Cytotoxic granule secretion by lymphocytes and its link to immune homeostasis

Gene editing of hematopoietic stem cells restores T-cell response in familial hemophagocytic lymphohistiocytosis

BACKGROUND

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory disorder characterized by a life-threatening cytokine storm and immunopathology. Familial HLH type 3 (FHL3) accounts for approximately 30% of all inborn HLH cases worldwide. It is caused by mutations in the UNC13D gene that result in impaired degranulation of cytotoxic vesicles and hence compromised T-cell- and natural killer-cell-mediated killing. Current treatment protocols, including allogeneic hematopoietic stem cell (HSC) transplantation, still show high mortality.

OBJECTIVE

We sought to develop and evaluate a curative genome editing strategy in the preclinical FHL3 Jinx mouse model. Jinx mice harbor a cryptic splice donor site in Unc13d intron 26 and develop clinical symptoms of human FHL3 upon infection with lymphocytic choriomeningitis virus (LCMV).

METHODS

We employed clustered regularly interspaced short palindromic repeats (CRISPR)-Cas technology to delete the disease-causing mutation in HSCs and transplanted Unc13d-edited stem cells into busulfan-conditioned Jinx recipient mice. Safety studies included extensive genotyping and chromosomal aberrations analysis by single targeted linker-mediated PCR sequencing (CAST-Seq)-based off-target analyses. Cure from HLH predisposition was assessed by LCMV infection.

RESULTS

Hematopoietic cells isolated from transplanted mice revealed efficient gene editing (>95%), polyclonality of the T-cell receptor repertoire, and neither signs of off-target effects nor leukemogenesis. Unc13d transcription levels of edited and wild-type cells were comparable. While LCMV challenge resulted in acute HLH in Jinx mice transplanted with mock-edited HSCs, Jinx mice grafted with Unc13d-edited cells showed rapid virus clearance and protection from HLH.

CONCLUSIONS

Our study demonstrates that transplantation of CRISPR-Cas edited HSCs supports the development of a functional polyclonal T-cell response in the absence of genotoxicity-associated clonal outgrowth.

Rapid Diagnosis of Hemophagocytic Lymphohistiocytosis Triggered by Disseminated BCG Infection in Infants With Severe Combined Immunodeficiency: Case Report

Hemophagocytic lymphohistiocytosis triggered by disseminated Bacillus Calmette-Guerin infection is rare. Targeted next-generation sequencing for tuberculosis can rapidly identify different strains of Mycobacterium tuberculosis complex as well as drug resistance genes. Herein we report 2 cases of hemophagocytic lymphohistiocytosis in whom targeted next-generation sequencing rapidly identified Bacillus Calmette-Guerin as the infectious trigger.

The Multifaceted Immunology of Cytokine Storm Syndrome

Cytokine storm syndromes (CSSs) are potentially fatal hyperinflammatory states that share the underpinnings of persistent immune cell activation and uninhibited cytokine production. CSSs can be genetically determined by inborn errors of immunity (i.e., familial hemophagocytic lymphohistiocytosis) or develop as a complication of infections, chronic inflammatory diseases (e.g., Still disease), or malignancies (e.g., T cell lymphoma). Therapeutic interventions that activate the immune system such as chimeric Ag receptor T cell therapy and immune checkpoint inhibition can also trigger CSSs in the setting of cancer treatment. In this review, the biology of different types of CSSs is explored, and the current knowledge on the involvement of immune pathways and the contribution of host genetics is discussed. The use of animal models to study CSSs is reviewed, and their relevance for human diseases is discussed. Lastly, treatment approaches for CSSs are discussed with a focus on interventions that target immune cells and cytokines.

Adoptive T cell therapy cures mice from active hemophagocytic lymphohistiocytosis (HLH)

Primary hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory syndrome caused by impaired lymphocyte cytotoxicity. First-line therapeutic regimens directed against activated immune cells or secreted cytokines show limited efficacy since they do not target the underlying immunological problem: defective lymphocyte cytotoxicity causing prolonged immune stimulation. A potential rescue strategy would be the adoptive transfer of ex vivo gene-corrected autologous T cells. However, transfusion of cytotoxicity-competent T cells under conditions of hyperinflammation may cause more harm than benefit. As a proof-of-concept for adoptive T cell therapy (ATCT) under hyperinflammatory conditions, we transferred syngeneic, cytotoxicity-competent T cells into mice with virally triggered active primary HLH. ATCT with functional syngeneic trigger-specific T cells cured Jinx mice from active HLH without life-threatening side effects and protected Perforin-deficient mice from lethal HLH progression by reconstituting cytotoxicity. Cured mice were protected long-term from HLH relapses. A threshold frequency of transferred T cells with functional differentiation was identified as a predictive biomarker for long-term survival. This study is the first proof-of-concept for ATCT in active HLH.

