Hypogammaglobulinemia and exacerbated CD8 T-cell–mediated immunopathology in SAP-deficient mice with chronic LCMV infection mimics human XLP disease

Murine Models of Familial Cytokine Storm Syndromes

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory disease caused by mutations in effectors and regulators of cytotoxicity in cytotoxic T cells (CTL) and natural killer (NK) cells. The complexity of the immune system means that in vivo models are needed to efficiently study diseases like HLH. Mice with defects in the genes known to cause primary HLH (pHLH) are available. However, these mice only develop the characteristic features of HLH after the induction of an immune response (typically through infection with lymphocytic choriomeningitis virus). Nevertheless, murine models have been invaluable for understanding the mechanisms that lead to HLH. For example, the cytotoxic machinery (e.g., the transport of cytotoxic vesicles and the release of granzymes and perforin after membrane fusion) was first characterized in the mouse. Experiments in murine models of pHLH have emphasized the importance of cytotoxic cells, antigen-presenting cells (APC), and cytokines in hyperinflammatory positive feedback loops (e.g., cytokine storms). This knowledge has facilitated the development of treatments for human HLH, some of which are now being tested in the clinic.

Cutting Edge: Polycomb Repressive Complex 1 Subunit Cbx4 Positively Regulates Effector Responses in CD8 T Cells

CTL differentiation is controlled by the crosstalk of various transcription factors and epigenetic modulators. Uncovering this process is fundamental to improving immunotherapy and designing novel therapeutic approaches. In this study, we show that polycomb repressive complex 1 subunit chromobox (Cbx)4 favors effector CTL differentiation in a murine model. Cbx4 deficiency in CTLs induced a transcriptional signature of memory cells and increased the memory CTL population during acute viral infection. It has previously been shown that besides binding to H3K27me3 through its chromodomain, Cbx4 functions as a small ubiquitin-like modifier (SUMO) E3 ligase in a SUMO-interacting motifs (SIM)-dependent way. Overexpression of Cbx4 mutants in distinct domains showed that this protein regulates CTL differentiation primarily in an SIM-dependent way and partially through its chromodomain. Our data suggest a novel role of a polycomb group protein Cbx4 controlling CTL differentiation and indicated SUMOylation as a key molecular mechanism connected to chromatin modification in this process.

Restriction of viral replication, rather than T cell immunopathology, drives lethality in MNV CR6-infected STAT1-deficient mice

Recent evidence indicates that viral components of the microbiota can contribute to intestinal homeostasis and protection from local inflammatory or infectious insults. However, host-derived mechanisms that regulate the virome remain largely unknown. Here, we use colonization with the model commensal murine norovirus (MNV CR6) to interrogate host-directed mechanisms of viral regulation, and show that STAT1 is a central coordinator of both viral replication and antiviral T cell responses. In addition to restricting CR6 replication to the intestinal tract, we show that STAT1 regulates antiviral CD4⁺ and CD8⁺ T cell responses, and prevents systemic viral-induced tissue damage and disease. Despite altered T cell responses that resemble those that mediate lethal immunopathology in systemic viral infections in STAT1-deficient mice, depletion of adaptive immune cells and their associated effector functions had no effect on CR6-induced disease. However, therapeutic administration of an antiviral compound limited viral replication, preventing viral-induced tissue damage and death without impacting the generation of inflammatory antiviral T cell responses. Collectively, our data show that STAT1 restricts MNV CR6 replication within the intestinal mucosa, and that uncontrolled viral replication mediates disease rather than the concomitant development of dysregulated antiviral T cell responses in STAT1-deficient mice. Importance The intestinal microbiota is a collection of bacteria, archaea, fungi and viruses that colonize the mammalian gut. Co-evolution of the host and microbiota has required development of immunological tolerance to prevent ongoing inflammatory responses against intestinal microbes. Breakdown of tolerance to bacterial components of the microbiota can contribute to immune activation and inflammatory disease. However, the mechanisms that are necessary to maintain tolerance to viral components of the microbiome, and the consequences of loss of tolerance, are less well understood. Here, we show that STAT1 is integral for preventing escape of a commensal-like virus, murine norovirus CR6 (MNV CR6) from the gut, and that in the absence of STAT1, mice succumb to infection-induced disease. In contrast to other systemic viral infections, mortality of STAT1-deficient mice is not driven by immune-mediated pathology. Our data demonstrates the importance of host-mediated geographical restriction of commensal-like viruses.

Kdm6b Regulates the Generation of Effector CD8+ T Cells by Inducing Chromatin Accessibility in Effector-Associated Genes

The transcriptional and epigenetic regulation of CD8+ T cell differentiation is critical for balancing pathogen eradication and long-term immunity by effector and memory CTLs, respectively. In this study, we demonstrate that the lysine demethylase 6b (Kdm6b) is essential for the proper generation and function of effector CD8+ T cells during acute infection and tumor eradication. We found that cells lacking Kdm6b (by either T cell-specific knockout mice or knockdown using short hairpin RNA strategies) show an enhanced generation of memory precursor and early effector cells upon acute viral infection in a cell-intrinsic manner. We also demonstrate that Kdm6b is indispensable for proper effector functions and tumor protection, and that memory CD8+ T cells lacking Kdm6b displayed a defective recall response. Mechanistically, we identified that Kdm6b, through induction of chromatin accessibility in key effector-associated gene loci, allows for the proper generation of effector CTLs. Our results pinpoint the essential function of Kdm6b in allowing chromatin accessibility in effector-associated genes, and identify Kdm6b as a potential target for therapeutics in diseases with dysregulated effector responses.

B Cell Diversification Is Uncoupled from SAP-Mediated Selection Forces in Chronic Germinal Centers within Peyer's Patches

Antibodies secreted within the intestinal tract provide protection from the invasion of microbes into the host tissues. Germinal center (GC) formation in lymph nodes and spleen strictly requires SLAM-associated protein (SAP)-mediated T cell functions; however, it is not known whether this mechanism plays a similar role in mucosal-associated lymphoid tissues. Here, we find that in Peyer’s patches (PPs), SAP-mediated T cell help is required for promoting B cell selection in GCs, but not for clonal diversification. PPs of SAP-deficient mice host chronic GCs that are absent in T cell-deficient mice. GC B cells in SAP-deficient mice express AID and Bcl6 and generate plasma cells in proportion to the GC size. Single-cell IgA sequencing analysis reveals that these mice host few diversified clones that were subjected to mild selection forces. These findings demonstrate that T cell-derived help to B cells in PPs includes SAP-dependent and SAP-independent functions.

