Changes in spleen cell proliferative responses and resistance to syngeneic tumor challenge in aging NBR rats

New Zealand Black rats are shown to have a mean longevity of 102.9 weeks. The 90%, 50% and 10% survival age are 81 weeks, 108 weeks and 126 weeks, respectively. Resistance to syngeneic tumor challenge is significantly lower in young rats (3-35 weeks) and very old rats (126-140 weeks) compared to middle-aged rats (36-85 weeks). The in vitro spleen cell proliferative response to syngeneic tumor cells is decreased in old rats. No decrease is observed in spleen cell proliferation induced by the mitogens concanavalin A or phytohemagglutinin. Young (4 weeks) rat spleens contain suppressor cells which affect the mitogen and syngeneic tumor cell induced proliferation of responsive spleen cells (from 50-week-old rats). Spleen cells from old rats (130 weeks) do not demonstrate analogous suppressor cell activity.