Genomics Driving Diagnosis and Treatment of Inborn Errors of Immunity With Cancer Predisposition

Inborn errors of immunity (IEI) are genetically and clinically heterogeneous disorders that, in addition to infection susceptibility and immune dysregulation, can have an enhanced cancer predisposition. The increasing availability of upfront next-generation sequencing diagnostics in immunology and oncology have uncovered substantial overlap of germline and somatic genetic conditions that can result in immunodeficiency and cancer. However, broad application of unbiased genetics in these neighboring disciplines still needs to be deployed, and joined therapeutic strategies guided by germline and somatic genetic risk factors are lacking. We illustrate the current difficulties encountered in clinical practice, summarize the historical development of pathophysiological concepts of cancer predisposition, and review select genetic, molecular, and cellular mechanisms of well-defined and illustrative disease entities such as DNA repair defects, combined immunodeficiencies with Epstein-Barr virus susceptibility, autoimmune lymphoproliferative syndromes, regulatory T-cell disorders, and defects in cell intrinsic immunity. We review genetic variants that, when present in the germline, cause IEI with cancer predisposition but, when arising as somatic variants, behave as oncogenes and cause specific cancer entities. We finally give examples of small molecular compounds that are developed and studied to target genetically defined cancers but might also proof useful to treat IEI.

Digenic Inheritance: Evidence and Gaps in Hemophagocytic Lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory disorder characterized by the inability to properly terminate an immune response. Familial HLH (FHLH) and related immune dysregulation syndromes are associated with mutations in the genes PRF1, UNC13D, STX11, STXBP2, LYST, AP3B1, and RAB27A, all of which are required for the assembly, exocytosis, and function of cytotoxic granules within CD8+ T cells and natural killer (NK) cells. Loss-of-function mutations in these genes render the cytotoxicity pathway ineffective, thereby failing to eradicate immune stimuli, such as infectious pathogens or malignant cells. The resulting persistent immune system stimulation drives hypercytokinemia, ultimately leading to severe tissue inflammation and end-organ damage. Traditionally, a diagnosis of FHLH requires the identification of biallelic loss-of-function mutations in one of these degranulation pathway genes. However, this narrow definition fails to encompass patients with other genetic mechanisms underlying degranulation pathway dysfunction. In particular, mounting clinical evidence supports a potential digenic mode of inheritance of FHLH in which single loss-of-function mutations in two different degranulation pathway genes cooperate to impair pathway activity. Here, we review the functions of the FHLH-associated genes within the degranulation pathway and summarize clinical evidence supporting a model in which cumulative defects along this mechanistic pathway may underlie HLH.

Natural Killer Cells: Potential Biomarkers and Therapeutic Target in Autoimmune Diseases?

Autoimmune diseases recognize a multifactorial pathogenesis, although the exact mechanism responsible for their onset remains to be fully elucidated. Over the past few years, the role of natural killer (NK) cells in shaping immune responses has been highlighted even though their involvement is profoundly linked to the subpopulation involved and to the site where such interaction takes place. The aberrant number and functionality of NK cells have been reported in several different autoimmune disorders. In the present review, we report the most recent findings regarding the involvement of NK cells in both systemic and organ-specific autoimmune diseases, including type 1 diabetes (T1D), primary biliary cholangitis (PBC), systemic sclerosis, systemic lupus erythematosus (SLE), primary Sjögren syndrome, rheumatoid arthritis, and multiple sclerosis. In T1D, innate inflammation induces NK cell activation, disrupting the Treg function. In addition, certain genetic variants identified as risk factors for T1D influenced the activation of NK cells promoting their cytotoxic activity. The role of NK cells has also been demonstrated in the pathogenesis of PBC mediating direct or indirect biliary epithelial cell destruction. NK cell frequency and number were enhanced in both the peripheral blood and the liver of patients and associated with increased NK cell cytotoxic activity and perforin expression levels. NK cells were also involved in the perpetuation of disease through autoreactive CD4 T cell activation in the presence of antigen-presenting cells. In systemic sclerosis (SSc), in addition to phenotypic abnormalities, patients presented a reduction in CD56^hi NK-cells. Moreover, NK cells presented a deficient killing activity. The influence of the activating and inhibitory killer cell immunoglobulin-like receptors (KIRs) has been investigated in SSc and SLE susceptibility. Furthermore, autoantibodies to KIRs have been identified in different systemic autoimmune conditions. Because of its role in modulating the immune-mediated pathology, NK subpopulation could represent a potential marker for disease activity and target for therapeutic intervention.

