Reina E. Mebius

Innate lymphoid cells — a proposal for uniform nomenclature

Innate lymphoid cells (ILCs) are a family of developmentally related cells that are involved in immunity and in tissue development and remodelling. Recent research has identified several distinct members of this family. Confusingly, many different names have been used to characterize these newly identified ILC subsets. Here, we propose that ILCs should be categorized into three groups based on the cytokines that they can produce and the transcription factors that regulate their development and function.

Structure and function of the spleen.

The spleen combines the innate and adaptive immune system in a uniquely organized way. The structure of the spleen enables it to remove older erythrocytes from the circulation and leads to the efficient removal of blood-borne microorganisms and cellular debris. This function, in combination with a highly organized lymphoid compartment, makes the spleen the most important organ for antibacterial and antifungal immune reactivity. A better understanding of the function of this complex organ has been gained from recent studies, as outlined in this Review article.

Developing Lymph Nodes Collect CD4+CD3− LTβ+ Cells That Can Differentiate to APC, NK Cells, and Follicular Cells but Not T or B Cells

For a brief period during fetal lymph node organogenesis in mice, lymph node postcapillary high endothelial venules surprisingly express the Peyers patch addressin MAdCAM-1. This expression allows initial seeding of this incipient structure by two unusual lymphocyte populations selectively expressing the Peyers patch homing receptor integrin alpha4beta7: CD4+CD3- oligolineage progenitors and TCR gammadelta+ T cells. We show here that CD4+CD3- cells are lineage-restricted progenitors that express surface lymphotoxin-beta (LTbeta) and the chemokine receptor BLR1 and that can become natural killer cells, dendritic antigen-presenting cells, and follicular cells of unknown outcome, but these cells do not become T or B lymphocytes. Since the necessity of lymphotoxin in lymphoid organ development has been shown, we propose that the novel subset of CD4+CD3-LTbeta+ fetal cells is instrumental in the development of lymphoid tissue architecture.

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-α1β2-expressing lymphoid-tissue inducer cells and lymphotoxin-β receptor-expressing stromal organizer cells. Here, we discuss new insights into the earliest phases of peripheral lymph node and Peyers patch formation that occur before lymphotoxin-β 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.

Conduits mediate transport of low-molecular-weight antigen to lymph node follicles.

To track drainage of lymph-borne small and large antigens (Ags) into the peripheral lymph nodes and subsequent encounter by B cells and follicular dendritic cells, we used the approach of multiphoton intravital microscopy. We find a system of conduits that extend into the follicles and mediate delivery of small antigens to cognate B cells and follicular dendritic cells. The follicular conduits provide an efficient and rapid mechanism for delivery of small antigens and chemokines such as CXCL13 to B cells that directly contact the conduits. By contrast, large antigens were bound by subcapsular sinus macrophages and subsequently transferred to follicular B cells as previously reported. In summary, the findings identify a unique pathway for the channeling of small lymph-borne antigens and chemoattractants from the subcapsular sinus directly to the B cell follicles. This pathway could be used for enhancing delivery of vaccines or small molecules for improvement of humoral immunity.

The Fetal Liver Counterpart of Adult Common Lymphoid Progenitors Gives Rise to All Lymphoid Lineages, CD45+CD4+CD3− Cells, As Well As Macrophages

We identified an IL-7Rα+Sca-1lowc-Kitlow population in E14 fetal liver, which is the phenotypical analog of common lymphoid progenitors (CLP) in adult bone marrow. After transfer into newborn mice, the IL-7Rα+Sca-1lowc-Kitlow population rapidly differentiated into CD45+CD4+CD3− cells, which are candidate cells for initiating lymph node and Peyer’s patch formation. In addition, this population also gave rise to B, T, NK, and CD8α+ and CD8α− dendritic cells. The fetal liver precursors expressed a significantly lower level of the myeloid-suppressing transcription factor Pax-5, than adult CLP, and retained differentiation activity for macrophages in vitro. We propose that the transition from fetal liver IL-7Rα+Sca-1lowc-Kitlow cells to adult CLP involves a regulated restriction of their developmental potential, controlled, at least in part, by Pax-5 expression.

