Construction of high level prokaryotic expression and purification system of PD-L1 extracellular domain by using Escherichia coli host cell machinery

Effects of combined cytotoxic T-lymphocyte antigen 4 and programed death 1 ligand-receptor blockade on interferon-gamma production in bovine leukemia virus-infected cattle

Background and Aim

In chronic viral infections, cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) and programmed death ligand 1 (PD-L1) significantly suppress immune responses. The CTLA-4 receptor abundance in regulatory T cells showed a positive association with viral load and a negative association with interferon-gamma (IFN-γ) production in bovine leukemia virus (BLV)-infected cattle. Blocking this receptor boosted IFN-γ production, recovering immune response against this illness. In human cancer patients, not everyone responded positively to non-immunotherapy using CTLA-4 receptor antibodies. The present study analyzed the synergistic effects of CTLA-4 and PD-L1 receptor blockade on IFN-γ production in BLV+ cattle in vitro.

Materials and Methods

The genes for bovine CTLA-4 and PD-L1 were artificially produced. The amino acid sequences of the extracellular receptor domains were sourced from the National Center for Biotechnology Information PubMed database. The western blotting and liquid chromatography with tandem mass spectrometry (LC-MS/MS) techniques were employed for the characterization of recombinant CTLA-4 (rCTLA-4) and recombinant PD-L1 (rPD-L1) proteins. The immunoinhibitory effects of recombinant proteins in Staphylococcus enterotoxin B (SEB)-stimulated cattle peripheral blood mononuclear cells (PBMCs) were investigated. Enzyme-linked immunosorbent assay (ELISA) was used to analyze monoclonal antibodies against rCTLA-4 and rPD-L1. Antibodies generated from peripheral blood mononuclear cells of healthy and BLV-seropositive cows were employed to evaluate their blocking capabilities.

Results

The resulting recombinant proteins specifically reacted with commercial homogeneous monoclonal antibodies (mAbs) using ELISA and anti-His-tag mAbs using western blotting. Analysis of the proteins using LC-MS/MS revealed correspondence with the sequences of rCTLA-4 and rPD-L1 located in the Mascot database. rCTLA-4 and rPD-L1 proteins inhibited IFN-γ production in bovine PBMCs of activated SEB. When PBMCs from cows were cultured with activated SEB containing rCTLA-4 and rPD-L1, the mAbs increased IFN-γ production in PBMCs. The combined cultivation of mAbs and PBMCs from BLV+ cattle enhanced IFN-γ production in the cells.

Conclusion

These findings suggest that the combined blockade of bovine CTLA-4 and PD-L1 receptors can be used as a therapy for bovine leukemia. However, it was shown that a single PBMC sample from a BLV-positive donor did not amplify the synergistic effect. Therefore, it is necessary to perform further studies on a larger population and assessing a wider range of cytokines.

Engineering the Mechanical Stability of a Therapeutic Affibody/PD-L1 Complex by Anchor Point Selection

Protein-protein complexes can vary in mechanical stability depending on the direction from which force is applied. Here we investigated the anisotropic mechanical stability of a molecular complex between a therapeutic non-immunoglobulin scaffold called Affibody and the extracellular domain of the immune checkpoint protein PD-L1. We used a combination of single-molecule AFM force spectroscopy (AFM-SMFS) with bioorthogonal clickable peptide handles, shear stress bead adhesion assays, molecular modeling, and steered molecular dynamics (SMD) simulations to understand the pulling point dependency of mechanostability of the Affibody:(PD-L1) complex. We observed diverse mechanical responses depending on the anchor point. For example, pulling from residue #22 on Affibody generated an intermediate unfolding event attributed to partial unfolding of PD-L1, while pulling from Affibody's N-terminus generated force-activated catch bond behavior. We found that pulling from residue #22 or #47 on Affibody generated the highest rupture forces, with the complex breaking at up to ~ 190 pN under loading rates of ~104-105 pN/sec, representing a ~4-fold increase in mechanostability as compared with low force N-terminal pulling. SMD simulations provided consistent tendencies in rupture forces, and through visualization of force propagation networks provided mechanistic insights. These results demonstrate how mechanostability of therapeutic protein-protein interfaces can be controlled by informed selection of anchor points within molecules, with implications for optimal bioconjugation strategies in drug delivery vehicles.

