Adaptive immune regulation in the gut: T cell-dependent and T cell-independent IgA synthesis.

In mammals, the gastrointestinal tract harbors an extraordinarily dense and complex community of microorganisms. The gut microbiota provide strong selective pressure to the host to evolve adaptive immune responses required for the maintenance of local and systemic homeostasis. The continuous antigenic presence in the gut imposes a dynamic remodeling of gut-associated lymphoid tissues (GALT) and the selection of multiple layered strategies for immunoglobulin (Ig) A production. The composite and dynamic gut environment also necessitates heterogeneous, versatile, and convertible T cells, capable of inhibiting (Foxp3(+) T cells) or helping (T(FH) cells) local immune responses. In this review, we describe recent advances in our understanding of dynamic pathways that lead to IgA synthesis, in gut follicular structures and in extrafollicular sites, by T cell-dependent and T cell-independent mechanisms. We discuss the finely tuned regulatory mechanisms for IgA production and emphasize the role of mucosal IgA in the selection and maintenance of the appropriate microbial composition that is necessary for immune homeostasis.

The transcriptional regulators IRF4, BATF and IL-33 orchestrate development and maintenance of adipose tissue-resident regulatory T cells.

Foxp3(+) regulatory T (Treg) cells in visceral adipose tissue (VAT-Treg cells) are functionally specialized tissue-resident cells that prevent obesity-associated inflammation and preserve insulin sensitivity and glucose tolerance. Their development depends on the transcription factor PPAR-γ; however, the environmental cues required for their differentiation are unknown. Here we show that interleukin 33 (IL-33) signaling through the IL-33 receptor ST2 and myeloid differentiation factor MyD88 is essential for development and maintenance of VAT-Treg cells and sustains their transcriptional signature. Furthermore, the transcriptional regulators BATF and IRF4 were necessary for VAT-Treg differentiation through direct regulation of ST2 and PPAR-γ expression. IL-33 administration induced vigorous population expansion of VAT-Treg cells, which tightly correlated with improvements in metabolic parameters in obese mice. Human omental adipose tissue Treg cells also showed high ST2 expression, suggesting an evolutionarily conserved requirement for IL-33 in VAT-Treg cell homeostasis.

The 6th international cell senescence association conference.

The 6th conference of the international cell senescence association (ICSA) on the theme of "A New Era of Senescence Research: The Challenge of Controlling Aging and Cancer" was held on December 12-15, 2021 in Osaka, Japan as a Hybrid Meeting. The conference brought together basic and translational scientists to discuss recent developments in the field of cellular senescence research. In recent years, the study of cellular senescence has become a very hot field of research. It is clear that the ICSA, founded in 2015, has played an important role in this process. The 6th ICSA conference has provided another opportunity for exchanges and new connections between basic and translational scientists. The scientific program consisted of keynote lectures, invited talks, short talks selected from abstracts, a poster session, and luncheon seminars sponsored by the Japanese Society of Anti-Aging Medicine. In the Meet the Editor session, Dr Christoph Schmitt, Editor-in-Chief of Nature Metabolism, gave a short presentation about the journal and answered questions from the audience. Being a hybrid meeting, there was only so much that could be done, but we hope that the meeting was fruitful.

Foxp3(+) T cells regulate immunoglobulin a selection and facilitate diversification of bacterial species responsible for immune homeostasis.

Foxp3(+) T cells play a critical role for the maintenance of immune tolerance. Here we show that in mice, Foxp3(+) T cells contributed to diversification of gut microbiota, particularly of species belonging to Firmicutes. The control of indigenous bacteria by Foxp3(+) T cells involved regulatory functions both outside and inside germinal centers (GCs), consisting of suppression of inflammation and regulation of immunoglobulin A (IgA) selection in Peyer's patches, respectively. Diversified and selected IgAs contributed to maintenance of diversified and balanced microbiota, which in turn facilitated the expansion of Foxp3(+) T cells, induction of GCs, and IgA responses in the gut through a symbiotic regulatory loop. Thus, the adaptive immune system, through cellular and molecular components that are required for immune tolerance and through the diversification as well as selection of antibody repertoire, mediates host-microbial symbiosis by controlling the richness and balance of bacterial communities required for homeostasis.

The sensing of environmental stimuli by follicular dendritic cells promotes immunoglobulin A generation in the gut.

