Aging in the lympho-hematopoietic stem cell compartment

Rejuvenation of the reconstitution potential and reversal of myeloid bias of aged HSCs upon pH treatment

Aged hematopoietic stem cells (HSCs) show reduced reconstitution potential, limiting their use in transplantation settings in the clinic. We demonstrate here that exposure of aged HSCs ex vivo to a pH of 6.9 instead of the commonly used pH of 7.4 results in enhanced HSCs potential that is consistent with rejuvenation, including attenuation of the myeloid bias of aged HSC and restoration of a youthful frequency of epigenetic polarity. Rejuvenation of aged HSCs by pH 6.9 is, at least in part, due to alterations in the polyamine/methionine pathway within pH 6.9 HSCs, and consequently, attenuation of the production of spermidine also attenuated aging of HSCs. Exposure of aged HSCs to pH 6.9, or pharmacological targeting of the polyamine pathway, might thus extend the use of HSCs from aged donors for therapeutic applications.

Tetrahydroxy stilbene glucoside rejuvenates aging hematopoietic stem cells with predilection for lymphoid differentiation via AMPK and Tet2

INTRODUCTION

Aging of hematopoietic stem cells (HSCs) has emerged as an important challenge to human health. Recent advances have raised the prospect of rejuvenating aging HSCs via specific medical interventions, including pharmacological treatments. Nonetheless, efforts to develop such drugs are still in infancy until now.

OBJECTIVES

We aimed to screen the prospective agents that can rejuvenate aging HSCs and explore the potential mechanisms.

METHODS

We screened a set of natural anti-aging compounds through oral administration to sub-lethally irradiated mice, and identified 2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside (TSG) as a potent rejuvenating agent for aging HSCs. Then naturally aged mice were used for the follow-up assessment to determine the HSC rejuvenating potential of TSG. Finally, based on the transcriptome and DNA methylation analysis, we validated the role of the AMP-activated protein kinase (AMPK)-ten-eleven-translocation 2 (Tet2) axis (the AMPK-Tet2 axis) as the underlying mechanisms of TSG for ameliorating HSCs aging.

RESULTS

TSG treatment not only significantly increased the absolute number of common lymphoid progenitors (CLPs) along with B lymphocytes, but also boosted the HSCs/CLPs repopulation potential of aging mice. Further elaborated mechanism research demonstrated that TSG supplementation restored the stemness of aging HSCs, as well as promoted an epigenetic reprograming that was associated with an improved regenerative capacity and an increased rate of lymphopoiesis. Such effects were diminished when the mice were co-treated with an AMPK inhibitor, or when it was performed in Tet2 knockout mice as well as senescent cells assay.

CONCLUSION

TSG is effective in rejuvenating aging HSCs by modulating the AMPK- Tet2 axis and thus represents a potential candidate for developing effective HSC rejuvenating therapies.

Considerations and Approaches for Cancer Immunotherapy in the Aging Host

Advances in cancer immunotherapy are improving treatment successes in many distinct cancer types. Nonetheless, most tumors fail to respond. Age is the biggest risk for most cancers, and the median population age is rising worldwide. Advancing age is associated with manifold alterations in immune cell types, abundance, and functions, rather than simple declines in these metrics, the consequences of which remain incompletely defined. Our understanding of the effects of host age on immunotherapy mechanisms, efficacy, and adverse events remains incomplete. A deeper understanding of age effects in all these areas is required. Most cancer immunotherapy preclinical studies examine young subjects and fail to assess age contributions, a remarkable deficit given the known importance of age effects on immune cells and factors mediating cancer immune surveillance and immunotherapy efficacy. Notably, some cancer immunotherapies are more effective in aged versus young hosts, while others fail despite efficacy in the young. Here, we review our current understanding of age effects on immunity and associated nonimmune cells, the tumor microenvironment, cancer immunotherapy, and related adverse effects. We highlight important knowledge gaps and suggest areas for deeper enquiries, including in cancer immune surveillance, treatment response, adverse event outcomes, and their mitigation.

A state-of-the-art review on the MicroRNAs roles in hematopoietic stem cell aging and longevity

Aging is a biological process determined through time-related cellular and functional impairments, leading to a decreased standard of living for the organism. Recently, there has been an unprecedented advance in the aging investigation, especially the detection that the rate of senescence is at least somewhat regulated via evolutionarily preserved genetic pathways and biological processes. Hematopoietic stem cells (HSCs) maintain blood generation over the whole lifetime of an organism. The senescence process influences many of the natural features of HSC, leading to a decline in their capabilities, independently of their microenvironment. New studies show that HSCs are sensitive to age-dependent stress and gradually lose their self-renewal and regeneration potential with senescence. MicroRNAs (miRNAs) are short, non-coding RNAs that post-transcriptionally inhibit translation or stimulate target mRNA cleavage of target transcripts via the sequence-particular connection. MiRNAs control various biological pathways and processes, such as senescence. Several miRNAs are differentially expressed in senescence, producing concern about their use as moderators of the senescence process. MiRNAs play an important role in the control of HSCs and can also modulate processes associated with tissue senescence in specific cell types. In this review, we display the contribution of age-dependent alterations, including DNA damage, epigenetic landscape, metabolism, and extrinsic factors, which affect HSCs function during aging. In addition, we investigate the particular miRNAs regulating HSCs senescence and age-associated diseases. Video Abstract.

Harnessing matrix stiffness to engineer a bone marrow niche for hematopoietic stem cell rejuvenation

Hematopoietic stem cell (HSC) self-renewal and aging are tightly regulated by paracrine factors from the bone marrow niche. However, whether HSC rejuvenation could be achieved by engineering a bone marrow niche ex vivo remains unknown. Here, we show that matrix stiffness fine-tunes HSC niche factor expression by bone marrow stromal cells (BMSCs). Increased stiffness activates Yap/Taz signaling to promote BMSC expansion upon 2D culture, which is largely reversed by 3D culture in soft gelatin methacrylate hydrogels. Notably, 3D co-culture with BMSCs promotes HSC maintenance and lymphopoiesis, reverses aging hallmarks of HSCs, and restores their long-term multilineage reconstitution capacity. In situ atomic force microscopy analysis reveals that mouse bone marrow stiffens with age, which correlates with a compromised HSC niche. Taken together, this study highlights the biomechanical regulation of the HSC niche by BMSCs, which could be harnessed to engineer a soft bone marrow niche for HSC rejuvenation.

