Effects of aging on proliferation and E47 transcription factor activity induced by different stimuli in murine splenic B cells

Aging and Interferons: Impacts on Inflammation and Viral Disease Outcomes

As highlighted by the COVID-19 global pandemic, elderly individuals comprise the majority of cases of severe viral infection outcomes and death. A combined inability to control viral replication and exacerbated inflammatory immune activation in elderly patients causes irreparable immune-mediated tissue pathology in response to infection. Key to these responses are type I, II, and III interferons (IFNs), which are involved in inducing an antiviral response, as well as controlling and suppressing inflammation and immunopathology. IFNs support monocyte/macrophage-stimulated immune responses that clear infection and promote their immunosuppressive functions that prevent excess inflammation and immune-mediated pathology. The timing and magnitude of IFN responses to infection are critical towards their immunoregulatory functions and ability to prevent immunopathology. Aging is associated with multiple defects in the ability of macrophages and dendritic cells to produce IFNs in response to viral infection, leading to a dysregulation of inflammatory immune responses. Understanding the implications of aging on IFN-regulated inflammation will give critical insights on how to treat and prevent severe infection in vulnerable individuals. In this review, we describe the causes of impaired IFN production in aging, and the evidence to suggest that these impairments impact the regulation of the innate and adaptive immune response to infection, thereby causing disease pathology.

Immune system and COVID-19 by sex differences and age

In COVID-19 disease, are reported gender differences in relation to severity and death. The aim of this review is to highlight gender differences in the immune response to COVID-19. The included studies were identified using PubMed, until 30 October 2020. The search included the following keywords: SARS-CoV-2, COVID-19, gender, age, sex, and immune system. Literature described that females compared to males have greater inflammatory, antiviral, and humoral immune responses. In female, estrogen is a potential ally to alleviate SARS-COV-2 disease. In male, testosterone reduces vaccination response and depresses the cytokine response. In the older patients, and in particular, in female older patients, it has been reported a progressive functional decline in the immune systems. Differences by gender were reported in infection diseases, including SARS-CoV-2. These data should be confirmed by the other epidemiological studies.

Immunesenescence: A Predisposing Risk Factor for the Development of COVID-19?

Bearing a strong resemblance to the phenotypic and functional remodeling of the immune system that occurs during aging (termed immunesenescence), the immune response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of Coronavirus disease 2019 (COVID-19), is characterized by an expansion of inflammatory monocytes, functional exhaustion of lymphocytes, dysregulated myeloid responses and the presence of highly activated senescent T cells. Alongside advanced age, male gender and pre-existing co-morbidities [e.g., obesity and type 2 diabetes (T2D)] are emerging as significant risk factors for COVID-19. Interestingly, immunesenescence is more profound in males when compared to females, whilst accelerated aging of the immune system, termed premature immunesenescence, has been described in obese subjects and T2D patients. Thus, as three distinct demographic groups with an increased susceptibility to COVID-19 share a common immune profile, could immunesenescence be a generic contributory factor in the development of severe COVID-19? Here, by focussing on three key aspects of an immune response, namely pathogen recognition, elimination and resolution, we address this question by discussing how immunesenescence may weaken or exacerbate the immune response to SARS-CoV-2. We also highlight how aspects of immunesenescence could render potential COVID-19 treatments less effective in older adults and draw attention to certain therapeutic options, which by reversing or circumventing certain features of immunesenescence may prove to be beneficial for the treatment of groups at high risk of severe COVID-19.

