locally contribute to chronic inflammation, is gradually evolving. These cells are believed to differ from B cells of primary and secondary lymphoid organs and may provide additional features besides autoantibody production, including cytokine expression and crosstalk to autoreactive T cells in an antigen-presenting manner. In chronically inflamed skin, B cells may appear in tertiary lymphoid structures. Those abnormal lymph node-like structures comprise a network of immune and stromal cells possibly enriched by vascular structures and thus constitute an ideal niche for local autoimmune responses. In this review, we describe current considerations of different B cell subsets and their assumed role in skin autoimmunity. Moreover, we discuss traditional and B cell-associated approaches for the treatment of autoimmune skin diseases, including drugs targeting B cells (e.g., CD19- and CD20-antibodies), plasma cells (e.g., proteasome inhibitors, CXCR4 antagonists), activated pathways (such as BTK- and PI3K-inhibitors) and associated activator molecules (BLyS, APRIL).

M Cells: Intelligent Engineering of Mucosal Immune Surveillance

M cells are specialized intestinal epithelial cells that provide the main machinery for sampling luminal microbes for mucosal immune surveillance. M cells are usually found in the epithelium overlying organized mucosal lymphoid tissues, but studies have identified multiple distinct lineages of M cells that are produced under different conditions, including intestinal inflammation. Among these lineages there is a common morphology that helps explain the efficiency of M cells in capturing luminal bacteria and viruses; in addition, M cells recruit novel cellular mechanisms to transport the particles across the mucosal barrier into the lamina propria, a process known as transcytosis. These specializations used by M cells point to a novel engineering of cellular machinery to selectively capture and transport microbial particles of interest. Because of the ability of M cells to effectively violate the mucosal barrier, the circumstances of M cell induction have important consequences. Normal immune surveillance insures that transcytosed bacteria are captured by underlying myeloid/dendritic cells; in contrast, inflammation can induce development of new M cells not accompanied by organized lymphoid tissues, resulting in bacterial transcytosis with the potential to amplify inflammatory disease. In this review, we will discuss our own perspectives on the life history of M cells and also raise a few questions regarding unique aspects of their biology among epithelia.

Co-localization of lymphoid aggregates and lymphatic networks in nose- (NALT) and lacrimal duct-associated lymphoid tissue (LDALT) of mice

Background

The lymphatic vascular pattern in the head of mice has rarely been studied, due to problems of sectioning and immunostaining of complex bony structures. Therefore, the association of head lymphoid tissues with the lymphatics has remained unknown although the mouse is the most often used species in immunology.

Results

Here, we studied the association of nasal and nasolacrimal duct lymphatics with lymphoid aggregates in 14-day-old and 2-month-old mice. We performed paraffin sectioning of whole, decalcified heads, and immunostaining with the lymphatic endothelial cell-specific antibodies Lyve-1 and Podoplanin. Most parts of the nasal mucous membrane do not contain any lymphatics. Only the region of the inferior turbinates contains lymphatic networks, which are connected to those of the palatine. Nose-associated lymphoid tissue (NALT) is restricted to the basal parts of the nose, which contain lymphatics. NALT is continued occipitally and can be found at both sides along the sphenoidal sinus, again in close association with lymphatic networks. Nasal lymphatics are connected to those of the ocular region via a lymphatic network along the nasolacrimal duct (NLD). By this means, lacrimal duct-associated lymphoid tissue (LDALT) has a dense supply with lymphatics.

Conclusions

NALT and LDALT play a key role in the immune system of the mouse head, where they function as primary recognition sites for antigens. Using the dense lymphatic networks along the NLD described in this study, these antigens reach lymphatics near the palatine and are further drained to lymph nodes of the head and neck region.

NALT and LDALT develop in immediate vicinity of lymphatic vessels. Therefore, we suggest a causative connection of lymphatic vessels and the development of lymphoid tissues.

Molecular and Cellular Requirements for the Assembly of Tertiary Lymphoid Structures

At sites of chronic inflammation, recruited immune cells form structures that resemble secondary lymphoid organs (SLOs). Those are characterized by segregated areas of prevalent T- or B-cell aggregation, differentiation of high endothelial venules (HEVs) and local activation of resident stromal cells. B-cell proliferation and affinity maturation towards locally displayed autoantigens have been demonstrated at those sites, known as tertiary lymphoid structures (TLSs). TLS formation has been associated with local disease persistence and progression as well as increased systemic manifestations. While bearing a similar histological structure to SLO, the signals that regulate TLS and SLO formation can diverge, and a series of pro-inflammatory cytokines has been ascribed as responsible for TLS formation at different anatomical sites. Here we review the structural elements as well as the signals responsible for TLS aggregation, aiming to provide an overview to this complex immunological phenomenon.

Cellular and Vascular Components of Tertiary Lymphoid Structures

Inflammatory immune cells recruited at the site of chronic inflammation form structures that resemble secondary lymphoid organs (SLO). These are characterized by segregated areas of prevalent T- or B-cell aggregation, differentiation of high endothelial venules, and local activation of resident stromal cells, including lymphatic endothelial cells. B-cell proliferation and affinity maturation toward locally displayed autoantigens have been demonstrated at these sites, known as tertiary lymphoid structures (TLS). TLS formation during chronic inflammation has been associated with local disease persistence and progression, as well as increased systemic manifestations. While bearing a similar histological structure to SLO, the signals that regulate TLS and SLO formation can diverge and a series of pro-inflammatory cytokines have been ascribed as responsible for TLS formation at different anatomical sites. Moreover, for a long time the structural compartment that regulates TLS homeostasis, including survival and recirculation of leucocytes has been neglected. In this chapter, we summarize the novel data available on TLS formation, structural organization, and the functional and anatomical links connecting TLS and SLOs.

