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

Summary The omentum is a site of B1 lymphopoiesis and immune responsiveness to T-independent antigens. However, it is unknown whether it supports immune responses independently of conventional lymphoid organs. We show that the omentum collects antigens and cells from the peritoneal cavity and supports T-dependent B cell responses, including isotype switching, somatic hypermutation and limited affinity maturation, despite the lack of identifiable follicular dendritic cells. The omentum also supports CD4 and CD8 responses to peritoneal antigens and recruits effector T cells primed in other locations. Unlike conventional lymphoid organs, milky spots in the omentum develop in the absence of lymphoid tissue inducer cells, but require CXCL13. Although the lymphoid architecture of milky spots is disrupted in lymphotoxin-deficient mice, normal architecture is 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.

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. 2. The development of LTi cells from precursors in the fetal liver and their local differentiation into LTαβ-expressing cells requires the activities of cytokines, such as TRANCE or IL-7. IL-7 seems to be most important for the differentiation of LTi cells at mucosal sites, such as the Peyer's patches, while TRANCE is most important at sites of peripheral lymph node development.3. LTi cells are attracted to sites of lymphoid organ development by homeostatic chemokines, including CXCL13, CCL19 and CCL21. Like TRANCE and IL-7, these chemokines also maintain surface LTαβ expression on LTi cells. 4.The lymphotoxin signalling pathway plays a central role in the development of secondary lymphoid organs due to its ability to trigger mesenchymal cell differentiation, elicit homeostatic chemokine expression and to promote the differentiation of HEVs, stromal cells and dendritic cells.

Current influenza vaccines elicit Abs to the hemagglutinin and neuraminidase envelope proteins. Due to antigenic drift, these vaccines must be reformulated annually to include the envelope proteins predicted to dominate in the following season. By contrast, vaccination with the conserved nucleoprotein (NP) elicits immunity against multiple serotypes (heterosubtypic immunity). NP vaccination is generally thought to convey protection primarily via CD8 effector mechanisms. However, significant titers of anti-NP Abs are also induced, yet the involvement of Abs in protection has largely been disregarded. To investigate how Ab responses might contribute to heterosubtypic immunity, we vaccinated C57BL/6 mice with soluble rNP. This approach induced high titers of NP-specific serum Ab, but only poorly detectable NP-specific T cell responses. Nevertheless, rNP immunization significantly reduced morbidity and viral titers after influenza challenge. Importantly, Ab-deficient mice were not protected by this vaccination strategy. Furthermore, rNP-immune serum could transfer protection to naive hosts in an Ab-dependent manner. Therefore, Ab to conserved, internal viral proteins, such as NP, provides an unexpected, yet important mechanism of protection against influenza. These results suggest that vaccines designed to elicit optimal heterosubtypic immunity to influenza should promote both Ab and T cell responses to conserved internal proteins.

Ectopic or tertiary lymphoid tissues develop at sites of inflammation or infection in peripheral, non-lymphoid organs. These tissues are architecturally similar to conventional secondary lymphoid organs, with separated B and T cell areas, specialized populations of dendritic cells, well-differentiated stromal cells and high endothelial venules. Ectopic lymphoid tissues are often associated with the local pathology that results from chronic infection or chronic inflammation. However, there are also examples in which ectopic lymphoid tissues appear to contribute to local protective immune responses. Here we review how ectopic lymphoid structures develop and function in the context of local immunity and pathology.

Immunity to heterosubtypic strains of influenza is thought to be mediated primarily by memory T cells, which recognize epitopes in conserved proteins. However, the involvement of B cells in this process is controversial. We show in this study that influenza-specific memory T cells are insufficient to protect mice against a lethal challenge with a virulent strain of influenza in the absence of B cells. B cells contribute to protection in multiple ways. First, although non-neutralizing Abs by themselves do not provide any protection to challenge infection, they do reduce weight loss, lower viral titers, and promote recovery of mice challenged with a virulent heterosubtypic virus in the presence of memory T cells. Non-neutralizing Abs also facilitate the expansion of responding memory CD8 T cells. Furthermore, in cooperation with memory T cells, naive B cells also promote recovery from infection with a virulent heterosubtypic virus by generating new neutralizing Abs. These data demonstrate that B cells use multiple mechanisms to promote resistance to heterosubtypic strains of influenza and suggest that vaccines that elicit both memory T cells and Abs to conserved epitopes of influenza may be an effective defense against a wide range of influenza serotypes.

