Lymphoid tissue inducer cells: architects of CD4 immune responses in mice and men

Lead suppresses interferon γ to induce splenomegaly via modification on splenic endothelial cells and lymphoid tissue organizer cells in mice

Splenomegaly is a symptom characterized by the presence of an enlarged spleen. The impact of environmental factors on splenomegaly is largely unknown. In this study, C57BL/6 mice were treated with 125 ppm or 1250 ppm lead (Pb) via drinking water for 8 wk, and the process of splenomegaly was evaluated. Treatment with 1250 ppm Pb, but not 125 ppm Pb, caused splenomegaly, which was associated with increased capacity for erythrocyte clearance. Intriguingly, Pb-caused splenomegaly was independent of lymphoid tissue inducer (LTi) cells, which produce lymphotoxins α and β (LTα/β) to activate endothelial cells and LT organizer (LTo) cells and drive the development of spleen physiologically. A direct action of Pb on endothelial cells and LTo cells did not impact their proliferation. On the other hand, during steady state, a tonic level of interferon (IFN)γ acted on endothelial cells and LTo cells to suppress splenomegaly, as IFNγ receptor (IFNγR)-deficient mice had enlarged spleens relative to wild-type mice; during Pb exposure, splenic IFNγ production was suppressed, thus leading to a loss of the inhibitory effect of IFNγ on splenomegaly. Mechanically, Pb acted on splenic CD4⁺ T cells to suppress IFNγ production, which impaired the Janus kinase (Jak)1/ signal transducer and activator of transcription (STAT)1 signaling in endothelial cells and LTo cells; the weakened Jak1/STAT1 signaling resulted in the enhanced nuclear factor-κB (NF-κB) signaling in endothelial cells and LTo cells, which drove their proliferation and caused splenomegaly. The present study reveals a previously unrecognized mechanism for the immunotoxicity of Pb, which may extend our current understanding for Pb toxicology.

Innate Lymphoid Cells in Autoimmune Diseases

Innate lymphoid cells (ILC) are a heterogeneous group of immune cells characterized by lymphoid morphology and cytokine profile similar to T cells but which do not express clonally distributed diverse antigen receptors. These particular cells express transcription factors and cytokines reflecting their similarities to T helper (Th)1, Th2, and Th17 cells and are therefore referred to as ILC1, ILC2, and ILC3. Other members of the ILC subsets include lymphoid tissue inducer (LTi) and regulatory ILC (ILCreg). Natural killer (NK) cells share a common progenitor with ILC and also exhibit a lymphoid phenotype without antigen specificity. ILC are found in low numbers in peripheral blood but are much more abundant at barrier sites such as the skin, liver, airways, lymph nodes, and the gastrointestinal tract. They play an important role in innate immunity due to their capacity to respond rapidly to pathogens through the production of cytokines. Recent evidence has shown that ILC also play a key role in autoimmunity, as alterations in their number or function have been identified in systemic lupus erythematosus, systemic sclerosis, and rheumatoid arthritis. Here, we review recent advances in the understanding of the role of ILC in the pathogenesis of autoimmune diseases, with particular emphasis on their role as a potential diagnostic biomarker and as therapeutic targets.

Pathological RANK signaling in B cells drives autoimmunity and chronic lymphocytic leukemia

Clinical evidence suggests alterations in receptor activator of NF-κB (RANK) signaling are key contributors to B cell autoimmunity and malignancy, but the pathophysiological consequences of aberrant B cell–intrinsic RANK signaling remain unknown. We generated mice that express a human lymphoma–derived, hyperactive RANK^K240E variant in B lymphocytes in vivo. Forced RANK signaling disrupted B cell tolerance and induced a fully penetrant systemic lupus erythematosus–like disease in addition to the development of chronic lymphocytic leukemia (CLL). Importantly, RANK^K240E transgenic CLL cells as well as CLL cells of independent murine and of human origin depend on microenvironmental RANK ligand (RANKL) for tumor cell survival. Consequently, inhibition of the RANKL–RANK axis with anti-RANKL antibodies killed murine and human CLL cells in vitro and in vivo. These results establish pathological B cell–intrinsic RANK signaling as a potential driver of autoimmunity and B cell malignancy, and they suggest the exploitation of clinically available anti-RANKL compounds for CLL treatment.

