The synapse and cytolytic machinery of cytotoxic T cells

The emerging role of protein kinase Cθ in cytoskeletal signaling

Cytoskeletal rearrangements often occur as the result of transduction of signals from the extracellular environment. Efficient awakening of this powerful machinery requires multiple activation and deactivation steps, which usually involve phosphorylation or dephosphorylation of different signaling units by kinases and phosphatases, respectively. In this review, we discuss the signaling characteristics of one of the nPKC isoforms, PKCθ, focusing on PKCθ-mediated signal transduction to cytoskeletal elements, which results in cellular rearrangements critical for cell type-specific responses to stimuli. PKCθ is the major PKC isoform present in hematopoietic and skeletal muscle cells. PKCθ plays roles in T cell signaling through the IS, survival responses in adult T cells, and T cell FasL-mediated apoptosis, all of which involve cytoskeletal rearrangements and relocation of this enzyme. PKCθ has been linked to the regulation of cell migration, lymphoid cell motility, and insulin signaling and resistance in skeletal muscle cells. Additional roles were suggested for PKCθ in mitosis and cell-cycle regulation. Comprehensive understanding of cytoskeletal regulation and the cellular "modus operandi" of PKCθ holds promise for improving current therapeutic applications aimed at autoimmune diseases.

Human leukocyte antigen (HLA) class I down-regulation by human immunodeficiency virus type 1 negative factor (HIV-1 Nef): what might we learn from natural sequence variants?

HIV-1 causes a chronic infection in humans that is characterized by high plasma viremia, progressive loss of CD4+ T lymphocytes, and severe immunodeficiency resulting in opportunistic disease and AIDS. Viral persistence is mediated in part by the ability of the Nef protein to down-regulate HLA molecules on the infected cell surface, thereby allowing HIV-1 to evade recognition by antiviral CD8+ T lymphocytes. Extensive research has been conducted on Nef to determine protein domains that are required for its immune evasion activities and to identify critical cellular co-factors, and our mechanistic understanding of this process is becoming more complete. This review highlights our current knowledge of Nef-mediated HLA class I down-regulation and places this work in the context of naturally occurring sequence variation in this protein. We argue that efforts to fully understand the critical role of Nef for HIV-1 pathogenesis will require greater analysis of patient-derived sequences to elucidate subtle differences in immune evasion activity that may alter clinical outcome.

Alloreactive effector T cells require the local formation of a proinflammatory environment to allow crosstalk and high avidity interaction with nonhematopoietic tissues to induce GVHD reactivity

Based on clinical observations that donor T cells specific for minor histocompatibility antigens (MiHA) ubiquitously expressed on both hematopoietic and nonhematopoietic cells were detected in patients showing evident graft-versus-leukemia/lymphoma (GVL) reactivity with no or limited coinciding graft-versus-host disease (GVHD), we hypothesized that nonhematopoietic tissues may be relatively unsusceptible to the cytotoxic effect of MiHA-specific T cells under normal, noninflammatory conditions. To test this hypothesis, we investigated the reactivity of alloreactive T cells specific for ubiquitously expressed MiHA against skin-derived primary human fibroblasts. We demonstrated that this reactivity was not merely determined by their antigen-specificity, but was highly dependent on adhesion molecule expression. ICAM-1 expression on the fibroblasts upregulated under proinflammatory conditions and induced during cross-talk with the T cells was demonstrated to be a crucial factor facilitating formation of high avidity interactions with the T cells and subsequent efficient target cell destruction. Furthermore, we provide supporting evidence for the role of ICAM-1 in vivo by demonstrating that ICAM-1 expression on nonhematopoietic target cells was dependent on the presence of infiltrating activated T cells, as was illustrated by restricted ICAM-1 expression at the sites of T cell infiltration in skin biopsies of patients with acute GVHD (aGVHD), by the absence of ICAM-1 expression in the same biopsies in areas without T cell infiltration and by the absence of ICAM-1 expression in biopsies of patients without GVHD independent of the presence of infiltrating nonactivated T cells. In conclusion, under noninflammatory conditions, nonhematopoietic tissues are unsusceptible to the GVHD reactivity of alloreactive T cells due to their inability to establish high avidity interactions.

