Granzyme B is recovered by natural killer cells via clathrin-dependent endocytosis

Lytic granule exocytosis at immune synapses: lessons from neuronal synapses

Regulated exocytosis is a central mechanism of cellular communication. It is not only the basis for neurotransmission and hormone release, but also plays an important role in the immune system for the release of cytokines and cytotoxic molecules. In cytotoxic T lymphocytes (CTLs), the formation of the immunological synapse is required for the delivery of the cytotoxic substances such as granzymes and perforin, which are stored in lytic granules and released via exocytosis. The molecular mechanisms of their fusion with the plasma membrane are only partially understood. In this review, we discuss the molecular players involved in the regulated exocytosis of CTL, highlighting the parallels and differences to neuronal synaptic transmission. Additionally, we examine the strengths and weaknesses of both systems to study exocytosis.

Locked and Loaded: Mechanisms Regulating Natural Killer Cell Lytic Granule Biogenesis and Release

NK cell-mediated cytotoxicity is a critical element of our immune system required for protection from microbial infections and cancer. NK cells bind to and eliminate infected or cancerous cells via direct secretion of cytotoxic molecules toward the bound target cells. In this review, we summarize the current understanding of the molecular regulations of NK cell cytotoxicity, focusing on lytic granule development and degranulation processes. NK cells synthesize apoptosis-inducing proteins and package them into specialized organelles known as lytic granules (LGs). Upon activation of NK cells, LGs converge with the microtubule organizing center through dynein-dependent movement along microtubules, ultimately polarizing to the cytotoxic synapse where they subsequently fuse with the NK plasma membrane. From LGs biogenesis to degranulation, NK cells utilize several strategies to protect themselves from their own cytotoxic molecules. Additionally, molecular pathways that enable NK cells to perform serial killing are beginning to be elucidated. These advances in the understanding of the molecular pathways behind NK cell cytotoxicity will be important to not only improve current NK cell-based anti-cancer therapies but also to support the discovery of additional therapeutic opportunities.

Advances in epidermal growth factor receptor specific immunotherapy: lessons to be learned from armed antibodies

The epidermal growth factor receptor (EGFR) has been recognized as an important therapeutic target in oncology. It is commonly overexpressed in a variety of solid tumors and is critically involved in cell survival, proliferation, metastasis, and angiogenesis. This multi-dimensional role of EGFR in the progression and aggressiveness of cancer, has evolved from conventional to more targeted therapeutic approaches. With the advent of hybridoma technology and phage display techniques, the first anti-EGFR monoclonal antibodies (mAbs) (Cetuximab and Panitumumab) were developed. Due to major limitations including host immune reactions and poor tumor penetration, these antibodies were modified and used as guiding mechanisms for the specific delivery of readily available chemotherapeutic agents or plants/bacterial toxins, giving rise to antibody-drug conjugates (ADCs) and immunotoxins (ITs), respectively. Continued refinement of ITs led to deimmunization strategies based on depletion of B and T-cell epitopes or substitution of non-human toxins leading to a growing repertoire of human enzymes capable of inducing cell death. Similarly, the modification of classical ADCs has resulted in the first, fully recombinant versions. In this review, we discuss significant advancements in EGFR-targeting immunoconjugates, including ITs and recombinant photoactivable ADCs, which serve as a blueprint for further developments in the evolving domain of cancer immunotherapy.

Human Granzyme B Based Targeted Cytolytic Fusion Proteins

Cancer immunotherapy aims to selectively target and kill tumor cells whilst limiting the damage to healthy tissues. Controlled delivery of plant, bacterial and human toxins or enzymes has been shown to promote the induction of apoptosis in cancerous cells. The 4th generation of targeted effectors are being designed to be as humanized as possible—a solution to the problem of immunogenicity encountered with existing generations. Granzymes are serine proteases which naturally function in humans as integral cytolytic effectors during the programmed cell death of cancerous and pathogen-infected cells. Secreted predominantly by cytotoxic T lymphocytes and natural killer cells, granzymes function mechanistically by caspase-dependent or caspase-independent pathways. These natural characteristics make granzymes one of the most promising human enzymes for use in the development of fusion protein-based targeted therapeutic strategies for various cancers. In this review, we explore research involving the use of granzymes as cytolytic effectors fused to antibody fragments as selective binding domains.

