Decreased E47 in senescent B cell precursors is stage specific and regulated posttranslationally by protein turnover

RNA binding proteins in senescence: A potential common linker for age-related diseases?

Aging represents the major risk factor for the onset and/or progression of various disorders including neurodegenerative diseases, metabolic disorders, and bone-related defects. As the average age of the population is predicted to exponentially increase in the coming years, understanding the molecular mechanisms underlying the development of aging-related diseases and the discovery of new therapeutic approaches remain pivotal. Well-reported hallmarks of aging are cellular senescence, genome instability, autophagy impairment, mitochondria dysfunction, dysbiosis, telomere attrition, metabolic dysregulation, epigenetic alterations, low-grade chronic inflammation, stem cell exhaustion, altered cell-to-cell communication and impaired proteostasis. With few exceptions, however, many of the molecular players implicated within these processes as well as their role in disease development remain largely unknown. RNA binding proteins (RBPs) are known to regulate gene expression by dictating at post-transcriptional level the fate of nascent transcripts. Their activity ranges from directing primary mRNA maturation and trafficking to modulation of transcript stability and/or translation. Accumulating evidence has shown that RBPs are emerging as key regulators of aging and aging-related diseases, with the potential to become new diagnostic and therapeutic tools to prevent or delay aging processes. In this review, we summarize the role of RBPs in promoting cellular senescence and we highlight their dysregulation in the pathogenesis and progression of the main aging-related diseases, with the aim of encouraging further investigations that will help to better disclose this novel and captivating molecular scenario.

Myc-Interacting Zinc Finger Protein 1 (Miz-1) Is Essential to Maintain Homeostasis and Immunocompetence of the B Cell Lineage

Aging of the immune system is described as a progressive loss of the ability to respond to immunologic stimuli and is commonly referred to as immunosenescence. B cell immunosenescence is characterized by a decreased differentiation rate in the bone marrow and accumulation of antigen-experienced and age-associated B cells in secondary lymphoid organs (SLOs). A specific deletion of the POZ-domain of the transcription factor Miz-1 in pro-B cells, which is known to be involved in bone marrow hematopoiesis, leads to premature aging of the B cell lineage. In mice, this causes a severe reduction in bone marrow-derived B cells with a drastic decrease from the pre-B cell stage on. Further, mature, naïve cells in SLOs are reduced at an early age, while post-activation-associated subpopulations increase prematurely. We propose that Miz-1 interferes at several key regulatory checkpoints, critical during B cell aging, and counteracts a premature loss of immunocompetence. This enables the use of our mouse model to gain further insights into mechanisms of B cell aging and it can significantly contribute to understand molecular causes of impaired adaptive immune responses to counteract loss of immunocompetence and restore a functional immune response in the elderly.

Patho-Physiology of Aging and Immune-Senescence: Possible Correlates With Comorbidity and Mortality in Middle-Aged and Old COVID-19 Patients

During the last 2 years, the entire world has been severely devastated by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic (COVID-19) as it resulted in several million deaths across the globe. While the virus infects people indiscriminately, the casualty risk is higher mainly in old, and middle-aged COVID-19 patients. The incidences of COVID-19 associated co-morbidity and mortality have a great deal of correlation with the weakened and malfunctioning immune systems of elderly people. Presumably, due to the physiological changes associated with aging and because of possible comorbidities such as diabetes, hypertension, obesity, cardiovascular, and lung diseases, which are more common in elderly people, may be considered as the reason making the elderly vulnerable to the infection on one hand, and COVID-19 associated complications on the other. The accretion of senescent immune cells not only contributes to the deterioration of host defense, but also results in elevated inflammatory phenotype persuaded immune dysfunction. In the present review, we envisage to correlate functioning of the immune defense of older COVID-19 patients with secondary/super infection, increased susceptibility or aggravation against already existing cancer, infectious, autoimmune, and other chronic inflammatory diseases. Moreover, we have discussed how age-linked modulations in the immune system affect therapeutic response against administered drugs as well as immunological response to various prophylactic measures including vaccination in the elderly host. The present review also provides an insight into the intricate pathophysiology of the aging and the overall immune response of the host to SARS-CoV-2 infection. A better understanding of age-related immune dysfunction is likely to help us in the development of targeted preemptive strategies for deadly COVID-19 in elderly patients.

Marrow failure and aging: The role of "Inflammaging"

Despite aging and the enormous cellular output required of the marrow every day of the lifespan, most aged patients do not suffer significant marrow failure or cytopenias, an attestation to the proliferative capacity of this system. However, as marrow and its hematopoietic stem cells age, a reduction in ability to maintain homeostasis after stress or with exposure to prolonged chronic inflammation, so-called "inflammaging," may contribute to cytopenias, inadequate immune responses, and dysplasia/leukemia. In some instances, these changes may be intrinsic to the stem cell but in others, there may be extrinsic environmental influences. In this review, the role of aging as it relates to stem cell changes, immune function, and anemia are reviewed.

The aging lung: Physiology, disease, and immunity

The population is aging at a rate never seen before in human history. As the number of elderly adults grows, it is imperative we expand our understanding of the underpinnings of aging biology. Human lungs are composed of a unique panoply of cell types that face ongoing chemical, mechanical, biological, immunological, and xenobiotic stress over a lifetime. Yet, we do not fully appreciate the mechanistic drivers of lung aging and why age increases the risk of parenchymal lung disease, fatal respiratory infection, and primary lung cancer. Here, we review the molecular and cellular aspects of lung aging, local stress response pathways, and how the aging process predisposes to the pathogenesis of pulmonary disease. We place these insights into context of the COVID-19 pandemic and discuss how innate and adaptive immunity within the lung is altered with age.

Hallmarks of aging and immunosenescence: Connecting the dots

Aging is a natural physiological process that features various and variable challenges, associated with loss of homeostasis within the organism, often leading to negative consequences for health. Cellular senescence occurs when cells exhaust the capacity to renew themselves and their tissue environment as the cell cycle comes to a halt. This process is influenced by genetics, metabolism and extrinsic factors. Immunosenescence, the aging of the immune system, is a result of the aging process, but can also in turn act as a secondary inducer of senescence within other tissues. This review aims to summarize the current state of knowledge regarding hallmarks of aging in relation to immunosenescence, with a focus on aging-related imbalances in the medullary environment, as well as the components of the innate and adaptive immune responses. Aging within the immune system alters its functionality, and has consequences for the person's ability to fight infections, as well as for susceptibility to chronic diseases such as cancer and cardiovascular disease. The senescence-associated secretory phenotype is described, as well as the involvement of this phenomenon in the paracrine induction of senescence in otherwise healthy cells. Inflammaging is discussed in detail, along with the comorbidities associated with this process. A knowledge of these processes is required in order to consider possible targets for the application of senotherapeutic agents - interventions with the potential to modulate the senescence process, thus prolonging the healthy lifespan of the immune system and minimizing the secondary effects of immunosenescence.

