Lytic granule exocytosis at immune synapses: lessons from neuronal synapses

IFT20 regulates TFEB-dependent lytic granule biogenesis in cytotoxic T lymphocytes by orchestrating the MPR-dependent transport of granzyme B

Cytotoxic T lymphocytes (CTL) exploit specialized secretory lysosomes, the lytic granules (LG) to kill target cells. The LGs carry a battery of apoptosis-inducing molecules enriched in granzymes (GZM), perforin and FasL, which are released at the immune synapse formed by CTLs with their cognate targets. Recent studies have revealed an unexpected diversity among LGs, suggesting the existence of multiple vesicular trafficking pathways in their biogenesis and exocytosis. We have previously implicated the ciliary protein IFT20 in the retrograde trafficking of the cation-independent mannose-6-phosphate receptor (MPR), which is required for the lysosomal targeting of the acid hydrolases. Here we investigate the role of IFT20 in LG biogenesis in CTLs, showing that IFT20 is essential for MPR recycling to the trans-Golgi network and ensures proper granzyme B (GZMB) localization to LGs. As a result, IFT20 deficiency impairs the killing capability of CTLs. In turn, to rescue the lysosome and LG defects, IFT20-deficient CTLs expresses higher levels of lysosomal genes and of components of the cytotoxic machinery of LGs. Interestingly, an in silico analysis suggests a transcriptional co-regulation of lysosome and LG genes by the master regulator of lysosome biogenesis TFEB. Accordingly, modulation of TFEB results in alterations in the expression of LG-related genes and CTL-mediated cytotoxicity. Collectively, our results identify IFT20 as a new player in the trafficking pathways that regulate LG biogenesis and highlight the existence in CTLs of an extended gene expression program regulated by TFEB, downstream of IFT20.

Functional T cell response to COVID-19 vaccination with or without natural infection with SARS-CoV-2 in adults and children

Severe COVID-19 is rare in children suggesting differences in immune response between children and adults. Limited information is available on how cellular immunity is modulated by COVID-19 vaccination and prior infection, and whether it is differentially modulated in children compared to adults. Here, we aimed to compare COVID-19 vaccine-induced functional T cell response between adults and children with and without previous SARS-CoV-2 infection. Adults (18-45 years; n = 45) and children (5-10 years; n = 51;), who received Pfizer-BioNTech COVID-19 vaccine or remained unvaccinated, and previously infected or not with SARS-CoV-2 were selected from two cross-sectional SARS-CoV-2 serosurveillance studies conducted in Bangladesh. Plasma nucleocapsid (N)-specific antibodies were measured by electrochemiluminescence immunoassay; IFN-γ, perforin and granzyme B secreting T cells were assessed using ELISpot assay. Vaccination in adults without previous infection, induced higher frequencies of IFN-γ and granzyme B secreting T lymphocytes compared to unvaccinated adults, while it increased only IFN-γ expression in vaccinated children. Previous infection increased IFN-γ response in unvaccinated adults only. Unvaccinated children showed higher granzyme B expression compared to adults irrespective of infection status. In vaccinated individuals, prior infection induced perforin expression in both adults and children. Children showed slightly different functional T cell response than adults in response to COVID-19 vaccination and infection. mRNA vaccination provided higher IFN-γ response in both adults and children, but induced cytotoxic T lymphocyte (CTL) response in adults only. Future studies may evaluate the impact of other types of COVID-19 vaccines on functional T cell immunity in children to confirm the findings.

Lysosome Functions in Atherosclerosis: A Potential Therapeutic Target

Lysosomes in mammalian cells are recognized as key digestive organelles, containing a variety of hydrolytic enzymes that enable the processing of both endogenous and exogenous substrates. These organelles digest various macromolecules and recycle them through the autophagy-lysosomal system. Recent research has expanded our understanding of lysosomes, identifying them not only as centers of degradation but also as crucial regulators of nutrient sensing, immunity, secretion, and other vital cellular functions. The lysosomal pathway plays a significant role in vascular regulation and is implicated in diseases such as atherosclerosis. During atherosclerotic plaque formation, macrophages initially engulf large quantities of lipoproteins, triggering pathogenic responses that include lysosomal dysfunction, foam cell formation, and subsequent atherosclerosis development. Lysosomal dysfunction, along with the inefficient degradation of apoptotic cells and the accumulation of modified low-density lipoproteins, negatively impacts atherosclerotic lesion progression. Recent studies have highlighted that lysosomal dysfunction contributes critically to atherosclerosis in a cell- and stage-specific manner. In this review, we discuss the mechanisms of lysosomal biogenesis and its regulatory role in atherosclerotic lesions. Based on these lysosomal functions, we propose that targeting lysosomes could offer a novel therapeutic approach for atherosclerosis, shedding light on the connection between lysosomal dysfunction and disease progression while offering new insights into potential anti-atherosclerotic strategies.

