T cell differentiation and functional maturation in aging mice

Reversal of apoptotic resistance by Lycium barbarum glycopeptide 3 in aged T cells

OBJECTIVE

To study whether Lycium barbarum glycopeptide 3 (LBGP3) affects T cell apoptosis in aged mice.

METHODS

LBGP3 was purified with DEAE cellulose and Sephadex columns. Apoptotic "sub-G1 peak" was detected by flow cytometry and DNA ladder was resolved by agarose gel electrophoresis. Levels of IFN-gamma and IL-10 were measured with specific kits and mRNA expression was detected by RT-PCR. Apoptosis-related proteins of FLIP, FasL, and Bcl-2 were determined by Western blotting.

RESULTS

LBGP3 was purified from Fructus Lycii water extracts and identified as a 41 kD glycopeptide. Treatment with 200 microg/mL LBGP3 increased the apoptotic rate of T cells from aged mice and showed a similar DNA ladder pattern to that in young T cells. The reversal of apoptotic resistance was involved in down-regulating the expression of Bcl-2 and FLIP, and up-regulating the expression of FasL.

CONCLUSION

Lycium barbarum glycopeptide 3 reverses apoptotic resistance of aged T cells by modulating the expression of apoptosis-related molecules.

A dilemma for mucosal vaccination: efficacy versus toxicity using enterotoxin-based adjuvants

In the development of mucosal vaccines, cholera toxin (CT) has been shown to be an effective adjuvant and to induce both mucosal and systemic immune responses via a Th2 cell-dependent pathway. However, a major concern for use of mucosal adjuvants such as CT is that this molecule is not suitable for use in humans because of its innate toxicity. Recent vaccine development efforts have emphasized nasal application of antigen and CT for the induction of mucosal IgA responses. When we examined potential toxicity of CT for the central nervous system (CNS), both CT and CT-B accumulated in the olfactory nerves/epithelium and olfactory bulbs of mice when given by the nasal route. The development of effective mucosal vaccines for the elderly is also an important issue; however, only limited information is available. When mucosal adjuvanticity of CT was evaluated in aged mice, an early immune dysregulation was evident in the mucosal immune system. The present review discusses these potential problems for effective mucosal vaccine development.

Evidence for early aging in the mucosal immune system

Despite recent advances in the cellular and molecular analysis of induction and regulation of mucosal immune responses, little is yet known about differences which occur in aging. To address this important issue, we have compared the mucosal and systemic immune responses of aged (12- to 14-mo- or 2-year-old) and young adult (6- to 8-wk-old) C57BL/6 mice. Both aged and young mice were immunized weekly with three oral doses of 1 mg of OVA and 10 microg of cholera toxin (CT) as mucosal adjuvant. Both groups of mice over 1 or 2 years of age showed reduced levels of Ag-specific mucosal or systemic immune responses at day 21. An Ag-specific B cell enzyme-linked immunospot assay confirmed these results at the cellular level. When the Ag-induced cytokine responses were examined at both protein and mRNA levels, CD4(+) T cells from spleen and Peyer's patches of young adult mice revealed elevated levels of IL-4 production; however, these cytokine responses were significantly diminished in aged mice. In contrast to mucosal immunization, mice s. c. immunized with OVA plus CT resulted in impaired OVA-specific but intact CT B subunit-specific immune responses in 12- to 14-mo-old mice although the responses to both Ags were depressed in 2-year-old mice. These results provide the first evidence that the development of age-associated alterations possibly occurs earlier in the mucosal immune system than in the systemic immune compartment.

Molecular steps of cell suicide: an insight into immune senescence

The cellular and molecular basis of immune senescence is unclear. A number of mechanisms have been proposed. In this issue of the Journal of Clinical Immunology, some of the mechanisms for various immunologic abnormalities in aging are presented. In this article, various molecular steps of both death receptor and mitochondrial pathways of apoptosis in general are reviewed. In particular, the role of apoptosis in T-cell immune senescence is discussed.