Cell signaling and the aging of B cells

The uniqueness of each B cell lies in the structural diversity of the B-cell antigen receptor allowing the virtually limitless recognition of antigens, a necessity to protect individuals against a range of challenges. B-cell development and response to stimulation are exquisitely regulated by a group of cell surface receptors modulating various signaling cascades and their associated genetic programs. The effects of these signaling pathways in optimal antibody-mediated immunity or the aberrant promotion of immune pathologies have been intensely researched in the past in young individuals. In contrast, we are only beginning to understand the contribution of these pathways to the changes in B cells of old organisms. Thus, critical transcription factors such as E2A and STAT5 show differential expression or activity between young and old B cells. As a result, B-cell physiology appears altered, and antibody production is impaired. Here, we discuss selected phenotypic changes during B-cell aging and attempt to relate them to alterations of molecular mechanisms.

Aging and the Immune System: the Impact of Immunosenescence on Viral Infection, Immunity and Vaccine Immunogenicity

Immunosenescence is characterized by a progressive deterioration of the immune system associated with aging. Multiple components of both innate and adaptive immune systems experience aging-related changes, such as alterations in the number of circulating monocytic and dendritic cells, reduced phagocytic activities of neutrophils, limited diversity in B/T cell repertoire, T cell exhaustion or inflation, and chronic production of inflammatory cytokines known as inflammaging. The elderly are less likely to benefit from vaccinations as preventative measures against infectious diseases due to the inability of the immune system to mount a successful defense. Therefore, aging is thought to decrease the efficacy and effectiveness of vaccines, suggesting aging-associated decline in the immunogenicity induced by vaccination. In this review, we discuss aging-associated changes in the innate and adaptive immunity and the impact of immunosenescence on viral infection and immunity. We further explore recent advances in strategies to enhance the immunogenicity of vaccines in the elderly. Better understanding of the molecular mechanisms underlying immunosenescence-related immune dysfunction will provide a crucial insight into the development of effective elderly-targeted vaccines and immunotherapies.

Immunosenescence: A systems-level overview of immune cell biology and strategies for improving vaccine responses

Immunosenescence contributes to a decreased capacity of the immune system to respond effectively to infections or vaccines in the elderly. The full extent of the biological changes that lead to immunosenescence are unknown, but numerous cell types involved in innate and adaptive immunity exhibit altered phenotypes and function as a result of aging. These manifestations of immunosenescence at the cellular level are mediated by dysregulation at the genetic level, and changes throughout the immune system are, in turn, propagated by numerous cellular interactions. Environmental factors, such as nutrition, also exert significant influence on the immune system during aging. While the mechanisms that govern the onset of immunosenescence are complex, systems biology approaches allow for the identification of individual contributions from each component within the system as a whole. Although there is still much to learn regarding immunosenescence, systems-level studies of vaccine responses have been highly informative and will guide the development of new vaccine candidates, novel adjuvant formulations, and immunotherapeutic drugs to improve vaccine responses among the aging population.

Inflammatory immune cells may impair the preBCR checkpoint, reduce new B cell production, and alter the antibody repertoire in old age

Aging impairs development of new B cells and diminishes the expression of protective antibodies. Reduced numbers of B cell precursors generally occur in old (~2 yrs.) mice. At the pro-B to pre-B cell transition, the pre-B cell receptor (preBCR) checkpoint directs pre-B cell expansion and selection of the pre-B cell immunoglobulin (Ig) μ heavy chain variable region repertoire. The preBCR is comprised of Ig μ heavy chain + surrogate light chains (SLC; λ5/VpreB). In old B cell precursors, SLC is decreased and fewer pre-B cells form the preBCR. In pro-B cells, SLC is complexed with cadherin 17 to form a "pro-B cell receptor" whose signaling is postulated to increase apoptotic sensitivity. We propose that inflammation in old mice, in part mediated by the age-associated B cells (ABC), promotes apoptosis among pro-B cells, particularly those relatively high in SLC. The remaining pro-B cells, with lower SLC, now generate pre-B cells with limited capacity to form the preBCR. Ig μ heavy chains vary in their capacity to associate with SLC and form the preBCR. We speculate that limited SLC restricts formation of the preBCR to a subset of Ig μ heavy chains. This likely impacts the composition of the antibody repertoire among B cells.

Inflamm-Aging of Hematopoiesis, Hematopoietic Stem Cells, and the Bone Marrow Microenvironment

All hematopoietic and immune cells are continuously generated by hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) through highly organized process of stepwise lineage commitment. In the steady state, HSCs are mostly quiescent, while HPCs are actively proliferating and contributing to daily hematopoiesis. In response to hematopoietic challenges, e.g., life-threatening blood loss, infection, and inflammation, HSCs can be activated to proliferate and engage in blood formation. The HSC activation induced by hematopoietic demand is mediated by direct or indirect sensing mechanisms involving pattern recognition receptors or cytokine/chemokine receptors. In contrast to the hematopoietic challenges with obvious clinical symptoms, how the aging process, which involves low-grade chronic inflammation, impacts hematopoiesis remains undefined. Herein, we summarize recent findings pertaining to functional alternations of hematopoiesis, HSCs, and the bone marrow (BM) microenvironment during the processes of aging and inflammation and highlight some common cellular and molecular changes during the processes that influence hematopoiesis and its cells of origin, HSCs and HPCs, as well as the BM microenvironment. We also discuss how age-dependent alterations of the immune system lead to subclinical inflammatory states and how inflammatory signaling might be involved in hematopoietic aging. Our aim is to present evidence supporting the concept of “Inflamm-Aging,” or inflammation-associated aging of hematopoiesis.

B cells in the aging immune system: time to consider B-1 cells

The investigation of immune senescence has uncovered many changes in B cell development, maintenance, and function with increasing age. However, most of these studies have focused on conventional B cell subsets in the spleen. The B-1 cell subset is an essential arm of the innate immune system, which in general has been understudied in terms of immune senescence. Here, we review what is currently known about B cells during aging and go on to describe why B-1 cell biology is an important component of the aging immune system in the context of diseases that most affect the aged population.

