Noriko Komatsu

Preferential Generation of Follicular B Helper T Cells from Foxp3+ T Cells in Gut Peyer's Patches

Most of the immunoglobulin A (IgA) in the gut is generated by B cells in the germinal centers of Peyers patches through a process that requires the presence of CD4+ follicular B helper T(TFH) cells. The nature of these TFH cells in Peyers patches has been elusive. Here, we demonstrate that suppressive Foxp3+CD4+ T cells can differentiate into TFH cells in mouse Peyers patches. The conversion of Foxp3+ T cells into TFH cells requires the loss of Foxp3 expression and subsequent interaction with B cells. Thus, environmental cues present in gut Peyers patches promote the selective differentiation of distinct helper T cell subsets, such as TFH cells.

Heterogeneity of natural Foxp3+ T cells: A committed regulatory T-cell lineage and an uncommitted minor population retaining plasticity

Natural regulatory T cells (Treg) represent a distinct lineage of T lymphocytes committed to suppressive functions, and expression of the transcription factor Foxp3 is thought to identify this lineage specifically. Here we report that, whereas the majority of natural CD4+Foxp3+ T cells maintain stable Foxp3 expression after adoptive transfer to lymphopenic or lymphoreplete recipients, a minor fraction enriched within the CD25− subset actually lose it. Some of those Foxp3− T cells adopt effector helper T cell (Th) functions, whereas some retain “memory” of previous Foxp3 expression, reacquiring Foxp3 upon activation. This minority “unstable” population exhibits flexible responses to cytokine signals, relying on transforming growth factor-β to maintain Foxp3 expression and responding to other cytokines by differentiating into effector Th in vitro. In contrast, CD4+Foxp3+CD25high T cells are resistant to such conversion to effector Th even after many rounds of cell division. These results demonstrate that natural Foxp3+ T cells are a heterogeneous population consisting of a committed Treg lineage and an uncommitted subpopulation with developmental plasticity.

Pathogenic conversion of Foxp3 + T cells into TH17 cells in autoimmune arthritis

Autoimmune diseases often result from an imbalance between regulatory T (Treg) cells and interleukin-17 (IL-17)-producing T helper (TH17) cells; the origin of the latter cells remains largely unknown. Foxp3 is indispensable for the suppressive function of Treg cells, but the stability of Foxp3 has been under debate. Here we show that TH17 cells originating from Foxp3+ T cells have a key role in the pathogenesis of autoimmune arthritis. Under arthritic conditions, CD25loFoxp3+CD4+ T cells lose Foxp3 expression (herein called exFoxp3 cells) and undergo transdifferentiation into TH17 cells. Fate mapping analysis showed that IL-17–expressing exFoxp3 T (exFoxp3 TH17) cells accumulated in inflamed joints. The conversion of Foxp3+CD4+ T cells to TH17 cells was mediated by synovial fibroblast-derived IL-6. These exFoxp3 TH17 cells were more potent osteoclastogenic T cells than were naive CD4+ T cell–derived TH17 cells. Notably, exFoxp3 TH17 cells were characterized by the expression of Sox4, chemokine (C-C motif) receptor 6 (CCR6), chemokine (C-C motif) ligand 20 (CCL20), IL-23 receptor (IL-23R) and receptor activator of NF-κB ligand (RANKL, also called TNFSF11). Adoptive transfer of autoreactive, antigen-experienced CD25loFoxp3+CD4+ T cells into mice followed by secondary immunization with collagen accelerated the onset and increased the severity of arthritis and was associated with the loss of Foxp3 expression in the majority of transferred T cells. We observed IL-17+Foxp3+ T cells in the synovium of subjects with active rheumatoid arthritis (RA), which suggests that plastic Foxp3+ T cells contribute to the pathogenesis of RA. These findings establish the pathological importance of Foxp3 instability in the generation of pathogenic TH17 cells in autoimmunity.

