Extensive sequence and structural evolution of Arginase 2 inhibitory antibodies enabled by an unbiased approach to affinity maturation

Affinity maturation is a powerful technique in antibody engineering for the in vitro evolution of antigen binding interactions. Key to the success of this process is the expansion of sequence and combinatorial diversity to increase the structural repertoire from which superior binding variants may be selected. However, conventional strategies are often restrictive and only focus on small regions of the antibody at a time. In this study, we used a method that combined antibody chain shuffling and a staggered-extension process to produce unbiased libraries, which recombined beneficial mutations from all six complementarity-determining regions (CDRs) in the affinity maturation of an inhibitory antibody to Arginase 2 (ARG2). We made use of the vast display capacity of ribosome display to accommodate the sequence space required for the diverse library builds. Further diversity was introduced through pool maturation to optimize seven leads of interest simultaneously. This resulted in antibodies with substantial improvements in binding properties and inhibition potency. The extensive sequence changes resulting from this approach were translated into striking structural changes for parent and affinity-matured antibodies bound to ARG2, with a large reorientation of the binding paratope facilitating increases in contact surface and shape complementarity to the antigen. The considerable gains in therapeutic properties seen from extensive sequence and structural evolution of the parent ARG2 inhibitory antibody clearly illustrate the advantages of the unbiased approach developed, which was key to the identification of high-affinity antibodies with the desired inhibitory potency and specificity.

Structural and functional characterization of C0021158, a high-affinity monoclonal antibody that inhibits Arginase 2 function via a novel non-competitive mechanism of action

Arginase 2 (ARG2) is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of L-arginine. The dysregulated expression of ARG2 within specific tumor microenvironments generates an immunosuppressive niche that effectively renders the tumor 'invisible' to the host's immune system. Increased ARG2 expression leads to a concomitant depletion of local L-arginine levels, which in turn leads to suppression of anti-tumor T-cell-mediated immune responses. Here we describe the isolation and characterization of a high affinity antibody (C0021158) that inhibits ARG2 enzymatic function completely, effectively restoring T-cell proliferation in vitro. Enzyme kinetic studies confirmed that C0021158 exhibits a noncompetitive mechanism of action, inhibiting ARG2 independently of L-arginine concentrations. To elucidate C0021158's inhibitory mechanism at a structural level, the co-crystal structure of the Fab in complex with trimeric ARG2 was solved. C0021158's epitope was consequently mapped to an area some distance from the enzyme's substrate binding cleft, indicating an allosteric mechanism was being employed. Following C0021158 binding, distinct regions of ARG2 undergo major conformational changes. Notably, the backbone structure of a surface-exposed loop is completely rearranged, leading to the formation of a new short helix structure at the Fab-ARG2 interface. Moreover, this large-scale structural remodeling at ARG2's epitope translates into more subtle changes within the enzyme's active site. An arginine residue at position 39 is reoriented inwards, sterically impeding the binding of L-arginine. Arg39 is also predicted to alter the pKA of a key catalytic histidine residue at position 160, further attenuating ARG2's enzymatic function. In silico molecular docking simulations predict that L-arginine is unable to bind effectively when antibody is bound, a prediction supported by isothermal calorimetry experiments using an L-arginine mimetic. Specifically, targeting ARG2 in the tumor microenvironment through the application of C0021158, potentially in combination with standard chemotherapy regimens or alternate immunotherapies, represents a potential new strategy to target immune cold tumors.

Hepatitis G virus genotypes in Australia, Papua New Guinea and the Solomon Islands: a possible new Pacific type identified

BACKGROUND

Hepatitis G Virus (HGV)/GB Virus-C (GBV-C) is a newly discovered RNA virus. Nucleotide sequence comparison and phylogenetic studies of the 5' untranslated region (5'UTR) within the viral genome have identified at least three different types which have provisionally been classified as type 1 (West African origin), type 2 (North American origin) and type 3 (Asian origin).

METHODS AND RESULTS

The products of RT-PCR were sequenced by using blood donors and patients infected with HGV/GBV-C in Australia, Papua New Guinea and the Solomon Islands to investigate the genotype distribution in this area of the world. All the Australian isolates showed strong sequence homology with type 2, while the Papua New Guinea and Solomon Islands sequences were more closely related, but differ from type 3, which has previously been reported from isolates studied within Asia.

CONCLUSIONS

Phylogenetic analysis suggests that these latter sequences are either a new HGV/GBV-C Pacific type or a subtype of the Asian type RNA virus. Isolates homologous with type 1 were not identified in these population groups.

Localisation of hepatitis C virus proteins in infected liver tissue by immunofluorescence

Hepatitis C virus (HCV) antigen expression was examined by immunohistochemical staining in liver tissue taken at biopsy from 8 anti-HCV positive patients. Frozen liver sections were stained by indirect immunofluorescence for capsid, E2/NS1, NS3, NS4 and NS5 using polyclonal antibodies raised to synthetic peptides from these regions. The antigens E2 and NS3 were localised in scattered hepatocytes and also in cells within and around areas of inflammation. A weaker signal was observed for NS4 and NS5 and no signal was seen for capsid antigen. No staining was seen in liver tissue from 9 individuals, including 3 hepatitis B virus-positive and 2 hepatitis delta virus/positive patients, who were negative for serological markers of HCV. The specificity of the staining reaction was also confirmed by the lack of staining in HCV-positive liver samples, after the antisera was pre-adsorbed against the specific peptide. Collectively, the data suggests that HCV may not only be hepatotropic but also lymphotropic, and this may be an important factor in the pathogenesis of HCV infection.