Mechanisms controlling granule-mediated cytolytic activity of cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTL) play a critical role in immunity against viruses and cancer. The antigen receptor or T-cell receptor (TCR) on CTL determines the specificity toward target cells. The CD8 co-receptor functions in concert with the TCR to enhance TCR-mediated signaling, accounting for the remarkable sensitivity and swift signaling kinetics of the CTL response. The latter ensures efficient delivery and release of lytic granules, resulting in sensitive and rapid destruction of target cells.

Terminal transport of lytic granules to the immune synapse is mediated by the kinesin-1/Slp3/Rab27a complex

Cytotoxic T lymphocytes kill target cells via the polarized secretion of cytotoxic granules at the immune synapse. The lytic granules are initially recruited around the polarized microtubule-organizing center. In a dynein-dependent transport process, the granules move along microtubules toward the microtubule-organizing center in the minus-end direction. Here, we found that a kinesin-1-dependent process is required for terminal transport and secretion of polarized lytic granule to the immune synapse. We show that synaptotagmin-like protein 3 (Slp3) is an effector of Rab27a in cytotoxic T lymphocytes and interacts with kinesin-1 through the tetratricopeptide repeat of the kinesin-1 light chain. Inhibition of the Rab27a/Slp3/kinesin-1 transport complex impairs lytic granule secretion. Our data provide further molecular insights into the key functional and regulatory mechanisms underlying the terminal transport of cytotoxic granules and the latter's secretion at the immune synapse.

Familial hemophagocytic lymphohistiocytosis: a model for understanding the human machinery of cellular cytotoxicity

Cytotoxic T lymphocytes, natural killer cells, and NKT cells are effector cells able to kill infected cells. In some inherited human disorders, a defect in selected proteins involved in the cellular cytotoxicity mechanism results in specific clinical syndromes, grouped under the name of familial hemophagocytic lymphohistiocytosis. Recent advances in genetic studies of these patients has allowed the identification of different genetic subsets. Additional genetic immune deficiencies may also induce a similar clinical picture. International cooperation and prospective trials resulted in refining the diagnostic and therapeutic approach to these rare diseases with improved outcome but also with improved knowledge of the mechanisms underlying granule-mediated cellular cytotoxicity in humans.

Remodelling of cortical actin where lytic granules dock at natural killer cell immune synapses revealed by super-resolution microscopy

Natural Killer (NK) cells are innate immune cells that secrete lytic granules to directly kill virus-infected or transformed cells across an immune synapse. However, a major gap in understanding this process is in establishing how lytic granules pass through the mesh of cortical actin known to underlie the NK cell membrane. Research has been hampered by the resolution of conventional light microscopy, which is too low to resolve cortical actin during lytic granule secretion. Here we use two high-resolution imaging techniques to probe the synaptic organisation of NK cell receptors and filamentous (F)-actin. A combination of optical tweezers and live cell confocal microscopy reveals that microclusters of NKG2D assemble into a ring-shaped structure at the centre of intercellular synapses, where Vav1 and Grb2 also accumulate. Within this ring-shaped organisation of NK cell proteins, lytic granules accumulate for secretion. Using 3D-structured illumination microscopy (3D-SIM) to gain super-resolution of ~100 nm, cortical actin was detected in a central region of the NK cell synapse irrespective of whether activating or inhibitory signals dominate. Strikingly, the periodicity of the cortical actin mesh increased in specific domains at the synapse when the NK cell was activated. Two-colour super-resolution imaging revealed that lytic granules docked precisely in these domains which were also proximal to where the microtubule-organising centre (MTOC) polarised. Together, these data demonstrate that remodelling of the cortical actin mesh occurs at the central region of the cytolytic NK cell immune synapse. This is likely to occur for other types of cell secretion and also emphasises the importance of emerging super-resolution imaging technology for revealing new biology.

