Liver regeneration stimulated by a loss of liver mass leads to hepatocyte and nonparenchymal cell proliferation and rapid restoration of liver parenchyma. Mice with targeted disruption of the interleukin-6 (IL-6) gene had impaired liver regeneration characterized by liver necrosis and failure. There was a blunted DNA synthetic response in hepatocytes of these mice but not in nonparenchymal liver cells. Furthermore, there were discrete G1 phase (prereplicative stage in the cell cycle) abnormalities including absence of STAT3 (signal transducer and activator of transcription protein 3) activation and depressed AP-1, Myc, and cyclin D1 expression. Treatment of IL-6-deficient mice with a single preoperative dose of IL-6 returned STAT3 binding, gene expression, and hepatocyte proliferation to near normal and prevented liver damage, establishing that IL-6 is a critical component of the regenerative response.

The involvement of complement activation products in promoting tumor growth has not yet been recognized. Here we show that generation of complement C5a in the tumor microenvironment enhanced tumor growth by suppressing the anti-tumor CD8+ T cell-mediated response. This suppression was associated with the recruitment of myeloid-derived suppressor cells (MDSCs) into tumors and augmentation of their T cell-directed suppressive capabilities. Amplification of MDSC suppressive capacity by C5a occurred through regulation of the production of reactive oxygen and nitrogen species. Pharmacological blockade of C5a receptor significantly impaired tumor growth to a degree comparable to the effect produced by the anti-cancer drug Taxol. Thus, this study demonstrates a therapeutic role for complement inhibition in the treatment of cancer.

Interleukin‐6 null (IL‐6−/−) mice have impaired liver regeneration and increased liver necrosis following partial hepatectomy that is corrected with IL‐6 treatment. Following acute carbon tetrachloride (CCl4 ) treatment, we found that IL‐6−/− mice developed increased hepatocellular injury and defective regeneration with significant blunting of signal transducer‐and‐activator of transcription protein 3 (STAT3) and nuclear factor‐κB (NF‐κB) activation and reduced hepatocyte DNA synthetic and mitotic responses. After CCl4 treatment, unlike partial hepatectomy, increased hepatocyte apoptosis was noted in IL‐6−/− livers. Pretreatment with IL‐6 before CCl4 reduced acute CCl4 injury and apoptosis and accelerated regeneration in both IL‐6+/+ and −/− livers. Repetitive doses of CCl4 in the presence or absence of phenobarbital resulted in increased injury and fibrosis in IL‐6 −/− compared with +/+ livers. After acute and chronic injury, IL‐6−/− livers showed the protracted presence of α‐smooth muscle actin associated with activated stellate cells, indicating a disturbed response in wound healing that progressed to fibrosis. These data provide evidence for an important role for IL‐6 in reducing CCl4 –induced acute and chronic liver injury and fibrosis.

Therapeutic modulation of the complement system has become increasingly important in line with the growing recognition of the role of complement in numerous diseases. Compstatin, a peptidic inhibitor that acts at the central level of the complement cascade, is currently in clinical evaluation but routes to improve its efficacy have not yet been fully explored. Here, we report improvements in both the inhibitory potency and pharmacokinetic parameters of compstatin that broaden its clinical applications. Selective modification of the compstatin N-terminus with non-proteinogenic amino acids resulted in the first analogue with subnanomolar binding affinity (KD = 0.5 nM) and other similarly potent derivatives with improved solubility in clinically relevant solvents. Detailed structure-activity relationship studies based on biophysical and computational methods revealed key structural determinants for the observed improvements. Importantly, pharmacokinetic evaluation in non-human primates revealed target-driven elimination kinetics with plasma half-life values exceeding expectations for peptidic drugs (close to 12 hours). This successful optimization strategy is expected to pave the way for systemic administration of compstatin in a range of clinical conditions.

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