Established and emergent roles for Ikaros transcription factors in lymphoid cell development and function

Ikaros zinc finger transcription factors are important regulators of the gene programs underlying the development of hematopoietic cell lineages. The family consists of five members: Ikaros, Helios, Aiolos, Eos, and Pegasus, which engage in both homo- and heterotypic intrafamilial interactions to exert diverse functional effects. Pioneering studies focused on the role of these factors in early lymphoid development, as their absence resulted in severe defects in lymphocyte populations. More recent work has now begun to define nuanced, stage-specific roles for Ikaros family members in the differentiation and function of mature T, B, and innate lymphoid cell populations including natural killer (NK) cells. The precise transcriptional mechanisms by which these factors function, both independently and collaboratively, is an area of active investigation. However, several key themes appear to be emerging regarding the pathways influenced by Ikaros family members, including the end-to-end regulation of cytokine signaling. Here, we review roles for Ikaros factors in lymphoid cell development, differentiation, and function, including a discussion of the current understanding of the transcriptional mechanisms they employ and considerations for the future study of this important transcription factor family.

Severe combined immunodeficiency in stimulator of interferon genes (STING) V154M/wild-type mice

BACKGROUND

Autosomal dominant gain-of-function mutations in human stimulator of interferon genes (STING) lead to a severe autoinflammatory disease called STING-associated vasculopathy with onset in infancy that is associated with enhanced expression of interferon-stimulated gene transcripts.

OBJECTIVE

The goal of this study was to analyze the phenotype of a new mouse model of STING hyperactivation and the role of type I interferons in this system.

METHODS

We generated a knock-in model carrying an amino acid substitution (V154M) in mouse STING, corresponding to a recurrent mutation seen in human patients with STING-associated vasculopathy with onset in infancy. Hematopoietic development and tissue histology were analyzed. Lymphocyte activation and proliferation were assessed in vitro. STING V154M/wild-type (WT) mice were crossed to IFN-α/β receptor (IFNAR) knockout mice to evaluate the type I interferon dependence of the mutant Sting phenotype recorded.

RESULTS

In STING V154M/WT mice we detected variable expression of inflammatory infiltrates in the lungs and kidneys. These mice showed a marked decrease in survival and developed a severe combined immunodeficiency disease (SCID) affecting B, T, and natural killer cells, with an almost complete lack of antibodies and a significant expansion of monocytes and granulocytes. The blockade in B- and T-cell development was present from early immature stages in bone marrow and thymus. In addition, in vitro experiments revealed an intrinsic proliferative defect of mature T cells. Although the V154M/WT mutant demonstrated increased expression of interferon-stimulated genes, the SCID phenotype was not reversed in STING V154M/WT IFNAR knockout mice. However, the antiproliferative defect in T cells was rescued partially by IFNAR deficiency.

CONCLUSIONS

STING gain-of-function mice developed an interferon-independent SCID phenotype with a T-cell, B-cell, and natural killer cell developmental defect and hypogammaglobulinemia that is associated with signs of inflammation in lungs and kidneys. Only the intrinsic proliferative defect of T cells was partially interferon dependent.

