Dendritic cells in old age--neglected by gerontology?

Age-associated impairment of antitumor immunity in carcinoma-bearing mice and restoration by oral administration of Lentinula edodes mycelia extract

Because cancer is associated with aging, immunological features in the aged should be considered in anticancer immunotherapy. In this study, we investigated antitumor immunity in aged mice using a CT26 colon carcinoma model. The tumor growth of CT26 was accelerated in aged mice compared with that in young mice, but this difference was not observed in nude mice. The serum levels of IL-6 and TNF-α were higher in aged mice than those in young mice, irrespective of the CT26-bearing state. The in vitro induction of CT26-specific CTLs from aged mice that were vaccinated with doxorubicin (DTX)-treated CT26 cells was impaired. In vivo neutralization of IL-6, but not TNF-α, showed a tendency to restore the in vitro induction of CT26-specific CTLs from vaccinated aged mice. Analyses on tumor-infiltrating immune cells as early as day 5 after CT26 inoculation revealed that monocytic and granulocytic MDSCs preferentially infiltrated into tumor sites in aged mice compared with young mice. Alternatively, oral administration of Lentinula edodes mycelia (L.E.M.) extract, which has the potential to suppress inflammation in tumor-bearing hosts, decreased the serum levels of IL-6 in aged mice. When administration of L.E.M. extract was started 1 week earlier, CT26 growth was retarded in aged mice and the in vivo priming of tumor-specific CTLs was improved in CT26-vaccinated aged mice. These results indicate early infiltration of MDSCs is related to impaired immunity of aged hosts and that oral administration of L.E.M. extract can mitigate the impairment.

Aging, imbalanced inflammation and viral infection

Older people experience enhanced susceptibility to viral infections and subsequent superimposed bacterial infections. Based on both experimental and clinical studies, this susceptibility is thought to be due to declining immune responses. However, our work indicates that older people may succumb to viral infection due to exaggerated immune responses as aged mice produce higher serum levels of the inflammatory mediator IL-17 than younger mice upon herpes viral infection. These age-elevated IL-17 responses induce a lethal immune pathology during viral infection. Early during the course of infection natural killer T-cells (NKT-cells) are major contributors to the elevated IL-17 response in aged mice. These responses synergize with defective viral clearance with aging noted by impaired IFN-α responses by plasmacytoid DCs. Our results indicate that novel anti-inflammatory drugs may resolve imbalanced inflammation and improve outcomes in older people infected with viruses.

B7-H1 expression on old CD8+ T cells negatively regulates the activation of immune responses in aged animals

T cell responses are compromised in the elderly. The B7-CD28 family receptors are critical in the regulation of immune responses. We evaluated whether the B7-family and CD28-family receptors were differentially expressed in dendritic cells, macrophages, and CD4(+) and CD8(+) T cells from young and old mice, which could contribute to the immune dysfunction in the old. Although most of the receptors were equally expressed in all cells, >85% of the old naive CD8(+) T cells expressed B7-H1 compared with 25% in the young. Considering that B7-H1 negatively regulates immune responses, we hypothesized that expression of B7-H1 would downregulate the function of old CD8(+) T cells. Old CD8(+) T cells showed reduced ability to proliferate, but blockade of B7-H1 restored the proliferative capacity of old CD8(+) T cells to a level similar to young CD8(+) T cells. In vivo blockade of B7-H1 restored antitumor responses against the B7-H1(-) BM-185-enhanced GFP tumor, such that old animals responded with the same efficiency as young mice. Our data also indicate that old CD8(+) T cells express lower levels of TCR compared with young CD8(+) T cells. However, following antigenic stimulation in the presence of B7-H1 blockade, the levels of TCR expression were restored in old CD8(+) T cells, which correlated with stronger T cell activation. These studies demonstrated that expression of B7-H1 in old CD8(+) T cells impairs the proper activation of these cells and that blockade of B7-H1 could be critical to optimally stimulate a CD8 T cell response in the old.

Cancer, aging and immunotherapy: lessons learned from animal models

Aging of the immune system is associated with a dramatic reduction in responsiveness as well as functional dysregulation. This deterioration of immune function with advancing age is associated with an increased incidence of cancer. Although there is a plethora of reports evaluating the effect of immunotherapy in stimulating antitumor immune responses, the majority of these studies do not pay attention to the effect aging has on the immune system. Studies from our group and others indicate that immunotherapies could be effective in the young, are not necessarily effective in the old. To optimally stimulate an antitumor immune response in the old, it is necessary to (1) identify and understand the intrinsic defects of the old immune system and (2) use relevant models that closely reflect those of cancer patients, where self-tolerance and aging are present simultaneously. The present review summarizes some defects found in the old immune system affecting the activation of antitumor immune responses, the strategies used to activate stronger antitumor immune response in the old and the use of a tolerant animal tumor model to target a self-tumor antigen for the optimization of immunotherapeutic interventions in the old.

