Overlapping Neuroimmune Mechanisms and Therapeutic Targets in Neurodegenerative Disorders

Many potential immune therapeutic targets are similarly affected in adult-onset neurodegenerative diseases, such as Alzheimer's (AD) disease, Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and frontotemporal dementia (FTD), as well as in a seemingly distinct Niemann-Pick type C disease with primarily juvenile onset. This strongly argues for an overlap in pathogenic mechanisms. The commonly researched immune targets include various immune cell subsets, such as microglia, peripheral macrophages, and regulatory T cells (Tregs); the complement system; and other soluble factors. In this review, we compare these neurodegenerative diseases from a clinical point of view and highlight common pathways and mechanisms of protein aggregation, neurodegeneration, and/or neuroinflammation that could potentially lead to shared treatment strategies for overlapping immune dysfunctions in these diseases. These approaches include but are not limited to immunisation, complement cascade blockade, microbiome regulation, inhibition of signal transduction, Treg boosting, and stem cell transplantation.

Neuroimmune characterization of optineurin insufficiency mouse model during ageing

Optineurin is a multifunctional polyubiquitin-binding protein implicated in inflammatory signalling. Optineurin mutations are associated with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), neurodegenerative diseases characterised by neuronal loss, neuroinflammation, and peripheral immune disbalance. However, the pathogenic role of optineurin mutations is unclear. We previously observed no phenotype in the unmanipulated young optineurin insufficiency mice (Optn^470T), designed to mimic ALS/FTD-linked truncations deficient in polyubiquitin binding. The purpose of this study was to investigate whether ageing would trigger neurodegeneration. We performed a neurological, neuropathological, and immunological characterization of ageing wild-type (WT) and Optn^470T mice. No motor or cognitive differences were detected between the genotypes. Neuropathological analyses demonstrated signs of ageing including lipofuscin accumulation and microglial activation in WT mice. However, this was not worsened in Optn^470T mice, and they did not exhibit TAR DNA-binding protein 43 (TDP-43) aggregation or neuronal loss. Spleen immunophenotyping uncovered T cell immunosenescence at two years but without notable differences between the WT and Optn^470T mice. Conventional dendritic cells (cDC) and macrophages exhibited increased expression of activation markers in two-year-old Optn^470T males but not females, although the numbers of innate immune cells were similar between genotypes. Altogether, a combination of optineurin insufficiency and ageing did not induce ALS/FTD-like immune imbalance and neuropathology in mice.

Emerging Trends in the Field of Inflammation and Proteinopathy in ALS/FTD Spectrum Disorder

Proteinopathy and neuroinflammation are two main hallmarks of neurodegenerative diseases. They also represent rare common events in an exceptionally broad landscape of genetic, environmental, neuropathologic, and clinical heterogeneity present in patients. Here, we aim to recount the emerging trends in amyotrophic lateral sclerosis (ALS) and frontotemporal degeneration (FTD) spectrum disorder. Our review will predominantly focus on neuroinflammation and systemic immune imbalance in ALS and FTD, which have recently been highlighted as novel therapeutic targets. A common mechanism of most ALS and ~50% of FTD patients is dysregulation of TAR DNA-binding protein 43 (TDP-43), an RNA/DNA-binding protein, which becomes depleted from the nucleus and forms cytoplasmic aggregates in neurons and glia. This, in turn, via both gain and loss of function events, alters a variety of TDP-43-mediated cellular events. Experimental attempts to target TDP-43 aggregates or manipulate crosstalk in the context of inflammation will be discussed. Targeting inflammation, and the immune system in general, is of particular interest because of the high plasticity of immune cells compared to neurons.

Ageing-Induced Decline in Primary Myeloid Cell Phagocytosis Is Unaffected by Optineurin Insufficiency

Optineurin is a ubiquitin-binding adaptor protein involved in multiple cellular processes, including innate inflammatory signalling. Mutations in optineurin were found in amyotrophic lateral sclerosis, an adult-onset fatal neurodegenerative disease that targets motor neurons. Neurodegeneration results in generation of neuronal debris, which is primarily cleared by myeloid cells. To assess the role of optineurin in phagocytosis, we performed a flow cytometry-based phagocytic assay of apoptotic neuronal debris and E. coli bioparticles in bone marrow-derived macrophages (BMDMs), and primary neonatal microglia from wild-type (WT) and optineurin-insufficient (Optn^470T) mice. We found no difference in phagocytosis efficiency and the accompanying cytokine secretion in WT and Optn^470T BMDMs and microglia. This was true at both steady state and upon proinflammatory polarization with lipopolysaccharide. When we analysed the effect of ageing as a major risk factor for neurodegeneration, we found a substantial decrease in the percentage of phagocytic cells and proinflammatory cytokine secretion in BMDMs from 2-year-old mice. However, this ageing-induced phagocytic decline was unaffected by optineurin insufficiency. All together, these results indicate that ageing is the factor that perturbs normal phagocytosis and proinflammatory cytokine secretion, but that optineurin is dispensable for these processes.

