Edited by: Xian Chang Li, Brigham and Women’s Hospital, USA
Reviewed by: Julian Dyson, Imperial College London, UK; Graham Anderson, University of Birmingham, UK
*Correspondence: Jonathan S. Bromberg, University of Maryland, 29 Greene Street, Suite 9200, Baltimore, MD 21201, USA. e-mail:
This article was submitted to Frontiers in Immunological Tolerance, a specialty of Frontiers in Immunology.
This is an open-access article subject to a non-exclusive license between the authors and Frontiers Media SA, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and other Frontiers conditions are complied with.
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It is first important to understand the structure, development, and regulatory mechanisms of lymphoid organs to provide the basis for understanding on how these control immunity and tolerance. The lymph node (LN) is an encapsulated, highly organized SLO (Figures
The spleen is surrounded by a capsule that extends many projections into the interior to form a compartmentalized structure (Figures
Lymph nodes develop during embryogenesis or in the first few weeks after birth through recruitment and interaction of lymphoid tissue inducer (LTi) cells and lymphoid tissue organizer (LTo) cells. The central role of the LTi cells in LN development is the expression and presentation of lymphotoxin alpha and beta (LTα1β2) to the LTβ receptor (LTβR) on LTo, and this interaction leads to organized lymphoid structures. The importance of this signal is demonstrated in mice deficient in LTβR or LTα, which lack LN and Peyer’s patches (PP; De Togni et al.,
In addition to organizing lymphoid structure during development, LTi cells are present in adult lymphoid tissues. Even after the maturation of SLO, a continuous interplay between lymphocytes and stromal cells is likely to be required for the maintenance of tissue architecture and the characteristics of adult stromal cells. Adult LTi cells express OX40L and CD30L, which are critical for memory CD4 T cell generation (Kim et al.,
Splenic LTi-like cells contribute the development of SLO and also to host defense, by producing IL-17 and IL-22 in response to pathogen or IL-23 stimulation (Takatori et al.,
LT is a tumor necrosis factor (TNF)-related cytokine required for the development and organization of SLO (Cyster,
The development of the spleen is independent of LT, however, the microarchitecture of the splenic white pulp requires LTβR signaling for its development and maintenance in the adult mice. Treatment with LTβRIg, which blocks LT signaling, dissolves discrete B cell follicles, alters the marginal zone, prevents germinal center formation in the spleen, and impairs antibody production in response to immunization (Mackay et al.,
The appropriate structure of the SLO is integral in immune fate decisions, as following SLO entry naïve lymphocytes must decide to remain naïve, or become activated, anergic, or deleted. The presence (or absence) of antigen, co-stimulation, cell interactions, and/or chemokines/cytokines are all instructive in these decisions. Stromal cells construct intricate scaffolding within the SLO and provide architectural support. In addition to defining structure, these cells also contribute to lymphocyte trafficking, antigen presentation and cellular interactions (Mueller and Germain,
In the LN, lymphocytes leave the blood and enter via the HEV before flowing to the cortical ridge and cortical sinuses (Grigorova et al.,
Following LN entry via HEV, or splenic entry via central arterioles in the marginal zone (Steiniger et al.,
Fibroblastic reticular cells not only form the physical structure of conduits to allow the flow of antigen and cells in the LN (Link et al.,
The chemokines CXCL12 (Wright et al.,
Fibroblastic reticular cells may also deliver inhibitory or negative signals to lymphocytes, preventing their homing to and within SLO. FRC produce CCL2 (Katakai et al.,
Fibroblastic reticular cells provide both cues and scaffolding, dictating the movements, and interactions of the diverse cell populations residing and migrating through the SLO. These observations suggest a model in which a transplant recipient encounters donor antigen, and as the recipient responds to these antigens lymphoid organ structure is remodeled. SLO structure affects both primary and subsequent immune responses to alloantigens, altering where antigen is presented, which cell types encounter antigen, the activation signals detected by these cell types, and the interactions of the various cell populations. Further levels of complexity include that SLO structure is influenced by previous inflammatory and antigen challenges, so the structure within the SLO of the transplant recipient may be as unique as a fingerprint. Hence, detection of these distinctive structural and environmental pressures may provide novel and unique targets for designing therapeutic protocols.