Cd226-/- natural killer cells fail to establish stable contacts with cancer cells and show impaired control of tumor metastasis in vivo

CD226 is an activating receptor expressed on natural killer (NK) cells, CD8⁺ T cells, and other immune cells. Upon binding to its ligands expressed on target cells, CD226 activates intracellular signaling that triggers cytokine production and degranulation in NK cells. However, the role of CD226 in contact dynamics between NK and cancer cells has remained unclear. Our time-lapse images showed that individual wild-type CD226⁺ NK cells contacted B16F10 melanoma cells for 23.7 min, but Cd226^-/- NK cells only for 12.8 min, although both NK cell subsets showed equal contact frequency over 4 h. On the surface of B16F10 cells, CD226⁺ cells stayed at the same site with oscillating movement (named stable contact), while Cd226^-/- NK cells moved around at a velocity of 4 μm/min (named unstable contact). Consequently, Cd226^-/- NK cells did not kill B16F10 cells in vitro and did not inhibit their metastasis into the lung in vivo. Taken together, our data demonstrate that CD226 enables prolonged stable interaction between NK and cancer cells, which is needed for efficient killing of cancer cells.

Endocytosis of Cytotoxic Granules Is Essential for Multiple Killing of Target Cells by T Lymphocytes

CTLs are serial killers that kill multiple target cells via exocytosis of cytotoxic granules (CGs). CG exocytosis is tightly regulated and has been investigated in great detail; however, whether CG proteins are endocytosed following exocytosis and contribute to serial killing remains unknown. By using primary CTLs derived from a knock-in mouse of the CG membrane protein Synaptobrevin2, we show that CGs are endocytosed in a clathrin- and dynamin-dependent manner. Following acidification, endocytosed CGs are recycled through early and late, but not recycling endosomes. CGs are refilled with granzyme B at the late endosome stage and polarize to subsequent synapses formed between the CTL and new target cells. Importantly, inhibiting CG endocytosis in CTLs results in a significant reduction of their cytotoxic activity. Thus, our data demonstrate that continuous endocytosis of CG membrane proteins is a prerequisite for efficient serial killing of CTLs and identify key events in this process.

Activation of Lymphocyte Cytolytic Machinery: Where are We?

Target cell recognition by cytotoxic lymphocytes implies the simultaneous engagement and clustering of adhesion and activating receptors followed by the activation of an array of signal transduction pathways. The cytotoxic immune synapse represents the highly specialized dynamic interface formed between the cytolytic effector and its target that allows temporal and spatial integration of signals responsible for a defined sequence of processes culminating with the polarized secretion of lytic granules. Over the last decades, much attention has been given to the molecular signals coupling receptor ligation to the activation of cytolytic machinery. Moreover, in the last 10 years the discovery of genetic defects affecting cytotoxic responses greatly boosted our knowledge on the molecular effectors involved in the regulation of discrete phases of cytotoxic process at post-receptor levels. More recently, the use of super resolution and total internal reflection fluorescence imaging technologies added new insights on the dynamic reorganization of receptor and signaling molecules at lytic synapse as well as on the relationship between granule dynamics and cytoskeleton remodeling. To date we have a solid knowledge of the molecular mechanisms governing granule movement and secretion, being not yet fully unraveled the machinery that couples early receptor signaling to the late stage of synapse remodeling and granule dynamics. Here we highlight recent advances in our understanding of the molecular mechanisms acting in the activation of cytolytic machinery, also discussing similarities and differences between Natural killer cells and cytotoxic CD8⁺ T cells.

