Age-associated Defects Research Articles

Defective CD4 T cell cognate helper functions in response to influenza immunization and infection in aged mice (47.8)

Abstract A poor immune response is a hallmark of aging. Older individuals are more susceptible to infections and respond poorly to new immunization. This is caused, at least in part, by a defective antibody response. We have shown that age-associated defects impair CD4 T cell functions. Since an optimal antibody response requires good CD4 cognate help through T follicular helper (Tfh) cells, we aimed to determine whether the impaired cognate help resulted from defects in the generation and/or functions of Tfh cells in aged mice. The work presented herein shows that the generation of Tfh cells in response to influenza immunization is delayed, reduced and shortened in aged mice compared to young mice. Also fewer Tfh cells are generated in aged mice than in young mice following influenza infection. The Tfh cells generated in aged mice, however, express similar levels of IL-21, BCL-6, CXCR5 and IL-4 mRNA than Tfh cells from young mice. These data suggest that the generation rather than the functions of Tfh cells is impaired in aged mice. The addition of poly(I:C) as an adjuvant to the influenza vaccine increases Tfh generation and antibody production in young and aged mice. We are investigating the mechanisms by which TLR ligands improve the Tfh response.

Age-related alterations in mesenchymal stem cells related to shift in differentiation from osteogenic to adipogenic potential: Implication to age-associated bone diseases and defects

Mesenchymal stem cells (MSC) have attracted considerable attention in the fields of cell and gene therapy due to their intrinsic ability to differentiate into multiple lineages. The various therapeutic applications involving MSC require initial expansion and/or differentiation in vitro prior to clinical use. However, serial passages of MSC in culture lead to decreased differentiation potential and stem cell characteristics, eventually inducing cellular aging which will limit the success of cell-based therapeutic interventions. Here we review the age-related changes that occur in MSC with a special focus on the shift of differentiation potential from osteogenic to adipogenic lineage during the MSC aging processes and how aging causes this preferential shift by oxidative stress and/or energy metabolism defect. Oxidative stress-related signals and some microRNAs affect the differentiation potential shift of MSC by directly targeting key regulatory factors such as Runx-2 or PPAR-γ, and energy metabolism pathway is involved as well. All information described here including transcription factors, microRNAs and FoxOs could be used towards development of treatment regimens for age-related bone diseases and related defects based on mutually exclusive lineage fate determination of MSC.

造血制御機構の加齢変化

The hematopoietic microenvironment is composed of heterogeneous mesenchymal cells, such as osteoblasts, epithelial cells, fibroblastoid cells, adipocytes and macrophages, which are referred to as stromal cells. Stromal cells regulate hematopoietic stem cell (HSC) by producing positive- and negative-regulators to supply mature hematopoietic cells for the lifetime. It is well known that B lymphopoiesis in the bone marrow (BM) is attenuated with aging, whereas myelopoiesis is not affected. One standing issue is whether age-associated defects in B lymphopoiesis reflect intrinsic defects and/or extrinsic defects of the cells, such as stromal cell defects. Although extensive studies have revealed that age-associated intrinsic defects occur in B cell development, the age-associated stromal cell defects have not been fully elucidated. Since B lymphopoiesis in the BM is more dependent on stromal cells than the other lineages, B lymphopoiesis is a good model to investigate the effect of aging on stromal cells. Senescence accelerated mice (SAMP1) exhibit premature senescence-like stromal cell impairment after 30 weeks of age. Thus, this model mouse is a useful tool to clarify the role of stromal cells during the development of senescence-associated defects in B lymphopoiesis. In the steady state, SAMP1 mice exhibit simultaneous down-regulation of positive- and negative-regulators of B lymphopoiesis in the BM during premature aging, resulting in suppressive homeostasis of B cell development. While both regulators are down-regulated, the relative cytokine levels are barely maintained in the steady-state. Under perturbed conditions induced by 5-fluorouracil or irradiation, aged SAMP1 mice exhibit prolonged dysregulation of cytokine production, resulting in a further diminution of B lymphopoiesis. These results suggest that, in part, age-related deterioration of B cell development may be due to functional impairment of stromal cells, which induces vicious suppressive homeostasis of B cell development. Taken together, age-associated changes in hematopoiesis seem to be due not only to intrinsic-defects but also to extrinsic defects of hematopoietic cells.

