Horses greater than 20 years of age exhibit alterations in their immune res
ponses similar to those observed in other aged individuals. The purpose of
this study was to characterize immunosenescence in a population of aged pon
ies. The peripheral blood mononuclear cells (PBMC) from aged ponies exhibit
ed a decreased proliferative response to various mitogens that was not over
come by the addition of interleukin 2 (IL-2) to the cultures. No difference
in overall expression of the IL-2 receptor was seen between young and aged
ponies, though CD8(+) cells from aged ponies exhibited increased levels of
IL-2 receptor expression. The kinetics of the response to both mitogen and
IL-2 did not appear to be affected in the aged PBMCs. These results indica
te that the age-related decrease in the proliferative response to mitogens
is not due to a failure to produce or respond to IL-2 but probably involves
some other process. (C) 2001 Elsevier Science Ltd. All rights reserved.