Suppression of bone formation by osteoclastic expression of semaphorin 4D

Most of the currently available drugs for osteoporosis inhibit osteoclastic bone resorption; only a few drugs promote osteoblastic bone formation. It is thus becoming increasingly necessary to identify the factors that regulate bone formation. We found that osteoclasts express semaphorin 4D (Sema4D), previously shown to be an axon guidance molecule, which potently inhibits bone formation. The binding of Sema4D to its receptor Plexin-B1 on osteoblasts resulted in the activation of the small GTPase RhoA, which inhibits bone formation by suppressing insulin-like growth factor-1 (IGF-1) signaling and by modulating osteoblast motility. Sema4d−/− mice, Plxnb1−/− mice and mice expressing a dominant-negative RhoA specifically in osteoblasts showed an osteosclerotic phenotype due to augmented bone formation. Notably, Sema4D-specific antibody treatment markedly prevented bone loss in a model of postmenopausal osteoporosis. Thus, Sema4D has emerged as a new therapeutic target for the discovery and development of bone-increasing drugs.

Fezf2 Orchestrates a Thymic Program of Self-Antigen Expression for Immune Tolerance

Self-tolerance to immune reactions is established via promiscuous expression of tissue-restricted antigens (TRAs) in medullary thymic epithelial cells (mTECs), leading to the elimination of T cells that respond to self-antigens. The transcriptional regulator Aire has been thought to be sufficient for the induction of TRAs, despite some indications that other factors may promote TRA expression in the thymus. Here, we show that the transcription factor Fezf2 directly regulates various TRA genes in mTECs independently of Aire. Mice lacking Fezf2 in mTECs displayed severe autoimmune symptoms, including the production of autoantibodies and inflammatory cell infiltration targeted to peripheral organs. These responses differed from those detected in Aire-deficient mice. Furthermore, Fezf2 expression and Aire expression are regulated by distinct signaling pathways and promote the expression of different classes of proteins. Thus, two independent factors, Fezf2 and Aire, permit the expression of TRAs in the thymus to ensure immune tolerance.

Full restoration of peripheral Foxp3+ regulatory T cell pool by radioresistant host cells in scurfy bone marrow chimeras

Mutations in the gene encoding the transcription factor Foxp3 lead to fatal autoimmune pathology in mice and humans, which is associated with a deficiency in Foxp3+ regulatory T cells (Treg). It has also been proposed that Foxp3 inactivation in nonhematopoietic tissues, particularly in thymic epithelium, is required for the pathogenesis, because Foxp3 mutant scurfy bone marrow cells fail to transmit the disease to lethally irradiated WT hosts. We demonstrate here that the lack of pathology in these radiation chimeras is due to the presence of radioresistant endogenous Foxp3+ Treg of the host. In addition, chimeras carrying the scurfy mutation only in nonhematopoietic cells exhibit no evidence of autoimmune pathology. Thus, Foxp3 deficiency in nonhematopoietic cells does not contribute to the scurfy disease. Furthermore, our analyses of radiation chimeras revealed that the peripheral Treg pool is fully and specifically restored and maintained by radioresistant endogenous Treg or adoptively transferred exogenous Treg through “homeostatic” proliferation in the absence of Treg production from scurfy donor bone marrow cells. These results thus provide evidence that the autoimmune pathology in scurfy mice results indeed from a Treg deficiency and illustrate a robust homeostatic mechanism that strictly controls the size of peripheral Treg pool by fine-tuning of homeostatic proliferation.

Agonist-Selected T Cell Development Requires Strong T Cell Receptor Signaling and Store-Operated Calcium Entry

T cell receptor (TCR) signaling driven by interaction of the TCR with specific complexes of self-peptide and the major histocompatibility complex determines T cell fate in thymic development. However, the signaling pathway through which TCR signal strength regulates distinct T cell lineages remains unknown. Here we have used mice lacking the endoplasmic reticulum Ca2+ sensors stromal interaction molecule 1 (STIM1) and STIM2 to show that STIM-induced store-operated Ca2+ entry is not essential for thymic development of conventional TCRαβ+ T cells but is specifically required for the development of agonist-selected T cells (regulatory T cells, invariant natural killer T cells, and TCRαβ+ CD8αα+ intestinal intraepithelial lymphocytes). The severe impairment of agonist-selected T cell development is mainly due to a defect in interleukin-2 (IL-2) or IL-15 signaling. Thus, STIM1 and STIM2-mediated store-operated Ca2+ influx, leading to efficient activation of NFAT (nuclear factor of activated T cells), is critical for the postselection maturation of agonist-selected T cells.