Regulation of Fas-mediated apoptosis in CD2-fas transgenic mice

Fas-mediated apoptosis is an essential mechanism for maintenance of immune homeostasis. The expression of Fas is regulated at transcriptional and protein levels. Furthermore, several death domain molecules and caspases are crucial downstream mediators and executioners of Fas-mediated apoptosis. A tightly regulated interaction of these molecules ensures normal immune functions, including the execution of activation-induced cell death, T-cell mediated cytotoxicity, and surveillance of immune privileged tissues. In contrast, abnormally increased or decreased Fas-mediated apoptosis is a major pathogenic mechanism of several diseases, including systemic or tissue-specific autoimmune diseases and immune deficiency. Two CD2-fas transgenic mouse lines are described here to demonstrate the importance of controlling Fas-mediated apoptosis. Correction of Fas in Fas-mutant mice restored apoptosis function and ameliorated autoimmune symptoms, whereas a long-term enhancement of Fas expression in Fas-normal mice resulted in an increased acute-phase response and renal amyloidosis in aged transgenic mice.

Autoantibodies to T-lineage cells in aged mice

Aging is accompanied by a marked decline in protective immune function. This loss of effective immunity is largely due to alterations in the T-cell compartment. There are major impairments in both the production of new T-cells within the thymus and in the functions of mature T-cells in peripheral lymphoid organs. The mechanism(s) underlying this age-related decline in T-lineage cells is not clear. Here, we demonstrate that aging is accompanied by the appearance of appreciable titers of anti-T-lineage autoantibodies. The autoantibodies, which are exclusively of the IgM class, begin to appear at 1 year of life and are universally found in the sera of 2-year-old mice. Among thymocytes, all CD4/CD8 subsets reacted with the autoantibodies, with the CD4+8+ subset showing the greatest reactivity. The autoantibodies also bound to resting peripheral CD4+ and CD8+ cells. Following activation with either anti-CD3 or with TCR-independent stimulators, reactivity to peripheral T-cells was diminished, suggesting that the determinants recognized by the autoantibodies are downregulated in response to activation signals. Lastly, thymocytes freshly isolated from old, but not young, mice have IgM antibodies bound to their surfaces. Thus, circulating autoantibodies in old mice have access to the thymus and bind to thymocytes in situ. These results lead to the proposal that the presence of anti-T-lineage autoantibodies in vivo interferes with normal T-cell development and/or function in aged animals.

Cell death and longevity: implications of Fas-mediated apoptosis in T-cell senescence

Two prominent features of immune senescence are altered T-cell phenotype and reduced T-cell response. We have previously shown that T-cell senescence is greatly reduced in CD2-fas transgenic mice, in which the Fas apoptosis molecule is constantly expressed on T cells. Using a different experimental approach, the relationship between T-cell senescence and apoptosis was analyzed on human peripheral blood mononuclear cells. The results indicate that there was increased apoptosis of CD45RO- (CD45RA+) T cells upon activation. We propose that this could account for the increase in CD45RO+ "memory" T cells with aging in humans. T-cell responsiveness remained high in CD2-fas transgenic aged mice, but there was no increase in overall life span of these mice. Increased T-cell responsiveness was associated with an increased acute-phase response and serum amyloid A deposition in the glomerulus of aged CD2-fas transgenic mice. Therefore, restoration of the T-cell immune function using a CD2-fas transgene produced undesirable side-effects to aged transgenic mice. In addition to its role in activation-induced cell death, Fas-mediated apoptosis may be important in deleting T cells in response to DNA damage. It may also inhibit cell-cycle progression by cleaving various kinases and DNA repair enzymes. We observed that cell lines derived from human premature aging diseases have a higher sensitivity to Fas-mediated apoptosis. The implications of these observations are discussed.

