Early antigen-specific response by naive CD8 T cells is not altered with aging

IL-6-mediated environmental conditioning of defective Th1 differentiation dampens antitumour immune responses in old age

Decline in immune function and inflammation concomitantly develop with ageing. Here we focus on the impact of this inflammatory environment on T cells, and demonstrate that in contrast to successful tumour elimination in young mice, replenishment of tumour-specific CD4⁺ T cells fails to induce tumour regression in aged hosts. The impaired antitumour effect of CD4⁺ T cells with their defective Th1 differentiation in an aged environment is restored by interleukin (IL)-6 blockade or IL-6 deficiency. IL-6 blockade also restores the impaired ability of CD4⁺ T cells to promote CD8⁺ T-cell-dependent tumour elimination in aged mice, which requires IFN-γ. Furthermore, IL-6-stimulated production of IL-4/IL-21 through c-Maf induction is responsible for impaired Th1 differentiation. IL-6 also contributes to IL-10 production from CD4⁺ T cells in aged mice, causing attenuated responses of CD8⁺ T cells. These findings suggest that IL-6 serves as an extrinsic factor counteracting CD4⁺ T-cell-mediated immunity against tumour in old age.

T-cell responses are dysregulated in aged humans and mice, which leads to poor antitumour responses. Here, the authors demonstrate that this phenomenon is at least partially due to an overproduction of IL-6 caused by ageing and its inhibitory effect on Th1 differentiation of tumour-specific CD4 T cells.

Optimization of immunotherapy in elderly cancer patients

Increasing evidence has revealed the incidence of cancer augments with aging, which could be attributed to a multitude of age-associated changes including the dysregulation of the immune system. Although many reports demonstrate the efficacy of cancer immunotherapies in numerous preclinical studies, most experiments have been performed in young animals. Studies from our group and others show that cancer immunotherapy could be ineffective in old mice, even though the same therapeutic treatment works efficiently in young mice. Given that cancer occurs mostly in the elderly, we should take age-associated immune dysregulation into consideration to achieve the effectiveness of immunotherapeutic interventions in the old. Understanding both age-related and tumor-related immune alterations might be equally important in improving the effectiveness of immunotherapy. This article reviews a number of age-associated immune alterations with specific attention given to the impact on antitumor responses, and also discusses possible strategies for optimization of immunotherapeutic interventions in the elderly.

Two separate defects affecting true naive or virtual memory T cell precursors combine to reduce naive T cell responses with aging

Naive T cell responses are eroded with aging. We and others have recently shown that unimmunized old mice lose ≥ 70% of Ag-specific CD8 T cell precursors and that many of the remaining precursors acquire a virtual (central) memory (VM; CD44(hi)CD62L(hi)) phenotype. In this study, we demonstrate that unimmunized TCR transgenic (TCRTg) mice also undergo massive VM conversion with age, exhibiting rapid effector function upon both TCR and cytokine triggering. Age-related VM conversion in TCRTg mice directly depended on replacement of the original TCRTg specificity by endogenous TCRα rearrangements, indicating that TCR signals must be critical in VM conversion. Importantly, we found that VM conversion had adverse functional effects in both old wild-type and old TCRTg mice; that is, old VM, but not old true naive, T cells exhibited blunted TCR-mediated, but not IL-15-mediated, proliferation. This selective proliferative senescence correlated with increased apoptosis in old VM cells in response to peptide, but decreased apoptosis in response to homeostatic cytokines IL-7 and IL-15. Our results identify TCR as the key factor in differential maintenance and function of Ag-specific precursors in unimmunized mice with aging, and they demonstrate that two separate age-related defects--drastic reduction in true naive T cell precursors and impaired proliferative capacity of their VM cousins--combine to reduce naive T cell responses with aging.