Optimized expression of Peptidyl-prolyl cis/transisomerase cyclophilinB with prokaryotic toxicity from Sporothrix globosa

Cyclophilin B (CypB), a significant member of immunophilins family with peptidyl-prolyl cis-trans isomerase (PPIase) activity, is crucial for the growth and metabolism of prokaryotes and eukaryotes. Sporothrix globosa (S. globosa), a principal pathogen in the Sporothrix complex, causes sporotrichosis. Transcriptomic analysis identified the cypB gene as highly expressed in S. globosa. Our previous study demonstrated that the recombinant Escherichia coli strain containing SgcypB gene failed to produce sufficient product when it was induced to express the protein, implying the potential toxicity of recombinant protein to the bacterial host. Bioinformatics analysis revealed that SgCypB contains transmembrane peptides within the 52 amino acid residues at the N-terminus and 21 amino acids near the C-terminus, and 18 amino acid residues within the cytoplasm. AlphaFold2 predicted a SgCypB 3D structure in which there is an independent PPIase domain consisting of a spherical extracellular part. Hence, we chose to express the extracellular domain to yield high-level recombinant protein with PPIase activity. Finally, we successfully produced high-yield, truncated recombinant CypB protein from S. globosa (SgtrCypB) that retained characteristic PPIase activity without host bacterium toxicity. This study presents an alternative expression strategy for proteins toxic to prokaryotes, such as SgCypB.

ONE-SENTENCE SUMMARY

The recombinant cyclophilin B protein of Sporothrix globosa was expressed successfully by retaining extracellular domain with peptidyl-prolyl cis-trans isomerase activity to avoid toxicity to the host bacterium.

Recombinant human TIM-3 ectodomain expressed in bacteria and recovered from inclusion bodies as a stable and active molecule

Introduction: Microbial systems, such as Escherichia coli, as host recombinant expression is the most versatile and the cheapest system for protein production, however, several obstacles still remain, such as recovery of soluble and functional proteins from inclusion bodies, elimination of lipopolysaccharides (LPS) contamination, incomplete synthesis, degradation by proteases, and the lack of post-translational modifications, which becomes even more complex when comes to membrane proteins, because they are difficult not only to produce but also to keep in solution in its active state. T-cell Immunoglobulin and Mucin domain 3 (TIM-3) is a type I transmembrane protein that is predominantly expressed on the surface of T lymphocytes, natural killer (NK) cells, dendritic cells, and macrophages, playing a role as a negative immune checkpoint receptor. TIM-3 comprises a single ectodomain for interaction with immune system soluble and cellular components, a transmembrane domain, and a cytoplasmic tail, responsible for the binding of signaling and scaffolding molecules. TIM-3 pathway holds potential as a therapeutic target for immunotherapy against tumors, autoimmunity, chronic virus infections, and various malignancies, however, many aspects of the biology of this receptor are still incompletely understood, especially regarding its ligands. Methods: Here we overcome, for the first time, the challenge of the production of active immune checkpoint protein recovered from bacterial cytoplasmic inclusion bodies, being able to obtain an active, and non-glycosylated TIM-3 ectodomain (TIM-3-ECD), which can be used as a tool to better understand the interactions and roles of this immune checkpoint. The TIM-3 refolding was obtained by the association of high pressure and alkaline pH. Results: The purified TIM-3-ECD showed the correct secondary structure and was recognized from anti-TIM-3 structural-dependent antibodies likewise commercial TIM-3-ECD was produced by a mammal cells system. Furthermore, immunofluorescence showed the ability of TIM-3-ECD to bind to the surface of lung cancer A549 cells and to provide an additional boost for the expression of the lymphocyte activation marker CD69 in anti-CD3/CD28 activated human PBMC. Discussion: Taken together these results validated a methodology able to obtain active checkpoint proteins from bacterial inclusion bodies, which will be helpful to further investigate the interactions of this and others not yet explored immune checkpoints.