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Chronic germinal centers in Peyer’s patches are formed in SAP-deficient mice

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SAP-independent germinal centers arise in response to influenza infection

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Few highly diversified clones dominate the SAP-independent germinal centers

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Germinal center B cells in SAP-deficient mice are subjected to mild selection forces

Follicular Helper T Cells are Essential for the Elimination of Plasmodium Infection

CD4⁺ follicular helper T (Tfh) cells have been shown to be critical for the activation of germinal center (GC) B-cell responses. Similar to other infections, Plasmodium infection activates both GC as well as non-GC B cell responses. Here, we sought to explore whether Tfh cells and GC B cells are required to eliminate a Plasmodium infection. A CD4 T cell-targeted deletion of the gene that encodes Bcl6, the master transcription factor for the Tfh program, resulted in complete disruption of the Tfh response to Plasmodium chabaudi in C57BL/6 mice and consequent disruption of GC responses and IgG responses and the inability to eliminate the otherwise self-resolving chronic P. chabaudi infection. On the other hand, and contrary to previous observations in immunization and viral infection models, Signaling Lymphocyte Activation Molecule (SLAM)-Associated Protein (SAP)-deficient mice were able to activate Tfh cells, GC B cells, and IgG responses to the parasite. This study demonstrates the critical role for Tfh cells in controlling this systemic infection, and highlights differences in the signals required to activate GC B cell responses to this complex parasite compared with those of protein immunizations and viral infections. Therefore, these data are highly pertinent for designing malaria vaccines able to activate broadly protective B-cell responses.

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Chronic Plasmodium infection cannot be eliminated in the absence of Tfh cell responses.

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SAP-deficient mice are able to activate GC Tfh and GC B-cell responses to Plasmodium infection.

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There is a hierarchical requirement for the control of chronic Plasmodium infection following IL-21R > Tfh cells > SAP.

Successful vaccines work through activation of protective B-cell responses. Malaria, caused by Plasmodium infection transmitted by mosquito bites, remains a global threat. Despite substantial efforts, a vaccine able to bring about high levels of protection from Plasmodium infection remains elusive. Here, using an experimental malaria model including natural mosquito transmission, we demonstrate that proper activation of follicular helper CD4⁺ T cells is essential for the control and eradication of chronic Plasmodium infection through protective B-cell responses. Thus, it is strongly advisable for novel vaccine efforts to monitor the robust activation of this important immune compartment.

2B4-SAP signaling is required for the priming of naive CD8+ T cells by antigen-expressing B cells and B lymphoma cells

Mutations in SH2D1A gene that encodes SAP (SLAM-associated protein) result in X-linked lymphoproliferative disease (XLP), a rare primary immunodeficiency disease defined by exquisite sensitivity to the B-lymphotropic Epstein-Barr virus (EBV) and B cell lymphomas. However, the precise mechanism of how the loss of SAP function contributes to extreme vulnerability to EBV and the development of B cell lymphomas remains unclear. Here, we investigate the hypothesis that SAP is critical for CD8⁺ T cell immune surveillance of antigen (Ag)-expressing B cells or B lymphoma cells under conditions of defined T cell receptor (TCR) signaling. Sh2d1a^-/- CD8⁺ T cells exhibited greatly diminished proliferation relative to wild type when Ag-presenting-B cells or -B lymphoma cells served as the primary Ag-presenting cell (APC). By contrast, Sh2d1a^-/- CD8⁺ T cells responded equivalently to wild-type CD8⁺ T cells when B cell-depleted splenocytes, melanoma cells or breast carcinoma cells performed Ag presentation. Through application of signaling lymphocyte activation molecule (SLAM) family receptor blocking antibodies or SLAM family receptor-deficient CD8⁺ T cells and APCs, we found that CD48 engagement on the B cell surface by 2B4 is crucial for initiating SAP-dependent signaling required for the Ag-driven CD8⁺ T cell proliferation and differentiation. Altogether, a pivotal role for SAP in promoting the expansion and differentiation of B cell-primed viral-specific naive CD8⁺ T cells may explain the selective immune deficiency of XLP patients to EBV and B cell lymphomas.

Responses to Microbial Challenges by SLAMF Receptors

The SLAMF family (SLAMF) of cell surface glycoproteins is comprised of nine glycoproteins and while SLAMF1, 3, 5, 6, 7, 8, and 9 are self-ligand receptors, SLAMF2 and SLAMF4 interact with each other. Their interactions induce signal transduction networks in trans, thereby shaping immune cell-cell communications. Collectively, these receptors modulate a wide range of functions, such as myeloid cell and lymphocyte development, and T and B cell responses to microbes and parasites. In addition, several SLAMF receptors serve as microbial sensors, which either positively or negatively modulate the function of macrophages, dendritic cells, neutrophils, and NK cells in response to microbial challenges. The SLAMF receptor-microbe interactions contribute both to intracellular microbicidal activity as well as to migration of phagocytes to the site of inflammation. In this review, we describe the current knowledge on how the SLAMF receptors and their specific adapters SLAM-associated protein and EAT-2 regulate innate and adaptive immune responses to microbes.