Fat-Associated Lymphoid Clusters in Inflammation and Immunity

Fat-associated lymphoid clusters (FALCs) are atypical lymphoid tissues that were originally identified in mouse and human mesenteries due to that they contain a high number of type 2 innate lymphoid cells/nuocytes/natural helper cells. FALCs are located on adipose tissues in mucosal surfaces such as the mediastinum, pericardium, and gonadal fat. Importantly, these clusters contain B1, B2 and T lymphocytes as well as myeloid and other innate immune cell populations. The developmental cues of FALC formation have started to emerge, showing that these clusters depend on a different set of molecules and cells than secondary lymphoid tissues for their formation. Here, we review the current knowledge on FALC formation, and we compare FALCs and omental milky spots and their responses to inflammation.

Lymphotoxin in physiology of lymphoid tissues - Implication for antiviral defense

Lymphotoxin (LT) is a member of the tumor necrosis factor (TNF) superfamily of cytokines which serves multiple functions, including the control of lymphoid organ development and maintenance, as well as regulation of inflammation and autoimmunity. Although the role of LT in organogenesis and maintenance of lymphoid organs is well established, the contribution of LT pathway to homeostasis of lymphoid organs during the immune response to pathogens is less understood. In this review, we highlight recent advances on the role of LT pathway in antiviral immune responses. We discuss the role of LT signaling in lymphoid organ integrity, type I IFN production and regulation of protection and immunopathology during viral infections. We further discuss the potential of therapeutic targeting LT pathway for controlling immunopathology and antiviral protection.

The evolution of nasal immune systems in vertebrates

The olfactory organs of vertebrates are not only extraordinary chemosensory organs but also a powerful defense system against infection. Nasopharynx-associated lymphoid tissue (NALT) has been traditionally considered as the first line of defense against inhaled antigens in birds and mammals. Novel work in early vertebrates such as teleost fish has expanded our view of nasal immune systems, now recognized to fight both water-borne and air-borne pathogens reaching the olfactory epithelium. Like other mucosa-associated lymphoid tissues (MALT), NALT of birds and mammals is composed of organized lymphoid tissue (O-NALT) (i.e., tonsils) as well as a diffuse network of immune cells, known as diffuse NALT (D-NALT). In teleosts, only D-NALT is present and shares most of the canonical features of other teleost MALT. This review focuses on the evolution of NALT in vertebrates with an emphasis on the most recent findings in teleosts and lungfish. Whereas teleost are currently the most ancient group where NALT has been found, lungfish appear to be the earliest group to have evolved primitive O-NALT structures.

Central Role of Core Binding Factor β2 in Mucosa-Associated Lymphoid Tissue Organogenesis in Mouse

Mucosa-associated lymphoid tissue (MALT) is a group of secondary and organized lymphoid tissue that develops at different mucosal surfaces. Peyer's patches (PPs), nasopharynx-associated lymphoid tissue (NALT), and tear duct-associated lymphoid tissue (TALT) are representative MALT in the small intestine, nasal cavity, and lacrimal sac, respectively. A recent study has shown that transcriptional regulators of core binding factor (Cbf) β2 and promotor-1-transcribed Runt-related transcription factor 1 (P1-Runx1) are required for the differentiation of CD3-CD4+CD45+ lymphoid tissue inducer (LTi) cells, which initiate and trigger the developmental program of PPs, but the involvement of this pathway in NALT and TALT development remains to be elucidated. Here we report that Cbfβ2 plays an essential role in NALT and TALT development by regulating LTi cell trafficking to the NALT and TALT anlagens. Cbfβ2 was expressed in LTi cells in all three types of MALT examined. Indeed, similar to the previous finding for PPs, we found that Cbfβ2-/- mice lacked NALT and TALT lymphoid structures. However, in contrast to PPs, NALT and TALT developed normally in the absence of P1-Runx1 or other Runx family members such as Runx2 and Runx3. LTi cells for NALT and TALT differentiated normally but did not accumulate in the respective lymphoid tissue anlagens in Cbfβ2-/- mice. These findings demonstrate that Cbfβ2 is a central regulator of the MALT developmental program, but the dependency of Runx proteins on the lymphoid tissue development would differ among PPs, NALT, and TALT.

Structure, Organization, and Development of the Mucosal Immune System of the Respiratory Tract

The respiratory tract is served by a variety of lymphoid tissues, including the tonsils, adenoids, nasal-associated lymphoid tissue (NALT), and bronchus-associated lymphoid tissue (BALT), as well as the lymph nodes that drain the upper and lower respiratory tract. Each of these tissues uses unique mechanisms to acquire antigens and respond to pathogens in the local environment and supports immune responses that are tailored to protect those locations. This chapter will review the important features of NALT and BALT and define how these tissues contribute to immunity in the upper and lower respiratory tract, respectively.

