Abnormalities of T cell signaling in systemic lupus erythematosus

Vaishali R Moulton 0 George C Tsokos 0 0 Division of Rheumatology, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School , Boston, MA 02115 , USA Systemic lupus erythematosus (SLE) is an autoimmune disease resulting from a loss of tolerance to multiple self antigens, and characterized by autoantibody production and inflammatory cell infiltration in target organs, such as the kidneys and brain. T cells are critical players in SLE pathophysiology as they regulate B cell responses and also infiltrate target tissues, leading to tissue damage. Abnormal signaling events link to defective gene transcription and altered cytokine production, contributing to the aberrant phenotype of T cells in SLE. Study of signaling and gene transcription abnormalities in SLE T cells has led to the identification of novel targets for therapy. - Lipid rafts are sphingolipid-cholesterol-GM1-rich microdomains bearing TCR-CD3 complexes and associated signaling molecules distributed on the T cell surface. In normal T cells, TCR stimulation leads to clustering of these rafts to aid formation of the immunological synapse, allowing for cognate interactions with corresponding molecules on APCs. Freshly isolated SLE T cells, however, display pre-clustered lipid rafts, indicating that the T cells are poised for activation. In addition, these lipid rafts contain an altered composition of residing molecules on their surface. Alterations include the increased expression of FcR, Syk, and phospholipase C (PLC), with decreased expression of the lymphocyte kinase Lck. The localization of tyrosine phosphatase CD45 within the lipid rafts and its association with and activation of Lck are abnormal, leading to the degradation and thus reduced expression of Lck [1-4]. The costimulatory molecule cytotoxic T lymphocyte associated antigen 4 (CTLA4), a signaling component of the lipid raft, is an important negative regulator of TCR activation. Expression of CTLA4 is found to be increased in freshly isolated T cells from SLE patients [5]; paradoxically, however, it is unable to control the aberrant T cell activation. Blocking the CTLA4-B7 signaling pathway appears to impede disease progression in animal models of lupus, although timing of treatment is important, such that early treatment prevents or ameliorates disease [6,7]. Continuous exposure of T cells to autoantigen and/or circulating anti-CD3/TCR autoantibodies [8] may account for the observed aggregated lipid rafts on freshly isolated T cells from the peripheral blood of SLE patients. The pre-aggregated lipid rafts contribute to the pathogenesis of SLE, as evidenced in the lupus-prone MRL/lpr mouse. In this mouse, the percentage of T cells with clustered lipid rafts increases with age and peaks before lupus pathology development. More importantly, acceleration of lipid raft aggregation leads to disease advancement, whereas disruption of the aggregates delays pathology [9]. Ex vivo treatment of T lymphocytes from SLE patients with atorvastatin, an inhibitor of 3-hyroxy-3-methylgluteryl CoA reductase that disrupts lipid rafts, showed reduced co-localization of CD45 and Lck, thus reducing the active form of Lck within the rafts. Furthermore, TCR activation not only restored the ERK phosphorylation but also decreased their production of the cytokines IL6 and IL10, which are implicated in SLE pathogenesis. These results show that statins may have therapeutic value in restoring signaling defects in SLE T cells and potentially disease [10]. TCR-CD3 complex The TCR is the surface sensor for antigens presented to lymphocytes in the context of the MHC molecule by APCs. The TCR and chains are closely coupled to the CD3 , , , and chains to form the TCR-CD3 complex. Each subunit of the chain bears three immunoreceptor tyrosine activation motifs (ITAMs); thus, the homodimer bears a total of six ITAMs and is a critical signaling transducer of T cells. In nave T cells, antigen recognition brings together the TCR, the co-receptor molecule (CD4 or CD8) and the tyrosine phosphatase CD45 on the T cell surface within cholesterol-rich domains called lipid rafts. CD45 removes inhibitory phosphates from the Src family lymphocyte kinase (Lck), and the CD3 chain is phosphorylated at the six ITAMs by Lck. The CD3 chain then recruits the zeta associated protein of 70 kDa (ZAP70) kinase, which is also phosphorylated by Lck. ZAP70 then phosphorylates the adaptor proteins Linker of activation in T cells (LAT) and SLP-76, thus transmitting the signal downstream into three distinct pathways. The adaptor proteins bind and activate the enzyme PLC on one hand and activate the Ras-mitogen-activated protein kinase (MAPK) pathway through guanine nucleotide exchange factors on the other. PLC cleaves phosphatidylinositol bisphosphate into diacyl glycerol and inositol trisphosphate. Diacyl glycerol activates protein kinase C (PKC), which activates the transcription factor NF-B. Inositol trisphosphate leads to opening of the calcium channels, increased intracellular calcium concentrations and activation of the phosphatase calcineurin, which dephosphorylates and activates the transcription factor Nuclear factor of activated T cells (NFAT). Finally, the Ras-MAPK cascade induces and activates fos protein, a component of the transcription factor Activated protein 1 (AP1). Activation of NF-B, NFAT and AP1 leads to nuclear translocation of these factors and activation of target gene transcription, cell proliferation and differentiation [11]. Triggering of the TCR in SLE T cells leads to an abnormally accelerated and heightened tyrosine phosphorylation of signaling intermediates, and increased calcium flux characterizing their hyper-responsive phenotype [12]. The stronger signaling is evidenced by the earlier and greater overall tyrosine phosphorylation of signaling intermediates. SLE T cells display a unique rewiring of the surface TCR-CD3 complex wherein expression of the CD3 chain is decreased in cells from a majority of patients [12] (Figure 1). The lack of the CD3 chain in the TCR-CD3 complex is structurally and functionally replaced by the homologous Fc receptor gamma (FcR) chain [13]. FcR was initially identified as the Fc portion of the IgE receptor in mast cells and has structural and functional similarity to the chain, although the CD3 chain has three ITAMs whereas FcR has only one. Upon stimulation of SLE T cells, the FcR chain recruits the spleen tyrosine kinase (Syk) instead of the normally recruited ZAP70. The FcR-Syk interaction is exponentially (>100-fold) stronger than that of the chain-ZAP 70 combination, rendering a stronger downstream intracellular signal [14]. While this leads to abnormally increased calcium influx, it does not translate into higher IL2-producing capacity of these cells. Rather, the SLE T cells are poor producers of IL2, rendering their somewhat anergic phenotype. Interestingly, replenishment of the CD3 chain in SLE T cells in vitr (...truncated)