Dysregulation of the complement system is linked to the pathogenesis of a variety of hematological disorders. Eculizumab, an anti-complement C5 monoclonal antibody, is the current standard of care for paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS). However, because of high levels of C5 in plasma, eculizumab has to be administered biweekly by intravenous infusion. By applying recycling technology through pH-dependent binding to C5, we generated a novel humanized antibody against C5, SKY59, which has long-lasting neutralization of C5. In cynomolgus monkeys, SKY59 suppressed C5 function and complement activity for a significantly longer duration compared to a conventional antibody. Furthermore, epitope mapping by X-ray crystal structure analysis showed that a histidine cluster located on C5 is crucial for the pH-dependent interaction with SKY59. This indicates that the recycling effect of SKY59 is driven by a novel mechanism of interaction with its antigen and is distinct from other known pH-dependent antibodies. Finally, SKY59 showed neutralizing effect on C5 variant p.Arg885His, while eculizumab does not inhibit complement activity in patients carrying this mutation. Collectively, these results suggest that SKY59 is a promising new anti-C5 agent for patients with PNH and other complement-mediated disorders.

The small hydrophobic (SH) protein from the human respiratory syncytial virus (hRSV) is a glycoprotein of ;64 amino acids with one putative a-helical transmembrane domain. Although SH protein is important for viral infectivity, its exact role during viral infection is not clear. Herein, we have studied the secondary structure, orientation, and oligomerization of the transmembrane domain of SH (SH-TM) in the presence of lipid bilayers. Only one oligomer, a pentamer, was observed in PFO-PAGE. Using polarized attenuated total reflection-Fourier transform infrared (PATR-FTIR) spectroscopy, we show that the SH-TM is a-helical. The rotational orientation of SH-TM was determined by site-specific infrared dichroism (SSID) at two consecutive isotopically labeled residues. This orientation is consistent with that of an evolutionary conserved pentameric model obtained from a global search protocol using 13 homologous sequences of RSV. Conductance studies of SH-TM indicate ion channel activity, which is cation selective, and inactive below the predicted pK a of histidine. Thus, our results provide experimental evidence that the transmembrane domain of SH protein forms pentameric a-helical bundles that form cation-selective ion channels in planar lipid bilayers. We provide a model for this pore, which should be useful in mutagenesis studies to elucidate its role during the virus cycle.

Modulating the complement system is a promising strategy in drug discovery for disorders with uncontrolled complement activation. Although some of these disorders can be effectively treated with an antibody that inhibits complement C5, the high plasma concentration of C5 requires a huge dosage and frequent intravenous administration. Moreover, a conventional anti-C5 antibody can cause C5 to accumulate in plasma by reducing C5 clearance when C5 forms an immune complex (IC) with the antibody, which can be salvaged from endosomal vesicles by neonatal Fc receptor (FcRn)-mediated recycling. In order to neutralize the increased C5, an even higher dosage of the antibody would be required. This antigen accumulation can be suppressed by giving the antibody a pH-dependent C5-binding property so that C5 is released from the antibody in the acidic endosome and then trafficked to the lysosome for degradation, while the C5-free antibody returns back to plasma. We recently demonstrated that a pH-dependent C5-binding antibody, SKY59, exhibited long-lasting neutralization of C5 in cynomolgus monkeys, showing potential for subcutaneous delivery or less frequent administration. Here we report the details of the antibody engineering involved in generating SKY59, from humanizing a rabbit antibody to improving the C5-binding property. Moreover, because the pH-dependent C5-binding antibodies that we first generated still accumulated C5, we hypothesized that the surface charges of the ICs partially contributed to a slow uptake rate of the C5–antibody ICs. This idea motivated us to engineer the surface charges of the antibody. Our surface-charge engineered antibody consequently exhibited a high capacity to sweep C5 and suppressed the C5 accumulation in vivo by accelerating the cycle of sweeping: uptake of ICs into cells, release of C5 from the antibody in endosomes, and salvage of the antigen-free antibody. Thus, our engineered anti-C5 antibody, SKY59, is expected to provide significant benefits for patients with complement-mediated disorders.

The p7 protein from the hepatitis C virus (HCV) is a 63 amino acid long polypeptide that is essential for replication, and is involved in protein trafficking and proton transport. Therefore, p7 is a possible target for antivirals. The consensus model for the channel formed by p7 protein is a hexameric or heptameric oligomer of α-helical hairpin monomers, each having two transmembrane domains, TM1 and TM2, where the N-terminal TM1 would face the lumen of this channel. A reported high-throughput functional assay to search for p7 channel inhibitors is based on carboxyfluorescein (CF) release from liposomes after p7 addition. However, the rationale for the dual ability of p7 to serve as an ion or proton channel in the infected cell, and to permeabilize membranes to large molecules like CF is not clear. We have recreated both activities in vitro, examining the conformation present in these assays using infrared spectroscopy. Our results indicate that an α-helical form of p7, which can transport protons, is not able to elicit CF release. In contrast, membrane permeabilization to CF is observed when p7 contains a high percentage of β-structure, or when using a C-terminal fragment of p7, encompassing TM2. We propose that the reported inhibitory effect of some small compounds, e.g., rimantadine, on both CF release and proton transport can be explained via binding to the membrane-inserted C-terminal half of p7, increasing its rigidity, in a similar way to the influenza A M2-rimantadine interaction.

No abstract