Novel technologies for applying immune checkpoint blockers

Cancer cells develop several ways to subdue the immune system among others via upregulation of inhibitory immune checkpoint (ICP) proteins. These ICPs paralyze immune effector cells and thereby enable unfettered tumor growth. Monoclonal antibodies (mAbs) that block ICPs can prevent immune exhaustion. Due to their outstanding effects, mAbs revolutionized the field of cancer immunotherapy. However, current ICP therapy regimens suffer from issues related to systemic administration of mAbs, including the onset of immune related adverse events, poor pharmacokinetics, limited tumor accessibility and immunogenicity. These drawbacks and new insights on spatiality prompted the exploration of novel administration routes for mAbs for instance peritumoral delivery. Moreover, novel ICP drug classes that are adept to novel delivery technologies were developed to circumvent the drawbacks of mAbs. We therefore review the state-of-the-art and novel delivery strategies of ICP drugs.

Size-optimized nuclear-targeting phototherapy enhances the type I interferon response for "cold" tumor immunotherapy

There is growing interest in the effect of innate immune silencing in "cold" tumors, which always fail in the immune checkpoint blockade monotherapy using PD-L1 monoclonal antibodies (aPD-L1). Combination of aPD-L1 with photodynamic therapy, i.e., photoimmunotherapy, is a promising strategy to improve the mono immunotherapy. Nuclear-targeting nanoparticles could elicit a type I interferon (IFN)-mediated innate immune response and reverse the immunosuppressive microenvironment for long-term immunotherapy of "cold" tumors. Photosensitizers such as zinc phthalocyanine (ZnPc) have limited ability to target the nucleus and activate innate sensing pathways to minimize tumor recurrence. Additionally, the relationship between nanoparticle size and nuclear entry capacity remains unclear. Herein, graphene quantum dots (GQDs) were employed as aPD-L1 and ZnPc carriers. Three particle sizes (200 nm, 32 nm and 5 nm) of aPD-L1/ZnPc/GQD-PEG (PZGE) were synthesized and tested. The 5 nm nanoparticles achieved the best nuclear enrichment capacity contributing to their ultrasmall size. Notably, 5 nm PZGE-based photodynamic therapy enabled an amplification of the type I IFN-mediated innate immune response and could convert "immune-cold" tumors into "immune-hot" ones. Utilizing their size advantage to target the nucleus, 5 nm nanoparticles induced DNA damage and activated the type I IFN-mediated innate immune response, subsequently promoting cytotoxic T-lymphocyte infiltration and reversing negative PD-L1 expression. Furthermore, the nanoplatform we designed is promising for the effective suppression of distant oral squamous cell carcinoma. Thus, for the first time, this study presents a size design strategy for nuclear-targeted photo-controlled immune adjuvants and the nuclear-targeted phototherapy-mediated immunomodulatory functions of type I IFN innate immune signalling for "immune-cold" tumors. STATEMENT OF SIGNIFICANCE: The potential of commonly used photosensitizers to activate innate sensing pathways for producing type I IFNs is limited due to the lack of nuclear targeting. Facilitating the nuclear-targeting of photosensitizers to enhance innate immune response and execute long-term tumor killing effect would be a promising strategy for "cold" tumor photoimmunotherapy. Herein, we report an optimal size of PZGE nanoparticles that enable the nuclear-targeting of ZnPc, which reinforces the type I IFN-mediated innate immune response, synergistically reversing "cold tumors" to "hot tumors" for effective primary and distant tumor photoimmunotherapy. This work highlights the marked efficacy of ultrasmall nuclear-located nanocarriers and offers new insight into "immune-cold tumors" via prominent innate immune activation mediated by nuclear-targeting photoimmunotherapy.

