Alteration in CD45RBhi/CD45RBlo T-cell ratio following CD45RB monoclonal-antibody therapy occurs by selective deletion of CD45RBhi effector cells

Tolerance induction by CD45RB monoclonal antibody (mAb) in murine allograft models is associated with an alteration in the CD45RB(lo)/CD45RB(hi) T-cell ratio in favor of CD45RB(lo) T cells, which can function as regulatory cells and promote tolerance. It has been proposed that inversion of the CD45RB(hi)/CD45RB(lo) normal T-cell ratio by mAb can occur by down-regulation of CD45RB surface molecules expressed by T cells. Because CD45RB mAb infusion can lead to a reduction in peripheral T cells, we tested whether other mechanisms might participate in the inversion of the CD45RB(hi)/CD45RB(lo) ratio, including apoptosis of CD45RB hi cells. We report that CD45RB mAb led to rapid elimination of both CD4(+) and CD8(+) T cells in vitro. Importantly, CD45RB mAb selectively eliminated CD45RB(hi)T cells without affecting the viability of CD45RB(lo) T cells. Furthermore, the death of T cells occurred with a reduction in mitochondrial transmembrane potential and DNA fragmentation but with little evidence of nuclear condensation and cell shrinkage typically found with cells undergoing apoptosis. We propose that CD45RB mAb therapy may promote a dominant regulatory T-cell population that has the capacity to inhibit rejection by the selective elimination of CD45RB(hi) effector T cells. This occurs by a process that does not involve the classic morphologic features of apoptosis. Strategies that facilitate an inversion of the CD45RB(hi)/CD45RB(lo) T-cell subset ratio may improve the efficacy of CD45RB mAb, and therapeutic measures that prevent deletion of CD45RB(hi) T cells may need to be avoided to achieve tolerance clinically.