The development and function of mucosal lymphoid tissues: a balancing act with micro-organisms

Mucosal surfaces are constantly exposed to environmental antigens, colonized by commensal organisms and used by pathogens as points of entry. As a result, the immune system has devoted the bulk of its resources to mucosal sites to maintain symbiosis with commensal organisms, prevent pathogen entry, and avoid unnecessary inflammatory responses to innocuous antigens. These functions are facilitated by a variety of mucosal lymphoid organs that develop during embryogenesis in the absence of microbial stimulation as well as ectopic lymphoid tissues that develop in adults following microbial exposure or inflammation. Each of these lymphoid organs samples antigens from different mucosal sites and contributes to immune homeostasis, commensal containment, and immunity to pathogens. Here we discuss the mechanisms, mostly based on mouse studies, that control the development of mucosal lymphoid organs and how the various lymphoid tissues cooperate to maintain the integrity of the mucosal barrier.

TNFα-dependent development of lymphoid tissue in the absence of RORγt⁺ lymphoid tissue inducer cells

Lymphoid tissue often forms within sites of chronic inflammation. Here we report that expression of the proinflammatory cytokine tumor necrosis factor α (TNFα) drives development of lymphoid tissue in the intestine. Formation of this ectopic lymphoid tissue was not dependent on the presence of canonical RORgt(+) lymphoid tissue-inducer (LTi) cells, because animals expressing increased levels of TNFα but lacking RORgt(+) LTi cells (TNF/Rorc(gt)(-/-) mice) developed lymphoid tissue in inflamed areas. Unexpectedly, such animals developed several lymph nodes (LNs) that were structurally and functionally similar to those of wild-type animals. TNFα production by F4/80(+) myeloid cells present within the anlagen was important for the activation of stromal cells during the late stages of embryogenesis and for the activation of an organogenic program that allowed the development of LNs. Our results show that lymphoid tissue organogenesis can occur in the absence of LTi cells and suggest that interactions between TNFα-expressing myeloid cells and stromal cells have an important role in secondary lymphoid organ formation.

Imaging murine NALT following intranasal immunization with flagellin-modified circumsporozoite protein malaria vaccines

Intranasal (IN) immunization with a Plasmodium circumsporozoite (CS) protein conjugated to flagellin, a Toll-like receptor 5 agonist, was found to elicit antibody-mediated protective immunity in our previous murine studies. To better understand IN-elicited immune responses, we examined the nasopharynx-associated lymphoid tissue (NALT) in immunized mice and the interaction of flagellin-modified CS with murine dendritic cells (DCs) in vitro. NALT of immunized mice contained a predominance of germinal center (GC) B cells and increased numbers of CD11c+ DCs localized beneath the epithelium and within the GC T-cell area. We detected microfold cells distributed throughout the NALT epithelial cell layer and DC dendrites extending into the nasal cavity, which could potentially function in luminal CS antigen uptake. Flagellin-modified CS taken up by DCs in vitro was initially localized within intracellular vesicles followed by a cytosolic distribution. Vaccine modifications to enhance delivery to the NALT and specifically target NALT antigen-presenting cell populations will advance development of an efficacious needle-free vaccine for the 40% of the world's population at risk of malaria.

Characteristics of nasal-associated lymphoid tissue (NALT) and nasal absorption capacity in chicken

As the main mucosal immune inductive site of nasal cavity, nasal-associated lymphoid tissue (NALT) plays an important role in both antigen recognition and immune activation after intranasal immunization. However, the efficiency of intranasal vaccines is commonly restricted by the insufficient intake of antigen by the nasal mucosa, resulting from the nasal mucosal barrier and the nasal mucociliary clearance. The distribution of NALT and the characteristic of nasal cavity have already been described in humans and many laboratory rodents, while data about poultry are scarce. For this purpose, histological sections of the chicken nasal cavities were used to examine the anatomical structure and histological characteristics of nasal cavity. Besides, the absorptive capacity of chicken nasal mucosa was also studied using the materials with different particle size. Results showed that the NALT of chicken was located on the bottom of nasal septum and both sides of choanal cleft, which mainly consisted of second lymphoid follicle. A large number of lymphocytes were distributed under the mucosal epithelium of inferior nasal meatus. In addition, there were also diffuse lymphoid tissues located under the epithelium of the concha nasalis media and the walls of nasal cavity. The results of absorption experiment showed that the chicken nasal mucosa was capable to absorb trypan blue, OVA, and fluorescent latex particles. Inactivated avian influenza virus (IAIV) could be taken up by chicken nasal mucosa except for the stratified squamous epithelium sites located on the forepart of nasal cavity. The intake of IAIV by NALT was greater than that of the nasal mucosa covering on non-lymphoid tissue, which could be further enhanced after intranasal inoculation combined with sodium cholate or CpG DNA. The study on NALT and nasal absorptive capacity will be benefit for further understanding of immune mechanisms after nasal vaccination and development of nasal vaccines for poultry.

