INBRX-120, a CD8α-targeted detuned IL-2 that selectively expands and activates tumoricidal effector cells for safe and durable in vivo responses.

BACKGROUND: As a major driver of lymphocyte proliferation and activation interleukin 2 (IL-2) is a crucial mediator for antitumor responses. Despite promising activity in a subset of patients, wider therapeutic utility of IL-2 (aldesleukin) has been hampered by severe dose-limiting toxicities, the expansion of immunosuppressive regulatory T cells and a poor pharmacokinetic (PK) profile. Recent engineering efforts, including non-α IL-2 variants, have lowered the toxicity profile, but have yet to induce meaningful antitumor activity in a wider patient population. METHODS: We engineered INBRX-120, a CD8α-targeted Cisleukin™ molecule consisting of an affinity tuned IL-2 (IL2-x) connected to two high affinity CD8α-specific single domain antibodies via an effector-silenced Fc domain. To show that this large affinity differential enables directed IL-2 cis-signaling exclusively on CD8α-expressing tumoricidal effector cell populations, INBRX-120 effects on target cell expansion, activation and antitumor activity were tested in vitro. In vivo antitumor efficacy was evaluated in syngeneic mouse models alone or in combination with programmed cell death protein-1 (PD-1) blockade. Preclinical safety, as well as pharmacodynamic (PD) and PK profiling was carried out in non-human primates. RESULTS: INBRX-120 effectively expanded and enhanced the cytotoxic capacity of CD8 T cells and natural killer cells towards tumor cells without affecting regulatory T cells in vitro and in vivo. In syngeneic mouse models, INBRX-120 surrogate showed safe, potent, and durable antitumor efficacy alone and in combination with PD-1 blockade. In non-human primates, INBRX-120 expanded and activated CD8α-expressing effector cells, showed a favorable PK profile, and was well tolerated up to a dose of 1 mg/kg. CONCLUSIONS: Through its unique cis-signaling activity on CD8α-expressing effector cells, INBRX-120 overcomes the major limitations of IL-2-based therapy and effectively harnesses IL-2's potent intrinsic antitumor activity. This novel therapeutic strategy promises safer clinical activity that could induce meaningful antitumor efficacy in a wider set of patients with various cancer indications.

Profiling constitutive proteolytic events in vivo.

Most known organisms encode proteases that are crucial for constitutive proteolytic events. In the present paper, we describe a method to define these events in proteomes from Escherichia coli to humans. The method takes advantage of specific N-terminal biotinylation of protein samples, followed by affinity enrichment and conventional LC (liquid chromatography)-MS/MS (tandem mass spectrometry) analysis. The method is simple, uses conventional and easily obtainable reagents, and is applicable to most proteomics facilities. As proof of principle, we demonstrate profiles of proteolytic events that reveal exquisite in vivo specificity of methionine aminopeptidase in E. coli and unexpected processing of mitochondrial transit peptides in yeast, mouse and human samples. Taken together, our results demonstrate how to rapidly distinguish real proteolysis that occurs in vivo from the predictions based on in vitro experiments.

Streptolysin O Rapidly Impairs Neutrophil Oxidative Burst and Antibacterial Responses to Group A Streptococcus.

Group A Streptococcus (GAS) causes a wide range of human infections, ranging from simple pharyngitis to life-threatening necrotizing fasciitis and toxic shock syndrome. A globally disseminated clone of M1T1 GAS has been associated with an increase in severe, invasive GAS infections in recent decades. The secreted GAS pore-forming toxin streptolysin O (SLO), which induces eukaryotic cell lysis in a cholesterol-dependent manner, is highly upregulated in the GAS M1T1 clone during bloodstream dissemination. SLO is known to promote GAS resistance to phagocytic clearance by neutrophils, a critical first element of host defense against invasive bacterial infection. Here, we examine the role of SLO in modulating specific neutrophil functions during their early interaction with GAS. We find that SLO at subcytotoxic concentrations and early time points is necessary and sufficient to suppress neutrophil oxidative burst, in a manner reversed by free cholesterol and anti-SLO blocking antibodies. In addition, SLO at subcytotoxic concentrations blocked neutrophil degranulation, interleukin-8 secretion and responsiveness, and elaboration of DNA-based neutrophil extracellular traps, cumulatively supporting a key role for SLO in GAS resistance to immediate neutrophil killing. A non-toxic SLO derivate elicits protective immunity against lethal GAS challenge in a murine infection model. We conclude that SLO exerts a novel cytotoxic-independent function at early stages of invasive infections (<30 min), contributing to GAS escape from neutrophil clearance.