In the Peyer's patches (PPs), germinal centers (GCs) are chronically induced by bacteria and are the major sites for generation of gut immunoglobulin A (IgA) immune responses. Whether follicular dendritic cells (FDCs) within the GCs directly contribute to the IgA production in PPs is unknown. We showed here that direct stimulation of FDCs by bacterial products and retinoic acid synergistically enhanced the expression of the chemokine CXCL13, the survival factor BAFF, and molecules that facilitate the secretion and activation of the cytokine TGF-beta1. A reduced production of these molecules by PP FDCs associated with deficiencies in the Toll-like receptor pathway or vitamin A resulted in decreased numbers of GC B cells and defective generation of IgA(+) B cells within PP GCs. Our data indicate that PP FDCs are conditioned by environmental stimuli to express key factors for B cell migration, survival, and preferential generation of IgA in gut.

The role of the adaptive immune system in regulation of gut microbiota.

The gut nourishes rich bacterial communities that affect profoundly the functions of the immune system. The relationship between gut microbiota and the immune system is one of reciprocity. The microbiota contributes to nutrient processing and the development, maturation, and function of the immune system. Conversely, the immune system, particularly the adaptive immune system, plays a key role in shaping the repertoire of gut microbiota. The fitness of host immune system is reflected in the gut microbiota, and deficiencies in either innate or adaptive immunity impact on diversity and structures of bacterial communities in the gut. Here, we discuss the mechanisms that underlie this reciprocity and emphasize how the adaptive immune system via immunoglobulins (i.e. IgA) contributes to diversification and balance of gut microbiota required for immune homeostasis.

Impact of senescence-associated secretory phenotype and its potential as a therapeutic target for senescence-associated diseases.

"Cellular senescence" is a state in which cells undergo irreversible cell cycle arrest in response to a variety of cellular stresses. Once cells senesce, they are strongly resistant to any mitogens, including oncogenic stimuli. Therefore, cellular senescence has been assumed to be a potent anticancer mechanism. Although irreversible cell-cycle arrest is traditionally considered the major characteristic of senescent cells, recent studies have revealed some additional functions. Most noteworthy is the increased secretion of various secretory proteins, such as inflammatory cytokines, chemokines, growth factors, and MMPs, into the surrounding extracellular fluid. These newly recognized senescent phenotypes, termed senescence-associated secretory phenotypes (SASPs), reportedly contribute to tumor suppression, wound healing, embryonic development, and even tumorigenesis promotion. Thus, SASPs appear to be beneficial or deleterious, depending on the biological context. As senescent cells are known to accumulate during the aging process in vivo, it is quite possible that their accumulation in aged tissues promotes age-associated functional decline and various diseases, including cancers, at least to some extent. Here, we focus on and discuss the functional and regulatory network of SASPs toward opening up new possibilities for controlling aging and aging-associated diseases.

Gut TFH and IgA: key players for regulation of bacterial communities and immune homeostasis.

The main function of the immune system is to protect the host against pathogens. However, unlike the systemic immune system, the gut immune system does not eliminate, but instead nourishes complex bacterial communities and establishes advanced symbiotic relationships. Immunoglobulin A (IgA) is the most abundant antibody isotype in mammals, produced mainly in the gut. The primary function of IgA is to maintain homeostasis at mucosal surfaces, and studies in mice have demonstrated that IgA diversification has an essential role in the regulation of gut microbiota. Dynamic diversification and constant adaptation of IgA responses to local microbiota require expression of activation-induced cytidine deaminase by B cells and control from T follicular helper and Foxp3(+) T cells in germinal centers (GCs). We discuss the finely tuned regulatory mechanisms for IgA synthesis in GCs of Peyer's patches and emphasize the roles of CD4(+) T cells for IgA selection and the maintenance of appropriate gut microbial communities required for immune homeostasis.

Histone chaperone Spt6 is required for class switch recombination but not somatic hypermutation.

Activation-induced cytidine deaminase (AID) is shown to be essential and sufficient to induce two genetic alterations in the Ig loci: class switch recombination (CSR) and somatic hypermutation (SHM). However, it is still unknown how a single-molecule AID differentially regulates CSR and SHM. Here we identified Spt6 as an AID-interacting protein by yeast two-hybrid screening and immunoprecipitation followed by mass spectrometry. Knockdown of Spt6 resulted in severe reduction of CSR in both the endogenous Ig locus in B cells and an artificial substrate in fibroblast cells. Conversely, knockdown of Spt6 did not reduce but slightly enhanced SHM in an artificial substrate in B cells, indicating that Spt6 is required for AID to induce CSR but not SHM. These results suggest that Spt6 is involved in differential regulation of CSR and SHM by AID.

Roles of B-1 and B-2 cells in innate and acquired IgA-mediated immunity.

The gut harbors an extremely dense and complex community of microorganisms that are in constant dialog with our immune cells. The gut bacteria provide strong selective pressure to the host to evolve innate and adaptive immune responses required for the maintenance of local and systemic homeostasis. One of the most conspicuous responses of the gut immune system following microbial colonization is the production of immunoglobulin A (IgA). In this review, we discuss the roles of B-1 and B-2 cells in IgA-mediated immunity and present an updated view for the sites and mechanisms of IgA synthesis in the gut. We summarize the role of secretory IgAs for regulation of microbial communities and provide clues as to how the gut microbiota contributes to the development of the gut-associated lymphoid tissues.

