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Patients with ectodermal dysplasia with immunodeficiency (ED-ID) caused by mutations in the inhibitor of NF-κB α (IκBα) are susceptible to severe recurrent infections, despite normal T and B cell numbers and intact in vitro lymphocyte function. Moreover, the outcome of hematopoietic stem cell transplantation (HSCT) in these patients is poor despite good engraftment. Mice heterozygous for the IκBα S32I mutation found in patients exhibited typical features of ED-ID. Strikingly, the mice lacked lymph nodes, Peyer's patches, splenic marginal zones, and follicular dendritic cells and failed to develop contact hypersensitivity (CHS) or form germinal centers (GCs), all features not previously recognized in patients and typical of defective noncanonical NF-κB signaling. Lymphotoxin β receptor (LTβR)-driven induction of chemokines and adhesion molecules mediated by both canonical and noncanonical NF-κB pathways was impaired, and levels of p100 were markedly diminished in the mutant. IκBα mutant → Rag2(-/-), but not WT→IκBα mutant, bone marrow chimeras formed proper lymphoid organs and developed CHS and GCs. Defective architectural cell function explains the immunodeficiency and poor outcome of HSCT in patients with IκBα deficiency and suggests that correction of this niche is critical for reconstituting their immune function., (© 2015 Mooster et al.)

Lymphoid tissue development is initiated during embryogenesis by the migration of lymphoid tissue inducer (LTi) cells from the fetal liver to the periphery, where they induce the formation of lymph nodes and Peyer's patches. In the fetal liver, a subset of common lymphoid progenitors (CLPs) that expresses the integrin α4β7 gives rise to LTi cells, a process strictly dependent on the expression of the transcriptional repressor Id2 and the nuclear hormone receptor retinoic acid-related orphan receptor γ t (RORγt). In this study, we show that Id2 and RORγt are sequentially up-regulated during LTi cell development, matching two waves of differentiation with opposite requirements for Notch signaling. Both the expression of Id2 and Notch are required for the generation of α4β7(+) RORγt(-) fetal progenitors, but Notch subsequently blocks progression to the RORγt(+) stage and final maturation of LTi cells. Notch is therefore a necessary switch to engage the LTi developmental pathway, but needs to be turned off later to avoid diversion to the T cell fate.

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 gammat, lymphotoxin (LT) alpha1beta2-LTbetaR, 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.

Lymph node (LN) function depends on T and B cell compartmentalization, antigen presenting cells, and high endothelial venules (HEVs) expressing mucosal addressin cell adhesion molecule (MAdCAM-1) and peripheral node addressin (PNAd), ligands for naive cell entrance into LNs. Luminal PNAd expression requires a HEV-restricted sulfotransferase (HEC-6ST). To investigate LT alpha beta's activities in lymphoid organogenesis, mice simultaneously expressing LT alpha and LT beta under rat insulin promoter II (RIP) control were compared with RIPLT alpha mice in a model of lymphoid neogenesis and with LT beta-/- mice. RIPLT alpha beta pancreata exhibited massive intra-islet mononuclear infiltrates that differed from the more sparse peri-islet cell accumulations in RIPLT alpha pancreata: separation into T and B cell areas was more distinct with prominent FDC networks, expression of lymphoid chemokines (CCL21, CCL19, and CXCL13) was more intense, and L-selectin+ cells were more frequent. In contrast to the predominant abluminal PNAd pattern of HEV in LT beta-/- MLN and RIPLT alpha pancreatic infiltrates, PNAd was expressed at the luminal and abluminal aspects of HEV in wild-type LN and in RIPLT alpha beta pancreata, coincident with HEC-6ST. These data highlight distinct roles of LT alpha and LT alpha beta in lymphoid organogenesis supporting the notion that HEC-6ST-dependent luminal PNAd is under regulation by LT alpha beta.