Autoimmune arthritis: the interface between the immune system and joints.

Rheumatoid arthritis (RA) is an autoimmune disease, characterized by chronic inflammation and synovial hyperplasia in the joints that ultimately lead to cartilage and bone destruction. A wealth of research has shown that CD4(+) T cells, especially IL-17 producing helper T (Th17) cells, play an important role in RA development. However, it still remains to be clarified how the systemic immune response results in the local joint disorders. Studies on animal models of RA have shed light on the importance of the interaction between immune cells and joint-specific mesenchymal cells. In particular, joint-specific mesenchymal cells contribute to the Th17-mediated augmentation of the inflammatory phase in RA by promoting the migration of Th17 cells to the inflammatory joint and then homeostatic proliferation with increase in IL-17 production. In addition, recent progress in osteoimmunology has provided new insights into the pathogenesis of the bone destruction phase in RA. Of note, Th17 cells have been shown to enhance the differentiation of osteoclasts via joint-specific mesenchymal cells. Thus, the interaction of CD4(+) T cells and nonhematopoietic mesenchymal cells in joints plays a key role in RA pathogenesis during both the inflammatory and bone destruction phases. Focusing on this interaction will lead to a better understanding of the mechanism by which the systemic immune response results in local joint disorders and also helps provide a molecular basis for novel therapeutic strategies.

RANKL expressed on synovial fibroblasts is primarily responsible for bone erosions during joint inflammation

Objective RANKL is mainly expressed by synovial fibroblasts and T cells within the joints of rheumatoid arthritis patients. The relative importance of RANKL expression by these cell types for the formation of bone erosions is unclear. We therefore aimed to quantify the contribution of RANKL by each cell type to osteoclast differentiation and bone destruction during inflammatory arthritis. Methods RANKL was specifically deleted in T cells (Tnfsf11flox/Δ Lck-Cre), in collagen VI expressing cells including synovial fibroblasts (Tnfsf11flox/Δ Col6a1-Cre) and in collagen II expressing cells including articular chondrocytes (Tnfsf11flox/Δ Col2a1-Cre). Erosive disease was induced using the collagen antibody-induced arthritis (CAIA) and collagen-induced arthritis (CIA) models. Osteoclasts and cartilage degradation were assessed by histology and bone erosions were assessed by micro-CT. Results The inflammatory joint score during CAIA was equivalent in all mice regardless of cell-targeted deletion of RANKL. Significant increases in osteoclast numbers and bone erosions were observed in both the Tnfsf11flox/Δ and the Tnfsf11flox/Δ Lck-Cre groups during CAIA; however, the Tnfsf11flox/Δ Col6a1-Cre mice showed significant protection against osteoclast formation and bone erosions. Similar results on osteoclast formation and bone erosions were obtained in CIA mice. The deletion of RANKL on any cell type did not prevent articular cartilage loss in either model of arthritis used. Conclusions The expression of RANKL on synovial fibroblasts rather than T cells is predominantly responsible for the formation of osteoclasts and erosions during inflammatory arthritis. Synovial fibroblasts would be the best direct target in RANKL inhibition therapies.

Immune complexes regulate bone metabolism through FcRγ signalling

Autoantibody production and immune complex (IC) formation are frequently observed in autoimmune diseases associated with bone loss. However, it has been poorly understood whether ICs regulate bone metabolism directly. Here we show that the level of osteoclastogenesis is determined by the strength of FcRγ signalling, which is dependent on the relative expression of positive and negative FcγRs (FcγRI/III/IV and IIB, respectively) as well as the availability of their ligands, ICs. Under physiological conditions, unexpectedly, FcγRIII inhibits osteoclastogenesis by depriving other osteoclastogenic Ig-like receptors of FcRγ. Fcgr2b(-/-) mice lose bone upon the onset of a hypergammaglobulinemia or the administration of IgG1 ICs, which act mainly through FcγRIII. The IgG2 IC activates osteoclastogenesis by binding to FcγRI and FcγRIV, which is induced under inflammatory conditions. These results demonstrate a link between the adaptive immunity and bone, suggesting a regulatory role for ICs in bone resorption in general, and not only in inflammatory diseases.