Strain specificities in influence of ageing on germinal centre reaction to inactivated influenza virus antigens in mice: Sex-based differences

Considering variability in vaccine responsiveness across human populations, in respect to magnitude and quality, and importance of vaccines in the elderly, the influence of recipient genetic background on the kinetics of age-related changes in the serum IgG antibody responses to seasonal trivalent inactivated split-virus influenza bulk (TIV) was studied in BALB/c and C57BL/6 mice showing quantitative and qualitative differences in this responses in young adult ages. With ageing the total serum IgG response to influenza viruses declined, in a strain-specific manner, so the strain disparity observed in young adult mice (the greater magnitude of IgG response in BALB/c mice) disappeared in aged mice. However, the sexual dimorphisms in this response (more prominent in females of both strains) remained in aged ones. The strain-specific differences in age-related decline in the magnitude of IgG response to TIV correlated with the number of germinal centre (GC) B splenocytes. The age-related decline in GC B cell number was consistent with the decrease in the proliferation of B cells and CD4+ cells in splenocyte cultures upon restimulation with TIV. Additionally, the age-related decrease in the magnitude of IgG response correlated with the increase in follicular T regulatory (fTreg)/follicular T helper (fTh) and fTreg/GC B splenocyte ratios (reflecting decrease in fTh and GC B numbers without changes in fTreg number), and the frequency of CD4+ splenocytes producing IL-21, a key factor in balancing the B cell and fTreg cell activity. With ageing the avidity of virus influenza-specific antibody increased in females of both strains. Moreover, ageing affected IgG2a/IgG1 and IgG2c/IgG1 ratios (reflecting Th1/Th2 balance) in male BALB/c mice and female C57BL/6 mice, respectively. Consequently, differently from young mice exhibiting the similar ratios in male and female mice, in aged female mice of both strains IgG2a(c)/IgG1 ratios were shifted towards a less effective IgG1 response (stimulated by IL-4 cytokines) compared with males. The age-related alterations in IgG subclass profiles in both strains correlated with those in IFN-γ/IL-4 production level ratio in splenocyte cultures restimulated with TIV. These findings stimulate further research to formulate sex-specific strategies to improve efficacy of influenza vaccine in the elderly.

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.

[Premature immunosenescence in triple-transgenic mice for Alzheimer's disease]

INTRODUCTION

A deterioration of the neuroimmunoendocrine network has been observed in Alzheimer's disease (AD). However, the peripheral immune response has hardly been investigated in this pathology. Since some immune function parameters have been established as good markers of the rate of ageing, and can predict longevity, the aim of the present work was to study some of these functions in splenic leucocytes in transgenic mice for AD of different ages.

MATERIAL AND METHODS

Young female (4 ± 1 months), adult (9 ± 1 months), and mature (12 ± 1 months) triple-transgenic mice for AD (3 xTgAD) and non-transgenic (NTg) control mice of the same ages were used. The chemotaxis, the anti-tumour activity of « natural killer » (NK) cells and the lymphoproliferative response in the presence of the mitogens concanavalin A and lipopolysaccharide, functions that decrease with age, were determined in splenic leucocytes. In addition, the differences in lifespan between 3 xTgAD and NTg were studied in parallel using other animals, until their death through natural causes.

RESULTS

In 3 xTgAD, with respect to NTg, chemotaxis decreased at all ages studied, whereas in lymphoproliferative response this reduction was shown at 4 months and 9 months. NK activity was diminished only in young 3 xTgAD with respect to NTg. The 3 xTgAD showed a shorter lifespan than the NTg control group.

CONCLUSIONS

The 3 xTgAD mice show a premature immunosenescence, which could explain their early mortality. The determination of these immune functions at peripheral level could serve as a marker of the progression of the Alzheimer's disease.

An age-related numerical and functional deficit in CD19(+) CD24(hi) CD38(hi) B cells is associated with an increase in systemic autoimmunity

