Factors affecting product association as a mechanism of host-cell protein persistence in bioprocessing.

Product association of host-cell proteins (HCPs) to monoclonal antibodies (mAbs) is widely regarded as a mechanism that can enable HCP persistence through multiple purification steps and even into the final drug substance. Discussion of this mechanism often implies that the existence or extent of persistence is directly related to the strength of binding but actual measurements of the binding affinity of such interactions remain sparse. Two separate avenues of investigation of HCP-mAb binding are reported here. One is the measurement of the affinity of binding of individual, commonly persistent Chinese hamster ovary (CHO) HCPs to each of a set of mAbs, and the other uses quantitative proteomic measurements to assess binding of HCPs in a null CHO harvested cell culture fluid (HCCF) to mAbs produced in the same cell line. The individual HCP measurements show that the binding affinities of individual HCPs to different mAbs can vary appreciably but are rarely very high, with only weak pH dependence. The measurements on the null HCCF allow estimation of individual HCP-mAb affinities; these are typically weaker than those seen in affinity measurements on isolated HCPs. Instead, the extent of binding appears correlated with the initial abundance of individual HCPs in the HCCF and the forms of the HCPs in the solution, i.e., whether HCPs are present as free molecules or as parts of large aggregates. Separate protein A chromatography experiments performed by feeding different fractions of a mAb-containing HCCF obtained by size-exclusion chromatography (SEC) showed clear differences in the number and identity of HCPs found in the protein A eluate. These results indicate a significant role for HCP-mAb association in determining HCP persistence through protein A chromatography, presumably through binding of HCP-mAb complexes to the resin. Overall, the results illustrate the importance of considering more fully the biophysical context of HCP-product association in assessing the factors that may affect the phenomenon and determine its implications. Knowledge of the abundances and the forms of individual or aggregated HCPs in HCCF are particularly significant, emphasizing the integration of upstream and downstream bioprocessing and the importance of understanding the collective properties of HCPs in addition to just the biophysical properties of individual HCPs.

Identification and characterization of CHO host-cell proteins in monoclonal antibody bioprocessing.

Host-cell proteins (HCPs) are the foremost class of process-related impurities to be controlled and removed in downstream processing steps in monoclonal antibody (mAb) manufacturing. However, some HCPs may evade clearance in multiple purification steps and reach the final drug product, potentially threatening drug stability and patient safety. This study extends prior work on HCP characterization and persistence in mAb process streams by using mass spectrometry (MS)-based methods to track HCPs through downstream processing steps for seven mAbs that were generated by five different cell lines. The results show considerable variability in HCP identities in the processing steps but extensive commonality in the identities and quantities of the most abundant HCPs in the harvests for different processes. Analysis of HCP abundance in the harvests shows a likely relationship between abundance and the reproducibility of quantification measurements and suggests that some groups of HCPs may hinder the characterization. Quantitative monitoring of HCPs persisting through purification steps coupled with the findings from the harvest analysis suggest that multiple factors, including HCP abundance and mAb-HCP interactions, can contribute to the persistence of individual HCPs and the identification of groups of common, persistent HCPs in mAb manufacturing.

Characterization and implications of host-cell protein aggregates in biopharmaceutical processing.

In the production of biopharmaceuticals such as monoclonal antibodies (mAbs) and vaccines, the residual amounts of host-cell proteins (HCPs) are among the critical quality attributes. In addition to overall HCP levels, individual HCPs may elude purification, potentially causing issues in product stability or patient safety. Such HCP persistence has been attributed mainly to biophysical interactions between individual HCPs and the product, resin media, or residual chromatin particles. Based on measurements on process streams from seven mAb processes, we have found that HCPs in aggregates, not necessarily chromatin-derived, may play a significant role in the persistence of many HCPs. Such aggregates may also hinder accurate detection of HCPs using existing proteomics methods. The findings also highlight that certain HCPs may be difficult to remove because of their functional complementarity to the product; specifically, chaperones and other proteins involved in the unfolded protein response (UPR) are disproportionately present in the aggregates. The methods and findings described here expand our understanding of the origins and potential behavior of HCPs in cell-based biopharmaceutical processes and may be instrumental in improving existing techniques for HCP detection and clearance.

Towards continuous mAb purification: Clearance of host cell proteins from CHO cell culture harvests via "flow-through affinity chromatography" using peptide-based adsorbents.

