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

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.

Considerations and Approaches for Cancer Immunotherapy in the Aging Host

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

Loss of early B cell protein λ5 decreases bone mass and accelerates skeletal aging

The early B cell protein λ5 is an essential component of the surrogate light chain and the preB cell receptor (preBCR), which is critical for optimal B cell development. To investigate the effect of λ5 and/or B cells on bone acquisition over time, we developed a panel of JH -/- , λ5-/-, JH -/- λ5-/-, and wild-type (WT) BALB/c mice and then studied postnatal bone development and aging in these mice at one, six, twelve, and twenty-two months of age. The trabecular bone volume over total volume (BV/TV) in JH -/- mice was similar to WT mice at all ages. In contrast, at six months of age and thereafter, λ5-/- and JH -/- λ5-/- mice demonstrated a severe decrease in trabecular bone mass. Surprisingly, bone mass in six-month-old λ5-/- and JH -/- λ5-/- mice was similar to or even lower than in aged (twenty-two-months) WT mice, suggesting accelerated skeletal aging. The postnatal development and the acquisition of cortical bone mass in JH -/- λ5-/- mice were generally comparable to WT. However, JH -/- λ5-/- mice showed a significant decrease in cortical BV/TV at six- and twelve months of age. To examine the contribution of λ5 and B cells to postnatal bone synthesis, we separately transplanted whole bone marrow cells from JH -/- λ5-/- and WT mice into irradiated JH -/- λ5-/- and WT recipients. WT recipients of JH -/- λ5-/- marrow cells failed to show acquisition of trabecular bone mass, whereas transplanting WT marrow cells into JH -/- λ5-/- recipients led to the recovery of trabecular bone mass. Transfer of WT marrow cells into JH -/- λ5-/- mice promoted synthesis of new cortical and trabecular bone. Our findings indicate that λ5 plays a major role in preserving bone mass during postnatal development and skeletal aging which is distinct from its role in B cell development. The absence of both λ5 and B cells in JH -/- λ5-/- mice leads to delayed acquisition of cortical bone during postnatal development. Dissecting the mechanism(s) by which λ5 regulates bone homeostasis may provide new avenues for the treatment of age-related loss of bone mass and osteoporosis.

The Influence of Heterochronic Non-Myeloablative Bone Marrow Transplantation on the Immune System, Frailty, General Health, and Longevity of Aged Murine Recipients

The stem cell theory of aging postulates that stem cells become inefficient at maintaining the original functions of the tissues. We, therefore, hypothesized that transplanting young bone marrow (BM) to old recipients would lead to rejuvenating effects on immunity, followed by improved general health, decreased frailty, and possibly life span extension. We developed a murine model of non-myeloablative heterochronic BM transplantation in which old female BALB/c mice at 14, 16, and 18(19) months of age received altogether 125.1 ± 15.6 million nucleated BM cells from young male donors aged 7–13 weeks. At 21 months, donor chimerism was determined, and the immune system’s innate and adaptive arms were analyzed. Mice were then observed for general health and frailty until spontaneous death, when their lifespan, post-mortem examinations, and histopathological changes were recorded. The results showed that the old mice developed on average 18.7 ± 9.6% donor chimerism in the BM and showed certain improvements in their innate and adaptive arms of the immune system, such as favorable counts of neutrophils in the spleen and BM, central memory Th cells, effector/effector memory Th and Tc cells in the spleen, and B1a and B1b cells in the peritoneal cavity. Borderline enhanced lymphocyte proliferation capacity was also seen. The frailty parameters, pathomorphological results, and life spans did not differ significantly in the transplanted vs. control group of mice. In conclusion, although several favorable effects are obtained in our heterochronic non-myeloablative transplantation model, additional optimization is needed for better rejuvenation effects.

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.

The Complexity of Microglial Interactions With Innate and Adaptive Immune Cells in Alzheimer's Disease

In the naïve mouse brain, microglia and astrocytes are the most abundant immune cells; however, there is a complexity of other immune cells present including monocytes, neutrophils, and lymphocytic cells, such as natural killer (NK) cells, T cells, and B cells. In Alzheimer’s disease (AD), there is high inflammation, reactive microglia, and astrocytes, leaky blood–brain barrier, the buildup of amyloid-beta (Aβ) plaques, and neurofibrillary tangles which attract infiltrating peripheral immune cells that are interacting with the resident microglia. Limited studies have analyzed how these infiltrating immune cells contribute to the neuropathology of AD and even fewer have analyzed their interactions with the resident microglia. Understanding the complexity and dynamics of how these immune cells interact in AD will be important for identifying new and novel therapeutic targets. Thus, this review will focus on discussing our current understanding of how macrophages, neutrophils, NK cells, T cells, and B cells, alongside astrocytes, are altered in AD and what this means for the disorder, as well as how these cells are affected relative to the resident microglia.