In aged mice, low surrogate light chain promotes pro-B-cell apoptotic resistance, compromises the PreBCR checkpoint, and favors generation of autoreactive, phosphorylcholine-specific B cells

In aged mice, new B-cell development is diminished and the antibody repertoire becomes more autoreactive. Our studies suggest that (i) apoptosis contributes to reduced B lymphopoiesis in old age and preferentially eliminates those B-cell precursors with higher levels of the surrogate light chain (SLC) proteins (λ5/VpreB) and (ii) λ5(low) B-cell precursors generate new B cells which show increased reactivity to the self-antigen/bacterial antigen phosphorylcholine (PC). Pro-B cells in old bone marrow as well as pro-B cells from young adult λ5-deficient mice are resistant to cytokine-induced apoptosis (TNFα; TGFβ), indicating that low λ5 expression in pro-B cells is sufficient to cause increased survival. Transfer of TNFα-producing 'age-associated B cells' (ABC; CD21/35(-) CD23(-)) or follicular (FO) B cells from aged mice into RAG-2 KO recipients led to preferential loss of λ5(high) pro-B cells, but retention of λ5(low), apoptosis-resistant pro-B cells. In old mice, there is increased reactivity to PC in both immature bone marrow B cells and mature splenic FO B cells. In young mice, absence of λ5 expression led to a similar increase in PC reactivity among bone marrow and splenic B cells. We propose that in old age, increased apoptosis, mediated in part by TNFα-producing B cells, results in preferential loss of SLC(high) pro-B cells within the bone marrow. Further B-cell development then occurs via an 'SLC(low)' pathway that not only impairs B-cell generation, but promotes autoreactivity within the naïve antibody repertoires in the bone marrow and periphery.

Salvador-Warts-Hippo pathway in a developmental checkpoint monitoring helix-loop-helix proteins

The E proteins and Id proteins are, respectively, the positive and negative heterodimer partners for the basic-helix-loop-helix protein family and as such contribute to a remarkably large number of cell-fate decisions. E proteins and Id proteins also function to inhibit or promote cell proliferation and cancer. Using a genetic modifier screen in Drosophila, we show that the Id protein Extramacrochaetae enables growth by suppressing activation of the Salvador-Warts-Hippo pathway of tumor suppressors, activation that requires transcriptional activation of the expanded gene by the E protein Daughterless. Daughterless protein binds to an intronic enhancer in the expanded gene, both activating the SWH pathway independently of the transmembrane protein Crumbs and bypassing the negative feedback regulation that targets the same expanded enhancer. Thus, the Salvador-Warts-Hippo pathway has a cell-autonomous function to prevent inappropriate differentiation due to transcription factor imbalance and monitors the intrinsic developmental status of progenitor cells, distinct from any responses to cell-cell interactions.

Age effects on mouse and human B cells

Our laboratory has contributed to the areas of B cell receptor (BCR) and pre-BCR gene identification and transcription and has focused on the problem of the aged immune system in mice and humans for the last 15 years. We have found biomarkers for the decrease in B cell function in aged mice and humans. These include decreases in immunoglobulin (Ig) class switch (e.g., IgM to IgG), decreases in the enzyme AID (activation-induced cytidine deaminase) and decreases in the transcription factor E47. The E47 mRNA stability is decreased in old B cells due to decreased phospho-MAPKinase and phospho-TTP (tristetraprolin). Inflammation, e.g., TNF-α, which increases with age, impacts B cells directly by increasing their TNF-α and NF-κB and leads to the above decreased pathway. Both class switch and affinity maturation are decreased in elderly responses to the influenza vaccine and biomarkers we have found (numbers and percentages of switched memory B cells and AID in stimulated B cells in culture) can predict a beneficial or decreased immune response to the vaccine. Current and future avenues to improve the humoral immune response in the elderly are discussed.

Impaired B lymphopoiesis in old age: a role for inflammatory B cells?

Continued generation of new B cells within the bone marrow is required throughout life. However, in old age, B lymphopoiesis is inhibited at multiple developmental stages from hematopoietic stem cells through the late stages of new B cell generation. While changes in B cell precursor subsets, as well as alterations in the supporting bone marrow microenvironment, in old age have been known for the last 20 years, only more recently have insights into the cellular and molecular mechanisms responsible become clarified. Our recent discovery that B cells in aged mice are pro-inflammatory and can diminish B cell generation within the bone marrow suggests a potential mechanism of inappropriate "B cell feedback" which contributes to a bone marrow microenvironment unfavorable to B lymphopoiesis. We hypothesize that the consequences of a pro-inflammatory microenvironment in old age are (1) reduced B cell generation and (2) alteration in the "read-out" of the antibody repertoire. Both of these likely ensue from reduced expression of the surrogate light chain (λ5 + VpreB) and consequently reduced expression of the pre-B cell receptor (preBCR), critical to pre-B cell expansion and Vh selection. In old age, B cell development may progressively be diverted into a preBCR-compromised pathway. These abnormalities in B lymphopoiesis likely contribute to the poor humoral immunity seen in old age.

A meta-analysis of Hodgkin lymphoma reveals 19p13.3 TCF3 as a novel susceptibility locus

Recent genome-wide association studies (GWAS) of Hodgkin lymphoma (HL) have identified associations with genetic variation at both HLA and non-HLA loci; however, much of heritable HL susceptibility remains unexplained. Here we perform a meta-analysis of three HL GWAS totaling 1,816 cases and 7,877 controls followed by replication in an independent set of 1,281 cases and 3,218 controls to find novel risk loci. We identify a novel variant at 19p13.3 associated with HL (rs1860661; odds ratio (OR)=0.81, 95% confidence interval (95% CI) = 0.76-0.86, P(combined) = 3.5 × 10(-10)), located in intron 2 of TCF3 (also known as E2A), a regulator of B- and T-cell lineage commitment known to be involved in HL pathogenesis. This meta-analysis also notes associations between previously published loci at 2p16, 5q31, 6p31, 8q24 and 10p14 and HL subtypes. We conclude that our data suggest a link between the 19p13.3 locus, including TCF3, and HL risk.

Transcriptional profiling of mRNAs and microRNAs in human bone marrow precursor B cells identifies subset- and age-specific variations

BACKGROUND

Molecular mechanisms explaining age-related changes in the bone marrow with reduced precursor B cell output are poorly understood.

METHODS

We studied the transcriptome of five precursor B cell subsets in individual bone marrow samples from 4 healthy children and 4 adults employing GeneChip® Human Exon 1.0 ST Arrays (Affymetrix®) and TaqMan® Array MicroRNA Cards (Life Technologies™).