Advanced Age Impairs Macrophage Polarization

Aging affects many aspects of the cellular function of macrophages. Macrophages play a critical role in innate immunity, acting as sentinels to fight pathogens, promoting wound healing, and orchestrating the development of the specific acquired immune response. However, little is known about how age influences the ability of macrophage to change phenotypes in response to environmental factors. This study examined the age-associated defects on macrophage polarization toward a pro-inflammatory (M1) or an anti-inflammatory (M2) phenotype. Adherent splenocytes enriched for macrophages were cultured with or without lipopolysaccharide (LPS), a combination of interferon (IFN)-γ and tumor necrosis factor (TNF)-α or interleukin (IL)-4. A panel of M1 markers, inducible nitric oxide synthase (iNOS), IL-6, IL-1β, and TNF-α, and M2 markers, including arginase-1 (Arg1), Ym1, and Found In Inflammatory Zone 1 (FIZZ1), were analyzed. IL-6 mRNA in cells from aged mice was decreased by 78% and 58% compared with young after stimulation with LPS or IFN-γ and TNF-α (P<0.05), respectively. Also, there was a marked reduction in the induced levels of iNOS, IL-1β, and TNF-α in cells from aged mice relative to young controls. Similarly, IL-4 exposure resulted in a reduction of M2 markers in adherent splenocytes from aged mice compared with younger animals. This was consistent with a 28% decrease in splenic F4/80(+)IL-4R(+) cells in aged mice relative to controls, although IL-4R expression on these cells did not vary between age groups. In contrast, levels of M1 and most M2 markers, save for FIZZ1, in bone marrow-derived macrophages were similar between the age groups, irrespective of stimuli. These data imply that impaired macrophage polarization in the elderly may dysregulate the development of the host response, making them more susceptible to infectious diseases and that the aging microenvironment may be a key modulator of these macrophage-elicited responses.

Age-Associated Reduction of the Count and Functional Activity of Stromal Precursor Cells Can Be Caused by Both True Reduction (Exhaustion) of Cell Pool and Regulatory Effects of the Organism

The study was carried out on CBA mice using the method of heterotopic transplantation. A fragment of the femoral bone marrow (1/2) or spleen (1/5 of the organ) was transplanted under the renal capsule of a recipient. The following donor-recipient cross-transplantation variants were studied: young-young (Y-Y), young-old (Y-O), old-old (O-O), and old-young (O-Y). Cell suspensions were prepared from 2-month transplants inoculated in monolayer cultures and the cloning efficiency (ECF-F) of stromal precursor cells (CFC-F) was evaluated. The bone marrow transplant ECF-F and the count of CFC-F in the O-O group were 8-fold lower than in the Y-Y group. In the O-Y group, ECF-F was 3-fold higher than in the O-O group, but by 2.5 times lower than in the Y-Y group. ECF-F in Y-O group was 2-fold lower than in Y-Y group. The ECF-F and CFC-F count in spleen transplants in the O-O group were 4- and 6-fold lower, respectively, than in Y-Y group. However, in O-Y group ECF-F was 7-fold higher than in O-O group and higher than even in Y-Y group. The weight of induced ectopic bone tissue after transplantation of the osteoinductor (fragments of the allogenic urinary bladder mucosa) was 2-fold lower in the O-O vs. Y-Y group. However, comparison of the ectopic bone tissue weights in different experimental groups showed that osteoinductor activity of the bladder epithelium did not decrease, but increased 3-fold with age (O-Y:Y-Y). A 5-fold reduction of this proportion in groups where the osteoinductor was transplanted from old donors to old and young recipients (O-Y:O-O) could be attributed to age-specific reduction of the count of inducible osteogenic precursor cells (IOPC). The data in general suggest that age-specific reduction of the stromal precursor count and functional activity could be caused by the true reduction (exhaustion) of cell pool (bone marrow CFC-F; presumably, IOPC) and by the regulatory effects of the organism (bone marrow and splenic CFC-F, IOPC). These data seem to be significant for understanding of the role of osteogenic stromal precursor cells in the development of age-associated bone tissue defects, for example, senile osteoporosis.