Crosstalk between gut microbiota and cellular senescence: a vicious cycle leading to aging gut.

Two phenomena, the accumulation of senescent cells and changes in the gut microbiota, are thought to contribute to the decline of biological functions and the development of diseases associated with aging. However, the relationship between these two phenomena and their effects on aging remains to be clarified. Recently, we have reported that gut bacteria induce cellular senescence in ileal germinal center (GC) B cells, resulting in decreased IgA production and diversity. This, in turn, leads to an imbalance in the gut microbiota. Thus, the crosstalk between the gut microbiota and cellular senescence via the host immune system may establish a vicious cycle and contribute to the disruption of gut homeostasis associated with aging.

Multiple ageing effects on testicular/epididymal germ cells lead to decreased male fertility in mice.

In mammals, females undergo reproductive cessation with age, whereas male fertility gradually declines but persists almost throughout life. However, the detailed effects of ageing on germ cells during and after spermatogenesis, in the testis and epididymis, respectively, remain unclear. Here we comprehensively examined the in vivo male fertility and the overall organization of the testis and epididymis with age, focusing on spermatogenesis, and sperm function and fertility, in mice. We first found that in vivo male fertility decreased with age, which is independent of mating behaviors and testosterone levels. Second, overall sperm production in aged testes was decreased; about 20% of seminiferous tubules showed abnormalities such as germ cell depletion, sperm release failure, and perturbed germ cell associations, and the remaining 80% of tubules contained lower number of germ cells because of decreased proliferation of spermatogonia. Further, the spermatozoa in aged epididymides exhibited decreased total cell numbers, abnormal morphology/structure, decreased motility, and DNA damage, resulting in low fertilizing and developmental rates. We conclude that these multiple ageing effects on germ cells lead to decreased in vivo male fertility. Our present findings are useful to better understand the basic mechanism behind the ageing effect on male fertility in mammals including humans.

Sesamin Metabolites Suppress the Induction of Cellular Senescence.

Cellular senescence induces inflammation and is now considered one of the causes of organismal aging. Accumulating evidence indicates that age-related deterioration of mitochondrial function leads to an increase in reactive oxygen species (ROS) and DNA damage, which in turn causes cellular senescence. Thus, it is important to maintain mitochondrial function and suppress oxidative stress in order to inhibit the accumulation of senescent cells. Sesamin and its isomer episesamin are types of lignans found in sesame oil, and after being metabolized in the liver, their metabolites have been reported to exhibit antioxidant properties. However, their effects on cellular senescence remain unknown. In this study, the effects of sesamin, episesamin, and their metabolites SC1 and EC1-2 on replicative senescence were evaluated using human diploid lung fibroblasts, and TIG-3 cells. The results showed that sesamin and episesamin treatment had no effect on proliferative capacity compared to the untreated late passage group, whereas SC1 and EC1-2 treatment improved proliferative capacity and mitigated DNA damage of TIG-3 cells. Furthermore, other cellular senescence markers, such as senescence-associated secretory phenotype (SASP), mitochondria-derived ROS, and mitochondrial function (ROS/ATP ratio) were also reduced by SC1 and EC1-2 treatment. These results suggest that SC1 and EC1-2 can maintain proper mitochondrial function and suppress the induction of cellular senescence.

Bacterial induction of B cell senescence promotes age-related changes in the gut microbiota.

The elucidation of the mechanisms of ageing and the identification of methods to control it have long been anticipated. Recently, two factors associated with ageing-the accumulation of senescent cells and the change in the composition of gut microbiota-have been shown to play key roles in ageing. However, little is known about how these phenomena occur and are related during ageing. Here we show that the persistent presence of commensal bacteria gradually induces cellular senescence in gut germinal centre B cells. Importantly, this reduces both the production and diversity of immunoglobulin A (IgA) antibodies that target gut bacteria, thereby changing the composition of gut microbiota in aged mice. These results have revealed the existence of IgA-mediated crosstalk between the gut microbiota and cellular senescence and thus extend our understanding of the mechanism of gut microbiota changes with age, opening up possibilities for their control.

Cellular senescence in white matter microglia is induced during ageing in mice and exacerbates the neuroinflammatory phenotype.

