Bone marrow precursor cells from aged mice generate CD4 T cells that function well in primary and memory responses

IgD+ age-associated B cells are the progenitors of the main T-independent B cell response to infection that generates protective Ab and can be induced by an inactivated vaccine in the aged

Age-associated B cells (ABC) accumulate with age and are associated with autoimmunity and chronic infection. However, their contributions to acute infection in the aged and their developmental pathways are unclear. We find that the response against influenza A virus infection in aged mice is dominated by a Fas+ GL7- effector B cell population we call infection-induced ABC (iABC). Most iABC express IgM and include antibody-secreting cells in the spleen, lung, and bone marrow. We find that in response to influenza, IgD+ CD21- CD23- ABC are the precursors of iABC and become memory B cells. These IgD+ ABC develop in germ-free mice, so are independent of foreign antigen recognition. The response of ABC to influenza infection, resulting in iABC, is T cell independent and requires both extrinsic TLR7 and TLR9 signals. In response to influenza infection, IgD+ ABC can induce a faster recovery of weight and higher total anti-influenza IgG and IgM titers that can neutralize virus. Immunization with whole inactivated virus also generates iABC in aged mice. Thus, in unimmunized aged mice, whose other B and T cell responses have waned, IgD+ ABC are likely the naive B cells with the potential to become Ab-secreting cells and to provide protection from infection in the aged.

Significant difference in Th1/Th2 paradigm induced by tuberculosis-specific antigens between IGRA-positive and IGRA-negative patients

False negative interferon-γ release assay (IGRA) results constitute the major dilemma for the diagnosis of tuberculosis (TB) infections. Herein, we conducted a cohort study to compare the host immunological response to TB-specific antigens between active TB patients with positive and negative IGRA results and control groups. A total of 274 laboratory-confirmed TB patients were included in our analysis, consisting of 221 were IGRA positive and 53 were IGRA negative. Patients with the elderly were identified as an independent risk factor for negative IGRA results. In addition, the elevated level of IL-4 and the decreased levels of IFN-γ, IL-2, IL-6, IL-1β, and IL-12 in IGRA negative TB relative to IGRA positive TB group, demonstrating a significant difference in Th1/Th2 paradigm between two groups. The IFN-γ&IL-2 based assay could correctly identify 247 out of 307 MTB-infected individuals [271 TB patients and 36 individuals with latent TB infection (LTBI)], demonstrating a sensitivity of 80.5%. Then the IFN-γ and IL-4 were applied to distinguish healthy control and IGRA-negative group. When using the stepwise algorithm, the sensitivity for detecting Mycobacterium tuberculosis (MTB) infections was significantly increased from 80.5% to 89.6%. Additionally, patients with negative IGRA results had a conversion to culture-negative status longer than those with positive IGRA results. In conclusion, a stepwise algorithm outperforms IGRA assays to accurately identify MTB infections by the combination IFN-γ, IL-2, and IL-4. Further study is needed to evaluate the accuracy of our diagnostic algorithm in the LTBI population.

Immune and stem cell compartments of acetabular and femoral bone marrow in hip osteoarthritis patients

OBJECTIVE

Hip osteoarthritis (OA) affects all components of the osteochondral unit, leading to bone marrow (BM) lesions, and unknown consequences on BM cell functionality. We analyzed the cellular composition in OA-affected acetabula compared to proximal femur shafts obtained of hip OA patients to reveal yet not explored immune and stem cell compartments.

DESIGN

Combining flow cytometry, cellular assays and transcription analyses, we performed extensive ex vivo phenotyping of acetabular BM cells from 18 hip OA patients, comparing them with their counterparts from patient-matched femoral shaft BM samples. Findings were related to differences in skeletal sites and age.

RESULTS

Acetabular BM had a greater frequency of T-lymphocytes, non-hematopoietic cells and colony-forming units fibroblastic potential than femoral BM. The incidence of acetabular CD45⁺CD3⁺ T-lymphocytes increased (95% CI: 0.1770 to 0.0.8416), while clonogenic hematopoietic progenitors declined (95% CI: -0.9023 to -0.2399) with age of patients. On the other side, in femoral BM, we observed higher B-lymphocyte, myeloid and erythroid cell frequencies. Acetabular mesenchymal stromal cells (MSCs) showed a senescent profile associated with the expression of survival and inflammation-related genes. Efficient osteogenic and chondrogenic differentiation was detected in acetabular MSCs, while adipogenesis was more pronounced in their femoral counterparts.

