Studying the biology of cytotoxic T lymphocytes in vivo with a fluorescent granzyme B-mTFP knock-in mouse

Detection of Granzyme B-associated Binding Targets in Peripheral Blood Samples of Hosts in Sickness and in Health Using a Granzyme B-like Peptide Fluorescent Conjugate (GP1R)

Granzyme B, mostly expressed by cytotoxic T lymphocytes in the fight against cancer and infection, is known to induce cell death based on its active enzymatic activity as a serine protease. Recent studies showed cytotoxicity of a non-enzymatic granzyme B-like peptide (also referred to as granzyme B-associated peptide or GP1 in this report) in tumor cells and presence of binding targets for GP1R (i.e., GP1 conjugated with rhodamine fluorochrome) in tumor cells, bacteria, and circulating platelets/neutrophils of healthy hosts. But there were no data on "sick" hosts to help substantiate any potential GP1 based medical applications. Thus, we adopted similar GP1R binding protocols to further study binding of GP1 in different biological samples (including different blood samples of hosts in sickness and in health, cancer cell lines, and trigeminal ganglia culture of infected hosts treated with and without GP1) and determine if any binding patterns might have any associations with different health conditions. The overall preliminary results appear to show certain GP1R + binding patterns in certain blood components (especially neutrophils) have potential correlations with certain health conditions of hosts at sampling times, indicating potential GP1R applications for diagnostic purposes. Findings of different GP1R binding patterns in different cancer cell lines, whole blood samples and trigeminal ganglia culture of experimental mice infected with HSV-1 virus (might cause neuropathy) within a week post-infection, and blood samples of GP1-treated mouse survivors on day 21 post-infection provided preliminary evidence of potential GP1-led tumor cell-specific cell death and treatment efficacy for greater survival.

Immunoengineering can overcome the glycocalyx armour of cancer cells

Cancer cell glycocalyx is a major line of defence against immune surveillance. However, how specific physical properties of the glycocalyx are regulated on a molecular level, contribute to immune evasion and may be overcome through immunoengineering must be resolved. Here we report how cancer-associated mucins and their glycosylation contribute to the nanoscale material thickness of the glycocalyx and consequently modulate the functional interactions with cytotoxic immune cells. Natural-killer-cell-mediated cytotoxicity is inversely correlated with the glycocalyx thickness of the target cells. Changes in glycocalyx thickness of approximately 10 nm can alter the susceptibility to immune cell attack. Enhanced stimulation of natural killer and T cells through equipment with chimeric antigen receptors can improve the cytotoxicity against mucin-bearing target cells. Alternatively, cytotoxicity can be enhanced through engineering effector cells to display glycocalyx-editing enzymes, including mucinases and sialidases. Together, our results motivate the development of immunoengineering strategies that overcome the glycocalyx armour of cancer cells.

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.

PD-L1 antibody enhanced β-glucan antitumor effects via blockade of the immune checkpoints in a melanoma model

In the tumor microenvironment (TME), one of the major functions of tumor-recruited CD11b⁺ cells are the suppression of the T-cell-mediated anti-tumor immune response. β-glucan could convert the phenotype of tumor-recruited CD11b⁺ cells from the suppressive to the promotive, and enhanced their anti-tumor effects. However, β-glucan could enhance the PD-1/PD-L1 expression on CD11b⁺ cells, while PD-1 could inhibit macrophage phagocytosis and PD-L1 could induce a co-inhibitory signal in T-cells and lead to T-cell apoptosis and anergy. These protumor effects may be reversed by PD-1/PD-L1 block therapy. In the present study, we focused on the efficacy of β-glucan anti-tumor therapy combined with anti-PD-L1 mAb treatment, and the mechanism of their synergistic effects could be fully verified. We verified the effect of β-glucan (i.e., inflammatory cytokine secretion of TNF-α, IL-12, IL-6, IL-1β and the expression of immune checkpoint PD-1/PD-L1) in naïve mouse peritoneal exudate CD11b⁺ cells. In our mouse melanoma model, treatment with a PD-L1 blocking antibody with β-glucan synergized tumor regression. After treatment with β-glucan and anti-PD-L1 mAb antibody, tumor infiltrating leukocyte (TILs) not only showed a competent T-cell function (CD107a, perforin, IL-2, IFN-γ and Ki67) and CTL population, but also showed enhanced tumor-recruited CD11b⁺ cell activity (IL-12, IL-6, IL-1β and PD-1). This effect was also verified in the peritoneal exudate CD11b⁺ cells of tumor-bearing mice. PD-1/PD-L1 blockade therapy enhanced the β-glucan antitumor effects via the blockade of tumor-recruited CD11b⁺ cell immune checkpoints in the melanoma model.

ALFA-PRF: a novel approach to detect murine perforin release from CTLs into the immune synapse

When killing through the granule exocytosis pathway, cytotoxic lymphocytes release key effector molecules into the immune synapse, perforin and granzymes, to initiate target cell killing. The pore-forming perforin is essential for the function of cytotoxic lymphocytes, as its pores disrupt the target cell membrane and allow diffusion of pro-apoptotic serine proteases, granzyme, into the target cell, where they initiate various cell death cascades. Unlike human perforin, the detection of its murine counterpart in a live cell system has been problematic due its relatively low expression level and the lack of sensitive antibodies. The lack of a suitable methodology to visualise murine perforin secretion into the synapse hinders the study of the cytotoxic lymphocyte secretory machinery in murine models of human disease. Here, we describe a novel recombinant technology, whereby a short ALFA-tag sequence has been fused with the amino-terminus of a mature murine perforin, and this allowed its detection by the highly specific FluoTag^®-X2 anti-ALFA nanobodies using both Total Internal Reflection Fluorescence (TIRF) microscopy of an artificial synapse, and confocal microscopy of the physiological immune synapse with a target cell. This methodology can have broad application in the field of cytotoxic lymphocyte biology and for the many models of human disease.

Identification of distinct cytotoxic granules as the origin of supramolecular attack particles in T lymphocytes

Cytotoxic T lymphocytes (CTL) kill malignant and infected cells through the directed release of cytotoxic proteins into the immunological synapse (IS). The cytotoxic protein granzyme B (GzmB) is released in its soluble form or in supramolecular attack particles (SMAP). We utilize synaptobrevin2-mRFP knock-in mice to isolate fusogenic cytotoxic granules in an unbiased manner and visualize them alone or in degranulating CTLs. We identified two classes of fusion-competent granules, single core granules (SCG) and multi core granules (MCG), with different diameter, morphology and protein composition. Functional analyses demonstrate that both classes of granules fuse with the plasma membrane at the IS. SCG fusion releases soluble GzmB. MCGs can be labelled with the SMAP marker thrombospondin-1 and their fusion releases intact SMAPs. We propose that CTLs use SCG fusion to fill the synaptic cleft with active cytotoxic proteins instantly and parallel MCG fusion to deliver latent SMAPs for delayed killing of refractory targets.