Role of phosphoinositide 3-kinase–Akt signaling pathway in the age-related cytokine dysregulation in splenic macrophages stimulated via TLR-2 or TLR-4 receptors

Age-associated defects in both B-lymphocytes and macrophages in elderly result in a reduction in the efficacy of vaccines to many Gram positive bacteria like Streptococcus pneumoniae. Splenic macrophages from aged mice have been shown to have a defect in production of pro-inflammatory cytokines (IL-6, IL-12, IL-1β, TNF-α) but exhibit increased production of IL-10 upon TLR-4 ligation. Here we showed that aged macrophages demonstrate similar cytokine dysregulation phenotype upon stimulation with TLR-2 ligands, or killed S. pneumoniae. We hypothesized that an age-associated increase in activity of phosphatidyl inositol 3-kinase (PI3K)-Akt signaling pathway may be playing a causal role in the age-associated cytokine dysregulation. We found that gene expression of both the regulatory (p85β) and the catalytic (p110δ) subunits of Class IA PI3K is higher in aged than in young splenic macrophages. The age-associated increase in the activity of PI3K was also demonstrated by an upregulation of P-Akt and its downstream target, glycogen synthase kinase-3 (GSK-3). Inhibition of PI3K enhanced induction of pro-inflammatory cytokines, by TLR-2/TLR-1, TLR-2/TLR-6 and TLR-4 ligands as well as heat killed S. pneumoniae (HKSP). Therefore, targeting PI3-Kinase could rescue cytokine dysregulation in aged macrophages and enhance the relevant pro-inflammatory cytokines needed to support B-cell activation and differentiation.

TLR-Activated Dendritic Cells Enhance the Response of Aged Naive CD4 T Cells via an IL-6–Dependent Mechanism

The most effective immunological adjuvants contain microbial products, such as TLR agonists, which bind to conserved pathogen recognition receptors. These activate dendritic cells (DCs) to become highly effective APCs. We assessed whether TLR ligand-treated DCs can enhance the otherwise defective response of aged naive CD4 T cells. In vivo administration of CpG, polyinosinic-polycytidylic acid, and Pam(3)CSK(4) in combination with Ag resulted in the increased expression of costimulatory molecules and MHC class II by DCs, increased serum levels of the inflammatory cytokines IL-6 and RANTES, and increased cognate CD4 T cell responses in young and aged mice. We show that, in vitro, preactivation of DCs by TLR ligands makes them more efficient APCs for aged naive CD4 T cells. After T-DC interaction, there are enhanced production of inflammatory cytokines, particularly IL-6, and greater expansion of the aged T cells, resulting from increased proliferation and greater effector survival with increased levels of Bcl-2. TLR preactivation of both bone marrow-derived and ex vivo DCs improved responses. IL-6 produced by the activated DCs during cognate T cell interaction was necessary for enhanced aged CD4 T cell expansion and survival. These studies suggest that some age-associated immune defects may be overcome by targeted activation of APCs by TLR ligands.