Regulation of Innate Lymphoid Cells by Aryl Hydrocarbon Receptor

With striking similarity to their adaptive T helper cell counterparts, innate lymphoid cells (ILCs) represent an emerging family of cell types that express signature transcription factors, including T-bet+ Eomes+ natural killer cells, T-bet+ Eomes- group 1 ILCs, GATA3+ group 2 ILCs, RORγt+ group 3 ILCs, and newly identified Id3+ regulatory ILC. ILCs are abundantly present in barrier tissues of the host (e.g., the lung, gut, and skin) at the interface of host-environment interactions. Active research has been conducted to elucidate molecular mechanisms underlying the development and function of ILCs. The aryl hydrocarbon receptor (Ahr) is a ligand-dependent transcription factor, best known to mediate the effects of xenobiotic environmental toxins and endogenous microbial and dietary metabolites. Here, we review recent progresses regarding Ahr function in ILCs. We focus on the Ahr-mediated cross talk between ILCs and other immune/non-immune cells in host tissues especially in the gut. We discuss the molecular mechanisms of the action of Ahr expression and activity in regulation of ILCs in immunity and inflammation, and the interaction between Ahr and other pathways/transcription factors in ILC development and function with their implication in disease.

Ikaros Inhibits Group 3 Innate Lymphoid Cell Development and Function by Suppressing the Aryl Hydrocarbon Receptor Pathway

Group 3 innate lymphoid cells (ILC3s) expressing the transcription factor (TF) RORγt are important for the defense and homeostasis of host intestinal tissues. The zinc finger TF Ikaros, encoded by Ikzf1, is essential for the development of RORγt(+) fetal lymphoid tissue inducer (LTi) cells and lymphoid organogenesis, but its role in postnatal ILC3s is unknown. Here, we show that small-intestinal ILC3s had lower Ikaros expression than ILC precursors and other ILC subsets. Ikaros inhibited ILC3s in a cell-intrinsic manner through zinc-finger-dependent inhibition of transcriptional activity of the aryl hydrocarbon receptor, a key regulator of ILC3 maintenance and function. Ablation of Ikzf1 in RORγt(+) ILC3s resulted in increased expansion and cytokine production of intestinal ILC3s and protection against infection and colitis. Therefore, in contrast to being required for LTi development, Ikaros inhibits postnatal ILC3 development and function to regulate gut immune responses at steady state and in disease.

The Role of Lymphoid Neogenesis in Allografts

De novo induction of organized lymphoid aggregates at nonlymphoid sites has been observed in many chronic inflammatory conditions where foreign antigens such as infectious agents, autoantigens or alloantigens, persist. The prevailing opinion in the field of transplantation is that lymphoid neogenesis within allografts is detrimental to the establishment of immune tolerance. These structures, commonly referred to as tertiary lymphoid organs (TLOs), are thought to contribute to graft rejection by generating and propagating local alloimmune responses. However, recent studies have shown that TLOs rich in regulatory Foxp3(+) cells are present in long-term accepting allografts. The notion that TLOs can contribute to the local downregulation of immune responses has been corroborated in other chronic inflammation models. These findings suggest that contrary to previous suggestions that the induction of TLOs in allografts is necessarily harmful, the induction of "tolerogenic" TLOs may prove advantageous. In this review, we discuss our current understanding of how TLOs are induced and how they regulate immune responses with a particular focus on alloimmunity.

Increased Lymphocyte Infiltration in Rheumatoid Arthritis Is Correlated with an Increase in LTi-like Cells in Synovial Fluid

In this study, we compared the immune cell populations in rheumatoid arthritis (RA) synovial fluid, which shows lymphoid tissue-like structure, with those in tonsils, which are normal secondary lymphoid tissues. Firstly, we found that CD4^-CD11b⁺ macrophages were the major population in RA synovial fluid and that B cells were the major population in tonsils. In addition, synovial fluid from patients with osteoarthritis, which is a degenerative joint disease, contained CD4⁺CD11b⁺ monocytes as the major immune cell population. Secondly, we categorized three groups based on the proportion of macrophages found in RA synovial fluid: (1) the macrophage-high group, which contained more than 80% macrophages; (2) the macrophage-intermediate group, which contained between 40% and 80% macrophages; and (3) the macrophage-low group, which contained less than 40% macrophages. In the macrophage-low group, more lymphoid tissue inducer (LTi)-like cells were detected, and the expression of OX40L and TRANCE in these cells was higher than that in the other groups. In addition, in this group, the suppressive function of regulatory T cells was downregulated. Finally, CXCL13 expression was higher in RA synovial fluid than in tonsils, but CCL21 expression was comparable in synovial fluid from all groups and in tonsils. These data demonstrate that increased lymphocyte infiltration in RA synovial fluid is correlated with an increase in LTi-like cells and the elevation of the chemokine expression.