Cellular senescence, a state of irreversible cell-cycle arrest caused by a variety of cellular stresses, is critically involved in age-related tissue dysfunction in various organs. However, the features of cells in the central nervous system that undergo senescence and their role in neural impairment are not well understood as yet. Here, through comprehensive investigations utilising single-cell transcriptome analysis and various mouse models, we show that microglia, particularly in the white matter, undergo cellular senescence in the brain and spinal cord during ageing and in disease models involving demyelination. Microglial senescence is predominantly detected in disease-associated microglia, which appear in ageing and neurodegenerative diseases. We also find that commensal bacteria promote the accumulation of senescent microglia and disease-associated microglia during ageing. Furthermore, knockout of p16INK4a, a key senescence inducer, ameliorates the neuroinflammatory phenotype in damaged spinal cords in mice. These results advance our understanding of the role of cellular senescence in the central nervous system and open up possibilities for the treatment of age-related neural disorders.

SARS-CoV-2 infection triggers paracrine senescence and leads to a sustained senescence-associated inflammatory response.

Reports of post-acute COVID-19 syndrome, in which the inflammatory response persists even after SARS-CoV-2 has disappeared, are increasing1, but the underlying mechanisms of post-acute COVID-19 syndrome remain unknown. Here, we show that SARS-CoV-2-infected cells trigger senescence-like cell-cycle arrest2,3 in neighboring uninfected cells in a paracrine manner via virus-induced cytokine production. In cultured human cells or bronchial organoids, these SASR-CoV-2 infection-induced senescent cells express high levels of a series of inflammatory factors known as senescence-associated secretory phenotypes (SASPs)4 in a sustained manner, even after SARS-CoV-2 is no longer detectable. We also show that the expression of the senescence marker CDKN2A (refs. 5,6) and various SASP factor4 genes is increased in the pulmonary cells of patients with severe post-acute COVID-19 syndrome. Furthermore, we find that mice exposed to a mouse-adapted strain of SARS-CoV-2 exhibit prolonged signs of cellular senescence and SASP in the lung at 14 days after infection when the virus was undetectable, which could be substantially reduced by the administration of senolytic drugs7. The sustained infection-induced paracrine senescence described here may be involved in the long-term inflammation caused by SARS-CoV-2 infection.

Gut bacteria identified in colorectal cancer patients promote tumourigenesis via butyrate secretion.

Emerging evidence is revealing that alterations in gut microbiota are associated with colorectal cancer (CRC). However, very little is currently known about whether and how gut microbiota alterations are causally associated with CRC development. Here we show that 12 faecal bacterial taxa are enriched in CRC patients in two independent cohort studies. Among them, 2 Porphyromonas species are capable of inducing cellular senescence, an oncogenic stress response, through the secretion of the bacterial metabolite, butyrate. Notably, the invasion of these bacteria is observed in the CRC tissues, coinciding with the elevation of butyrate levels and signs of senescence-associated inflammatory phenotypes. Moreover, although the administration of these bacteria into ApcΔ14/+ mice accelerate the onset of colorectal tumours, this is not the case when bacterial butyrate-synthesis genes are disrupted. These results suggest a causal relationship between Porphyromonas species overgrowth and colorectal tumourigenesis which may be due to butyrate-induced senescence.

Expansion of senescent megakaryocyte-lineage cells maintains CML cell leukemogenesis.

BCR-ABL, an oncogenic fusion gene, plays a central role in the pathogenesis of chronic myeloid leukemia (CML). Oncogenic signaling induces oncogene-induced senescence and senescence-associated secretory phenotype (SASP), which is characterized by enhanced production of various cytokines. BCR-ABL gene transduction confers senescent phenotype in vitro; however, the in vivo relevance of senescence has not been explored in this context. Transplantation of BCR-ABL-expressing hematopoietic stem/progenitor cells caused CML in mice with an increase in bone marrow BCR-ABL+CD41+CD150+ leukemic megakaryocyte-lineage (MgkL) cells, which exhibited enhanced senescence-associated ß-galactosidase staining and increased expression of p16 and p21, key molecules that are crucially involved in senescence. Moreover, knockout of p16 and p21 genes reduced both BCR-ABL-induced abnormal megakaryopoiesis and the maintenance of CML cell leukemogenic capacity, as evidenced by attenuated leukemogenic capacity at secondary transplantation. The expression of transforming growth factor-ß1 (TGF-ß1), a representative SASP molecule, was enhanced in the leukemic MgkL cells, and TGF-ß1 inhibition attenuated CML cell leukemogenic capacity both in vitro and in vivo. Furthermore, BCR-ABL-expressing MgkL cells displayed enhanced autophagic activity, and autophagy inhibition reduced bone marrow MgkL cell number and prolonged the survival of CML mice, which had transiently received the tyrosine kinase inhibitor, imatinib, earlier. Thus, BCR-ABL induced the expansion of senescent leukemic MgkL cells, which supported CML leukemogenesis by providing TGF-ß1.