Autoimmunity increases with aging indicative of reduced immune tolerance, but the mechanisms involved are poorly defined. In recent years, subsets of B cells with immunoregulatory properties have been identified in murine models of autoimmune disorders, and these cells downregulate immune responses via secretion of IL10. In humans, immature transitional B cells with a CD19(+) CD24(hi) CD38(hi) phenotype have been reported to regulate immune responses via IL10 production. We found the frequency and numbers of CD19(+) CD24(hi) CD38(hi) cells were reduced in the PBMC pool with age. IL10 expression and secretion following activation via either CD40, or Toll-like receptors was also impaired in CD19(+) CD24(hi) CD38(hi) B cells from healthy older donors. When investigating the mechanisms involved, we found that CD19(+) CD24(hi) CD38(hi) B-cell function was compromised by age-related effects on both T cells and B cells: specifically, CD40 ligand expression was lower in CD4 T cells from older donors following CD3 stimulation, and signalling through CD40 was impaired in CD19(+) CD24(hi) CD38(hi) B cells from elders as evidenced by reduced phosphorylation (Y705) and activation of STAT3. However, there was no age-associated change in expression of costimulatory molecules CD80 and CD86 on CD19(+) CD24(hi) CD38(hi) cells, suggesting IL10-dependent immune suppression is impaired, but contact-dependent suppressive capacity is intact with age. Finally, we found a negative correlation between CD19(+) CD24(hi) CD38(hi) B-cell IL10 production and autoantibody (Rheumatoid factor) levels in older adults. We therefore propose that an age-related decline in CD19(+) CD24(hi) CD38(hi) B cell number and function may contribute towards the increased autoimmunity and reduced immune tolerance seen with aging.

Limited efficacy of inactivated influenza vaccine in elderly individuals is associated with decreased production of vaccine-specific antibodies

During seasonal influenza epidemics, disease burden is shouldered predominantly by the very young and the elderly. Elderly individuals are particularly affected, in part because vaccine efficacy wanes with age. This has been linked to a reduced ability to induce a robust serum antibody response. Here, we show that this is due to reduced quantities of vaccine-specific antibodies, rather than a lack of antibody avidity or affinity. We measured levels of vaccine-specific plasmablasts by ELISPOT 1 week after immunization of young and elderly adults with inactivated seasonal influenza vaccine. Plasmablast-derived polyclonal antibodies (PPAbs) were generated from bulk-cultured B cells, while recombinant monoclonal antibodies (re-mAbs) were produced from single plasmablasts. The frequency of vaccine-specific plasmablasts and the concentration of PPAbs were lower in the elderly than in young adults, whereas the yields of secreted IgG per plasmablast were not different. Differences were not detected in the overall vaccine-specific avidity or affinity of PPAbs and re-mAbs between the 2 age groups. In contrast, reactivity of the antibodies induced by the inactivated seasonal influenza vaccine toward the 2009 pandemic H1N1 virus, which was not present in the vaccine, was higher in the elderly than in the young. These results indicate that the inferior antibody response to influenza vaccination in the elderly is primarily due to reduced quantities of vaccine-specific antibodies. They also suggest that exposure history affects the cross-reactivity of vaccination-induced antibodies.

Alterations in marginal zone macrophages and marginal zone B cells in old mice

Marginal zones (MZs) are architecturally organized for clearance of and rapid response against blood-borne Ags entering the spleen. MZ macrophages (MZMs) and MZ B cells are particularly important in host defense against T-independent pathogens and may be crucial for the prevention of diseases, such as streptococcal pneumonia, that are devastating in older patients. Our objective was to determine whether there are changes in the cellular components of the MZ between old and young mice. Using immunocytochemistry and a blinded scoring system, we observed gross architectural changes in the MZs of old mice, including reduction in the abundance of MZMs surrounding the MZ sinus as well as disruptions in positioning of mucosal addressin cell adhesion molecule 1 (MAdCAM-1)(+) sinus lining cells and metallophilic macrophages. Loss of frequency of MZMs was corroborated by flow cytometry. A majority of old mice also showed reduced frequency of MZ B cells, which correlated with decreased abundance of MZM in individual old mice. The spleens of old mice showed less deposition of intravenously injected dextran particles within the MZ, likely because of the decreased frequency in MZMs, because SIGN-R1 expression was not reduced on MZM from old mice. The phagocytic ability of individual MZMs was examined using Staphylococcus aureus bioparticles, and no differences in phagocytosis were found between macrophages from young or old spleens. In summary, an anatomical breakdown of the MZ occurs in advanced age, and a reduction in frequency of MZM may affect the ability of the MZM compartment to clear blood-borne Ags and mount proper T-independent immune responses.

