Complement system activation plays an important role in both innate and acquired immunity. Activation of complement and the subsequent formation of C5b-9 channels (the membrane attack complex) on cell membranes lead to cell death. However, when the number of channels assembled on the surface of nucleated cells is limited, sublytic C5b-9 can induce cell cycle progression by activating signal transduction pathways and transcription factors and inhibiting apoptosis. This induction by C5b-9 is dependent upon the activation of the phosphatidylinositol 3-kinase/Akt/FOXO1 and ERK1 pathways in a Gi protein-dependent manner. C5b-9 induces sequential activation of CDK4 and CDK2, enabling the G1/S-phase transition and cellular proliferation. In addition, it induces RGC-32, a novel gene that plays a role in cell cycle activation by interacting with Akt and the cyclin B1-CDC2 complex. C5b-9 also inhibits apoptosis by inducing the phosphorylation of Bad and blocking the activation of FLIP, caspase-8, and Bid cleavage. Thus, sublytic C5b-9 plays an important role in cell activation, proliferation, and differentiation, thereby contributing to the maintenance of cell and tissue homeostasis.

Proliferation of aortic smooth muscle cells contributes to atherogenesis and neointima formation. Sublytic activation of complement, particularly C5b-9, induces cell cycle progression in aortic smooth muscle cells. RGC-32 is a novel protein that may promote cell cycle progression in response to complement activation. We cloned human RGC-32 cDNA from a human fetal brain cDNA library. The human RGC-32 cDNA encodes a 117-amino acid protein with 92% similarity to the rat and mouse protein. Human RGC-32 maps to chromosome 13 and is expressed in most tissues. Sublytic complement activation enhanced RGC-32 mRNA expression in human aortic smooth muscle cells and induced nuclear translocation of the protein. RGC-32 was physically associated with cyclin-dependent kinase p34 CDC2 and increased the kinase activity in vivo and in vitro. In addition, RGC-32 was phosphorylated by p34 CDC2 -cyclin B1 in vitro. Mutation of RGC-32 protein at Thr-91 prevented the p34 CDC2 -mediated phosphorylation and resulted in loss of p34 CDC2 kinase enhancing activity. Overexpression of RGC-32 induced quiescent aortic smooth muscle cells to enter S-phase. These data indicate that cell cycle activation by C5b-9 may involve p34 CDC2 activity through RGC-32. RGC-32 appears to be a cell cycle regulatory factor that mediates cell proliferation, both as an activator and substrate of p34 CDC2 .C5b-9, the membrane attack complex of complement, causes cell death by forming transmembrane pores (1). When the number of C5b-9 molecules is limited to a sublytic level, nucleated cells are able to escape cell death by eliminating membrane-inserted terminal complement complexes (TCC 1 ; C5b-7, C5b-8, and C5b-9) by endocytosis and/or membrane shedding (2-4). Among these complexes, C5b-9 is most potent in activating target cells. C5b-9 causes a Ca 2ϩ influx and generates intracellular second messengers, including phosphatidylinositol triphosphates, diacylglycerol, and ceramide (5-8). Membrane-inserted TCC activates the G i /G o family of G proteins (9). Activation of G i /G o by TCC is responsible for the G␤␥-mediated activation of cell cycle through activation of Ras, Raf-1, extracellular signal regulated kinase-1 (10), and activation of phosphatidylinositol 3-phosphate kinase (10, 11).Cell cycle activation by C5b-9 is associated with an increase in CDK4, CDK2, and p34 CDC2 activities, and this is followed by an increase in DNA synthesis and cell proliferation (11, 12, 24 -26). The C5b-9-induced DNA synthesis is abolished by inhibitors of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase-1 and phosphatidylinositol 3-phosphate kinase (11). Cell cycle activation by C5b-9 in postmitotic cells, such as oligodendrocytes (OLG) and myotubes, is associated with expression of c-JUN and c-FOS protooncogenes and loss of differentiation (12)(13)(14).In an effort to find novel C5b-9-induced genes involved in cell cycle regulation, we cloned the rat Response Gene to Complement (RGC-32) using mRNA differential display PCR in OLG (15, 16). C5b-9 en...

Neutralization of TNF-α in humans with rheumatoid arthritis or Crohn’s disease has been associated with the development of humoral autoimmunity. To determine the effect of TNF-α neutralization on cell-mediated and humoral-mediated responses, we administered anti-TNF-α mAb to mice undergoing acute graft-vs-host disease (GVHD) using the parent-into-F1 model. In vivo neutralization of TNF-α blocked the lymphocytopenic features characteristic of acute GVHD and induced a lupus-like chronic GVHD phenotype (lymphoproliferation and autoantibody production). These effects resulted from complete inhibition of detectable antihost CTL activity and required the presence of anti-TNF-α mAb for the first 4 days after parental cell transfer, indicating that TNF-α plays a critical role in the induction of CTL. Moreover, an in vivo blockade of TNF-α preferentially inhibited the production of IFN-γ and blocked IFN-γ-dependent up-regulation of Fas; however, cytokines such as IL-10, IL-6, or IL-4 were not inhibited. These results suggest that a therapeutic TNF-α blockade may promote humoral autoimmunity by selectively inhibiting the induction of a CTL response that would normally suppress autoreactive B cells.

Diminished expression of TCR ζ and reciprocal up-regulation and association of FcRγ with the TCR/CD3 complex is a hallmark of systemic lupus erythematosus (SLE) T cells. In this study we explored whether differential molecular associations of the spleen tyrosine kinase Syk that preferentially binds to FcRγ contribute to pathological amplification of signals downstream of this “rewired TCR” in SLE. We detected higher amounts of Syk expression and activity in SLE compared with normal T cells. Selective inhibition of the activity of Syk reduced the strength of TCR-induced calcium responses and slowed the rapid kinetics of actin polymerization exclusively in SLE T cells. Syk and ZAP-70 also associated differently with key molecules involved in cytoskeletal and calcium signaling in SLE T cells. Thus, while Vav-1 and LAT preferentially bound to Syk, phospholipase C-γ1 bound to both Syk and ZAP-70. Our results show that differential associations of Syk family kinases contribute to the enhanced TCR-induced signaling responses in SLE T cells. Thus, we propose molecular targeting of Syk as a measure to control abnormal T cell responses in SLE.

No abstract