Enhanced CD8 T cell responses through GITR-mediated costimulation resolve chronic viral infection

Chronic infections are characterized by the inability to eliminate the persisting pathogen and often associated with functional impairment of virus-specific T-cell responses. Costimulation through Glucocorticoid-induced TNFR-related protein (GITR) can increase survival and function of effector T cells. Here, we report that constitutive expression of GITR-ligand (GITRL) confers protection against chronic lymphocytic choriomeningitis virus (LCMV) infection, accelerating recovery without increasing pathology. Rapid viral clearance in GITRL transgenic mice coincided with increased numbers of poly-functional, virus-specific effector CD8+ T cells that expressed more T-bet and reduced levels of the rheostat marker PD-1. GITR triggering also boosted the helper function of virus-specific CD4 T cells already early in the infection, as was evidenced by increased IL-2 and IFNγ production, and more expression of CD40L and T-bet. Importantly, CD4-depletion experiments revealed that the expanded pool of virus-specific effector CD8 T cells and the ensuing viral clearance in LCMV-infected GITRL tg mice was entirely dependent on CD4 T cells. We found no major differences for NK cell and regulatory T cell responses, whereas the humoral response to the virus was increased in GITRL tg mice, but only in the late phase of the infection when the virus was almost eradicated. Based on these findings, we conclude that enhanced GITR-triggering mediates its protective, anti-viral effect on the CD8 T cell compartment by boosting CD4 T cell help. As such, increasing costimulation through GITR may be an attractive strategy to increase anti-viral CTL responses without exacerbating pathology, in particular to persistent viruses such as HIV and HCV.

The ability of the immune system to rapidly respond to a viral infection is a prerequisite for the survival of an individual. The immediate reaction of innate immune cells and the subsequent response of antigen-specific lymphocytes is usually effective for rapid neutralization and removal of the invading virus. Yet, such protective immune responses need to be well controlled, as they can cause severe tissue damage that may disable the host more than the infection itself. One way that has evolutionarily been proven effective to deal with this balancing act between protective immunity and prevention of immunopathology is to render virus-specific T cells “exhausted” when the virus cannot be eradicated and the host becomes chronically infected. Exhausted T cells progressively lose their ability to kill other cells and produce different cytokines. The benefit of this exhausted state of anti-viral immunity is that it induces less tissue damage, but the downside is obviously less efficient control over the viral infection. Many immunotherapeutic and vaccination strategies against chronic viral infections are currently dedicated to overcome the exhausted state of the virus-specific T cells and thereby clear the virus. However, the accompanying risk is an exaggerated immune response with overt immunopathology. Here we describe in a mouse model that enhanced triggering through the costimulatory molecule GITR on T cells is able to provide protection upon viral infection and clear an otherwise persistent virus, but importantly without the development of collateral damage due to immunopathology. We show that GITR-mediated costimulation enhances a protective CD8 T cell response, for which CD4 T cell help is required. Our study provides new insights in how a particular costimulatory pathway can be utilized to boost anti-viral immunity, which is highly relevant for the development of safe immunotherapeutic strategies against chronic viral infections in humans.

Hemophagocytic lymphohistiocytosis (HLH): A heterogeneous spectrum of cytokine-driven immune disorders

Hemophagocytic lymphohistiocytosis (HLH) comprises a group of life-threatening immune disorders classified into primary or secondary HLH. The former is caused by mutations in genes involved in granule-mediated cytotoxicity, the latter occurs in a context of infections, malignancies or autoimmune/autoinflammatory disorders. Both are characterized by systemic inflammation, severe cytokine storms and immune-mediated organ damage. Despite recent advances, the pathogenesis of HLH remains incompletely understood. Animal models resembling different subtypes of HLH are therefore of great value to study this disease and to uncover novel treatment strategies. In this review, all known animal models of HLH will be discussed, highlighting findings on cell types, cytokines and signaling pathways involved in disease pathogenesis and extrapolating therapeutic implications for the human situation.

Exogenous OX40 stimulation during lymphocytic choriomeningitis virus infection impairs follicular Th cell differentiation and diverts CD4 T cells into the effector lineage by upregulating Blimp-1

T cell costimulation is a key component of adaptive immunity to viral infection but has also been associated with pathology because of excessive or altered T cell activity. We recently demonstrated that the TNFR family costimulatory molecule OX40 (CD134) is critically required to sustain antiviral T cell and Ab responses that enable control of viral replication in the context of chronic lymphocytic choriomeningitis virus (LCMV) infection. In this study, we investigated whether reinforcing OX40 stimulation through an agonist Ab had the potential to prevent LCMV persistence. We observed that anti-OX40 injection early after LCMV clone 13 infection increased CD8 T cell-mediated immunopathology. More strikingly, OX40 stimulation of virus-specific CD4 T cells promoted expression of the transcriptional repressor Blimp-1 and diverted the majority of cells away from follicular Th cell differentiation. This occurred in both acute and chronic infections, and resulted in dramatic reductions in germinal center and Ab responses to the viral infection. The effect of the OX40 agonist was dependent on IL-2 signaling and the timing of OX40 stimulation. Collectively, our data demonstrate that excessive OX40 signaling can result in deleterious consequences in the setting of LCMV infection.

Simultaneous TCR and CD244 signals induce dynamic downmodulation of CD244 on human antiviral T cells

Various cosignaling molecules on T cells can contribute to activation, inhibition, or exhaustion, depending on context. The surface receptor signaling lymphocytic activation molecule (SLAM) family receptor CD244 (2B4/SLAMf4) has been shown to be capable of either inhibitory or enhancing effects upon engagement of its ligand CD48 (SLAMf2). We examined phenotypes of CD8 T cells from HIV(+) and HIV(neg) human donors, specific for HIV and/or respiratory syncytial virus. Cultured and ex vivo CD8 T cells expressed PD-1, CD244, and TIM-3. We found that ex vivo CD8 T cells downregulated CD244 in response to superantigen. Furthermore, cognate peptide induced rapid downregulation of both CD244 and TIM-3, but not PD-1, on CD8 T cell clones. CD244 downmodulation required simultaneous signaling via both TCR and CD244 itself. Using a pH-sensitive fluorophore conjugated to avidin-Ab tetramers, we found that CD244 crosslinking in the presence of TCR signaling resulted in rapid transport of CD244 to an acidic intracellular compartment. Downregulation was not induced by PMA-ionomycin, or prevented by PI3K inhibition, implicating a TCR-proximal signaling mechanism. CD244 internalization occurred within hours of TCR stimulation and required less peptide than was required to induce IFN-γ production. The degree of CD244 internalization varied among cultured CD8 T cell lines of different specificities, and correlated with the enhancement of IFN-γ production in response to CD48 blockade in HIV(+), but not HIV(neg), subjects. Our results indicate that rapid CD244 internalization is induced by a two-signal mechanism and plays a role in modulation of antiviral CD8 T cell responses by CD48-CD244 signaling.