Implications of aging and self-tolerance on the generation of immune and antitumor immune responses

Cancer statistics show a disproportionately higher burden of tumors in the old. Most of the studies evaluating vaccination strategies have not taken into consideration the effect that aging has on the immune system. For the first time, we describe an animal tumor model in which self-tolerance and aging are present at the same time. FVB-Her-2/neu mice which are tolerant to neu antigens crossed with HLA-A2/Kb mice (A2xneu) develop spontaneous tumors when they are more than 22 months old. Analysis of CD8(+) T-cell-specific responses in A2xneu mice indicated that the priming activity of old A2xneu mice to induce an immune response was diminished compared with young animals. Following intratumoral injections of CpG-ODN, approximately 30% of young A2xneu mice rejected the tumor; however, no antitumor effect was observed in old A2xneu mice. Analysis of T regulatory cells (Treg) indicated that there are significantly more Tregs in old animals. After CpG-ODN vaccination plus Treg depletion, 70% of young A2xneu mice rejected the tumor. The same treatment prolonged survival in old A2xneu mice, but none of the animals rejected the tumor. Even though CpG-ODN injections plus Treg depletion could rescue the antitumor responses against self-tumor antigens in young tolerant mice, the same therapy is not as effective in old tolerant hosts. Relevant tumor models such as the A2xneu mice in which self-tolerance and aging are present at the same time are critical to allow the optimization of vaccination strategies to effectively stimulate immune responses against self-tumor antigens in the young and the old.

CpG-ODN but not other TLR-ligands restore the antitumor responses in old mice: the implications for vaccinations in the aged

AIM

There is accumulative evidence indicating that targeting antigen presenting cells (APCs) with different types of adjuvants could result in the induction of antitumor immune responses. It has been hypothesized that APCs function may be altered in the elderly contributing to a decline in the immune function. We evaluated whether targeting APCs following injection with Poly I:C, LPS, flagellin, imiquimod and CpG-ODN would induce an antitumor response in the old.

MATERIALS AND METHODS

The immune and antitumor responses induce Poly I:C, LPS, flagellin, imiquimod and CpG-ODN were compared in young (2 month old) and old (18 months) mice.

RESULTS

Our results indicated that only intratumoral (i.t.) injections of CpG-ODN completely rejected the tumor in both young and old mice. Injections of Poly I:C also induced the rejection of tumors in the young but not in the old. Furthermore, i.t. injections of CpG-ODN promoted the development of protective memory responses in the young and the old. Analysis of the immune responses in the old indicated that CpG-ODN but not Poly-I:C induces: a pro-inflammatory Th1 type response; accumulation and activation of CD4+, CD8+ T and, NK cell responses; activation of APCs; and reduction in the number of Tregs. The activation of these immune-parameters positively correlates with the induction of an antitumor response.

CONCLUSIONS

These studies indicate that there are differences in the level of stimulation with TLR-ligands between young and old APCs and that the aged immune responses can be rescued and exploited for the induction of tumor immunity by targeting APCs with specific TLR-ligands. These results have important clinical implications for developing immunization strategies containing TLR-ligands that will be effective in both the young and old.

Aging and the dendritic cell system: implications for cancer

The immune system shows a decline in responsiveness to antigens both with aging, as well as in the presence of tumors. The malfunction of the immune system with age can be attributed to developmental and functional alterations in several cell populations. Previous studies have shown defects in humoral responses and abnormalities in T cell function in aged individuals, but have not distinguished between abnormalities in antigen presentation and intrinsic T cell or B cell defects in aged individuals. Dendritic cells (DC) play a pivotal role in regulating immune responses by presenting antigens to naïve T lymphocytes, modulating Th1/Th2/Th3/Treg balance, producing numerous regulatory cytokines and chemokines, and modifying survival of immune effectors. DC are receiving increased attention due to their involvement in the immunobiology of tolerance and autoimmunity, as well as their potential role as biological adjuvants in tumor vaccines. Recent advances in the molecular and cell biology of different DC populations allow for addressing the issue of DC and aging both in rodents and humans. Since DC play a crucial role in initiating and regulating immune responses, it is reasonable to hypothesize that they are directly involved in altered antitumor immunity in aging. However, the results of studies focusing on DC in the elderly are conflicting. The present review summarizes the available human and experimental animal data on quantitative and qualitative alterations of DC in aging and discusses the potential role of the DC system in the increased incidence of cancer in the elderly.