B-cell responses to vaccination at the extremes of age

Infants and the elderly share a high vulnerability to infections and therefore have specific immunization requirements. Inducing potent and sustained B-cell responses is as challenging in infants as it is in older subjects. Several mechanisms to explain the decreased B-cell responses at the extremes of age apply to both infants and the elderly. These include intrinsic B-cell limitations as well as numerous microenvironmental factors in lymphoid organs and the bone marrow. This Review describes the mechanisms that shape B-cell responses at the extremes of age and how they could be taken into account to design more effective immunization strategies for these high-risk age groups.

A role for Fli-1 in B cell proliferation: implications for SLE pathogenesis

Transgenic overexpression of Fli-1 in normal mice leads to SLE-like disease and increased expression was reported in SLE-affected human and murine lymphocytes. Reducing Fli-1 expression in MRL/lpr mice decreased antibody production, proteinuria, renal pathology, and mortality. Compared to those with wild-type expression of Fli-1, we report here that proliferative responses of Fli-1-deficient naïve B cells to several mitogens were reduced in lupus-prone and control mice. Expression of mitogen receptors, including BCR, TLR4, and TLR9, was not significantly impacted in Fli-1-deficient naïve B cells. IL12a transcripts were upregulated and NFAT transcripts were downregulated in Fli-1-deficient MRL/lpr B cells. These results demonstrate that Fli-1 deficiency affects B cell proliferative responses to mitogens, independent of BCR and TLR expression. IL12a and NFAT, known to influence proliferation, were identified as potential mediators of this effect. This may be a mechanism by which overexpression of Fli-1 contributes to B cell hyperactivity and subsequent SLE pathogenesis.

[Immunological markers of ageing]

Ageing is a process involving morphological and physiological modifications that gradually appear with time and lead to death. Given the heterogeneous nature of the process among individuals and among the different organs, tissues, and systems in the same individual, the concept of <<biological age>> has been developed. The search for parameters that enable us to evaluate biological age--and therefore longevity--and the analysis of the efficacy of strategies to retard the ageing process are the objectives of gerontology. At present, one of the most important theories of ageing is the <<oxidative-inflammatory>> theory. Given that immune cell function is an excellent marker of health, we review the concepts that enable different functional and oxidative stress parameters in immune cells to be identified as markers of biological age and longevity. None of these parameters is universally accepted as a biomarker of ageing, although they are becoming increasingly important.

Perichromatin fibrils as early markers of transcriptional alterations

Perichromatin fibrils represent the morphological expression of transcription and co-transcriptional processing of pre-mRNA. They can be considered, hence, an example of work in progress. High resolution techniques such as electron microscopy demonstrate that perichromatin fibrils play a role as early markers of transcriptional alterations. In this paper, we review some experimental and physiological conditions impairing or modulating transcription as well as their effects on perichromatin fibrils. In all the situations reported, perichromatin fibrils show modifications in their amount and/or their associated proteins. Their movements are also affected, as well as their export or their intra-nuclear storage forms. Perichromatin fibrils therefore represent highly sensitive markers not only for monitoring transcriptional and processing rate but also for identifying the maturation level of pre-mRNA/mRNA occurring in the cell nucleus and the functional correlation with the cellular metabolic state.

Tristetraprolin, a negative regulator of mRNA stability, is increased in old B cells and is involved in the degradation of E47 mRNA

We have previously shown that the E2A-encoded transcription factor E47, which regulates class switch in splenic B cells, is down-regulated in old B cells, due to increased E47 mRNA decay. At least part of the decreased stability of E47 mRNA seen in aged B cells is mediated by proteins. We have herein looked at the specific proteins responsible for the degradation of the E47 mRNA and found that tristetraprolin (TTP), a physiological regulator of mRNA expression and stability, is involved in the degradation of the E47 mRNA. Although many studies have characterized TTP expression and function in macrophages, monocytes, mast cells, and T cells, little is known about the expression and function of TTP in primary B cells. We show herein that TTP mRNA and protein expression are induced by LPS in B cells from young and old mice, the levels of TTP in old B cells always being higher than those in young B cells. Although TTP mRNA is degraded at a significantly higher rate in old B cells, TTP mRNA expression is higher in old than in young, likely due to its increased transcription. Like in macrophages, TTP protein expression and function in B cells are dependent upon p38 MAPK. We found that there is less phospho-TTP (inactive form), as well as phospho-p38, in old than in young splenic-activated B cells. This is the first report showing that TTP is involved in the degradation of the E47 mRNA and is up-regulated in old B cells.

