Specific protein-ligand interactions are critical for cellular function, and most proteins select their partners with sharp discrimination. However, the oligopeptide-binding protein of Salmonella typhimurium (OppA) binds peptides of two to five amino acid residues without regard to sequence. The crystal structure of OppA reveals a three-domain organization, unlike other periplasmic binding proteins. In OppA-peptide complexes, the ligands are completely enclosed in the protein interior, a mode of binding that normally imposes tight specificity. The protein fulfills the hydrogen bonding and electrostatic potential of the ligand main chain and accommodates the peptide side chains in voluminous hydrated cavities.

Dendritic cells (DC) are potent antigen-presenting cells and understanding their mechanisms of antigen uptake is important for loading DC with antigen for immunotherapy. The multilectin receptors, DEC-205 and macrophage mannose receptor (MMR), are potential antigen-uptake receptors; therefore, we examined their expression and FITC-dextran uptake by various human DC preparations. The RT-PCR analysis detected low levels of DEC-205 mRNA in immature blood DC, Langerhans cells (LC) and immature monocyte-derived DC (Mo-DC). Its mRNA expression increased markedly upon activation, indicating that DEC-205 is an activation-associated molecule. In Mo-DC, the expression of cell-surface DEC-205 increased markedly during maturation. In blood DC, however, the cell-surface expression of DEC-205 did not change during activation, suggesting the presence of a large intracellular pool of DEC-205 or post-transcriptional regulation. Immature Mo-DC expressed abundant MMR, but its expression diminished upon maturation. Blood DC and LC did not express detectable levels of the MMR. FITC-dextran uptake by both immature and activated blood DC was 30- to 70-fold less than that of LC, immature Mo-DC and macrophages. In contrast to immature Mo-DC, the FITC-dextran uptake by LC was not inhibited effectively by mannose, an inhibitor for MMR-mediated FITC-dextran uptake. Thus, unlike Mo-DC, blood DC and LC do not use the MMR for carbohydrate-conjugated antigen uptake and alternative receptors may yet be defined on these DC. Therefore, DEC-205 may have a different specificity as an antigen uptake receptor or contribute to an alternative DC function.

Dendritic cells (DC) are specialist antigen presenting cells which capture antigens in the periphery, migrate centrally, and present the processed antigens in the context of major histocompatibility complex and appropriate co-stimulatory molecules to T lymphocytes for the initiation of an immune response. DEC-205 has been identified as a putative antigen-uptake receptor, which is expressed abundantly on mouse DC. The recently cloned mouse DEC-205 cDNA predicts a molecular structure which has a marked similarity to the macrophage mannose receptor. Using reverse transcriptase-polymerase chain reaction (RT-PCR) and cDNA library screening, we obtained the full coding region of human DEC-205 cDNA from the Hodgkin's disease-derived L428 cell line. The predicted protein structure is a type I transmembrane protein of 1722 amino acids consisting of a signal peptide, cysteine-rich domain, fibronectin type II domain, ten carbohydrate recognition-like domains, transmembrane domain, and a cytoplasmic tail. Human DEC-205 is 77% identical to the mouse protein with completely conserved cysteines. The DEC-205 gene (LY75) was mapped to chromosome band 2q24 by somatic cell hybrid panel analysis and fluorescent in situ hybridization. Northern blot analysis detected 7.8 and 9.5 kilobase DEC-205 transcripts in myeloid, B lymphoid, and Hodgkin's disease-derived cell lines. RT-PCR analysis indicated that immature blood DC contain a barely detectable amount of DEC-205 transcripts but these were markedly increased upon differentiation/activation.

Heme oxygenase, the rate-limiting enzyme in the degradation of heme to bile-pigments and carbon monoxide, is induced in response to increased oxidative stress and is believed to provide a cytoprotective effect. We investigated the role of heme oxygenase in cultured rabbit corneal epithelial cells (RCE), and its potential to alleviate oxidative stress-induced cell damage. Heme oxygenase in RCE was effectively and potently induced by most metals tested, including tin, silver, and gold, and cytokines such as IL-6, and TGF beta. Stannous chloride and heme-induced heme oxygenase mRNA by 40 and 100 fold within 1-3 hours and increased enzyme activity by 9.2- and 10-fold, respectively, over a 24 hour period. IL-6, TGF beta and H2O2 induced heme oxygenase by 2-3 fold. Zinc protoporphyrins were effective inhibitors of heme oxygenase activity in vitro. However, when incubated with cells for 24 h they induced heme oxygenase mRNA but decreased or had no effect on its activity. Administration of heme, SnCl2, and H2O2 resulted in some degree of glutathione perturbation (GSH/GSSG). However, in all cases, depletion of glutathione was exacerbated if heme oxygenase was simultaneously inhibited. Conversely, perturbation of glutathione levels was minimized if heme oxygenase was induced by heme or stannous chloride. These results demonstrate that RCE cells exhibit functional heme oxygenase activity which is inducible in response to inflammatory cytokines and oxidative stress agents and suggest a cytoprotective role for heme oxygenase against cell injury.

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