Immunological biomarkers of ageing in man: changes in both innate and adaptive immunity are associated with health and longevity

Scientific and clinical advances in the last century have led to increased numbers of individuals living to older ages. Thus a major concern is how to live these years with a high quality of life. The ageing immune system is less well able to cope with infectious diseases than the youthful immune system probably as a consequence of altered immune response to pathogens. Thus, both innate and adaptive immune responses show age-related changes that could be decisive for healthy ageing and survival. Longitudinal studies in healthy elderly have allowed the definition of the ''immune risk phenotype" (IRP) a predictor of mortality in elderly individuals that is based on several parameters of the adaptive immune response. Here, we hypothesize that failures in innate immunity observed in frail elderly are related to those alterations described in adaptive immunity defined as the IRP. It will be important to include assays of NK cell markers and functions in future longitudinal studies in order to investigate this point in detail as well as to consider the trace element zinc as an essential co-factor for optimal NK cell activity.

Effect of aging on bone marrow-derived murine CD11c+CD4-CD8alpha- dendritic cell function

Dendritic cells (DCs) are actively used as cellular adjuvant in cancer immunotherapy. However, although DC immunotherapies primarily target the elderly population, little is known about the effect of aging on DC functions. Here, we compared the T-cell stimulation, cytokine production, and tumor surveillance functions of bone marrow-derived CD11c(+)CD4(-)CD8alpha(-) DCs of old and young C57BL/6 mice. Old immature bone marrow-derived CD4(-)CD8alpha(-) DCs (imDCs) were 4 times less effective than were young DCs in stimulating syngeneic CD4(+) T-cell proliferation. Old imDCs also have decreased DC-specific/intracellular adhesion molecule type 3-grabbing, nonintegrin (DC-SIGN) expression compared to young DCs. Interestingly, mice treated with the ovalbumin peptide-pulsed young DCs exhibited significantly greater tumor regression than with ovalbumin peptide-pulsed old DCs. Old terminally differentiated bone marrow-derived DCs (tDC) also have increased interleukin-10, but decreased interleukin-6 and tumor necrosis factor-alpha production. Taken together, these results have important implications in the clinical application of DC-based tumor immunotherapy in elderly persons.

Murine [corrected] myeloid dendritic cell-dependent toll-like receptor immunity is preserved with aging

The immune response is the result of the interplay between innate and adaptive immunity, yet the impact of aging on this interaction is unclear. Addressing this fundamental question will be critical for the development of effective vaccines for the rapidly rising older subpopulation that manifests increased prevalence of malignancies and infections. Therefore, we undertook the current study to investigate whether aging impairs toll-like receptor (TLR) function in myeloid dendritic cells and whether this leads to reduced T-cell priming. Our results demonstrate that innate TLR immune priming function of myeloid bone marrow derived and splenic dendritic cells (DC) is preserved with aging using both allogeneic and infectious murine experimental systems. In contrast, aging impairs in vitro and in vivo intrinsic T-cell function. Therefore, our results demonstrate that myeloid DCs manifest preserved TLR-mediated immune responses with aging. However, aging critically impairs intrinsic adaptive T-cell function.

Diversity of the Ig Repertoire is Maintained With Age In Spite of Reduced Germinal Centre Cells in Human Tonsil Lymphoid Tissue

Humans and almost all species studied to date exhibit a decreased responsiveness to immunization and increased autoimmunity with age. While this has been observed clinically for decades, only recently has an understanding of the molecular basis for these changes begun to be appreciated. Studies of the B-cell aspects of these changes in ageing mice and the very few reports in ageing humans have not been conclusive. Here we examine the nucleotide sequence of over 1250 VH transcripts from the tonsils of individuals of various ages for changes to the VH4 immunoglobulin repertoire. An exhaustive examination of VH, DH and JH gene segment utilization revealed a remarkable similarity of the repertoires. The extent of somatic hypermutation was fully maintained or even increased by some measures into the eighth decade of life. However, we found by middle age that the representation of naïve and germinal centre B-cell subpopulations changed relative to total B lymphocytes in the tonsil. While the percentage of naïve and germinal centre B-cell subpopulations changes during the second half of life, these findings suggest that even with advancing age, humans remain capable of generating an extremely diverse Ig repertoire while maintaining a similar spectrum of Ig rearrangements once the germinal centre reaction begins.