Role of innate immunity in the pathogenesis of chronic rhinosinusitis: progress and new avenues

Chronic rhinosinusitis is a heterogeneous and multifactorial disease with unknown etiology. Aberrant responses to microorganisms have been suggested to play a role in the pathophysiology of the disease. Research has focused on the presence, detection, response to, and eradication of these potential threats. Main topics seem to center on the contribution of structural cells such as epithelium and fibroblasts, on the consequences of activation of pattern-recognition receptors, and on the role of antimicrobial agents. This research should be viewed not only in the light of a comparison between healthy and diseased individuals, but also in a comparison between patients who do or do not respond to treatment. New players that could play a role in the pathophysiology seem to surface at regular intervals, adding to our understanding (and the complexity) of the disease and opening new avenues that may help fight this incapacitating disease.

NF-κB subunit specificity in hemopoiesis

Although the diverse functions served by the nuclear factor-κB (NF-κB) pathway in virtually all cell types are typically employed to deal with stress responses, NF-κB transcription factors also play key roles in the development of hemopoietic cells. This review focuses on how NF-κB transcription factors control various aspects of thymic T-cell and myeloid cell differentiation that include its roles in hemopoietic precursors, conventional αβ T cells, CD4(+) regulatory T cells, natural killer T cells, γδ T cells, macrophages, and dendritic cells.

Tolerance and lymphoid organ structure and function

This issue of Frontiers in Immunologic Tolerance explores barriers to tolerance from a variety of views of cells, molecules, and processes of the immune system. Our laboratory has spent over a decade focused on the migration of the cells of the immune system, and dissecting the signals that determine how and where effector and suppressive regulatory T cells traffic from one site to another in order to reject or protect allografts. These studies have led us to a greater appreciation of the anatomic structure of the immune system, and the realization that the path taken by lymphocytes during the course of the immune response to implanted organs determines the final outcome. In particular, the structures, microanatomic domains, and the cells and molecules that lymphocytes encounter during their transit through blood, tissues, lymphatics, and secondary lymphoid organs are powerful determinants for whether tolerance is achieved. Thus, the understanding of complex cellular and molecular processes of tolerance will not come from “96-well plate immunology,” but from an integrated understanding of the temporal and spatial changes that occur during the response to the allograft. The study of the precise positioning and movement of cells in lymphoid organs has been difficult since it is hard to visualize cells within their three-dimensional setting; instead techniques have tended to be dominated by two-dimensional renderings, although advanced confocal and two-photon systems are changing this view. It is difficult to precisely modify key molecules and events in lymphoid organs, so that existing knockouts, transgenics, inhibitors, and activators have global and pleiotropic effects, rather than precise anatomically restricted influences. Lastly, there are no well-defined postal codes or tracking systems for leukocytes, so that while we can usually track cells from point A to point B, it is exponentially more difficult or even impossible to track them to point C and beyond. We believe this represents one of the fundamental barriers to understanding the immune system and devising therapeutic approaches that take into account anatomy and structure as major controlling principles of tolerance.

Manifestation of spontaneous and early autoimmune gastritis in CCR7-deficient mice

