Affinity and cross-reactivity engineering of CTLA4-Ig to modulate T cell costimulation

A review of fetal cell lines used during drug development: Focus on COVID-19 vaccines, transplant medications, and biologics

PURPOSE

The recent coronavirus disease 2019 (COVID-19) pandemic and vaccine mandates have increased the number of patient questions related to how fetal cell lines are used during drug development and final manufacturing. This article describes our literature search and review of COVID-19 vaccines, transplant medications, and biologics whose development included use of fetal cell lines.

SUMMARY

A detailed literature search was conducted to identify the common fetal cell lines used in COVID-19 vaccine development; the two most prevalent fetal cell lines identified were HEK-293 and PER.C6. Subsequent literatures searches were conducted to identify transplant medications and biologics whose development included use of the HEK-293 or PER.C6 cell lines. For the COVID-19 vaccines, only the viral vector vaccine by Janssen was found to contain proteins produced by PER.C6 in the final preparation administered to patients, and Novavax is the only vaccine for which fetal cell lines were not directly involved in any portion of drug development. For transplant medications, many medications were studied in fetal cell lines in postmarketing studies after Food and Drug Administration approval; however, none of these medications contained fetal cells or would expose a patient to a fetal cell line. Many new biologics and cellular therapies for genetic diseases and malignancies have been directly developed from HEK-293 fetal cells or contain proteins produced directly from fetal cell lines.

CONCLUSION

There were very few drugs reviewed that were found to contain HEK-293 or PER.C6 fetal cells or proteins derived directly from fetal cell lines; however, use of fetal cell lines in biologics and gene therapies will continue to increase. Healthcare providers should be mindful of patients' beliefs while also correcting common misconceptions about how these fetal cell lines are used throughout drug development and manufacturing.

Co-stimulatory pathway competitive assay development using Liquid chromatography-tandem mass spectrometry (LC-MS/MS)

T-cells play a significant role in the development of autoimmune diseases. The CD28-B7 costimulatory pathway is crucial for activating T-cells, and blocking this pathway is essential for treating autoimmune diseases. Therapeutic antibodies and fusion proteins that target costimulatory molecules like CD80, CD86, CTLA-4, and CD28 have been developed to explore the costimulation process and as targeted treatments. To advance our understanding of costimulation in autoimmunity and the inhibition of the costimulatory pathway, it is crucial to have an accurate, precise, and direct method for detecting and quantifying the soluble form of these molecules in body fluids and various biological systems. Herein, we developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for quantifying the four costimulatory proteins depending on the signature peptides derived from the soluble isoform of these proteins in multiple reaction monitoring (MRM) mode. The method was validated using the US FDA guidelines. The LOQ was determined as ∼0.5 nM for the four analytes, with quantification extended to 20 nM with a correlation coefficient of R2>0.998. The developed MRM method was used to analyze on-bead digested protein mixtures to establish a competitive assay for the CD28-B7 costimulatory pathway using CTLA4-Ig (Abatacept ™) as an FDA-approved drug for rheumatoid arthritis. The IC50 was determined to be 2.99 and 159.8 nM for sCD80 and sCD86, respectively. A straightforward MRM-based competitive assay will advance the knowledge about the costimulatory role in autoimmunity and the autoimmune therapeutic drug discovery, with the need for broad application on different in vitro and in vivo models to discover new targeted inhibitors.

A diagnostic electrochemical aptasensor development for sCD80 protein detection in human serum

Elevating soluble CD80 (sCD80) in human serum is a natural response to autoimmune diseases such as rheumatoid arthritis (RA). The level of sCD80 is associated with RA development and prognosis; therefore, it is potentially used as a biomarker. sCD80 is commonly measured in human serum using immunoassays (e.g., ELISA) with multiple drawbacks, mainly cross-reactivity. Aptamer-based biosensors (aptasensors) development for quantifying and detecting different biological molecules demonstrates applicability in next-generation medicine and biomarker detection. Herein, we selected a specific aptamer for sCD80 by conventional in-vitro selection process (SELEX) with the high-affinity aptamer (Kd = 47.69 nM). A sensitive aptasensor, for the first time, was developed on a screen-printed gold electrode (AuSPE) platform compatible with easy-to-use label-free electrochemical impedance spectroscopy. The immobilization of the aptamer on the gold surface and the presence of sCD80 in a complex with the aptamer were characterized by photo-induced force microscopy, which revealed the uniform assembly of the aptamer monolayer and the distribution of sCD80 on the electrode surface. The developed aptasensor showed a linear performance (0.025-10.0 nM of protein) with a detection limit of 8.0 pM. Furthermore, the aptasensor was tested in a biological matrix, where a linear signal was observed for the increased amount of spiked sCD80 (R2 = 0.9887). The recovery of the spiked amounts ranged from 105 to 125% with coefficient of variation (CV%) <7%, which supported the applicability of this sensor in detecting sCD80 for diagnosis.