Intrinsic defects in CD8 T cells with aging contribute to impaired primary antiviral responses

Aging is associated with altered immune responses, particularly with a diminished CD8 T cell response. Although both intrinsic and extrinsic factors are hypothesized to impact this decreased T cell response, the direct evidence of an intrinsic deficiency in virus-specific CD8 T cells is limited. In this study, a TCR transgenic (Tg) P14 mouse model was utilized to compare the activation and proliferation of the Tg CD8 T cells of young and aged P14 mice upon stimulation with antigen or infection with virus. The proliferation of purified Tg CD8 T cells of aged mice was significantly lower than that of young mice when cultured in vitro with both the LCMV specific peptide and antigen presenting cells from young wild type mice. In addition, expression of the activation markers, CD69, CD25, and CD44, was delayed on Tg T cells of aged mice after stimulation. Importantly, while adoptive transfer of purified Tg CD8 T cells of young or aged mice into young wild type mice resulted in expansion of the Tg CD8 T cells of both ages after LCMV infection, the expansion of the Tg T cells from aged mice was significantly decreased compared with that of the Tg T cells from young mice. However, while the number of IFN-γ secreting Tg CD8 T cells from aged mice was significantly decreased compared to that of young mice, the percentages of Tg CD8 T cells producing IFN-γ were similar in young and aged mice, demonstrating that proliferation, but not function, of the Tg CD8 T cells of aged mice was impaired. Importantly, chronological age alone was not sufficient to predict an altered proliferative response; rather, expression of high levels of CD44 on CD8 T cells of aged mice reflected a decreased proliferative response. These results reveal that alterations intrinsic to CD8 T cells can contribute to the age-associated defects in the primary CD8 T cell response during viral infection.

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.

Bim dictates naive CD4 T cell lifespan and the development of age-associated functional defects

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

Cell-intrinsic defects in the proliferative response of antiviral memory CD8 T cells in aged mice upon secondary infection

Although previous studies have demonstrated delayed viral clearance and blunted effector T cell responses in aged mice during infection, memory CD8 T cells and especially secondary responses have received less attention. In this study, we show that modest differences in the number of memory CD8 T cells formed in aged versus young animals were associated with altered memory CD8 T cell differentiation. Aged immune mice had increased morbidity and mortality upon secondary viral challenge, suggesting changes in T cell immunity. Indeed, virus-specific memory CD8 T cells from aged mice showed substantially reduced proliferative expansion upon secondary infection using multiple challenge models. In addition, this defect in recall capacity of aged memory CD8 T cells was cell-intrinsic and persisted upon adoptive transfer into young mice. Thus, the poor proliferative potential of memory T cells and altered memory CD8 T cell differentiation could underlie age-related defects in antiviral immunity.

Limited expansion of virus-specific CD8 T cells in the aged environment

The mechanisms responsible for the diminished immune response seen with aging are unclear. In this study, we investigate the contributions of alterations in the lymphoid microenvironment to this decrease. Using adoptive transfer of virus-specific transgenic CD8 T cells, we demonstrate that the aged environment inhibits the clonal expansion of specific CD8 T cells from young mice during virus infection. Transferred specific CD8 T cells from young mice demonstrated a response reflecting the CD8 T cell response of the intact aged host: the CD8 T cells expand more slowly and have a decreased maximal expansion in an aged compared to a young environment. While isolated DCs (MHC II(+) CD11c(+)) of aged mice maintain their ability to support CD8 T cell Ag-specific expansion in vitro, splenocytes demonstrated an age-associated decrease in this ability. Since the percentages of various populations of DCs in splenocytes demonstrate no significant alteration with age, this diminished APC activity of splenocytes of aged mice may reflect inhibitory activity of other cell populations. The results of this study demonstrate that elements of the aged environment play an important role in the alteration of T cell response to virus infection in the aged.

The effect of aging on OX40 agonist-mediated cancer immunotherapy

Agents that enhance T cell co-stimulatory signaling have emerged as promising cancer immunotherapies. Our laboratory has been evaluating the TNF receptor co-stimulatory molecule, OX40, which has the capacity to augment critical aspects of T cell function and induce tumor regression in animal models. Effective stimulation of OX40 expressing T cells was accomplished with agonist antibodies to OX40 that were eventually translated into a clinical trial for cancer patients. A recent attempt to assess the affect of immune senescence on OX40 therapy, revealed a dramatic loss of efficacy of the agonist therapy in older tumor-bearing mice. The deficiency in OX40-enhanced anti-tumor responses in older mice correlated with a decrease in the number of differentiated effector T cells. Further investigation suggests that the underlying age-related decline in the agonist OX40-mediated T cell responses was not inherent to the T cells themselves, but related to the host environment. Thus, effective use of immunotherapies based on T cell co-stimulatory molecules may require additional modifications, such as immune stimulants to increase innate immunity, to address age-related defects that reside outside of the T cell and within the host environment.