Human aging alters the first phase of the molecular response to stress in T-cells

This study examines how age affects the first phase of the heat shock response in human T-cells. To understand how age alters transcriptional regulation of the heat shock genes, a cross-sectional study was conducted utilizing human T-cells enriched from peripheral blood lymphocytes of healthy young (20-40 years old) and old (>70 years old) donors. Nuclear run-on analysis revealed a 66% reduction in hsp70 transcription rates in old compared to young nuclei harvested from T-cells exposed to a brief 42 degrees C heat shock. To determine if one or more protein transactivators of the proximal and distal promoter regions of the hsp70 gene were affected by age, gel shift analysis was performed. Both HSF1 and SP1 DNA-binding were reduced with age but no reduction was noted in CCAAT-DNA binding. Western blot analysis indicated that HSF1 but not HSF2 protein levels were reduced in aged donor samples. These data suggest that human T-cell senescence involves a multi-factorial mechanism that diminishes an important transcriptional response to thermal stress. The results are discussed relative to recent studies that support a multi-factorial mechanism for age-dependent attenuation of the heat shock transcription factor.

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.

Modulation of neuropeptide Y and norepinephrine on several leucocyte functions in adult, old and very old mice

The age-related changes in the communication between the nervous and the immune system have been scarcely investigated, especially in very aged subjects. The present work deals with the in vitro effects of norepinephrine and neuropeptide Y, separately and jointly, on functions such as lymphoproliferation, NK activity, and IL-2 and TNF-alpha release of peritoneal leucocytes from adult (24+/-2 weeks), old (72+/-2 weeks) and very old (128+/-2 weeks) mice. The old mice showed a decrease in proliferation, NK activity and IL-2 release, and an increase in TNF-alpha, whereas in the very old mice these functions were more similar to those of the adults. The effects of neurotransmitters on these functions were different depending on the age of the animals.

The aging of early B-cell precursors

B-cell genesis in the bone marrow declines with advancing age. In this review, we discuss our current understanding of why B-cell production rates decline with age with a special emphasis on why age-related factors might target very early lymphoid precursors. We consider the impact of aging on cytokine responsiveness and how current models for lineage relationships for very early B- and T-cell precursors might influence interpretations of experiments addressing age-associated declines in B- and T-cell differentiation. This discussion centers on the notion that aging affects events associated with the process by which hematopoietic stem cells are guided toward the B-cell pathway. Finally, we present a model in which the age-associated loss of early B-cell precursors is linked to suboptimal function of key transcriptional regulators of very early B-cell development.

Identification and functional clustering of global gene expression differences between age-related cataract and clear human lenses and aged human lenses

We have examined the gene expression profiles of young, old and cataractous human lenses in order to differentiate those gene expression changes specific for cataract from those also associated with lens aging. Differentially expressed transcripts were identified by oligonucleotide microarray analysis and clustered according to their known functions. Four hundred and twelve transcripts that are increased and 919 transcripts that are decreased were identified at the 2-fold or greater level between epithelia isolated from cataract relative to clear lenses while 182 transcripts that are increased and 547 transcripts that are decreased were identified at the 2-fold or greater level between young and old lens epithelia. Comparison of the cataract gene expression changes with those detected in lens aging revealed that only 3 transcripts exhibited similar trends in gene expression. These data suggest that cataract- and age-specific changes in gene expression do not overlap and provide evidence for multiple cataract- and age-specific gene expression changes in the human lens.

Age-associated alterations in CXCL1 chemokine expression by murine B cells

Background

The CXCL1 chemokines, macrophage inflammatory protein-2 (MIP-2) and cytokine-induced neutrophil chemoattractant (KC), have been shown to play a role in a number of pathophysiological disease states including endotoxin-induced inflammation and bacterial meningitis. While the expression of these chemokines has been identified in a variety of cell types in the mouse, little is known about their expression with murine B-lymphocytes.