Imaging of lytic granule exocytosis in CD8+ cytotoxic T lymphocytes reveals a modified form of full fusion

Here we imaged the exocytosis of lytic granules from human CD8(+) cytotoxic T lymphocytes using rapid total internal reflection microscopy, Lamp-1 tagged with mGFP to follow the fate of the lytic granule membrane, and granzyme A, granzyme B or serglycin tagged with mRFP to follow the fate of lytic granule cargo. Lytic granules were released by full fusion with the plasma membrane, such that the entire granule content for all three cargos visualized was released on a subsecond time scale. The behavior of GFP-Lamp-1 was, however, more complex. While it entered the plasma membrane in all cases, the extent to which it then diffused away from the site of exocytosis varied from nearly complete to highly restricted. Finally, the diffusion properties upon release of the three cargos examined put an upper limit on the size of the macromolecular complex of granzyme and serglycin that is presented to the target cell.

Physiological roles of NAADP-mediated Ca2+ signaling

Nicotinic acid dinucleotide phosphate (NAADP) is unique amongst Ca(2+) mobilizing messengers in that its principal function is to mobilize Ca(2+) from acidic organelles. Early studies indicated that it was likely that NAADP activates a novel Ca(2+) release channel distinct from the well characterized Ca(2+) release channels on the (sarco)-endoplasmic reticulum (ER), inositol trisphosphate and ryanodine receptors. In this review, we discuss the emergence of a novel family of endolysosomal channels, the two-pore channels (TPCs), as likely targets for NAADP, and how molecular and pharmacological manipulation of these channels is enhancing our understanding of the physiological roles of NAADP as an intracellular Ca(2+) mobilizing messenger.

Biological responses to spider silk-antibiotic fusion protein

The development of a new generation of multifunctional biomaterials is a continual goal for the field of materials science. The in vivo functional behaviour of a new fusion protein that combines the mechanical properties of spider silk with the antimicrobial properties of hepcidin was addressed in this study. This new chimeric protein, termed 6mer + hepcidin, fuses spider dragline consensus sequences (6mer) and the antimicrobial peptide hepcidin, as we have recently described, with retention of bactericidal activity and low cytotoxicity. In the present study, mouse subcutaneous implants were studied to access the in vivo biological response to 6mer + hepcidin, which were compared with controls of silk alone (6mer), polylactic-glycolic acid (PLGA) films and empty defects. Along with visual observations, flow cytometry and histology analyses were used to determine the number and type of inflammatory cells at the implantation site. The results show a mild to low inflammatory reaction to the implanted materials and no apparent differences between the 6mer + hepcidin films and the other experimental controls, demonstrating that the new fusion protein has good in vivo biocompatibility, while maintaining antibiotic function.

Cyclosporine-resistant, Rab27a-independent mobilization of intracellular preformed CD40 ligand mediates antigen-specific T cell help in vitro

CD40L is critically important for the initiation and maintenance of adaptive immune responses. It is generally thought that CD40L expression in CD4(+) T cells is regulated transcriptionally and made from new mRNA following Ag recognition. However, recent studies with two-photon microscopy revealed that most cognate interactions between effector CD4(+) T cells and APCs are too short for de novo synthesis of CD40L. Given that effector and memory CD4(+) T cells store preformed CD40L (pCD40L) in lysosomal compartments and that pCD40L comes to the cell surface within minutes of antigenic stimulation, we and others have proposed that pCD40L might mediate T cell-dependent activation of cognate APCs during brief encounters in vivo. However, it has not been shown that this relatively small amount of pCD40L is sufficient to activate APCs, owing to the difficulty of separating the effects of pCD40L from those of de novo CD40L and other cytokines in vitro. In this study, we show that pCD40L surface mobilization is resistant to cyclosporine or FK506 treatment, while de novo CD40L and cytokine expression are completely inhibited. These drugs thus provide a tool to dissect the role of pCD40L in APC activation. We find that pCD40L mediates selective activation of cognate but not bystander APCs in vitro and that mobilization of pCD40L does not depend on Rab27a, which is required for mobilization of lytic granules. Therefore, effector CD4(+) T cells deliver pCD40L specifically to APCs on the same time scale as the lethal hit of CTLs but with distinct molecular machinery.