RESULTS

A total of 1796 mRNAs (11%) were at least once differentially expressed between the various precursor B cell subsets in either age group (FDR 0.1%, p≤1.13×10(-4)) with more marked cell stage specific differences than those related to age. In contrast, microRNA profiles of the various precursor B cell subsets showed less hierarchical clustering as compared to the corresponding mRNA profiles. However, 17 of the 667 microRNA assays (2.5%) were at least once differentially expressed between the subsets (FDR 10%, p≤0.004). From target analysis (Ingenuity® Systems), functional assignment between postulated interacting mRNAs and microRNAs showed especially association to cellular growth, proliferation and cell cycle regulation. One functional network connected up-regulation of the differentiation inhibitor ID2 mRNA to down-regulation of the hematopoiesis- or cell cycle regulating miR-125b-5p, miR-181a-5p, miR-196a-5p, miR-24-3p and miR-320d in adult PreBII large cells. Noteworthy was also the stage-dependent expression of the growth promoting miR-17-92 cluster, showing a partly inverse trend with age, reaching statistical significance at the PreBII small stage (up 3.1-12.9 fold in children, p = 0.0084-0.0270).

CONCLUSIONS

The global mRNA profile is characteristic for each precursor B cell developmental stage and largely similar in children and adults. The microRNA profile is much cell stage specific and not changing much with age. Importantly, however, specific age-dependent differences involving key networks like differentiation and cellular growth may indicate biological divergence and possibly also altered production potential with age.

Expression of calpain-calpastatin system (CCS) member proteins in human lymphocytes of young and elderly individuals; pilot baseline data for the CALPACENT project

BACKGROUND

Ubiquitous system of regulatory, calcium-dependent, cytoplasmic proteases - calpains - and their endogenous inhibitor - calpastatin - is implicated in the proteolytic regulation of activation, proliferation, and apoptosis of many cell types. However, it has not been thoroughly studied in resting and activated human lymphocytes yet, especially in relation to the subjects' ageing process. The CALPACENT project is an international (Polish-Italian) project aiming at verifying the hypothesis of the role of calpains in the function of peripheral blood immune cells of Polish (Pomeranian) and Italian (Sicilian) centenarians, apparently relatively preserved in comparison to the general elderly population. In this preliminary report we aimed at establishing and comparing the baseline levels of expression of μ- and m-calpain and calpastatin in various, phenotypically defined, populations of human peripheral blood lymphocytes for healthy elderly Sicilians and Poles, as compared to these values observed in young cohort.

RESULTS

We have found significant differences in the expression of both μ- and m-calpain as well as calpastatin between various populations of peripheral blood lymphocytes (CD4+, CD8+ and CD19+), both between the age groups compared and within them. Interestingly, significantly higher amounts of μ- and m-calpains but not of calpastatin could be demonstrated in the CD4+CD28- and CD8+CD28- lymphocytes of old subjects (but not in the cells of young individuals), as compared to their CD28+ counterparts. Finally, decreased expression of both calpains in the elderly T cells is not related to the accumulation of effector/memory (CD45RO+) cells in the latter, as the expression of both calpains does not differ significantly between the naïve and memory T cells, while is significantly lower for elderly lymphocytes if both populations are taken separately.

CONCLUSIONS

Observed differences in the amounts of CCS member proteins between various populations of lymphocytes of young and elderly subjects may participate in the impaired proliferative activity of these cells in the elderly.

Increased ID2 levels in adult precursor B cells as compared with children is associated with impaired Ig locus contraction and decreased bone marrow output

Precursor B cell production from bone marrow in mice and humans declines with age. Because the mechanisms behind are still unknown, we studied five precursor B cell subsets (ProB, PreBI, PreBII large, PreBII small, immature B) and their differentiation-stage characteristic gene expression profiles in healthy individual toddlers and middle-aged adults. Notably, the composition of the precursor B cell compartment did not change with age. The expression levels of several transcripts encoding V(D)J recombination factors were decreased in adults as compared with children: RAG1 expression was significantly reduced in ProB cells, and DNA-PKcs, Ku80, and XRCC4 were decreased in PreBI cells. In contrast, TdT was 3-fold upregulated in immature B cells of adults. Still, N-nucleotides, P-nucleotides, and deletions were similar for IGH and IGK junctions between children and adults. PreBII large cells in adults, but not in children, showed highly upregulated expression of the differentiation inhibitor, inhibitor of DNA binding 2 (ID2), in absence of changes in expression of the ID2-binding partner E2A. Further, we identified impaired Ig locus contraction in adult precursor B cells as a likely mechanism by which ID2-mediated blocking of E2A function results in reduced bone marrow B cell output in adults. The reduced B cell production was not compensated by increased proliferation in adult immature B cells, despite increased Ki67 expression. These findings demonstrate distinct regulatory mechanisms in B cell differentiation between adults and children with a central role for transcriptional regulation of ID2.

In senescence, age-associated B cells secrete TNFα and inhibit survival of B-cell precursors

Aged mice exhibit ~ 5-10-fold increases in an ordinarily minor CD21/35(-) CD23(-) mature B-cell subset termed age-associated B cells (ABCs). ABCs from old, but not young, mice induce apoptosis in pro-B cells directly through secretion of TNFα. In addition, aged ABCs, via TNFα, stimulate bone marrow cells to suppress pro-B-cell growth. ABC effects can be prevented by the anti-inflammatory cytokine IL-10. Notably, CD21/35(+) CD23(+) follicular (FO) splenic and FO-like recirculating bone marrow B cells in both young and aged mice contain a subpopulation that produces IL-10. Unlike young adult FO B cells, old FO B cells also produce TNFα; however, secretion of IL-10 within this B-cell population ameliorates the TNFα-mediated effects on B-cell precursors. Loss of B-cell precursors in the bone marrow of old mice in vivo was significantly associated with increased ABC relative to recirculating FO-like B cells. Adoptive transfer of aged ABC into RAG-2 KO recipients resulted in significant losses of pro-B cells within the bone marrow. These results suggest that alterations in B-cell composition during old age, in particular, the increase in ABC within the B-cell compartments, contribute to a pro-inflammatory environment within the bone marrow. This provides a mechanism of inappropriate B-cell 'feedback' that promotes down-regulation of B lymphopoiesis in old age.