The transcription factor nuclear factor (NF)-kappaB has been suggested to be a key mediator of the development of lymph nodes and Peyer's patches. However, targeted deletion of NF-kappaB/ Rel family members has not yet corroborated such a function. Here we report that when mice lacking the RelA subunit of NF-kappaB are brought to term by breeding onto a tumor necrosis factor receptor (TNFR)1-deficient background, the mice that are born lack lymph nodes, Peyer's patches, and an organized splenic microarchitecture, and have a profound defect in T cell-dependent antigen responses. Analyses of TNFR1/RelA-deficient embryonic tissues and of radiation chimeras suggest that the dependence on RelA is manifest not in hematopoietic cells but rather in radioresistant stromal cells needed for the development of secondary lymphoid organs.

We show that the mesenchymal cells that surround the 12-d mouse embryo thymus are necessary for T cell differentiation. Thus, epithelial lobes with attached mesenchyme generate all T cell populations in vitro, whereas lobes from which mesenchyme has been removed show poor lymphopoiesis with few cells progressing beyond the CD4(-)CD8(-) stage of development. Interestingly, thymic mesenchyme is derived from neural crest cells, and extirpation of the region of the neural crest involved results in impaired thymic development and craniofacial abnormalities similar to the group of clinical defects found in the DiGeorge syndrome. Previous studies have suggested an inductive effect of mesenchyme on thymic epithelial morphogenesis. However, we have found that mesenchyme-derived fibroblasts are still required for early T cell development in the presence of mature epithelial cells, and hence mesenchyme might have a direct role in lymphopoiesis. We provide an anatomical basis for the role of mesenchyme by showing that mesenchymal cells migrate into the epithelial thymus to establish a network of fibroblasts and associated extracellular matrix. We propose that the latter might be important for T cell development through integrin and/or cytokine interactions with immature thymocytes.

For more than a decade, the biological roles and the apparent redundancy of the cytokines tumor necrosis factor (TNF) and lymphotoxin (LT) have been debated. LT alpha exists in its soluble form as a homotrimer, which like TNF only binds the TNF receptors, TNF-R55 or TNF-R75. The cell surface form of LT exists as a heteromer of LT alpha and LT beta subunits and this complex specifically binds the LT beta receptor (LT beta-R). To discriminate the functions of the LT and TNF systems, soluble LT beta-R-immunoglobulin (Ig) or TNF-R-Ig fusion proteins were introduced into embryonic circulation by injecting pregnant mice. Exposure to LT beta-R-Ig during gestation disrupted lymph node development and splenic architecture in the progeny indicating that both effects are mediated by the surface LT alpha/beta complex. These data are the first to identify a cell surface ligand involved in immune organ morphogenesis. Moreover, they unambiguously discriminate the functions of the various TNF/LT ligands, provide a unique model to study compartmentalization of immune responses and illustrate the generic utility of receptor-Ig fusion proteins for dissecting/ordering ontogenetic events in the absence of genetic modifications.

Virtually all human granular lymphocytes expressed the HNK-1 differentiation antigen when examined in lymphoid compartments from adults, neonates, and fetuses. The HNK-1+ cells were distinguishable into three subsets having distinct antigenic phenotypes: HNK+T3-M1-, HNK+T3+M1-, and HNK+T3-M1+. Thus, greater than 70% of the HNK-1+ cells from 13-17 wk fetuses (less than 0.2% of nucleated cells) lacked T cell antigens (e.g., T3, T8, T4, and T6) and the M1 myeloid antigen. Morphologically, the HNK+T3-M1- cells consisted of three different types: small granular lymphocytes (less than 10% of HNK-1+ cells), agranular small lymphocytes with a narrow rim of cytoplasm (70-80%), and agranular giant cells (greater than 15 micrometers) with considerable neutrophilic cytoplasm (15%). The purified fetal HNK-1+ cells exhibited a low level of cytotoxicity against K562 target cells. On the other hand, almost all of HNK-1+ cells in neonatal tissues as well as adult bone marrow, lymph node, and thymus, exhibited the HNK+T3+M1- phenotype, contained sparse cytoplasmic granules, and had an intermediate level of NK functional activity. Only adult blood and spleen contained a majority of mature HNK-1+ cells. These cells had an HNK+T3-M1- phenotype, abundant cytoplasmic granules, and maximum NK function. We propose that human NK cells may generate from a separate cell lineage and that they alter their phenotype, morphology, and functional capability during differentiation.