Cleavage of anti-apoptotic Bcl-2 family members after TCR stimulation contributes to the decision between T cell activation and apoptosis.

Engagement of the TCR induces activation-induced cell death (AICD) of T cells that have been previously stimulated. However, a portion of these T cells can survive and undergo further activation. The molecular mechanism that decides whether a T cell will live or die after TCR re-engagement is unclear. We found that cross-linking of TCR in preactivated primary mouse T cells led to the cleavage of anti-apoptotic Bcl-2 and Bcl-xL in dying cells. Cleavage-resistant Bcl-2 and Bcl-xL were more efficient than their wild-type counterparts in the inhibition of apoptosis in primary mouse T cells and in the H9 T cell line after TCR cross-linking. In contrast, the surviving T cells after TCR re-engagement displayed upregulation of Bcl-xL, and knockdown of Bcl-xL promoted AICD. This indicates that caspase-mediated cleavage of anti-apoptotic Bcl-2 or Bcl-xL facilitates AICD in T cells, whereas upregulation of Bcl-xL promotes T cell survival and allows further T cell activation. Our data suggest that cleavage of anti-apoptotic Bcl-2 and Bcl-xL contributes to the decision between T cell activation and apoptosis after TCR re-engagement.

Interleukin-2-mediated inhibition of dendritic cell development correlates with decreased CD135 expression and increased monocyte/macrophage precursors.

We have previously shown that interleukin-2 (IL-2) inhibits dendritic cell (DC) development from mouse bone marrow (BM) precursors stimulated with the ligand for FMS-like tyrosine kinase 3 receptor (Flt3L), and have provided evidence that this inhibition occurs at the monocyte DC precursor stage of DC development. Here, we explored the mechanism of IL-2-mediated inhibition of DC development. First, we showed that these in vitro cultures accurately model DCs that develop in vivo by comparing gene and protein expression of the three main Flt3L-induced DC subsets from the BM, CD11b(+) and CD24(+) conventional DCs (cDCs) and plasmacytoid DCs (pDCs) with their respective ex vivo spleen DC subsets (CD11b(+), CD8(+) and pDCs). Next, gene expression changes were quantified in Flt3L DC subsets that developed in the presence of IL-2. These changes included increased expression of Bcl2l11, which encodes the apoptosis-inducing protein Bim, and decreased expression of Flt3 (CD135), the receptor that initiates DC development. Interleukin-2 also significantly reduced Flt3 protein expression on all three Flt3L DC subsets, and attenuated Flt3L-induced STAT3 phosphorylation in DCs. Based on these data, we hypothesized that decreased Flt3 signalling may divert BM precursors down monocyte and macrophage lineages. Indeed, addition of IL-2 led to increases in Flt3(-) cells, including cKit(+) Ly6C(+) CD11b(-) populations consistent with the recently identified committed monocyte/macrophage progenitor. Therefore, IL-2 can inhibit DC development via decreased signalling through Flt3 and increased monocyte/macrophage development.

The human application of gene therapy to re-program T-cell specificity using chimeric antigen receptors.

The adoptive transfer of T cells is a promising approach to treat cancers. Primary human T cells can be modified using viral and non-viral vectors to promote the specific targeting of cancer cells via the introduction of exogenous T-cell receptors (TCRs) or chimeric antigen receptors (CARs). This gene transfer displays the potential to increase the specificity and potency of the anticancer response while decreasing the systemic adverse effects that arise from conventional treatments that target both cancerous and healthy cells. This review highlights the generation of clinical-grade T cells expressing CARs for immunotherapy, the use of these cells to target B-cell malignancies and, particularly, the first clinical trials deploying the Sleeping Beauty gene transfer system, which engineers T cells to target CD19+ leukemia and non-Hodgkin's lymphoma.

Promotion of Caspase Activation by Caspase-9-mediated Feedback Amplification of Mitochondrial Damage.

Mitochondrial disruption during apoptosis results in the activation of caspase-9 and a downstream caspase cascade. Triggering this caspase cascade leads to the cleavage of anti-apoptotic Bcl-2 family proteins, resulting in feedback amplification of mitochondrial disruption. However, whether such a feedback loop plays an important role in the promotion of caspase activation and execution of apoptosis has not been well established. We observed that mutated Bcl-2 or Bcl-xL that are resistant to cleavage by caspases inhibited caspase-9-induced caspase activation in human H9 T cells. The release of Smac after the activation of caspase-9 was also inhibited by cleavage-resistant Bcl-2 or Bcl-xL. Consistently, caspase-9-deficient cells were defective in the release of Smac after induction of apoptosis. Moreover, addition of a Smac mimetic overcame the inhibitory effects of cleavage-resistant Bcl-2/Bcl-xL, and restored caspase-9-mediated cell death. Our data suggest that caspase-9-induced feedback disruption of mitochondria plays an important role in promoting the activation of caspases, while a defect in this process can be overcome by promoting Smac functions.

Delineation of the caspase-9 signaling cascade.

In the intrinsic apoptosis pathway, mitochondrial disruption leads to the release of multiple apoptosis signaling molecules, triggering both caspase-dependent and -independent cell death. The release of cytochrome c induces the formation of the apoptosome, resulting in caspase-9 activation. Multiple caspases are activated downstream of caspase-9, however, the precise order of caspase activation downstream of caspase-9 in intact cells has not been completely resolved. To characterize the caspase-9 signaling cascade in intact cells, we employed chemically induced dimerization to activate caspase-9 specifically. Dimerization of caspase-9 led to rapid activation of effector caspases, including caspases-3, -6 and -7, as well as initiator caspases, including caspases-2, -8 and -10, in H9 and Jurkat cells. Knockdown of caspase-3 suppressed caspase-9-induced processing of the other caspases downstream of caspase-9. Silencing of caspase-6 partially inhibited caspase-9-mediated processing of caspases-2, -3 and -10, while silencing of caspase-7 partially inhibited caspase-9-induced processing of caspase-2, -3, -6 and -10. In contrast, deficiency in caspase-2, -8 or -10 did not significantly affect the caspase-9-induced caspase cascade. Our data provide novel insights into the ordering of a caspase signaling network downstream of caspase-9 in intact cells during apoptosis.

Caspase-9-induced mitochondrial disruption through cleavage of anti-apoptotic BCL-2 family members.

Mitochondrial disruption during apoptosis results in the release of cytochrome c that forms apoptosomes with Apaf-1 and caspase-9. Activation of caspase-9 by dimerization in apoptosomes then triggers a caspase signaling cascade. In addition, other apoptosis signaling molecules released from the mitochondrion, such as apoptosis-inducing factor and endonuclease G, may induce caspase-9-independent apoptosis. To determine the signaling events induced by caspase-9, we used chemically induced dimerization for specific activation of caspase-9. We observed that caspase-9 dimerization resulted in the loss of mitochondrial membrane potential and the cleavage of anti-apoptotic Bcl-2, Bcl-xL, and Mcl-1. Moreover, cleavage-resistant Bcl-2, Bcl-xL, or Mcl-1 potently inhibited caspase-9-dependent loss of mitochondrial membrane potential and the release of cytochrome c. Our data suggest that a caspase-9 signaling cascade induces feedback disruption of the mitochondrion through cleavage of anti-apoptotic Bcl-2, Bcl-xL, and Mcl-1.