Défaut d’exocytose des granules lytiques : Plusieurs causes, un même effet

Une réponse immune exagérée, incontrôlée et le plus souvent fatale, connue sous le nom de syndrome hémophagocytaire (SH), est associée à un défaut de la fonction cytotoxique des lymphocytes T et natural killer (NK). Les anomalies moléculaires responsables, qui sont multiples, mettent en cause dans la plupart des cas un effecteur indispensable au fonctionnement de la machinerie lytique des lymphocytes. L’étude des lymphocytes cytotoxiques déficients en l’un ou l’autre de ces effecteurs apporte des éléments nouveaux quant à l’agencement des étapes clés de la sécrétion du contenu des granules lytiques au contact de la cellule cible. Des mécanismes moléculaires proches semblent contrôler la sécrétion vésiculaire au niveau des synapses immunologique et neurologique. D’autres effecteurs de la cytotoxicité ou du contrôle de l’homéostasie lymphocytaire à l’origine de SH doivent encore être caractérisés. Quant aux mécanismes précis de l’intervention de cette voie cytotoxique dans le maintien de l’homéostasie lymphocytaire (terminaison d’une réponse immune), ils demeurent à élucider.An in vivo disturbance of lymphocyte homeostasis occurs during the course of the hemophagocytic syndrome (HS). HS is a severe and often fatal syndrome resulting from potent and uncontrolled activation and proliferation of T-lymphocytes, mainly polyclonal CD8 lymphocytes, leading to excessive macrophage activation, high level of proinflammatory cytokine production and multiple deleterious effects. The onset of HS characterizes several inherited disorders in humans. In most of these conditions, the molecular defect impairs the granule-dependent cytotoxic activity of lymphocytes, thus highlighting the determinant role of this function in driving back the immune system to a state of equilibrium following infection. Several lines of evidence suggest that an increase in the expansion phase rather than a decrease in the contraction phase of the CD8+ T cells population characterizes the HS. Failure to kill antigen presenting cells through a transaction mechanism of cytotoxic cells should favor a sustained response, although the mechanism may be more complex than simple decrease of antigen load. Defect in the granule dependent cytotoxic function of lymphocytes result from perforin mutation in familial hemophagocytic lymphohistiocytosis type 2, from Munc13-4 (UNC13D) mutation in familial hemophagocytic lymphohistiocytosis type 3, from Rab27a mutation in Griscelli syndrome type 2, and from CHS/LYST mutation in Chediak-Higashi syndrome. The characterization of the molecular causes leading to these conditions identified Rab27a and Munc13-4 as two critical effectors of the exocytic machinery, required for the terminal transport/docking or priming of the cytotoxic granules, respectively. Different members of the Rab and Munc13 family of proteins are also used in neurotransmitter release at the neurological synapse, highlighting the similarity of the mechanisms regulating both secretory pathways. Future investigations regarding HS will continue to elucidate this exocytic pathway machinery and improve our understanding of how it finely regulates the immune response, an area that is likely to be useful for therapeutic intervention

Premature birth, respiratory distress, intracerebral hemorrhage, and silvery-gray hair: differential diagnosis of the 3 types of Griscelli syndrome

A preterm neonate, born to consanguineous parents, presented with respiratory distress, intracerebral hemorrhage, and a silvery-gray sheen of the hair and eyelashes. Griscelli syndrome (GS) type 3 was diagnosed after the detection of a novel homozygous mutation of the melanophilin gene. Thus, only the hypopigmentation, but not the patient's other clinical features, were attributable to this form of GS. Differential diagnosis of the various forms of GS must be performed as early as possible as GS2 is associated with a life threatening but curable immune disorder

Rab27A and its effector MyRIP link secretory granules to F-actin and control their motion towards release sites

The GTPase Rab27A interacts with myosin-VIIa and myosin-Va via MyRIP or melanophilin and mediates melanosome binding to actin. Here we show that Rab27A and MyRIP are associated with secretory granules (SGs) in adrenal chromaffin cells and PC12 cells. Overexpression of Rab27A, GTPase-deficient Rab27A-Q78L, or MyRIP reduced secretory responses of PC12 cells. Amperometric recordings of single adrenal chromaffin cells revealed that Rab27A-Q78L and MyRIP reduced the sustained component of release. Moreover, these effects on secretion were partly suppressed by the actin-depolymerizing drug latrunculin but strengthened by jasplakinolide, which stabilizes the actin cortex. Finally, MyRIP and Rab27A-Q78L restricted the motion of SGs in the subplasmalemmal region of PC12 cells, as measured by evanescent-wave fluorescence microscopy. In contrast, the Rab27A-binding domain of MyRIP and a MyRIP construct that interacts with myosin-Va but not with actin increased the mobility of SGs. We propose that Rab27A and MyRIP link SGs to F-actin and control their motion toward release sites through the actin cortex

Actin dynamics regulation by TTC7A/PI4KIIIα limits DNA damage and cell death under confinement