Disruption of the centromere protein J gene, CENPJ (CPAP, MCPH6, SCKL4), which is a highly conserved and ubiquitiously expressed centrosomal protein, has been associated with primary microcephaly and the microcephalic primordial dwarfism disorder Seckel syndrome. The mechanism by which disruption of CENPJ causes the proportionate, primordial growth failure that is characteristic of Seckel syndrome is unknown. By generating a hypomorphic allele of Cenpj, we have developed a mouse (Cenpjtm/tm) that recapitulates many of the clinical features of Seckel syndrome, including intrauterine dwarfism, microcephaly with memory impairment, ossification defects, and ocular and skeletal abnormalities, thus providing clear confirmation that specific mutations of CENPJ can cause Seckel syndrome. Immunohistochemistry revealed increased levels of DNA damage and apoptosis throughout Cenpjtm/tm embryos and adult mice showed an elevated frequency of micronucleus induction, suggesting that Cenpj-deficiency results in genomic instability. Notably, however, genomic instability was not the result of defective ATR-dependent DNA damage signaling, as is the case for the majority of genes associated with Seckel syndrome. Instead, Cenpjtm/tm embryonic fibroblasts exhibited irregular centriole and centrosome numbers and mono- and multipolar spindles, and many were near-tetraploid with numerical and structural chromosomal abnormalities when compared to passage-matched wild-type cells. Increased cell death due to mitotic failure during embryonic development is likely to contribute to the proportionate dwarfism that is associated with CENPJ-Seckel syndrome.

Lymph nodes provide specialized stromal microenvironments that support the recruitment and organization of T cells and B cells, enabling them to effectively participate in immune responses. While CD4 ؉ 3 ؊ lymphoid tissue inducer cells (LTic's) are known to play a key role in influencing lymph node (LN) development, the mechanisms that regulate the development of stromal organizer cells are unclear. Here, we define an ontogenetic program of lymph node stromal cell maturation in relation to the requirement for LTic's. We also describe a lymph node reaggregation assay to study cell-cell interactions and lymphocyte recruitment to these organs that reproduces the in vivo events during lymph node development. In addition, analysis of the lymph node anlagen in normal and lymphotoxin a (LTa)-deficient embryos shows that LTamediated signaling is required to sustain proliferation and survival of stromal cells in vivo. Our data identify LTa-independent and LTa-dependent stages of lymph node development, and provide direct evidence for the role of LTic's during LN organogenesis.( IntroductionOne of the main features of the immune system is the cross-talk interaction between different cell types that ensures full activation of immune responses. The exchange of information between cells of the immune system occurs within specific microenvironments in secondary lymphoid organs such as the spleen, lymph nodes (LNs), and Peyer patches (PPs). Disruption or abnormal development of these microenvironments leads to ineffective adaptive immune responses, resulting in repetitive infections and decrease survival of the host.LN organogenesis during embryo development depends on successful interactions between bone marrow-derived lymphoid tissue inducer cells (LTic's) and mesenchymal organizer stromal cells. [1][2][3] The signals involved in these interactions are transmitted through ligands and receptors of the tumor necrosis factor family that converge in activation of the NF-B transcription factors, resulting in marked changes in gene expression. 4,5 Genetic approaches had shown the crucial roles of lymphotoxin ␤ receptor (Lt␤R) and receptor activator of NF-B (RANK) and their ligands in the development of LNs. For instance, Lta Ϫ/Ϫ mice and Ltbr Ϫ/Ϫ mice lack all LNs, Ltb Ϫ/Ϫ mice form only mesenteric LNs, and Rank Ϫ/Ϫ and Rank-l Ϫ/Ϫ mice develop only rudimentary mesenteric LNs. 3,[6][7][8][9][10] Similarly, mice deficient in the NF-B family proteins RelA and RelB lack all LNs, while Nfkb2 Ϫ/Ϫ and Ikka aa knock-in mice present with defects in peripheral LNs. [11][12][13][14][15][16] While LTic's express the membrane-bound LT␣ 1 ␤ 2 ligand, stromal cells express the LT␤R receptor. Engagement of LT␤R receptor on stromal cells results in production of chemokines such as CXCL13, CCL21, and CCL19, and cell adhesion molecules that will attract further LTic's that express the chemokine receptors CXCR5 and CCR7. [17][18][19] Subsequent formation of cell clusters between LTic's and organizer cells results in organization of specific areas in...