Cross-linking of P-selectin glycoprotein ligand-1 induces death of activated T cells

Abstract

Increasing evidence has shown that death signaling in T cells is regulated in a complicated way. Molecules other than death receptors can also trigger T-cell death. Here, we demonstrate for the first time that P-selectin glycoprotein ligand-1 (PSGL-1) or CD162 molecules crosslinked by an anti-PSGL-1 monoclonal antibody, TAB4, can trigger a death signal in activated T cells. In contrast to classic cell death, PSGL-1-mediated T-cell death is caspase independent. It involves translocation of apoptosis-inducing factor from mitochondria to nucleus and mitochondrial cytochrome c release. Ultrastructurally, both peripheral condensation of chromatin and apoptotic body were observed in PSGL-1-mediated T-cell death. Collectively, this study demonstrates a novel role for PSGL-1 in controlling activated T-cell death and, thus, advances our understanding of immune regulation. IntroductionDeath of T cells is fundamental in proper immune responses. In central and peripheral lymphoid organs, T-cell death occurs as a regulated event to ensure self-tolerance. [1][2][3][4] As the consequence of an immune response in normal conditions, most antigen-activated T cells die subsequently. [5][6][7] Activated T-cell death plays an important role in shaping and maintaining the T-cell repertoire and in avoiding undesired immune responses. Therefore, clarification of molecular mechanisms that determine death of activated T cells becomes imperative.According to current understanding, activated T-cell death occurs in at least 2 conditions: cytokine withdrawal and repeated antigenic stimulation. 8 In the former, when there is no further antigen stimulation, both interleukin 2 (IL-2) production and IL-2 receptor expression decline, and the death because of cytokine withdrawal ensues. 9,10 The latter is antigen driven and mediated through death receptors such as Fas and tumor necrosis factor (TNF) receptor. [11][12][13] Both conditions of activated T-cell death occur by way of activation of caspases and are strictly dependent on caspase-3. 14,15 Such classic caspase-activating death pathways may culminate mitochondrial dysfunction and cytochrome c release. 16 There is now increasing evidence for the existence of alternative, caspase-independent machinery of cell death in T-cell development and activation. [17][18][19][20][21][22][23] Fas-deficient lpr mice are able to eliminate T cells. 24 Additionally, by repeated antigenic stimulation, activated T-cell death in FLIP (FLICE [FADD (Fas receptor-associated death domain)-like IL-1␤-converting enzyme]-inhibitory protein) transgenic mice does occur. These mice do not develop lymphoproliferative disease despite their resistance to Fas-triggered death. 25 In line with these observations, several reports have shown that caspase inhibition fails to prevent cell death on some death stimuli, 17,18,26 suggesting the involvement of other alternative mechanisms. However, these caspase-independent pathways may be triggered in activated T cells when subjected to some death stimuli other than death receptors. ...

A novel apoptosis-inducing anti-PSGL-1 antibody for T cell-mediated diseases

“…Contrasting their data, we failed to observe any significant adverse effect of TAB4 on the repopulation of lymphocytes in our animal studies. Our in vitro and in vivo results indicated that TAB4 must target T cells at a particular stage of activation, yet such a preferential susceptibility is not correlated with expression levels of PSGL-1 from the previous data [6]. It was reported that P-selectin binds to an exclusive form of PSGL-1 on Th1 cells, but not on Th2 cells [35].…”

“…The physiological importance of PSGL-1 was believed to be confined to mediating leukocyte recruitment, until we reported recently a novel function of PSGL-1 which can be cross-linked by antibody and can lead to death of activated T cells [6]. Levesque et al [34] previously demonstrated the role of PSGL-1 in mediating hematopoiesis by inducing apoptosis of a subpopulation of CD34 + CD38 -progenitor cells.…”

“…Based on our earlier finding that TAB4 can induce apoptosis of activated T cells in vitro [6], the current studies were directed at examining the in vivo effect of TAB4 in treating T cell-mediated diseases. Our data demonstrated that a short-term administration of TAB4 ameliorated the lethality of GVHD and resulted in the establishment of donor-derived functional T cells in the recipient mice.…”

“…In the previous report, we showed that TAB4, which recognizes mouse PSGL-1 (CD162), can induce apoptosis of Con A-activated or antigen-stimulated mouse T cells [6]. To clarify whether TAB4 has differential effects on activated and resting T cells when both populations are present, apoptosis was examined in a mixed culture of Con A-activated T cells and freshly isolated naive splenocytes, incubated with TAB4 or control antibody.…”

“…Recently, we described a novel property of P-selectin glycoprotein ligand-1 (PSGL-1) that can be triggered by antibody ligation and that can induce the activated T cells to undergo apoptosis [6]. Using the monoclonal antibody TAB4 generated in our laboratory, we demon-strated that cross-linking of PSGL-1 on activated T cells initiates a non-classical apoptosis pathway, which is caspase independent and involves mitochondrial release of apoptosis-inducing factors (AIF) and cytochrome c. Taking advantage of this important discovery, we further explored the potential of anti-PSGL-1 antibody in elimination of disease-causing T cells in vivo as a therapeutic strategy for GVHD and autoimmune diabetes.…”