Strategies to induce marked prolongation of secondary skin allograft survival in alloantigen-primed mice

Alloreactive memory T cells mediate accelerated rejection. We investigated the effect of polyclonal anti-T-cell antibody (ALS) and rapamycin (RAPA) on skin allograft survival in naive or alloantigen-primed mice. ALS prolonged graft survival in both naive and alloantigen-primed mice. T-cell depletion by ALS was associated with increased CD4(+)CD44(hi)OX40(+) and CD8(+)CD44(hi)CD122(+) memory T cells. Addition of RAPA to ALS extended graft survival in naive mice, but had no effect on secondary allograft survival in alloantigen-primed mice. In adoptive transfer experiments, RAPA inhibited alloantigen-stimulated proliferation and allograft rejection by naive T cells. In contrast, alloantigen-primed memory T cells, particularly CD4(+)CD44(hi)OX40(+) and CD8(+)CD44(hi)CD122(+) T cells, were resistant to RAPA in response to alloantigen and mediated accelerated rejection in the presence of RAPA. Resistance to RAPA by alloantigen-primed mice was overcome by the use of high-dose ALS, which achieved marked prolongation of secondary skin allograft survival (> 100 days). Inhibition of CD122(+) T cells and/or OX40/OX40L costimulation blockade, combined with low-dose ALS and RAPA, was also effective. These results demonstrate that tolerance may be achieved in allosensitized individuals by T-cell depletion- and RAPA-based strategies employing high-dose ALS or targeting CD122(+)CD8(+) T cells and/or the OX40/OX40L costimulatory pathway.