Background: The actin cytoskeleton has a crucial role in the maintenance of the immune homeostasis by controlling various cellular processes, including cell migration. Mutations in TTC7A have been described as the cause of a primary immunodeficiency associated to different degrees of gut involvement and alterations in the actin cytoskeleton dynamics. Objectives: This study investigates the impact of TTC7A deficiency in immune homeostasis. In particular, the role of the TTC7A/phosphatidylinositol 4 kinase type III α pathway in the control of leukocyte migration and actin dynamics. Methods: Microfabricated devices were leveraged to study cell migration and actin dynamics of murine and patient-derived leukocytes under confinement at the single-cell level. Results: We show that TTC7A-deficient lymphocytes exhibit an altered cell migration and reduced capacity to deform through narrow gaps. Mechanistically, TTC7A-deficient phenotype resulted from impaired phosphoinositide signaling, leading to the downregulation of the phosphoinositide 3-kinase/AKT/RHOA regulatory axis and imbalanced actin cytoskeleton dynamics. TTC7A-associated phenotype resulted in impaired cell motility, accumulation of DNA damage, and increased cell death in dense 3-dimensional gels in the presence of chemokines. Conclusions: These results highlight a novel role of TTC7A as a critical regulator of lymphocyte migration. Impairment of this cellular function is likely to contribute to the pathophysiology underlying progressive immunodeficiency in patients.</p

Molecular mechanisms of biogenesis and exocytosis of cytotoxic granules

International audienceCytotoxic T cells and natural killer cells are crucial for immune surveillance against virus-infected cells and tumour cells. Molecular studies of individuals with inherited defects that impair lymphocyte cytotoxic function have also highlighted the importance of cytotoxicity in the regulation and termination of immune responses. As discussed in this Review, characterization of these defects has contributed to our understanding of the key steps that are required for the maturation of cytotoxic granules and the secretion of their contents at the immunological synapse during target cell killing. This has revealed a marked similarity between cytotoxic granule exocytosis at the immunological synapse and synaptic vesicle exocytosis at the neurological synapse. We explore the possibility that comparison of these two kinetically and spatially regulated secretory pathways will provide clues to uncover additional effectors that regulate the cytotoxic function of lymphocytes

Regulation of T-cell antigen receptor-mediated inside-out signaling by cytosolic adapter proteins and Rap1 effector molecules

International audienceIntegrins are critical for the migration of T cells to lymphoid organs and to sites of inflammation and are also necessary for productive interactions between T cells and antigen-presenting cells. Integrin activation is enhanced following T-cell receptor (TCR) engagement, as signals initiated by the TCR increase affinity and avidity of integrins for their ligands. This process, known as inside-out signaling, has been shown to require several molecular components including the cytosolic adapter proteins adhesion and degranulation-promoting adapter protein and Src homology 2 domain-containing adapter protein of 55 kDa, the low molecular weight guanosine triphosphatase Rap1, and the Rap1 effector proteins Rap1 guanosine triphosphate-interacting adapter molecule, regulator of adhesion and cell polarization enriched in lymphoid tissues, and protein kinase D1. Herein, we review recent findings about how the TCR is linked to integrin activation through inside-out signaling

EBAG9 tempers lymphocyte killing activity

The cytotoxic activity of lymphocytes is crucial for immune surveillance and homeostasis. Several independent, naturally occurring genetic models characterized by defects in granule trafficking or exocytosis have helped to decipher the multiple steps and molecules that regulate the cytotoxic process. The study by Rüder and colleagues in this issue of the JCI shows that an engineered absence of EBAG9, previously reported as a tumor-associated antigen, enhances cytotoxic activity of CTLs but not NK cells, likely acting on the endosomal-lysosomal trafficking of the cytotoxic effectors (see the related article beginning on page 2184). This finding adds a new piece to the puzzle of complex mechanisms that tightly regulate the capacity of the cytotoxic response and suggests a new target to negatively modulate CTL responsiveness

Inherited defects causing hemophagocytic lymphohistiocytic syndrome

International audienceHemophagocytic lymphohistiocytosis (HLH) manifests as the uncontrolled activation of T lymphocytes and macrophages infiltrating multiple organs. Molecular studies of individuals with HLH have demonstrated in most of these conditions a critical role of granule-dependent cytotoxic activity in the regulation of lymphocyte homeostasis, and have allowed the characterization of key effectors regulating cytotoxic granule release. The cytolytic process may now be considered a multistep process, including cell activation; the polarization of cytotoxic granules toward the conjugated target cell; the tethering, priming, and fusion of the cytotoxic granules with the plasma membrane; and the release of their contents (perforin and granzymes) into the intercellular cleft, leading to target cell death. Cytolytic cells have a second effector function involving the production of cytokines, principally γ-interferon, which is secreted independently of the exocytosis cytotoxic granule pathway. An analysis of the mechanisms underlying HLH has identified γ-interferon as a key cytokine inducing uncontrolled macrophage activation, and thus represents a potential therapeutic target