Enhancement of serum and mucosal immune responses to a Haemophilus influenzae Type B vaccine by intranasal delivery

Intranasal (i.n.) vaccination is potentially the most direct method for conveying upper respiratory and mucosal immunity to respiratory pathogens. However, for unclear reasons, vaccines introduced into the nasal sinuses often have lower efficacy than vaccines administered by the more frequently used parenteral routes. We examined i.n. vaccination in a mouse immune-response model with a commonly used Haemophilus influenzae type B vaccine (Hibv) composed of the polyribosylribitol phosphate (PRP) capsule antigen conjugated to tetanus toxoid. Intranasal vaccination with Hibv using a Toll-like receptor 4 (TLR4) agonist as an adjuvant significantly increased the levels of IgA specific for the PRP capsule antigen in blood serum, saliva, and mucosal secretion specimens. In contrast, control mice vaccinated transdermally (t.d.) with Hibv did not produce significant levels of PRP-specific IgA in the blood serum and saliva, and anti-PRP IgG was increased only in serum. The i.n. and t.d. vaccinations resulted in equivalent bactericidal antibody responses in blood serum, suggesting that vaccine-derived IgG is protective against infection. Elevated levels of IgG specific for the tetanus toxoid carrier protein were measured in nasal sinuses and vaginal secretions in mice vaccinated by either the t.d. or i.n. route. Tissue culture studies confirmed that the nasopharynx-associated lymphoid tissue (NALT) was at least one of the sources of PRP-specific IgA and carrier-specific IgG within the nasal sinuses. We conclude that i.n. vaccination aided by a TLR4 agonist results in robust immune responses to both the carrier protein and bacterial polysaccharide components of the Hibv.

Examining the role of nasopharyngeal-associated lymphoreticular tissue (NALT) in mouse responses to vaccines

The nasopharyngeal-associated lymphoreticular tissues (NALT) found in humans, rodents, and other mammals, contribute to immunity in the nasal sinuses^1-3. The NALT are two parallel bell-shaped structures located in the nasal passages above the hard palate, and are usually considered to be secondary components of the mucosal-associated lymphoid system^4-6. Located within the NALT are discrete compartments of B and T lymphocytes interspersed with antigen-presenting dendritic cells^4,7,8. These cells are surrounded by an epithelial cell layer intercalated with M-cells that are responsible for antigen retrieval from the mucosal surfaces of the air passages^9,10. Naive lymphocytes circulating through the NALT are poised to respond to first encounters with respiratory pathogens⁷. While NALT disappear in humans by the age of two years, the Waldeyer's Ring and similarly structured lymphatic organs continue to persist throughout life⁶. In contrast to humans, mice retain NALT throughout life, thus providing a convenient animal model for the study of immune responses originating within the nasal sinuses¹¹.

Cultures of single-cell suspensions of NALT are not practical due to low yields of mononuclear cells. However, NALT biology can be examined by ex vivo culturing of the intact organ, and this method has the additional advantage of maintaining the natural tissue structure. For in vivo studies, genetic knockout models presenting defects limited to NALT are not currently available due to a poor understanding of the developmental pathway. For example, while lymphotoxin-α knockout mice have atrophied NALT, the Peyer's patches, peripheral lymph nodes, follicular dendritic cells and other lymphoid tissues are also altered in these genetically manipulated mice^12,13. As an alternative to gene knockout mice, surgical ablation permanently eliminates NALT from the nasal passage without affecting other tissues. The resulting mouse model has been used to establish relationships between NALT and immune responses to vaccines^1,3. Serial collection of serum, saliva, nasal washes and vaginal secretions is necessary for establishing the basis of host responses to vaccination, while immune responses originating directly from NALT can be confirmed by tissue culture. The following procedures outline the surgeries, tissue culture and sample collection necessary to examine local and systemic humoral immune responses to intranasal (IN) vaccination.

Pulmonary infection with influenza A virus induces site-specific germinal center and T follicular helper cell responses

Protection from influenza A virus (IAV) challenge requires switched, high affinity Abs derived from long-lived memory B cells and plasma cells. These B cell subsets are generated in germinal centers (GCs), hallmark structures of T helper cell-driven B cell immunity. A full understanding of the GC reaction after respiratory IAV infection is lacking, as is the characterization of T follicular helper (T_FH) cells that support GCs. Here, GC B cell and T_FH cell responses were studied in mice following pulmonary challenge with IAV. Marked GC reactions were induced in draining lymph nodes (dLNs), lung, spleen and nasal-associated lymphoid tissue (NALT), although the magnitude and kinetics of the response was site-specific. Examination of switching within GCs demonstrated IgG2⁺ cells to compose the largest fraction in dLNs, lung and spleen. IgA⁺ GC B cells were infrequent in these sites, but composed a significant subset of the switched GC population in NALT. Further experiments demonstrated splenectomized mice to withstand a lethal recall challenge, suggesting the spleen to be unnecessary for long-term protection in spite of strong GC responses in this organ. Final studies showed that T_FH cell numbers were highest in dLNs and spleen, and peaked in all sites prior to the height of the GC reaction. T_FH cells purified from dLNs generated IL-21 and IFNγ upon activation, although CD4⁺CXCR5⁻ T effector cells produced higher levels of all cytokines. Collectively, these findings reveal respiratory IAV infection to induce strong T helper cell-driven B cell responses in various organs, with each site displaying unique attributes.

The mucosal immune system of the respiratory tract

Most viruses use host mucosal surfaces as their initial portals of infection. The respiratory tract has the body's second-largest mucosal surface area after the digestive tract. An understanding of the unique nature of the mucosal immune system of respiratory organs is therefore extremely important for the development of new-generation vaccines and novel methods of preventing and treating respiratory infectious diseases, including viral infections.

Expression and function of interleukin-7 in secondary and tertiary lymphoid organs

Interleukin-7 (IL-7) is known since many years as stromal-cell derived cytokine that plays a key role for the adaptive immune system. It promotes lymphocyte development in the bone marrow and thymus as well as naive and memory T cell homeostasis in the periphery. More recently, IL-7 reporter mice and other approaches have led to the further characterization of the various stromal cell sources of IL-7 in secondary lymphoid organs (SLO) and other tissues. We will review these advances along with a discussion of the regulation of IL-7 and its receptor, and compare the biological effects IL-7 has on adaptive as well as innate immune cells in SLO. Finally, we will review the role of IL-7 in development of SLO and tertiary lymphoid tissues that frequently are associated with sites of chronic inflammation.