Evolving role of 2B4/CD244 in T and NK cell responses during virus infection

The signaling lymphocyte activation molecule (SLAM) family receptor, 2B4/CD244, was first implicated in anti-viral immunity by the discovery that mutations of the SLAM-associated protein, SAP/SH2D1A, impaired 2B4-dependent stimulation of T and natural killer (NK) cell anti-viral functions in X-linked lymphoproliferative syndrome patients with uncontrolled Epstein-Barr virus infections. Engagement of 2B4 has been variably shown to either activate or inhibit lymphocytes which express this receptor. While SAP expression is required for stimulatory functions of 2B4 on lymphocytes, it remains unclear whether inhibitory signals derived from 2B4 can predominate even in the presence of SAP. Regardless, mounting evidence suggests that 2B4 expression by NK and CD8 T cells is altered by virus infection in mice as well as in humans, and 2B4-mediated signaling may be an important determinant of effective immune control of chronic virus infections. In this review, recent findings regarding the expression and function of 2B4 as well as SAP on T and NK cells during virus infection is discussed, with a focus on the role of 2B4-CD48 interactions in crosstalk between innate and adaptive immunity.

Intronic SH2D1A mutation with impaired SAP expression and agammaglobulinemia

X-linked lymphoproliferative (XLP) disease is a primary immunodeficiency syndrome associated with the inability to control Epstein-Barr virus (EBV), lymphoma, and hypogammaglobulinemia. XLP is caused by mutations in the SH2D1A gene, which encodes the SLAM-associated protein (SAP), or in the BIRC4 gene, which encodes the X-linked inhibitor of apoptosis protein (XIAP). Here we report a patient with recurrent respiratory tract infections and early onset agammaglobulinemia who carried a unique disease-causing intronic loss-of-function mutation in SH2D1A. The intronic mutation affected SH2D1A gene transcription but not mRNA splicing, and led to markedly reduced level of SAP protein. Despite undetectable serum immunoglobulins, the patient's B cells replicated and differentiated into antibody producing cells normally in vitro.

OX40 facilitates control of a persistent virus infection

During acute viral infections, clearance of the pathogen is followed by the contraction of the anti-viral T cell compartment. In contrast, T cell responses need to be maintained over a longer period of time during chronic viral infections in order to control viral replication and to avoid viral spreading. Much is known about inhibitory signals such as through PD-1 that limit T cell activity during chronic viral infection, but little is known about the stimulatory signals that allow maintenance of anti-viral T cells. Here, we show that the co-stimulatory molecule OX40 (CD134) is critically required in the context of persistent LCMV clone 13 infection. Anti-viral T cells express high levels of OX40 in the presence of their cognate antigen and T cells lacking the OX40 receptor fail to accumulate sufficiently. Moreover, the emergence of T cell dependent germinal center responses and LCMV-specific antibodies are severely impaired. Consequently, OX40-deficient mice fail to control LCMV clone 13 infection over time, highlighting the importance of this signaling pathway during persistent viral infection.

A robust T cell response is the hallmark of an effective immune response to a variety of invading viruses. In many acute infections, the clearance of the viral pathogen is associated with a short and vigorous T cell response followed by development of pathogen-specific immune memory. However, some viruses can establish persistent infection in their respective host, during which an ongoing T cell response is required in order to prevent overwhelming viral replication. Little is known about the factors that sustain the T cell response in the persistent phase of a viral infection. In this report, we demonstrate that ligation of the OX40 molecule, which is expressed on T cells targeting the virus, is critically required in order to sustain the anti-viral immune response. We show that virus-specific, OX40-deficient T cells fail to accumulate sufficiently and consequently, mice lacking the OX40 receptor are incapable of controlling viral replication. Collectively our data establish OX40 as a crucial signaling molecule during a persistent viral infection.

The receptor Ly108 functions as a SAP adaptor-dependent on-off switch for T cell help to B cells and NKT cell development

Humans and mice deficient in the adaptor protein SAP (Sh2d1a) have a major defect in humoral immunity, resulting from a lack of T cell help for B cells. The role of SAP in this process is incompletely understood. We found that deletion of receptor Ly108 (Slamf6) in CD4(+) T cells reversed the Sh2d1a(-/-) phenotype, eliminating the SAP requirement for germinal centers. This potent negative signaling by Ly108 required immunotyrosine switch motifs (ITSMs) and SHP-1 recruitment, resulting in high amounts of SHP-1 at the T cell:B cell synapse, limiting T cell:B cell adhesion. Ly108-negative signaling was important not only in CD4(+) T cells; we found that NKT cell differentiation was substantially restored in Slamf6(-/-)Sh2d1a(-/-) mice. The ability of SAP to regulate both positive and negative signals in T cells can explain the severity of SAP deficiency and highlights the importance of SAP and SHP-1 competition for Ly108 ITSM binding as a rheostat for the magnitude of T cell help to B cells.

Late interleukin-6 escalates T follicular helper cell responses and controls a chronic viral infection

Multiple inhibitory molecules create a profoundly immunuosuppressive environment during chronic viral infections in humans and mice. Therefore, eliciting effective immunity in this context represents a challenge. Here, we report that during a murine chronic viral infection, interleukin-6 (IL-6) was produced by irradiation-resistant cells in a biphasic manner, with late IL-6 being absolutely essential for viral control. The underlying mechanism involved IL-6 signaling on virus-specific CD4 T cells that caused up-regulation of the transcription factor Bcl6 and enhanced T follicular helper cell responses at late, but not early, stages of chronic viral infection. This resulted in escalation of germinal center reactions and improved antibody responses. Our results uncover an antiviral strategy that helps to safely resolve a persistent infection in vivo.