Optineurin Deficiency and Insufficiency Lead to Higher Microglial TDP-43 Protein Levels

Mutations in optineurin, a ubiquitin-binding adaptor protein, cause amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease of motor neurons linked to chronic inflammation and protein aggregation. The majority of ALS patients, including those carrying the optineurin mutations, exhibit cytoplasmic mislocalization, ubiquitination, and aggregation of nuclear TAR DNA-binding protein 43 kDa (TDP-43). To address the crosstalk between optineurin and TDP-43, we generated optineurin knockout (KO) neuronal and microglial cell lines using the CRISPR/Cas9 approach. Interestingly, we observed that loss of optineurin resulted in elevated TDP-43 protein expression in microglial BV2 but not neuronal Neuro 2a and NSC-34 cell lines. No changes were observed at the mRNA level, suggesting that this increase was post-translationally regulated. To confirm this observation in primary cells, we then used microglia and macrophages from an optineurin loss-of-function mouse model that lacks the C-terminal ubiquitin-binding region (Optn^470T), mimicking optineurin truncations in ALS patients. As observed in the BV2 cells, we also found elevated basal levels of TDP-43 protein in Optn^470T microglia and bone marrow-derived macrophages. To test if inflammation could further enhance TDP-43 accumulation in cells lacking functional optineurin, we stimulated them with lipopolysaccharide (LPS), and we observed a significant increase in TDP-43 expression following LPS treatment of WT cells. However, this was absent in both BV2 Optn KO and primary Optn^470T microglia, which exhibited the same elevated TDP-43 levels as in basal conditions. Furthermore, we did not observe nuclear TDP-43 depletion or cytoplasmic aggregate formation in either Optn^470T microglia or LPS-treated WT or Optn^470T microglia. Taken together, our results show that optineurin deficiency and insufficiency post-translationally upregulate microglial TDP-43 protein levels and that elevated TDP-43 levels in cells lacking functional optineurin could not be further increased by an inflammatory stimulus, suggesting the presence of a plateau.

Interplay between immunity and amyotrophic lateral sclerosis: Clinical impact

Amyotrophic lateral sclerosis (ALS) is a debilitating and rapidly fatal neurodegenerative disease. Despite decades of research and many new insights into disease biology over the 150 years since the disease was first described, causative pathogenic mechanisms in ALS remain poorly understood, especially in sporadic cases. Our understanding of the role of the immune system in ALS pathophysiology, however, is rapidly expanding. The aim of this manuscript is to summarize the recent advances regarding the immune system involvement in ALS, with particular attention to clinical translation. We focus on the potential pathophysiologic mechanism of the immune system in ALS, discussing local and systemic factors (blood, cerebrospinal fluid, and microbiota) that influence ALS onset and progression in animal models and people. We also explore the potential of Positron Emission Tomography to detect neuroinflammation in vivo, and discuss ongoing clinical trials of therapies targeting the immune system. With validation in human patients, new evidence in this emerging field will serve to identify novel therapeutic targets and provide realistic hope for personalized treatment strategies.

Crosstalk of TDP-43 and Optineurin in in vitro and ex vivo Models of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease marked by protein aggregation and neuroinflammation. Protein aggregates in >95% of ALS cases are cytoplasmic and contain ubiquitinated and phosphorylated TAR DNA binding protein 43 kDa (TDP-43). Mutations in a ubiquitin-binding adaptor protein optineurin have recently been found in a subset of ALS cases. Its mutations are thought to act by loss of function, leading to disbalanced inflammatory signaling and/or impaired disposal of aggregated proteins by autophagy. Here we addressed the putative link between TDP-43 proteinopathy and loss of optineurin and/or its function. To this end we generated optineurin knockout (KO) neuronal and microglial cell lines by CRISPR/Cas9 technology, and (2) primary cells from a mouse optineurin insufficiency model lacking the ubiquitin-binding region (Optn470T). Elevated TDP-43 protein levels were found in optineurin-deficient BV2 microglial cell line, and in primary Optn470T bone marrow-derived macrophages (BMDM) and microglia. No differences were detected at TDP-43 mRNA levels, arguing for post-translational regulation. Elevated TDP-43 protein levels were not caused by autophagy blockade, and were specific to myeloid cells as they were not observed in neuronal NSC-34 and Neuro2A optineurin-deficient cell lines. Upon lipopolysaccharide (LPS) stimulation, TDP-43 levels increased in WT BV2 cells and BMDM, but in BV2 KO and OPTN470T BMDM they remained at the same elevated state as in basal conditions. Our results show that optineurin directly influences basal TDP-43 protein levels in myeloid but not in neuronal cells. Further studies are needed to determine if this could be a mechanistic link to protein aggregation in ALS.