The spleen’s role in graft rejection is different between models such as vascularized or non-vascularized organ transplantation models. In some vascularized models, it contributes to graft rejection; however, in non-vascularized transplantation models, LN but not the spleen are essential for the rejection of skin allografts. Rather, the spleen appears to enhance graft prolongation (Souther et al.,
In non-vascularized grafts, DC migrate from the graft to the regional draining LN. Activated DC are trapped in the LN where they generate effector T cells from naïve T cells. Regulatory or immature DC may pass through the LN, reach the spleen, and generate Treg. Several studies indicate that the expression pattern of chemokines and cytokine receptors between LN and spleen is different, explaining the different roles in transplant models (Tang and Cyster,
Suppression and regulatory mechanisms of the spleen are shown in several studies. A subset of splenic red pulp F4/80hiMac-1low macrophages, whose differentiation is regulated by CSF-1, regulates CD4+ T cell responses using TGFβ and IL-10 and inducing differentiation of Foxp3+ Treg (Kurotaki et al.,
In addition to dictating movement of lymphocytes within the LN by supplying both structure and directional cues, stromal cells may also dictate lymphocyte survival. Stromal cells affect T cell viability, and correct positioning within the LN is integral to T cell survival (Table
Name | Location | Phenotype | Function | |
---|---|---|---|---|
Follicular dendritic cell (FDC) | Cortex, B cell primary follicle (LN and spleen) | CD45−CD35+ FDC-M1+ | Regulate B cell homeostasis, migration, and survival (Gunn et al., |
|
Fibroblastic reticular cell (FRC) | Paracortex, T cell area (LN and spleen) | CD45− gp38(podoplanin)+ CD31− ER-TR7+ VCAM-1high CD44high | Support B cell, T cell, and DC interactions (Katakai et al., |
|
Lymphatic endothelial cell (LEC) | LN | CD45− gp38+CD31+ VCAM-1low CD44low | Directly induce tolerance of responding naïve CD8 T cell (Cohen et al., |
|
Blood endothelial cell (BEC) | LN and spleen | CD45−gp38− CD31+ VCAM-1low CD44low | Express PTAs (Cohen et al., |
|
Double negative (DN) | CD45−gp38−CD31− | Express PTAs (Cohen et al., |
Stromal cells also participate in peripheral tolerance by expressing autoimmune regulator gene (
Different subsets of stromal cells express
Dendritic cell and lymphocytes enter the LN through HEV and afferent lymphatics (von Andrian and Mempel,
Whether HEV enhance immune regulatory capacity of Treg is not certain. In an islet allograft model, adoptively transferred Treg that enter LN through HEV do not acquire an activated phenotype to suppress the alloimmune response (Zhang et al.,
Lymphatics are lined with LEC expressing LYVE-1. Lymphatics are found throughout the LN and have different functions and phenotypes in different regions. DC and T cells in peripheral tissues enter LN through afferent lymphatics, which typically end in the SCS, a hollow space below the fibrous capsule of the LN (Randolph et al.,
When leaving the LN, lymphocytes in the parenchyma enter medullary networks of lymphatic sinuses and from there the efferent lymphatics (von Andrian and Mempel,
Lymphatic vessels play an important role in immune tolerance. In an islet allograft model, Treg that are adoptively transferred and migrate from the graft to LN via afferent lymphatics prevent graft rejection. However, Treg which enter LN via HEV do not (Zhang et al.,
Vascular endothelial growth factors (VEGF), especially VEGF-A and VEGF-C, are involved in LEC proliferation and lymphangiogenesis (Shibuya and Claesson-Welsh,
In an islet allograft model, anti-VEGFR-3 mAb inhibits lymphangiogenesis and prolongs allograft survival, suggesting that inhibition of lymphangiogenesis may prevent immunity and inflammation (Yin et al.,
