TREM1 activation of myeloid cells promotes antitumor immunity.

Myeloid cells in the tumor microenvironment (TME) can exist in immunosuppressive and immunostimulatory states that impede or promote antitumor immunity, respectively. Blocking suppressive myeloid cells or increasing stimulatory cells to enhance antitumor immune responses is an area of interest for therapeutic intervention. Triggering receptor expressed on myeloid cells-1 (TREM1) is a proinflammatory receptor that amplifies immune responses. TREM1 is expressed on neutrophils, subsets of monocytes and tissue macrophages, and suppressive myeloid populations in the TME, including tumor-associated neutrophils, monocytes, and tumor-associated macrophages. Depletion or inhibition of immunosuppressive myeloid cells, or stimulation by TREM1-mediated inflammatory signaling, could be used to promote an immunostimulatory TME. We developed PY159, an afucosylated humanized anti-TREM1 monoclonal antibody with enhanced FcγR binding. PY159 is a TREM1 agonist that induces signaling, leading to up-regulation of costimulatory molecules on monocytes and macrophages, production of proinflammatory cytokines and chemokines, and enhancement of T cell activation in vitro. An antibody against mouse TREM1, PY159m, promoted antitumor efficacy in syngeneic mouse tumor models. These results suggest that PY159-mediated agonism of TREM1 on tumoral myeloid cells can promote a proinflammatory TME and offer a promising strategy for immunotherapy.

Targeting TREM2 on tumor-associated macrophages enhances immunotherapy.

Converting checkpoint inhibitor (CPI)-resistant individuals to being responsive requires identifying suppressive mechanisms. We identify TREM2+ tumor-associated macrophages (TAMs) as being correlated with exhausted CD8+ tumor-infiltrating lymphocytes (TILs) in mouse syngeneic tumor models and human solid tumors of multiple histological types. Fc domain-enhanced anti-TREM2 monoclonal antibody (mAb) therapy promotes anti-tumor immunity by elimination and modulation of TAM populations, which leads to enhanced CD8+ TIL infiltration and effector function. TREM2+ TAMs are most enriched in individuals with ovarian cancer, where TREM2 expression corresponds to disease grade accompanied by worse recurrence-free survival. In an aggressive orthotopic ovarian cancer model, anti-TREM2 mAb therapy drives potent anti-tumor immunity. These results highlight TREM2 as a highly attractive target for immunotherapy modulation in individuals who are refractory to CPI therapy and likely have a TAM-rich tumor microenvironment.

Biophysical and Immunological Characterization and In Vivo Pharmacokinetics and Toxicology in Nonhuman Primates of the Anti-PD-1 Antibody Pembrolizumab.

The programmed cell death 1 (PD-1) pathway represents a major immune checkpoint, which may be engaged by cells in the tumor microenvironment to overcome active T-cell immune surveillance. Pembrolizumab (Keytruda®, MK-3475) is a potent and highly selective humanized mAb of the IgG4/kappa isotype designed to directly block the interaction between PD-1 and its ligands, PD-L1 and PD-L2. This blockade enhances the functional activity of T cells to facilitate tumor regression and ultimately immune rejection. Pembrolizumab binds to human and cynomolgus monkey PD-1 with picomolar affinity and blocks the binding of human and cynomolgus monkey PD-1 to PD-L1 and PD-L2 with comparable potency. Pembrolizumab binds both the C'D and FG loops of PD-1. Pembrolizumab overcomes human and cynomolgus monkey PD-L1-mediated immune suppression in T-cell cultures by enhancing IL2 production following staphylococcal enterotoxin B stimulation of healthy donor and cancer patient cells, and IFNγ production in human primary tumor histoculture. Ex vivo and in vitro studies with human and primate T cells show that pembrolizumab enhances antigen-specific T-cell IFNγ and IL2 production. Pembrolizumab does not mediate FcR or complement-driven effector function against PD-1-expressing cells. Pembrolizumab displays dose-dependent clearance and half-life in cynomolgus monkey pharmacokinetic and toxicokinetic studies typical for human IgG4 antibodies. In nonhuman primate toxicology studies, no findings of toxicologic significance were observed. The preclinical data for pembrolizumab are consistent with the clinical anticancer activity and safety that has been demonstrated in human clinical trials.

Liprin phosphorylation regulates binding to LAR: evidence for liprin autophosphorylation.