Bioengineering and computational analysis of programmed cell death ligand-1 monoclonal antibody

The trans-membrane proteins of the B7 family programmed cell death ligand-1 (PD-L1) and programmed death-1 (PD-1) play important roles in inhibiting immune responses and enhancing self-tolerance via T-cell modulation. Several therapeutic antibodies are used to promote T-cell proliferation by preventing interactions between PD-1/PD-L1. Recombinant technology appears to be quite useful in the production of such potent antibodies. In this study, we constructed recombinant molecules by cloning variable regions of the PD-L1 molecule into pMH3 vectors and transferring them into mammalian cell lines for expression. G418 supplementation was used to screen the recombinant clones, which were then maintained on serum-free medium. The full-length antibody was isolated and purified from the medium supernatant at a concentration of 0.5-0.8 mg/ml. Antibody binding affinity was investigated using ELISA and immunofluorescence methods. The protein-protein interactions (PPI) were determined using a docking approach. The SWISS model was utilized for homology modeling, while ZDOCK, Chimera, and PyMOL were used to validate 3D models. The Ramachandran plots were constructed using the SWISS model, which revealed that high-quality structures had a value of more than 90%. Current technologies allow for the accurate determination of antigen-antibody interactions.

Programmed cell death ligand-1: A dynamic immune checkpoint in cancer therapy

Antibody-based immunotherapies play a pivotal role in cancer research with efficient achievements in tumor suppression. Tumor survival is assisted by modulation of immune checkpoints to create imbalances between immune cells and cancer cell's environment. The modulation results in T-cell signal inhibition ultimately inert its proliferation and activation against various tumor cells. PD-L1, a 40 kDa transmembrane protein of B7 family, binds with PD-1 on the membrane of T cells which results in inhibition of T-cell proliferation and activation. PD-L1/PD-1 pathway has generated novel target sites for antibodies that can block PD-L1/PD-1 interactions. The blockage results in T-cell proliferation and tumor cell suppression. The PD-L1 immune checkpoint strategies' development, expression and regulations, signal inhibitions, and developmental stages of PD-L1/PD-1 antibodies are briefly discussed here in this review. All this information will provide a base for new therapeutic development against PD-L1 and PD-1 immune checkpoint interactions and will make available promising treatment options.

Construction of Anti-hPD-L1 HCAb Nb6 and in Situ 124I Labeling for Noninvasive Detection of PD-L1 Expression in Human Bone Sarcoma

Immunotherapy is considered the fourth major treatment mode for cancer following surgery, chemotherapy, and radiotherapy. In recent years, tumor immunotherapy has achieved breakthrough progress; therefore, it is important to screen patients to identify those who will respond to tumor immunotherapy. Here, we report the construction of a novel heavy chain-only antibody (HCAb) and its corresponding ¹²⁴I-labeled probe. Using phage display technology, we generated a novel anti-hPD-L1-specific HCAb named Nb6 (selected from 95 monoclones) with high affinity for hPD-L1. The positron-emitting ¹²⁴I-labeled hPD-L1-targeted HCAb probe was prepared for further evaluation, and nonradioactive natural iodine (^natI)-labeled anti-hPD-L1 Nb6 was synthesized as a reference compound. ¹²⁵I-anti-hPD-L1 Nb6 uptake in OS-732 cells in vitro can be blocked by the precursor. The binding affinity of ¹²⁵I-anti-hPD-L1 Nb6 to OS-732 cell lines was 2.19 nM. For in vivo studies, an osteosarcoma OS-732 tumor-bearing mouse model was successfully constructed. Polymerase chain reaction (PCR) and Western blot analyses were performed to confirm the presence of the hPD-L1 gene and antigen in the tumor tissue of the OS-732 mouse model. Biodistribution showed that uptake of ¹²⁴I-anti-hPD-L1 Nb6 probes at 24 h was 4.43 ± 0.33% ID/g in OS-732 tumor tissues. Tumor lesions can be clearly delineated on micro-PET (positron emission tomography)/CT (computed tomography) imaging 24 h after injection of ¹²⁴I-anti-hPD-L1 Nb6, while the blocking group shows substantially decreased uptake on imaging. Pathological staining validated hPD-L1 expression on the surface of the tumor cell membrane; thus, ¹²⁴I-anti-hPD-L1 Nb6 can be used for in vivo noninvasive PET imaging. When administered in tandem, Nb6 and ¹²⁴I-anti-hPD-L1 Nb6 may provide a novel strategy to clinically screen patients for hPD-L1 to identify those who would benefit from immunotherapy of malignant tumors such as osteosarcoma.