A molecular mechanism for TNF-α-mediated downregulation of B cell responses

B cell function with age is decreased in class switch recombination (CSR), activation-induced cytidine deaminase (AID), and stability of E47 mRNA. The latter is regulated, at least in part, by tristetraprolin (TTP), which is increased in aged B cells and also negatively regulates TNF-α. In this study, we investigated whether B cells produce TNF-α, whether this changes with age, and how this affects their function upon stimulation. Our hypothesis is that in aging there is a feedback mechanism of autocrine inflammatory cytokines (TNF-α) that lowers the expression of AID and CSR. Our results showed that unstimulated B cells from old BALB/c mice make significantly more TNF-α mRNA and protein than do B cells from young mice, but after stimulation the old make less than the young; thus, they are refractory to stimulation. The increase in TNF-α made by old B cells is primarily due to follicular, but not minor, subsets of B cells. Incubation of B cells with TNF-α before LPS stimulation decreased both young and old B cell responses. Importantly, B cell function was restored by adding anti-TNF-α Ab to cultured B cells. To address a molecular mechanism, we found that incubation of B cells with TNF-α before LPS stimulation induced TTP, a physiological regulator of mRNA stability of the transcription factor E47, which is crucial for CSR. Finally, anti-TNF-α given in vivo increased B cell function in old, but not in young, follicular B cells. These results suggest new molecular mechanisms that contribute to reduced Ab responses in aging.

E47 retroviral rescue of intrinsic B-cell defects in senescent mice

In aging, immune responses are dramatically impaired, specifically the ability to produce protective antibodies. We previously showed that with age there is a B-cell intrinsic decrease in class switch recombination (CSR) because of a decrease in activation-induced cytidine deaminase (AID). One mechanism we have demonstrated for decreased AID includes increased mRNA degradation of the transcription factor E47, critical for AID transcription. Here, we show by means of a retroviral construct containing the DsRED reporter and the 3'UTR of E47 that the 3'UTR lowers mRNA expression, and particularly in B cells from old mice. This is the first demonstration that the E47 3'UTR directly regulates its degradation. The AID mRNA was not differentially regulated by degradation in aging. Therefore, we have here further established critical components for decreased AID with age. The major aim of this study was to establish conditions for the rescue of the intrinsic defect of aged B cells with retroviral addition of the coding region of E47 in splenic B cells to restore their ability to produce optimal AID and class switch to IgG. In this study, we show that young and old primary B cells overexpressing a stable E47 mRNA up-regulate E47, AID, and CSR and improve B-cell immune responses in senescent murine B cells. Our results provide a proof of principle for the rescue of intrinsic B-cell defects and the humoral immune response in senescence.

Chronic exposure to a TLR ligand injures hematopoietic stem cells

Hematopoietic stem cells (HSC) can be harmed by disease, chemotherapy, radiation, and normal aging. We show in this study that damage also occurs in mice repeatedly treated with very low doses of LPS. Overall health of the animals was good, and there were relatively minor changes in marrow hematopoietic progenitors. However, HSC were unable to maintain quiescence, and transplantation revealed them to be myeloid skewed. Moreover, HSC from treated mice were not sustained in serial transplants and produced lymphoid progenitors with low levels of the E47 transcription factor. This phenomenon was previously seen in normal aging. Screening identified mAbs that resolve HSC subsets, and relative proportions of these HSC changed with age and/or chronic LPS treatment. For example, minor CD150(Hi)CD48(-) populations lacking CD86 or CD18 expanded. Simultaneous loss of CD150(Lo/-)CD48(-) HSC and gain of the normally rare subsets, in parallel with diminished transplantation potential, would be consistent with age- or TLR-related injury. In contrast, HSC in old mice differed from those in LPS-treated animals with respect to VCAM-1 or CD41 expression and lacked proliferation abnormalities. HSC can be exposed to endogenous and pathogen-derived TLR ligands during persistent low-grade infections. This stimulation might contribute in part to HSC senescence and ultimately compromise immunity.

B-cell depletion reactivates B lymphopoiesis in the BM and rejuvenates the B lineage in aging

Aging is associated with a decline in B-lymphopoiesis in the bone marrow and accumulation of long-lived B cells in the periphery. These changes decrease the body's ability to mount protective antibody responses. We show here that age-related changes in the B lineage are mediated by the accumulating long-lived B cells. Thus, depletion of B cells in old mice was followed by expansion of multipotent primitive progenitors and common lymphoid progenitors, a revival of B-lymphopoiesis in the bone marrow, and generation of a rejuvenated peripheral compartment that enhanced the animal's immune responsiveness to antigenic stimulation. Collectively, our results suggest that immunosenescence in the B-lineage is not irreversible and that depletion of the long-lived B cells in old mice rejuvenates the B-lineage and enhances immune competence.

E2A is a transcriptional regulator of CD38 expression in chronic lymphocytic leukemia

CD38, a nucleotide-metabolizing ectoenzyme and a receptor, is a negative prognostic marker for chronic lymphocytic leukemia (CLL) patients. CD38 has a genetic polymorphism, with a C → G variation in a putative E-box located in a regulatory region. E2A, the predominant E-box factor in B lymphocytes, was found to be highly expressed by CD38(+) CLL patients. The highest CD38 levels scored by E2A(+)/G carrier patients suggested that E2A is (i) directly associated with CD38 expression, and that (ii) the binding of the transcription factor is influenced by the CD38 genotype. Chromatin immunoprecipitation indicated that E2A directly interacts with the CD38 regulatory region. Furthermore, E2A binding was stronger in the presence of the G allele. Experiments of E2A silencing led to a significant reduction of surface levels of CD38, confirming the working hypothesis. A direct functional interplay between E2A and CD38 was shown by exposing CLL cells to interleukin-2 and TLR-9 ligands, both inducers of CD38 expression. Under these conditions, CD38 upregulation was primarily conditioned by the presence of E2A and then by the G allele. The results of this study link E2A and CD38 expression within a common pathway, in which E-protein activity is required for the efficient induction of CD38 transcription.

Reduced EBF expression underlies loss of B-cell potential of hematopoietic progenitors with age

Aging is accompanied by a reduction in the generation of B lymphocytes leading to impaired immune responses. In this study, we have investigated whether the decline in B lymphopoiesis is due to age-related defects in the hematopoietic stem cell compartment. The ability of hematopoietic stem cells from old mice to generate B cells, as measured in vitro, is decreased 2-5-fold, while myeloid potential remains unchanged. This age-related decrease in B-cell potential is more marked in common lymphoid progenitors (CLP) and was associated with reduced expression of the B-lineage specifying factors, EBF and Pax5. Notably, retrovirus-mediated expression of EBF complemented the age-related loss of B-cell potential in CLP isolated from old mice. Furthermore, transduction of CLP from old mice with a constitutively active form of STAT5 restored both EBF and Pax5 expression and increased B-cell potential. These results are consistent with a mechanism, whereby reduced expression of EBF with age decreases the frequency with which multipotent hematopoietic progenitors commit to a B-cell fate, without altering their potential to generate myeloid cells.