The growth of advanced analytics in manufacturing monoclonal antibodies (mAbs) has highlighted the challenges associated with the clearance of host cell proteins (HCPs). Of special concern is the removal of "persistent" HCPs, including immunogenic and mAb-degrading proteins, that co-elute from the Protein A resin and can escape the polishing steps. Responding to this challenge, we introduced an ensemble of peptide ligands that target the HCPs in Chinese hamster ovary (CHO) cell culture fluids and enable mAb purification via flow-through affinity chromatography. This study describes their integration into LigaGuard™, an affinity adsorbent featuring an equilibrium binding capacity of ~30 mg of HCPs per mL of resin as well as dynamic capacities up to 16 and 22 mg/ml at 1- and 2-min residence times, respectively. When evaluated against cell culture harvests with different mAb and HCP titers and properties, LigaGuard™ afforded high HCP clearance, with logarithmic removal values (LRVs) up to 1.5, and mAb yield above 90%. Proteomic analysis of the effluents confirmed the removal of high-risk HCPs, including cathepsins, histones, glutathione-S transferase, and lipoprotein lipases. Finally, combining LigaGuard™ for HCP removal with affinity adsorbents for product capture afforded a global mAb yield of 85%, and HCP and DNA LRVs > 4.

Platelet aggregation in whole blood is not impaired by a platelet-sparing leukoreduction filter and instead depends upon the presence of leukocytes.

BACKGROUND: Recent studies characterizing in vitro hemostatic properties of whole blood (WB) leukoreduced (LR) with a platelet-sparing filter have described subtle, if any, changes to viscoelastic clotting; however, reductions in platelet (PLT) content and impedance aggregometry (IA) responses have been noted. The effects of filtration of WB (i.e., filter-contact effects, reduction in platelet and leukocyte count) have not been rigorously investigated as to their individual impacts on platelet IA responses. STUDY DESIGN AND METHODS: WB units from healthy donors were collected and characterized to assess the effects of platelet-sparing leukoreduction (LR) upon the in vitro hemostatic measures of platelet IA and thromboelastometry. Further characterization of platelet IA responses was carried out in WB samples to delineate the effects of platelet count and leukocyte presence/absence upon the response. RESULTS: WB filtration reduced the platelet count and IA responses but had no impact on viscoelastic clotting measures in fresh WB. Experiments revealed that IA responses have a linear correlation with platelet count in both apheresis platelets and WB and that passage of platelets through the WB-LR filter has no impact upon the strength of this response. Further experiments in LR WB showed that addition of autologous leukocytes back to the platelets fully restored the platelet aggregation response to pre-filtration levels. CONCLUSION: WB filtration results in platelet count reduction and leukocyte removal; however, platelet IA is not degraded by passage through the filter. Apparent declines in platelet IA responses can be fully attributed to the reduction in platelet count and the removal of leukocytes.

Continuous CD34+ cell collection by a new device is safe and more efficient than by a standard collection procedure: results of a two-center, crossover, randomized trial.

BACKGROUND: Peripheral blood stem cells (PBSCs) collected from granulocyte-colony-stimulating factor (G-CSF)-mobilized donors are routinely used for hematopoietic stem cell transplantation. Most PBSC collections worldwide have used the COBE Spectra (COBE) platform that is being replaced by the Spectra Optia (OPTIA). This study compared the PBSC collection performance and safety of the OPTIA using a single-step, continuous mononuclear cell (CMNC) collection procedure and the standard COBE MNC procedure. STUDY DESIGN AND METHODS: A prospective, noninferiority, randomized, open-label, crossover, multicenter study was conducted in G-CSF-mobilized donors randomized to undergo MNC collection on Days 5 and 6. The primary endpoint was CD34+ cell collection efficiency (CE1%) with a noninferiority margin of 10%. The secondary endpoint was CD34+ cell CE2%. Product purity and safety were also assessed. RESULTS: Twenty-three healthy donors (87% male) participated in the study. Mean (±SD) CD34+ CE1% was 84.4% (±16.4%) and 66.2% (±15.3%) for the OPTIA and COBE, respectively (p < 0.001 for noninferiority and superiority). Mean (±SD) CD34+ CE2% was 62.4 (±11.6) and 48.4 (±11.2) for the OPTIA and COBE, respectively (p < 0.001 for superiority). Granulocyte and platelet (PLT) contamination were lower in OPTIA-collected products. There were no unexpected adverse events (AEs) and no significant differences in the incidence of AEs between study arms. PLT loss was less with the OPTIA than with the COBE. CONCLUSION: The OPTIA CMNC collection procedure is safe and effective for the collection of CD34+ cells in G-CSF-mobilized donors and was superior to the COBE for CE1% and CE2%, collecting approximately 19 and 16% higher, respectively.