CONCLUSION

Our results suggest that distinctions in BM cellular compartments and MSCs may be due to the influence of the OA-stressed microenvironment, but also acetabular vs femoral shaft-specific peculiarities cannot be excluded. These results bring new knowledge on acetabular BM cell populations and may be addressed as novel pathogenic mechanisms and therapeutic targets in OA.

Sex-specific remodeling of T-cell compartment with aging: Implications for rat susceptibility to central nervous system autoimmune diseases

The incidence of multiple sclerosis (MS) and susceptibility of animals to experimental autoimmune encephalomyelitis (EAE), the most commonly used experimental model of MS, decrease with aging. Generally, autoimmune diseases develop as the ultimate outcome of an imbalance between damaging immune responses against self and regulatory immune responses (keeping the former under control). Thus, in this review the age-related changes possibly underlying this balance were discussed. Specifically, considering the central role of T cells in MS/EAE, the impact of aging on overall functional capacity (reflecting both overall count and individual functional cell properties) of self-reactive conventional T cells (Tcons) and FoxP3+ regulatory T cells (Tregs), as the most potent immunoregulatory/suppressive cells, was analyzed, as well. The analysis encompasses three distinct compartments: thymus (the primary lymphoid organ responsible for the elimination of self-reactive T cells - negative selection and the generation of Tregs, compensating for imperfections of the negative selection), peripheral blood/lymphoid tissues ("afferent" compartment), and brain/spinal cord tissues ("target" compartment). Given that the incidence of MS and susceptibility of animals to EAE are greater in women/females than in age-matched men/males, sex as independent variable was also considered. In conclusion, with aging, sex-specific alterations in the balance of self-reactive Tcons/Tregs are likely to occur not only in the thymus/"afferent" compartment, but also in the "target" compartment, reflecting multifaceted changes in both T-cell types. Their in depth understanding is important not only for envisaging effects of aging, but also for designing interventions to slow-down aging without any adverse effect on incidence of autoimmune diseases.

Acute Myeloid Leukemia: Is It T Time?

Acute myeloid leukemia (AML) is a heterogeneous disease driven by impaired differentiation of hematopoietic primitive cells toward myeloid lineages (monocytes, granulocytes, red blood cells, platelets), leading to expansion and accumulation of "stem" and/or "progenitor"-like or differentiated leukemic cells in the bone marrow and blood. AML progression alters the bone marrow microenvironment and inhibits hematopoiesis' proper functioning, causing sustained cytopenia and immunodeficiency. This review describes how the AML microenvironment influences lymphoid lineages, particularly T lymphocytes that originate from the thymus and orchestrate adaptive immune response. We focus on the elderly population, which is mainly affected by this pathology. We discuss how a permissive AML microenvironment can alter and even worsen the thymic function, T cells' peripheral homeostasis, phenotype, and functions. Based on the recent findings on the mechanisms supporting that AML induces quantitative and qualitative changes in T cells, we suggest and summarize current immunotherapeutic strategies and challenges to overcome these anomalies to improve the anti-leukemic immune response and the clinical outcome of patients.

T-cell senescence accelerates angiotensin II-induced target organ damage

AIMS

Aging is a risk factor for cardiovascular diseases and adaptive immunity has been implicated in angiotensin (Ang) II-induced target organ dysfunction. Herein, we sought to determine the role of T-cell senescence in Ang II-induced target organ impairment and to explore the underlying mechanisms.

METHODS AND RESULTS

Flow cytometric analysis revealed that T cell derived from aged mice exhibited immunosenescence. Adoptive transfer of aged T cells to immunodeficient RAG1 KO mice accelerates Ang II-induced cardiovascular and renal fibrosis compared with young T-cell transfer. Aged T cells also promote inflammatory factor expression and superoxide production in these target organs. In vivo and in vitro studies revealed that Ang II promotes interferon-gamma (IFN-γ) production in the aged T cells comparing to young T cells. Importantly, transfer of senescent T cell that IFN-γ KO mitigates the impairment. Aged T-cell-conditioned medium stimulates inflammatory factor expression and oxidative stress in Ang II-treated renal epithelial cells compared with young T cells, and these effects of aged T-cell-conditioned medium are blunted after IFN-γ-neutralizing antibody pre-treatment.