Efficient development and expression of scFv recombinant proteins against PD-L1 surface domain and potency in cancer therapy

PD-L1 is a 40 kDa trans-membrane protein of B7 family and an important T cell regulator. Binding of PD-L1 and PD-1 inhibits proliferation and activation of T cell results cell exhaustion. This phenomenon can be reversed by blocking PD-L1/PD-1 interactions with single chain variables fragment (scFv) fusion proteins and by direct inhibition of tumor cells with drug conjugates. The human phage-displayed scFv library was utilized to generate scFv against the PD-L1 antigen by affinity bio-panning. The positive clones were selected by continuous transfection of bacterial cells and sequence analysis. The binding affinity and specificity of the scFv and antibody fragments were determined by using surface plasmon resonance biosensor, western blot analysis, and immunofluorescence assay. After three rounds of panning selection, about 30% of clones have a binding affinity with targeted PD-L1 antigen. Eight positive clones with accurate sequences were isolated and analyzed for binding affinity with PD-L1 antigen. Three of those with accurate sequences and binding affinity were selected for the recombinant formation and soluble expression by Escherichia coli host machinery. The highly positive recombinant clones with the exact orientation of FR and CDR domains were developed and can be used as a drug carrier tools in ADC formation or direct inhibition of immune checkpoint in cancer immunotherapy. The conjugate achieved its initial potency and need efficient improvement to enhance direct tumor suppression and bio-therapeutics strategies enrichment.

Construction of small molecular CTLA4 analogs with CD80-binding affinity

A variety of CTLA4-Fc fusion proteins and anti-CTLA4 monoclonal antibody have been approved. Given the shortcomings of macromolecular antibodies, recombinant proteins derived from the tenth unit of human type III fibronectin (FN3) termed monobody were studied as CTLA4 analogs in this study. A peptide EL16¹ derived from CD80-binding domain (MYPPPY motifs) in the complementarity determining region (CDR) 3 of CTLA4 was found to inhibit the interaction of CTLA4 with CD80 significantly. Afterward, the peptide EL16 as well as the CDR1 of CTLA4 which is also critical for its binding to CD80 were grafted onto FN3 and obtained a novel CD80 binding monobody protein CFN13.² CFN13 showed 80% binding affinity compared to CTLA4. In addition, to increase the half-life, CFN13 was fused to human IgG1 Fc to generate CFN13-Fc fusion protein. As expected, CFN13-Fc bound to CD80 in a dosage-dependent manner as CFN13 did, and displayed 41.0% and 31.4% inhibition on the interaction of CTLA4-Fc with CD80 at 200 μg/ml and 100 μg/ml respectively. Moreover, peptide EL16 could inhibit CFN13-Fc binding to CD80 significantly, with the inhibition ratio of 64.3% and 52.8% at 100 and 50 μg/ml respectively, indicating that the peptide EL16 and CFN13-Fc shared the similar binding sites with CD80 and the CDR3 motif of CTLA4 contributed more than CDR1 in binding to CD80. In summary, our study provides insights into small molecular analogs of CTLA4.

Polyclonal antibody production against rGPC3 and their application in diagnosis of hepatocellular carcinoma

Glypican-3 (GPC3) is an integral membrane proteoglycan, which contains a core protein anchored to the cytoplasmic membrane through a glycosylphosphatidylinositol linkage. The glypican-3 can regulate the signaling pathways, thereby enhances cell division, growth, and apoptosis in certain cell types. It is almost nonexistent on the surface of the human normal cell membrane and highly expresses on the membrane of hepatocellular carcinoma (HCC) cells. It has been well established that GPC3 provides a useful diagnostic marker. For generating the polyclonal antibody of GPC3, we expected that GPC3 N-terminal region (amino acid sequence 26-358) could be expressed in Escherichia coli system, however, no active expression was observed after IPTG induction. Interestingly, after deletion of six proline residues from position 26 to 31 in the N-terminus, expression of recombinant GPC3 was clearly detected. We further analyzed the expressed protein deprived of six prolines, to immunize the New Zealand male rabbits for production of active antibodies. The binding affinity of antibody was analyzed by immunofluorescence analysis, immunohistochemical detection, and western blotting. The functional GPC3 N-terminal protein recombinant development, expression, purification, and the polyclonal antibody have been generated provide the basis for the diagnosis of HCC in cancer therapy.