Bim Dictates Naive CD4 T Cell Lifespan and the Development of Age-Associated Functional Defects

With age, peripheral naive CD4 T cells become both longer lived and functionally impaired and they express reduced levels of Bim, a proapoptotic Bcl family member. In this study, we show that reduced Bim expression by naive CD4 T cells intrinsically mediates their longer lifespan in the periphery. Moreover, using mixed bone marrow chimeras reconstituted with Bim(+/+) and Bim(+/-) bone marrow cells, Bim(+/-) naive CD4 T cells exhibit accelerated development of age-associated dysfunctions, including reduced proliferation and IL-2 production and defective helper function for B cells, without any increase in their turnover. However, newly generated Bim(+/-) naive CD4 T cells in middle-aged mice are not defective, indicating an additional requirement for their persistence in the periphery. These age-associated immune defects develop independently of the "aged" host environment and without extensive division, distinguishing them from classic "senescence." We suggest that the reduction of Bim levels with age in naive CD4 T cell is the initiating step that leads to increased cellular lifespan and development of age-associated functional defects.

Age-related deregulation of naive T cell homeostasis in elderly humans

Immunosenescence is characterized by phenotypic and functional changes of effector memory T cells. In spite of the well-described senescent defects of these experienced T cells, immune responses to new pathogens are also deeply affected in elderly humans, suggesting that naive T cells could also show age-related defects. It has been reported in both, animal models and humans, alterations of the naive T cell turnover associated to advanced age or low thymic function. However, as far as we know, homeostatic mechanisms involved in the deregulation of naive T cell peripheral dynamics and their consequences are still not well understood. Thus, the aim of our study was to analyze homeostatic parameters of peripheral naive T cells and their relationship with thymic function in young and elderly humans. Our results show that lower naive T cell numbers were associated with a lower thymic function and higher activation and proliferating naive T cell levels. We then analyzed sjTREC numbers and relative telomere length from sorted naive T cells. Our results show that the aberrant activation and proliferation status was related to lower sjTREC numbers (a peripheral proliferation marker) and both, higher CD57 expression levels and shortened telomeres (replicative senescence-related markers). Elderly individuals show a greater contraction of the CD8 naive T cell numbers and all homeostatic alterations were more severe in this compartment. In addition, we found that low functional thymus show a CD4-biased thymocyte production. Taken together, our results suggest a homeostatic deregulation, affecting mostly the naive CD8 T cell subset, leading to the accumulation of age-associated defects in, otherwise, phenotypically naive T cells.

The role of inflammatory and fibrogenic pathways in heart failure associated with aging

Heart failure is strongly associated with aging. Elderly patients with heart failure often have preserved systolic function exhibiting left ventricular hypertrophy accompanied by a decline in diastolic function. Experimental studies have demonstrated that age-related cardiac fibrosis plays an important role in the pathogenesis of diastolic heart failure in senescent hearts. Reactive oxygen species and angiotensin II are critically involved in fibrotic remodeling of the aging ventricle; their fibrogenic actions may be mediated, at least in part, through transforming growth factor (TGF)-beta. The increased prevalence of heart failure in the elderly is also due to impaired responses of the senescent heart to cardiac injury. Aging is associated with suppressed inflammation, delayed phagocytosis of dead cardiomyocytes, and markedly diminished collagen deposition following myocardial infarction, due to a blunted response of fibroblasts to fibrogenic growth factors. Thus, in addition to a baseline activation of fibrogenic pathways, senescent hearts exhibit an impaired reparative reserve due to decreased responses of mesenchymal cells to stimulatory signals. Impaired scar formation in senescent hearts is associated with accentuated dilative remodeling and worse systolic dysfunction. Understanding the pathogenesis of interstitial fibrosis in the aging heart and dissecting the mechanisms responsible for age-associated healing defects following cardiac injury are critical in order to design new strategies for prevention of adverse remodeling and heart failure in elderly patients.