CONCLUSION

These results provide a significant insight into the contribution of senescent T cells to Ang II-induced cardiovascular dysfunction and provide an attractive possibility that targeting T cell specifically might be a potential strategy to treat elderly hypertensive patients with end-organ dysfunction.

Understanding immunosenescence and its impact on vaccination of older adults

Older adults are more susceptible to viral and bacterial infection, and experience higher incidence and severity of infectious diseases. Although vaccination is the most logical solution in preventing infectious diseases, primary vaccine responses in individuals aged ≥65 years-old fail to generate complete protection. This is presumably attributed to immunosenescence, a term that describes functional differences associated with the immune system and natural age advancement. Both the innate and adaptive immune systems experience age-related impairments that contribute to insufficient protection following vaccination. This review addresses current knowledge of age-related changes that affect vaccine responsiveness; including the deficits in innate cell functions, dampened humoral and cell-mediated immune responses, current vaccination schedules for older adults, and concludes with potential strategies for improving vaccine efficacy specifically for this age group. Due to an age-related decline in immunity and poor vaccine responses, infectious diseases remain a burden among the aged population.

Stochastic non-enzymatic modification of long-lived macromolecules - A missing hallmark of aging

Damage accumulation in long-living macromolecules (especially extracellular matrix (ECM) proteins, nuclear pore complex (NPC) proteins, and histones) is a missing hallmark of aging. Stochastic non-enzymatic modifications of ECM trigger cellular senescence as well as many other hallmarks of aging affect organ barriers integrity and drive tissue fibrosis. The importance of it for aging makes it a key target for interventions. The most promising of them can be AGE inhibitors (chelators, O-acetyl group or transglycating activity compounds, amadorins and amadoriases), glucosepane breakers, stimulators of elastogenesis, and RAGE antagonists.

Thymic Engraftment by in vitro-Derived Progenitor T Cells in Young and Aged Mice

T cells play a critical role in mediating antigen-specific and long-term immunity against viral and bacterial pathogens, and their development relies on the highly specialized thymic microenvironment. T cell immunodeficiency can be acquired in the form of inborn errors, or can result from perturbations to the thymus due to aging or irradiation/chemotherapy required for cancer treatment. Hematopoietic stem cell transplant (HSCT) from compatible donors is a cornerstone for the treatment of hematological malignancies and immunodeficiency. Although it can restore a functional immune system, profound impairments exist in recovery of the T cell compartment. T cells remain absent or low in number for many months after HSCT, depending on a variety of factors including the age of the recipient. While younger patients have a shorter refractory period, the prolonged T cell recovery observed in older patients can lead to a higher risk of opportunistic infections and increased predisposition to relapse. Thus, strategies for enhancing T cell recovery in aged individuals are needed to counter thymic damage induced by radiation and chemotherapy toxicities, in addition to naturally occurring age-related thymic involution. Preclinical results have shown that robust and rapid long-term thymic reconstitution can be achieved when progenitor T cells, generated in vitro from HSCs, are co-administered during HSCT. Progenitor T cells appear to rely on lymphostromal crosstalk via receptor activator of NF-κB (RANK) and RANK-ligand (RANKL) interactions, creating chemokine-rich niches within the cortex and medulla that likely favor the recruitment of bone marrow-derived thymus seeding progenitors. Here, we employed preclinical mouse models to demonstrate that in vitro-generated progenitor T cells can effectively engraft involuted aged thymuses, which could potentially improve T cell recovery. The utility of progenitor T cells for aged recipients positions them as a promising cellular therapy for immune recovery and intrathymic repair following irradiation and chemotherapy, even in a post-involution thymus.

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.

