Low-Affinity Memory CD8+ T Cells Mediate Robust Heterologous Immunity.

Heterologous immunity is recognized as a significant barrier to transplant tolerance. Whereas it has been established that pathogen-elicited memory T cells can have high or low affinity for cross-reactive allogeneic peptide-MHC, the role of TCR affinity during heterologous immunity has not been explored. We established a model with which to investigate the impact of TCR-priming affinity on memory T cell populations following a graft rechallenge. In contrast to high-affinity priming, low-affinity priming elicited fully differentiated memory T cells with a CD45RB(hi) status. High CD45RB status enabled robust secondary responses in vivo, as demonstrated by faster graft rejection kinetics and greater proliferative responses. CD45RB blockade prolonged graft survival in low affinity-primed mice, but not in high affinity-primed mice. Mechanistically, low affinity-primed memory CD8(+) T cells produced more IL-2 and significantly upregulated IL-2Rα expression during rechallenge. We found that CD45RB(hi) status was also a stable marker of priming affinity within polyclonal CD8(+) T cell populations. Following high-affinity rechallenge, low affinity-primed CD45RB(hi) cells became CD45RB(lo), demonstrating that CD45RB status acts as an affinity-based differentiation switch on CD8(+) T cells. Thus, these data establish a novel mechanism by which CD45 isoforms tune low affinity-primed memory CD8(+) T cells to become potent secondary effectors following heterologous rechallenge. These findings have direct implications for allogeneic heterologous immunity by demonstrating that despite a lower precursor frequency, low-affinity priming is sufficient to generate memory cells that mediate potent secondary responses against a cross-reactive graft challenge.