Surface CD107a/LAMP-1 protects natural killer cells from degranulation-associated damage

Cytotoxic lymphocytes are important for immune responses against viral infections and cancer. They are able to kill target cells through the release of cytotoxic granules (CGs) without being harmed in the process. Because the lysosomal-associated membrane proteins (LAMPs) appear on the cell surface after CG exocytosis, we hypothesized that some of these proteins might be involved in transiently protecting cytotoxic lymphocytes from self-destruction. Intracellular expression of CD107a/LAMP-1, and to a lesser extent that of CD107b/LAMP-2, correlated with lymphocyte CG content. Engineered surface expression of CD107a/LAMP-1, but not of CD107b/LAMP-2, reduced the granule-mediated killing of transfected target cells. This was dependent on glycosylation of the CD107a/LAMP-1 hinge. Moreover, surface expression of CD107a/LAMP-1 reduced binding of perforin to cells. Importantly, knockdown of CD107a/LAMP-1 in primary human natural killer (NK) cells and deficiency of CD107a/LAMP-1 in mice resulted in increased NK cell apoptosis upon target cell-induced degranulation. Thus, our data support a novel role of CD107a/LAMP-1 in the protection of NK cells from degranulation-associated suicide, which may represent a general mechanism to transiently limit self-destruction by cytotoxic lymphocytes upon target cell killing.

Classification of human natural killer cells based on migration behavior and cytotoxic response

Despite intense scrutiny of the molecular interactions between natural killer (NK) and target cells, few studies have been devoted to dissection of the basic functional heterogeneity in individual NK cell behavior. Using a microchip-based, time-lapse imaging approach allowing the entire contact history of each NK cell to be recorded, in the present study, we were able to quantify how the cytotoxic response varied between individual NK cells. Strikingly, approximately half of the NK cells did not kill any target cells at all, whereas a minority of NK cells was responsible for a majority of the target cell deaths. These dynamic cytotoxicity data allowed categorization of NK cells into 5 distinct classes. A small but particularly active subclass of NK cells killed several target cells in a consecutive fashion. These "serial killers" delivered their lytic hits faster and induced faster target cell death than other NK cells. Fast, necrotic target cell death was correlated with the amount of perforin released by the NK cells. Our data are consistent with a model in which a small fraction of NK cells drives tumor elimination and inflammation.

Perforin is recaptured by natural killer cells following target cells stimulation for cytotoxicity

When encountering target cells, NK (natural killer) cells exocytose Pfn (perforin) and granzyme B to kill challengers. We previously reported that granzyme B is recycled and reused by NK cells via clathrin-dependent endocytosis. However, whether Pfn, a main secretory vesicle content, indispensible to granzyme B killing, undergoes endocytosis remains unknown. We demonstrate that Pfn is recaptured by early endosomes of NK cells via a clathrin-dependent endocytosis after target cell stimulation. Inhibition of clathrin-dependent endocytosis significantly attenuated the cytotoxicity of NK cells. The data suggest that the recovery of Pfn contributes to the cytotoxicity of NK cells. The assay of endocytosis of lytic molecule presents a particular focus for exploring the mechanism of abnormal cytotoxicity of NK cells.

Recycling and reutilization of cytotoxic molecules, a new type of energy conservation of NK cells?

Natural killer (NK) cells play critical roles in defense against tumors and viral infections. They exert their cytotoxic functions through the secretion of granules containing cytotoxic molecules, such as perforin and granzymes. These cytotoxic molecules are stored within dual-functional organelles, known as secretory lysosomes. Target cell recognition induces the formation of an "immunological synapse", between the NK cell and its target, into which cytotoxic granules release their contents. However the post-exocytosis regulation of the process is still largely unknown. Recent research and the data accumulated therefrom lead to new hypotheses that suggest that, not unlike synaptic vesicle recycling in neuronal terminals, NK cells also recycle not just their secretory lysosome membranes but their correlated cytotoxic molecules (perforin and granzymes). The newly endocytosed vesicles are used to replenish the "reserve pool" of vesicles for continued NK cell serial killings. These hypotheses, if proved to be correct, will significantly improve our understanding of NK cell cytotoxicity mechanisms and might even suggest new NK cell-based therapies that rely on NK serial killing abilities for overcoming tumors.