SLAM family receptors and SAP adaptors in immunity

The signaling lymphocyte activation molecule (SLAM)-associated protein, SAP, was first identified as the protein affected in most cases of X-linked lymphoproliferative (XLP) syndrome, a rare genetic disorder characterized by abnormal responses to Epstein-Barr virus infection, lymphoproliferative syndromes, and dysgammaglobulinemia. SAP consists almost entirely of a single SH2 protein domain that interacts with the cytoplasmic tail of SLAM and related receptors, including 2B4, Ly108, CD84, Ly9, and potentially CRACC. SLAM family members are now recognized as important immunomodulatory receptors with roles in cytotoxicity, humoral immunity, autoimmunity, cell survival, lymphocyte development, and cell adhesion. In this review, we cover recent findings on the roles of SLAM family receptors and the SAP family of adaptors, with a focus on their regulation of the pathways involved in the pathogenesis of XLP and other immune disorders.

The power and the promise of restimulation-induced cell death in human immune diseases

Controlled expansion and contraction of lymphocytes both during and after an adaptive immune response are imperative to sustain a healthy immune system. Both extrinsic and intrinsic pathways of lymphocyte apoptosis are programmed to eliminate cells at the proper time to ensure immune homeostasis. Genetic disorders of apoptosis described in mice and humans have established Fas and Bim as critical pro-apoptotic molecules responsible for T-cell death in response to T-cell receptor restimulation and cytokine withdrawal, respectively. Emerging evidence prompts revision of this classic paradigm, especially for our understanding of restimulation-induced cell death (RICD) and its physiological purpose. Recent work indicates that RICD employs both Fas and Bim for T-cell deletion, dispelling the notion that these molecules are assigned to mutually exclusive apoptotic pathways. Furthermore, new mouse model data combined with our discovery of defective RICD in X-linked lymphoproliferative disease (XLP) patient T cells suggest that RICD is essential for precluding excess T-cell accumulation and associated immunopathology during the course of certain infections. Here, we review how these advances offer a refreshing new perspective on the phenomenon of T-cell apoptosis induced through antigen restimulation, including its relevance to immune homeostasis and potential for therapeutic interventions.

Absence of mouse 2B4 promotes NK cell-mediated killing of activated CD8+ T cells, leading to prolonged viral persistence and altered pathogenesis

Persistent viral infections are often associated with inefficient T cell responses and sustained high-level expression of inhibitory receptors, such as the NK cell receptor 2B4 (also known as CD244), on virus-specific T cells. However, the role of 2B4 in T cell dysfunction is undefined, and it is unknown whether NK cells contribute to regulation of these processes. We show here that persistent lymphocytic choriomeningitis virus (LCMV) infection of mice lacking 2B4 resulted in diminished LCMV-specific CD8+ T cell responses, prolonged viral persistence, and spleen and thymic pathologies that differed from those observed in infected wild-type mice. Surprisingly, these altered phenotypes were not caused by 2B4 deficiency in T cells. Rather, the entire and long-lasting pathology and viral persistence were regulated by 2B4-deficient NK cells acting early in infection. In the absence of 2B4, NK cells lysed activated (defined as CD44hi) but not naive (defined as CD44lo) CD8+ T cells in a perforin-dependent manner in vitro and in vivo. These results illustrate the importance of NK cell self-tolerance to activated CD8+ T cells and demonstrate how an apparent T cell-associated persistent infection can actually be regulated by NK cells.

SLAM family receptors and the SLAM-associated protein (SAP) modulate T cell functions

One or more of the signaling lymphocytic activation molecule (SLAM) family (SLAMF) of cell surface receptors, which consists of nine transmembrane proteins, i.e., SLAMF1-9, are expressed on most hematopoietic cells. While most SLAMF receptors serve as self-ligands, SLAMF2 and SLAMF4 use each other as counter structures. Six of the receptors carry one or more copies of a unique intracellular tyrosine-based switch motif, which has high affinity for the single SH2-domain signaling molecules SLAM-associated protein and EAT-2. Whereas SLAMF receptors are costimulatory molecules on the surface of CD4+, CD8+, and natural killer (NK) T cells, they also involved in early phases of lineage commitment during hematopoiesis. SLAMF receptors regulate T lymphocyte development and function and modulate lytic activity, cytokine production, and major histocompatibility complex-independent cell inhibition of NK cells. Furthermore, they modulate B cell activation and memory generation, neutrophil, dendritic cell, macrophage and eosinophil function, and platelet aggregation. In this review, we will discuss the role of SLAM receptors and their adapters in T cell function, and we will examine the role of these receptors and their adapters in X-linked lymphoproliferative disease and their contribution to disease susceptibility in systemic lupus erythematosus.

The smallpox vaccine induces an early neutralizing IgM response

The antibody response elicited after immunization with vaccinia virus (VacV) is known to be sufficient to confer host protection against VacV or smallpox. In humans it has been shown that such anti-VacV antibody production can be sustained for decades. Nevertheless, little is known about the kinetics and the role in protection of the early antibody response after vaccination. In this study we identify VacV neutralizing IgM antibodies as early as 4 days after infection of C57BL/6 mice. Most of this IgM production is T cell dependent and predominantly independent of the germinal center reaction (SAP/SH2D1A independent). Importantly, the IgM neutralized both infectious forms of VacV: the intracellular mature virion (MV, IMV) and the extracellular enveloped virion (EV, EEV). Moreover, in mice primed with MHCII restricted peptides, an increase in the total VacV neutralizing antibody titers was seen, a large component of which was neutralizing IgM against the same protein from which the priming peptide was derived. To further demonstrate the biological relevance of this early neutralizing response, we examined anti-VacV antibodies in humans after vaccination. Human subjects could be divided into two groups early after immunization: IgG(hi) and IgG(lo). VacV IgM neutralizing antibodies were detected in the IgG(lo) group. Taken together these results indicate that both in a small animal model and in humans an early neutralizing IgM response after VacV immunization is present and likely contributes to control of the infection prior to the development of a robust IgG response.

