Thrombalexins: Cell-Localized Inhibition of Thrombin and Its Effects in a Model of High-Risk Renal Transplantation.

Allograft transplantation into sensitized recipients with antidonor antibodies results in accelerated antibody-mediated rejection (AMR), complement activation, and graft thrombosis. We have developed a membrane-localizing technology of wide applicability that enables therapeutic agents, including anticoagulants, to bind to cell surfaces and protect the donor endothelium. We describe here how this technology has been applied to thrombin inhibitors to generate a novel class of drugs termed thrombalexins (TLNs). Using a rat model of hyperacute rejection, we investigated the potential of one such inhibitor (thrombalexin-1 [TLN-1]) to prevent acute antibody-mediated thrombosis in the donor organ. TLN-1 alone was able to reduce intragraft thrombosis and significantly delay rejection. The results confirm a pivotal role for thrombin in AMR in vivo. This approach targets donor organs rather than the recipient and is intended to be directly translatable to clinical use.

Complotype affects the extent of down-regulation by Factor I of the C3b feedback cycle in vitro.

Sera from a large panel of normal subjects were typed for three common polymorphisms, one in C3 (R102G) and two in Factor H (V62I and Y402H), that influence predisposition to age-related macular degeneration and to some forms of kidney disease. Three groups of sera were tested; those that were homozygous for the three risk alleles; those that were heterozygous for all three; and those homozygous for the low-risk alleles. These groups vary in their response to the addition of exogenous Factor I when the alternative complement pathway is activated by zymosan. Both the reduction in the maximum amount of iC3b formed and the rate at which the iC3b is converted to C3dg are affected. For both reactions the at-risk complotype requires higher doses of Factor I to produce similar down-regulation. Because iC3b reacting with the complement receptor CR3 is a major mechanism by which complement activation gives rise to inflammation, the breakdown of iC3b to C3dg can be seen to have major significance for reducing complement-induced inflammation. These findings demonstrate for the first time that sera from subjects with different complement alleles behave as predicted in an in-vitro assay of the down-regulation of the alternative complement pathway by increasing the concentration of Factor I. These results support the hypothesis that exogenous Factor I may be a valuable therapeutic aid for down-regulating hyperactivity of the C3b feedback cycle, thereby providing a treatment for age-related macular degeneration and other inflammatory diseases of later life.

Taking complement to the clinic--has the time finally come?

Complement has been studied for over a century and its role in promoting the effector side of antibody-mediated immune reactions and of inducing inflammation is well understood. Nevertheless, it has proved surprisingly difficult to translate this information into pharmaceutical agents that can be used to treat immunopathological and inflammatory disease. There are, however, now clear signs that this situation will change. New types of therapeutic agents to interfere with complement function are being developed and it has become apparent quite recently that some common and otherwise untreatable diseases such as age-related macular degeneration are very largely due to mutations in the complement system that leads to a hyperinflammatory state. This has stimulated a renaissance of interest in the complement system as a therapeutic target and in this short review we discuss the possible ways of taking complement to the clinic, and the indications for which this may be carried out.

Membrane reinsertion of a myristoyl-peptidyl anchored extracellular domain growth hormone receptor.

The actions of GH are mediated through a cell surface cytokine receptor. We previously demonstrated that naturally occurring truncated membrane bound GH receptors (GHRs) can block GH receptor signaling. We have now investigated whether recombinant extracellular GHR can be conjugated to a myristoylated-peptide (mp) tail and inserted into cell membranes to modulate GHR signaling. Recombinant human extracellular domain (1-241) GHR was expressed in Escherichia coli, purified, and refolded from cell lysate. The free C-terminal cysteine was then reduced and conjugated to an activated preformed mp tail. The properties of the purified tailed GHR (GHR-mp) were then compared with those of the untailed purified GHR 1-241. Fluorescence-activated cell sorter analysis and cell surface binding assays demonstrated that GHR-mp inserted into the cell surface membranes of CHO cells, whereas untailed GHR 1-241 showed no insertion. In a cell-based bioassay GHR-mp partially inhibited wild-type GHR signaling, whereas GHR 1-241 had no effect. Truncated extracellular domain GHR can, when specifically modified with a membrane-localizing mp unit, insert into cell surface membranes and modulate GHR signaling.

Complement regulation at the molecular level: the structure of decay-accelerating factor.

The human complement regulator CD55 is a key molecule protecting self-cells from complement-mediated lysis. X-ray diffraction and analytical ultracentrifugation data reveal a rod-like arrangement of four short consensus repeat (SCR) domains in both the crystal and solution. The stalk linking the four SCR domains to the glycosylphosphatidylinositol anchor is extended by the addition of 11 highly charged O-glycans and positions the domains an estimated 177 A above the membrane. Mutation mapping and hydrophobic potential analysis suggest that the interaction with the convertase, and thus complement regulation, depends on the burial of a hydrophobic patch centered on the linker between SCR domains 2 and 3.