Too old to fight? Aging and its toll on innate immunity

Elderly individuals display increased susceptibility to chronic inflammatory diseases and microbial infections, such as periodontitis and oral aspiration pneumonia. The resurgent interest in innate immunity in the 2000s has been accompanied by parallel studies to understand the impact of aging on the function of the innate immune system, which not only provides first-line defense but is essential for the development of adaptive immunity. This review summarizes and discusses our current understanding of age-associated molecular alterations in neutrophils and macrophages, key inflammatory phagocytes implicated in both protective and destructive host responses. The analysis of recent literature suggests that, in advanced age, phagocytes undergo significant changes in signal transduction pathways that may affect their ability to perform antimicrobial functions or regulate the inflammatory response. These abnormalities are expected to contribute to the pathology of oral infection-driven inflammatory diseases in the elderly. Moreover, the elucidation of age-associated defects in the innate immune system will facilitate the development of intervention therapeutic strategies to promote or restore innate immune function and improve the quality of health in old age.

Elevated Mitochondrial Superoxide Generation Causes Early Onset of Ageassociated Disorders and Adult Stem Cell Defects

Elevated Mitochondrial Superoxide Generation Causes Early Onset of Ageassociated Disorders and Adult Stem Cell Defects

Effects of Aging on the Adaptive Immune Response to Respiratory Virus Infections

Severe acute respiratory disease caused by respiratory virus infections in individuals aged 65 years and older and in high-risk adults, such as those with chronic cardiopulmonary disorders, is associated with increased hospitalization and mortality rates. Epidemiological studies have identified influenza virus and respiratory syncytial virus as the most frequent causes of virus-induced respiratory disease in elderly and high-risk adults. Studies in both humans and animal models have established fundamental defects in cell-mediated and humoral immune responses in aged individuals. However, it is not well understood how age specifically alters the immune response to respiratory pathogens. In this review, we will focus our discussion on the major causative agents of severe respiratory virus infections in elderly and high-risk adults and the age-associated defects in the immune response that probably contribute to the increased disease severity observed in these populations.

Age-associated defects in Tcell metabolism (46.10)

Abstract Aging at the cellular and molecular level causes a decline in immune function. We are characterizing the responses of naïve T-cells to activation. Activated T-cells of aged mice initiate proliferation as well as their young counterparts, but have increased apoptotic rates. However, the mechanisms behind this post- activation death are not completely understood. We will show alterations in metabolic activities in T-cells of aged mice through differences in the expression of glucose transporter (Glut 1), glucose uptake and lactate production. Additionally, we will correlate these findings with autophagocytic vesicle formation by analyzing microtubule-associated protein -1 light chain 3 (LC3) as well as comparing relative amounts of actively respiring mitochondria using Mitotracker Red CM-H2XRos, a membrane potential dependant dye, and total mitochondrial mass by staining with the mitochondrial-selective fluorescence label Mitotracker Green. Levels of reactive oxygen species will be then quantified by staining with ROS specific dye DCF-DA and correlated to the ratio of mitochondrial mass to membrane potential.

“Functional mapping of the promoter region of the GNB2L1 human gene coding for RACK1 scaffold protein”