The Function of Memory CD8+ T Cells in Immunotherapy for Human Diseases

Memory T (Tm) cells protect against Ags that they have previously contacted with a fast and robust response. Therefore, developing long-lived Tm cells is a prime goal for many vaccines and therapies to treat human diseases. The remarkable characteristics of Tm cells have led scientists and clinicians to devise methods to make Tm cells more useful. Recently, Tm cells have been highlighted for their role in coronavirus disease 2019 vaccines during the ongoing global pandemic. The importance of Tm cells in cancer has been emerging. However, the precise characteristics and functions of Tm cells in these diseases are not completely understood. In this review, we summarize the known characteristics of Tm cells and their implications in the development of vaccines and immunotherapies for human diseases. In addition, we propose to exploit the beneficial characteristics of Tm cells to develop strategies for effective vaccines and overcome the obstacles of immunotherapy.

Old plasma dilution reduces human biological age: a clinical study

This work extrapolates to humans the previous animal studies on blood heterochronicity and establishes a novel direct measurement of biological age. Our results support the hypothesis that, similar to mice, human aging is driven by age-imposed systemic molecular excess, the attenuation of which reverses biological age, defined in our work as a deregulation (noise) of 10 novel protein biomarkers. The results on biological age are strongly supported by the data, which demonstrates that rounds of therapeutic plasma exchange (TPE) promote a global shift to a younger systemic proteome, including youthfully restored pro-regenerative, anticancer, and apoptotic regulators and a youthful profile of myeloid/lymphoid markers in circulating cells, which have reduced cellular senescence and lower DNA damage. Mechanistically, the circulatory regulators of the JAK-STAT, MAPK, TGF-beta, NF-κB, and Toll-like receptor signaling pathways become more youthfully balanced through normalization of TLR4, which we define as a nodal point of this molecular rejuvenation. The significance of our findings is confirmed through big-data gene expression studies.

Heterochronic parabiosis induces stem cell revitalization and systemic rejuvenation across aged tissues

The young circulatory milieu capable of delaying aging in individual tissues is of interest as rejuvenation strategies, but how it achieves cellular- and systemic-level effects has remained unclear. Here, we constructed a single-cell transcriptomic atlas across aged tissues/organs and their rejuvenation in heterochronic parabiosis (HP), a classical model to study systemic aging. In general, HP rejuvenated adult stem cells and their niches across tissues. In particular, we identified hematopoietic stem and progenitor cells (HSPCs) as one of the most responsive cell types to young blood exposure, from which a continuum of cell state changes across the hematopoietic and immune system emanated, through the restoration of a youthful transcriptional regulatory program and cytokine-mediated cell-cell communications in HSPCs. Moreover, the reintroduction of the identified rejuvenating factors alleviated age-associated lymphopoiesis decline. Overall, we provide comprehensive frameworks to explore aging and rejuvenating trajectories at single-cell resolution and revealed cellular and molecular programs that instruct systemic revitalization by blood-borne factors.

Cellular and Molecular Mechanisms Involved in Hematopoietic Stem Cell Aging as a Clinical Prospect

There is a hot topic in stem cell research to investigate the process of hematopoietic stem cell (HSC) aging characterized by decreased self-renewal ability, myeloid-biased differentiation, impaired homing, and other abnormalities related to hematopoietic repair function. It is of crucial importance that HSCs preserve self-renewal and differentiation ability to maintain hematopoiesis under homeostatic states over time. Although HSC numbers increase with age in both mice and humans, this cannot compensate for functional defects of aged HSCs. The underlying mechanisms regarding HSC aging have been studied from various perspectives, but the exact molecular events remain unclear. Several cell-intrinsic and cell-extrinsic factors contribute to HSC aging including DNA damage responses, reactive oxygen species (ROS), altered epigenetic profiling, polarity, metabolic alterations, impaired autophagy, Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, nuclear factor- (NF-) κB pathway, mTOR pathway, transforming growth factor-beta (TGF-β) pathway, and wingless-related integration site (Wnt) pathway. To determine how deficient HSCs develop during aging, we provide an overview of different hallmarks, age-related signaling pathways, and epigenetic modifications in young and aged HSCs. Knowing how such changes occur and progress will help researchers to develop medications and promote the quality of life for the elderly and possibly alleviate age-associated hematopoietic disorders. The present review is aimed at discussing the latest advancements of HSC aging and the role of HSC-intrinsic factors and related events of a bone marrow niche during HSC aging.

Extramedullary nasal plasmacytoma arising after polyp excision and the role of the inflammation in tumor development: A case report

A correlation between inflammation and cancer has been identified in the case of nasal cancer, however a specific connection between nasal inflammation and extramedullary nasal plasmacytoma (ENP), to the best of our knowledge, has not yet been determined. The present case report describes a patient affected by ENP, in who the tumor arose in the same area, from which a nasal polyp was previously surgically removed, five months after the polyp excision. The patient underwent surgical endoscopic tumor asportation without being treated with radio-chemotherapy. ENP was totally removed via surgery and no signs of recurrence were identified by endoscopy or magnetic resonance imaging during the last check-up 1 year after tumor asportation. It was hypothesized that in this elderly patient, who was exposed to viral infections and pollution for several years, ENP may be correlated to the inflammatory process that occurred after surgery, and this likely contributed to a neoplastic mutation in B-cells.

Community-acquired pneumonia in patients with bacterial meningitis: a prospective nationwide cohort study

OBJECTIVE

Pneumonia is considered a focus of infection in patients presenting with community-acquired bacterial meningitis but the impact on disease course is unclear. The aim was to study presenting characteristics, clinical course and outcome of meningitis patients with co-existing pneumonia on admission.

METHODS

We evaluated adult patients with community-acquired bacterial meningitis with pneumonia on admission in a nationwide, prospective cohort performed from March 2006 to June 2017. We performed logistic regression analysis to identify clinical characteristics predictive of pneumonia on admission, and to quantify the effect of pneumonia on outcome.

RESULTS

Pneumonia was diagnosed on admission in 315 of 1852 (17%) bacterial meningitis episodes and confirmed by chest X-ray in 256 of 308 (83%) episodes. Streptococcus pneumoniae was the causative organism in 256 of 315 episodes (81%). Pneumonia on admission was associated with advanced age (OR 1.03 per year increase, 95% CI 1.02-1.04, p < 0.001), alcoholism (OR 1.96, 95% CI 1.23-3.14, p 0.004), cancer (OR 1.54, 95% CI 1.12-2.13, p 0.008), absence of otitis or sinusitis (OR 0.44, 95% CI 0.32-0.59, p < 0.001) and S. pneumoniae (OR 2.14, 95% CI 1.55-2.95, p < 0.001) in the multivariate analysis. An unfavourable outcome defined as a score of 1-4 on the Glasgow Outcome Scale was observed in 172 (55%) episodes and 87 patients (28%) died. Pneumonia on admission was independently associated with unfavourable outcome and mortality in the multivariate analysis (OR 1.48, 95% CI 1.12-1.96; p 0.005).

