The effect of age on the cognate function of CD4+ T cells.

With increasing age, the ability to produce protective antibodies in response to immunization declines, resulting in reduced efficacy of vaccination. We have examined how reductions in CD4(+) T-cell function contribute to reduced humoral responses, using a model that allows us to compare identical numbers of antigen-specific naive T cells from young and aged T-cell receptor transgenic mice. Naive cells from aged mice exhibit reduced responses, both in vitro and in vivo. In vitro, responses of aged T cells can be enhanced by addition of interleukin (IL)-2. In vivo, using an adoptive transfer model with young hosts, naive cells from aged mice exhibit significant reductions in cognate helper function, leading to reduced B-cell expansion and differentiation. These age-related defects could be overcome by prior in vitro T helper 2 effector generation with aged T cells. This improvement in cognate function of the aged effectors may be related to the enhancement of CD154 expression, which occurs on aged T cells in the presence of exogenous IL-2. We also found no difference in B-cell expansion and differentiation when young cells were transferred to young or aged hosts. Our results indicate that age-related reductions in humoral responses are mainly due to defects in the cognate helper function of naive CD4(+) T cells from aged individuals.