Increasing parvovirus filter throughput of monoclonal antibodies using ion exchange membrane adsorptive pre-filtration.

Pre-filtration using ion exchange membrane adsorbers can improve parvovirus filter throughput of monoclonal antibodies (mAbs). The membranes work by binding trace foulants, and although some antibody product also binds, yields > or =99% are easily achieved by overloading. Results show that foulant adsorption is dependent on pH and conductivity, but independent of scale and adsorber brand. The ability to use ion exchange membranes as pre-filters is significant because it provides a clean, well defined, chemically stable option for enhancing throughput. Additionally, ion exchange membranes facilitate characterization of parvovirus filter foulants. Examination of adsorber elution samples using sedimentation velocity analysis and SEC-MALS/QELS revealed the presence of high molecular weight species ranging from 8 to 13 nm in hydrodynamic radius, which are similar in size to parvoviruses and thus would be expected to plug the pores of a parvovirus filter. A study of two identical membranes in-series supports the hypothesis that the foulants are soluble, trace level aggregates in the feed. This study's significance lies in a previously undiscovered application of membrane chromatography, leading to a more cost effective and robust approach to parvovirus filtration for the production of monoclonal antibodies.

Overloading ion-exchange membranes as a purification step for monoclonal antibodies.

The present study examined the overloading of ion-exchange membrane adsorbers, a form of frontal chromatography, as the final purification step in the production of mAbs (monoclonal antibodies) produced from CHO (Chinese-hamster ovary) cells. Preferential binding of impurities over antibody product was exploited using commercially available cation- and anion-exchange membranes. Three different antibody feedstreams previously purified over Protein A and ion-exchange column chromatography were tested. Feedstream conductivity and pH were adjusted to induce product and impurity adsorption. Membranes were then overloaded in a normal flow mode, resulting in retention of impurities and breakthrough of purified antibody. Although some amount of the product also binds to the membranes (usually < or =30 g mAb/l membrane), yields of > or =99% were achieved by marginalizing the losses, typically by loading more than 3 kg mAb/l membrane. Analyses of the purified pools show consistent removal of impurities despite strong mAb-ligand interactions and high membrane loadings. The clearance of host cell proteins was affected by pH and conductivity, but was unaffected by flow rate, membrane properties or scale. The importance of the present study lies in our demonstration of an alternative use of ion-exchange membranes for fast, effective and high yielding purification of mAbs.

Yeast surface display of a noncovalent MHC class II heterodimer complexed with antigenic peptide.

Microbial protein display technologies have enabled directed molecular evolution of binding and stability properties in numerous protein systems. In particular, dramatic improvements to antibody binding affinity and kinetics have been accomplished using these tools in recent years. Examples of successful application of display technologies to other immunological proteins have been limited to date. Herein, we describe the expression of human class II major histocompatibility complex allele (MHCII) HLA-DR4 on the surface of Saccharomyces cerevisiae as a noncovalently associated heterodimer. The yeast-displayed MHCII is fully native as assessed by binding of conformationally specific monoclonal antibodies; failure of antibodies specific for empty HLA-DR4 to bind yeast-displayed protein indicates antigenic peptide is bound. This report represents the first example of a noncovalent protein dimer displayed on yeast and of successful display of wild-type MHCII. Results further point to the potential for using yeast surface display for engineering and analyzing the antigen binding properties of MHCII.

Class II major histocompatibility complex-peptide tetramer staining in relation to functional avidity and T cell receptor diversity in the mouse CD4(+) T cell response to a rheumatoid arthritis-associated antigen.