The properties of the unique age-associated B cell subset reveal a shift in strategy of immune response with age

In aged mice, conventional naive B cells decrease and a new population of age-associated B cells (ABC)³ develops. When aged unprimed mice are infected with influenza virus, there is a reduced generation of helper CD4 T cell subsets and germinal center B cells, leading to limited production of IgG Ab and less generation of conventional long-lived plasma cells, compared to young. However, we find an enhanced non-follicular (GL7^-) ABC response that is helper T cell-independent, but requires high viral dose and pathogen recognition pathways. The infection-induced ABC (iABC) include IAV-specific Ab-secreting cells, some of which relocate to the bone marrow and lung, and persist for >4wk., suggesting they may provide significant protection. We also speculate there is a shift with increased age to dependence on TLR-mediated pathogen-recognition in both B and CD4 T cell responses.

The emerging role of ECM crosslinking in T cell mobility as a hallmark of immunosenescence in humans

Immunosenescence is thought to result from cellular aging and to reflect exposure to environmental stressors and antigens, including cytomegalovirus (CMV). However, not all of the features of immunosenescence are consistent with this view, and this has led to the emergence of the sister theory of "inflammaging". The recently discovered diffuse tissue distribution of resident memory T cells (T_RM) which don't recirculate, calls these theories into question. These cells account for most T cells residing in barrier epithelia which sit in and travel through the extracellular matrix (ECM). With almost all studies to date carried out on peripheral blood, the age-related changes of the ECM and their consequences for T cell mobility, which is crucial for the function of these cells, have been largely ignored. We propose an update of the theoretical framework of immunosenescence, based on a novel hypothesis: the increasing stiffness and cross-linking of the senescent ECM lead to a progressive immunodeficiency due to an age-related decrease in T cell mobility and eventually the death of these cells. A key element of this mechanism is the mechanical stress to which the cell cytoplasm and nucleus are subjected during passage through the ECM. This hypothesis is based on an "evo-devo" perspective bringing together some major characteristics of aging, to create a single interpretive framework for immunosenescence.

IL-6 Production by TLR-Activated APC Broadly Enhances Aged Cognate CD4 Helper and B Cell Antibody Responses In Vivo

Naive CD4 T cell responses, especially their ability to help B cell responses, become compromised with aging. We find that using APC pretreated ex vivo with TLR agonists, polyinosinic-polycytidylic acid and CpG, to prime naive CD4 T cells in vivo, restores their ability to expand and become germinal center T follicular helpers and enhances B cell IgG Ab production. Enhanced helper responses are dependent on IL-6 production by the activated APC. Aged naive CD4 T cells respond suboptimally to IL-6 compared with young cells, such that higher doses are required to induce comparable signaling. Preactivating APC overcomes this deficiency. Responses of young CD4 T cells are also enhanced by preactivating APC with similar effects but with only partial IL-6 dependency. Strikingly, introducing just the activated APC into aged mice significantly enhances otherwise compromised Ab production to inactivated influenza vaccine. These findings reveal a central role for the production of IL-6 by APC during initial cognate interactions in the generation of effective CD4 T cell help, which becomes greater with age. Without APC activation, aging CD4 T cell responses shift toward IL-6-independent Th1 and CD4 cytotoxic Th cell responses. Thus, strategies that specifically activate and provide Ag to APC could potentially enhance Ab-mediated protection in vaccine responses.

B Cell Development in the Bone Marrow Is Regulated by Homeostatic Feedback Exerted by Mature B Cells

Cellular homeostasis in the B cell compartment is strictly imposed to balance cell production and cell loss. However, it is not clear whether B cell development in the bone marrow is an autonomous process or subjected to regulation by the peripheral B cell compartment. To specifically address this question, we used mice transgenic for human CD20, where effective depletion of B lineage cells is obtained upon administration of mouse anti-human CD20 antibodies, in the absence of any effect on other cell lineages and/or tissues. We followed the kinetics of B cell return to equilibrium by BrdU labeling and flow cytometry and analyzed the resulting data by mathematical modeling. Labeling was much faster in depleted mice. Compared to control mice, B cell-depleted mice exhibited a higher proliferation rate in the pro-/pre-B compartment, and higher cell death and lower differentiation in the immature B cell compartment. We validated the first result by analysis of the expression of Ki67, the nuclear protein expressed in proliferating cells, and the second using Annexin V staining. Collectively, our results suggest that B lymphopoiesis is subjected to homeostatic feedback mechanisms imposed by mature B cells in the peripheral compartment.