Clinical features associated with correction of T-cell senescence: increased acute-phase response, amyloidosis and arthritis

Two prominent features associated with immunosenescence are thymic involution and altered T-cell phenotype and responsiveness. We have shown previously that in CD2-fas transgenic mice, in which the Fas apoptosis molecule is constituatively expressed on T cells, T-cell senescence is greatly reduced. Using a different experimental approach, the relationship between T-cell senescence and apoptosis was analyzed on human PBMCs. The results indicate that there was increased apoptosis of CD45RO- (CD45RA+) T cells upon activation. We propose that this could account for the increase in CD45RO+ 'memory' T cells with aging in humans. Together these results are consistent with the notion that T-cell senescence is associated with altered apoptosis and decreased T-cell responsiveness. T-cell responsiveness remained high in CD2-fas transgenic aged mice, but there was no increase in overall life span of the mice. Increased T-cell responsiveness was associated with an increased acute-phase response and amyloid A deposition in the glomerulus of these mice. These data suggest that restoration of the T-cell immune function in aged individuals must be carried out in concert with correction of other immune factors that down modulate the acute-phase response to prevent undesirable side-effects.

Differential immunologic modulatory effects of tetrahydrocannabinol as a function of age

Tetrahydrocannabinol (THC) the major psychoactive component of marijuana, differentially modulates the immune response of lymphoid cells from young and old mice. This effect was previously shown when cells from mice of different ages were treated with THC in vitro. In the present study THC was given in vivo to mice of different ages. Lymphoid cells from the young and old mice had different immunologic potential in terms of ability to produce cytokines following stimulation with either Con A or anti-CD3 antibody. IL-4 and IL-10 production was consistently up-regulated in spleen cell cultures from the older animals. In addition, in vivo administration of THC up-regulated the proliferative response of lymphoid cells from young adult mice. Such enhancement was not evident for cells from older animals. Although the proliferative response of spleen cells in the old mice tended to be suppressed, statistical significance was not evident possibly because of marked variation of the responses of cells from the older mice. Since marijuana is used by persons of a wide range of ages, aging should be an important variable that must be considered when assessing the immunomodilatory effect of this drug.

Thymocyte development in vitro: implications for studies of ageing and thymic involution

Functional defects that accumulate in the T cell compartment are thought to be responsible for the pronounced immunodeficiency that develops during ageing, and reduced production of T cells by the thymus as it undergoes involution has been suggested to contribute to this phenomenon. Understanding the mechanisms responsible for thymic involution requires a thorough knowledge of how thymopoiesis is regulated. Obtaining such information is dependent upon the availability of defined experimental systems that permit analysis of thymopoiesis at the cellular and molecular levels. Recent advances have been made in the development of such human and murine in vitro systems, and their analysis has the potential to identify thymic microenvironmental signals that regulate T cell production. This information should, in turn, provide a basis for understanding changes in thymopoiesis that occur during ageing. The features of these culture systems are reviewed in this article, and their potential application to the study of T cell production during ageing is discussed.

Prevention of age-related T cell apoptosis defect in CD2-fas-transgenic mice

T cell dysfunction and thymic involution are major immunologic abnormalities associated with aging. Fas (CD95) is a bifunctional molecule that is critical for apoptosis and stimulation during T cell development, but the role of Fas during aging has not been determined. Fas expression and function on T cells from old (22-26-mo-old) mice was compared with young (2-mo-old) mice and old CD2-fas-transgenic mice. Fas expression and ligand-induced apoptosis were decreased on T cells from old mice compared with young mice. This correlated with an age-related increase in CD44+Fas- T cells. There was a marked decrease in the proliferation of T cells from old mice after anti-CD3 stimulation compared with young mice. Anti-CD3-stimulated T cells from young mice exhibited increased production of interleukin (IL)-2 and decreased production of interferon-gamma and IL-10 compared with old mice. There was an age-related decrease in the total thymocyte count from 127 +/- 10 cells in young mice compared with 26 +/- 8 x 10(6) in old mice. In 26-mo-old CD2-fas-transgenic mice, Fas and CD44 expression, Fas-induced apoptosis, T cell proliferation, and cytokine production were comparable to that of the young mice. These results suggest that T cell senescence with age is associated with defective apoptosis, and that the CD2-fas transgene allows maintenance of Fas apoptosis function and T cell function in aged mice comparable to that of young mice.