Immunity in amyotrophic lateral sclerosis: blurred lines between excessive inflammation and inefficient immune responses

Despite wide genetic, environmental and clinical heterogeneity in amyotrophic lateral sclerosis, a rapidly fatal neurodegenerative disease targeting motoneurons, neuroinflammation is a common finding. It is marked by local glial activation, T cell infiltration and systemic immune system activation. The immune system has a prominent role in the pathogenesis of various chronic diseases, hence some of them, including some types of cancer, are successfully targeted by immunotherapeutic approaches. However, various anti-inflammatory or immunosuppressive therapies in amyotrophic lateral sclerosis have failed. This prompted increased scrutiny over the immune-mediated processes underlying amyotrophic lateral sclerosis. Perhaps the biggest conundrum is that amyotrophic lateral sclerosis pathogenesis exhibits features of three otherwise distinct immune dysfunctions-excessive inflammation, autoimmunity and inefficient immune responses. Epidemiological and genome-wide association studies show only minimal overlap between amyotrophic lateral sclerosis and autoimmune diseases, so excessive inflammation is usually thought to be secondary to protein aggregation, mitochondrial damage or other stresses. In contrast, several recently characterized amyotrophic lateral sclerosis-linked mutations, including those in TBK1, OPTN, CYLD and C9orf72, could lead to inefficient immune responses and/or damage pile-up, suggesting that an innate immunodeficiency may also be a trigger and/or modifier of this disease. In such cases, non-selective immunosuppression would further restrict neuroprotective immune responses. Here we discuss multiple layers of immune-mediated neuroprotection and neurotoxicity in amyotrophic lateral sclerosis. Particular focus is placed on individual patient mutations that directly or indirectly affect the immune system, and the mechanisms by which these mutations influence disease progression. The topic of immunity in amyotrophic lateral sclerosis is timely and relevant, because it is one of the few common and potentially malleable denominators in this heterogenous disease. Importantly, amyotrophic lateral sclerosis progression has recently been intricately linked to patient T cell and monocyte profiles, as well as polymorphisms in cytokine and chemokine receptors. For this reason, precise patient stratification based on immunophenotyping will be crucial for efficient therapies.

Glia in amyotrophic lateral sclerosis and spinal cord injury: common therapeutic targets

The toolkit for repairing damaged neurons in amyotrophic lateral sclerosis (ALS) and spinal cord injury (SCI) is extremely limited. Here, we reviewed the in vitro and in vivo studies and clinical trials on nonneuronal cells in the neurodegenerative processes common to both these conditions. Special focus was directed to microglia and astrocytes, because their activation and proliferation, also known as neuroinflammation, is a key driver of neurodegeneration. Neuroinflammation is a multifaceted process that evolves during the disease course, and can be either beneficial or toxic to neurons. Given the fundamental regulatory functions of glia, pathogenic mechanisms in neuroinflammation represent promising therapeutic targets. We also discussed neuroprotective, immunosuppressive, and stem-cell based approaches applicable to both ALS and SCI.

Optineurin in amyotrophic lateral sclerosis: Multifunctional adaptor protein at the crossroads of different neuroprotective mechanisms