Results

Here, we demonstrate that highly purified murine splenic B cells are capable of expressing both MIP-2 and KC protein and mRNA upon activation with lipopolysaccharide (LPS) but not in response to anti-μ and anti-CD40 in combination with interleukin-4 (IL-4) stimulation. Moreover, these chemokines are expressed at higher levels in B cells derived from young (4 m) compared to old (24–29 m) mice. Upon fractionation into distinct B-cell subsets, we found that the expression of MIP-2 and KC by aged follicular (FO) B cells is significantly decreased when compared to the same cells from younger mice, while only MIP-2 production was found to be diminished in aged marginal zone (MZ) B cells. Interestingly, MIP-2 and KC production by newly formed (NF) B cells did not significantly differ with age. Moreover, the potential relevance of these findings is supported by the poor ability of LPS-activated aged B cells to specifically mediate CXCL1-dependent leukocyte recruitment when compared to younger B cells.

Conclusion

Overall, the decreased expression of CXCL1 chemokines by aged B cells in response to LPS may have potential implications on the secondary recruitment of leukocytes to sites of microbial infections and inflammation possibly contributing to the increased susceptibility of older subjects to pathogen challenge.

Age-related differences in the E2A-encoded transcription factor E47 in bone marrow-derived B cell precursors and in splenic B cells

We have investigated the effects of aging on the E2A-encoded transcription factor E47, a key regulator of B cell functions, in B cell precursors and in splenic B cells. Here, we show that old mice can be classified as severely depleted, moderately depleted or not depleted mice, according to the percentage of pre-B cells in their bone marrow. IL-7-expanded populations of pro-B/early pre-B cells from bone marrow of both severely depleted and moderately depleted old mice exhibit a reduced E47 DNA-binding and expression compared to young mice, and this defect in severely depleted old mice is more dramatic than that in moderately depleted old mice. However, mRNA levels were comparable, suggesting that E47 in the bone marrow is not transcriptionally regulated. In the spleen, activated B cells from both severely depleted and moderately depleted old mice show a lower E47 DNA-binding and expression than young mice. However, in contrast to precursor B cells, E47 DNA-binding and expression are similarly and only moderately reduced in both severely depleted and in moderately depleted mice. The mRNA levels were found to be decreased in stimulated splenic B cells from old as compared to young mice, suggesting that E47 mRNA in the spleen may be both transcriptionally and/or post-transcriptionally regulated.

Reduced Ig Class Switch in Aged Mice Correlates with Decreased E47 and Activation-Induced Cytidine Deaminase

The capacity to class switch the IgH chain is critical to the effectiveness of humoral immune responses. We show that in vitro-stimulated splenic B cells from senescent mice are deficient in production of multiple class switch isotypes (IgG1, G2a, G3, and E), class switch recombination (CSR), and induction of the E2A-encoded transcription factor E47. E47 has previously been shown to be required for CSR, at least in part via expression of the activation-induced cytidine deaminase. Our studies show that impaired induction of E47, and subsequently activation-induced cytidine deaminase, contribute to poor CSR and production of secondary isotypes in senescence.

Effects of aging on DNA-binding activity of the E47 transcription factor in splenic B cells

In the present paper, we have investigated the DNA-binding activity of the E2A-encoded transcription factor E47 in LPS-activated splenic B cells from young and old BALB/c mice. E47 is a key regulator of B cell differentiation and function: it binds to the E-box site, found in the regulatory regions of several B cell-specific genes (immunoglobulin (Ig), mb-1, and lambda5), promotes cell survival of early pre-B cells, helps to initiate Ig rearrangements, promotes class switch and is perhaps involved in somatic mutation in mature splenic and lymph node B cells. Results show that LPS-stimulated splenic B cells from old mice display decreased E47 DNA-binding activity as compared to young mice and that in splenic activated B cells only E47 homodimers bind DNA.