Rapid CD40‐mediated rescue from CD95‐induced apoptosis requires TNFR‐associated factor‐6 and PI3K

Abstract

The activation molecule CD40 and the death receptor CD95/Fas play important roles in regulating B cells so that effective antimicrobial immunity occurs without autoimmunity. CD40 signaling increases CD95 expression, sensitizing cells to apoptosis, but sustained CD40 signals rescue B cells from CD95 killing. Here we describe a mechanism of early CD40-mediated rescue from CD95-induced apoptosis in B cells. Maximal rescue was achieved when CD40 signals were given within 1-2 h of initiating CD95 apoptosis. CD40 signaling did not block association of Fas-associated death domain-containing protein with CD95, but decreased CD95-induced activation of caspases 3 and 8. Rapid CD40 rescue did not require NF-jB activation and was independent of de novo protein synthesis, but was dependent upon active PI3 K. Signaling via a CD40 mutant that does not bind TNFR-associated factor (TRAF)1, TRAF2, and TRAF3 rescued B cells from CD95-induced apoptosis. TRAF1/2/3-independent rescue was confirmed in B cell lines made deficient in these TRAF molecules by gene targeting. In contrast, CD40 rescue was completely abrogated in TRAF6-deficient B cells, which showed reduced activation of Akt in response to CD40 engagement. These results reveal a new rapid mechanism to balance B cell activation and apoptosis. IntroductionB cells integrate many signals that direct proliferation and differentiation into Ig-secreting plasma cells or longlived memory cells. Ultimately, activated B cells can be eliminated through apoptosis, which is important for maintaining immune tolerance and homeostasis [1,2]. Molecules of the TNFR family play critical roles in regulating the balance between B cell activation and apoptosis. CD40 signals allow B cells to proliferate, differentiate, switch isotype, form GC, and become memory cells [3][4][5]. CD40 stimulation also induces upregulation of other members of the TNFR family, including the death receptor CD95/Fas [6].The importance of CD95 in maintaining lymphocyte homeostasis and tolerance is highlighted by the human disease autoimmune lymphoproliferative syndrome (ALPS), caused by mutations in the CD95 signaling pathway [7,8] autoantibodies are produced, and systemic autoimmunity develops [9]. Expression of the CD95 transgene exclusively in T cells of lpr/lpr mice is sufficient to decrease lymphadenopathy, splenomegaly, and the accumulation of abnormal T cells, but these mice still produce autoantibodies, causing deposition of immune complexes in kidney glomeruli [10]. In contrast, expression of the CD95 transgene in both B and T cells reduces the levels of serum Ig, autoimmune symptoms, and mortality [11]. Thus, CD95 signaling in B cells is crucial for maintaining peripheral tolerance and preventing autoimmunity. When CD95 is oligomerized, either by binding to its ligand on T cells (CD95L) or by Ab against its extracellular domain, it recruits the adaptor molecule Fas-associated death domain-containing protein (FADD) and procaspase 8 into a complex called the deathinducing signaling complex (DISC) that cleaves pro...

BAFF enhances chemotaxis of primary human B cells: a particular synergy between BAFF and CXCL13 on memory B cells

“…41,48 In addition, we show that BAFF, through the triggering of BAFF-R, induces the activation of the PI3K/PDK1 pathway (evidenced by AKT and PKC phosphorylation) and the phosphorylation of IB␣. In the light of recent studies of TRAF6-dependent PI3K recruitment downstream from CD40, 49 our results suggest that a similar mechanism occurs downstream from BAFF-R. Patke et al also report the activation of AKT downstream from BAFF-R in mice. 50 As a consequence, activated PI3K/PDK1 may participate in the activation of RhoA/ROCK I and IB␣.…”

A novel polymorphism of the human CD40 receptor with enhanced function

“…44 CD40 can also rescue B cells from CD95/Fas-mediated apoptosis. 45 The work presented here suggests that hCD40-P227A may have an increased capacity to participate in this process.…”

A proliferation-inducing ligand mediates follicular lymphoma B-cell proliferation and cyclin D1 expression through phosphatidylinositol 3-kinase–regulated mammalian target of rapamycin activation

“…TRAF6 in particular, which binds to the TACI cytoplasmic tail, has recently been shown to be important in both CD40-and TNF-mediated PI3K activation. [40][41][42] Although we cannot fully exclude a potential role for BCMA in APRIL-mediated signaling in FL patient specimens, inhibition of TACI expression significantly reduced both APRIL-induced proliferation and PI3K activation, whereas inhibition of BCMA had little effect, suggesting that TACI is the primary APRIL receptor responsible for these effects. Alignment of TACI and BCMA sequences reveals significant homology between the 2 receptors, 43 but they do differ in their TRAF-binding profile, suggesting that their cytoplasmic tails may transmit unique signals.…”