Deep mutational scanning of proteins in mammalian cells

Protein mutagenesis is essential for unveiling the molecular mechanisms underlying protein function in health, disease, and evolution. In the past decade, deep mutational scanning methods have evolved to support the functional analysis of nearly all possible single-amino acid changes in a protein of interest. While historically these methods were developed in lower organisms such as E. coli and yeast, recent technological advancements have resulted in the increased use of mammalian cells, particularly for studying proteins involved in human disease. These advancements will aid significantly in the classification and interpretation of variants of unknown significance, which are being discovered at large scale due to the current surge in the use of whole-genome sequencing in clinical contexts. Here, we explore the experimental aspects of deep mutational scanning studies in mammalian cells and report the different methods used in each step of the workflow, ultimately providing a useful guide toward the design of such studies.

A Novel CTLA-4 affinity peptide for cancer immunotherapy by increasing the integrin αvβ3 targeting

Immune checkpoint inhibitors (ICIs) are changing all aspects of malignant tumour therapy as an immunotherapy subverter in oncology. However, the current ICIs might induce systemic immune activation in other tissues and organs since they are not tumour-specific, causing the immune system to attack some normal tissues and organs of the human body. The toxicity can also amplify greatly although combined immunotherapy for cancer has increased the curative efficacy. The LC4 peptide was modified to improve its tumour-targeting ability and reduce peripheral immune system activation, which was obtained through phage display peptide library screening and could block the CTLA-4/CD80 interaction. The LC4 peptide as a result, like other ICIs, exerts anti-tumour effects by refreshing T cell function, and also activates the peripheral immune system. We used the PLGLAG peptide as a linker at the C-terminal of LC4 to connect with a tumour-targeting peptide RGD to increase the tumour tissue targeting ability, and obtain LC4-PLG-RGD. Further experiments demonstrated that the anti-tumour LC4-PLG-RGD activity was better than LC4 in vivo, and the ability to activate the peripheral immune system was weakened. In conclusion, LC4-PLG-RGD can increase the ICIs tumour-targeting and reduce excessive peripheral tissue immune activation, thereby reducing the side effects of ICIs, while increasing their anti-tumour efficacy. This study confirmed that enhanced ICI tumour targeting can effectively reduce immune-related adverse reaction occurrence.

Structure Prediction and Expression of Modified rCTLA4-Ig as a Blocker for B7 Molecules

CTLA4-Ig (Abatacept) has been produced to suppress immune response by inhibition of T cells functions in autoimmune disease. A new drug, which is called belatacept, has recently been recently developed that is more efficient. The development has been occurred by two substitutions (A29Y, L104E) in the extracellular domain of CTLA4. In the present study, the bioinformatics analysis was used in order to make a new structure that has a better function in comparison with belatacept. Firstly, eight different structures were designed. Thereafter, the secondary and 3D structures, mRNA structure, docking of chimeric proteins with CD80/CD86, antigenicity and affinity of designed chimeric molecules were predicted. Based on the criteria, a new candidate molecule was selected and its gene synthesized. The gene was cloned and expressed in E. coli BL21 (DE3) successfully. The purified rCTLA4-Ig was analyzed by SDS-PAGE, western blotting, and ELISA. Circular dichroism analysis (CD analysis) was used for characterization of the rCTLA4-Ig. Affinity of rCTLA4-Ig was also evaluated by the flow cytometry method. Finally, its biological activity was determined by T cell inhibition test. The results showed rCTLA4-Ig and the belatacept protein have some similarities in structure and function. In addition, rCTLA4-Ig was able to bind CD80/CD86 and inhibit T cell function. Although flow cytomery results showed that the standard protein (CTLA4-Ig), represented better affinity than rCTLA4-Ig, the recombinant protein was able to inhibit T cell proliferation as well as CTLA4-Ig.