Lymphotoxin β receptor signaling promotes tertiary lymphoid organogenesis in the aorta adventitia of aged ApoE−/− mice

Atherosclerosis involves a macrophage-rich inflammation in the aortic intima. It is increasingly recognized that this intimal inflammation is paralleled over time by a distinct inflammatory reaction in adjacent adventitia. Though cross talk between the coordinated inflammatory foci in the intima and the adventitia seems implicit, the mechanism(s) underlying their communication is unclear. Here, using detailed imaging analysis, microarray analyses, laser-capture microdissection, adoptive lymphocyte transfers, and functional blocking studies, we undertook to identify this mechanism. We show that in aged apoE−/− mice, medial smooth muscle cells (SMCs) beneath intimal plaques in abdominal aortae become activated through lymphotoxin β receptor (LTβR) to express the lymphorganogenic chemokines CXCL13 and CCL21. These signals in turn trigger the development of elaborate bona fide adventitial aortic tertiary lymphoid organs (ATLOs) containing functional conduit meshworks, germinal centers within B cell follicles, clusters of plasma cells, high endothelial venules (HEVs) in T cell areas, and a high proportion of T regulatory cells. Treatment of apoE−/− mice with LTβR-Ig to interrupt LTβR signaling in SMCs strongly reduced HEV abundance, CXCL13, and CCL21 expression, and disrupted the structure and maintenance of ATLOs. Thus, the LTβR pathway has a major role in shaping the immunological characteristics and overall integrity of the arterial wall.

Maternal retinoids control type 3 innate lymphoid cells and set the offspring immunity

The impact of nutritional status during fetal life on the overall health of adults has been recognized; however, dietary effects on the developing immune system are largely unknown. Development of secondary lymphoid organs occurs during embryogenesis and is considered to be developmentally programmed. Secondary lymphoid organ formation depends on a subset of type 3 innate lymphoid cells (ILC3) named lymphoid tissue inducer (LTi) cells. Here we show that mouse fetal ILC3s are controlled by cell-autonomous retinoic acid (RA) signalling in utero, which pre-sets the immune fitness in adulthood. We found that embryonic lymphoid organs contain ILC progenitors that differentiate locally into mature LTi cells. Local LTi cell differentiation was controlled by maternal retinoid intake and fetal RA signalling acting in a haematopoietic cell-autonomous manner. RA controlled LTi cell maturation upstream of the transcription factor RORγt. Accordingly, enforced expression of Rorgt restored maturation of LTi cells with impaired RA signalling, whereas RA receptors directly regulated the Rorgt locus. Finally, we established that maternal levels of dietary retinoids control the size of secondary lymphoid organs and the efficiency of immune responses in the adult offspring. Our results reveal a molecular link between maternal nutrients and the formation of immune structures required for resistance to infection in the offspring.

Chemokine CXCL13 is essential for lymph node initiation and is induced by retinoic acid and neuronal stimulation

The location of embryonic lymph node development is determined by the initial clustering of lymphoid tissue–inducer (LTi) cells. Here we demonstrate that both the chemokine CXCL13 and the chemokine CCL21 attracted LTi cells at embryonic days 12.5–14.5 and that initial clustering depended exclusively on CXCL13. Retinoic acid (RA) induced early CXCL13 expression in stromal organizer cells independently of lymphotoxin signaling. Notably, neurons adjacent to the lymph node anlagen expressed enzymes essential for RA synthesis. Furthermore, stimulation of parasymphathetic neural output in adults led to RA receptor (RAR)-dependent induction of CXCL13 in the gut. Therefore, our data show that the initiation of lymph node development is controlled by RA-mediated expression of CXCL13 and suggest that RA may be provided by adjacent neurons.

A conduit system distributes chemokines and small blood-borne molecules through the splenic white pulp.

Access to the splenic white pulp is restricted to lymphocytes and dendritic cells. Here we show that movement of molecules from the blood into these confined areas is also limited. Large molecules, such as bovine serum albumin (68 kD), immunoglobulin G (150 kD), and 500 kD dextran are unable to enter the white pulp, whereas smaller blood-borne molecules can directly permeate this compartment. The distribution is restricted to a stromal network that we refer to as the splenic conduit system. The small lumen of the conduit contains collagen fibers and is surrounded in the T cell areas by reticular fibroblasts that express ER-TR7. It also contains the chemokine CCL21. Conversely, in B cell follicles the B cell–attracting chemokine CXCL13 was found to be associated with the conduit and absence of ER-TR7+ fibroblasts. These results show heterogeneity of reticular fibroblasts that enfold the conduit system and suggest that locally produced chemokines are transported through and presented on this reticular network. Therefore, the conduit plays a role in distribution of both blood-borne and locally produced molecules and provides a framework for directing lymphocyte migration and organization of the splenic white pulp.