Protein phosphatase 2A (PP2A) is increased in old murine B cells and mediates p38 MAPK/tristetraprolin dephosphorylation and E47 mRNA instability

The transcription factor E47, which regulates immunoglobulin class switch in murine splenic B cells, is down-regulated in aged B cells due to reduced mRNA stability. Part of the decreased stability of E47 mRNA is mediated by tristetraprolin (TTP), a physiological regulator of mRNA stability. We have previously shown that TTP mRNA and protein expression are higher in old B cells, and the protein is less phosphorylated in old B cells, both of which lead to more binding of TTP to the 3'-UTR of E47 mRNA, thereby decreasing its stability. PP2A is a protein phosphatase that plays an important role in the regulation of a number of major signaling pathways. Herein we show that not only the amount but also the activity of PP2A is increased in old B cells. As a consequence of this higher phosphatase activity in old B cells, p38 MAPK and TTP (either directly or indirectly by PP2A) are less phosphorylated as compared with young B cells. PP2A dephosphorylation of p38 MAPK and/or TTP likely generates more binding of the hypophosphorylated TTP to the E47 mRNA, inducing its degradation. This mechanism may be at least in part responsible for the age-related decrease in class switch.

Intra-bone marrow bone marrow transplantation rejuvenates the B-cell lineage in aged mice

Age-related reductions in the frequency and absolute number of early B lineage precursors in the bone marrow of aged mice have been reported. Reversal of B-cell lineage senescence has not been achieved. Age-related impairment of the B-cell lineage is caused by the decreasing functionality of hematopoietic and B lineage precursors, and reduced efficacy of bone marrow stromal cells that constitute the bone marrow microenvironment. To induce rejuvenation of aged B cells, we injected whole bone marrow from young donors to irradiated aged recipients through the tibia and analyzed B-cell development and immune responsiveness. In aged mice, we found significant reductions in the frequencies and absolute numbers of pro-B cells (B220(+)CD43(+)CD24(+)BP-1(-) and B220(+)CD43(+)CD24(int)BP-1(+)) and pre-B cells (B220(+)CD43(+)CD24(high)BP-1(+) and B220(+)CD43(-)IgM(-)IgD(-)). Intra-bone marrow bone marrow transplantation (IBM-BMT) of young marrow cells including both hematopoietic stem cells and bone marrow stromal cells reversed the reduction of pro-B cells and pre-B cells. In the periphery, the frequency and absolute number of marginal zone-B cell were not significantly different between young, old and IBM-BMT group. The frequency of follicular-B cells in the IBM-BMT group was significantly increased compared to old group. The frequency of B1a B cells in the peritoneal cavity was significantly decreased in the IBM-BMT group. Antibody production against T-independent antigens was not different among the young, the aged and IBM-BMT groups.

Effects of aging on B cell function

Ability to make an optimal immune response to vaccines and infectious agents declines with age in humans and animal models. Recent advances have shown intrinsic B cell defects in aged mice and humans, including decreases in Ig class switch recombination (CSR), activation-induced cytidine deaminase (AID), and E47 transcription factor. Effects on somatic hypermutation (SHM) have been varied depending on the system studied. Increase of AID in mice has shown improved CSR but not SHM. The reported microarray analysis of human B cell subsets may now be used to delineate B cell defects with aging and all the advances presented should lead to selecting agents for improved immune response in the elderly.

B cells and aging: molecules and mechanisms

Recent advances allow aging-associated changes in B-cell function to be approached at a mechanistic level. Reduced expression of genes crucial to lineage commitment and differentiation yield diminished B-cell production. Moreover, intrinsic differences in the repertoire generated by B-cell precursors in aged individuals, coupled with falling B-cell generation rates and life-long homeostatic competition, result in narrowed clonotypic diversity. Similarly, reductions in gene products crucial for immunoglobulin class switch recombination and somatic hypermutation impact the efficacy of humoral immune responses. Together, these findings set the stage for integrated analyses of how age-related changes at the molecular, cellular and population levels interact to yield the overall aging phenotype.

Aging and cancer resistance in lymphoid progenitors are linked processes conferred by p16Ink4a and Arf

Lymphoid progenitors exhibit severe growth defects during aging while myelopoiesis is relatively unperturbed. These effects are due in part to the preferential expression of p16(Ink4a) and Arf in aged lymphoid progenitors. Their increased expression contributes to reduced growth and survival of lymphoid progenitors and makes them refractory to malignant transformation. Down-regulation of p16(Ink4a) and Arf in aged lymphoid progenitors reverted the senescent phenotype and restored susceptibility to transformation. These data provide a molecular explanation for the preferential effects of aging on lymphopoiesis, suggest that inhibiting p16(Ink4a) and Arf expression can rejuvenate B lymphopoiesis, and link aging and cancer resistance.

NK cells in the CD19- B220+ bone marrow fraction are increased in senescence and reduce E2A and surrogate light chain proteins in B cell precursors

E2A encoded proteins, key transcriptional regulators in B lineage specification and commitment, have been shown to decrease in B cell precursors in old age. E2A regulates genes encoding the surrogate light chain proteins lambda5 and VpreB. In old age, B cell precursors express less surrogate light chain and this results in compromised pre-B cell receptor function and diminished expansion of new pre-B cells in senescence. Herein, we show that aged bone marrow has increased Hardy Fraction A (CD19(-) B220(+)) cells, including NK cells, that can inhibit both E47 (E2A) protein and surrogate light chain protein expression in B cell precursors. In vitro, NK-associated inhibition of E47 protein is contact-independent and partially reversed by neutralization of TNFalpha. In vivo, depletion of NK cells in aged mice by treatment with anti-asialo GM1 antibody led to restoration of surrogate light chain protein levels to that typical of young B cell precursors. These studies suggest that NK cells, within the CD19(-) B220(+) bone marrow cell fraction, may contribute to a bone marrow microenvironment that has the potential to negatively regulate E47 (E2A) as well as surrogate light chain levels in B cell precursors in old age.