Lymph node lymphatics and HEV seem to be in synchrony. B cell-derived VEGF-A promotes HEV expansion as well as lymphangiogenesis in LN (Shrestha et al.,
When T cells enter LN through HEV or lymphatics, they are under the influence of chemokines secreted by LEC or vascular endothelial cells such as CCL19 and CCL21, which can inhibit the proliferation of T cells (Ziegler et al.,
T cell homing to LN plays a critical role in tolerance to alloantigen because Treg develop and are required within the LN during tolerance induction. If T cell homing to LN is inhibited by anti-L-selectin (CD62L) mAb, cardiac allograft survival is prevented despite a tolerogenic regimen of anti-CD40L mAb plus donor-specific transfusion (Ochando et al.,
Primary lymphoid organ | Thymus | Treg development | Involved cells: |
Bone marrow | Treg recruitment | CXCR4/CXCL12 interaction | |
Treg role | Providing immune-privileged sites for HSPC |
||
Secondary lymphoid organ | Spleen | Treg generation | Involved cells: |
LN | Treg generation | Involved cells: |
|
Treg role | Treg migrated from peripheral tissues more potent than LN-resident Tregs |
||
Tertiary lymphoid organ | Treg detected; role not certain |
The thymus is a lymphoid organ that is critical for tolerance. It is not only the site for eliminating self-reactive T cells through negative selection, but also for controlling self-reactive T cells through CD4+Foxp3+ regulatory T cell (Treg) development. Delayed generation of Treg by thymectomy at day 3, but not day 7, results in autoimmune disease development (Fontenot et al.,
A number of observations show that Treg develop in the medulla: expression of
Foxp3 expression does not commence until day 3 in neonates, suggesting that only organized thymic architecture provides proper co-stimulatory signals in the early thymus. CD28, IL-2R, TSLP receptor, CD154, glucocorticoid-induced TNF receptor, and Stat5 signals are all implicated in the development and lineage commitment of thymus-derived nTreg (Bettini and Vignali,
Various experimental models demonstrate that the thymus is required for Treg development and consequent tolerance induction to alloantigens. In a rat model using immature myeloid DC (imDC) primed with immune-dominant allopeptide
Bone marrow is an essential part of the immature and mature lymphocyte recirculation network, and it harbors mature CD4+CD25+ Treg and serves as a Treg reservoir. Studies show that CXCR4/CXCL12 signals play an important role in regulating Treg trafficking from bone marrow and in maintaining homeostatic levels of Treg in the periphery. G-CSF treatment decreases bone marrow CXCL12 expression, and results in Treg mobilization from bone marrow into the periphery, and is consistent with the low prevalence of acute GVHD and the improvement in autoimmune diseases following G-CSF treatment (Zou et al.,
The bone marrow is the primary site for B cell maturation. Naive B cells then migrate to SLO, become plasmablasts upon antigenic stimulation in antigen-activated T cell areas, secrete low-affinity antibody and eventually undergo apoptosis. Some activated B cells enter into the long-lived memory compartment as either memory B cells or long-lived plasma cells (PC). Long-lived PC remain in either the LN or spleen, but most home to and reside in the bone marrow. Long-lived PC in the marrow are a major source of persistent donor-specific alloantibody (Stegall et al.,
Name | Subset | Phenotype | Location | Function |
|
---|---|---|---|---|---|
Activated or mature | In the steady state or tolerance | ||||
Lymphoid organ-resident | CD8+ | CD11c+CD11b−CD8+CD4− (also CD103+CD207+ subset) | Spleen, LN and thymus | Promote cytotoxic T cell responses | CD8+ T cell tolerance Induce Treg |
CD4+ | CD11c+CD11b+CD8−CD4+ | Spleen and LN | Promote CD4+ T cell responses | CD4+ T cell tolerance |
|
CD8−CD4− | CD11c+CD8−CD4− (CD11b+ and CD11b− subsets) | Spleen and LN | |||
