Phyllis-Jean Linton, Ph.D., Associate Professor

Immunology Program

THE DECLINE OF T CELL RESPONSES IN THE AGED

Laboratory Staff: Chinna Pulla Avula, PhD, Beverly Bautista, Evan Bradley, Jay Liu and Yu Zhang, PhD

INTRODUCTION

The aging of the immune system, referred to as immunosenescence, is associated with a dramatic reduction in responsiveness as well as functional dysregulation. This deterioration of immune function with advancing age contributes to the increased incidence among the elderly of morbidity and mortality from infectious disease, and possibly autoimmunity and cancer. Though marginal alterations in B lymphocytes are apparent, the dramatic decline in humoral and cell-mediated responses is predominantly the consequence of senescent T cells. Superimposed on the general reduction in T cell function are the observations of similar numbers of CD4 and CD8 T cells in the periphery and a dramatic shift toward increased representation of T cells expressing a memory phenotype. Altogether these findings are believed to be the consequence of the accumulation of potentially irrelevant memory cells that are generated from a lifetime exposure to antigen in conjunction with thymic involution and a marked diminution in naive T cell output from the thymus. My laboratory's overall goal is to define those functional alterations in T cell responses that are attributable to the accumulation of anergic or defective CD4 and CD8 T cells vs. those alterations attributable to a shift in subset representation. Moreover, we are interested in defining changes in the "aged environment", such as those in dendritic cell recruitment and function, that would impact T cell function and lead to the diminished T cell responses that are observed.

MODEL

Until recently studies comparing the function of young and old T cells have been ambiguous due to changes in the overall representation of naïve vs. memory T cell subsets compounded by differences in the requirements for the optimal immunization of naïve vs. memory T cells. The use of T cell receptor (TcR) transgenic mice is key toward understanding the age-associated alterations in particular T cell populations since a direct comparison of responses to cognate antigen by naïve vs. memory phenotype CD4 and CD8 cells from young and old animals can be achieved. Thus, the generation of responses to cognate antigen by purified transgene expressing T cell populations overcomes previous barriers and allows for a better determination of the effects of aging on T cell function.

OUR FINDINGS

PHENOTYPE:
To this end, we have been using the AND TcR transgenic mice, in which the CD4 T cells express the transgenic TcR that recognizes a pigeon cytochrome c peptide in the context of IEk. Initial findings with the AND mice revealed that the number of transgene positive CD4 T cells is reduced with aging. Given that the pigeon cytochrome c response is of limited heterogeneity and that few, if any, transgene expressing memory cells develop in young AND mice without intentional antigenic stimulation, the chance of exposure to a cross-reacting antigen found in the environment is likely to be minimal. Indeed, we have shown that the transgene positive cells of old AND mice retain the naïve phenotype. This finding supports the hypothesis that the shift to a more memory phenotype within the T cell population in conventional aged animals is likely to be antigen driven and does not happen if antigen is not available. Similar findings were obtained with the 2C TcR transgenic mice, in which the transgene expressing CD8 cells recognize allo-Ld with a peptide found in the Kreb's cycle. Findings with the 2C model were similar to the AND model in that the transgene positive CD8 cells of aged 2C mice is reduced in number and the naïve phenotype is retained. Thus the findings from both the AND and 2C mice support the theory that the predominance of memory cells in the aged is due to a lifetime accumulation of environmental/foreign antigenic stimulation and the reduction of naïve transgene positive cells in the periphery coincides with reduced thymic output.

RESPONSIVENESS:
CD4 T Cells. Under similar conditions of optimal antigen presentation, a decrease in antigen responsiveness by naïve transgene positive CD4 T cells from aged AND mice was observed. This was revealed by a lower secretion of IL-2 and IL-3 and a lower proliferative capacity. The deficiencies in IL-2 production by naïve CD4 T cells of aged AND mice lead to less differentiated effector cells with decreased IL-2 receptor a (CD25) expression and a decreased proliferative capacity. Importantly, deficiencies in effector cell generation could be reversed by the addition of IL-2 but not other gamma receptor binding cytokines. These findings were the first demonstration of a decreased antigen-specific response by naïve CD4 T cells in the aged and suggest that intrinsic changes in the naïve CD4 T cell population occur independent of encounter with antigen. Our working model is shown in Figure 1.

CD8 T Cells. Using similar conditions of antigen stimulation, we have examined the responses generated by CD8+ cells isolated from aged 2C T cell receptor transgenic mice. MHC class I-restricted 2C TCR Tg mic provide a well-defined antigen-specific system to study deficiencies that may be intrinsic to the CD8 cells of the aged since the binding affinities involved in the TCR-MHC/peptide interaction are well defined, the in vitro responses by CD8 cells from 2C mice are CD4-independent, and a variety of Drosophila cell transfectants expressing varying costimulation/adhesion molecules are available as APCs. Using this model, we found that the naïve transgene positive CD8 cells from aged 2C mice expressed activation markers, produced IL-2, proliferated, and differentiated into cytotoxic T cells as efficiently as their young counterparts. The extent of responsiveness and the level of the responses were comparable in both age groups regardless of the stimulatory conditions used, i.e., partial costimulation/adhesion molecule expression on APCs, or presentation of lower affinity peptide or diminished peptide concentrations. By day 4 after antigen stimulation, no significant age-related differences were observed in the number of effector cells generated nor in the levels of secreted IL-2 or IFNg. Upon restimulation of effector cells, IL-2 secretion and to a lesser extent TNFa expression, but not IFNg secretion, were diminished with age. These findings suggest that age-associated alterations in naïve CD8 cell function is not found after primary stimulation but may become apparent upon antigen restimulation.

Figure 1: Responses of naive CD4 T cells from young and aged mice (a hypothesis). When CD4 cells from young mice encounter antigen on an appropriate antigen presenting cell, they secrete high levels of IL-2 and generate a large effector population which is highly susceptible to activation-induced cell death. When CD4 cells from aged mice encounter antigen on an antigen presenting cell, they secrete much less IL-2 and generate a much smaller effector population which is less susceptible to cell death. We suggest that the reduced production of cytokines, such as IL-2 and perhaps IL-3, by aged naive T cells will lead to less expansion during effector generation and altered properties of effectors obtained, including a decreased susceptibility to cell death. We predict this defect alone will lead to smaller primary responses to antigen and to the accumulation of T cells with an activated/memory phenotype which may become nonfunctional or anergic with time.