When optineurin mutations showed up on the amyotrophic lateral sclerosis (ALS) landscape in 2010, they differed from most other ALS-causing genes. They seemed to act by loss- rather than gain-of-function, and it was unclear how a polyubiquitin-binding adaptor protein, which was proposed to regulate a variety of cellular functions including cell signaling and vesicle trafficking, could mediate neuroprotection. This review discusses the considerable progress that has been made since then. A large number of mutations in optineurin and optineurin-interacting proteins TANK-binding kinase (TBK1) and p62/SQSTM-1 have been found in the ALS patients, suggesting a common neuroprotective pathway. Moreover, functional studies of the ALS-causing optineurin mutations and the recently established optineurin ubiquitin-binding deficient and knockout mouse models helped identify three major mechanisms likely to mediate neuroprotection: regulation of autophagy, mitigation of (chronic) inflammatory signaling, and blockade of necroptosis. These three processes crosstalk, and require multiple levels of control, many of which can be mediated by optineurin. Based on the role of optineurin in multiple processes and the unexpected finding that targeted optineurin deletion in microglia and oligodendrocytes ultimately leads to the same phenotype of axonal degeneration despite different initial defects, we propose that the failure of the weakest link in the optineurin neuroprotective network is sufficient to disturb homeostasis and set-off the domino effect that could ultimately lead to neurodegeneration.

Porphyrin-based cationic amphiphilic photosensitisers as potential anticancer, antimicrobial and immunosuppressive agents

Photodynamic therapy (PDT) combines a photosensitiser, light and molecular oxygen to induce oxidative stress that can be used to kill pathogens, cancer cells and other highly proliferative cells. There is a growing number of clinically approved photosensitisers and applications of PDT, whose main advantages include the possibility of selective targeting, localised action and stimulation of the immune responses. Further improvements and broader use of PDT could be accomplished by designing new photosensitisers with increased selectivity and bioavailability. Porphyrin-based photosensitisers with amphiphilic properties, bearing one or more positive charges, are an effective tool in PDT against cancers, microbial infections and, most recently, autoimmune skin disorders. The aim of the review is to present some of the recent examples of the applications and research that employ this specific group of photosensitisers. Furthermore, we will highlight the link between their structural characteristics and PDT efficiency, which will be helpful as guidelines for rational design and evaluation of new PSs.

The Golgi apparatus acts as a platform for TBK1 activation after viral RNA sensing

BACKGROUND

After viral infection and the stimulation of some pattern-recognition receptors, TANK-binding kinase I (TBK1) is activated by K63-linked polyubiquitination followed by trans-autophosphorylation. While the activated TBK1 induces type I interferon production by phosphorylating the transcription factor IRF3, the precise molecular mechanisms underlying TBK1 activation remain unclear.

RESULTS

We report here the localization of the ubiquitinated and phosphorylated active form of TBK1 to the Golgi apparatus after the stimulation of RIG-I-like receptors (RLRs) or Toll-like receptor-3 (TLR3), due to TBK1 K63-linked ubiquitination on lysine residues 30 and 401. The ubiquitin-binding protein optineurin (OPTN) recruits ubiquitinated TBK1 to the Golgi apparatus, leading to the formation of complexes in which TBK1 is activated by trans-autophosphorylation. Indeed, OPTN deficiency in various cell lines and primary cells impairs TBK1 targeting to the Golgi apparatus and its activation following RLR or TLR3 stimulation. Interestingly, the Bluetongue virus NS3 protein binds OPTN at the Golgi apparatus, neutralizing its activity and thereby decreasing TBK1 activation and downstream signaling.

CONCLUSIONS

Our results highlight an unexpected role of the Golgi apparatus in innate immunity as a key subcellular gateway for TBK1 activation after RNA virus infection.

The TBK1-binding domain of optineurin promotes type I interferon responses

Pathogen-associated molecular pattern (PAMP) recognition leads to TANK-binding kinase (TBK1) polyubiquitination and activation by transautophosphorylation, resulting in IFN-β production. Here, we describe a mouse model of optineurin insufficiency (OptnΔ(157) ) in which the TBK1-interacting N-terminus of optineurin was deleted. PAMP-stimulated cells from OptnΔ(157) mice had reduced TBK1 activity, no phosphorylation of optineurin Ser(187) , and mounted low IFN-β responses. In contrast to pull-down assays where the presence of N-terminus was sufficient for TBK1 binding, both the N-terminal and the ubiquitin-binding regions of optineurin were needed for PAMP-induced binding. This report establishes optineurin as a positive regulator TBK1 via a bipartite interaction between these molecules.