Autoimmune gastritis is a common autoimmune disorder characterized by chronic inflammatory cell infiltrates, atrophy of the corpus and fundus, and the occurrence of autoantibodies to parietal cell antigen. In CCR7-deficient mice, autoimmune gastritis developed spontaneously and was accompanied by metaplasia of the gastric mucosa and by the formation of tertiary lymphoid organs at gastric mucosal sites. T cells of CCR7-deficient mice showed an activated phenotype in the gastric mucosa, mesenteric lymph nodes, and peripheral blood. In addition, elevated serum IgG levels specific to gastric parietal cell antigen were detected. Because the role of organized lymphocytic aggregates at this inflammatory site is not completely understood, we first analyzed the cellular requirements for the formation of these structures. Autoreactive CD4(+) T cells were pivotal for tertiary lymphoid follicle formation, most likely in cooperation with dendritic cells, macrophages, and B cells. Second, we analyzed the necessity of secondary lymph nodes and tertiary lymphoid organs for the development of autoimmune gastritis using CCR7 single- and CCR7/lymphotoxin α double-deficient mice. Strikingly, manifestation of autoimmune gastritis was observed in the absence of secondary lymph nodes and preceded the development of tertiary lymphoid organs. Taken together, these findings identify an inflammatory process where gastric autoreactive T cells independent of organized tertiary lymphoid organs and classic lymph nodes can induce and maintain autoimmune gastritis.

Mucocutaneous candidiasis and autoimmunity against cytokines in APECED and thymoma patients: clinical and pathogenetic implications

Much has been learnt about the mechanisms of thymic self-tolerance induction from work on both the rare autosomal recessive disease autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED) and the autoimmune regulator (AIRE) protein mutated in this disease. Normally, AIRE drives low-level expression of huge numbers of peripheral tissue-specific antigens (TSAgs) in medullary thymic epithelial cells (mTECs), leading to the deletion of TSAg-reactive thymocytes maturing nearby. The very recently discovered neutralizing autoantibodies (autoAbs) against Th17-related cells and cytokines in two autoimmunity-related syndromes associated with AIRE-mutant thymi or AIRE-deficient thymomas help to explain the chronic mucocutaneous candidiasis (CMC) seen in both syndromes. The surprising parallels between these syndromes also demand new hypotheses and research into the consequences of AIRE deficiency and the ensuing autoimmunizing pathways, and suggest more appropriate treatment regimens as discussed in this review.

CD117⁺ CD3⁻ CD56⁻ OX40Lhigh cells express IL-22 and display an LTi phenotype in human secondary lymphoid tissues

Here, we identify cells within human adult secondary lymphoid tissues that are comparable in phenotype and location to the lymphoid tissue inducer (LTi) cells that persist in the adult mouse. Identified as CD117(+) CD3(-) CD56(-) cells, like murine LTi cells, they lack expression of many common lineage markers and express CD127, OX40L and TRANCE. These cells were detected at the interface between the B- and T- zones, as well as at the subcapsular sinus in LNs, the location where LTi cells reside in murine spleen and LNs. Furthermore, like murine LTi cells, these cells expressed high levels of IL-22 and upregulated IL-22 expression upon IL-23 stimulation. Importantly, these cells were not an NK cell subset since they showed no expression of IFN-γ and perforin. Interestingly, a subset of the CD117(+) CD3(-) CD56(-) OX40L(+) population expressed NKp46, again similar to recent findings in mice. Finally, these cells supported memory CD4(+) T-cell survival in an OX40L-dependent manner. Combined, these data indicate that the CD117(+) CD3(-) CD56(-) OX40L(+) cells in human secondary lymphoid tissues are comparable in phenotype, location and function to the LTi cells that persist within adult murine secondary lymphoid tissues.

CD4(+) lymphoid tissue-inducer cells promote innate immunity in the gut

Fetal CD4(+) lymphoid tissue inducer (LTi) cells play a critical role in the development of lymphoid tissues. Recent studies identified that LTi cells persist in adults and are related to a heterogeneous population of innate lymphoid cells that have been implicated in inflammatory responses. However, whether LTi cells contribute to protective immunity remains poorly defined. We demonstrate that after infection with Citrobacter rodentium, CD4(+) LTi cells were a dominant source of interleukin-22 (IL-22) early during infection. Infection-induced CD4(+) LTi cell responses were IL-23 dependent, and ablation of IL-23 impaired innate immunity. Further, depletion of CD4(+) LTi cells abrogated infection-induced expression of IL-22 and antimicrobial peptides, resulting in exacerbated host mortality. LTi cells were also found to be essential for host protective immunity in lymphocyte-replete hosts. Collectively these data demonstrate that adult CD4(+) LTi cells are a critical source of IL-22 and identify a previously unrecognized function for CD4(+) LTi cells in promoting innate immunity in the intestine.