Restimulation-induced apoptosis of T cells is impaired in patients with X-linked lymphoproliferative disease caused by SAP deficiency

X-linked lymphoproliferative disease (XLP) is a rare congenital immunodeficiency that leads to an extreme, usually fatal increase in the number of lymphocytes upon infection with EBV. It is most commonly defined molecularly by loss of expression of SLAM-associated protein (SAP). Despite this, there is little understanding of how SAP deficiency causes lymphocytosis following EBV infection. Here we show that T cells from individuals with XLP are specifically resistant to apoptosis mediated by TCR restimulation, a process that normally constrains T cell expansion during immune responses. Expression of SAP and the SLAM family receptor NK, T, and B cell antigen (NTB-A) were required for TCR-induced upregulation of key pro-apoptotic molecules and subsequent apoptosis. Further, SAP/NTB-A signaling augmented the strength of the proximal TCR signal to achieve the threshold required for restimulation-induced cell death (RICD). Strikingly, TCR ligation in activated T cells triggered increased recruitment of SAP to NTB-A, dissociation of the phosphatase SHP-1, and colocalization of NTB-A with CD3 aggregates. In contrast, NTB-A and SHP-1 contributed to RICD resistance in XLP T cells. Our results reveal what we believe to be novel roles for NTB-A and SAP in regulating T cell homeostasis through apoptosis and provide mechanistic insight into the pathogenesis of lymphoproliferative disease in XLP.

The proapoptotic function of SAP provides a clue to the clinical picture of X-linked lymphoproliferative disease

Deletion or mutation of the SAP gene is associated with the X-linked lymphoproliferative disease (XLP) that is characterized by extreme sensitivity to Epstein-Barr virus (EBV). Primary infection of the affected individuals leads to serious, sometimes fatal infectious mononucleosis (IM) and proneness to lymphoma. Our present results revealed a proapoptotic function of SAP by which it contributes to the maintenance of T-cell homeostasis and to the elimination of potentially dangerous DNA-damaged cells. Therefore, the loss of this function could be responsible for the uncontrolled T-cell proliferation in fatal IM and for the generation of lymphomas. We show now the role of SAP in apoptosis in T and B lymphocyte-derived lines. Among the clones of T-ALL line, the ones with higher SAP levels succumbed more promptly to activation induced cell death (AICD). Importantly, introduction of SAP expression into lymphoblastoid cell lines (LCL) established from XLP patients led to elevated apoptotic response to DNA damage. Similar results were obtained in the osteosarcoma line, Saos-2. We have shown that the anti-apoptotic protein VCP (valosin-containing protein) binds to SAP, suggesting that it could be instrumental in the enhanced apoptotic response modulated by SAP.

Gamma interferon signaling in macrophage lineage cells regulates central nervous system inflammation and chemokine production

Intracranial (i.c.) infection of mice with lymphocytic choriomeningitis virus (LCMV) results in anorexic weight loss, mediated by T cells and gamma interferon (IFN-gamma). Here, we assessed the role of CD4(+) T cells and IFN-gamma on immune cell recruitment and proinflammatory cytokine/chemokine production in the central nervous system (CNS) after i.c. LCMV infection. We found that T-cell-depleted mice had decreased recruitment of hematopoietic cells to the CNS and diminished levels of IFN-gamma, CCL2 (MCP-1), CCL3 (MIP-1alpha), and CCL5 (RANTES) in the cerebrospinal fluid (CSF). Mice deficient in IFN-gamma had decreased CSF levels of CCL3, CCL5, and CXCL10 (IP-10), and decreased activation of both resident CNS and infiltrating antigen-presenting cells (APCs). The effects of IFN-gamma signaling on macrophage lineage cells was assessed using transgenic mice, called "macrophages insensitive to interferon gamma" (MIIG) mice, that express a dominant-negative IFN-gamma receptor under the control of the CD68 promoter. MIIG mice had decreased levels of CCL2, CCL3, CCL5, and CXCL10 compared to controls despite having normal numbers of LCMV-specific CD4(+) T cells in the CNS. MIIG mice also had decreased recruitment of infiltrating macrophages and decreased activation of both resident CNS and infiltrating APCs. Finally, MIIG mice were significantly protected from LCMV-induced anorexia and weight loss. Thus, these data suggest that CD4(+) T-cell production of IFN-gamma promotes signaling in macrophage lineage cells, which control (i) the production of proinflammatory cytokines and chemokines, (ii) the recruitment of macrophages to the CNS, (iii) the activation of resident CNS and infiltrating APC populations, and (iv) anorexic weight loss.

SAP binds to CD22 and regulates B cell inhibitory signaling and calcium flux

The signaling lymphocyte activation molecule (SLAM)-associated protein (SAP or SH2D1A) is an important regulator of immune function which, when mutated or deleted, causes the X-linked lymphoproliferative syndrome (XLP). Because B cell lymphoma is a major phenotype of XLP, it is important to understand the function of SAP in B cells. Here we report that SAP is expressed endogenously in mouse splenic B cells, is inducibly expressed in the human BJAB cells, and co-localizes and interacts with CD22. We also show that SAP binding to the inhibitory immunoreceptor CD22 regulates calcium mobilization in B cells. Moreover, forced expression of SAP leads to constitutive CD22 tyrosine phosphorylation and decreased Ca(2+) response in B cells. Biochemical analysis reveals that, in response to IgM cross-linking, the phosphorylation of Syk, Blnk, or PLCgamma2 and their interactions with one another were either diminished or completely abolished in SAP-expressing cells compared to cells that lack SAP. Collectively our work identifies a novel role for SAP in B cells and extends its function to inhibitory immunoreceptor signaling and calcium mobilization.

SAP-controlled T-B cell interactions underlie germinal centre formation

Generation of long-term antibody-mediated immunity depends on the germinal centre (GC) reaction, which requires cooperation between antigen-specific T and B lymphocytes. In the human X-linked lymphoproliferative disease and its gene-targeted mouse model, loss-of-function mutations in signalling lymphocyte activation molecule-associated protein (SAP, encoded by SH2D1a) cause a profound defect in GC formation by an as yet unknown mechanism. Using two-photon intravital imaging, here we show that SAP deficiency selectively impairs the ability of CD4⁺ T cells to stably interact with cognate B cells but not antigen-presenting dendritic cells. This selective defect results in a failure of antigen-specific B cells to receive adequate levels of contact-dependent T cell help to expand normally, despite sap^−/− T cells exhibiting the known characteristics of otherwise competent helper T cells. Furthermore, lack of stable interactions with B cells renders sap^−/− T cells unable to be efficiently recruited to and retained in a nascent GC to sustain the GC reaction. These results offer a compelling explanation for the GC defect due to SAP deficiency and provide novel insights into the bi-directional communication between cognate T and B cells in vivo.