Global transcriptional profiling reveals distinct functions of thymic stromal subsets and age-related changes during thymic involution

Age-associated thymic involution results in diminished T cell output and function in aged individuals. However, molecular mediators contributing to the decline in thymic function during early thymic involution remain largely unknown. Here, we present transcriptional profiling of purified thymic stromal subsets from mice 1, 3, and 6 months of age spanning early thymic involution. The data implicate unanticipated biological functions for a subset of thymic epithelial cells. The predominant transcriptional signature of early thymic involution is decreased expression of cell-cycle-associated genes and E2F3 transcriptional targets in thymic epithelial subsets. Also, expression of proinflammatory genes increases with age in thymic dendritic cells. Many genes previously implicated in late involution are already deregulated by 3-6 months of age. We provide these thymic stromal data sets, along with thymocyte data sets, in a readily searchable web-based platform, as a resource for investigations into thymocyte:stromal interactions and mechanisms of thymic involution.

The effect of age on thymic function

Age-related regression of the thymus is associated with a decline in naïve T cell output. This is thought to contribute to the reduction in T cell diversity seen in older individuals and linked with increased susceptibility to infection, autoimmune disease, and cancer. Thymic involution is one of the most dramatic and ubiquitous changes seen in the aging immune system, but the mechanisms which underlying this process are poorly understood. However, a picture is emerging, implicating the involvement of both extrinsic and intrinsic factors. In this review we assess the role of the thymic microenvironment as a potential target that regulates thymic involution, question whether thymocyte development in the aged thymus is functionally impaired, and explore the kinetics of thymic involution.

Causes, consequences, and reversal of immune system aging

The effects of aging on the immune system are manifest at multiple levels that include reduced production of B and T cells in bone marrow and thymus and diminished function of mature lymphocytes in secondary lymphoid tissues. As a result, elderly individuals do not respond to immune challenge as robustly as the young. An important goal of aging research is to define the cellular changes that occur in the immune system and the molecular events that underlie them. Considerable progress has been made in this regard, and this information has provided the rationale for clinical trials to rejuvenate the aging immune system.

Use it or lose it: establishment and persistence of T cell memory

Pre-existing T cell memory provides substantial protection against viral, bacterial, and parasitic infections. The generation of protective T cell memory constitutes a primary goal for cell-mediated vaccines, thus understanding the mechanistic basis of memory development and maintenance are of major importance. The widely accepted idea that T cell memory pools are directly descended from the effector populations has been challenged by recent reports that provide evidence for the early establishment of T cell memory and suggest that the putative memory precursor T cells do not undergo full expansion to effector status. Moreover, it appears that once the memory T cells are established early in life, they can persist for the lifetime of an individual. This is in contrast to the reported waning of naïve T cell immunity with age. Thus, in the elderly, immune memory that was induced at an early age may be more robust than recently induced memory, despite the necessity for long persistence. The present review discusses the mechanisms underlying the early establishment of immunological memory and the subsequent persistence of memory T cell pools in animal models and humans.

Aged-related shifts in T cell homeostasis lead to intrinsic T cell defects

Our recent studies indicate that the longer peripheral persistence of naïve CD4 T cells that occurs with age is necessary for the development of the key aging defects that lead to compromised responses to vaccination and to new pathogens or new strains of circulating infectious agents. This longer persistence is in turn is linked to the decrease in development of new thymic emigrants and thymic involution that occur at adolescence. Therefore the process of development of naïve CD4 aging defects, is closely tied to the homeostasis of T cells and the shifts that occur in their homeostasis with age. Here we review this connection between age-related changes in T cell homeostasis and the development of T cell defects and discuss the implication for approaches to better vaccinating the elderly.