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.

Understanding the Heterogeneous Population of Age-Associated B Cells and Their Contributions to Autoimmunity and Immune Response to Pathogens

In humans and mice, susceptibility to infections and autoimmunity increases with age due to age-associated changes in innate and adaptive immune responses. Aged innate cells are also less active, leading to decreased naive T- and B-cell responses. Aging innate cells contribute to an overall heightened inflammatory environment. Naive T and B cells undergo cell-intrinsic age-related changes that result in reduced effector and memory responses. However, previously established B- and T-cell memory responses persist with age. One dramatic change is the appearance of a newly recognized population of age-associated B cells (ABCs) that has a unique cluster of differentiation (CD)21-CD23- phenotype. Here, we discuss the discovery and origins of the naive phenotype immunoglobulin (Ig)D+ versus activated CD11c+T-bet+ ABCs, with a focus on protective and pathogenic properties. In humans and mice, antigen-experienced CD11c+T-bet+ ABCs increase with autoimmunity and appear in response to bacterial and viral infections. However, our analyses indicate that CD21-CD23- ABCs include a resting, naive, progenitor ABC population that expresses IgD. Similar to generation of CD11c+T-bet+ ABCs, naive ABC response to pathogens depends on toll-like receptor stimulation, making this a key feature of ABC activation. Here, we put forward a potential developmental map of distinct subsets from putative naive ABCs. We suggest that defining signals that can harness the naive ABC response may contribute to protection against pathogens in the elderly. CD11c+T-bet+ ABCs may be useful targets for therapeutic strategies to counter autoimmunity.

Inflammation and the Alteration of B-Cell Physiology in Aging

Aging results for the immune system in a departure from the optimal homeostatic state seen in young organisms. This divergence regrettably contributes to a higher frequency of compromised responses to infections and inefficient classical vaccination in aged populations. In B cells, the cornerstone of humoral immunity, the development and distribution of the various mature B cell subsets are impacted by aging in both humans and mice. In addition, aged mature B cells demonstrate limited capacity to mount efficient antibody responses. An expected culprit for the decline in effective immunity is the rise of the systemic levels of pro-inflammatory molecules during aging, establishing a chronic low-grade inflammation. Indeed, numerous alterations affecting directly or indirectly B cells in old people and mice are reminiscent of various effects of acute inflammation on this cell type in young adults. The present mini-review will highlight the possible adverse contributions of the persistent low-level inflammation observed in susceptible older organisms to the inadequate B-cell physiology.

B Cell Dysfunction Associated With Aging and Autoimmune Diseases

Impaired humoral responses, as well as an increased propensity for autoimmunity, play an important role in the development of immune system dysfunction associated with aging. Accumulation of a subset of atypical B cells, termed age-associated B cells (ABCs), is one of the key age-related changes in B cell compartments. ABCs are characterized by their distinct phenotypes, gene expression profiles, special survival requirements, variations in B cell receptor repertoires, and unique functions. Here, we summarize recent progress in the knowledge base related to the features of ABCs, their potential role in immune senescence, and their relationship with autoimmune diseases.

Genome organization and chromatin analysis identify transcriptional downregulation of insulin-like growth factor signaling as a hallmark of aging in developing B cells

BACKGROUND

Aging is characterized by loss of function of the adaptive immune system, but the underlying causes are poorly understood. To assess the molecular effects of aging on B cell development, we profiled gene expression and chromatin features genome-wide, including histone modifications and chromosome conformation, in bone marrow pro-B and pre-B cells from young and aged mice.

RESULTS

Our analysis reveals that the expression levels of most genes are generally preserved in B cell precursors isolated from aged compared with young mice. Nonetheless, age-specific expression changes are observed at numerous genes, including microRNA encoding genes. Importantly, these changes are underpinned by multi-layered alterations in chromatin structure, including chromatin accessibility, histone modifications, long-range promoter interactions, and nuclear compartmentalization. Previous work has shown that differentiation is linked to changes in promoter-regulatory element interactions. We find that aging in B cell precursors is accompanied by rewiring of such interactions. We identify transcriptional downregulation of components of the insulin-like growth factor signaling pathway, in particular downregulation of Irs1 and upregulation of Let-7 microRNA expression, as a signature of the aged phenotype. These changes in expression are associated with specific alterations in H3K27me3 occupancy, suggesting that Polycomb-mediated repression plays a role in precursor B cell aging.

CONCLUSIONS

Changes in chromatin and 3D genome organization play an important role in shaping the altered gene expression profile of aged precursor B cells. Components of the insulin-like growth factor signaling pathways are key targets of epigenetic regulation in aging in bone marrow B cell precursors.

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.