Inflammatory Cell Patterns in the Nasal Mucosa of Patients with Idiopathic Rhinitis

Background

Nonallergic rhinitis comprises many subgroups of rhinitis (vasomotor rhinitis, nonallergic rhinitis with eosinophilia syndrome, occupational rhinitis, idiopathic rhinitis, etc. in which its main feature is known to be free of allergy. We evaluate the cellular infiltrate of subjects with idiopathic rhinitis and compare them with allergic and control counterparts for detecting underlying pathophysiology.

Methods

Subjects selected from patients admitted to Istanbul University Medical Faculty between 2006 and 2009 were classified into idiopathic rhinitis (n = 16; mean age, 26.5 years), allergic rhinitis (n = 17; mean age, 31.1 years), and the control (n = 25, mean age, 28.8 years) groups. Inferior turbinate specimens were collected using Gerritsma forceps. Skin-prick testing was performed. Immunohistochemical detection was performed using B7 (chymase, clone CC1) and G3 (tryptase, clone AA1) primary antibodies for mast cells, human eosinophil major basic protein (clone BMK-13) for eosinophils, and immunoglobulin E (IgE) Ab-1 for mast and plasma cells in epithelium, superficial, and deep submucosa.

Results

We found significantly higher levels of mast cells within the different sites of nasal mucosa of allergic and idiopathic subjects compared with normal mucosa (p < 0.05). Additionally, a significant increase was observed in IgE+ cells of the patients with allergic and idiopathic rhinitis compared with the controls (p < 0.05 for each) Eosinophils were significantly increased within the epithelium of allergic patients’ mucosa.

Conclusion

We seem to have supportive data about possible mechanisms of “idiopathic rhinitis” that suggests local allergic inflammation. The study results provided important information for further provocation and immunohistochemical studies analyzing the shared mechanism of allergic and idiopathic rhinitis.

The development of inducible bronchus-associated lymphoid tissue depends on IL-17

Ectopic or tertiary lymphoid tissues, such as inducible bronchus-associated lymphoid tissue (iBALT), form in nonlymphoid organs after local infection or inflammation. However, the initial events that promote this process remain unknown. Here we show that iBALT formed in mouse lungs as a consequence of pulmonary inflammation during the neonatal period. Although we found CD4(+)CD3(-) lymphoid tissue-inducer cells (LTi cells) in neonatal lungs, particularly after inflammation, iBALT was formed in mice that lacked LTi cells. Instead, we found that interleukin 17 (IL-17) produced by CD4(+) T cells was essential for the formation of iBALT. IL-17 acted by promoting lymphotoxin-α-independent expression of the chemokine CXCL13, which was important for follicle formation. Our results suggest that IL-17-producing T cells are critical for the development of ectopic lymphoid tissues.

Stromal cell-immune cell interactions

Interaction between different types of hematopoietic cells is essential for proper functioning of the immune system. For instance, the cytokines produced by antigen-presenting dendritic cells will determine the type of T cell response that is induced. However, hematopoietic cells are also strongly influenced by the surrounding nonhematopoietic cells. The cells that form these microenvironments are collectively called stromal cells. Here, we focus on the stromal cells present within secondary lymphoid organs and discuss their importance for various aspects of the immune system.

Nasal immunity to staphylococcal toxic shock is controlled by the nasopharynx-associated lymphoid tissue

The nasopharynx-associated lymphoid tissue (NALT) of humans and other mammals is associated with immunity against airborne infections, though it is generally considered to be a secondary component of the mucosa-associated lymphoid system. We found that protective immunity to a virulence factor of nasal mucosa-colonizing Staphylococcus aureus, staphylococcal enterotoxin B (SEB), requires a functional NALT. We examined the role of NALT using intranasal (IN) vaccination with a recombinant SEB vaccine (rSEBv) combined with an adjuvant in a mouse model of SEB-induced toxic shock. The rSEBv was rapidly internalized by NALT cells at the mucosal barrier, and transport into NALT was accelerated by inclusion of a Toll-like receptor 4 (TLR4) agonist. Vaccine-induced germinal centers of B cells formed within NALT, accompanied by elevated levels of IgA(+) and IgG(+) cells, and these were further increased by TLR4 activation. The NALT was the site of specific anti-rSEBv IgA and IgG production but was also influenced by intraperitoneal (IP) inoculation and perhaps other isolated lymphoid follicles observed within the nasal cavity. Vaccination by the IN route generated robust levels of anti-rSEBv IgA in saliva, nasal secretions, and blood compared to much lower levels after IP vaccination. IN vaccination also induced secretion of anti-rSEBv IgG in the blood and nasal secretions. Significantly, the efficacy of IN vaccination was dependent on NALT, as surgical removal resulted in greater sensitivity to IN challenge with wild-type SEB. Thus, protective immunity to SEB within the nasal sinuses was elicited by responses originating in NALT.