Detection of hepatitis C virus RNA by in situ hybridization

Persistent infection with hepatitis C virus (HCV) is associated with chronic hepatitis and cirrhosis which may eventually develop into primary hepatocellular carcinoma. The mechanism of pathogenesis is ill-defined and nothing is known of the distribution, frequency or type of infected cell in the liver of HCV-infected individuals. In this study we have examined liver tissue taken at autopsy from 2 anti-HCV-positive patients by in situ hybridization for the presence of HCV RNA. Viral RNA was detected by autoradiography after hybridization with 125I-labelled riboprobes, representing approximately 35% of the HCV genome. Only a few positive cells were identified in the HCV-infected liver samples, but not in a normal liver sample. Hybridization with an unrelated probe was negative in all samples. The HCV RNA-positive cells were detected with anti-sense but not sense RNA probes, suggesting that they contained a high ratio of genomic:antigenomic RNA. The appearance and distribution of the HCV RNA-positive cells suggested that they were not hepatocytes and were more likely to be lymphocytes or macrophages.

A dipstick immunobinding enzyme-linked immunosorbent assay for serodiagnosis of hepatitis B and delta virus infections

A simple, specific and economical dipstick immunobinding enzyme-linked immunosorbent assay (DIA) for detecting hepatitis B surface antigen (HBsAg) and antibodies to hepatitis delta virus (anti-HDV), utilizing cellulose nitrate membrane is described. Screening of 815 serum specimens for HBsAg by DIA and micro ELISA revealed a positivity of 22.69% and 22.94% respectively. In the detection of antibodies to delta antigen, DIA was compared with an indirect immunofluorescence technique using A3 cell line as antigen substrate and a commercial macro ELISA. Of the 143 HBsAg positive sera tested for anti-HDV, 59 (41.25%) were positive by both immunofluorescence and macro ELISA and 61 (42.65%) by DIA. While the positive and negative predictive values of DIA for HBsAg were 100% and 99.6%, for anti-HDV by DIA these were 96.7% and 100% respectively. Based on the simplicity of performance and the economical nature of the test system, DIA is recommended as a diagnostic tool for field surveys and small laboratories in developing countries.

Bloody needles: the volumes of blood transferred in simulations of needlestick injuries and shared use of syringes for injection of intravenous drugs

Residual HIV-infected blood in needles and syringes is a source of HIV infection. Using radiolabelled blood we have stimulated needlestick injuries and sharing of syringes by intravenous drug users and quantified the volumes of blood which could be transferred to recipients in these situations. Up to 0.75 microliters of blood was transferred in needlestick simulations, but there was a large variation. In simulations of needlesharing, seven to ten times more blood was transferred from the index user to the first sharer when 2 ml syringes were used compared with 1 ml syringes. Washing with water was not effective in removing 'infected' blood from a syringe.

Effect of Phyllanthus amarus on duck hepatitis B virus replication in vivo

Nine ducks congenitally infected with the duck hepatitis B virus (DHBV) were treated either orally (four ducks for 10 weeks) or intraperitoneally (five ducks for 12 weeks) with the Indian traditional herbal remedy Phyllanthus amarus. Compared to placebo-treated control ducks, these treatments did not result in a reduction of circulating viral DNA in the serum or in the level of viral DNA replication in the liver. In two of the five intraperitoneal-treated ducks, a reduction in the levels of duck hepatitis B surface antigenaemia (DHBsAg) was observed. The data strongly suggest that Phyllanthus amarus has no significant inhibitory effect on DHBV DNA replication and only a minor effect on DHBsAg production.

Replication of human hepatitis delta virus in primary cultures of woodchuck hepatocytes

We obtained two lines of evidence that monolayer cultures of primary woodchuck hepatocytes support replication of the genome of human hepatitis delta virus (HDV). (i) From a Northern (RNA blot) analysis of the HDV-related RNA in infected cultures, both genomic and antigenomic 1.7-kilobase RNA species were detected at 11 days after infection. The ratio of genomic RNA to antigenomic RNA was 2:1 to 10:1, comparable to that previously reported in studies of experimentally infected chimpanzees and woodchucks. (ii) Replication in culture was also demonstrated by in situ hybridization with a strand-specific probe. Such studies showed that only a small fraction of the cultured cells supported replication and that in such cells the relative and absolute levels of the HDV RNAs were comparable to those in liver cells infected in vivo. Furthermore, as with the in vivo studies, the HDV RNAs were predominantly localized to the nucleus. In summary, we demonstrated that cultured cells supported both the early events of HDV adsorption and penetration and the intermediate events of genome replication.