Required minimal protein domain of flower for synaptobrevin2 endocytosis in cytotoxic T cells

Flower, a highly conserved protein, crucial for endocytosis and cellular fitness, has been implicated in cytotoxic T lymphocyte (CTL) killing efficiency through its role in cytotoxic granule (CG) endocytosis at the immune synapse (IS). This study explores the molecular cues that govern Flower-mediated CG endocytosis by analyzing uptake of Synaptobrevin2, a protein specific to CG in mouse CTL. Using immunogold electron microscopy and total internal fluorescence microscopy, we found that Flower translocates in a stimulus-dependent manner from small vesicles to the IS, thereby ensuring specificity in CG membrane protein recycling. Using confocal live-cell imaging, we assessed the ability of a range of naturally occurring mouse, human and Drosophila isoforms to rescue defective endocytosis in Flower KO CTLs. This analysis demonstrated that the N-terminal portion of the protein, encompassing amino acids 1-106 in mice, is the minimal domain necessary for Synaptobrevin2 endocytosis. Additionally, we identified two pivotal sites through site-specific mutation: a putative AP2-binding site, and a tyrosine at position 104 in mouse Flower. These findings provide insights into Flower's specific functional domain essential for CG endocytosis, which is a key process in mediating T cell serial killing required for the effective fight against cancer.

Apicomplexan Pore-Forming Toxins

Pore-forming toxins (PFTs) are released by one cell to directly inflict damage on another cell. Hosts use PFTs, including members of the membrane attack complex/perforin protein family, to fight infections and cancer, while bacteria and parasites deploy PFTs to promote infection. Apicomplexan parasites secrete perforin-like proteins as PFTs to egress from infected cells and traverse tissue barriers. Other protozoa, along with helminth parasites, utilize saposin-like PFTs prospectively for nutrient acquisition during infection. This review discusses seminal and more recent advances in understanding how parasite PFTs promote infection and describes how they are regulated and fulfill their roles without causing parasite self-harm. Although exciting progress has been made in defining mechanisms of pore formation by PFTs, many open questions remain to be addressed to gain additional key insights into these remarkable determinants of parasitic infections.

SNARE proteins: Core engines of membrane fusion in cancer

Vesicles are loaded with a variety of cargoes, including membrane proteins, secreted proteins, signaling molecules, and various enzymes, etc. Not surprisingly, vesicle transport is essential for proper cellular life activities including growth, division, movement and cellular communication. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate membrane fusion of vesicles with their target compartments that is fundamental for cargo delivery. Recent studies have shown that multiple SNARE family members are aberrantly expressed in human cancers and actively contribute to malignant proliferation, invasion, metastasis, immune evasion and treatment resistance. Here, the localization and function of SNARE proteins in eukaryotic cells are firstly mapped. Then we summarize the expression and regulation of SNAREs in cancer, and describe their contribution to cancer progression and mechanisms, and finally we propose engineering botulinum toxin as a strategy to target SNAREs for cancer treatment.

A solution for highly efficient electroporation of primary cytotoxic T lymphocytes

BACKGROUND

Cytotoxic T lymphocytes (CTLs) are central players in the adaptive immune response. Their functional characterization and clinical research depend on efficient and reliable transfection. Although various methods have been utilized, electroporation remains the preferred technique for transient gene over-expression. However, the efficiency of electroporation is reduced for human and mouse primary CTLs. Lonza offers kits that effectively improve plasmid DNA transfection quality. Unfortunately, the removal of key components of the cell recovery medium considerably reduced the efficiency of their kit for CTLs. Our aim was to develop a new recovery medium to be used with Lonza's Nucleofector system that would significantly enhance transfection rates.

RESULTS

We assessed the impact of different media in which the primary CTLs were placed to recover after electroporation on cell survival, transfection rate and their ability to form an immunological synapse and to perform exocytosis. We transfected the cells with pmax-GFP and large constructs encoding for either CD81-super ecliptic pHluorin or granzyme B-pHuji. The comparison of five different media for mouse and two for human CTLs demonstrated that our new recovery medium composed of Opti-MEM-GlutaMAX supplemented with HEPES, DMSO and sodium pyruvate gave the best result in cell survival (> 50%) and transfection rate (> 30 and 20% for mouse and human cells, respectively). More importantly, the functionality of CTLs was at least twice as high as with the original Lonza recovery medium. In addition, our RM significantly improved transfection efficacy of natural killer cells that are notoriously hard to electroporate.

CONCLUSION

Our results show that successful transfection depends not only on the electroporation medium and pulse sequence but also on the medium applied for cell recovery. In addition, we have reduced our reliance on proprietary products by designing an effective recovery medium for both mouse and human primary CTLs and other lymphocytes that can be easily implemented by any laboratory. We expect that this recovery medium will have a significant impact on both fundamental and applied research in immunology.