Runx1/Cbfβ2 complexes are required for lymphoid tissue inducer cell differentiation at two developmental stages

Hematopoietic lymphoid tissue inducer (LTi) cells are essential for the development of secondary lymphoid tissues including lymph nodes and Peyer's patches. Two transcription factors, the helix-loop-helix inhibitor Id2 and the retinoic acid-related orphan receptor γt (Rorγt), have been shown to be crucial for LTi cell development. However, it remains unclear how the specification of multipotent hematopoietic progenitor cells toward the LTi lineage is programmed. In this study, we report impaired lymphoid tissue organogenesis in mice in which the function of Runx1/Cbfβ transcription factor complexes was attenuated by the loss of either the distal promoter-derived Runx1 or Cbfβ2 variant protein. We found that LTi progenitors in fetal liver, defined previously as a lineage marker-negative α4β7 integrin (α4β7)(+) IL-7R α-chain (IL-7Rα)(+) population, can be subdivided into Rorγt-expressing IL-7Rα(high) cells and nonexpressing IL-7Rα(mid) cells. Whereas Id2 and Rorγt are required to direct α4β7(+)IL-7Rα(mid) cells to become α4β7(+)IL-7Rα(high) cells, Runx1/Cbfβ2 complexes are necessary for the emergence of α4β7(+)IL-7Rα(mid) cells. In addition, the loss of Cbfβ2, but not P1-Runx1, resulted in an inefficient upregulation of Rorγt in residual α4β7(+)IL-7Rα(+) LTi cells at anlagen. Our results thus revealed that Runx1/Cbfβ2 complexes regulate the differentiation of LTi cells at two stages: an early specification of hematopoietic progenitors toward the LTi lineage and a subsequent activation of Rorγt expression at anlagen.

Cytokine/stromal cell networks and lymphoid tissue environments

Initiation of an effective adaptive immune response against a foreign pathogen requires orchestrated encounters between lymphocytes and antigen-presenting cells. The tissues of the lymphoid system provide the ideal environment for increasing the efficiency of these encounters. Within the spleen, the mucosal-associated lymphoid tissues, and the lymph nodes, an intricate network of stromal cells, collagen fibers, and extracellular matrix exists that effectively compartmentalizes immune cells as they transit through these tissues. The stromal cells within lymphoid tissues are by no means homogenous, and it is now clear that these cells are not merely sessile bystanders during immune responses. Indeed, stromal cells within lymphoid tissues are the source of important cytokines and chemokines that guide and polarize immune cells. Here, we review the cytokines that maintain the integrity of this important stromal scaffold system within the lymphoid tissue, paying particular attention to the Lymphotoxin pathway, which is an important player in stromal cell biology. How cytokines maintain the organization of lymphoid tissues during development, in the adult animal, during inflammation and during disease will be discussed in sequence, and the clinical implications of targeting cytokines that regulate lymphoid tissue stroma will be considered.

Adaptive immunity and adipose tissue biology

Studies of immunity typically focus on understanding how hematopoietic cells interact within conventional secondary lymphoid tissues. However, immune reactions and their regulation occur in various environments within the body. Adipose tissue is one tissue that can influence and be influenced by adjacent and embedded lymphocytes. Despite the abundance and wide distribution of such tissue, and despite a growing obesity epidemic, studies of these interactions have been only marginally appreciated in the past. Here, we review advances in understanding of lymphoid structures within adipose tissue, the relationship between adipose tissue and adaptive immune function, and evidence for how this relationship contributes to obesity-associated diseases.

Lymphoid tissue inducer cells and the evolution of CD4 dependent high-affinity antibody responses

Phylogeny indicates that in mammals memory CD4-dependent antibody responses evolved after monotremes split from the common ancestor of marsupial and eutherian mammals. This was strongly associated with the development of segregated B and T cell areas and the development of a linked lymph node network. The evolution of the lymphotoxin beta receptor in these higher mammals was key to the development of these new functions. Here, we argue that lymphoid tissue inducer cells played a pivotal role not only in the development of organized lymphoid structures but also in the subsequent genesis of the CD4-dependent class-switched memory antibody responses that depend on an organized infrastructure to work. In this review, we concentrate on the role of this cell type in the making of a tolerant CD4 T cell repertoire and in the sustenance of CD4 T cell responses for protective immunity.

New insights into the development of lymphoid tissues

Secondary lymphoid organs are important locations for the initiation of adaptive immune responses. They develop before birth, and their formation requires interaction between lymphotoxin-α₁ß₂-expressing lymphoid-tissue inducer cells and lymphotoxin-ß receptor-expressing stromal organizer cells. Here, we discuss new insights into the earliest phases of peripheral lymph node and Peyer's patch formation that occur before lymphotoxin-ß receptor signalling and suggest a role for the developing nervous system. In addition, we discuss the differing requirements for the postnatal formation of mucosa-associated lymphoid tissues and tertiary lymphoid structures that develop at sites of chronic inflammation.

Function of mucosa-associated lymphoid tissue in antibody formation

Abundant evidence supports the notion that human intestinal plasma cells are largely derived from B cells initially activated in gut-associated lymphoid tissue (GALT). Nevertheless, insufficient knowledge exists about the uptake, processing, and presentation of luminal antigens occurring in GALT to accomplish priming and sustained expansion of mucosal B cells. Also, it is unclear how the germinal center reaction so strikingly promotes class switch to IgA and expression of J chain, although the commensal microbiota appears to contribute to both diversification and memory. B-cell migration from GALT to the intestinal lamina propria is guided by rather well-defined adhesion molecules and chemokines/chemokine receptors, but the cues directing homing to secretory effector sites beyond the gut require better definition. In this respect, the role of human Waldeyer's ring (including adenoids and the palatine tonsils) as a regional mucosa-associated lymphoid tissue must be better defined, although the balance of evidence suggests that it functions as nasopharynx-associated lymphoid tissue (NALT) like the characteristic NALT structures in rodents. Altogether, data suggest a remarkable compartmentalization of the mucosal immune system that must be taken into account in the development of effective local vaccines to protect specifically the airways, small and large intestines, and the female genital tract.

