A critical precursor frequency of donor-reactive CD4⁺ T cell help is required for CD8⁺ T cell-mediated CD28/CD154-independent rejection

Ag-specific precursor frequency is increasingly being appreciated as an important factor in determining the kinetics, magnitude, and degree of differentiation of T cell responses, and recently was found to play a critical role in determining the relative requirement of CD8⁺ T cells for CD28- and CD154-mediated costimulatory signals during transplantation. We addressed the possibility that variations in CD4⁺ T cell precursor frequency following transplantation might affect CD4⁺ T cell proliferation, effector function, and provision of help for donor-reactive B cell and CD8⁺ T cell responses. Using a transgenic model system wherein increasing frequencies of donor-reactive CD4⁺ T cells were transferred into skin graft recipients, we observed that a critical CD4 ⁺ T cell threshold precursor frequency was necessary to provide help following blockade of the CD28 and CD154 costimulatory pathways, as measured by increased B cell and CD8⁺ T cell responses and precipitation of graft rejection. In contrast to high-frequency CD8⁺ T cell responses, this effect was observed even though the proliferative and cytokine responses of Ag-specific CD4⁺ T cells were inhibited. Thus, we conclude that an initial high frequency of donor-reactive CD4⁺ T cells uncouples T cell proliferative and effector cytokine production from the provision of T cell help.