CTLA4-Ig-Based Bifunctional Costimulation Inhibitor Blocks CD28 and ICOS Signaling to Prevent T Cell Priming and Effector Function

CTLA4-Ig/abatacept dampens activation of naive T cells by blocking costimulation via CD28. It is an approved drug for rheumatoid arthritis but failed to deliver efficacy in a number of other autoimmune diseases. One explanation is that activated T cells rely less on CD28 signaling and use alternate coreceptors for effector function. ICOS is critical for activation of T-dependent humoral immune responses, which drives pathophysiology of IgG-mediated autoimmune diseases. In this study, we asked whether CD28 and ICOS play nonredundant roles for maintenance of T-dependent responses in mouse models. Using a hapten-protein immunization model, we show that during an ongoing germinal center response, combination treatment with CTLA4-Ig and ICOS ligand (ICOSL) blocking Ab completely dissolves ongoing germinal center responses, whereas single agents show only partial activity. Next, we took two approaches to engineer a therapeutic molecule that blocks both pathways. First, we engineered CTLA4-Ig to enhance binding to ICOSL while retaining affinity to CD80/CD86. Using a library approach, binding affinity of CTLA4-Ig to human ICOSL was increased significantly from undetectable to 15-42 nM; however, the affinity was still insufficient to completely block binding of ICOSL to ICOS. Second, we designed a bispecific costimulation inhibitor with high-affinity CTLA4 extracellular domains fused to anti-ICOSL Ab termed bifunctional costimulation inhibitor. With this bispecific approach, we achieved complete inhibition of CD80 and CD86 binding to CD28 as well as ICOS binding to ICOSL. Such bispecific molecules may provide greater therapeutic benefit in IgG-mediated inflammatory diseases compared with CTLA4-Ig alone.

Deep mutagenesis in the study of COVID-19: a technical overview for the proteomics community

INTRODUCTION

The spike (S) of SARS coronavirus 2 (SARS-CoV-2) engages angiotensin-converting enzyme 2 (ACE2) on a host cell to trigger viral-cell membrane fusion and infection. The extracellular region of ACE2 can be administered as a soluble decoy to compete for binding sites on the receptor-binding domain (RBD) of S, but it has only moderate affinity and efficacy. The RBD, which is targeted by neutralizing antibodies, may also change and adapt through mutation as SARS-CoV-2 becomes endemic, posing challenges for therapeutic and vaccine development.

AREAS COVERED

Deep mutagenesis is a Big Data approach to characterizing sequence variants. A deep mutational scan of ACE2 expressed on human cells identified mutations that increase S affinity and guided the engineering of a potent and broad soluble receptor decoy. A deep mutational scan of the RBD displayed on the surface of yeast has revealed residues tolerant of mutational changes that may act as a source for drug resistance and antigenic drift.

EXPERT OPINION

Deep mutagenesis requires a selection of diverse sequence variants; an in vitro evolution experiment that is tracked with next-generation sequencing. The choice of expression system, diversity of the variant library and selection strategy have important consequences for data quality and interpretation.

Differential influence on antibody dependent cellular phagocytosis by different glycoforms on therapeutic Monoclonal antibodies

Therapeutic monoclonal antibodies (mAbs), particularly of the IgG1 subclass, are capable of effector function activities that may be important for their mechanism of action. One such effector function activity is Antibody Dependent Cellular Phagocytosis (ADCP), which has been shown to be mediated primarily through the activating FcγR, FcγRIIa, on macrophages and neutrophils. The critical quality attributes that are the most impactful and predictive of ADCP activity, and therefore most suitable to monitor during IgG1 antibody manufacturing, are not well established. Primary cell assays for ADCP are often laborious and subject to donor to donor variability, making such assays less desirable for product characterization. By developing and employing an ADCP reporter gene assay, we have been able to determine with high sensitivity the glycan structures that can impact FcγRIIa mediated ADCP across multiple different IgG1 antibodies. Interestingly we observed that some IgG1 antibodies are very potent mediators of ADCP while others do not mediate ADCP even though they possess other effector function activities (ADCC and CDC). Additionally, we find that ADCP by different IgG1 antibodies has markedly different sensitivity to glycan species, with one antibody demonstrating a surprisingly strong influence of β-galactosylation and high mannose levels.