Aging of the CD4 T Cell Compartment

Higher morbidity and mortality following infections, particularly influenza, is observed in the elderly population. Because of this, people over 65 years old are often targeted for preventive immunization. Many vaccines, however, are not as effective in generating protective antibodies in older individuals. CD4⁺ T cells, through their B cell helper functions, play a central role in the humoral response. Aging has deleterious effects on the immune system, and understanding how aging impairs CD4⁺ T cell functions is of critical importance to design new immunization and treatment strategies targeted to the elderly population. In this paper, we review some of the qualitative and quantitative changes in the CD4⁺ T cell compartment that arise with aging. We also summarize the age-related intrinsic defects that impact naïve, memory and regulatory CD4⁺ T cell functions.

Persistent degenerative changes in thymic organ function revealed by an inducible model of organ regrowth

The thymus is the most rapidly aging tissue in the body, with progressive atrophy beginning as early as birth and not later than adolescence. Latent regenerative potential exists in the atrophic thymus, because certain stimuli can induce quantitative regrowth, but qualitative function of T lymphocytes produced by the regenerated organ has not been fully assessed. Using a genome-wide computational approach, we show that accelerated thymic aging is primarily a function of stromal cells, and that while overall cellularity of the thymus can be restored, many other aspects of thymic function cannot. Medullary islet complexity and tissue-restricted antigen expression decrease with age, representing potential mechanisms for age-related increases in autoimmune disease, but neither of these is restored by induced regrowth, suggesting that new T cells produced by the regrown thymus will probably include more autoreactive cells. Global analysis of stromal gene expression profiles implicates widespread changes in Wnt signaling as the most significant hallmark of degeneration, changes that once again persist even at peak regrowth. Consistent with the permanent nature of age-related molecular changes in stromal cells, induced thymic regrowth is not durable, with the regrown organ returning to an atrophic state within 2 weeks of reaching peak size. Our findings indicate that while quantitative regrowth of the thymus is achievable, the changes associated with aging persist, including potential negative implications for autoimmunity.

Age-related Deficiencies in Antigen-Specific CD4 T cell Responses: Lessons from Mouse Models

Infectious diseases contribute to significant morbidity and mortality in elderly populations. One of the major contributing factors to this is age-related declines in the immune system that diminish the response o both infections and vaccinations. In order to understand how specific changes in the immune system influence the generation of immunity in older individuals, immunologists have developed aging mouse models that allow for experimental manipulation of immune system components. These models have shown that there are dramatic age-related changes in naive CD4 T cell function that have the potential to impact a myriad of immune responses. In this review, we will summarize these findings on the intrinsic changes in CD4 T cell function and discuss how these changes influence immunity.

The aging immune system and its relationship with cancer

The incidence of most common cancers increases with age. This occurs in association with, and is possibly caused by a decline in immune function, termed immune senescence. Although the size of the T-cell compartment is quantitatively maintained into older age, several deleterious changes (including significant changes to T-cell subsets) occur over time that significantly impair immunity. This article highlights some of the recent findings regarding the aging immune system, with an emphasis on the T-cell compartment and its role in cancer.

Reversing B cell aging

Age-related alterations in the cellular composition of the B lineage are a major cause of the poor antibody response to vaccination and to infectious agents among the elderly population. The mechanisms leading to these changes are poorly understood. Recently, we have shown that these changes reflect, at least in part, homeostatic pressures imposed by long-lived B cells that accumulate with aging, and that aging in the B lineage can be reversed upon alteration of B cell homeostasis by depletion. Here we discuss homeostatic causes for B lineage immunosenescence, and the potential for its rejuvenation.

Plasmacytoid DC from aged mice down-regulate CD8 T cell responses by inhibiting cDC maturation after Encephalitozoon cuniculi infection

