Hallmarks of human immunosenescence: adaptation or dysregulation?

Is immunosenescence an intrinsic ageing process leading to dysregulation of immunity or an adaptive response of the individual to pathogen exposure? Age-associated differences in bone marrow immune cell output and thymic involution suggest the former. Accepted hallmarks of immunosenescence (decreased numbers and percentages of peripheral nave T cells, especially CD8+ cells, and accumulations of memory T cells, especially late-stage differentiated CD8+ cells) suggest the latter, viewed as the result of depletion of the reservoir of nave cells over time by contact with pathogens and their conversion to memory cells, the basis of adaptive immunity. Thymic involution beginning early in life limits the generation of naive cells such that the adult is believed to rely to a great extent on the nave cell pool produced mostly before puberty. Thus, these hallmarks of immunosenescence would be markedly affected by the history of the individual's exposure to pathogens. It would be predicted that in modern industrialized populations, the cumulative effects of antigenic stressors would be lower than in less hygienic societies, whereas intrinsic processes might be more similar in different populations. Identifying such stressors and taking steps to nullify their impact could therefore result in delayed immunosenescence and contribute significantly to improving public health. Here, I discuss some of the available data bearing on this prediction.