Computational Redesign of PD-1 Interface for PD-L1 Ligand Selectivity

Chronic or persistent stimulation of the programmed cell death-1 (PD-1) pathway prevents T cells from mounting anti-tumor and anti-viral immune responses. Blockade of this inhibitory checkpoint pathway has shown therapeutic importance by rescuing T cells from their exhausted state. Cognate ligands of the PD-1 receptor include the tissue-specific PD-L1 and PD-L2 proteins. Engineering a human PD-1 interface specific for PD-L1 or PD-L2 can provide a specific reagent and therapeutic advantage for tissue-specific disruption of the PD-1 pathway. We utilized ProtLID, a computational framework, which constitutes a residue-based pharmacophore approach, to custom-design a human PD-1 interface specific to human PD-L1 without any significant affinity to PD-L2. In subsequent cell assay experiments, half of all single-point mutant designs proved to introduce a statistically significant selectivity, with nine of these maintaining a close to wild-type affinity to PD-L1. This proof-of-concept study suggests a general approach to re-engineer protein interfaces for specificity.

Toxicological and pharmacological assessment of AGEN1884, a novel human IgG1 anti-CTLA-4 antibody

CTLA-4 and CD28 exemplify a co-inhibitory and co-stimulatory signaling axis that dynamically sculpts the interaction of antigen-specific T cells with antigen-presenting cells. Anti-CTLA-4 antibodies enhance tumor-specific immunity through a variety of mechanisms including: blockade of CD80 or CD86 binding to CTLA-4, repressing regulatory T cell function and selective elimination of intratumoral regulatory T cells via an Fcγ receptor-dependent mechanism. AGEN1884 is a novel IgG1 antibody targeting CTLA-4. It potently enhanced antigen-specific T cell responsiveness that could be potentiated in combination with other immunomodulatory antibodies. AGEN1884 was well-tolerated in non-human primates and enhanced vaccine-mediated antigen-specific immunity. AGEN1884 combined effectively with PD-1 blockade to elicit a T cell proliferative response in the periphery. Interestingly, an IgG2 variant of AGEN1884 revealed distinct functional differences that may have implications for optimal dosing regimens in patients. Taken together, the pharmacological properties of AGEN1884 support its clinical investigation as a single therapeutic and combination agent.

The Effect of ASP2409, a Novel CD86-Selective Variant of CTLA4-Ig, on Renal Allograft Rejection in Nonhuman Primates

BACKGROUND

Blockade of CD28-mediated T cell costimulation by a modified cytotoxic T lymphocyte-associated antigen 4 (CTLA4-Ig), belatacept, is a clinically effective immunosuppressive therapy for the prevention of renal allograft rejection. Use of belatacept-based calcineurin inhibitor-free immunosuppression, however, has demonstrated an increased frequency of cellular rejection episodes and immunosuppression-related safety issues relative to conventional regimens. Furthermore, belatacept typically requires infusion for its administration chronically, which may present an inconvenience to patients. To address these issues, a novel CTLA4-Ig variant, ASP2409, with improved CD86 binding selectivity and affinity relative to belatacept was created using DNA shuffling directed evolution methods.

METHODS

We evaluated the immunosuppressive effect of ASP2409 on in vitro alloimmune T cell responses, in vivo tetanus toxoid (TTx)-induced immunological responses and renal transplantation in cynomolgus monkeys.

RESULTS

ASP2409 had 6.1-fold higher and 2.1-fold lower binding affinity to monkey CD86 and CD80 relative to belatacept, respectively. ASP2409 was 18-fold more potent in suppressing in vitro alloimmune T cell responses relative to belatacept. In a cynomolgus monkey TTx immunization model, ASP2409 inhibited anti-TTx immune responses at a 10-fold lower dose level than belatacept. In a cynomolgus monkey renal transplantation model, subcutaneous injection of 1 mg/kg ASP2409 prevented allograft rejection through complete CD86 and partial CD80 receptor occupancies and dramatically prolonged renal allograft survival in combination with tacrolimus or mycophenolate mofetil/methylprednisolone.

CONCLUSIONS

These results support the potential of ASP2409 as an improved CTLA4-Ig for maintenance immunosuppression in organ transplantation.