CONCLUSION

Pneumonia on admission in bacterial meningitis is a frequent coexisting infection and is independently associated with unfavourable outcome and mortality.

Causes and Mechanisms of Hematopoietic Stem Cell Aging

Many elderly people suffer from hematological diseases known to be highly age-dependent. Hematopoietic stem cells (HSCs) maintain the immune system by producing all blood cells throughout the lifetime of an organism. Recent reports have suggested that HSCs are susceptible to age-related stress and gradually lose their self-renewal and regeneration capacity with aging. HSC aging is driven by cell-intrinsic and -extrinsic factors that result in the disruption of the immune system. Thus, the study of HSC aging is important to our understanding of age-related immune diseases and can also provide potential strategies to improve quality of life in the elderly. In this review, we delineate our understanding of the phenotypes, causes, and molecular mechanisms involved in HSC aging.

Induction and Detection of Autophagy in Aged Hematopoietic Stem Cells by Exposing Them to Microvesicles Secreted by HSC-Supportive Mesenchymal Stromal Cells

Autophagy is an important cellular process for maintenance of quality and functionality of cells. This happens through repair and renewal of cellular components like proteins and mitochondria. Reduction in autophagy process in aged hematopoietic stem cells (HSCs) leads to their compromised stemness and self-renewal capacity, and consequently, their applicability in various regenerative therapies also reduces. HSC functions are regulated by their microenvironment, known as "HSC niche," which comprises of mesenchymal stromal cells (MSCs), osteoblasts, endothelial cells, etc. In this niche, the MSCs are known to closely interact with the HSCs, and therefore, they can directly influence the stem cell fate. In our earlier studies, we have demonstrated that young MSCs or aged MSCs rejuvenated by treating them with LY294002, a PI3K inhibitor (rescued aged MSCs), rejuvenate aged HSCs via intercellular transfer of microvesicles (MVs) harboring autophagy-inducing mRNAs.Here, we describe the protocol for induction of autophagy in aged HSCs by incubating them with microvesicles (MVs) collected from young MSCs or rescued aged MSCs. We also describe the protocols for determination of autophagy levels in these HSCs.

Signaling Pathways Regulating Hematopoietic Stem Cell and Progenitor Aging

Purpose of Review

Functional decline of hematopoiesis that occurs in the elderly, or in patients who receive therapies that trigger cellular senescence effects, results in a progressive reduction in the immune response and an increased incidence of myeloid malignancy. Intracellular signals in hematopoietic stem cells and progenitors (HSC/P) mediate systemic, microenvironment, and cell-intrinsic effector aging signals that induce their decline. This review intends to summarize and critically review our advances in the understanding of the intracellular signaling pathways responsible for HSC decline during aging and opportunities for intervention.

Recent Findings

For a long time, aging of HSC has been thought to be an irreversible process imprinted in stem cells due to the cell intrinsic nature of aging. However, recent murine models and human correlative studies provide evidence that aging is associated with molecular signaling pathways, including oxidative stress, metabolic dysfunction, loss of polarity and an altered epigenome. These signaling pathways provide potential targets for prevention or reversal of age-related changes.

Summary

Here we review our current understanding of the signalling pathways that are differentially activated or repressed during HSC/P aging, focusing on the oxidative, metabolic, biochemical and structural consequences downstream, and cell-intrinsic, systemic, and environmental influences.

Aged neutrophils accumulate in lymphoid tissues from healthy elderly mice and infiltrate T- and B-cell zones

The average age of the human population is rising, leading to an increasing burden of age-related diseases, including increased susceptibility to infection. However, immune function can decrease with age which could impact on processes that require a functional immune system. Aging is also characterized by chronic low-grade inflammation which could further impact immune cell function. While changes to neutrophils in blood during aging have been described, little is known in aging lymphoid organs. This study used female C57BL/6J mice comparing bone marrow (BM), spleen and lymph nodes from young mice aged 2-3 months (equivalent to 18 human years) with healthy elderly mice aged 22-24 months (equivalent to 60-70 human years). Neutrophil proportions increased in BM and secondary lymphoid organs of elderly mice relative to their younger counterparts and presented an atypical phenotype. Interestingly, neutrophils from elderly spleen and lymph nodes were long lived (with decreased apoptosis via Annexin V staining and increased proportion of BrdU^neg mature cells) with splenic neutrophils also demonstrating a hypersegmented morphology. Furthermore, splenic neutrophils of elderly mice expressed a mixed phenotype with increased expression of activation markers, CD11b and ICAM-1, increased proinflammatory TNFα, yet increased anti-inflammatory transforming growth factor-beta. Elderly splenic architecture was compromised, as the marginal zone (required for clearing infections) was contracted. Moreover, neutrophils from elderly but not young mice accumulated in lymph node and splenic T- and B-cell zones. Overall, the expansion of functionally compromised neutrophils could contribute to increased susceptibility to infection observed in the elderly.

Considerations for successful cancer immunotherapy in aged hosts

Improvements in understanding cancer immunopathogenesis have now led to unprecedented successes in immunotherapy to treat numerous cancers. Although aging is the most important risk factor for cancer, most pre-clinical cancer immunotherapy studies are undertaken in young hosts. This review covers age-related immune changes as they affect cancer immune surveillance, immunopathogenesis and immune therapy responses. Declining T cell function with age can impede efficacy of age-related cancer immunotherapies, but examples of successful approaches to breach this barrier have been reported. It is further recognized now that immune functions with age do not simply decline, but that they change in potentially detrimental ways. For example, detrimental immune cell populations can become predominant during aging (notably pro-inflammatory cells), the prevalence or function of suppressive cells can increase (notably myeloid derived suppressor cells), drugs can have age-specific effects on immune cells, and attributes of the aged microenvironment can impede or subvert immunity. Key advances in these and related areas will be reviewed as they pertain to cancer immunotherapy in the aged, and areas requiring additional study and some speculations on future research directions will be addressed. We prefer the term Age Related Immune Dysfunction (ARID) as most encompassing the totality of age-associated immune changes.

