Jiu Jiang

Response of aged mice to primary virus infections

Summary:  Aging is associated with an increased morbidity to virus infections as well as a delay in clearance of symptoms after infection. Studies of sublethal virus infections of aged mice closely mirror the human situation: there is a delay in clearance of virus. The delay in virus clearance is accompanied by a delay and a decrease in T‐cell response, particularly of CD8+ T cells. Intrinsic alterations of T cells of aged mice contribute to this decrease in virus‐specific T‐cell response; however, evidence suggests that environmental or innate components of the aged host also influence this age‐associated decline in clearance of virus. While the changes in the adaptive immune response have been carefully described, the early events in the generation of the T‐cell response after virus infection have received limited attention. Importantly, age‐associated changes in the innate response to virus infection, particularly production of and response to interferon (IFN)‐α/β, cytotoxicity and IFN‐γ production by natural killer cells, interleukin‐12 induction, and depletion of non‐specific T cells early during virus infection need further evaluation.

An Essential Role of Th1 Responses and Interferon-gamma in Infection-Mediated Suppression of Neoplastic Growth

We had previously demonstrated that in mice acute toxoplasmosis leads to systemic inhibition of angiogenesis and, consequently, strong suppression of neoplastic growth. Here we investigated the role of Th1 cytokines, in particular interferon gamma (IFNg), in this phenomenon. Besides toxoplasma, neoplastic growth was readily blocked during acute infection with other Th1 response-inducing pathogens such as Listeria monocytogenes and lymphocytic choriomeningitis virus (LCMV). In contrast, chronic infection with LCMV (when Th1 responses were strongly suppressed) and acute infection with Schistosoma mansoni (when Th2 responses predominated) afforded no anti-tumor protection. To corroborate the involvement of Th1 cytokines in infection-mediated suppression of neoplastic growth, we utilized mice deficient in interleukin-10 (IL10), a suppressor of Th1 responses. When challenged with B16 cells concomitantly with toxoplasma infection, both IL10-null and wild type mice exhibited resistance to neoplastic growth. However, tumors borne by IL10-null animals were even smaller than those borne by their wild type counterparts. This enhanced resistance correlated with dramatically elevated levels of circulating IFNg, a principal Th1 cytokine. Furthermore, while interleukin-12 and tumor necrosis factor g were dispensable for tumor suppression, in animals deficient in IFNg production or signaling, tumor growth and neovascularization were markedly enhanced. Interestingly, the enhancement was also apparent in uninfected animals suggesting that IFNg and its anti-angiogenic effects underlie both infection-dependent and -independent tumor surveillance.

Characterization of the Metabolic Phenotype of Rapamycin-Treated CD8+ T Cells with Augmented Ability to Generate Long-Lasting Memory Cells

Background Cellular metabolism plays a critical role in regulating T cell responses and the development of memory T cells with long-term protections. However, the metabolic phenotype of antigen-activated T cells that are responsible for the generation of long-lived memory cells has not been characterized. Design and Methods Using lymphocytic choriomeningitis virus (LCMV) peptide gp33-specific CD8+ T cells derived from T cell receptor transgenic mice, we characterized the metabolic phenotype of proliferating T cells that were activated and expanded in vitro in the presence or absence of rapamycin, and determined the capability of these rapamycin-treated T cells to generate long-lived memory cells in vivo. Results Antigen-activated CD8+ T cells treated with rapamycin gave rise to 5-fold more long-lived memory T cells in vivo than untreated control T cells. In contrast to that control T cells only increased glycolysis, rapamycin-treated T cells upregulated both glycolysis and oxidative phosphorylation (OXPHOS). These rapamycin-treated T cells had greater ability than control T cells to survive withdrawal of either glucose or growth factors. Inhibition of OXPHOS by oligomycin significantly reduced the ability of rapamycin-treated T cells to survive growth factor withdrawal. This effect of OXPHOS inhibition was accompanied with mitochondrial hyperpolarization and elevation of reactive oxygen species that are known to be toxic to cells. Conclusions Our findings indicate that these rapamycin-treated T cells may represent a unique cell model for identifying nutrients and signals critical to regulating metabolism in both effector and memory T cells, and for the development of new methods to improve the efficacy of adoptive T cell cancer therapy.