OBJECTIVE: Although studies have suggested that human cartilage (HC) gp-39 may be an antigen recognized by autoreactive CD4(+) T cells in rheumatoid arthritis, we previously failed to identify specific CD4(+) T cells in patients' synovial fluid or blood using a class II major histocompatibility complex-peptide tetramer composed of the immunodominant HC gp-39(263-275) epitope covalently linked to DR4. We undertook this study to better understand the parameters for specific binding of this tetramer. METHODS: DR4-transgenic mice were immunized with the HC gp-39 peptide, and a set of peptide-responsive hybridomas was derived. Hybridomas were stained with the DR4-gp-39 tetramer and cultured with increasing amounts of peptide in the presence of DR4-expressing antigen-presenting cells to determine functional avidity. RESULTS: Great variability was apparent in the ability of the tetramer to stain the hybridomas, and there was a strong correlation between the intensity of tetramer staining and functional avidity. Importantly, nearly 30% of the hybridomas did not stain with tetramer, and these cells exhibited relatively low functional avidity. Although the addition of an anti-T cell receptor (anti-TCR) monoclonal antibody during the staining procedure enhanced binding of the tetramer to a number of the hybridomas, a significant percentage remained unstainable. Analysis of TCR expression showed that >90% of the hybridomas expressed the same TCR beta-chain variable region (V(beta)10), and sequencing of the TCR junctional regions showed diversity in the third complementarity-determining region. CONCLUSION: These results suggest that immune responses dominated by relatively low-affinity TCR interactions, such as those that may occur in autoimmune disease, will be difficult to detect using standard tetramer techniques.

Beryllium presentation to CD4+ T cells is dependent on a single amino acid residue of the MHC class II beta-chain.

Chronic beryllium disease (CBD) is characterized by a CD4+ T cell alveolitis and granulomatous inflammation in the lung. Genetic susceptibility to this disease has been linked with HLA-DP alleles, particularly those possessing a glutamic acid at position 69 (Glu69) of the beta-chain. However, 15% of CBD patients do not possess a Glu69-containing HLA-DP allele, suggesting that other MHC class II alleles may be involved in disease susceptibility. In CBD patients without a Glu69-containing HLA-DP allele, an increased frequency of HLA-DR13 alleles has been described, and these alleles possess a glutamic acid at position 71 of the beta-chain (which corresponds to position 69 of HLA-DP). Thus, we hypothesized that beryllium presentation to CD4+ T cells was dependent on a glutamic acid residue at the identical position of both HLA-DP and -DR. The results show that HLA-DP Glu69- and HLA-DR Glu71-expressing molecules are capable of inducing beryllium-specific proliferation and IFN-gamma expression by lung CD4+ T cells. Using fibroblasts expressing mutated HLA-DP2 and -DR13 molecules, beryllium recognition was dependent on the glutamic acid at position 69 of HLA-DP and 71 of HLA-DR, suggesting a critical role for this amino acid in beryllium presentation to Ag-specific CD4+ T cells. Thus, these results demonstrate that a single amino acid residue of the MHC class II beta-chain dictates beryllium presentation and potentially, disease susceptibility.

Use of soluble MHC class II/peptide multimers to detect antigen-specific T cells in human disease.

Most techniques that identify antigen-specific T cells are dependent on the response of these cells to the relevant antigen in culture. Soluble multimers of MHC molecules, when occupied with the same peptide, will bind selectively to T cells specific for that MHC/peptide complex. Techniques to produce fluorescent MHC class II/peptide multimers have recently been developed. These reagents provide a method to facilitate detection and isolation of antigen-specific CD4+ T cells and they represent a new research tool to study these cells in patients with immune-mediated diseases.

Excess apoptosis of mononuclear cells contributes to the depressed cytomegalovirus-specific immunity in HIV-infected patients on HAART.

HIV-infected patients on highly active antiretroviral therapy (HAART) have persistently decreased cytomegalovirus (CMV)-specific proliferative responses [lymphocyte proliferation assay (LPA)] in spite of increases in CD4+ T cell counts. Here we demonstrate an association between apoptosis of unstimulated peripheral blood mononuclear cells (uPBMC) and decreased CMV-LPA. HAART recipients had more apoptosis of uPBMC than controls when measured by caspases 3, 8, and 9 activities and by annexin V binding. Patients with undetectable HIV replication maintained significantly higher apoptosis of CD4+ and CD14+ cells compared to controls. CMV-LPA decreased with higher apoptosis of uPBMC in patients only. This association was independent of CD4+ cell counts or HIV replication. Furthermore, rescuing PBMC from apoptosis with crmA, but not with TRAIL- or Fas-pathway blocking agents or with other caspase inhibitors, increased CMV-LPA in HAART recipients. This effect was not observed in uninfected controls, further indicating that the down regulatory effect of apoptosis on cell-mediated immunity (CMI) was specifically associated with the HIV-infected status.