Low-dose antigen-experienced CD4⁺ T cells display reduced clonal expansion but facilitate an effective memory pool in response to secondary exposure

The strength and duration of an antigenic signal at the time of initial stimulation were assumed to affect the development and response of effectors and memory cells to secondary stimulation with the same antigen. To test this assumption, we used T-cell receptor (TCR)-transgenic CD4⁺ T cells that were stimulated in vitro with various antigen doses. The primary effector CD4⁺ T cells generated in response to low-dose antigen in vitro exhibited reduced clonal expansion upon secondary antigenic exposure after adoptive transfer to hosts. However, the magnitude of their contraction was much smaller than both those generated by high-dose antigen stimulation and by naïve CD4⁺ T cells, resulting in higher numbers of antigen-specific CD4⁺ T cells remaining until the memory stage. Moreover, secondary effectors and memory cells developed by secondary antigen exposure were not functionally impaired. In hosts given the low-dose antigen-experienced CD4⁺ T cells, we also observed accelerated recall responses upon injection of antigen-bearing antigen-presenting cells. These results suggest that primary TCR stimulation is important for developing optimal effectors during initial antigen exposure to confer long-lasting memory CD4⁺ T cells in response to secondary exposure.