Effects of aging on the adaptive immune response to respiratory virus infections

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

Prospects for developing an effective particle-mediated DNA vaccine against influenza

Vaccine strategies capable of conferring broad protection against both seasonal and pandemic strains of influenza are urgently needed. DNA vaccines are an attractive choice owing to their capacity to induce robust humoral and cellular immune responses at low doses and because they can be developed and manufactured rapidly to more effectively meet the threat of an influenza epidemic or pandemic. Particle-mediated epidermal delivery (PMED), or the gene gun, is a DNA vaccine delivery technology shown to induce protective levels of antibody and T-cell responses in animals and humans against a wide variety of diseases, including influenza. This review focuses on current advances toward the development of an effective PMED DNA vaccine against influenza, including strategies to enhance vaccine immunogenicity, the potential for PMED-based DNA vaccines to improve protection in the vulnerable elderly population, and the prospects for a vaccine capable of providing cross-protection against both seasonal and pandemic strains of influenza.

OX40-enhanced tumor rejection and effector T cell differentiation decreases with age

OX40 agonists have potent immunotherapeutic effects against a variety of murine tumors, yet it is unclear the role that age-related immune senescence plays on their efficacy. We found that middle-aged and elderly tumor-bearing mice (12 and 20 mo old, respectively) treated with anti-OX40 were less responsive compared with young mice 6 mo or less of age. Decreased tumor-free survival was observed in both male and female mice, and was not due to changes in the surface expression of OX40 on T cells in older animals. Enumeration of cytokine-producing effector T cells in tumor-bearing mice revealed a significant decline in these cells in the older mice treated with anti-OX40 compared with their younger counterparts. The decrease of this critical T cell population in middle-aged mice was not a result of inherent T cell deficiencies, but was revealed to be T cell extrinsic. Finally, combining IL-12, an innate cytokine, with anti-OX40 boosted levels of differentiated effector T cells in the older anti-OX40-treated mice and partially restored the defective antitumor responses in the middle-aged mice. Our data show that the anti-OX40-enhancement of tumor immunity and effector T cell numbers is decreased in middle-aged mice and was partially reversed by coadministration of the proinflammatory cytokine IL-12.

Development of effective vaccines for old mice in a tumor model

Vaccines are often inefficient in old people and old mice. Few studies have focused on testing vaccines in old populations. Here we used DNA tumor antigen vaccines against melanoma and showed that old mice were not protected. Vaccines incorporating fusions of the tumor antigen with microbial adjuvant proteins OmpA (E. Coli) or Vp22 (Herpes simplex virus-1) dramatically improved protection of old mice. The mechanisms by which these adjuvant proteins act are distinct. TLR2 was not required for either OmpA or Vp22. Antigen processing and presentation were not boosted by these fusion constructs. However, fusion constructs with Vp22 gave a strong CD4 response to B16 melanoma and the OmpA response is MHC-II dependent. Both adjuvant fusion constructs stimulated CD4 and CD8 responses otherwise diminished in old mice.

Age-associated decline in T cell repertoire diversity leads to holes in the repertoire and impaired immunity to influenza virus

A diverse T cell repertoire is essential for a vigorous immune response to new infections, and decreasing repertoire diversity has been implicated in the age-associated decline in CD8 T cell immunity. In this study, using the well-characterized mouse influenza virus model, we show that although comparable numbers of CD8 T cells are elicited in the lung and lung airways of young and aged mice after de novo infection, a majority of aged mice exhibit profound shifts in epitope immunodominance and restricted diversity in the TCR repertoire of responding cells. A preferential decline in reactivity to viral epitopes with a low naive precursor frequency was observed, in some cases leading to “holes” in the T cell repertoire. These effects were also seen in young thymectomized mice, consistent with the role of the thymus in maintaining naive repertoire diversity. Furthermore, a decline in repertoire diversity generally correlated with impaired responses to heterosubtypic challenge. This study formally demonstrates in a mouse infection model that naturally occurring contraction of the naive T cell repertoire can result in impaired CD8 T cell responses to known immunodominant epitopes and decline in heterosubtypic immunity. These observations have important implications for the design of vaccine strategies for the elderly.