Natural Killer Cells in Systemic Autoinflammatory Diseases: A Focus on Systemic Juvenile Idiopathic Arthritis and Macrophage Activation Syndrome

Natural killer (NK) cells are innate immune lymphocytes with potent cytolytic and immune-regulatory activities. NK cells are well-known for their ability to kill infected and malignant cells in a fast and non-specific way without prior sensitization. For this purpose, NK cells are equipped with a set of cytotoxic molecules such as perforin and apoptosis-inducing proteins. NK cells also have the capacity to produce large amounts of cytokines and chemokines that synergize with their cytotoxic function and that ensure interaction with other immune cells. A less known feature of NK cells is their capacity to kill non-infected autologous cells, such as immature dendritic cells and activated T cells and monocytes. Via the release of large amounts of TNF-α and IFN-γ, NK cells may contribute to disease pathology. Conversely they may exert a regulatory role through secretion of immuno-regulatory cytokines such as GM-CSF, IL-13, and IL-10. Thus, NK cells may be important target and effector cells in the pathogenesis of autoinflammatory diseases, in particular in those disorders associated with a cytokine storm or in conditions where immune cells are highly activated. Key examples of such diseases are systemic juvenile idiopathic arthritis (sJIA) and its well-associated complication, macrophage activation syndrome (MAS). sJIA is a chronic childhood immune disorder of unknown etiology, characterized by arthritis and systemic inflammation, including a daily spiking fever and evanescent rash. MAS is a potentially fatal complication of autoimmune and autoinflammatory diseases, and most prevalently associated with sJIA. MAS is considered as a subtype of hemophagocytic lymphohistiocytosis (HLH), a systemic hyperinflammatory disorder characterized by defective cytotoxic pathways of cytotoxic T and NK cells. In this review, we describe the established features of NK cells and provide the results of a literature survey on the reported NK cell abnormalities in monogenic and multifactorial autoinflammatory disorders. Finally, we discuss the role of NK cells in the pathogenesis of sJIA and MAS.

IFN-γ and CD25 drive distinct pathologic features during hemophagocytic lymphohistiocytosis

BACKGROUND

Inflammatory activation of CD8+ T cells can, when left unchecked, drive severe immunopathology. Hyperstimulation of CD8+ T cells through a broad set of triggering signals can precipitate hemophagocytic lymphohistiocytosis (HLH), a life-threatening systemic inflammatory disorder.

OBJECTIVE

The mechanism linking CD8+ T-cell hyperactivation to pathology is controversial, with excessive production of IFN-γ and, more recently, excessive consumption of IL-2, which are proposed as competing hypotheses. We formally tested the proximal mechanistic events of each pathway in a mouse model of HLH.

METHODS

In addition to reporting a complete autosomal recessive IFN-γ receptor 1-deficient patient with multiple aspects of HLH pathology, we used the mouse model of perforin (Prf1)KO mice infected with lymphocytic choriomeningitis virus to genetically eliminate either IFN-γ production or CD25 expression and assess the immunologic, hematologic, and physiologic disease measurement.

RESULTS

We found a striking dichotomy between the mechanistic basis of the hematologic and inflammatory components of CD8+ T cell-mediated pathology. The hematologic features of HLH were completely dependent on IFN-γ production, with complete correction after loss of IFN-γ production without any role for CD8+ T cell-mediated IL-2 consumption. By contrast, the mechanistic contribution of the immunologic features was reversed, with no role for IFN-γ production but substantial correction after reduction of IL-2 consumption by hyperactivated CD8+ T cells. These results were complemented by the characterization of an IFN-γ receptor 1-deficient patients with HLH-like disease, in whom multiple aspects of HLH pathology were observed in the absence of IFN-γ signaling.

CONCLUSION

These results synthesize the competing mechanistic models of HLH pathology into a dichotomous pathogenesis driven through discrete pathways. A holistic model provides a new paradigm for understanding HLH and, more broadly, the consequences of CD8+ T-cell hyperactivation, thereby paving the way for clinical intervention based on the features of HLH in individual patients.