Suppression of Dendritic Cell-Derived IL-12 by Endogenous Glucocorticoids Is Protective in LPS-Induced Sepsis

Sepsis, an exaggerated systemic inflammatory response, remains a major medical challenge. Both hyperinflammation and immunosuppression are implicated as causes of morbidity and mortality. Dendritic cell (DC) loss has been observed in septic patients and in experimental sepsis models, but the role of DCs in sepsis, and the mechanisms and significance of DC loss, are poorly understood. Here, we report that mice with selective deletion of the glucocorticoid receptor (GR) in DCs (GR^CD11c-cre) were highly susceptible to LPS-induced septic shock, evidenced by elevated inflammatory cytokine production, hypothermia, and mortality. Neutralizing anti-IL-12 antibodies prevented hypothermia and death, demonstrating that endogenous GC-mediated suppression of IL-12 is protective. In LPS-challenged GR^CD11c-cre mice, CD8⁺ DCs were identified as the major source of prolonged IL-12 production, which correlated with elevations of NK cell-derived IFN-γ. In addition, the loss of GR in CD11c⁺ cells rescued LPS-induced loss of CD8⁺ DCs but not other DC subsets. Unlike wild-type animals, exposure of GR^CD11c-cre mice to low-dose LPS did not induce CD8⁺ DC loss or tolerance to subsequent challenge with high dose, but neutralization of IL-12 restored the ability of low-dose LPS to tolerize. Therefore, endogenous glucocorticoids blunt LPS-induced inflammation and promote tolerance by suppressing DC IL-12 production.

Lipopolysaccharide (LPS) from bacteria causes the increased production of endogenous glucocorticoids, protecting mice from sepsis and contributing to LPS tolerance by suppressing production of interleukin-12 (IL-12) by dendritic cells and causing the death of the primary producers of IL-12. Read the Synopsis.

Sepsis refers to life-threatening systemic inflammation, often caused by infection with bacteria that produce lipopolysaccharide (LPS). Glucocorticoids, immunosuppressive hormones produced by the adrenals, have been used to treat sepsis for over 50 y, but little is known about the role of endogenous (naturally occurring) glucocorticoids in systemic inflammation. Macrophages have been considered the primary source of inflammatory mediators (cytokines) and a target for glucocorticoid-mediated suppression. The possible role of another immune cell population, dendritic cells, has not been explored in detail. We created a mouse model in which the glucocorticoid receptor is selectively deleted in dendritic cells (DCs). We found that the elevation of glucocorticoids that accompanies sepsis protects mice from LPS-induced septic shock by suppressing DC production of IL-12, a cytokine that causes the secretion of other inflammatory mediators. In addition, LPS-induced glucocorticoids caused the death of a subset of DCs that are the primary producers of IL-12. Glucocorticoids were also found to be important for the phenomenon of "LPS tolerance", in which inoculation with low-dose LPS makes mice resistant to rechallenge with a high dose. This unexpected role of DC-produced IL-12 and its suppression by endogenous glucocorticoids may account, at least in part, for the known association of adrenal insufficiency and prolonged sepsis.

c-IAP ubiquitin protein ligase activity is required for 4-1BB signaling and CD8(+) memory T-cell survival

Cellular inhibitor of apoptosis proteins (c-IAP) 1 and 2 are widely expressed ubiquitin protein ligases that regulate a variety of cellular functions, including the sensitivity of T cells to costimulation. 4-1BB is a TNF receptor family member that signals via a complex that includes TRAF family members and the c-IAPs to upregulate NF-κB and ERK, and has been implicated in memory T-cell survival. Here, we show that effector and memory T cells from mice expressing a dominant negative E3-inactive c-IAP2 (c-IAP2(H570A)) have impaired signaling downstream of 4-1BB. When infected with lymphocytic choriomeningitis virus, unlike mice in which c-IAPs were acutely downregulated by c-IAP antagonists, the primary response of c-IAP2(H570A) mice was normal. However, the number of antigen-specific CD8(+) but not CD4(+) T cells declined more rapidly and to a greater extent in c-IAP2(H570A) mice than in WT controls. Studies with T-cell adoptive transfer demonstrated that the enhanced decay of memory cells was T-cell intrinsic. Thus, c-IAP E3 activity is required for 4-1BB coreceptor signaling and maintenance of CD8(+) T-cell memory.

Optineurin is dispensable for LPS- and Salmonella typhimurium-induced autophagy (INM3P.412)

In vitro studies demonstrated that optineurin, a ubiquitin (Ub)-binding protein, becomes an autophagy cargo receptor upon being phosphorylated on Ser177 by TANK binding kinase 1 (TBK1) in response to cytosolic Salmonella typhimurium. Phosphorylated optineurin then regulates autophagy by delivering ubiquinated bacteria to autophagosomal membranes via light chain 3 (LC3), hence restricting their growth. To test the in vivo relevance of these findings, we used a mouse model with C-terminal optineurin truncation, which lacks the Ub-binding region (Optn470T). No difference was found between WT and Optn470T macrophages in LC3 lipidation during LPS-induced autophagy. Similarly, p62+LC3+ aggresome-like structures generated upon LPS or Salmonella typhimurium stimulation in Optn470T macrophages colocalized with lysosomal marker LAMP1 to the same extent as in WT cells, demonstrating no perturbation in autophagosome fusion to lysosomes. Importantly, upon in vivo infection with Salmonella typhimurium, no difference was found in bacterial replication in WT and Optn470T mice. These results argue against the role of optineurin in Salmonella typhimurium infection in vivo.

