33 Development of a clinical ex vivo assay for the assessment of therapeutic CD28 costimulatory pathway engagement

BackgroundPreclinical evidence supports combining checkpoint inhibition (CPI) with T cell costimulatory agonism to improve the breadth and durability of anti-tumor responses relative to CPI alone. Currently, there are a number of therapeutic approaches combining costimulatory receptor agonists (e.g. CD28, 4-1BB, OX40L, etc.) with tumor targeting agents and/or CPI. Identification of a pharmacodynamically-justified therapeutic dose can be challenging because traditional duration of target occupancy does not necessarily correlate with immunological activity in the case of costimulatory molecules, and an ‘always on’ dose risks immune exhaustion. ALPN-202, a variant CD80 vIgD-Fc fusion protein that mediates PD-L1-dependent CD28 costimulation and inhibits the PD-L1 and CTLA-4 checkpoints, is in development for the treatment of multiple advanced malignancies. To assess clinical CD28 agonism in the context of ALPN-202 treatment, we developed a novel, ex vivo whole blood target-dependent costimulation (TDC) assay.MethodsA TDC assay was developed using clinical samples from NEON-1 (NCT04186637), an ongoing dose escalation and expansion clinical trial of ALPN-202 for patients with advanced malignancies. The assay uses patient blood stimulated with paraformaldehyde-fixed, artificial antigen presenting cells (aAPC) expressing both cell-surface anti-CD3 and PD-L1. Pre-dose and end-of-infusion (EOI) blood was drawn from trial participants and co-cultured for 24 hours with the aAPCs in a pre-made assay plate. Plasma was collected and secreted IL-2 was quantified and used as a measure of PD-L1-dependent CD28 costimulation. Nonlinear regression was used to calculate area under the curve (AUC) for each condition, and sample AUC values were compared to a positive control (pre-dose blood stimulated with a fixed concentration of ALPN-202). Serum concentration of ALPN-202 and CD28 target saturation analyses were conducted concurrently to evaluate the exposure-response relationship.ResultsUsing the ex vivo TDC assay, ALPN-202 demonstrated PD-L1-dependent T cell costimulation at all dose levels tested to date in the NEON-1 clinical trial, consistent with preclinical assay development data. Similarly, CD28 target saturation levels on circulating T cells correlated with serum concentration of ALPN-202.ConclusionsWe have developed a novel ex vivo assay to assess induction of PD-L1-dependent CD28 costimulation in a therapeutic setting. This assay has been successfully employed to monitor controlled CD28 costimulation by the CD28 agonist therapeutic candidate ALPN-202, helping to establish a PK/PD relationship that is consistent with preclinical data. More broadly, this type of cell-based, ex vivo TDC assay could be adapted to assess costimulatory receptor engagement, particularly target-dependent costimulation, for other therapeutic agonists in clinical development.Ethics ApprovalThis study was approved by WCG IRB’s Human Subjects Review, approval number: 20211877.

First-in-human dose escalation of ALPN-202, a conditional CD28 costimulator and dual checkpoint inhibitor, in advanced malignancies.