Aging affects initiation and continuation of T cell proliferation

Aging is associated with a decline in immune responses, particularly within the T cell compartment. While the expansion of specific T cells in response to virus infections is consistently decreased in aged mice, the differences in T cell activation between young and aged mice as demonstrated in each round of proliferation remain poorly defined. In the present study, we utilized the T cell mitogen, ConA, to explore if fewer T cells of aged mice initiate proliferation upon mitogen stimulation or if similar numbers of T cells of aged mice begin proliferation but undergo fewer rounds of division. We also examined whether these age-associated changes in proliferation are reflected by differences in T cell activation by comparing activation markers (CD25, CD69, CD44, and CD62L) on T cells of young and aged mice at each round of proliferation. Not only was the kinetics of the expression of these markers greatly different between young and aged mice on the entire CD8 T cell population, but also at each round of proliferation. Our results demonstrate that a larger percentage of CD8 T cells of aged mice do not proliferate at all upon stimulation. Of the CD8 T cells of aged mice that do proliferate, a larger percentage start later and stop sooner. These results suggest that multiple levels of alteration may need to be considered when trying to maximize the immune response of aged individuals.

Depletion of T Cells by Type I Interferon: Differences between Young and Aged Mice

Type I IFN (IFN-I or IFN-αβ) plays an important role in the innate immune response against viral infection. Here we report that a potent inducer of IFN-αβ, polyinosinic-polycytidylic acid [poly(I:C)], led to the depletion of T cells in young, but not aged mice, and that this depletion was limited to central memory, but not effector memory, T cells. Although early activation of T cells in vivo by poly(I:C), as demonstrated by CD69, was not impaired with aging, the expression of active caspase-3 was higher in young compared with aged mice. This depletion of T cells and induction of active caspase-3 in young mice and of CD69 in both young and aged mice by poly(I:C) were blocked by anti-IFN-αβ Ab. Although poly(I:C) stimulated lower circulating levels of IFN-αβ in aged mice, administration of IFN-αβ after poly(I:C) did not induce depletion of T cells in aged mice. These results indicate that IFN-αβ plays a critical role in the depletion of T cells of young mice, and further suggest that the lower level of functional IFN-αβ and decreased induction of active caspase-3 in T cells of aged mice after poly(I:C) may be responsible for the increased resistance of T cells of aged mice to depletion.

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.

Cutting Edge: T Cells from Aged Mice Are Resistant to Depletion Early During Virus Infection

Aging is associated with decreased expansion of T cells upon stimulation. In young mice, infection induces a transient T cell depletion followed by the development of an Ag-specific T cell response that controls the infection. We found that T cells were depleted early after infection with E55 + murine leukemia retrovirus in young, but not aged, mice. Adoptive transfer experiments showed donor T cells of young, but not aged, mice were depleted due to apoptosis in various tissues of young recipients. However, T cells of neither young nor aged donors were depleted in aged recipients. These results indicate that both environmental and intrinsic cellular properties limit depletion of T cells of aged mice and suggest a novel explanation for the decreased T cell response associated with aging.

Expansion of Regulatory T cells in Aged Mice following Influenza Infection

While it has been established that Treg cells can down-modulate an immune response, no study has addressed if the observed increase in Treg cells in aged mice is related to the decreased and delayed specific CD8 T cell responses seen following primary influenza infection. In this study, phenotypic characteristics and function of Treg cells were analyzed in young (4-6 months) and aged (18-22 months) mice prior to and during the course of primary influenza infection. Upon infection, aged, but not young, mice have a significant expansion of Treg cells. In addition, Treg cells of aged mice demonstrate both a higher percentage and higher expression per cell of CD69 both at baseline and during infection compared to young mice. However, Treg cells isolated from young and aged mice comparably suppress CD8 T cells and suppression is dose dependent. These results suggest that the increase in the percentage of Treg cells in aged mice may contribute to the diminished CD8 T cell response to primary influenza infection.

CD8 T cell responses to influenza virus infection in aged mice

Influenza is one of the most common infectious diseases afflicting humans, particularly the elderly. The murine model has been widely employed for investigation of immunity to influenza virus infection. In this paper, we review the recent advances in understanding the diminished CD8 T cell immune response to influenza virus infection in aged mice. Possible mechanisms of impaired CD8 T cell responses with aging are addressed, including: (1) the role of dendritic cells (DCs); (2) the effect of age-associated changes in the T cell repertoire; and (3) the interactions with CD4 T cells, including T regulatory (Treg) cells and CD4 T helper cells. The aged murine model of the CD8 T cell response to influenza virus is helping to elucidate the mechanisms of immunosenescence which can lead to therapeutic improvements in the primary CD8 T cell response to new infections, as well as the development of new strategies for immunization to prevent influenza in the elderly.

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.