Antigen-dependent rescue of nose-associated lymphoid tissue (NALT) development independent of LTbetaR and CXCR5 signaling

Nose-associated lymphoid tissue (NALT) in the rodent upper respiratory tract develops postnatally and is considered to be independent of several factors known to be involved in the organogenesis of LN and Peyer's patches (PP). In this study we demonstrate that at least two different pathways result in NALT development. Following NALT anlage formation the intrinsic pathway relies on a signaling cascade including those mediated through the chemokine receptor CXCR5 and the lymphotoxin beta receptor (LTbetaR). This allows for the formation of high endothelial venules and thereby the recruitment of lymphocytes into NALT. Alternatively, high endothelial venule formation and lymphocyte recruitment can be induced in the NALT anlage by environmental signals, which are independent of LT-betaR and chemokine receptor CXCR5 signaling but in part rely on CD40 ligand. Thus, our study identifies a novel mechanism that facilitates the rescue of NALT development at late stages in adult life independent of the canonical LTbetaR-CXCR5 signaling axis.

Lymphoid tissue inducer cells: bridges between the ancient innate and the modern adaptive immune systems

Phylogeny indicates that adaptive immunity evolved first in diffusely distributed lymphoid tissues found in the lamina propria (LP) of the gut. B follicular structures appeared later, probably initially in isolated lymphoid follicles in the LP and then in organized lymphoid tissues such as lymph nodes and Peyer's patches. The development of these new lymphoid structures was enabled by gene duplication and evolution of new tumor necrosis family members. Here, we argue that lymphoid tissue inducer cells (LTis) had a pivotal role, not only in the development of organized lymphoid structures, but also in the subsequent genesis of the CD4-dependent class-switched memory antibody responses. In this review, we concentrate on the latter function: the sustenance by LTis of CD4 T-cell responses for protective immunity.

Id2-, RORgammat-, and LTbetaR-independent initiation of lymphoid organogenesis in ocular immunity

The eye is protected by the ocular immunosurveillance system. We show that tear duct–associated lymphoid tissue (TALT) is located in the mouse lacrimal sac and shares immunological characteristics with mucosa-associated lymphoid tissues (MALTs), including the presence of M cells and immunocompetent cells for antigen uptake and subsequent generation of mucosal immune responses against ocularly encountered antigens and bacteria such as Pseudomonas aeruginosa. Initiation of TALT genesis began postnatally; it occurred even in germ-free conditions and was independent of signaling through organogenesis regulators, including inhibitor of DNA binding/differentiation 2, retinoic acid–related orphan receptor γt, lymphotoxin (LT) α1β2–LTβR, and lymphoid chemokines (CCL19, CCL21, and CXCL13). Thus, TALT shares immunological features with MALT but has a distinct tissue genesis mechanism and plays a key role in ocular immunity.

Secondary lymphoid organs: responding to genetic and environmental cues in ontogeny and the immune response

Secondary lymphoid organs (SLOs) include lymph nodes, spleen, Peyer's patches, and mucosal tissues such as the nasal-associated lymphoid tissue, adenoids, and tonsils. Less discretely anatomically defined cellular accumulations include the bronchus-associated lymphoid tissue, cryptopatches, and isolated lymphoid follicles. All SLOs serve to generate immune responses and tolerance. SLO development depends on the precisely regulated expression of cooperating lymphoid chemokines and cytokines such as LTalpha, LTbeta, RANKL, TNF, IL-7, and perhaps IL-17. The relative importance of these factors varies between the individual lymphoid organs. Participating in the process are lymphoid tissue initiator, lymphoid tissue inducer, and lymphoid tissue organizer cells. These cells and others that produce crucial cytokines maintain SLOs in the adult. Similar signals regulate the transition from inflammation to ectopic or tertiary lymphoid tissues.

Immunological commonalities and distinctions between airway and digestive immunity

Airway and digestive tissues are the frontlines of the body's defense, being continuously exposed to the outside environment and encountering large numbers of antigens and microorganisms. To achieve immunosurveillance and immunological homeostasis in the harsh environments of the mucosal surfaces, the mucosal immune system tightly regulates a state of opposing but harmonized immune activation and quiescence. Recently, accumulating evidence has revealed that although the respiratory and intestinal immune systems share common mucosa-associated immunological features that are different from those of the systemic immune system, they also show distinctive immunological phenotypes, functions, and developmental pathways. We describe here the common and distinct immunological features of respiratory and intestinal immune systems and its application to the development of mucosal vaccines.

Omental milky spots develop in the absence of lymphoid tissue-inducer cells and support B and T cell responses to peritoneal antigens

The omentum is a site of B1 cell lymphopoiesis and immune responsiveness to T cell-independent antigens. However, it is unknown whether it supports immune responses independently of conventional lymphoid organs. We showed that the omentum collected antigens and cells from the peritoneal cavity and supported T cell-dependent B cell responses, including isotype switching, somatic hypermutation, and limited affinity maturation, despite the lack of identifiable follicular dendritic cells. The omentum also supported CD4+ and CD8+ T cell responses to peritoneal antigens and recruited effector T cells primed in other locations. Unlike conventional lymphoid organs, milky spots in the omentum developed in the absence of lymphoid tissue-inducer cells, but required the chemokine CXCL13. Although the lymphoid architecture of milky spots was disrupted in lymphotoxin-deficient mice, normal architecture was restored by reconstitution with lymphotoxin-sufficient hematopoietic cells. These results indicate that the milky spots of the omentum function as unique secondary lymphoid organs that promote immunity to peritoneal antigens.