RACK1 (Receptor for Activated C Kinase 1) is a scaffold protein for different kinases and membrane receptors. Previously, we characterized an age-dependent decline of RACK1 protein expression which could be counteracted with DHEA (dehydroepiandrosterone) [Corsini, E., et al. 2002. In vivo dehydroepiandrosterone restores age-associated defects in the protein kinase C signal transduction pathway and related functional responses. J. Immunol. 168, 1753–1758. and Corsini, E., et al. 2005. Age-related decline in RACK-1 expression in human leukocytes is correlated to plasma levels of dehydroepiandrosterone. J. Leukoc. Biol. 77, 247–256.]. Hypothesizing a direct control of RACK1 expression by DHEA we studied the not yet characterized human promoter region of its coding gene GNB2L1. The FLOE (Fluorescently Labeled Oligonucleotide Extension) was used to map the transcription start site and a novel Gateway luciferase vector (GW luc basic; Del Vecchio, I., Zuccotti, A., Canneva, F., Lenzken, S.C., Racchi, M., 2007. Development of the first Gateway firefly luciferase vector and use of reverse transcriptase in FLOE (Fluorescently Labeled Oligonucleotide Extension) reactions. Plasmid 58, 269–274.) to obtain promoter region mutants. Human SH-SY5Y, THP1 and lymphoblastoid cells were used for transient transfections and treatments with lipopolysaccharide (LPS), phorbol myristate acetate (PMA), DHEA and cortisol (the first two molecules to differently activate NF-kB, a transcription complex able to regulate the murine Gnb2l1 gene expression, whereas DHEA and cortisol since they are known to be imbalanced during the aging and possess counteracting actions on the immune function). The primer extension demonstrated the existence of two alternative start sites of transcription respectively located at about 230 and 300 nt 5′ of the Genbank mRNA entry for GNB2L1. Moreover, as a result of the luciferase study we were able to demonstrate that a little region of approximately 300 nt conserved sufficient elements for reporter expression. We also reported that the DHEA modulation of GNB2L1 endogenous expression could not be recapitulated with the luciferase assays. Indeed, the promoter was significantly modulated by means of LPS and PMA treatments but not using DHEA. Differently the use of cortisol led us to demonstrate a biologically significant decrease of luciferase activity only in the presence of a binding site for nuclear receptors of glucocorticoids. Interestingly, other binding sites for transcriptional factors were identified in silico: different c-Rel (NF-kB) and some cardiomyocitic specific cis-acting elements. All this data suggest that the DHEA mediated GNB2L1 regulation is modulated by distant elements (enhancers/silencers), whereas LPS, PMA and cortisol effect can act directly on the mapped GNB2L1 promoter. In conclusion we hypothesize that the imbalance between DHEA and cortisol during aging could be important in the previously demonstrated recovery of the RACK1 expression.

Mononuclear Phagocyte-Derived IL-10 Suppresses the Innate IL-12/IFN-γ Axis in Lung-Challenged Aged Mice

Previously, we reported that IL-10-producing mononuclear phagocytes increase in lungs of aged mice, causing impaired innate cytokine expression. Since dendritic cells (DCs) contribute to innate NK cell and adaptive T cell immunity, we tested the hypothesis that age-related IL-10 might influence DC function with effects on NK and T cell activation. The results showed that DC recruitment to sites of lung inflammation was normal in aged mice (>20 mo). However, IFN-gamma-producing NK cells in LPS-challenged lungs were decreased in aged as compared with young mice, which was associated with increased IL-10(+)CD11b(+)Gr-1(low)CD11c(-) cells consistent with mononuclear phagocytes. In vivo or in vitro blockade of IL-10 signaling restored IFN-gamma-producing NK cells. This restoration was reversed by IL-12 neutralization, indicating that IL-10 suppressed sources of IL-12 in aged mice. To probe DC function in adaptive immunity, we transferred young naive OVA-specific TCR transgenic T cells to old mice. Following challenge with OVA plus LPS, Ag presentation in the context of MHC-I and MHC-II occurred with similar kinetics and intensity in draining lymph nodes of young and old recipients as measured by proliferation. Despite this, aged hosts displayed impaired induction of IFN-gamma(+)CD4(+), but not IFN-gamma(+)CD8(+), effector T cells. Blockade of IL-10 signaling reversed age-associated defects. These studies indicate that the innate IL-12/IFN-gamma axis is not intrinsically defective in lungs of aged mice, but is rather suppressed by enhanced production of mononuclear phagocyte-derived IL-10. Our data identify a novel mechanism of age-associated immune deficiency.