Optineurin insufficiency impairs IRF3 but not NF-κB activation in immune cells

Optineurin is a widely expressed polyubiquitin-binding protein that has been implicated in regulating cell signaling via its NF-κB essential modulator-homologous C-terminal ubiquitin (Ub)-binding region. Its functions are controversial, with in vitro studies finding that optineurin suppressed TNF-mediated NF-κB activation and virus-induced activation of IFN regulatory factor 3 (IRF3), whereas bone marrow-derived macrophages (BMDMs) from mice carrying an optineurin Ub-binding point mutation had normal TLR-mediated NF-κB activation and diminished IRF3 activation. We have generated a mouse model in which the entire Ub-binding C-terminal region is deleted (Optn(470T)). Akin to C-terminal optineurin mutations found in patients with certain neurodegenerative diseases, Optn(470T) was expressed at substantially lower levels than the native protein, allowing assessment not only of the lack of Ub binding, but also of protein insufficiency. Embryonic lethality with incomplete penetrance was observed for 129 × C57BL/6 Optn(470T/470T) mice, but after further backcrossing to C57BL/6, offspring viability was restored. Moreover, the mice that survived were indistinguishable from wild type littermates and had normal immune cell distributions. Activation of NF-κB in Optn(470T) BMDM and BM-derived dendritic cells with TNF or via TLR4, T cells via the TCR, and B cells with LPS or anti-CD40 was normal. In contrast, optineurin and/or its Ub-binding function was necessary for optimal TANK binding kinase 1 and IRF3 activation, and both Optn(470T) BMDMs and bone marrow-derived dendritic cells had diminished IFN-β production upon LPS stimulation. Importantly, Optn(470T) mice produced less IFN-β upon LPS challenge. Therefore, endogenous optineurin is dispensable for NF-κB activation but necessary for optimal IRF3 activation in immune cells.

CD70 is downregulated by interaction with CD27

Engagement of the receptor CD27 by CD70 affects the magnitude and quality of T cell responses in a variety of infection models, and exaggerated signaling via this pathway results in enhanced immune responses and autoimmunity. One means by which signaling is regulated is tight control of cell surface CD70, which is expressed on dendritic cells (DCs), T cells, and B cells only upon activation. In this article, we show that a second level of regulation also is present. First, although undetectable on the cell surface by flow cytometry, immature DCs have a small pool of CD70 that continuously recycles from the plasma membrane. In addition, surface levels of CD70 on DCs and T cells were higher in mice deficient in CD27, or on DCs for which the interaction between CD70 and CD27 was precluded by blocking Abs. Binding of CD70 by its receptor resulted in downregulation of CD70 transcription and protein levels, suggesting that CD70-mediated "reverse signals" regulate its own levels. Therefore, the ability of CD70 to trigger costimulation is self-regulated when it binds its complementary receptor.

CD70 deficiency impairs effector CD8 T cell generation and viral clearance but is dispensable for the recall response to lymphocytic choriomeningitis virus

CD27 interactions with its ligand, CD70, are thought to be necessary for optimal primary and memory adaptive immune responses to a variety of pathogens. Thus far, all studies addressing the function of the CD27-CD70 axis have been performed in mice lacking CD27, in those overexpressing CD70, or in those in which these molecules were blocked or mimicked by Abs or recombinant soluble CD70. Because these methods have in some cases led to divergent results, we generated CD70-deficient mice to directly assess its role in vivo. We find that lack of CD70-mediated stimulation during primary responses to lymphocytic choriomeningitis virus lowered the magnitude of CD8 Ag-specific T cell response, resulting in impaired viral clearance, without affecting CD4 T cell responses. Unexpectedly, CD70-CD27 costimulation was not needed for memory CD8 T cell generation or the ability to mount a recall response to lymphocytic choriomeningitis virus. Adoptive transfers of wild-type memory T cells into CD70(-/-) or wild-type hosts also showed no need for CD70-mediated stimulation during the course of the recall response. Moreover, CD70 expression by CD8 T cells could not rescue endogenous CD70(-/-) cells from defective expansion, arguing against a role for CD70-mediated T:T help in this model. Therefore, CD70 appears to be an important factor in the initiation of a robust and effective primary response but dispensable for CD8 T cell memory responses.