Deviation of the B cell pathway in senescent mice is associated with reduced surrogate light chain expression and altered immature B cell generation, phenotype, and light chain expression

B lymphopoiesis in aged mice is characterized by reduced B cell precursors and an altered Ab repertoire. This likely results, in part, from reduced surrogate L chains in senescent B cell precursors and compromised pre-BCR checkpoints. Herein, we show that aged mice maintain an ordinarily minor pool of early c-kit(+) pre-B cells, indicative of poor pre-BCR expression, even as pre-BCR competent early pre-B cells are significantly reduced. Therefore, in aged mice, B2 B lymphopoiesis shifts from dependency on pre-BCR expansion and selection to more pre-BCR-deficient pathways. B2 c-kit(+) B cell precursors, from either young or aged mice, generate new B cells in vitro that are biased to larger size, higher levels of CD43, and decreased kappa L chain expression. Notably, immature B cells in aged bone marrow exhibit a similar phenotype in vivo. We hypothesize that reduced surrogate L chain expression contributes to decreased pre-B cells in aged mice. The B2 pathway is partially blocked with limited B cell development and reduced pre-BCR expression and signaling. In old age, B2 pathways have limited surrogate L chain and increasingly generate new B cells with altered phenotype and L chain expression.

Mechanisms for decreased function of B cells in aged mice and humans

The immune system has been known for some time to be compromised in aged individuals, e.g., both mice and humans, and in both humoral and cellular responses. Our studies have begun to elucidate intrinsic B lymphocyte defects in Ig class switch recombination, activation-induced cytidine deaminase, and E47 transcription factor expression. These defects occur in both mice and humans. Our studies have also shown that tristetraprolin is one of the key players in regulating the decreased E47 mRNA stability in aged B lymphocytes. These and current studies should lead to improvements in B lymphocyte function in aged populations.

Accelerated Notch-Dependent Degradation of E47 Proteins in Aged B Cell Precursors Is Associated with Increased ERK MAPK Activation

The transcriptional regulator E47, encoded by the E2A gene, is crucial to B lymphopoiesis. In BALB/c senescent mice (approximately 2 years old), the incidence of E47-expressing pro-B cells in vivo and E47 protein steady state levels in B cell precursors in vitro were reduced. Poor expression of E47 protein was a consequence of accelerated proteasome-mediated turnover and was associated with heightened ubiquitin modification of E2A-encoded proteins in aged B cell precursors. Both MAPK and Notch activity have been previously associated with E2A-encoded protein stability in lymphocytes. Aged B cell precursors exhibited heightened levels of MAPK activity reflected in increased levels of phospho-ERK proteins. Phosphorylation of E2A-encoded proteins was also increased in aged B cell precursors and pharmacologic inhibition of MEK-1 resulted in a partial restoration of their E47 protein. Both Notch proteins and their Delta-like ligands were detected comparably in young and aged B cell precursors. Either inhibition of Notch activation via gamma-secretase or Ab blockade of Notch-Delta-like ligand interactions partially restored E47 expression in aged B cell precursors. We hypothesize that increased MAPK activity promotes phosphorylation of E2A-encoded protein in aged B cell precursors. Subsequently, E2A-encoded proteins undergo ubiquitination and accelerated degradation in a Notch-dependent process. The dysregulation of E2A-encoded protein expression may contribute to the reductions seen in early B lymphopoiesis during murine senescence.

Immunosenescence: emerging challenges for an ageing population

It is now becoming apparent that the immune system undergoes age-associated alterations, which accumulate to produce a progressive deterioration in the ability to respond to infections and to develop immunity after vaccination, both of which are associated with a higher mortality rate in the elderly. Immunosenescence, defined as the changes in the immune system associated with age, has been gathering interest in the scientific and health-care sectors alike. The rise in its recognition is both pertinent and timely given the increasing average age and the corresponding failure to increase healthy life expectancy. This review attempts to highlight the age-dependent defects in the innate and adaptive immune systems. While discussing the mechanisms that contribute to immunosenescence, with emphasis on the extrinsic factors, particular attention will be focused on thymic involution. Finally, we illuminate potential therapies that could be employed to help us live a longer, fuller and healthier life.

Aging, B lymphopoiesis, and patterns of leukemogenesis

The production of B lymphocytes begins to decline steadily early in adult life and is severely compromised in the elderly. This occurrence has been attributed to intrinsic defects in early hematopoietic progenitors and B cell precursors as well as to microenvironmental changes in aged bone marrow. The aim of this review is to present an overview of B lymphocyte senescence and its underlying causes and to discuss its impact on immune function and leukemogenesis in aged individuals.

E2A protein degradation by the ubiquitin-proteasome system is stage-dependent during muscle differentiation

The E2A proteins are basic helix-loop-helix transcription factors that regulate proliferation and differentiation in many cell types. In muscle cells, the E2A proteins form heterodimers with muscle regulatory factors such as MyoD, which then bind to DNA and regulate the transcription of target genes essential for muscle differentiation. We now demonstrate that E2A proteins are primarily localized in the nucleus in both C2C12 myoblasts and myotubes, and are degraded by the ubiquitin proteasome system evidenced by stabilization following treatment with the proteasome inhibitor, MG132. During the differentiation from myoblast to myotube, the cellular abundance of E2A proteins is relatively unaltered, despite significant changes (each approximately 5-fold) in the relative rates of protein synthesis and protein degradation via the ubiquitin-proteasome system. The rate of ubiquitin-proteasome-mediated E2A protein degradation depends on the myogenic differentiation state (t 1/2 approximately 2 h in proliferating myoblasts versus t 1/2 > 10 h in differentiated myotubes), and is also associated with cell cycle in non-muscle cells. Our findings reveal an important role for both translational and post-translational regulatory mechanisms in mediating the complex program of muscle differentiation determined by the E2A proteins.

Evolving patterns of lymphopoiesis from embryogenesis through senescence

Lymphopoiesis occurs throughout adult life, but the number of lymphocytes produced is substantially reduced in old individuals. This commentary will highlight observations suggesting that common mechanisms underlie declines in both B and T cell production and that reductions in lymphopoiesis are initiated much earlier than has generally been appreciated.

Effects of Aging on the Common Lymphoid Progenitor to Pro-B Cell Transition

The number of common lymphoid progenitors (CLP) and their pre-pro-B and pro-B cell progeny is reduced in old mice, but the age-related changes responsible for these declines have not been fully elucidated. The aim of this study was to provide additional insights into the impact of senescence on early B cell development by analyzing the CLP and pro-B cell compartments under steady-state conditions and after cytoablation with 5-fluorouracil. 5-Fluorouracil subjects the hemopoietic system to acute stress and has the advantage of revealing defects in progenitors that may otherwise be subtle. The data demonstrate significant, age-related defects in the proliferative potential of early B cell precursors and suggest that the ability of CLP to differentiate into pre-pro-B cells is also compromised by senescence. These age-related changes in early B lymphopoiesis do not result from a general defect in HSC or the bone marrow microenvironment that impairs development in all hemopoietic lineages. Instead, data demonstrating that myeloid progenitor number and developmental potential do not decline with age indicate that B lymphopoiesis is particularly sensitive to defects that accumulate during senescence.