2547 Background: Strong preclinical rationale has emerged for combining checkpoint inhibition (CPI) with T cell costimulatory agonists, particularly CD28, a critical T cell costimulatory molecule recently recognized as a key target of checkpoint inhibition. ALPN-202 is a variant CD80 vIgD-Fc fusion that mediates PD-L1-dependent CD28 costimulation and inhibits the PD-L1 and CTLA-4 checkpoints. It has demonstrated superiority to CPI-only therapies in tumor models, while demonstrating favorable safety in preclinical toxicology studies. Methods: This is a cohort-based, open-label dose escalation and expansion study of ALPN-202 in adults with advanced solid tumors or lymphoma (NCT04186637). Subjects with cancers refractory to standard therapies (including approved CPIs), or cancers without available standard or curative therapy are eligible. After two planned single-subject cohorts, a standard 3+3 dose escalation has been implemented with two dose schedules in parallel, Q1W and Q3W. Objectives include evaluation of safety and tolerability, PK, PD and preliminary anticancer activity of ALPN-202. Disease assessments are evaluated by RECIST v1.1 for solid tumors or by Lugano Classification for lymphoma. Results: As of January 2021, 20 subjects with advanced malignancies have received ALPN-202. Dose-dependent PK and target saturation have been preliminarily observed. So far, ALPN-202 has been well tolerated at dose levels ranging from 0.001 to 1 mg/kg weekly, with no DLTs. Low-grade skin toxicities (grade 1-2 rash) have been observed in 4 subjects (20%). Among 11 evaluable subjects, an unconfirmed partial response has been observed in one subject with colorectal carcinoma, while stable disease has been observed in 5 subjects with colorectal carcinoma, mesothelioma (2), cholangiocarcinoma, and renal cell carcinoma -- for a preliminary clinical benefit (PR+SD) rate of 100% (4/4) at dose levels of 0.3 mg/kg and higher, or 54% (5/11) overall (table). The meeting presentation will update this data, which is expected to include the conclusion of Q1W dose escalation, as well as immune correlates. Conclusions: First-in-human dose escalation with ALPN-202 has been well tolerated at doses capable of engaging CD28 costimulation in vivo in association with dual PD-L1/CTLA-4 checkpoint inhibition, with early signs of anti-tumor activity. These findings suggest that CD28 agonism can be safely achieved in humans, and further suggest that dose expansion with ALPN-202 is warranted to assess the relevance of controlled CD28 costimulation as a novel approach to cancer immunotherapy. Clinical trial information: NCT04186637. [Table: see text]

CD28 engagement inhibits CD73-mediated regulatory activity of CD8+ T cells

CD28 is required for T cell activation as well as the generation of CD4+Foxp3+ Treg. It is unclear, however, how CD28 costimulation affects the development of CD8+ T cell suppressive function. Here, by use of Hepa1.6.gp33 in vitro killing assay and B16.gp33 tumor mouse model we demonstrate that CD28 engagement during TCR ligation prevents CD8+ T cells from becoming suppressive. Interestingly, our results showed that ectonucleotidase CD73 expression on CD8+ T cells is upregulated in the absence of CD28 costimulation. In both murine and human tumor-bearing hosts, CD73 is upregulated on CD28−CD8+ T cells that infiltrate the solid tumor. UPLC-MS/MS analysis revealed that CD8+ T cells activation without CD28 costimulation produces elevated levels of adenosine and that CD73 mediates its production. Adenosine receptor antagonists block CD73-mediated suppression. Our data support the notion that CD28 costimulation inhibits CD73 upregulation and thereby prevents CD8+ T cells from becoming suppressive. This study uncovers a previously unidentified role for CD28 costimulation in CD8+ T cell activation and suggests that the CD28 costimulatory pathway can be a potential target for cancer immunotherapy.

CD80 on Human T Cells Is Associated With FoxP3 Expression and Supports Treg Homeostasis

CD80 and CD86 are expressed on antigen presenting cells (APCs) and their role in providing costimulation to T cells is well established. However, it has been shown that these molecules can also be expressed by T cells, but the significance of this observation remains unknown. We have investigated stimuli that control CD80 and CD86 expression on T cells and show that in APC-free conditions around 40% of activated, proliferating CD4+ T cells express either CD80, CD86 or both. Expression of CD80 and CD86 was strongly dependent upon provision of CD28 costimulation as ligands were not expressed following TCR stimulation alone. Furthermore, we observed that CD80+ T cells possessed the hallmarks of induced regulatory T cells (iTreg), expressing Foxp3 and high levels of CTLA-4 whilst proliferating less extensively. In contrast, CD86 was preferentially expressed on INF-γ producing cells, which proliferated more extensively and had characteristics of effector T cells. Finally, we demonstrated that CD80 expressed on T cells inhibits CTLA-4 function and facilitates the growth of iTreg. Together these data establish endogenous expression of CD80 and CD86 by activated T cells is not due to ligand capture by transendocytosis and highlight clear differences in their expression patterns and associated functions.