Gamma(c) deficiency precludes CD8+ T cell memory despite formation of potent T cell effectors

Several cytokines (including IL-2, IL-7, IL-15, and IL-21) that signal through receptors sharing the common gamma chain (gamma(c)) are critical for the generation and peripheral homeostasis of naive and memory T cells. Recently, we demonstrated that effector functions fail to develop in CD4(+) T cells that differentiate in the absence of gamma(c). To assess the role of gamma(c) cytokines in cell-fate decisions that condition effector versus memory CD8(+) T cell generation, we compared the response of CD8(+) T cells from gamma(c)(+) or gamma(c)(-) P14 TCR transgenic mice after challenge with lymphocytic choriomeningitis virus. The intrinsic IL-7-dependent survival defect of gamma(c)(-) naive CD8(+) T cells was corrected by transgenic expression of human Bcl-2. We demonstrated that although gamma(c)-dependent signals are dispensable for the initial expansion and the acquisition of cytotoxic functions following antigenic stimulation, they condition the terminal proliferation and differentiation of CD8(+) effector T cells (i.e., KLRG1(high) CD127(low) short-lived effector T cells) via the transcription factor, T-bet. Moreover, the gamma(c)-dependent signals that are critical for memory T cell formation are not rescued by Bcl2 overexpression. Together, these data reveal an unexpected divergence in the requirement for gamma(c) cytokines in the differentiation of CD4(+) versus CD8(+) cytotoxic T lymphocytes.

The CD8+ memory T-cell state of readiness is actively maintained and reversible

The ability of the adaptive immune system to respond rapidly and robustly upon repeated antigen exposure is known as immunologic memory, and it is thought that acquisition of memory T-cell function is an irreversible differentiation event. In this study, we report that many phenotypic and functional characteristics of antigen-specific CD8 memory T cells are lost when they are deprived of contact with dendritic cells. Under these circumstances, memory T cells reverted from G(1) to the G(0) cell-cycle state and responded to stimulation like naive T cells, as assessed by proliferation, dependence upon costimulation, and interferon-gamma production, without losing cell surface markers associated with memory. The memory state was maintained by signaling via members of the tumor necrosis factor receptor superfamily, CD27 and 4-1BB. Foxo1, a transcription factor involved in T-cell quiescence, was reduced in memory cells, and stimulation of naive CD8 cells via CD27 caused Foxo1 to be phosphorylated and emigrate from the nucleus in a phosphatidylinositol-3 kinase-dependent manner. Consistent with these results, maintenance of G(1) in vivo was compromised in antigen-specific memory T cells in vesicular stomatitis virus-infected CD27-deficient mice. Therefore, sustaining the functional phenotype of T memory cells requires active signaling and maintenance.

The CD8+ T cell memory state of readiness is actively maintained and reversible (83.18)

The ability of the adaptive immune system to respond rapidly and robustly upon repeated antigen exposure is known as immunologic memory, and it is thought that T cell progression from the naïve to the memory state is an irreversible differentiation event. Here we report that unlike naïve T cells, which are in the G0 phase of the cell cycle, CD8+ memory T cells exist in a state of readiness in G1. This was maintained by occupancy of members the TNF receptor superfamily, CD27 and 4-1BB, which when disrupted caused memory T cells to revert to G0 and respond like naïve T cells to stimulation. Furthermore, maintenance of the G1 state was compromised in antigen-specific memory T cells of VSV-infected CD27-deficient mice. Manipulating the memory T cell state of readiness might benefit disorders involving chronic ongoing immune responses.