Molecular and physiological manifestations and measurement of aging in humans

Biological aging is associated with a reduction in the reparative and regenerative potential in tissues and organs. This reduction manifests as a decreased physiological reserve in response to stress (termed homeostenosis) and a time-dependent failure of complex molecular mechanisms that cumulatively create disorder. Aging inevitably occurs with time in all organisms and emerges on a molecular, cellular, organ, and organismal level with genetic, epigenetic, and environmental modulators. Individuals with the same chronological age exhibit differential trajectories of age-related decline, and it follows that we should assess biological age distinctly from chronological age. In this review, we outline mechanisms of aging with attention to well-described molecular and cellular hallmarks and discuss physiological changes of aging at the organ-system level. We suggest methods to measure aging with attention to both molecular biology (e.g., telomere length and epigenetic marks) and physiological function (e.g., lung function and echocardiographic measurements). Finally, we propose a framework to integrate these molecular and physiological data into a composite score that measures biological aging in humans. Understanding the molecular and physiological phenomena that drive the complex and multifactorial processes underlying the variable pace of biological aging in humans will inform how researchers assess and investigate health and disease over the life course. This composite biological age score could be of use to researchers seeking to characterize normal, accelerated, and exceptionally successful aging as well as to assess the effect of interventions aimed at modulating human aging.

Considerations for successful cancer immunotherapy in aged hosts

Immunotherapy is now experiencing unprecedented successes in treating various cancers based on new understandings of cancer immunopathogenesis. Nonetheless, although ageing is the biggest risk factor for cancer, the majority of cancer immunotherapy preclinical studies are conducted in young hosts. This review will explore age-related changes in immunity as they relate to cancer immune surveillance, immunopathogenesis and responses to immunotherapy. Although it is recognized that declining T cell function with age poses a great challenge to developing effective age-related cancer immunotherapies, examples of successful approaches to overcome this hurdle have been developed. Further, it is now recognized that immune functions do not simply decline with age, but rather change in ways than can be detrimental. For example, with age, specific immune cell populations with detrimental functions can become predominant (such as cells producing proinflammatory cytokines), suppressive cells can become more numerous or more suppressive (such as myeloid-derived suppressor cells), drugs can affect aged immune cells distinctly and the aged microenvironment is becoming recognized as a significant barrier to address. Key developments in these and other areas will be surveyed as they relate to cancer immunotherapy in aged hosts, and areas in need of more study will be assessed with some speculations for the future. We propose the term 'age-related immune dysfunction' (ARID) as best representative of age-associated changes in immunity.

Lymphocyte generation and population homeostasis throughout life

Immune aging is a multi-faceted process that manifests as reduced competence to fight infections and malignant cells, as well as diminished tissue repair, unprovoked inflammation, and increased autoreactivity. The aging adaptive immune system, with its high complexity in functional cell subpopulations and diversity of B- and T-cell receptors, has to cope with the challenge of maintaining homeostasis while responding to exogenous stimuli and compensating for reduced generative capacity. With thymic involution, naïve T cells begin to function as quasi-stem cells and maintain the compartment through peripheral homeostatic proliferation that shapes the T-cell repertoire through peripheral selection and the activation of differentiation pathways. Similarly, reduced generation of early B-cell progenitors alters the composition of the peripheral B-cell compartment with the emergence of a unique, auto-inflammatory B-cell subset, termed age-associated B cells (ABCs). These changes in T- and B-cell composition and function are core manifestations of immune aging.

Aging, Clonality, and Rejuvenation of Hematopoietic Stem Cells

Aging is associated with reduced organ function and increased disease incidence. Hematopoietic stem cell (HSC) aging driven by both cell intrinsic and extrinsic factors is linked to impaired HSC self-renewal and regeneration, aging-associated immune remodeling, and increased leukemia incidence. Compromised DNA damage responses and the increased production of reactive oxygen species (ROS) have been previously causatively attributed to HSC aging. However, recent paradigm-shifting concepts, such as global epigenetic and cytoskeletal polarity shifts, cellular senescence, as well as the clonal selection of HSCs upon aging, provide new insights into HSC aging mechanisms. Rejuvenating agents that can reprogram the epigenetic status of aged HSCs or senolytic drugs that selectively deplete senescent cells provide promising translational avenues for attenuating hematopoietic aging and, potentially, alleviating aging-associated immune remodeling and myeloid malignancies.

Per2 induction limits lymphoid-biased haematopoietic stem cells and lymphopoiesis in the context of DNA damage and ageing

Ageing-associated impairments in haemato-lymphopoiesis are associated with DNA damage accumulation and reduced maintenance of lymphoid-biased (Ly-biased) compared with myeloid-biased (My-biased) haematopoietic stem cells (HSCs). Here, in vivo RNAi screening identifies period circadian clock 2 (Per2) as a critical factor limiting the maintenance of HSCs in response to DNA damage and ageing. Under these conditions, Per2 is activated predominantly in Ly-biased HSCs and stimulates DNA damage signalling and p53-dependent apoptosis in haematopoietic cells. Per2 deletion ameliorates replication stress and DNA damage responses in haematopoietic cells, thereby improving the maintenance of Ly-biased HSCs, lymphopoiesis, and immune function in ageing mice without increasing the accumulation of DNA damage. Per2-deficient mice retain Batf/p53-dependent induction of differentiation of HSCs in response to DNA damage and exhibit an elongated lifespan. Together, these results identify Per2 as a negative regulator of Ly-biased HSCs and immune functions in response to DNA damage and ageing.

Flagellin-dependent TLR5/caveolin-1 as a promising immune activator in immunosenescence

The age-associated decline of immune responses causes high susceptibility to infections and reduced vaccine efficacy in the elderly. However, the mechanisms underlying age-related deficits are unclear. Here, we found that the expression and signaling of flagellin (FlaB)-dependent Toll-like receptor 5 (TLR5), unlike the other TLRs, were well maintained in old macrophages, similar to young macrophages. The expression and activation of TLR5/MyD88, but not TLR4, were sensitively regulated by the upregulation of caveolin-1 in old macrophages through direct interaction. This interaction was also confirmed using macrophages from caveolin-1 or MyD88 knockout mice. Because TLR5 and caveolin-1 were well expressed in major old tissues including lung, skin, intestine, and spleen, we analyzed in vivo immune responses via a vaccine platform with FlaB as a mucosal adjuvant for the pneumococcal surface protein A (PspA) against Streptococcus pneumoniae infection in young and aged mice. The FlaB-PspA fusion protein induced a significantly higher level of PspA-specific IgG and IgA responses and demonstrated a high protective efficacy against a lethal challenge with live S. pneumoniae in aged mice. These results suggest that caveolin-1/TLR5 signaling plays a key role in age-associated innate immune responses and that FlaB-PspA stimulation of TLR5 may be a new strategy for a mucosal vaccine adjuvant against pneumococcal infection in the elderly.