The SLAM and SAP gene families control innate and adaptive immune responses

The nine SLAM-family genes, SLAMF1-9, a subfamily of the immunoglobulin superfamily, encode differentially expressed cell-surface receptors of hematopoietic cells. Engagement with their ligands, which are predominantly homotypic, leads to distinct signal transduction events, for instance those that occur in the T or NK cell immune synapse. Upon phosphorylation of one or more copies of a unique tyrosine-based signaling motif in their cytoplasmic tails, six of the SLAM receptors recruit the highly specific single SH2-domain adapters SLAM-associated protein (SAP), EAT-2A, and/or EAT-2B. These adapters in turn bind to the tyrosine kinase Fyn and/or other protein tyrosine kinases connecting the receptors to signal transduction networks. Individuals deficient in the SAP gene, SH2D1A, develop an immunodeficiency syndrome: X-linked lympho-proliferative disease. In addition to operating in the immune synapse, SLAM receptors initiate or partake in multiple effector functions of hematopoietic cells, for example, neutrophil and macrophage killing and platelet aggregation. Here we discuss the current understanding of the structure and function of these recently discovered receptors and adapter molecules in the regulation of adaptive and innate immune responses.

Epstein-Barr virus persistence in the absence of conventional memory B cells: IgM+IgD+CD27+ B cells harbor the virus in X-linked lymphoproliferative disease patients

Epstein-Barr virus (EBV) persists in healthy virus carriers within the immunoglobulin (Ig)D−CD27+ (class-switched) memory B-cell compartment that normally arises through antigen stimulation and germinal center transit. Patients with X-linked lymphoproliferative disease (XLP) lack such class-switched memory B cells but are highly susceptible to EBV infection, often developing fatal symptoms resembling those seen in EBV-associated hemophagocytic syndrome (EBV-AHS), a disease caused by aberrant virus entry into the NK- or T-cell system. Here we show that XLP patients who survive primary EBV exposure carry relatively high virus loads in the B-cell, but not the NK- or T-cell, compartment. Interestingly, in the absence of conventional class-switched memory B cells, the circulating EBV load was concentrated within a small population of IgM+IgD+CD27+ (nonswitched) memory cells rather than within the numerically dominant naive (IgM+IgD+CD27−) or transitional (CD10+CD27−) subsets. In 2 prospectively studied patients, the circulating EBV load was stable and markers of virus polymorphism detected the same resident strain over time. These results provide the first definitive evidence that EBV can establish persistence in the B-cell system in the absence of fully functional germinal center activity and of a class-switched memory B-cell compartment.

A gammaherpesvirus-secreted activator of Vbeta4+ CD8+ T cells regulates chronic infection and immunopathology

Little is known about herpesvirus modulation of T cell activation in latently infected individuals or the implications of such for chronic immune disorders. Murine gammaherpesvirus 68 (MHV68) elicits persistent activation of CD8(+) T cells bearing a Vbeta4(+) T cell receptor (TCR) by a completely unknown mechanism. We show that a novel MHV68 protein encoded by the M1 gene is responsible for Vbeta4(+) CD8(+) T cell stimulation in a manner reminiscent of a viral superantigen. During infection, M1 expression induces a Vbeta4(+) effector T cell response that resists functional exhaustion and appears to suppress virus reactivation from peritoneal cells by means of long-term interferon-gamma (IFNgamma) production. Mice lacking an IFNgamma receptor (IFNgammaR(-/-)) fail to control MHV68 replication, and Vbeta4(+) and CD8(+) T cell activation by M1 instead contributes to severe inflammation and multiorgan fibrotic disease. Thus, M1 manipulates the host CD8(+) T cell response in a manner that facilitates latent infection in an immunocompetent setting, but promotes disease during a dysregulated immune response. Identification of a viral pathogenecity determinant with superantigen-like activity for CD8(+) T cells broadens the known repertoire of viral immunomodulatory molecules, and its function illustrates the delicate balance achieved between persistent viruses and the host immune response.

Extralymphatic virus sanctuaries as a consequence of potent T-cell activation

T helper cells can support the functions of CD8(+) T cells against persistently infecting viruses such as murine lymphocytic choriomeningitis virus (LCMV), cytomegalovirus, hepatitis C virus and HIV. These viruses often resist complete elimination and remain detectable at sanctuary sites, such as the kidneys and other extralymphatic organs. The mechanisms underlying this persistence are not well understood. Here we show that mice with potent virus-specific T-cell responses have reduced levels and delayed formation of neutralizing antibodies, and these mice fail to clear LCMV from extralymphatic epithelia. Transfer of virus-specific B cells but not virus-specific T cells augmented virus clearance from persistent sites. Virus elimination from the kidneys was associated with the formation of IgG deposits in the interstitial space, presumably from kidney-infiltrating B cells. CD8(+) T cells in the kidneys of mice that did not clear virus from this site were activated but showed evidence of exhaustion. Thus, we conclude that in this model of infection, site-specific virus persistence develops as a consequence of potent immune activation coupled with reductions in virus-specific neutralizing antibodies. Our results suggest that sanctuary-site formation depends both on organ anatomy and on the induction of different adaptive immune effector mechanisms. Boosting T-cell responses alone may not reduce virus persistence.

Quantitative PCR technique for detecting lymphocytic choriomeningitis virus in vivo

Quantitative PCR (QPCR, or real time PCR (rtPCR)) has emerged as a powerful virologic technique for measuring viral replication and viral loads in humans and animal models. We have developed a QPCR assay to accurately quantify lymphocytic choriomeningitis virus (LCMV) in infected mice. We first validated this assay using plasmid DNA and LCMV viral stocks. We then demonstrated that the LCMV QPCR assay can detect LCMV in serum and tissues of chronically infected mice (LCMV-clone 13), with greater sensitivity than conventional plaque assay. Subsequently, we demonstrated that the QPCR assay can detect LCMV in tissues of CD40L(-/-) mice during a low grade chronic infection with LCMV Armstrong. Finally, we improved the assay further such that it was approximate 1000-fold more sensitive than plaque assay for detection of the presence of LCMV in tissue.