Dysregulation of IL-15-mediated T-cell homeostasis in TGF-beta dominant-negative receptor transgenic mice

T-cell subpopulations, defined by their expression of CD4, CD8, naive, and memory cell-surface markers, occupy distinct homeostatic compartments that are regulated primarily by cytokines. CD8+ memory T cells, as defined by CD44(hi) surface expression, are dependent on IL-15 as a positive regulator of their homeostatic maintenance. Manipulation of IL-15 signaling through gene aberration, overexpression, or receptor alterations has been shown to dramatically affect T-cell homeostasis, with overexpression leading to fatal leukemia. Here we show that TGF-beta is the critical negative regulator of murine CD8+ memory T-cell homeostasis with direct opposition to the positive effects of IL-15. This negative regulation is mediated, at least in part, by the ability of TGF-beta to modulate expression of the beta-chain of the IL-15 receptor, thus establishing a central axis between these 2 cytokines for homeostatic control of CD8+ memory T-cell populations. These data establish TGF-beta as a critical and dominant tumor-suppressor pathway opposing IL-15-mediated CD8+ T-cell expansion and potential malignant transformation.

Immunity and age: living in the past?

Increasing age is associated with a decreasing ability to mediate effective immune responses to newly encountered antigens. It is generally believed that this reflects the age-associated decline in the number, repertoire and function of available naive T cells. Here, we propose that naive T cells become increasingly irrelevant to the immune system, and that responses to newly encountered antigens are progressively dominated by cross-reactive memory T cells as the individual ages. In addition, we propose that the majority, if not all, of the response to newly encountered antigens in the elderly is mediated by cross-reactive memory T cells. This predicts highly stochastic responses to new infections that should vary between individuals, and has important implications for vaccination strategies in the elderly.

Immune receptor signaling, aging, and autoimmunity

With advancing age, the immune system undergoes changes that predispose to autoimmune reactivity. Aging reduces the efficiency of physical barriers, decreasing protection against invasive pathogens, and exposing previously hidden antigens in the body's own tissues. Self-antigens acquire alterations that increase their immunogenicity. In addition, the ability of innate immunity to eliminate infectious agents deteriorates, resulting in inappropriate persistence of immune stimulation and antigen levels exceeding the threshold for the activation of B or T cells. B cell turnover is reduced and numbers of naïve T cells decline to the advantage of increasing numbers of memory T cells. In parallel, the loss of co-stimulatory T cell molecules may increase reactivity of T cells, and render them less susceptible to downregulation. Since optimal immune reactivity requires a tight balance of transduction pathways in both T and B lymphocytes, and because these pathways are altered in systemic autoimmune diseases, we would like to propose that, with age, alterations of the immune receptor signaling machinery underlie the higher incidence of autoimmune phenomena in the elderly. Consistently, aging is associated with alterations in several components of the signaling complex in B cells, memory and naïve T cells, and a reduced activation of several lipid rafts-associated proteins. Because the coincidence of autoimmune disease with other ailments increases the burden of disease and limits therapeutic options in the aged, further investigation of these pathways in the elderly represents a challenge that will need to be addressed in order to devise effective preventive and therapeutic interventions.

Intrinsic versus environmental influences on T-cell responses in aging

A decline in T-cell responses and a switch to memory T-cell predominance occur with aging. We have used the T-cell receptor (TCR) transgenic mouse model to study age-associated changes in T-cell responses that are a consequence of shifts in subset representation versus changes intrinsic to T cells versus changes in the 'aged' microenvironment. We found that naive transgene-expressing (Tg(+)) CD4(+) T cells from aged mice respond to antigen with reduced interleukin-2 (IL-2) production, decreased cell expansion, and limited differentiation to effectors. Comparable to the characteristic accumulation of memory phenotype T cells in aged humans and conventional rodents, Tg(+) CD4(+) T cells from old OTII and 6.5 TCR transgenic mice acquire a memory phenotype without immunization and become hyporesponsive. The naive Tg(+) CD8(+) T cells from aged 2C mice expressed activation markers, produced IL-2, proliferated, and differentiated into cytotoxic T lymphocytes as efficiently as their young counterparts. Responses by adoptive transferred Tg(+) cells from young mice, immunized in young and old conventional hosts, indicated that the host age influences the onset of cell division, level of cell expansion, and number of cytokine-producing cells. Co-transfer of dendritic cells (DCs) from young and less so from aged conventional mice partially restored responses. Furthermore, DCs and T-cell migration to draining lymphoid organs was reduced due to deficiencies intrinsic to aged cells and the aged environment. Thus, alterations in T-cell responses in aging are attributable to intrinsic and environmental influences.