Characterization of Mandibular Bones in Senile Osteoporotic Mice

At present, little is known about the age-related changes in jaw bones. The aim of this study was to characterize the mandibles of 6 month-old senile osteoporotic mice, SAMP6, and compare with those of age-matched controls, SAMR1. In comparison to SAMR1, SAMP6 showed thinner cortical bone, lower bone volume, and poorly organized collagen matrix. The collagen fibril diameter in SAMP6 was significantly smaller than that of SAMR1. In SAMP6 both collagen content and cross-links were lower than those of SAMR1, but the ratio of the major mature cross-link (pyridinoline) to its precursor reducible cross-link (dehydrodihydroxylysinonorleucine/its ketoamine) was higher in comparison to SAMR1. In addition, the extent of lysine hydroxylation of collagen was higher in SAMP6 than that of SAMR1. These results indicate that not only the quantity of collagen but also its quality are altered in SAMP6 and may result in the age-associated osteoporotic defects of mandibles.

Environmental and Intrinsic Factors Lead to Antigen Unresponsiveness in CD4+ Recent Thymic Emigrants from Aged Mice

Naive CD4 cells from aged mice respond inefficiently to Ag, but the factors that underlie the age-associated defects remain unclear. We have used two approaches to isolate recent thymic emigrants (RTE) in young and aged mice and have compared their capacity to respond to antigenic stimulation ex vivo. An in situ intrathymic CFSE injection labeled developing thymocytes and allowed the identification of RTE in secondary lymphoid tissues. Analysis of CFSE-labeled RTE and control unlabeled naive CD4 cells indicated that cells from aged mice were defective in their ability to increase intracellular Ca(2+) concentration following TCR cross-linking. Aged naive and RTE CD4 also secreted less IL-2 and proliferated less than that of comparable young CD4 populations. Defects in effector generation in aged RTE were overcome by the addition of IL-2 to cultures. RTE from both polyclonal and TCR transgenic mice were compromised, indicating that defects were independent of TCR specificity. In the second model, the cotransfer of congenic marker-labeled young and aged BM cells into young and aged syngeneic hosts revealed that hyporesponsiveness in aged RTE was caused by a combination of defects intrinsic to CD4 progenitors and defects induced by the aged environment. Depletion of peripheral CD4 cells in aged mice led to production of new RTE that were not defective. The results of this study suggest that defects induced by environmental and lineage intrinsic factors act together to reduce responses to Ag in aged naive CD4 cells and that these defects can be overcome in aged CD4 cells produced during recovery from lymphopenia.

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.

Downregulation of renal vasopressin V2 receptor and aquaporin-2 expression parallels age-associated defects in urine concentration.

Renal concentrating ability is known to be impaired with aging. The antidiuretic hormone AVP plays an important role in renal water excretion by regulating the membrane insertion and abundance of the water channel aquaporin-2 (AQP2); this effect is primarily mediated via the V2 subtype of the AVP receptor (V2R). This study evaluated the hypothesis that decreased renal sensitivity to AVP, with subsequent altered renal AQP2 expression, contributes to the reduced urinary concentrating ability with aging. Our results show that under baseline conditions, urine osmolality is significantly lower in aged Fischer 344 and Brown-Norway F1 hybrid (F344BN) rats despite equivalent plasma AVP concentrations as in young rats. Levels of kidney V2R mRNA expression and AQP2 abundances were also significantly decreased in aged F344BN rats, as was AQP2 immunostaining in collecting duct cells. In response to moderate water restriction, urine osmolality increased by significantly lesser amounts in aged F344BN rats compared with young rats despite similar increases in plasma AVP levels. Moderate water restriction induced equivalent relative increases in renal AQP2 abundances in all age groups but resulted in significantly lower abundances in total kidney AQP2 protein in aged compared with young F344BN rats. These results therefore demonstrate a functional impairment of renal concentrating ability in aged F344BN rats that is not due to impaired secretion of AVP but rather appears to be related to impaired responsiveness of the kidney to AVP that is secondary, at least in part, to a downregulation of renal V2R expression and AQP2 abundance.