The LAR transmembrane tyrosine phosphatase associates with liprin-alpha proteins and colocalizes with liprin-alpha1 at focal adhesions. LAR has been implicated in axon guidance, and liprins are involved in synapse formation and synapse protein trafficking. Several liprin mutants have weaker binding to LAR as assessed by yeast interaction trap assays, and the extents of in vitro and in vivo phosphorylation of these mutants were reduced relative to that of wild-type liprin-alpha1. Treatment of liprin-alpha1 with calf intestinal phosphatase weakened its interaction with the recombinant GST-LAR protein. A liprin LH region mutant that inhibited liprin phosphorylation did not bind to LAR as assessed by coprecipitation studies. Endogenous LAR was shown to bind phosphorylated liprin-alpha1 from MDA-486 cells labeled in vivo with [32P]orthophosphate. In further characterizing the phosphorylation of liprin, we found immunoprecipitates of liprin-alpha1 expressed in COS-7 cells to incorporate phosphate after washes of up to 4 M NaCl. Additionally, purified liprin-alpha1 derived from Sf-9 insect cells retained the ability to incorporate phosphate in in vitro phosphorylation assays, and a liprin-alpha1 truncation mutant incorporated phosphate after denaturation and/or renaturation in SDS gels. Finally, binding assays showed that liprin binds to ATP-agarose and that the interaction is challenged by free ATP, but not by free GTP. Moreover, liprin LH region mutations that inhibit liprin phosphorylation stabilized the association of liprin with ATP-agarose. Taken together, our results suggest that liprin autophosphorylation regulates its association with LAR.

Neurotransmitter release regulated by a MALS-liprin-alpha presynaptic complex.

Synapses are highly specialized intercellular junctions organized by adhesive and scaffolding molecules that align presynaptic vesicular release with postsynaptic neurotransmitter receptors. The MALS/Veli-CASK-Mint-1 complex of PDZ proteins occurs on both sides of the synapse and has the potential to link transsynaptic adhesion molecules to the cytoskeleton. In this study, we purified the MALS protein complex from brain and found liprin-alpha as a major component. Liprin proteins organize the presynaptic active zone and regulate neurotransmitter release. Fittingly, mutant mice lacking all three MALS isoforms died perinatally with difficulty breathing and impaired excitatory synaptic transmission. Excitatory postsynaptic currents were dramatically reduced in autaptic cultures from MALS triple knockout mice due to a presynaptic deficit in vesicle cycling. These findings are consistent with a model whereby the MALS-CASK-liprin-alpha complex recruits components of the synaptic release machinery to adhesive proteins of the active zone.

Signaling between focal adhesion kinase and trio.

The Trio guanine nucleotide exchange factor functions in neural development in Caenorhabditis elegans and Drosophila and in the development of neural tissues and skeletal muscle in mouse. The association of Trio with the Lar tyrosine phosphatase led us to study the role of tyrosine phosphorylation in Trio function using focal adhesion kinase (FAK). The Lar-interacting domain of Trio is constitutively tyrosine-phosphorylated when expressed in COS-7 cells and was highly phosphorylated when it was co-transfected with FAK. Co-precipitation studies indicated that Trio binds to the FAK amino-terminal domain and to the FAK kinase domain via its SH3 and kinase domains, respectively. Tyrosine-phosphorylated FAK and Trio were present mainly in the detergent-insoluble fraction of cell lysates, and co-expression of Trio and FAK resulted in increased amounts of Trio present in the detergent-insoluble fraction. Immunofluorescence of cells co-transfected with FAK and Trio revealed significant co-localization of the proteins at the cell periphery, indicating that they form a stable complex in vivo. A FAK phosphorylation site, tyrosine residue 2737, was identified in subdomain I of the Trio kinase domain. Additionally, in vitro phosphorylation assays and in vivo co-expression studies indicated that Trio enhances FAK kinase activity. These results suggest Trio may be involved in the regulation of focal adhesion dynamics in addition to effecting changes in the actin cytoskeleton through the activation of Rho family GTPases.

Association of the kinesin motor KIF1A with the multimodular protein liprin-alpha.

Liprin-alpha/SYD-2 is a multimodular scaffolding protein important for presynaptic differentiation and postsynaptic targeting of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid glutamate receptors. However, the molecular mechanisms underlying these functions remain largely unknown. Here we report that liprin-alpha interacts with the neuron-specific kinesin motor KIF1A. KIF1A colocalizes with liprin-alpha in various subcellular regions of neurons. KIF1A coaccumulates with liprin-alpha in ligated sciatic nerves. KIF1A cofractionates and coimmunopreciptates with liprin-alpha and various liprin-alpha-associated membrane, signaling, and scaffolding proteins including alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptors, GRIP/ABP, RIM, GIT1, and beta PIX. These results suggest that liprin-alpha functions as a KIF1A receptor, linking KIF1A to various liprin-alpha-associated proteins for their transport in neurons.

Interaction between GRIP and liprin-alpha/SYD2 is required for AMPA receptor targeting.

Interaction with the multi-PDZ protein GRIP is required for the synaptic targeting of AMPA receptors, but the underlying mechanism is unknown. We show that GRIP binds to the liprin-alpha/SYD2 family of proteins that interact with LAR receptor protein tyrosine phosphatases (LAR-RPTPs) and that are implicated in presynaptic development. In neurons, liprin-alpha and LAR-RPTP are enriched at synapses and coimmunoprecipitate with GRIP and AMPA receptors. Dominant-negative constructs that interfere with the GRIP-liprin interaction disrupt the surface expression and dendritic clustering of AMPA receptors in cultured neurons. Thus, by mediating the targeting of liprin/GRIP-associated proteins, liprin-alpha is important for postsynaptic as well as presynaptic maturation.