Deficient B lymphopoiesis in murine senescence: potential roles for dysregulation of E2A, Pax-5, and STAT5

B lymphopoiesis in senescent mice is typically diminished and characterized by low pre-B cell numbers. The transcription factors E2A, Pax-5, and STAT5 have been implicated in the differentiation, proliferation, and survival of B cell precursors. In this review, we discuss the impairment of B lymphopoiesis during old age in the context of mechanisms at the molecular level responsible for the handling and turnover of these key transcriptional proteins. Alterations in the expression of E2A, Pax-5, and STAT5 may affect multiple stages of B cell development, contribute to reduced B lymphopoiesis, and preface changes in the "read-out" of the BCR repertoire during murine senescence.

RNA Stability of the E2A-Encoded Transcription Factor E47 Is Lower in Splenic Activated B Cells from Aged Mice

We have demonstrated previously that DNA binding and protein expression of the E2A-encoded transcription factor E47 are lower in nuclear extracts of activated splenic B cells from old mice. In the present study, we address how E47 protein expression is regulated in aging. Results herein show that E2A mRNA levels were decreased in stimulated splenic B cells from old as compared with young mice. RNA stability assays showed that the rate of E2A mRNA decay was accelerated in stimulated splenic B cells from old mice, but E47 protein degradation rates were comparable in young vs aged B cells, indicating that the regulation of E47 expression in activated splenic B cells occurs primarily by mRNA stability. The rates of decay of other mRNAs showed that the increased mRNA degradation in aged splenic activated B cells is not a general phenomenon but restricted to a subset of mRNAs. We next investigated the signal transduction pathways controlling E2A mRNA expression and stability and found that p38 MAPK regulates E2A mRNA expression through increased mRNA stability and is down-regulated in aged activated B cells. Results show that inhibition of p38 MAPK significantly reduces E2A mRNA stability in both young and old B cells, further stressing the role of p38 MAPK in E2A RNA stabilization. These studies demonstrate that the transcription factor E2A, critical for many aspects of B cell function, is regulated by a novel mechanism in aging.

Helix-loop-helix proteins and lymphocyte development

Helix-loop-helix (HLH) proteins are transcriptional regulators that control a wide variety of developmental pathways in both invertebrate and vertebrate organisms. Results obtained in the past decade have shown that HLH proteins also contribute to the development of lymphoid lineages. A subset of HLH proteins, the 'E proteins', seems to be particularly important for proper lymphoid development. Members of the E protein family include E12, E47, E2-2 and HEB. The E proteins contribute to B lineage- and T lineage-specific gene expression programs, regulate lymphocyte survival and cellular proliferation, activate the rearrangement of antigen receptor genes and control progression through critical developmental checkpoints. This review discusses HLH proteins in lymphocyte development and homeostasis.

Linking age-related defects in B lymphopoiesis to the aging of hematopoietic stem cells

B cell genesis declines with age, but at what stage and why remains unclear. Previous studies attribute the decline in B cell production in aged mice to both environmental and cell-intrinsic defects that impact mid-to-late stream B cell precursors. However, mounting evidence suggests that the aging process may also negatively affect the earliest phases of B cell development. We review past studies on the B cells and aging question, discuss recent data suggesting that age-associated defects in B cell development reflect deficiencies in hematopoietic stem cell-proximal progenitor pools, and provide an integrative model that will hopefully facilitate further studies into this complex problem.

Bone marrow microenvironmental changes in aged mice compromise V(D)J recombinase activity and B cell generation

B cell generation and immunoglobulin (Ig) diversity in mice is compromised with aging. Our recent work sought to understand mechanism(s) that contribute to reduced B cell production in aged mice. Using in vivo labeling, we found that reduction in marrow pre-B cells reflects increased attrition during passage from the pro-B to pre-B cell pool. Analyses of reciprocal bone marrow (BM) chimeras reveal that the production rates of pre-B cells are controlled primarily by microenvironmental factors, rather than intrinsic events. To understand changes in pro-B cells that could diminish production of pre-B cells, we evaluated rag2 expression and V(D)J recombinase activity in pro-B cells at the single cell level. The percentage of pro-B cells that express rag2 is reduced in aged mice and is correlated with both a loss of V(D)J recombinase activity in pro-B cells and reduced numbers of pre-B cells. Reciprocal BM chimeras revealed that the aged microenvironment also determines rag2 expression and recombinase activity in pro-B cells. These observations suggest that extrinsic factors in the BM that decline with age are largely responsible for less efficient V(D)J recombination in pro-B cells and diminished progression to the pre-B cell stage. These extrinsic factors may include cytokines and chemokines derived from BM stromal cells that are essential to the development of B cell precursors. The changes during aging within the BM hematopoietic microenvironment most likely are linked to the physiology of aging bone. Bone degrades with age (osteoporosis) due to decreased formation of new bone by osteoblasts. Marrow stem cells (MSC) are considered the progenitor of both adipocytes, osteoblasts and hematopoietic stromal cells and a controlled reciprocal regulation exists of osteoblast versus adipocyte differentiation; with age adipocytes increase, and osteoblast decrease. It is possible that stromal cell generation from MSC is compromised during aging. Currently, understanding of BM microenvironmental factors that regulate rag gene expression is very limited. However, as early progenitors differentiate, it is increasing clear that a limited set of transcription factors (e.g. ikaros, PU.1, E2A, EBF, pax5) regulate B-lineage specific genes, and that expression and stability of these factors is responsive to the microenvironment. Current and future work by several groups will strive to understand mechanisms that regulate these factors and how aging impacts these regulatory circuits.

Dwindling competition with constant demand: Can homeostatic adjustments explain age-associated changes in peripheral B cell selection?

The close relationship between specificity-based selection and homeostatic processes in maintaining peripheral B cell pools has become increasingly evident. Thus, age-associated changes observed within these pools may reflect homeostatic responses to proximal primary lesions. Marked shifts in the size and dynamics of most B lymphocyte subsets and their progenitors occur with age: perturbations in B lineage precursors result in reduced production of immature B lymphocytes in the bone marrow and transitional pools in the periphery, but these effects appear to be offset by compensatory homeostatic processes at the marrow-periphery interface. We propose a model whereby these "distal" homeostatic adjustments relax the stringency of specificity based selection, affording a potential explanation for the increased frequency of autoreactive specificities with age.