MECHANISMS OF COMPLEMENT ATTACK ON NUCLEATED CELLS

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The long term objective of our research is to understand the biology of the terminal complement complex (TCC) which includes C5b-7, C5b-8, and C5b-9. C5b-9 mediated nucleated cell killing is governed by a multi-step process: sequential assembly of TCC in the membrane, elimination of potentially lytic C5b-9 from the cell surface, and the process of cell death elicited by C5b-9. Once TCC is assembled, it is subjected to rapid elimination from the cell surface. TCC elimination, vital for cell survival, is enhanced by increased cytosolic Ca2+, [Ca2+] i, in a protein kinase C (PKC) -dependent manner when the number of TCC is limiting. Since elimination of complement channels depends on signals generated by TCC and TCC is known to be a potent inducer of cell activation, the effect of C5b-7, C5b-8, and sublytic C5b-9 channels may be of major biological importance in cells surviving from limited complement attack in vivo. Other than Ca2+ influx and increase in [Ca 2+ ]i, little is known about whether and how TCC generates other signal messengers. Interestingly, some of the biological activities of C5b-9 can be achieved by C5b-7 and/or C5b-8 with little or no Ca2+ influx, which suggests that mediators other than Ca2+ may be involved. We have been able to demonstrate that TCC, especially C5b-7, increased the mass levels of diacylglycerol (DAG) and ceramide, potent second messengers, in a human JY B cell line and in a murine C2 muscle cell line. These findings are potentially significant to understand the pathobiology of affected cells in autoimmune diseases in which functionally important cells such as B-cells and muscle cells are targeted by auto-antibodies. In this proposal, we will examine (1) TCC-induced signal messengers, specifically DAG and ceramide, and the mechanisms of their generation by analyzing G protein activation, (2) the regulatory effect of TCC on the expression of muscle-specific proteins in C2 mytotubes by examining post-transcriptional stability of mRNAs encoding these proteins, and (3) the mechanisms of nucleated cell killing mediated by C5b-9 by exploring the effect of uncontrolled calcium influx on the functional status of mitochondria.