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

Ageing-related bone and immunity changes: insights into the complex interplay between the skeleton and the immune system

Ageing as a natural irreversible process inherently results in the functional deterioration of numerous organ systems and tissues, including the skeletal and immune systems. Recent studies have elucidated the intricate bidirectional interactions between these two systems. In this review, we provide a comprehensive synthesis of molecular mechanisms of cell ageing. We further discuss how age-related skeletal changes influence the immune system and the consequent impact of immune system alterations on the skeletal system. Finally, we highlight the clinical implications of these findings and propose potential strategies to promote healthy ageing and reduce pathologic deterioration of both the skeletal and immune systems.

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.

MicroRNAs miR-155 and miR-16 Decrease AID and E47 in B Cells from Elderly Individuals

Our research in the past few years has identified B cell-specific biomarkers able to predict optimal Ab responses in both young and elderly individuals. These biomarkers are activation-induced cytidine deaminase (AID), the enzyme of class switch recombination and somatic hypermutation; the transcription factor E47, crucial for AID expression; and the ability to generate optimal memory B cells. Moreover, we have found that the increased proinflammatory status of the elderly, both in sera and intrinsic to B cells, negatively impacts B cell function. We have now investigated whether particular inflammatory microRNAs (miRs) contribute to decreased E47 and AID in aged B cells. Our data indicate that E47 and AID mRNA stability is lower in stimulated B cells from elderly individuals. We measured the expression of two miRs crucial for class switch recombination, miR-155 and miR-16, in human unstimulated B cells from young and elderly individuals with the rationale that increases in these before stimulation would decrease E47/AID upon cell activation. We found these miRs and B cell-intrinsic inflammation upregulated in aged unstimulated B cells and negatively associated with AID in the same B cells after stimulation with CpG. We propose that the downregulation of AID in aged human B cells may occur through binding of miR-155 to the 3'-untranslated regions of AID mRNA and/or binding of miR-16 to the 3'-untranslated regions of E47 mRNA, as well as at the transcriptional level of less E47 for AID. Our results indicate novel molecular pathways leading to reduced B cell function with aging.

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.

How T follicular helper cells and the germinal centre response change with age

Normal ageing is accompanied by a decline in the function of the immune system that causes an increased susceptibility to infections and an impaired response to vaccination in older individuals. This results in an increased disease burden in the aged population, even with good immunisation programmes in place. The decreased response to vaccination is partly due to the diminution of the germinal centre response with age, caused by impaired T-cell help to B cells. Within the germinal centre, T-cell help is provided by a specialised subset of CD4(+) T cells; T follicular helper (Tfh) cells. Tfh cells provide survival and selection signals to germinal centre B cells, allowing them to egress from the germinal centre and become long-live plasma cells or memory B cells, and provide life-long protection against subsequent infection. This review will discuss the cellular and molecular changes in both Tfh cells and germinal centre B cells that occur with advancing age, which result in a smaller germinal centre response and a less effective response to immunisation.

Aging and neoteny in the B lineage

Aging and the physiologic decline of tissues and cells were once thought to be irreversible. However, recent studies suggest that various tissues, especially parts of the hematopoietic system, can be rejuvenated. Here we review potential mechanisms for this process and how they may be used to reverse age-related disorders and aging in general. We propose the novel hypothesis that altering the homeostatic process during cellular depletion can reverse aging in the hematopoietic system.

Aging impairs murine B cell differentiation and function in primary and secondary lymphoid tissues

Age-related changes in humoral immunity are responsible for the reduced vaccine responses observed in elderly individuals. Although aging has been shown to affect T cells, dendritic cells and macrophages and these effects significantly impact humoral responses, intrinsic alterations in B cells also occur. We here provide an overview of age-related changes in mouse B cells. In particular, we summarize data from the literature showing age-related changes in B cell differentiation in the bone marrow, in B cell marker expression and cell survival in the periphery and in the ability to make specific antibodies in both splenic and mucosal tissues. Moreover, we summarize the results from our studies showing that the ability to undergo class switch recombination, the enzyme activation-induced cytidine deaminase and the transcription factor E47 are all decreased in stimulated B cells from old mice. The defects presented in this review for aged B cells should allow the discovery of strategies for improvement of humoral immune responses in both humans and mice in the near future.