Immunorejuvenation in the elderly

Abstract

Dysregulated T-cell-mediated immunity contributes materially to the increased susceptibility to infectious disease, and possibly cancer, in the elderly. One hallmark of this state of "immunosenescence" in humans is the predominance of large clones of peripheral T cells with limited antigen receptor heterogeneity and a corresponding contraction of diversity in the T-cell antigen recognition repertoire. Surprisingly, a major driving force for these clonal expansions in humans is cytomegalovirus. The lifelong chronic antigenic stress caused by infection with this persistent activating virus results in clonal exhaustion of specific CD8 T cells, and their acquisition of a state of anergy and apoptosis resistance similar in many respects, and, the authors believe for similar reasons, to that commonly seen in the tumor-specific T cells of cancer patients. This excess of dysfunctional cells is indirectly immunosuppressive by filling the "immunologic space" and shrinking the T-cell repertoire for new antigens, as well as directly suppressive via cytokine secretion. It is associated with the "immunologic risk profile" predicting 2- and 4-year mortality in longitudinal studies of very old people. Therefore, it is hypothesized that deletion of such accumulations of dysfunctional cells would be beneficial to the individual. It may be possible to distinguish functional CMV-specific cells (which are essential to maintain immunosurveillance) from dysfunctional ones by their expression of certain surface molecules. This, coupled with methods directed at reinvigorating the thymus (e.g., use of interleukin 7), and targeting CMV by pharmacologic and immunotherapeutic interventions might result in a degree of "immunorejuvenation" sufficient to take elderly individuals out of the risk category and thereby extend healthy longevity.