Age associated impairment of immune function results in inefficient vaccination, tumor surveillance and increased severity of infections. Several alterations in adaptive immunity have been observed and recent studies report age related declines in innate immune responses to opportunistic pathogens including Encephalitozoon cuniculi. We previously demonstrated that conventional dendritic cells (cDC) from 9-month-old animals exhibit sub-optimal response to E. cuniculi infection, suggesting that age associated immune senescence begins earlier than expected. We focused this study on how age affects plasmacytoid DC (pDC) function. More specifically how aged pDC affect cDC function as we observed that the latter are the predominant activators of CD8 T cells during this infection. Our present study demonstrates that pDC from middle-aged mice (12 months) suppress young (8 week old) cDC driven CD8 T cell priming against E. cuniculi infection. The suppressive effect of pDC from older mice decreased maturation of young cDC via cell contact. Aged mouse pDC exhibited higher expression of PD-L1 and blockade of their interaction with cDC via this molecule restored cDC maturation and T cell priming. Furthermore, the PD-L1 dependent suppression of cDC T cell priming was restricted to effector function of antigen-specific CD8 T cells not their expansion. To the best of our knowledge, the data presented here is the first report highlighting a cell contact dependent, PD-L1 regulated, age associated defect in a DC subpopulation that results in a sub-optimal immune response against E. cuniculi infection. These results have broad implications for design of immunotherapeutic approaches to enhance immunity for aging populations.

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

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

Age-related defects in the cytoskeleton signaling pathways of CD4 T cells

It has been postulated that the cytoskeleton controls many aspects of T cell function, including activation, proliferation and apoptosis. Recent advances in our understanding of F-actin polymerization and the Ezrin-Radixin-Moesin (ERM) family of cytoskeleton signal proteins have provided new insights into immunological synapse formation during T cell activation. During aging there is a significant decline of T cell function largely attributable to declines in activation of CD4 T cells and defects in the formation of the immunological synapse. Here we discuss recent progress in the understanding of how aging alters F-actin and ERM proteins in mouse CD4 T cells, and the implications of these changes for the T cell activation process.

Age-related changes in immune function: effect on airway inflammation

Immunosenescence is defined as changes in the innate and adaptive immune response associated with increased age. The clinical consequences of immunosenescence include increased susceptibility to infection, malignancy and autoimmunity, decreased response to vaccination, and impaired wound healing. However, there are several immune alterations that might facilitate persistence of asthma into late adulthood or development of asthma after the age of 50 to 60 years. Asthma in older patients is not uncommon, and this is a growing population as the average lifespan increases. Specific innate changes that might affect severity of asthma in older patients or be involved in the development of late-onset asthma include impaired mucociliary clearance and changes in airway neutrophil, eosinophil, and mast cell numbers and function. Additionally, age-related altered antigen presentation and decreased specific antibody responses might increase the risk of respiratory tract infections. Respiratory tract infections exacerbate asthma in older patients and possibly play a role in the pathogenesis of late-onset asthma. Furthermore, cytokine profiles might be modified with aging, with some investigators suggesting a trend toward T(H)2 cytokine expression. This review examines specific innate and adaptive immune responses affected by aging that might affect the inflammatory response in older adults with asthma.

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.

Neutrophils are immune cells preferentially targeted by retinoic acid in elderly subjects

Background

The immune system gradually deteriorates with age and nutritional status is a major factor in immunosenescence. Of the many nutritional factors implicated in age-related immune dysfunction, vitamin A may be a good candidate, since vitamin A concentrations classically decrease during aging whereas it may possess important immunomodulatory properties via its active metabolites, the retinoic acids. This prompted us to investigate the immune response induced by retinoids in adults and elderly healthy subjects. Before and after oral supplementation with 13cis retinoic acid (0.5 mg/kg/day during 28 days), whole blood cells were phenotyped, and functions of peripheral blood mononuclear cells (PBMC) and polymorphonuclear cells (PMN) were investigated by flow cytometry and ELISA tests.

Results

In both young adults (n = 20, 25 ± 4 years) and older subjects (n = 20, 65 ± 4 years), retinoic acid supplementation had no effect on the distribution of leukocyte subpopulations or on the functions of PBMC (Il-2 and sIl-2R production, membrane expression of CD25). Concerning PMN, retinoic acid induced an increase in both spontaneous migration and cell surface expression of CD11b in the two different age populations, whereas bactericidal activity and phagocytosis remained unchanged.

Conclusions

We demonstrated that retinoic acid induces the same intensity of immune response between adult and older subjects, and more specifically affects PMN functions, i.e. adhesion and migration, than PBMC functions.