This work was supported by the Intramural Research Program of the National Institutes of Health, Center for Cancer Research,

National Cancer Institute

CD8 single-cell gene coexpression reveals three different effector types present at distinct phases of the immune response

To study in vivo CD8 T cell differentiation, we quantified the coexpression of multiple genes in single cells throughout immune responses. After in vitro activation, CD8 T cells rapidly express effector molecules and cease their expression when the antigen is removed. Gene behavior after in vivo activation, in contrast, was quite heterogeneous. Different mRNAs were induced at very different time points of the response, were transcribed during different time periods, and could decline or persist independently of the antigen load. Consequently, distinct gene coexpression patterns/different cell types were generated at the various phases of the immune responses. During primary stimulation, inflammatory molecules were induced and down-regulated shortly after activation, generating early cells that only mediated inflammation. Cytotoxic T cells were generated at the peak of the primary response, when individual cells simultaneously expressed multiple killer molecules, whereas memory cells lost killer capacity because they no longer coexpressed killer genes. Surprisingly, during secondary responses gene transcription became permanent. Secondary cells recovered after antigen elimination were more efficient killers than cytotoxic T cells present at the peak of the primary response. Thus, primary responses produced two transient effector types. However, after boosting, CD8 T cells differentiate into long-lived killer cells that persist in vivo in the absence of antigen.

T cells in G1 provide a memory-like response to secondary stimulation

The commitment of naive T cells to proliferate is a function of the strength and duration of stimuli mediated by the TCR and coreceptors. Ranges of 2-20 h of stimulation have been reported as necessary in vitro. Whether T cells actually experience uninterrupted stimulation for such long periods under physiological conditions is controversial. Here we ask whether commitment to proliferate requires continuous stimulation, or can T cells integrate intermittent periods of stimulation. T cells were stimulated for two short-term (subthreshold) periods (5-7 h) either sequentially or separated by an interval of rest. Naive lymph node T cells were able to integrate interrupted stimulation, even when the duration of rest was as long as 2 days. Furthermore, when short-term-stimulated T cells were separated by density, three populations were observed: low density blasts, intermediate density G(1) cells, and high density G(0) cells. Low density cells progressed to division without further stimulation, whereas G(0) and G(1) cells remained undivided. However, after a period of rest, a second subthreshold stimulation caused the G(1) but not the G(0) fraction to quickly proceed through the cell cycle. We conclude that noncycling T cells in the G(1) phase of the cell cycle remain in a state of readiness for prolonged periods of time, and may represent a population of memory-like effectors capable of responding rapidly to antigenic challenge.

Dynamic regulation of IL-7 receptor expression is required for normal thymopoiesis

Interleukin-7 receptor (IL-7R) levels are tightly controlled during ontogeny: high on double-negative (DN) cells, absent on double-positive (DP) cells, and high once again on thymocytes undergoing positive selection. To determine if loss of IL-7–mediated survival signals in DP cells is necessary for normal antigen-specific selection, we created T-lineage–specific IL-7R α chain (IL-7Rα) transgenic (Tg) mice in which IL-7R is expressed throughout ontogeny. There was no effect of the IL-7Rα Tg on negative selection. Surprisingly, however, although the thymi of IL-7Rα Tg mice were comparable at birth, there was a decrease in thymocyte number as the mice aged. This was found to be due to competition between DN and IL-7R–expressing DP cells for endogenous IL-7, which resulted in decreased levels of Bcl-2 in DN cells, increased DN apoptosis, and decreased DN cell number. Therefore, the down-regulation of IL-7R on DP cells is an “altruistic” act required for maintaining an adequate supply of local IL-7 for DN cells.

Regulation of constitutive TCR internalization by the zeta-chain

The ability of a T cell to be activated is critically regulated by the number of TCRs expressed on the plasma membrane. Cell surface TCR expression is influenced by dynamic processes such as synthesis and transport of newly assembled receptors, endocytosis of surface TCR, and recycling to the plasma membrane of internalized receptors. In this study, the internalization of fluorescently labeled anti-TCR Abs was used to analyze constitutive endocytosis of TCRs on T cells, and to investigate the role of the zeta-chain in this process. We found that cell surface TCRs lacking zeta were endocytosed more rapidly than completely assembled receptors, and that reexpression of full-length zeta led to a dose-dependent decrease in the rate of TCR internalization. Rapid TCR internalization was also observed with CD4(+)CD8(+) thymocytes from zeta-deficient mice, whereas TCR internalization on thymocytes from CD3-delta deficient animals was slow, similar to that of wild-type thymocytes. This identifies a specific role for zeta in the regulation of constitutive receptor internalization. Furthermore, chimeric zeta molecules containing non-native intracellular amino acid sequences also led to high levels of TCR expression and reduced TCR cycling. These effects were dependent solely on the length of the intracellular tail, ruling out a role for intracellular zeta-specific interactions with other molecules as a mechanism for regulating TCR internalization. Rather, these findings strongly support a model in which the zeta-chain stabilizes TCR residency on the cell surface, and functions to maintain cell surface receptor expression by sterically blocking internalization sequences in other TCR components.