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.

Mucosal immunity and tolerance in the elderly

Age-associated dysregulation of the immune system of the gastrointestinal (GI) tract has been well documented for both secretory (S)-IgA immunity and oral tolerance. Thus, impaired antigen-specific Ab responses in aged animals and the elderly have been reported. Further, it has been shown that gut-associated lymphoreticular tissue (GALT) mediated immune responses are more susceptible to aging than are lymphoid tissues involved in peripheral immunity. Aging also impairs oral tolerance, which may be of central importance for maintaining GI homeostasis. Thus, as early as 6-8-month-old mice failed to establish systemic unresponsiveness to orally introduced antigens. Despite these studies, the precise mechanisms for impaired GI tract immune system responses remain unclear. The evidence of reduced sizes of Peyer's patches through aging suggests that age-associated mucosal dysregulation may be the result of mucosal inductive tissue dysfunction. Indeed, the frequencies of naive CD4+ T cells and dendritic cells (DCs) in Peyer's patches of aged mice were reduced and this led to a lack of essential cytokine synthesis for the induction of either S-IgA immunity or oral tolerance.

Regulation of dendritic cell expansion in aged athymic nude mice by FLT3 ligand

This report describes age-related alterations of dendritic cells (DC) distribution in nude athymic mice in vivo and reversal of certain age-dependent defects by an in vivo administration of hematopoietic growth factor FLT3 ligand (FLT3L). There are decreased percentages of CD11c(+) DC in the bone marrow and spleen and a reduced expression of MHC class II and CD86 molecules on DC in old nude mice. The decreased levels of CD11c(+) DC were due to the CD8alpha(-) DC subset. The distribution of CD11c(+) CD8alpha(+) DC in the lymphoid tissues was not different in young and old mice. The effect of in vivo administration of FLT3L on the generation and distribution of DC in the lymphoid tissues in young and old nude mice was also evaluated. Although, FLT3L had a higher inductive potential on the expansion of DC from the bone marrow in the elderly mice, the total level of CD11c(+) DC in the young animals was still significantly higher as compared to the old animals. Interestingly, FLT3L induced a pronounced redistribution and accumulation of MHC class II(+) DC in the lymphoid tissues in old mice, markedly increased the accumulation of CD8alpha(-) DC in the bone marrow in both young and old nude mice, and elevated both CD8alpha(-) and CD8alpha(+) DC in the spleen in young mice. However, only the level of CD8alpha(+) DC was up regulated in the spleen in old athymic mice after FLT3L-based therapy. In summary, abnormalities in DC generation and distribution in old athymic mice could be, in part, circumvented by the in vivo administration of FLT3L.

Dramatic increase of telomerase activity during dendritic cell differentiation and maturation

Telomerase, the reverse transcriptase that maintains telomere DNA, is usually undetectable in most adult tissues but is positive in embryonic tissues and in cancers. In addition, freshly islolated or in vitro-activated lymphocytes were shown to express high levels of telomerase activity, although its expression in myeloid cells including dendritic cells (DCs) is largely unknown. Here, we investigated telomerase activity during the differentiation and maturation process of DCs. In vitro culture of bone marrow (BM) cells with granulocyte macrophage-colony stimulating factor and interleukin-4 induced a dramatic increase of telomerase activity accompanied with their differentiation into DCs. Furthermore, stimulation with microbial components such as lipopolysaccharide (LPS), which triggers maturation of DCs, augmented the activity. In vivo responses of telomerase activity were also observed in splenic DCs by injection of LPS intraperitoneally. It is interesting that in old mice, telomerase activity of splenic DCs was significantly higher than young mice but rather decreased after LPS stimulation. By measuring expression of cell-surface activation markers, splenic DCs of old mice responded poorly to LPS stimulation. Such poor responses to LPS were also observed in BM-derived DCs. These different features of DCs between young and old mice may contribute to a pathogenesis to microbial infections.