Considerations for successful cancer immunotherapy in aged hosts

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

Age-associated B cells (ABC) inhibit B lymphopoiesis and alter antibody repertoires in old age

With old age (∼2y old), mice show substantial differences in B cell composition within the lymphoid tissues. In particular, a novel subset of IgM⁺ CD21/35^lo/- CD23^- mature B cells, the age-associated B cells or ABC, increases numerically and proportionately. This occurs at the expense of other B cell subsets, including B2 follicular B cells in spleen and recirculating primary B cells in bone marrow. Our studies suggest that ABC have a distinctive antibody repertoire, as evidenced by relatively high reactivity to the self-antigens phosphorylcholine (PC) and malondialdehyde (MDA). While PC and MDA are found on apoptotic cells and oxidized lipoproteins, antibodies to these antigens are also cross-reactive with epitopes on bacterial species. In old mice, ABC express TNFα and are pro-inflammatory. ABC can inhibit growth and/or survival in pro-B cells as well as common lymphoid progenitors (CLP). In particular, ABC cause apoptosis in pro-B cells with relatively high levels of the surrogate light chain (SLC) and, consequently, promote an "SLC low" pathway of B cell differentiation in old mice. SLC together with μ heavy chain comprises the pre-B cell receptor (preBCR) critical for pre-B cell expansion and selection of the μ heavy chain Vh repertoire. The low level of SLC likely impairs normal preBCR driven proliferation and alters μ heavy chain Vh selection thereby affecting the antibody specificities of new B cells. In this manner, ABC may contribute to both qualitative and quantitative disruptions of normal B lymphopoiesis in old age.

Aging effects on T-bet expression in human B cell subsets

In order to compare human and mouse B cell subset markers, we evaluated T-bet expression in human B cell subsets from individuals of different ages. We found T-bet expressed in unstimulated memory more than naïve B cells, and more in young individuals. TLR7 stimulation up-regulated T-bet in all B cell subsets from young and elderly individuals, and more in the elderly. By fold-increase the best effect was seen in subsets of the elderly and especially in those that undergo class switch (naïve and IgM). We also evaluated CD11c expression, as T-bet+CD11c+ B cells are expanded in healthy elderly individuals and also in patients with autoimmunity. Similar to T-bet, CD11c expression was higher in memory than in naïve B cells, but no differences were observed between young and elderly individuals. After TLR7 stimulation, CD11c increases in all B cell subsets (especially in naïve and IgM) from the elderly.

In old BALB/c mice, bone marrow pre-B cell and surrogate light chain reduction is associated with increased B cell reactivity to phosphorylcholine, but reduced T15 idiotype dominance

In young adult BALB/c mice, antibodies to phosphorylcholine (PC) bearing the T15 (TEPC 15) idiotype confer protection against pneumococcal infections. In old age, even though PC reactive B cells are often increased, the proportion of T15+ antibodies declines. We hypothesize that limited surrogate light chain (SLC) and compromise of the pre-B cell receptor checkpoint in old mice contribute to both reduced new B cell generation and changes in the anti-PC antibodies seen in old age. In old mice: 1) early pre-B cell loss is most pronounced at the preBCR checkpoint; however, the reduced pool of early pre-B cells continues to proliferate consistent with preBCR signaling; 2) increased PC reactivity is seen in bone marrow immature B cells; 3) deficient SLC promotes increased B cell PC reactivity and diminished T15 idiotype expression; and 4) as pre-B cell losses and reduced SLC become progressively more severe, increased T15 negative PC reactive B cells occur. These results associate a reduction in pre-B cells, imposed at the preBCR checkpoint, with increased reactivity to PC, but more limited expression of the protective T15 idiotype among PC reactive antibodies in old age.

Age-Associated B Cells Express a Diverse Repertoire of VH and Vκ Genes with Somatic Hypermutation

The origin and nature of age-associated B cells (ABCs) in mice are poorly understood. In this article, we show that their emergence required MHC class II and CD40/CD40L interactions. Young donor B cells were adoptively transferred into congenic recipients and allowed to remain for 1 mo in the absence of external Ag. B cells expressing the T-bet transcription factor, a marker for ABCs, were generated after multiple cell divisions from C57BL/6 donors but not from MHC class II- or CD40-deficient donors. Furthermore, old CD154 (CD40L)-deficient mice did not accrue ABCs, confirming that they arise primarily through T-dependent interactions. To determine what Igs ABCs express, we sequenced V_H and Vκ rearranged genes from unimmunized 22-mo-old C57BL/6 mice and showed that they had a heterogeneous repertoire, which was comparable to that seen in old follicular and marginal zone B cell subsets. However, in contrast to the follicular and marginal zone cells, ABCs displayed significant somatic hypermutation. The mutation frequency was lower than found in germinal center cells after deliberate immunization, suggesting that ABCs have undergone mild stimulation from endogenous Ags over time. These observations show that quiescent ABCs are Ag-experienced cells that accumulate during T cell-dependent responses to diverse Ags during the life of an individual.