A CD80-Biased CTLA4-Ig Fusion Protein with Superior In Vivo Efficacy by Simultaneous Engineering of Affinity, Selectivity, Stability, and FcRn Binding

Affinity- and stability-engineered variants of CTLA4-Ig fusion molecules with enhanced pharmacokinetic profiles could yield improved therapies with the potential of higher efficacy and greater convenience to patients. In this study, to our knowledge, we have, for the first time, used in vitro evolution to simultaneously optimize CTLA4 affinity and stability. We selected for improved binding to both ligands, CD80 and CD86, and screened as dimeric Fc fusions directly in functional assays to identify variants with stronger suppression of in vitro T cell activation. The majority of CTLA4 molecules showing the largest potency gains in primary in vitro and ex vivo human cell assays, using PBMCs from type 1 diabetes patients, had significant improvements in CD80, but only modest gains in CD86 binding. We furthermore observed different potency rankings between our lead molecule MEDI5265, abatacept, and belatacept, depending on which type of APC was used, with MEDI5265 consistently being the most potent. We then created fusions of both stability- and potency-optimized CTLA4 moieties with human Fc variants conferring extended plasma t1/2 In a cynomolgus model of T cell-dependent Ab response, the CTLA4-Ig variant MEDI5265 could be formulated at >100 mg/ml for s.c. administration and showed superior efficacy and significantly prolonged serum t1/2 The combination of higher stability and potency with prolonged pharmacokinetics could be compatible with very infrequent, s.c. dosing while maintaining a similar level of immune suppression to more frequently and i.v. administered licensed therapies.

Affinity Maturation of a Cyclic Peptide Handle for Therapeutic Antibodies Using Deep Mutational Scanning

Meditopes are cyclic peptides that bind in a specific pocket in the antigen-binding fragment of a therapeutic antibody such as cetuximab. Provided their moderate affinity can be enhanced, meditope peptides could be used as specific non-covalent and paratope-independent handles in targeted drug delivery, molecular imaging, and therapeutic drug monitoring. Here we show that the affinity of a recently reported meditope for cetuximab can be substantially enhanced using a combination of yeast display and deep mutational scanning. Deep sequencing was used to construct a fitness landscape of this protein-peptide interaction, and four mutations were identified that together improved the affinity for cetuximab 10-fold to 15 nm Importantly, the increased affinity translated into enhanced cetuximab-mediated recruitment to EGF receptor-overexpressing cancer cells. Although in silico Rosetta simulations correctly identified positions that were tolerant to mutation, modeling did not accurately predict the affinity-enhancing mutations. The experimental approach reported here should be generally applicable and could be used to develop meditope peptides with low nanomolar affinity for other therapeutic antibodies.

T lymphocyte antigen 4-modified dendritic cell therapy for asthmatic mice guided by the CCR7 chemokine receptor

The CD80/CD86-CD28 axis is a critical pathway for immuno-corrective therapy, and the cytotoxic T lymphocyte antigen 4 (CTLA4) is a promising immunosuppressor targeting the CD80/CD86-CD28 axis; however, its use for asthma therapy needs further optimization. A human CTLA4 fused with the IgCγ Fc (CTLA4Ig) and mouse CC chemokine receptor type7 (CCR7) coding sequences were inserted into a recombinant adenovirus (rAdV) vector to generate rAdV-CTLA4Ig and rAdV-CCR7. The naive dendritic cells (DCs) were infected with these rAdVs to ensure CCR7 and CTLA4Ig expression. The therapeutic effects of modified DCs were evaluated. rAdV-CTLA4Ig and rAdV-CCR7 infected DCs improved all asthma symptoms. Inflammatory cell infiltration and cytokine analysis showed that rAdV-CTLA4Ig and rAdV-CCR7-modified DC therapy reduced the number of eosinophils and lymphocyte and neutrophil infiltration in the lung. Interestingly, assessment of the humoral immunity showed that the IL-4 and IFNγ levels of the rAdV-CTLA4Ig and rAdV-CCR7-modified DC-treated mice decreased significantly and did not reverse the Th1/Th2 balance. DCs expressing CCR7 displayed guidance ability for DC migration, primarily for DCs in the inflammatory lung. Additionally, the rAdVs caused an inflammatory response by inducing DC differentiation, inflammatory cell infiltration and changes in cytokines; however, mice transplanted with rAdV-green fluorescent protein (GFP)-infected DCs displayed no asthma manifestations. In conclusion, CTLA4Ig-modified DCs exhibited a therapeutic effect on asthma, and CCR7 may guide DC homing. The combination of these two molecules may be a model for precision-guided immunotherapy.

Considerations for combined immune checkpoint modulation and radiation treatment

Recent advances indicate that new therapeutic strategies for the treatment of malignancies will be realized from combined radiation treatment and immune checkpoint modulation. Numerous biophysical properties must be considered for effective biologic development, including affinity, selectivity, oligomeric state and valency. High-resolution structural characterization contributes to our understanding of these properties and can lead to the realization of proteins with unique in vitro activities and novel in vivo therapeutic functions. In this article we focus on the importance of these factors for new potential biologics and consider these in the context of combination therapies with physical modalities, including radiation therapy. In particular, we examine the consequences of altered avidities and subset-specific ligand density on the rational modification of biological function in the immunoglobulin and tumor necrosis factor superfamilies and for new optimized combination therapies.

