The complement system plays a crucial role in the innate defense against common pathogens. Activation of complement leads to robust and efficient proteolytic cascades, which terminate in opsonization and lysis of the pathogen as well as in the generation of the classical inflammatory response through the production of potent proinflammatory molecules. More recently, however, the role of complement in the immune response has been expanded due to observations that link complement activation to adaptive immune responses. It is now appreciated that complement is a functional bridge between innate and adaptive immune responses that allows an integrated host defense to pathogenic challenges. As such, a study of its functions allows insight into the molecular underpinnings of host-pathogen interactions as well as the organization and orchestration of the host immune response. This review attempts to summarize the roles that complement plays in both innate and adaptive immune responses and the consequences of these interactions on host defense.

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Ailene oxide synthases convert ilpoxygenasederived fatty acid hydroperoxides to unstable allene epoxides. In plants, an allene oxide is a precursor of the growth regulator jasmonic acid. Previously, we showed that an allene oxide synthase from flaiseed has the spectral properties of a cytochrome P450. The relationship to the P450 gene famly is now established from the primary structure deduced from the cDNA. The encoded protein of 536 amino acids has segments at the C tenus that match certain well conserved regions in cytochrome P450s. The heme-binding cysteine is recognizable at position 489. However, there are unprecedented moftions in this region, with substution of two of the three most highly conserved amino acids. Also very unusual is the absence of a conserved threonine that normally helps form the 02-biding pocket in cytchrome P450s. Notably, O2 is not involved in the allene oxide synthase reaction and, furthermore, the enzyme is known to have a weak interaction with CO. While allene oxide synthases are usually described as mkrosomal, the flax cDNA encodes a 58-amino acid sinal sequence characterisc of a mitochondrial or chloroplast tranit peptide.

Multidrug resistance protein 1 (MRP1) is an ATPbinding cassette (ABC) transporter that transports a range of hydrophobic xenobiotics, as well as relatively hydrophilic organic anion conjugates. The protein is present at high levels in testicular Leydig and Sertoli cells. Studies with knockout mice suggest that MRP1 may protect germ cells from exposure to some cytotoxic xenobiotics, but potential endobiotic substrates in this organ have not been identified. Previously, we have shown certain D-ring, but not A-ring, estrogen glucuronides can act as competitive inhibitors of MRP1 mediated transport, suggesting that they are potential substrates for the protein. In the case of 17␤-estradiol-17␤-D-glucuronide, this has been confirmed by direct transport studies. The Leydig cell is the major site of estrogen conjugation in the testis. However, the principal products of conjugation are A-ring estrogen sulfates, which are then effluxed from the cell by an unknown transporter. To determine whether MRP1/mrp1 could fulfill this function, we used membrane vesicles from MRP1-transfected HeLa cells to assess this possibility. We found that estradiol and estrone 3-sulfate alone were poor competitors of MRP1-mediated transport of the cysteinyl leukotriene, leukotriene C 4 . However, in the presence of reduced glutathione (GSH), their inhibitory potency was markedly increased. Direct transport studies using [ 3 H]estrone 3-sulfate confirmed that the conjugated estrogen could be efficiently transported (K m ‫؍‬ 0.73 M, V max ‫؍‬ 440 pmol mg ؊1 protein min ؊1 ), but only in the presence of either GSH or the nonreducing alkyl derivative, S-methyl GSH. In contrast to previous studies using vincristine as a substrate, we detected no reciprocal increase in MRP1-mediated GSH transport. These results provide the first example of GSH-stimulated, MRP1-mediated transport of a potential endogenous substrate and expand the range of MRP1 substrates whose transport is stimulated by GSH to include certain hydrophilic conjugated endobiotics, in addition to previously identified hydrophobic xenobiotics.

Fatty acid hydroperoxides (lipoxygenase products) are metabolized to allene oxides by a type of dehydrase that has been detected in plants, corals, and starfish oocytes. The allene oxides are unstable epoxide precursors of more complex products such as jasmonic acid, the plant growth hormone. Characterization of the dehydrase enzyme of flaxseed revealed that it is a 55-kilodalton hemoprotein. The spectral characteristics of this dehydrase revealed it to be a cytochrome P-450. It operates with the remarkable activity of greater than or equal to 1000 turnovers per second. The results establish a new catalytic activity for a cytochrome P-450 and illustrate the cooperation of different oxygenases in pathways of fatty acid metabolism.

Both inflammatory diseases and cancer are associated with heightened protein translation. However, the mechanisms of translational regulation and the roles of translation factors in these diseases are not clear. Programmed cell death 4 (PDCD4) is a newly described inhibitor of protein translation. To determine the roles of PDCD4 in vivo, we generated PDCD4-deficient mice by gene targeting. We report here that mice deficient in PDCD4 develop spontaneous lymphomas and have a significantly reduced life span. Most tumors are of the B lymphoid origin with frequent metastasis to liver and kidney. However, PDCD4-deficient mice are resistant to inflammatory diseases such as autoimmune encephalomyelitis and diabetes. Mechanistic studies reveal that upon activation, PDCD4-deficient lymphocytes preferentially produce cytokines that promote oncogenesis but inhibit inflammation. These results establish that PDCD4 controls lymphoma genesis and autoimmune inflammation by selectively inhibiting protein translation in the immune system.

Factor H (fH) and properdin both modulate complement; however, fH inhibits activation, and properdin promotes activation of the alternative pathway of complement. Mutations in fH associate with several human kidney diseases, but whether inhibiting properdin would be beneficial in these diseases is unknown. Here, we found that either genetic or pharmacological blockade of properdin, which we expected to be therapeutic, converted the mild C3 GN of an fH-mutant mouse to a lethal C3 GN with features of human dense deposit disease. We attributed this phenotypic change to a differential effect of properdin on the dynamics of alternative pathway complement activation in the fluid phase and the cell surface in the fHmutant mice. Thus, in fH mutation-related C3 glomerulopathy, additional factors that impact the activation of the alternative pathway of complement critically determine the nature and severity of kidney pathology. These results show that therapeutic manipulation of the complement system requires rigorous diseasespecific target validation.

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