Retinoid-related orphan receptors (RORs): critical roles in development, immunity, circadian rhythm, and cellular metabolism

The last few years have witnessed a rapid increase in our knowledge of the retinoid-related orphan receptors RORα, -β, and -γ (NR1F1-3), their mechanism of action, physiological functions, and their potential role in several pathologies. The characterization of ROR-deficient mice and gene expression profiling in particular have provided great insights into the critical functions of RORs in the regulation of a variety of physiological processes. These studies revealed that RORα plays a critical role in the development of the cerebellum, that both RORα and RORβ are required for the maturation of photoreceptors in the retina, and that RORγ is essential for the development of several secondary lymphoid tissues, including lymph nodes. RORs have been further implicated in the regulation of various metabolic pathways, energy homeostasis, and thymopoiesis. Recent studies identified a critical role for RORγ in lineage specification of uncommitted CD4⁺ T helper cells into Th17 cells. In addition, RORs regulate the expression of several components of the circadian clock and may play a role in integrating the circadian clock and the rhythmic pattern of expression of downstream (metabolic) genes. Study of ROR target genes has provided insights into the mechanisms by which RORs control these processes. Moreover, several reports have presented evidence for a potential role of RORs in several pathologies, including osteoporosis, several autoimmune diseases, asthma, cancer, and obesity, and raised the possibility that RORs may serve as potential targets for chemotherapeutic intervention. This prospect was strengthened by recent evidence showing that RORs can function as ligand-dependent transcription factors.

A combination of Flt3 ligand cDNA and CpG ODN as nasal adjuvant elicits NALT dendritic cells for prolonged mucosal immunity

We explore cellular and molecular mechanisms of nasal adjuvant of a combination of a plasmid encoding the Flt3 ligand cDNA (pFL) and CpG oligodeoxynucleotides (CpG ODN). The double DNA adjuvant given with OVA maintained prolonged OVA-specific secretory IgA (S-IgA) Ab responses in external secretions for more than 25 weeks after the final immunization. Further, both Th1- and Th2-type cytokine responses were induced by this combined adjuvant regimen. The frequencies of plasmacytoid DCs (pDCs) and CD8(+) DCs were significantly increased in nasopharyngeal-associated lymphoreticular tissue (NALT) of mice given the combined adjuvant. Importantly, when we examined adjuvanticity of pFL plus CpG ODN in 2-year-old mice, significant levels of mucosal IgA Ab responses were also induced. These results demonstrate that nasal delivery of a combined DNA adjuvant offers an attractive possibility for the development of an effective mucosal vaccine for the elderly.

Development of secondary lymphoid organs

Secondary lymphoid organs develop during embryogenesis or in the first few weeks after birth according to a highly coordinated series of interactions between newly emerging hematopoietic cells and immature mesenchymal or stromal cells. These interactions are orchestrated by homeostatic chemokines, cytokines, and growth factors that attract hematopoietic cells to sites of future lymphoid organ development and promote their survival and differentiation. In turn, lymphotoxin-expressing hematopoietic cells trigger the differentiation of stromal and endothelial cells that make up the scaffolding of secondary lymphoid organs. Lymphotoxin signaling also maintains the expression of adhesion molecules and chemokines that govern the ultimate structure and function of secondary lymphoid organs. Here we describe the current paradigm of secondary lymphoid organ development and discuss the subtle differences in the timing, molecular interactions, and cell types involved in the development of each secondary lymphoid organ.

ROR gamma t is dispensable for the development of intestinal mucosal T cells

To examine the origin of intestinal mucosal T cells and, in particular, unconventional CD8 alpha alpha(+) T cells, we have undertaken a thorough analysis of the gut immune compartment in euthymic and athymic mice carrying either wild-type or mutant transcription factor retinoic acid-related orphan receptor-gamma t (ROR gamma t). We identified a previously unrealized complexity of gut cryptopatch (CP) cells that challenges the previous assertion that CP cells comprise ROR gamma t-expressing adult counterparts of fetal lymphoid tissue inducer (Lti) cells. We showed that many CP cells express intermediate T cell differentiation markers, whether or not they express ROR gamma t, and found that CPs are not completely dependent on ROR gamma t, as previously reported, but merely fewer in number in the ROR gamma t-deficient condition. Indeed, c-kit(+)IL-7R(+)Lin(-)ROR gamma t(-) cells inside the CP and c-kit(+)IL-7R(+)Lin(-)ROR gamma t(-) and c-kit(+)IL-7R(+)Lin(-)ROR gamma t(low) cells outside the CP basically remain in the gut mucosa of ROR gamma t-deficient ROR gamma t(EGFP/EGFP) mice. Consistent with these non-Lti-like c-kit(+)IL-7R(+)Lin(-) cells being gut T cell progenitors, ROR gamma t-deficient mice develop the normal number of intestinal mucosal T cells. These results clearly reassert the intraintestinal differentiation of the body's largest peripheral T cell subpopulation.