Functionally Distinct Subsets of Lineage-Biased Multipotent Progenitors Control Blood Production in Normal and Regenerative Conditions

Despite great advances in understanding the mechanisms underlying blood production, lineage specification at the level of multipotent progenitors (MPPs) remains poorly understood. Here, we show that MPP2 and MPP3 are distinct myeloid-biased MPP subsets that work together with lymphoid-primed MPP4 cells to control blood production. We find that all MPPs are produced in parallel by hematopoietic stem cells (HSCs), but with different kinetics and at variable levels depending on hematopoietic demands. We also show that the normally rare myeloid-biased MPPs are transiently overproduced by HSCs in regenerating conditions, hence supporting myeloid amplification to rebuild the hematopoietic system. This shift is accompanied by a reduction in self-renewal activity in regenerating HSCs and reprogramming of MPP4 fate toward the myeloid lineage. Our results support a dynamic model of blood development in which HSCs convey lineage specification through independent production of distinct lineage-biased MPP subsets that, in turn, support lineage expansion and differentiation.

Infection, sepsis, and immune function in the older adult receiving critical care

The elderly are vulnerable to developing sepsis due to functional and immune changes, and frequent instrumentation and contact with the health care system. Those infected often present with nonspecific complaints and are at risk for underrecognition and undertreatment, with greater likelihood of rapid progression to severe sepsis and septic shock; however, often respond well to early, organized, and aggressive interventions. Survivors may not return to baseline level of function and may require long-term care facilities after discharge from the hospital. Patient and family preferences for goals of care should be explored as early as possible and incorporated into treatment plans.

Redox signaling in cardiac renewal

SIGNIFICANCE

Utilizing oxygen (O2) through mitochondrial oxidative phosphorylation enables organisms to generate adenosine triphosphate (ATP) with a higher efficiency than glycolysis, but it results in increased reactive oxygen species production from mitochondria, which can result in stem cell dysfunction and senescence.

RECENT ADVANCES

In the postnatal organism, the hematopoietic system represents a classic example of the role of stem cells in cellular turnover and regeneration. However, in other organs such as the heart, both the degree and source of cellular turnover have been heavily contested.

CRITICAL ISSUES

Although recent evidence suggests that the major source of the limited cardiomyocyte turnover in the adult heart is cardiomyocyte proliferation, the identity and potential role of undifferentiated cardiac progenitor cells remain controversial. Several types of cardiac progenitor cells have been identified, and several studies have identified an important role of redox and metabolic regulation in survival and differentiation of cardiac progenitor cells. Perhaps a simple way to approach these controversies is to focus on the multipotentiality characteristics of a certain progenitor population, and not necessarily its ability to give rise to all cell types within the heart. In addition, it is important to note that cycling cells in the heart may express markers of differentiation or may be truly undifferentiated, and for the purpose of this review, we will refer to these cycling cells as progenitors.

FUTURE DIRECTIONS

We propose that hypoxia, redox signaling, and metabolic phenotypes are major regulators of cardiac renewal, and may prove to be important therapeutic targets for heart regeneration.

Early B lymphocyte development: Similarities and differences in human and mouse

B lymphocytes differentiate from hematopoietic stem cells through a series of distinct stages. Early B cell development proceeds in bone marrow until immature B cells migrate out to secondary lymphoid tissues, such as a spleen and lymph nodes, after completion of immunoglobulin heavy and light chain rearrangement. Although the information about the regulation by numerous factors, including signaling molecules, transcription factors, epigenetic changes and the microenvironment, could provide the clinical application, our knowledge on human B lymphopoiesis is limited. However, with great methodological advances, significant progress for understanding B lymphopoiesis both in human and mouse has been made. In this review, we summarize the experimental models for studies about human adult B lymphopoiesis, and the role of microenvironment and signaling molecules, such as cytokines, transforming growth factor-β superfamily, Wnt family and Notch family, with point-by-point comparison between human and mouse.

Regulation of hematopoietic stem cell aging by the small RhoGTPase Cdc42

Aging of stem cells might be the underlying cause of tissue aging in tissue that in the adult heavily rely on stem cell activity, like the blood forming system. Hematopoiesis, the generation of blood forming cells, is sustained by hematopoietic stem cells. In this review article, we introduce the canonical set of phenotypes associated with aged HSCs, focus on the novel aging-associated phenotype apolarity caused by elevated activity of the small RhoGTPase in aged HSCs, discuss the role of Cdc42 in hematopoiesis and describe that pharmacological inhibition of Cdc42 activity in aged HSCs results in functionally young and thus rejuvenated HSCs.

Hematopoietic stem cell aging

Aging is organized in a hierarchy, in which aging of cells results in aged tissues, ultimately limiting lifespan. For organ systems that also in the adult depend on stem cells for tissue homeostasis like the hematopoietic system that forms immune cells, it is believed that aging of the stem cells strongly contributes to aging-associated dysfunction. In this review, we summarize current aspects on cellular and molecular mechanisms that are associated with aging of hematopoietic stem cells, the role of the stem cell niche for stem cell aging as well as novel and encouraging experimental approaches to attenuate aging of hematopoietic stem cells to target immunosenescence.

Telomere dysfunction in peripheral blood mononuclear cells from patients with primary biliary cirrhosis

BACKGROUND

Chromosomal instability in peripheral blood mononuclear cells has a role in the onset of primary biliary cirrhosis. We hypothesized that patients with primary biliary cirrhosis may harbour telomere dysfunction, with consequent chromosomal instability and cellular senescence.

AIM

To evaluate the clinical significance of telomerase activity and telomere length in peripheral blood mononuclear cells from patients with primary biliary cirrhosis.

STUDY DESIGN

In this population-based case control study, 48 women with primary biliary cirrhosis (25 with cirrhosis), 12 with chronic hepatitis C matched by age and severity of disease, and 55 age-matched healthy women were identified. Mononuclear cells from the peripheral blood of patients and controls were isolated. Telomere length and telomerase activity were measured.

RESULTS

Telomere length and telomerase activity did not differ between cases (5.9 ± 1.5 kb) and controls (6.2 ± 1.4 kb, pc=0.164). Telomere shortening and advanced-stage disease strongly correlated with telomerase activity. Patients with advanced disease retained significantly less telomerase activity than those with early-stage disease (0.6 ± 0.9 OD vs. 1.5 ± 3.7 OD, p=0.03). Telomere loss correlated with age, suggesting premature cellular ageing in patients with primary biliary cirrhosis.

CONCLUSION

Our data strongly support the telomere hypothesis of human cirrhosis, indicating that telomere shortening and telomerase activity represent a molecular mechanism in the evolution of human cirrhosis in a selected population of patients.