N-terminomics: a high-content screen for protease substrates and their cleavage sites.

Proteases play vital roles in many cellular processes and signaling cascades through specific limited cleavage of their targets. It is important to identify what proteins are substrates of proteases and where their cleavage sites are so as to reveal the molecular mechanisms and specificity of signaling. We have developed a method to achieve this goal using a strategy that chemically tags the substrate's alpha amine generated by proteolysis, enriches for tagged peptides, and identifies them using liquid chromatography-coupled tandem mass spectrometry (LC-MS/MS). Peptide MS/MS data are searched against a database to reveal what proteins are cleaved, whereby peptide N-termini demarcate sites of protease cleavage.

Streptolysin O promotes group A Streptococcus immune evasion by accelerated macrophage apoptosis.

Group A Streptococcus (GAS) is a leading human bacterial pathogen capable of producing invasive infections even in previously healthy individuals. As frontline components of host innate defense, macrophages play a key role in control and clearance of GAS infections. We find GAS induces rapid, dose-dependent apoptosis of primary and cultured macrophages and neutrophils. The cell death pathway involves apoptotic caspases, is partly dependent on caspase-1, and requires GAS internalization by the phagocyte. Analysis of GAS virulence factor mutants, heterologous expression, and purified toxin studies identified the pore-forming cytolysin streptolysin O (SLO) as necessary and sufficient for the apoptosis-inducing phenotype. SLO-deficient GAS mutants induced less macrophage apoptosis in vitro and in vivo, allowed macrophage cytokine secretion, and were less virulent in a murine systemic infection model. Ultrastructural evidence of mitochondrial membrane remodeling, coupled with loss of mitochondrial depolarization and cytochrome c release, suggests a direct attack of the toxin initiates the intrinsic apoptosis pathway. A general caspase inhibitor blocked SLO-induced apoptosis and enhanced macrophage killing of GAS. We conclude that accelerated, caspase-dependent macrophage apoptosis induced by the pore-forming cytolysin SLO contributes to GAS immune evasion and virulence.

Structural and kinetic determinants of protease substrates.

Two fundamental questions with regard to proteolytic networks and pathways concern the structural repertoire and kinetic threshold that distinguish legitimate signaling substrates. We used N-terminal proteomics to address these issues by identifying cleavage sites within the Escherichia coli proteome that are driven by the apoptotic signaling protease caspase-3 and the bacterial protease glutamyl endopeptidase (GluC). Defying the dogma that proteases cleave primarily in natively unstructured loops, we found that both caspase-3 and GluC cleave in alpha-helices nearly as frequently as in extended loops. Notably, biochemical and kinetic characterization revealed that E. coli caspase-3 substrates are greatly inferior to natural substrates, suggesting protease and substrate coevolution. Engineering an E. coli substrate to match natural catalytic rates defined a kinetic threshold that depicts a signaling event. This unique combination of proteomics, biochemistry, kinetics and substrate engineering reveals new insights into the structure-function relationship of protease targets and their validation from large-scale approaches.

Identification of proteolytic cleavage sites by quantitative proteomics.

The identification of natural substrates and their cleavage sites is pivotal to defining proteolytic pathways. Here we report a novel strategy for the identification of the signature of proteolytic cleavage events based on quantitative proteomics. Lysine residues in proteins are blocked by guanidination so that free N-terminals can be labeled with amine-specific iTRAQ reagents. The quantitative nature of iTRAQ reagents allows us to distinguish N-terminals newly formed by proteolytic treatment (neoepitopes) from original N-terminals in proteins. Proteins are digested with trypsin and analyzed using MALDI-TOF/TOF mass spectrometry. Peptides labeled with iTRAQ reagents are distinguished from other peptides by exhibiting intense signature ions in tandem mass spectrometry analysis. A corresponding data acquisition strategy was developed to specifically analyze iTRAQ tagged N-terminal peptides. To validate the procedure, we examined a set of recombinant Escherichia coli proteins that have predicted caspase-3 cleavage motifs. The protein mixture was treated with active or inactive caspase-3 and subsequently labeled with two different iTRAQ reagents. Mass spectrometric analysis located 10 cleavage sites, all corresponding to caspase-3 consensus. Spiking caspase-cleaved substrate into a human cell lysate demonstrated the high sensitivity of the procedure. Moreover, we were able to identify proteolytic cleavage products associated with the induction of cell-free apoptosis. Together, these data reveal a novel application for iTRAQ technology for the detection of proteolytic substrates.