The immunological synapse: a cause or consequence of T-cell receptor triggering?

The immunological synapse forms as a result of the tight apposition of a T cell with an antigen-presenting cell (APC) and it is the site where the T-cell receptor (TCR) is triggered by its antigen ligand, the peptide-MHC complex present in the APC membrane. The immunological synapse was initially characterized in the T-cell membrane as three concentric rings of membrane receptors and their underlying cytoskeletal and signalling proteins. The inner circle, or central supramolecular activation cluster (cSMAC), concentrates most of the TCR and CD28, and it is surrounded by the peripheral SMAC that is formed by integrins. Finally, the most external ring or distal SMAC (dSMAC) is where proteins with large ectodomains are located, such as CD43 and CD45, far from the cSMAC. This arrangement was initially thought to be responsible for maintaining sustained TCR signalling, however, this typical concentric bull's-eye pattern is not found in the immunological synapses formed with the APCs of dendritic cells. Interestingly, TCR signalling has been detected in microclusters formed in the dSMAC area and it extinguishes as the TCRs reach the cSMAC. Hence, it appears that TCR signalling and full T-cell activation do not require the formation of the cSMAC and that this structure may rather play a role in TCR down-regulation, as well as participating in the polarized secretion of lytic granules. Here, we shall review the historical evolution of the role of the cSMAC in T-cell activation, finally discussing our most recent data indicating that the cSMAC serves to internalize exhausted TCRs by phagocytosis.

Role of cytotoxic protease granzyme-b in neuronal degeneration during human stroke

Infiltration of leukocytes into post-ischemic cerebrum is a well-described phenomenon in stroke injury. Because CD-8(+) T-lymphocytes secrete cytotoxic proteases, including granzyme-b (Gra-b) that exacerbates post-ischemic brain damage, we investigated roles of Gra-b in human stroke. To study the role of Gra-b in stroke, ischemic and non-ischemic tissues (from post-mortem stroke patients) were analyzed using immunoblotting, co-immunoprecipitation, terminal deoxy uridine nick end labeling (TUNEL) and Annexin-V immunostaining, and in vitro neuron survival assays. Activated CG-SH cells and supernatants were used to model leukocyte-dependent injury. Non-ischemic brain tissues were used as non-pathological controls. Non-activated CG-SH cells and supernatants were used as controls for in vitro experiments. Human stroke (ischemic) samples contained significantly higher levels of Gra-b and interferon-gamma inducible protein-10 (IP-10/CXCL10) than non-ischemic controls. In stroke, poly (ADP-ribose) polymerase-1 and heat shock protein-70 were cleaved to canonical proteolytic "signature" fragments by Gra-b. Gra-b was also found to bind to Bid and caspase-3. Gra-b also co-localized with Annexin-V(+) /TUNEL(+) in degenerating neurons. Importantly, Gra-b inhibition protected both normal and ischemia-reperfused neurons against in vitro neurotoxicity mediated by activated CG-SH cells and supernatants. These results suggest that increased leukocyte infiltration and elevated Gra-b levels in the post-stroke brain can induce contact-dependent and independent post-ischemic neuronal death to aggravate stroke injury.

Metabolism, migration and memory in cytotoxic T cells

The transcriptional and metabolic programmes that control CD8(+) T cells are regulated by a diverse network of serine/threonine kinases. The view has been that the kinases AKT and mammalian target of rapamycin (mTOR) control T cell metabolism. Here, we challenge this paradigm and discuss an alternative role for these kinases in CD8(+) T cells, namely to control cell migration. Another emerging concept is that AMP-activated protein kinase (AMPK) family members control T cell metabolism and determine the effector versus memory fate of CD8(+) T cells. We speculate that one link between metabolism and immunological memory is provided by kinases that originally evolved to control T cell metabolism and have subsequently acquired the ability to control the expression of key transcription factors that regulate CD8(+) T cell effector function and migratory capacity.