Alcam regulates long-term hematopoietic stem cell engraftment and self-renewal

Hematopoietic stem cells (HSCs) reside in a specialized bone marrow (BM) microenvironment that supports the maintenance and functional integrity of long-term (LT)-HSCs throughout postnatal life. The objective of this work is to study the role of activated leukocyte cell adhesion molecule (Alcam) in HSC differentiation and self-renewal using an Alcam-null (Alcam(-/-) ) mouse model. We show here that Alcam is differentially regulated in adult hematopoiesis and is highly expressed in LT-HSCs where its level progressively increases with age. Young adult Alcam(-/-) mice had normal homeostatic hematopoiesis and normal numbers of phenotypic HSCs. However, Alcam(-/-) HSCs had reduced long-term replating capacity in vitro and reduced long-term engraftment potential upon transplantation. We show that Alcam(-/-) BM contain a markedly lower frequency of long-term repopulating cells than wild type. Further, the long-term repopulating potential and engraftment efficiency of Alcam(-/-) LT-HSCs was greatly compromised despite a progressive increase in phenotypic LT-HSC numbers during long-term serial transplantation. In addition, an age-associated increase in phenotypic LT-HSC cellularity was observed in Alcam(-/-) mice. This increase was predominately within the CD150(hi) fraction and was accompanied by significantly reduced leukocyte output. Consistent with an aging-like phenotype, older Alcam(-/-) LT-HSCs display myeloid-biased repopulation activity upon transplantation. Finally, Alcam(-/-) LT-HSCs display premature elevation of age-associated gene expression, including Selp, Clu, Cdc42, and Foxo3. Together, this study indicates that Alcam regulates functional integrity and self-renewal of LT-HSCs.

Cdc42 and aging of hematopoietic stem cells

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) continuously provide mature blood cells during the lifespan of a mammal. The functional decline in hematopoiesis in the elderly, which involves a progressive reduction in the immune response and an increased incidence of myeloid malignancy, is partly linked to HSC aging. Molecular mechanisms of HSC aging remain unclear, hindering rational approaches to slow or reverse the decline of HSC function with age. Identifying conditions under which aged HSCs become equivalent to young stem cells might result in treatments for age-associated imbalances in lymphopoiesis and myelopoiesis and in blood regeneration.

RECENT FINDINGS

Aging of HSCs has been for a long time thought to be an irreversible process imprinted in stem cells due to the intrinsic nature of HSC aging. Mouse model studies have found that aging is associated with elevated activity of the Rho GTPase Cdc42 in HSCs that is causative for loss of polarity, altered epigenetic modifications and functional deficits of aged HSCs. The work suggests that inhibition of Cdc42 activity in aged HSCs may reverse a number of phenotypes associated with HSC aging.

SUMMARY

Maintaining the regenerative capacity of organs or organ systems may be a useful way to ensure healthy aging. A defined set of features phenotypically separate young from aged HSCs. Aging of HSCs has been thought to be irreversible. Recent findings support the hypothesis that functional decline of aged HSCs may be reversible by pharmacological intervention of age altered signaling pathways and epigenetic modifications.

A canonical to non-canonical Wnt signalling switch in haematopoietic stem-cell ageing

Many organs with a high cell turnover (for example, skin, intestine and blood) are composed of short-lived cells that require continuous replenishment by somatic stem cells. Ageing results in the inability of these tissues to maintain homeostasis and it is believed that somatic stem-cell ageing is one underlying cause of tissue attrition with age or age-related diseases. Ageing of haematopoietic stem cells (HSCs) is associated with impaired haematopoiesis in the elderly. Despite a large amount of data describing the decline of HSC function on ageing, the molecular mechanisms of this process remain largely unknown, which precludes rational approaches to attenuate stem-cell ageing. Here we report an unexpected shift from canonical to non-canonical Wnt signalling in mice due to elevated expression of Wnt5a in aged HSCs, which causes stem-cell ageing. Wnt5a treatment of young HSCs induces ageing-associated stem-cell apolarity, reduction of regenerative capacity and an ageing-like myeloid-lymphoid differentiation skewing via activation of the small Rho GTPase Cdc42. Conversely, Wnt5a haploinsufficiency attenuates HSC ageing, whereas stem-cell-intrinsic reduction of Wnt5a expression results in functionally rejuvenated aged HSCs. Our data demonstrate a critical role for stem-cell-intrinsic non-canonical Wnt5a signalling in HSC ageing.

Investigation of the role of interleukin-6 and hepcidin antimicrobial peptide in the development of anemia with age

Anemia is common in older adults and associated with adverse health outcomes in epidemiological studies. A thorough understanding of the complex pathophysiological mechanisms driving anemia in the elderly is lacking; but inflammation, iron restriction, and impaired erythroid maturation are thought to influence the phenotype. We hypothesized that interleukin-6 contributes to this anemia, given its pro-inflammatory activities, its ability to induce hepcidin antimicrobial peptide, and its negative impact on several tissues in older adults. We tested this hypothesis by comparing changes in indices of inflammation, iron metabolism and erythropoiesis in aged C57BL/6 mice to aged mice with targeted deletions of interleukin-6 or hepcidin antimicrobial peptide. Circulating neutrophil and monocyte numbers and inflammatory cytokines increased with age. Decline in hemoglobin concentration and red blood cell number indicated that C57BL/6, interleukin-6 knockout mice, and hepcidin antimicrobial peptide knockout mice all demonstrated impaired erythropoiesis by 24 months. However, the interleukin-6 knock out genotype and the hepcidin antimicrobial peptide knock out genotype resulted in improved erythropoiesis in aged mice. Increased erythropoietic activity in the spleen suggested that the erythroid compartment was stressed in aged C57BL/6 mice compared to aged interleukin-6 knockout mice. Our data suggest C57BL/6 mice are an appropriate mammalian model for the study of anemia with age. Furthermore, although interleukin-6 and hepcidin antimicrobial peptide are not required, they can participate in the development of anemia in aging mice, and could be targeted, pre-clinically, with existing interventions to determine the feasibility of such agents for the treatment of anemia in older adults.

Sirt1 ablation promotes stress-induced loss of epigenetic and genomic hematopoietic stem and progenitor cell maintenance

Loss of Sirt1 causes increased Hoxa9 expression and expansion of HSPC subsets under hematopoietic stress, resulting in increased DNA damage and exhaustion of long-term progenitors.

The (histone) deacetylase Sirt1 is a mediator of genomic and epigenetic maintenance, both of which are critical aspects of stem cell homeostasis and tightly linked to their functional decline in aging and disease. We show that Sirt1 ablation in adult hematopoietic stem and progenitor cells (HSPCs) promotes aberrant HSPC expansion specifically under conditions of hematopoietic stress, which is associated with genomic instability as well as the accumulation of DNA damage and eventually results in a loss of long-term progenitors. We further demonstrate that progenitor cell expansion is mechanistically linked to the selective up-regulation of the HSPC maintenance factor and polycomb target gene Hoxa9. We show that Sirt1 binds to the Hoxa9 gene, counteracts acetylation of its histone target H4 lysine 16, and in turn promotes polycomb-specific repressive histone modification. Together, these findings demonstrate a dual role for Sirt1 in HSPC homeostasis, both via epigenetic regulation of a key developmental gene and by promoting genome stability in adult stem cells.