IL-17 family member cytokines: regulation and function in innate immunity

Recently, the IL-17 family member cytokines have become prominent subjects of investigation. IL-17 (IL-17A) is the best-described member of this family where its production has been mainly attributed to a specialized T helper subset of the adaptive immune response termed Th17. However, recent research on this and other Th17 cytokines has revealed new sources and functions of IL-17 family members in the innate immune response. This review will highlight recent advances in the field of IL-17 family member cytokines and will predominantly focus on the innate regulation and function of IL-17, IL-17F, and IL-25.

Synthesis of artificial lymphoid tissue with immunological function

The ability to generate functional artificial lymphoid tissue to induce specific immunity at ectopic sites could offer a potential breakthrough for treatment of diseases such as cancer and severe infection using immunotherapy. Artificial lymphoid tissue could also offer an informative tool to study further lymphoid tissue development and function in vivo. Here, we review the process of secondary and tertiary lymphoid organization, of which an understanding is essential for artificial lymphoid tissue synthesis. Using this knowledge, we consider the combination of cell types, soluble factors and scaffold properties that will enable proper accumulation and organization of lymphocytes into tissue grafts. Recent success in in vivo induction of artificial lymphoid tissue are also considered.

Heterogeneity of IL-22-producing Lymphoid Tissue Inducer-like Cells in Human and Mouse

Lymphoid tissue inducer (LTi) cells have been characterized in mouse as a key cell when secondary lymphoid tissues are organized during development and memory T cells are formed after birth. In addition to their involvement in adaptive immune responses, recent studies show that they contribute to innate immune responses by producing large amount of interleukin (IL)-22 against microbial attack. Here, we compare IL-22-producing LTi and LTi-like cells in human and mouse and discuss their heterogeneity in different tissues.

Effects of interleukin-15 on human CD3(-)CD117(+)CD56(-)OX40L(+) cell differentiation

Lymphoid tissue inducer (LTi) cells are essential for secondary lymphoid tissue development, and recently identified human LTi cells are closely related to natural killer (NK) cells. In this study, we investigate whether human CD3(-)CD117(+)CD56(-) cells that include LTi and immature NK cells respond to interleukin (IL)-15, which is an NK cell growth factor. In the presence of IL-15, CD3(-)CD117(+)CD56(-) cells proliferate and downregulate the expression of OX40L and mRNA for IL-22, lymphotoxin-alpha, and aryl hydrocarbon receptor, but not Id2. To examine whether CD(-)CD117(+)CD56(-) cells differentiate into CD3(-)CD117(+)CD56(+) NK cells by IL-15, we sorted CD3(-)CD117(+)CD56(-)OX40L(+) cells and cultured with IL-15 for 7 days. Approximately 75% of the cells differentiated into imterferon-gamma-expressing CD56(+) cells and approximately 25% of the cells did not. In addition, the latter population expressed LTi markers, including lymphotoxin-alpha and retinoid-related orphan receptor-gamma (RORC). These results show that approximately 25% of CD3(-)CD117(+)CD56(-)OX40L(+) cells are LTi cells and do not differentiate into CD56(+) NK cells by IL-15.

Roles of the NF-kappaB pathway in lymphocyte development and function

This article focuses on the functions of NF-kappaB that vitally impact lymphocytes and thus adaptive immunity. NF-kappaB has long been known to be essential for many of the responses of mature lymphocytes to invading pathogens. In addition, NF-kappaB has important functions in shaping the immune system so it is able to generate adaptive responses to pathogens. In both contexts, NF-kappaB executes critical cell-autonomous functions within lymphocytes as well as within supportive cells, such as antigen-presenting cells or epithelial cells. It is these aspects of NF-kappaB's physiologic impact that we address in this article.