The effect of age on the phenotype and function of developing thymocytes

The immune system declines with age leading to a progressive deterioration in the ability to respond to infection and vaccination. Age-associated thymic involution is one of the most recognized changes in the ageing immune system and is believed to be a major contributor towards immunosenescence; however, the precise mechanisms involved in age-associated thymic involution remain unclear. In order to gain further insight into the effect of ageing on T-cell development, steady-state thymopoiesis was studied in mice ranging from 1 to 18 months of age. There was a decrease in thymic cellularity with age, but the most dramatic loss occurred early in life. Although there were no alterations in the proportion of the major thymocyte subsets, there was a significant decline in the expression of other key molecules including CD3 and CD24. There was a decline in the ability of thymocytes from older mice to respond to mitogens, which was demonstrated by a failure to up-regulate expression of the activation marker CD69 and to enter the G(2)--M phase of the cell cycle. This was concurrent with an increased resistance to apoptosis in thymocytes from aged animals. Together, these results suggest that T cells may be flawed even before exiting to the periphery and that this could contribute to the age-associated decline in immune function.

Age-associated increase in lifespan of naive CD4 T cells contributes to T-cell homeostasis but facilitates development of functional defects

With age, T-cell generation from the thymus is much reduced, yet a substantial naïve T-cell pool is maintained even in aged animals, suggesting that naïve T cells either persist longer or turn over faster to maintain T-cell homeostasis. We found that with age, naïve CD4 T cells became progressively longer-lived. Their longer lifespan did not depend on recognition of self-peptide/class II. Newly generated naïve T cells derived from aged stem cells had a shorter lifespan, like that of young naïve T cells. Conversely, naïve CD4 T cells derived from middle-aged thymectomized mice were longer-lived in vivo, and their development of functional defects was accelerated. These observations suggest that naïve T cells develop their longer lifespan during their sojourn in the periphery. Increased longevity of naïve CD4 T cells correlated well with reduced expression of proapoptotic molecule Bim. We suggest that the intrinsic increase in longevity helps maintain naïve T-cell homeostasis but facilitates the development of functional defects in mice.

T-cell immunosenescence: lessons learned from mouse models of aging

It is well established that increasing age is associated with a decreased capacity of the immune system to mediate effective immune responses to vaccination and invading pathogens. Because of the inherent limitations of conducting experiments in humans, much of what we have learned is owed to the utility of experimental mouse models of aging. Recent studies performed in the mouse have demonstrated mechanisms responsible for age-related declines in the function of CD4(+) and CD8(+) cells. This review describes key findings regarding age-related defects in T-cell function and discusses the impact these defects have on vaccine efficacy and immunity.

An evolutionary perspective on the mechanisms of immunosenescence

There is an accumulating body of evidence that a decline in immune function with age is common to most if not all vertebrates. For instance, age-associated thymic involution seems to occur in all species that possess a thymus, indicating that this process is evolutionary ancient and conserved. The precise mechanisms regulating immunosenescence remain to be resolved, but much of what we do know is consistent with modern evolutionary theory. In this review, we assess our current knowledge from an evolutionary perspective on the occurrence of immunosenescence, we show that life history trade-offs play a key role and we highlight the possible advantages of the age-related decline in thymic function.

Is thymocyte development functional in the aged?

T cells are an integral part of a functional immune system with the majority being produced in the thymus. Of all the changes related to immunosenescence, regression of the thymus is considered one of the most universally recognised alterations. Despite the reduction of thymic size, there is evidence to suggest that T cell output is still present into old age, albeit much diminished; leading to the assumption that thymocyte development is normal. However, current data suggests that recent thymic emigrant from the aged thymus are functionally less responsive, giving rise to the possibility that the generation of naïve T cell may be intrinsically impaired in the elderly. In light of these findings we discuss the evidence that suggest aged T cells may be flawed even before exiting to the periphery and could contribute to the age-associated decline in immune function.

The ageing immune system: is it ever too old to become young again?

Ageing is accompanied by a decline in the function of the immune system, which increases susceptibility to infections and can decrease the quality of life. The ability to rejuvenate the ageing immune system would therefore be beneficial for elderly individuals and would decrease health-care costs for society. But is the immune system ever too old to become young again? We review here the promise of various approaches to rejuvenate the function of the immune system in the rapidly growing ageing population.