Aging induced decline in T-lymphopoiesis is primarily dependent on status of progenitor niches in the bone marrow and thymus

Age-related decline in the generation of T cells is associated with two primary lymphoid organs, the bone marrow (BM) and thymus. Both organs contain lympho-hematopoietic progenitor/stem cells (LPCs) and non-hematopoietic stromal/niche cells. Murine model showed this decline is not due to reduced quantities of LPCs, nor autonomous defects in LPCs, but rather defects in their niche cells. However, this viewpoint is challenged by the fact that aged BM progenitors have a myeloid skew. By grafting young wild-type (WT) BM progenitors into aged IL-7R^−/− hosts, which possess WT-equivalent niches although LPCs are defect, we demonstrated that these young BM progenitors also exhibited a myeloid skew. We, further, demonstrated that aged BM progenitors, recruited by a grafted fetal thymus in the in vivo microenvironment, were able to compete with their young counterparts, although the in vitro manipulated old BM cells were not able to do so in conventional BM transplantation. Both LPCs and their niche cells inevitably get old with increasing organismal age, but aging in niche cells occurred much earlier than in LPCs by an observation in thymic T-lymphopoiesis. Therefore, the aging induced decline in competence to generate T cells is primarily dependent on status of the progenitor niche cells in the BM and thymus.

The ageing haematopoietic stem cell compartment

Stem cell ageing underlies the ageing of tissues, especially those with a high cellular turnover. There is growing evidence that the ageing of the immune system is initiated at the very top of the haematopoietic hierarchy and that the ageing of haematopoietic stem cells (HSCs) directly contributes to changes in the immune system, referred to as immunosenescence. In this Review, we summarize the phenotypes of ageing HSCs and discuss how the cell-intrinsic and cell-extrinsic mechanisms of HSC ageing might promote immunosenescence. Stem cell ageing has long been considered to be irreversible. However, recent findings indicate that several molecular pathways could be targeted to rejuvenate HSCs and thus to reverse some aspects of immunosenescence.

Nrf2 regulates haematopoietic stem cell function

Coordinating the balance between haematopoietic stem cell (HSC) quiescence and self-renewal is crucial for maintaining haematopoiesis lifelong. Equally important for haematopoietic function is modulating HSC localization within the bone marrow niches, as maintenance of HSC function is tightly controlled by a complex network of intrinsic molecular mechanisms and extrinsic signalling interactions with their surrounding microenvironment. In this study we demonstrate that nuclear factor erythroid 2-related factor 2 (Nfe2l2, or Nrf2), well established as a global regulator of the oxidative stress response, plays a regulatory role in several aspects of HSC homeostasis. Nrf2 deficiency results in an expansion of the haematopoietic stem and progenitor cell compartment due to cell-intrinsic hyperproliferation, which was accomplished at the expense of HSC quiescence and self-renewal. We further show that Nrf2 modulates both migration and retention of HSCs in their niche. Moreover, we identify a previously unrecognized link between Nrf2 and CXCR4, contributing, at least partially, to the maintenance of HSC function.

Immunosenescence-associated microRNAs in age and heart failure

AIMS

Ageing of the immune system, immunosenescence, is characterized by impaired lymphopoiesis, especially B-lymphocyte maturation, and is a hallmark of chronic heart failure (CHF). MicroRNAs (miRNAs) are non-coding, small RNAs, which post-transcriptionally control gene expression of multiple target genes. The miR-181 family is known to control haematopoietic lineage differentiation. Here, we study the role of the miR-181 family in immunosenescence and CHF.

METHODS AND RESULTS

We conducted a clinical study analysing peripheral blood (PB) for miRNA expression and leucocyte distribution of young healthy controls (25 ± 4 years; n = 30), aged healthy controls (64 ± 5 years; n = 13), and age-matched CHF patients (64 ± 11years; n = 18). The expression of miR-181 family members was reduced, whereas miR-34a was increased in PB of aged individuals. In particular, miR-181c was further reduced in age-matched CHF patients. In PB, we observed reduced numbers of lymphocytes, in particular cytotoxic T cells and B cells, with rising age, and the expression of miR-181 correlated with the number of B cells. Notably, in CHF patients, ischaemic heart failure was associated with a further reduction of total B cells as well as their subpopulations, such as memory B cells, compared with age-matched healthy volunteers.

CONCLUSIONS

Ageing- and CHF-associated changes in PB leucocyte subsets are paralleled by alterations in the expression of miRNAs involved in lymphopoiesis, which might play an important role in the age-related and CHF-mediated dysregulation of immune functions resulting in immunosenescence. Furthermore, miR-181c may serve as a marker for reduced immune functions in CHF patients.

Rapamycin increases oxidative stress response gene expression in adult stem cells

Balancing quiescence with proliferation is of paramount importance for adult stem cells in order to avoid hyperproliferation and cell depletion. In some models, stem cell exhaustion may be reversed with the drug rapamycin, which was shown can suppress cellular senescence in vitro and extend lifespan in animals. We hypothesized that rapamycin increases the expression of oxidative stress response genes in adult stem cells, and that these gene activities diminish with age. To test our hypothesis, we exposed mice to rapamycin and then examined the transcriptome of their spermatogonial stem cells (SSCs). Gene expression microarray analysis revealed that numerous oxidative stress response genes were upregulated upon rapamycin treatment, including superoxide dismutase 1, glutathione reductase, and delta-aminolevulinate dehydratase. When we examined the expression of these genes in 55-week-old wild type SSCs, their levels were significantly reduced compared to 3-week-old SSCs, suggesting that their downregulation is coincident with the aging process in adult stem cells. We conclude that rapamycin-induced stimulation of oxidative stress response genes may promote cellular longevity in SSCs, while a decline in gene expression in aged stem cells could reflect the SSCs' diminished potential to alleviate oxidative stress, a hallmark of aging.

Aging of the microenvironment influences clonality in hematopoiesis

The mechanisms of the age-associated exponential increase in the incidence of leukemia are not known in detail. Leukemia as well as aging are initiated and regulated in multi-factorial fashion by cell-intrinsic and extrinsic factors. The role of aging of the microenvironment for leukemia initiation/progression has not been investigated in great detail so far. Clonality in hematopoiesis is tightly linked to the initiation of leukemia. Based on a retroviral-insertion mutagenesis approach to generate primitive hematopoietic cells with an intrinsic potential for clonal expansion, we determined clonality of transduced hematopoietic progenitor cells (HPCs) exposed to a young or aged microenvironment in vivo. While HPCs displayed primarily oligo-clonality within a young microenvironment, aged animals transplanted with identical pool of cells displayed reduced clonality within transduced HPCs. Our data show that an aged niche exerts a distinct selection pressure on dominant HPC-clones thus facilitating the transition to mono-clonality, which might be one underlying cause for the increased age-associated incidence of leukemia.

