CTL-mediated cytolysis: perforin and alternative pathways?

Structural biology of the membrane attack complex

The complement system is an intricate network of serum proteins that mediates humoral innate immunity through an amplification cascade that ultimately leads to recruitment of inflammatory cells or opsonisation or killing of pathogens. One effector arm of this network is the terminal pathway of complement, which leads to the formation of the membrane attack complex (MAC) composed of complement components C5b, C6, C7, C8 and C9. Upon formation of C5 convertases via the classical or alternative pathways of complement activation, C5b is generated from C5 by proteolytic cleavage, nucleating a series of association and polymerisation reactions of the MAC-constituting complement components that culminate in pore formation of pathogenic membranes. Recent structures of MAC components and homologous proteins significantly increased our understanding of oligomerisation, membrane association and integration, shedding light onto the molecular mechanism of this important branch of the innate immune system.

Granzymes, cytotoxic granules and cell death: the early work of Dr. Jurg Tschopp

Within the powerful legacy left by Jurg Tschopp, we should not forget his early work that helped to elucidate the molecular pathways responsible for the clearance of virus-infected and transformed cells by cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. Jurg's skilful biochemical approach formed a firm platform upon which the work of so many other biochemists, cell biologists and immunologists would come to rely. Jurg coined the shorthand term 'granzyme' to denote the individual members of a family of serine proteases sequestered in and secreted from the cytotoxic granules of CTL/NK cells. He was also one of the first to describe the lytic properties of purified perforin and to postulate the synergy of perforin and granzymes, which we now know to underpin target cell apoptosis. Jurg was a major protagonist in the debate that raged throughout the 1980's and early 1990's on the physiological relevance of the 'granule exocytosis' pathway. Ultimately, resolving this issue led Jurg and his colleagues to even greater and impactful discoveries in the broader field of apoptosis research. Jurg Tschopp ranks with other pioneers, particularly Gideon Berke, Chris Bleackley, Pierre Golstein, Pierre Henkart and Eckhard Podack for making seminal discoveries on our understanding of how the immune system eliminates dangerous cells.

Omega-3 fatty acid modification of membrane structure and function. II. Alteration by docosahexaenoic acid of tumor cell sensitivity to immune cytolysis

Docosahexaenoic acid (DHA, 22:6) is a long-chain omega-3 fatty acid abundant in cold water fish; it is the most unsaturated fatty acid found in biologic systems and is reported to alter membrane structure. To explore DHA's effect on membrane function, we have fused tumor cells with synthetic phosphatidylcholine (PC) containing stearic acid in the sn-1 position and DHA in the sn-2 position (18:0, 22:6 PC) and have found the lipid-modified tumor cells to be more sensitive to cytolysis by alloreactive cytotoxic T lymphocytes. Cold target competition experiments suggested that fusion of tumor plasma membranes with 18:0, 22:6 PC produced a qualitative change in expression of surface antigens recognized by cytotoxic T lymphocytes. We monitored the expression of various epitopes on tumor cells by complement-mediated lysis and radioimmunoassay with monoclonal antibodies against H-2 class I antigens. Our results suggest that membrane-bound DHA increases the expression of some epitopes while decreasing the expression of others and that different tumor lines vary in the magnitude of DHA's effect. Our findings are consistent with a model in which DHA-containing phospholipids segregate into membrane domains, in turn altering the expression of membrane proteins.