From the "missing self" hypothesis to adaptive NK cells: Insights of NK cell-mediated effector functions in immune surveillance

The original discovery of NK cells approximately 40 yr ago was based on their unique capability to kill tumor cells without prior sensitization or priming, a process named natural cytotoxicity. Since then, several studies have documented that NK cells can kill hematopoietic and nonhematopoietic cancer cells. NK cells also recognize and kill cells that have undergone viral infections. Besides natural cytotoxicity, NK cells are also major effectors of antibody-dependent cell cytotoxicity (ADCC). Therefore, NK cells are well "armed" to recognize and mount immune responses against "insults" that result from cell transformation and viral infections. Because of these attributes, an essential role of NK cells in tumor surveillance was noted. Indeed, several studies have shown a correlation between impaired NK cell cytotoxicity and a higher risk of developing cancer. This evidence led to the idea that cancer initiation and progress is intimately related to an abnormal or misdirected immune response. Whereas all these ideas remain current, it is also true that NK cells represent a heterogeneous population with different abilities to secrete cytokines and to mediate cytotoxic functions. In addition, recent data has shown that NK cells are prone to suffer epigenetic modifications resulting in the acquisition of previously unrecognized attributes such as memory and long-term survival. Such NK cells, referred as "adaptive" or "memory-like," also display effector functions that are not necessarily equal to those observed in conventional NK cells. Given the new evidence available, it is essential to discuss the conceptual reasoning and misconceptions regarding the role of NK cells in immune surveillance and immunotherapy.

Natural killer cells in inflammation and autoimmunity

First described 40 years ago, natural killer (NK) cells represent the founding members of the innate lymphoid cell (ILC) family. They were initially defined by their ability to kill cancer cells of hematopoietic origin. More recently, NK cells are recognized not only for their ability to kill infected or malignant cells, but also for mediating cytotoxicity against a range of normal immune cells. They thereby play an important physiological role in controlling immune responses and maintaining homeostasis. Besides cytotoxic activity, NK cells activation is accompanied by secretion of pro-inflammatory cytokines. Hence, NK cells have the potential to act both in driving inflammation and in restricting adaptive immune responses that may otherwise lead to excessive inflammation or even autoimmunity. Here, we highlight how NK cell activity is linked to inflammasome activation and review new molecular insights to the roles of NK cells in inflammation and autoimmunity. Furthermore, in light of new insights to NK cell differentiation and memory, we deliberate on how distinct NK cell subsets may impact immunoregulatory functions. Hypothetically, memory-like or adaptive NK cells could drive NK cell-mediated autoreactive diseases. Together, new findings underscore the complex yet important physiological roles of NK cells in both promoting inflammation and exerting immunoregulation and maintenance of immune homeostasis. Insights raise intriguing questions as to how NK cells themselves maintain self-tolerance.

Prognostic Factors and Outcomes in Adults With Secondary Hemophagocytic Lymphohistiocytosis: A Single-center Experience

INTRODUCTION

Hemophagocytic lymphohistiocytosis (HLH) is a disorder caused by severe immune activation. There are no specific criteria to establish the diagnosis in adults; however, the HLH-04 criteria are among the most commonly used. The HScore is a non-validated tool that can also be useful for HLH diagnosis.

PATIENTS AND METHODS

We describe the prognostic factors and outcomes of 64 adults diagnosed with HLH in a reference medical center in Mexico City. We included patients ≥ 18 years with HLH, diagnosed and treated at our institution from 1998 to 2016.

RESULTS

The median age was 35 years (range, 18-77 years). The underlying cause of HLH was lymphoma in 33 (51.56%) patients (MA-HLH). Cutaneous involvement was more frequent in MA-HLH (33.33%), when compared with patients with non-malignancy associated HLH (NM-HLH) (9.68%) (P = .022). Neurologic symptoms were more frequent in NM-HLH (25.81%) versus MA-HLH (6.06%) (P = .032). After a median follow-up of 14 months (range, 0-216 months), 30-day mortality was 26.56%. Three-year overall survival (OS) was higher for patients with MA-HLH compared with patients with NM-HLH (41% vs. 22.5%; P = .046). Multivariate analysis showed that the presence of nosocomial infection and neurologic symptoms were statistically significant predictors of inferior OS (P = .034 and P = .033, respectively).

CONCLUSION

In this series of adults with HLH, patients with nosocomial infections and neurologic symptoms had a statistically significant worse OS. In the largest series in Latin America, the most common cause of HLH was T-cell lymphoma. In our population, NM-HLH presented a higher mortality.