Aged mice develop protective antitumor immune responses with appropriate costimulation

There is a clear decrease in CD8(+) T cell effector function with aging, a loss once thought to be intrinsic to the CD8(+) T cells. Recent studies suggest, however, that this decline may be a consequence of altered stimulatory signals within the aged lymphoid microenvironment. In this study, we compared the immune responses of young and old mice against the BM-185 pre-B cell lymphoma expressing enhanced GFP (EGFP) as a surrogate tumor Ag. Young animals develop protective immune responses when immunized with BM-185-EGFP, but aged mice do not and ultimately succumb to the tumor. However, expression of CD80 (B7.1) on the BM-185-EGFP (BM-185-EGFP-CD80) results in rejection of the tumor by both young and old animals. Additionally, injection of BM-185-EGFP-CD80 cells in young mice promotes the development of long-lasting memory responses capable of rejecting BM-185 wild-type tumors. Aged animals similarly injected did not develop antitumor memory responses. Interestingly, old animals immunized with the BM-185-EGFP-CD80 cells plus injections of the agonist anti-OX40 mAb did develop long-lasting memory responses capable of rejecting the BM-185 wild-type tumors with the same vigor as the young animals. We show that old mice have the capacity to develop strong antitumor responses and protective memory responses as long as they are provided with efficient costimulation. These results have important implications for the development of vaccination strategies in the elderly, indicating that the aged T cell repertoire can be exploited for the induction of tumor immunity.

Is thymus redundant after adulthood?

Thymus is considered to involute with age with a decline in thymic function. However, this generality is not universally and incontrovertibly true. Many studies performed in animals and men have proved to the contrary that thymic activity and function appear to be well maintained in the old age and may be indispensable for T cell reconstitution in different immunological settings. During some clinical situations where T cell pool needs to be regenerated, renewal of thymic activity and mass has been observed in an otherwise dormant thymic remnant. New studies have revealed a dynamic interplay between postnatal thymus output and peripheral T cell pool. Moreover, age-related loss of thymic function appears to be only quantitative and not qualitative. This review, thus, focuses on the different conditions that lead to thymic involution and attempts to bring about the emerging notion and the clinical relevance of continuous thymic activity well beyond the adulthood to optimise the function of the immune system in the context of cancer and infectious diseases.

No intrinsic deficiencies in CD8+ T cell-mediated antitumor immunity with aging

Aging is associated with a decline in immune function, particularly within the T cell compartment. Because CD8(+) T cells are critical mediators of protective immunity against cancer, which arises more frequently with advancing age, it is important to understand how aging affects T cell-based antitumor responses. We used our DUC18 T cell/CMS5 tumor model system to examine the ability of both aged APCs and aged, tumor-specific CD8(+) T cells to mount protective responses to tumors in vivo. An assessment of aged DUC18 T cells in vitro showed a naive phenotype, but impaired proliferation in response to anti-CD3 and anti-CD28 stimulation. We found that DCs from young and old recipient mice are comparable phenotypically, and endogenous APCs in these mice are equally able to prime adoptively transferred young DUC18 T cells. Even when aged DUC18 T cells are transferred into aged CMS5-challenged mice, Ag-specific proliferation and CD25 expression are similar to those found when young DUC18 T cells are transferred into young mice. Although trafficking to tumor sites appears unequal, old and young DUC18 T cells reject primary CMS5 challenges to the same degree and with similar kinetics. Overall, we found no loss of endogenous APC function or intrinsic defects in CD8(+) DUC18 T cells with advanced age. Therefore, when young and old tumor-specific T cell populations are equivalently sized, CD8(+) T cell-mediated antitumor immunity in our system is not impaired by age, a finding that has positive implications for T cell-based immunotherapies.