Chronic lymphocytic choriomeningitis virus infection actively down-regulates CD4+ T cell responses directed against a broad range of epitopes

Activation of CD4(+) T cells helps establish and sustain CD8(+) T cell responses and is required for the effective clearance of acute infection. CD4-deficient mice are unable to control persistent infection and CD4(+) T cells are usually defective in chronic and persistent infections. We investigated the question of how persistent infection impacted pre-existing lymphocytic choriomeningitis virus (LCMV)-specific CD4(+) T cell responses. We identified class II-restricted epitopes from the entire set of open reading frames from LCMV Armstrong in BALB/c mice (H-2(d)) acutely infected with LCMV Armstrong. Of nine epitopes identified, six were restricted by I-A(d), one by I-E(d) and two were dually restricted by both I-A(d) and I-E(d) molecules. Additional experiments revealed that CD4(+) T cell responses specific for these epitopes were not generated following infection with the immunosuppressive clone 13 strain of LCMV. Most importantly, in peptide-immunized mice, established CD4(+) T cell responses to these LCMV CD4 epitopes as well as nonviral, OVA-specific responses were actively suppressed following infection with LCMV clone 13 and were undetectable within 12 days after infection, suggesting an active inhibition of established helper responses. To address this dysfunction, we performed transfer experiments using both the Smarta and OT-II systems. OT-II cells were not detected after clone 13 infection, indicating physical deletion, while Smarta cells proliferated but were unable to produce IFN-gamma, suggesting impairment of the production of this cytokine. Thus, multiple mechanisms may be involved in the impairment of helper responses in the setting of early persistent infection.

Perturbation of B cell activation in SLAM-associated protein-deficient mice is associated with changes in gammaherpesvirus latency reservoirs

Signaling lymphocyte activation molecule (SLAM)-associated protein (SAP)) interactions with SLAM family proteins play important roles in immune function. SAP-deficient mice have defective B cell function, including impairment of germinal center formation, production of class-switched Ig, and development of memory B cells. B cells are the major reservoir of latency for both EBV and the homologous murine gammaherpesvirus, gammaherpesvirus 68. There is a strong association between the B cell life cycle and viral latency in that the virus preferentially establishes latency in activated germinal center B cells, which provides access to memory B cells, a major reservoir of long-term latency. In the current studies, we have analyzed the establishment and maintenance of gammaHV68 latency in wild-type and SAP-deficient mice. The results show that, despite SAP-associated defects in germinal center and memory B cell formation, latency was established and maintained in memory B cells at comparable frequencies to wild-type mice, although the paucity of memory B cells translated into a 10-fold reduction in latent load. Furthermore, there were defects in normal latency reservoirs within the germinal center cells and IgD(+)"naive" B cells in SAP-deficient mice, showing a profound effect of the SAP mutation on latency reservoirs.

The role of SLAM family receptors in immune cell signaling

The signaling lymphocyte-activating molecule (SLAM) family immunoreceptors are expressed in a wide array of immune cells, including both T and B lymphocytes. By virtue of their ability to transduce tyrosine phosphorylation signals through the so-called ITSM (immunoreceptor tyrosine-based switch motif) sequences, they play an important part in regulating both innate and adaptive immune responses. The critical role of the SLAM immunoreceptors in mediating normal immune reactions was highlighted in recent findings that SAP, a SLAM-associated protein, modulates the activities of various immune cells through interactions with different members of the SLAM family expressed in these cells. Importantly, mutations or deletions of the sap gene in humans result in the X-linked lymphoproliferative syndrome. In this review, we summarize current knowledge and survey the latest developments in signal transduction events triggered by the activation of SLAM family receptors in different cell types.

SAP regulation of follicular helper CD4 T cell development and humoral immunity is independent of SLAM and Fyn kinase

Mutations in SH2D1A resulting in lack of SLAM-associated protein (SAP) expression cause the human genetic immunodeficiency X-linked lymphoproliferative disease. A severe block in germinal center development and lack of long-term humoral immunity is one of the most prominent phenotypes of SAP(-) mice. We show, in this study, that the germinal center block is due to an essential requirement for SAP expression in Ag-specific CD4 T cells to develop appropriate follicular helper T cell functions. It is unknown what signaling molecules are involved in regulation of SAP-dependent CD4 T cell help functions. SAP binds to the cytoplasmic tail of SLAM, and we show that SLAM is expressed on resting and activated CD4 T cells, as well as germinal center B cells. In addition, SAP can recruit Fyn kinase to SLAM. We have now examined the role(s) of the SLAM-SAP-Fyn signaling axis in in vivo CD4 T cell function and germinal center development. We observed normal germinal center development, long-lived plasma cell development, and Ab responses in SLAM(-/-) mice after a viral infection (lymphocytic choriomeningitis virus). In a separate series of experiments, we show that SAP is absolutely required in CD4 T cells to drive germinal center development, and that requirement does not depend on SAP-Fyn interactions, because CD4 T cells expressing SAP R78A are capable of supporting normal germinal center development. Therefore, a distinct SAP signaling pathway regulates follicular helper CD4 T cell differentiation, separate from the SLAM-SAP-Fyn signaling pathway regulating Th1/Th2 differentiation.

The role of SAP and the SLAM family in autoimmunity

The signaling lymphocyte activation molecule (SLAM) family of receptors and their associated signaling adaptors play a pivotal role in the regulation of various stages of cellular immunity. They regulate lymphocyte-lymphocyte interactions involved in both cell-mediated and humoral immune responses. Recent evidence indicates that members of this family of receptors and signaling intermediates are also involved in autoimmunity. These include strictly correlative studies showing increased expression of various family members in immune effectors involved in rheumatoid arthritis and in inflammatory bowel disease, as well as more direct evidence (from various knockout strains of mice) for their role in autoimmune processes such as experimental allergic encephalomyelitis and lupus. Additional studies defining naturally occurring polymorphic variations in the SLAM family show a direct role in initiating the break in tolerance that is an essential step in the progression towards autoimmunity.