As human beings and mice age, their immune response decline, increasing vulnerability to infectious disease. This project's long-term objectives are to understand why immune responses decline with age and to study treatments that retard or reverse this decline. The degree to which immune aging is intrinsically timed in the lymphoid cell precursor with the most repopulating and differentiating ability, the primitive stem cell (PSC), must be defined in order to accomplish these objectives. Three approaches will be used to determine how age affects to accomplish these objectives. Three approaches will be used to determine how age affects PSCs: A. PSC numbers, long-term functional abilities, and proliferative patterns (continuous function or sequential activation - inactivation) will be studied using old or young marrow mixed with genetically distinguishable cells in competitive repopulation analyses. B. Numbers and functions when only one, or a very few, PSCs are present will be compared using limiting dilution analyses. C. The functional abilities of individual PSC clones will be tested with PSCs identified by unique retroviral insertions. This will show whether each PSC contributes equally to the differentiated cell population. Thus how PSCs are affected by age will be determined by three independent approaches. Initially mice of the C57BL/6J (B6) strain and congenic competitors with allotypic markers will be tested. However contradictory findings on effects of age may result from genetic differences. Therefore effects of age on PSCs numbers, functional abilities and proliferative patterns will also be determined using mice of the DBA/2J (D2), BALB/c, and B6D2F1 strains, to detect and analyze genetic effects. All studies of PSC function require marrow transplantation, which may cause damage mediated through proliferation, reseeding or both. Their effects on PSCs will be compared in young and old mice, by analyzing recovery of repopulation ability, after a drug causing only proliferation, or after lethal irradiation while shielding different amounts of marrow. To test effects of age not intrinsic to PSCs, the hypothesis that young marrow in mixtures provides accessory cells that enhance functions of old PSCs will be tested. The question of how old marrow restores immune responses in young, but not in old, irradiated recipients will also be studied. In all of this work, a well-defined mouse colony is vital to avoid confusing primary effects of aging with effects caused by stress and disease, which are more common in aging individuals.