Mechanistic peculiarities of activation-induced mobilization of cytotoxic effector proteins in human T cells

It is widely accepted that cytotoxic T and NK cells store effector proteins including granzymes, perforin and Fas ligand (FasL) in intracellular granules, often referred to as secretory lysosomes. Upon target cell encounter, these organelles are transported to the cytotoxic immunological synapse, where they fuse with the plasma membrane to release the soluble effector molecules and to expose transmembrane proteins including FasL on the cell surface. We previously described two distinct species of secretory vesicles in T and NK cells that differ in size, morphology and protein loading, most strikingly regarding FasL and granzyme B. We now show that the signal requirements for the mobilization of one or the other granule also differ substantially. We report that prestored FasL can be mobilized independent of extracellular Ca2+, whereas the surface exposure of lysosome-associated membrane proteins (Lamps; CD107a and CD63) and the release of granzyme B are calcium-dependent. The use of selective inhibitors of actin dynamics unequivocally points to different transport mechanisms for individual vesicles. While inhibitors of actin polymerization/dynamics inhibit the surface appearance of prestored FasL, they increase the activation-induced mobilization of CD107a, CD63 and granzyme B. In contrast, inhibition of the actin-based motor protein myosin 2a facilitates FasL-, but impairs CD107a-, CD63- and granzyme B mobilization. From our data, we conclude that distinct cytotoxic effector granules are differentially regulated with respect to signaling requirements and transport mechanisms. We suggest that a T cell might 'sense' which effector proteins it needs to mobilize in a given context, thereby increasing efficacy while minimizing collateral damage.

Lymphocyte-independent pathways underlie the pathogenesis of murine cytomegalovirus-associated secondary haemophagocytic lymphohistiocytosis

Haemophagocytic lymphohistiocytosis (HLH) constitutes a spectrum of immunological disorders characterized by uncontrolled immune activation and key symptoms such as fever, splenomegaly, pancytopenia, haemophagocytosis, hyperferritinaemia and hepatitis. In genetic or primary HLH, hyperactivated CD8+ T cells are the main drivers of pathology. However, in acquired secondary HLH, the role of lymphocytes remains vague. In the present study the involvement of lymphocytes in the pathogenesis of a cytomegalovirus-induced model of secondary HLH was explored. We have previously reported CD8+ T cells to be redundant in this model, and therefore focused on CD4+ helper and regulatory T cells. CD4+ T cells were activated markedly and skewed towards a proinflammatory T helper type 1 transcription profile in mice displaying a severe and complete HLH phenotype. Counter to expectations, regulatory T cells were not reduced in numbers and were, in fact, more activated. Therapeutic strategies targeting CD25high hyperactivated T cells were ineffective to alleviate disease, indicating that T cell hyperactivation is not a pathogenic factor in cytomegalovirus-induced murine HLH. Moreover, even though T cells were essential in controlling viral proliferation, CD4+ T cells, in addition to CD8+ T cells, were dispensable in the development of the HLH-like syndrome. In fact, no T or B cells were required for induction and propagation of HLH disease, as evidenced by the occurrence of cytomegalovirus-associated HLH in severe combined immunodeficient (SCID) mice. These data suggest that lymphocyte-independent mechanisms can underlie virus-associated secondary HLH, accentuating a clear distinction with primary HLH.

The cytotoxic T lymphocyte immune synapse at a glance

The immune synapse provides an important structure for communication with immune cells. Studies on immune synapses formed by cytotoxic T lymphocytes (CTLs) highlight the dynamic changes and specialised mechanisms required to facilitate focal signalling and polarised secretion in immune cells. In this Cell Science at a Glance article and the accompanying poster, we illustrate the different steps that reveal the specialised mechanisms used to focus secretion at the CTL immune synapse and allow CTLs to be such efficient and precise serial killers.

Advances in the pathogenesis of primary and secondary haemophagocytic lymphohistiocytosis: differences and similarities

Haemophagocytic lymphohistiocytosis (HLH) comprises a heterogeneous spectrum of hyperinflammatory conditions that are inherited (primary HLH) or acquired in a context of infections, malignancies or autoimmune/autoinflammatory disorders (secondary HLH). Genetic defects in the cytotoxic machinery of natural killer and CD8(+) T cells underlie primary HLH, with residual cytotoxicity determining disease severity. Improved sequencing techniques have expanded the range of causal mutations and have redefined many cases of secondary HLH as primary HLH and vice versa, blurring the distinction between both subtypes. These insights allow HLH to be conceptualized as a threshold disease, in which interplay between various genetic and environmental factors causes progressive inflammation into a critical point, beyond which uncontrolled activation of immune cells and excessive cytokine production give rise to the cardinal symptoms of HLH. Various pathogenic pathways may thus converge to a common end stage of fulminant HLH.