Increased proliferation of CD8+ T cells in SAP-deficient mice is associated with impaired activation-induced cell death.

Defective signaling lymphocyte activation molecule (SLAM)-associated protein (SAP) is responsible for the human X-linked lymphoproliferative syndrome. Defects in T helper 2, natural killer, natural killer T and B cells have been demonstrated in SAP-deficient humans and mice, and increased proliferation of CD8+ T cells has been observed. In the current study, we investigated the properties of CD8+ T cell proliferation and activation-induced cell death (AICD), using OT-I T cell receptor (TCR)-transgenic mice on either wild-type (WT) or SAP-/- background. Interestingly, we found that ovalbumin peptide-activated SAP-/- CD8+ T cells have lower AICD compared to their WT counterparts. Furthermore, the induction of p73, a key mediator of TCR-induced apoptosis through the mitochondrial apoptotic pathway, was significantly reduced at both the mRNA and protein levels in the activated mutant cells. Meanwhile, a reduced level of activated caspase 9 was observed in the mutant cells. We conclude that reduced AICD in activated SAP-/- CD8+ T cells is associated with impaired p73 induction, indicating that the initiation of the mitochondrial apoptotic pathway might be impaired. Our data demonstrate an intrinsic defect in SAP-/- CD8+ T cells and shed light on the increased responsiveness of CD8+ T cells in SAP-/- mice.