Migratory | CCR7+ | LC(CD11c+ CD207+CD103−) |
LN | Transport the pathogen to the draining LN and promote T cell responses Up-regulate homing receptors of activated T cells | Carry PTA from periphery into LN CD8+ T cell tolerance |
CD11b+? (CD11c+CD11b+CD103−) | Not clear | Not clear | Not clear | ||
Plasmacytoid | CD11cmid CD11b−CD8± CD4+Gr-1+ (produce type I IFNs) | Thymus, bone marrow, and secondary lymphoid tissue | Anti-viral immunity | Induce Tregs |
|
Inflammatory (TIP and monocyte) | CD11c+CD11b+Ly6C+ (produce TNF and express iNOS) | Inflammatory lesions | Induction of adaptive immunity | Tezuka et al. ( |
Ectopic or tertiary lymphoid organs (TLO) are often induced at sites of chronic infection or inflammation in peripheral non-lymphoid organs. These tissues are architecturally similar to SLO, with separate B and T cell areas, specialized populations of DC, well-differentiated stromal cells, and HEV (Carragher et al.,
Tertiary lymphoid organs have been described in a variety of autoimmune diseases including gastritis, thyroiditis, and systemic lupus erythematosus (Carragher et al.,
The ectopic accumulation of lymphoid cells has been considered to signify destructive inflammation that is accompanied by tissue damage (Drayton et al.,
While great progress has been made in understanding molecules, signaling pathways, and cells important for alloimmunity, and how to manipulate, interfere with, and suppress immune cell activation, migration, and effectors functions to benefit transplantation, evidence has emerged that various lymphoid organs, their anatomic structures, and particular microenvironments that lymphocytes encounter or reside in during the course of the alloresponse are also critical in determining the final outcome of graft acceptance and transplant tolerance. The appropriate tissue architecture may decide whether immune cells remain naïve, or become activated, anergic, or deleted by affecting antigen presentation, adhesion molecule expression, co-stimulatory signal activation, cytokine and chemokine production, thus affect regulatory and effector cell differentiation, trafficking and effector activities. In order to further overcome barriers to transplant tolerance, precise models and investigations on the arrangement of cells and molecules in lymphoid structures and anatomic pathways are required. For example, how LTi/LTo interactions, LT signal cascades, stromal cells, and specific microdomains affect immune responses and transplant tolerance induction and maintenance will all be productive areas for investigation.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Aire, autoimmune regulator; APC, antigen-presenting cell; DC, dendritic cell; ER-TR7, Erasmus University Rotterdam- thymic reticulum antibody 7; FDC, follicular dendritic cell; FRC, fibroblastic reticular cell; FTreg, follicular regulatory T cells; GITR, glucocorticoid-induced tumor necrosis factor receptor; HEV, high endothelial venules; HSPC, hematopoietic stem/progenitor cells; HVEM, herpes simplex virus (HSV) glycoprotein D for HSV entry mediator; imDC, immature myeloid DC; LEC, lymphatic endothelial cells; LN, lymph node; LT, lymphotoxin; LTβR, Lymphotoxin β receptor; LTi, lymphoid tissue inducer; LTo, lymphoid tissue organizer; LYVE-1, lymphatic vessel endothelial hyaluronan receptor 1; nTreg, natural Treg; PALS, periarteiolar lymphoid sheath; pDC, plasmacytoid DC; PD-L1, programmed death ligand 1; PNAd, peripheral lymph node addressin; PP, Peyer’s patches; PTA, peripheral tissue-restricted antigen; SCS, subcapsular sinus; SIV, Simian immunodeficiency virus; SLO, secondary lymphoid organs; TCR, T cell receptor; TFH, follicular helper T cells; TGFβR, TGFβ receptor; TLO, tertiary lymphoid organs; TNF, tumor necrosis factor; Treg, regulatory T cells; TSLP, thymic stromal lymphopoietin; VEGF, vascular endothelial growth factor.