CD86 Is a Selective CD28 Ligand Supporting FoxP3+ Regulatory T Cell Homeostasis in the Presence of High Levels of CTLA-4

CD80 and CD86 are expressed on antigen presenting cells and are required to engage their shared receptor, CD28, for the costimulation of CD4 T cells. It is unclear why two stimulatory ligands with overlapping roles have evolved. CD80 and CD86 also bind the regulatory molecule CTLA-4. We explored the role of CD80 and CD86 in the homeostasis and proliferation of CD4+FoxP3+ regulatory T cells (Treg), which constitutively express high levels of CTLA-4 yet are critically dependent upon CD28 signals. We observed that CD86 was the dominant ligand for Treg proliferation, survival, and maintenance of a regulatory phenotype, with higher expression of CTLA-4, ICOS, and OX40. We also explored whether CD80-CD28 interactions were specifically compromised by CTLA-4 and found that antibody blockade, clinical deficiency of CTLA-4 and CRISPR-Cas9 deletion of CTLA-4 all improved Treg survival following CD80 stimulation. Taken together, our data suggest that CD86 is the dominant costimulatory ligand for Treg homeostasis, despite its lower affinity for CD28, because CD80-CD28 interactions are selectively impaired by the high levels of CTLA-4. These data suggest a cell intrinsic role for CTLA-4 in regulating CD28 costimulation by direct competition for CD80, and indicate that that CD80 and CD86 have discrete roles in CD28 costimulation of CD4 T cells in the presence of high levels of CTLA-4.

Genomic profiling of T-cell activation suggests increased sensitivity of memory T cells to CD28 costimulation

T-cell activation is a critical driver of immune responses. The CD28 costimulation is an essential regulator of CD4 T-cell responses, however, its relative importance in naive and memory T cells is not fully understood. Using different model systems, we observe that human memory T cells are more sensitive to CD28 costimulation than naive T cells. To deconvolute how the T-cell receptor (TCR) and CD28 orchestrate activation of human T cells, we stimulate cells using varying intensities of TCR and CD28 and profiled gene expression. We show that genes involved in cell cycle progression and division are CD28-driven in memory cells, but under TCR control in naive cells. We further demonstrate that T-helper differentiation and cytokine expression are controlled by CD28. Using chromatin accessibility profiling, we observe that AP1 transcriptional regulation is enriched when both TCR and CD28 are engaged, whereas open chromatin near CD28-sensitive genes is enriched for NF-kB motifs. Lastly, we show that CD28-sensitive genes are enriched in GWAS regions associated with immune diseases, implicating a role for CD28 in disease development. Our study provides important insights into the differential role of costimulation in naive and memory T-cell responses and disease susceptibility.

The non-coding RNA miR-17~92 is a central mediator of T cell activation

SummaryT cell activation is paramount for productive adaptive immune responses. CD28 is a key clinically targeted immunoregulatory receptor because it provides the prototypical costimulatory signal required for T cell activation. Therefore, a precise understanding of the molecular consequences of CD28 costimulation has direct therapeutic relevance. Here, we uncover that the microRNA cluster miR-17~92 is part of the molecular program triggered by CD28 costimulation and hence T cell activation. Combining genetics, transcriptomics, bioinformatics and biochemical miRNA:mRNA interaction maps we demonstrate that transgenic miR-17~92 can cell-intrinsically largely overcome defects caused by CD28-deficiency. miR-17~92 promotes transcription of a proinflammatory gene signature by enhancing the calcineurin/NFAT pathway. miR-17~92 binds to and represses a network of genes including several negative regulators of T cell activation. Finally, CD28-deficient T cells exhibit derepressed miR-17~92 target genes during activation, demonstrating that this non-coding RNA is required to shape the transcriptome. Thus, we propose that miR-17~92 constitutes a central mediator for T cell activation, integrating signals by the TCR and CD28 costimulation. In this model miR-17~92 facilitates T cell activation by dampening the breaks that prevent T cell activation.