Costimulatory pathways: physiology and potential therapeutic manipulation in systemic lupus erythematosus

System lupus erythematosus (SLE) is an immune-complex-mediated autoimmune condition with protean immunological and clinical manifestation. While SLE has classically been advocated as a B-cell or T-cell disease, it is unlikely that a particular cell type is more pathologically predominant than the others. Indeed, SLE is characterized by an orchestrated interplay amongst different types of immunopathologically important cells participating in both innate and adaptive immunity including the dendritic cells, macrophages, neutrophils and lymphocytes, as well as traditional nonimmune cells such as endothelial, epithelial, and renal tubular cells. Amongst the antigen-presenting cells and lymphocytes, and between lymphocytes, the costimulatory pathways which involve mutual exchange of information and signalling play an essential role in initiating, perpetuating, and, eventually, attenuating the proinflammatory immune response. In this review, advances in the knowledge of established costimulatory pathways such as CD28/CTLA-4-CD80/86, ICOS-B7RP1, CD70-CD27, OX40-OX40L, and CD137-CD137L as well as their potential roles involved in the pathophysiology of SLE will be discussed. Attempts to target these costimulatory pathways therapeutically will pave more potential treatment avenues for patients with SLE. Preliminary laboratory and clinical evidence of the potential therapeutic value of manipulating these costimulatory pathways in SLE will also be discussed in this review.

Deep mutational scanning of an antibody against epidermal growth factor receptor using mammalian cell display and massively parallel pyrosequencing

We developed a method for deep mutational scanning of antibody complementarity-determining regions (CDRs) that can determine in parallel the effect of every possible single amino acid CDR substitution on antigen binding. The method uses libraries of full length IgGs containing more than 1000 CDR point mutations displayed on mammalian cells, sorted by flow cytometry into subpopulations based on antigen affinity and analyzed by massively parallel pyrosequencing. Higher, lower and neutral affinity mutations are identified by their enrichment or depletion in the FACS subpopulations. We applied this method to a humanized version of the anti-epidermal growth factor receptor antibody cetuximab, generated a near comprehensive data set for 1060 point mutations that recapitulates previously determined structural and mutational data for these CDRs and identified 67 point mutations that increase affinity. The large-scale, comprehensive sequence-function data sets generated by this method should have broad utility for engineering properties such as antibody affinity and specificity and may advance theoretical understanding of antibody-antigen recognition.

Immune suppression in cynomolgus monkeys by XPro9523: an improved CTLA4-Ig fusion with enhanced binding to CD80, CD86 and neonatal Fc receptor FcRn

The CTLA4-Ig fusion proteins abatacept and belatacept are clinically proven immunosuppressants used for rheumatoid arthritis and renal transplant, respectively. Given that both biologics are typically administered chronically by infusion, a need exists for a next-generation CTLA4-Ig with more convenient dosing. We used structure-based protein engineering to optimize the affinity of existing CTLA4-Ig therapeutics for the ligands CD80 and CD86, and for the neonatal Fc receptor, FcRn. From a rationally designed library, we identified four substitutions that enhanced binding to human CD80 and CD86. Coupled with two IgG1 Fc substitutions that enhanced binding to human FcRn, these changes comprise the novel CTLA4-Ig fusion protein, XPro9523. Compared with abatacept, XPro9523 demonstrated 5.9-fold, 23-fold, and 12-fold increased binding to CD80, CD86, and FcRn, respectively; compared with belatacept, CD80, CD86, and FcRn binding increased 1.5-fold, 7.7-fold, and 11-fold, respectively. XPro9523 and belatacept suppressed human T cell proliferation and IL-2 production more potently than abatacept. XPro9523 also suppressed inflammation in the mouse collagen-induced arthritis model. In cynomolgus monkeys, XPro9523 saturated CD80 and CD86 more effectively than abatacept and belatacept, potently inhibited IgM and IgG immunization responses, and demonstrated longer half-life. Pharmacokinetic modeling of its increased potency and persistence suggests that, in humans, XPro9523 may demonstrate superior efficacy and dosing convenience compared with abatacept and belatacept.