Collapse of telomere homeostasis in hematopoietic cells caused by heterozygous mutations in telomerase genes

Telomerase activity is readily detectable in extracts from human hematopoietic stem and progenitor cells, but appears unable to maintain telomere length with proliferation in vitro and with age in vivo. We performed a detailed study of the telomere length by flow FISH analysis in leukocytes from 835 healthy individuals and 60 individuals with reduced telomerase activity. Healthy individuals showed a broad range in average telomere length in granulocytes and lymphocytes at any given age. The average telomere length declined with age at a rate that differed between age-specific breakpoints and between cell types. Gender differences between leukocyte telomere lengths were observed for all cell subsets studied; interestingly, this trend could already be detected at birth. Heterozygous carriers for mutations in either the telomerase reverse transcriptase (hTERT) or the telomerase RNA template (hTERC) gene displayed striking and comparable telomere length deficits. Further, non-carrier relatives of such heterozygous individuals had somewhat shorter leukocyte telomere lengths than expected; this difference was most profound for granulocytes. Failure to maintain telomere homeostasis as a result of partial telomerase deficiency is thought to trigger cell senescence or cell death, eventually causing tissue failure syndromes. Our data are consistent with these statements and suggest that the likelihood of similar processes occurring in normal individuals increases with age. Our work highlights the essential role of telomerase in the hematopoietic system and supports the notion that telomerase levels in hematopoietic cells, while limiting and unable to prevent overall telomere shortening, are nevertheless crucial to maintain telomere homeostasis with age.

Human blood cells all originate from a common precursor, the hematopoietic stem cell. Telomerase, the enzyme responsible for adding telomere repeats to chromosome ends, is active in human hematopoietic stem cells but appears unable to maintain a constant telomere length with age. We first document the telomere length of different blood cell subsets from 835 healthy individuals between birth and 100 years, to delineate the normal rate of telomere attrition with age. Telomere lengths of blood cells were found to be slightly longer in women than in men, from birth and throughout life. We then compared this reference data to the telomere length in similar blood cell subsets from individuals with reduced telomerase activity as a result of a mutation in one of the genes encoding telomerase and from their direct relatives. Strikingly short telomeres were found in telomerase-deficient individuals, consistent with their cellular pathology and disease susceptibility, and somewhat shorter telomeres than expected were found in cells of relatives with normal telomerase maintenance. Our data can be used as a reference for blood cell telomere length in studies of normal and accelerated aging.

Cdc42 activity regulates hematopoietic stem cell aging and rejuvenation

The decline in hematopoietic function seen during aging involves a progressive reduction in the immune response and an increased incidence of myeloid malignancy, and has been linked to aging of hematopoietic stem cells (HSCs). The molecular mechanisms underlying HSC aging remain unclear. Here we demonstrate that elevated activity of the small RhoGTPase Cdc42 in aged HSCs is causally linked to HSC aging and correlates with a loss of polarity in aged HSCs. Pharmacological inhibition of Cdc42 activity functionally rejuvenates aged HSCs, increases the percentage of polarized cells in an aged HSC population, and restores the level and spatial distribution of histone H4 lysine 16 acetylation to a status similar to that seen in young HSCs. Our data therefore suggest a mechanistic role for Cdc42 activity in HSC biology and epigenetic regulation, and identify Cdc42 activity as a pharmacological target for ameliorating stem cell aging.

B cell maintenance and function in aging

In this review we discuss the changes that occur in the B lymphocyte compartment of mice and humans as they progress to old age, focusing on recent advances in this important area of research. Primary areas considered include increased morbidity and mortality in the elderly following infection, and decreased responsiveness to vaccines that evoke primary humoral immune responses, as well as those that evoke responses by memory B cells generated following vaccination and natural infection earlier in life. We then consider some of the mechanisms that may underlie these observed declines in humoral immune function. This includes a discussion of alterations in B cell repertoire and subcompartment distribution, as well as defects in B lymphopoiesis, cell development and homeostasis that may contribute to these alterations, and ultimately to declining protective quality of antibodies produced in the elderly.

Near-to-perfect homeostasis: examples of universal aging rule which germline evades

Aging is considered to be a progressive decline in an organism's functioning over time and is almost universal throughout the living world. Currently, many different aging mechanisms have been reported at all levels of biological organization, with a variety of biochemical, metabolic, and genetic pathways involved. Some of these mechanisms are common across species, and others work different, but each of them is constitutive. This review describes the common characteristics of the aging processes, which are consistent changes over time that involve either the accumulation or depletion of particular system components. These accumulations and depletions may result from imperfect homeostasis, which is the incomplete compensation of a particular biological process with another process evolved to compensate it. In accordance with disposable-soma theory, this imperfection in homeostasis may originate as a function of cell differentiation as early as in yeasts. It may result either from antagonistic pleiotropy mechanisms, or be simply negligible as a subject of natural selection if an adverse effect of the accumulation phenotypically manifests in organism's post-reproductive age. If this phenomenon holds true for many different functions it would lead to the occurrence of a wide variety of aging mechanisms, some of which are common among species, while others unique, because aging is the inherent property of most biological processes that have not yet evolved to be perfectly in balance. Examples of imperfect homeostasis mechanisms of aging, the ways in which germ line escapes from them, and the possibilities of anti-aging treatment are discussed in this review.

Stress and stem cells

The unique properties and functions of stem cells make them particularly susceptible to stresses and also lead to their regulation by stress. Stem cell division must respond to the demand to replenish cells during normal tissue turnover as well as in response to damage. Oxidative stress, mechanical stress, growth factors, and cytokines signal stem cell division and differentiation. Many of the conserved pathways regulating stem cell self-renewal and differentiation are also stress-response pathways. The long life span and division potential of stem cells create a propensity for transformation (cancer) and specific stress responses such as apoptosis and senescence act as antitumor mechanisms. Quiescence regulated by CDK inhibitors and a hypoxic niche regulated by FOXO transcription factor function to reduce stress for several types of stem cells to facilitate long-term maintenance. Aging is a particularly relevant stress for stem cells, because repeated demands on stem cell function over the life span can have cumulative cell-autonomous effects including epigenetic dysregulation, mutations, and telomere erosion. In addition, aging of the organism impairs function of the stem cell niche and systemic signals, including chronic inflammation and oxidative stress.