Memory T lymphocyte hyporesponsiveness to non-cognate stimuli: a key factor in age-related immunodeficiency

Previous studies from our laboratory have suggested that aging leads to an accumulation of cells expressing high levels of CD44, thought to be a marker for memory lymphocytes, and that positively selected CD44hi T cells, from mice of any age, respond poorly to concanavalin A (Con A) in limiting dilution estimates of interleukin (IL)-2-producing cells. We now report the results of a more comprehensive analysis of memory T cell function, in old and young mice, to non-cognate activators (Con A and the staphylococcal enterotoxin SEB). We report that memory T cells, isolated by removing cells bearing the CD45RB determinant, contain very few cells able to respond to either Con A or SEB under limiting dilution culture conditions, whether the responses are measured by IL-2 or by IL-3 accumulation. As a control, we show that memory T cells do respond strongly, at limiting dilution, to recently encountered priming antigens, i.e. Schistosoma mansoni egg antigen; the limiting dilution culture protocol thus does not preclude activation of memory T cells when cognate stimuli are presented to antigen-specific cells. These data suggest that virgin and memory T cells may differ fundamentally in their activation requirements, and suggest further that the accumulation, with age, of memory T cells accounts for the low responsiveness of old mice to non-cognate mitogens.