Pannexin hemichannel–mediated release of ATP provides an autocrine, costimulatory signal for T cell activation.

IntroductionChronic lymphocytic leukemia (CLL) is the most common B-cell neoplasm in Europe and the United States, characterized by progressive accumulation of monoclonal CD5 ϩ B cells in peripheral blood, bone marrow, and peripheral lymphoid organs. The clinical course of CLL is highly variable, ranging from an indolent disease that may never require treatment to a rapidly progressive disease. 1 One of the principal prognostic features is the mutational status of the immunoglobulin heavy chain variable region genes (IGHV). CLL with poor prognosis has unmutated IGHV genes (U-CLL), whereas CLL with good prognosis carries somatic mutations in these genes (M-CLL). 2,3 Neoplastic CLL cells are typically arrested in the G 0 /G 1 phase of the cell cycle and accumulate in tissues because of prolonged survival. 1 Although extrinsic factors, such as escape from immune surveillance and chemotaxis to a favorable microenvironment, contribute to the extended survival of CLL B cells, 4 CLL is primarily a disease of defective apoptosis, and intrinsic defects in a number of components of the apoptotic circuitry have been identified, including overexpression of antiapoptotic proteins (eg, Bcl-2, Mcl-1, XIAP) and reduction in the expression of proapoptotic proteins (eg, Bax, DAPK-1). [5][6][7] Other apoptosis defects, including abnormalities in the p53 and ATM pathways in the most aggressive subsets, have also been implicated in the prolonged survival of malignant cells. [8][9][10] In this respect, apoptosis is emerging as a key therapeutic target in CLL, as witnessed by the ongoing clinical trials on Bcl-2 inhibitors. 11 p66Shc, a member of the Shc family of protein adapters, acts as antagonist of mitogenic signaling and positive regulator of oxidative stress-induced apoptosis in fibroblasts. 12,13 In T cells, where its expression is epigenetically controlled, 14,15 p66Shc uncouples the T-cell receptor from activation of the Ras/mitogen-activated protein kinase pathway by competitively inhibiting recruitment to the T-cell receptor of the mitogenic isoform, p52Shc. 16 Furthermore, p66Shc enhances T-cell susceptibility to apoptotic stimuli by increasing reactive oxygen species (ROS) production and impairing Ca 2ϩ homeostasis. 17 We have recently reported that p66Shc is expressed in murine B cells and that p66Shc deficiency results in enhanced proliferative responses of mouse B cells to B-cell receptor (BCR) engagement, 18 suggesting that p66Shc may display similar activities in T and B cells.Here, we have investigated the role of p66Shc in B-cell survival. We show that p66Shc promotes B-cell apoptosis by uncoupling the BCR from the survival pathways mediated by Akt and Erk. On the basis of these findings, we have investigated p66Shc expression and function in B cells from patients with CLL. The results identify a role for p66Shc in the imbalance among proapoptotic and antiapoptotic Bcl-2 family members in CLL and An Inside Blood analysis of this article appears at the front of this issue.The online version of this article contains...

Rai (ShcC) belongs to the family of Shc adaptor proteins and is expressed in neuronal cells, where it acts as a survival factor activating the PI3K/Akt survival pathway. In vivo, Rai protects the brain from ischemic damage. In this study, we show that Rai is expressed in T and B lymphocytes. Based on the finding that Rai−/− mice consistently develop splenomegaly, the role of Rai in lymphocyte homeostasis and proliferation was addressed. Surprisingly, as opposed to neurons, Rai was found to impair lymphocyte survival. Furthermore, Rai deficiency results in a reduction in the frequency of peripheral T cells with a concomitant increase in the frequency of B cells. Rai−/− lymphocytes display enhanced proliferative responses to Ag receptor engagement in vitro, which correlates with enhanced signaling by the TCR and BCR, and more robust responses to allergen sensitization in vivo. A high proportion of Rai−/− mice develop a lupus-like autoimmune syndrome characterized by splenomegaly, spontaneous peripheral T and B cell activation, autoantibody production, and deposition of immune complexes in the kidney glomeruli, resulting in autoimmune glomerulonephritis. The data identify Rai as a negative regulator of lymphocyte survival and activation and show that loss of this protein results in breaking of immunological tolerance and development of systemic autoimmunity.

Besides lowering circulating cholesterol, statins act as immunomodulators. Although the effects of statins on lymphocyte activation and differentiation have been clearly defined, there is no consensus as to effects of these drugs on phagocytes. We have addressed the outcome of simvastatin treatment on the activation and effector function of human macrophages in the pathophysiologically relevant context of challenge with an opportunistic pathogen. We provide evidence that: simvastatin blocks the biological effects rapidly triggered by IgG-opsonized bacteria (phagocytosis and oxidative burst) while enhancing the delayed effects elicited by FcgammaR stimulation (production of proinflammatory mediators); these opposite effects of simvastatin result from enhancement of the JNK pathway and concomitant impairment of other signaling modules activated by FcgammaR engagement; and these activities are dependent on the capacity of simvastatin to block protein prenylation. The results provide novel mechanistic insight into the activities of statins on phagocytes and are of relevance to the assessment of potential side-effects in patients undergoing long-term hypocholesterolemic therapy.

Rai/ShcC is a member of the Shc family of protein adaptors expressed with the highest abundance in the central nervous system, where it exerts a protective function by coupling neurotrophic receptors to the PI3K/Akt survival pathway. Rai is also expressed, albeit at lower levels, in other cell types, including T and B lymphocytes. We have previously reported that in these cells Rai attenuates antigen receptor signaling, thereby impairing not only cell proliferation but also, opposite to neurons, cell survival. Here we have addressed the mechanism underlying the inhibitory activity of Rai on TCR signaling. We show that Rai interferes with the TCR signaling cascade one of the earliest steps –recruitment of the initiating kinase ZAP-70 to the phosphorylated subunit of the TCR/CD3 complex, which results in a generalized dampening of the downstream signaling events. The inhibitory activity of Rai is associated to its inducible recruitment to phosphorylated CD3, which occurs in the physiological signaling context of the immune synapse. Rai is moreover found as a pre-assembled complex with ZAP-70 and also constitutively interacts with the regulatory p85 subunit of PI3K, similar to neuronal cells, notwithstanding the opposite biological outcome, i.e. impairment of PI-3K/Akt activation. The data highlight the ability of Rai to establish interactions with the TCR and key signaling mediators which, either directly (e.g. by inhibiting ZAP-70 recruitment to the TCR or sequestering ZAP-70/PI3K in the cytosol) or indirectly (e.g. by promoting the recruitment of effectors responsible for signal extinction) prevent full triggering of the TCR signaling cascade.