Immune dysfunction in the elderly and its reversal by antihistamines

The decline in immunity seen in the elderly is a significant contributor to disease burden. This decline has largely been attributed to alterations in T cell immunity and contributes to an overall increased risk and severity of infection in the elderly. A key component of T cell immunity involves antigen presentation, an event where an antigen is processed and presented to specific immune cells for destruction. This event has been found to be crucial to immune function. Recent research has focused on a key antigen presenting cell (APC), the dendritic cell (DC), and changes within its function associated with aging. DCs are considered to be the most professional APCs, and are responsible for the initiation and outcome of effector T cells and their resultant immune response. DCs capture antigens and undergo a maturation process and polarize into either type 1 dendritic cells (DC1) or type 2 dendritic cells (DC2), based upon their ability to favor a T helper1 (Th1) or T helper 2 (Th2) T cell response, respectively. Evidence suggests that in normal healthy adults, a Th1 type response predominates, and in frail elders, a Th2 response predominates. It has been proposed that this change from a predominately Th1 type to a predominate Th2 type response is a possible mechanism for age-associated immune dysfunction. In addition, recent research has focused on how histamine, an inflammatory mediator, promotes a Th2 response. Histamine has also been shown to polarize human DCs into Th2 cell-promoting effector DCs or DC2s. This has been shown to occur via interaction with the H2 receptor. Therefore, we theorize that use of an H2 selective antihistamine will reverse this polarization back to a Th1 type response and therefore improve immune function of the frail elderly.

Circulating, interferon-producing plasmacytoid dendritic cells decline during human ageing

Increased frequency and severity of infections in the elderly have been taken as indicative of declining immune function. Dendritic cells (DCs), the most important antigen-presenting cells, play a central role in initiating and modulating immune responses. One type, DC2, arises from precursor plasmacytoid DCs (pDCs), a rare population of circulating blood cells, whose hallmark function is rapid and copious production of interferon-alpha (IFN-alpha) upon microbial challenge. We found significant decreases of the circulating pDCs during ageing in healthy adult humans, as defined both by flow cytometry and IFN-alpha generation. Mean pDC/mm3 in peripheral blood declined from 7.8 for the youngest age group (18-39 years) to 4.2 for the oldest (60-91 years; P = 0.017). IFN-alpha generation declined similarly, from 3537 to 1201 IU/ml, respectively (P = 0.006). There was also a slight decline over the age range in the amount of IFN generated per pDC (slope = -0.0087; P = 0.046). CD4+ T cells decreased by approximately 20% over the same age range (P = 0.001), while there was no change in the total lymphocyte or monocyte counts.

Enhanced allostimulatory activity of host antigen-presenting cells in old mice intensifies acute graft-versus-host disease

Older bone marrow transplantation (BMT) recipients are at heightened risk for acute graft-versus-host disease (GVHD) after allogeneic BMT, but the causes of this association are poorly understood. Using well-characterized murine BMT models we have explored the mechanisms of increased GVHD in older mice. GVHD mortality, morbidity, and pathologic and biochemical indices were all worse in old recipients. Donor T cell responses were significantly increased in old recipients both in vivo and in vitro when stimulated by antigen-presenting cells (APCs) from old mice, which also secreted more TNF-alpha and IL-12 after LPS stimulation. In a B6 --> B6D2F1 model, CD4(+) donor T cells but not CD8(+) T cells mediated more severe GVHD in old mice. We confirmed the role of aged APCs in GVHD using B6D2F1 BM chimeras created with either old or young BM. Four months after chimera creation, allogeneic BMT from B6 donors caused significantly worse GVHD in old BM chimeras. APCs from these mice also stimulated greater responses from allogeneic cells in vitro. These data demonstrate a hitherto unsuspected mechanism of amplified donor T cell responses by aged allogeneic host APCs that increases acute GVHD in aged recipients in this BMT model.

The frail elderly: role of dendritic cells in the susceptibility of infection

The decline in immunity in the elderly has largely been attributed to impairment of T cell mechanisms. This seems reasonable since the thymus involutes with age, so that the number of naïve cells to respond to new foreign antigens also declines. However, little is known about how aging affects antigen-presenting cells (APC) that are responsible for the initiation and outcome of effector T cell immune responses. This review focuses on the age-related alterations of a key APC, the dendritic cell (DC). Recent findings suggest that interleukin-10, a key cytokine that can suppress cell mediated immunity and maturation of DC subsets, is elevated in the very healthy elderly. However, production of IL-12, required for the initiation of T cell immune responses, declines in frail elderly along with DC antigen presenting function. These findings suggest that shifts in IL-10 and IL-12 may not only directly influence immune response but may also alter the balance and maturation of DC subsets. Finally, study of immunologic differences between the very healthy and frail elderly may reveal important changes in DC function and regulation influenced by age and/or environment (disease, nutrition, medications, etc.).