Role of the immune modulator programmed cell death-1 during development and apoptosis of mouse retinal ganglion cells

PD-1: A New Candidate Target for Analgesic Peptide Design

Chronic pain is a common health problem in humans. The unique properties and valuable clinical applications of analgesic peptides make them attractive pharmacotherapy options for pain control. Numerous targets for pain modulation processes are currently known, including opioid receptors, transient receptor potential (TRP) channels, voltage-gated ion channels, neuronal nicotinic receptors, and neurotensin receptors. However, these targets are not able to address the development needs of peptide-based drugs. Recent studies revealed that programmed cell death 1 (PD-1) is widely expressed in the dorsal root ganglia (DRG), spinal cord, and cerebral cortex. PD-1 signaling in neurons is involved in the regulation of neuronal excitability, synaptic transmission, and synaptic plasticity. PD-1 is able to silence nociceptive neurons upon activation. Consistently, Pd1 deficiency or blockade increases the pain sensitivity in naïve mice. PD-1 agonists, including PD-L1 and H-20, evoke Src homology 2 domain-containing tyrosine phosphatase-1 (SHP-1) phosphorylation, modulate neuronal excitability, and attenuate acute and chronic pain with minimal opioid-related adverse effects, suggesting a superior therapeutic index and a sound strategy for the development novel nonopioid analgesics. In addition, PD-1 signaling in non-neuronal cells could alleviate chronic pain by regulating neuroinflammation. Here, we review the potential and challenges of PD-1 as a candidate target for the development of analgesic peptides. PERSPECTIVE: This review paper aims to review recent advances in research on PD-1 in the domain of pain interference, explore how to obtain more promising PD-1 receptor-targeting analgesic peptides based on PD-L1 and analgesic peptide H-20 for relieving pathological pain, and offer potential optimization strategies for follow-up work of H-20.

Protective effects of blocking PD-1 pathway on retinal ganglion cells in a mouse model of chronic ocular hypertension

Purpose

In this study, we aimed to investigate whether Programmed cell death 1 ligand 1/programmed cell death 1 ligand 2 (PD-L1/PD-L2) double knockout (dKO) has a protective effect on RGCs in a mouse model of chronic ocular hypertension (COHT).

Methods

We used superparamagnetic iron oxide to induce COHT in mice. Apoptosis of retinal ganglion cells (RGCs) and activation of microglia were evaluated using western blotting (WB) and immunofluorescence staining of the mouse retina. In addition, we also conducted transcriptome sequencing and further gene expression analyses using the gene ontology (GO) and Kyoto Encyclopedia of Genes (KEGG) database.

Results

In the mouse model of COHT, PD-L1/PD-L2 prevented the apoptosis of RGCs to some extent. Blocking the programmed cell death 1 (PD-1) pathway also increased the number of anti-inflammatory M2-activated microglia and enhanced the phosphorylation of its related pathway signal transducer and activator of transcription (STAT)6. Sequencing results showed that this protective effect may have been achieved by regulating the NF-B, tumour necrosis factor (TNF), PI3K/Akt and toll-like receptor signaling pathway etc.

Conclusion

Blocking the PD-1 pathway has a protective effect on RGCs in the mouse model of COHT induced by superparamagnetic iron oxide.

Blockade of PD-L1/PD-1 signaling promotes osteo-/odontogenic differentiation through Ras activation

The programmed cell death ligand 1 (PD-L1) and its receptor programmed cell death 1 (PD-1) deliver inhibitory signals to regulate immunological tolerance during immune-mediated diseases. However, the role of PD-1 signaling and its blockade effect on human dental pulp stem cells (hDPSCs) differentiation into the osteo-/odontogenic lineage remain unknown. We show here that PD-L1 expression, but not PD-1, is downregulated during osteo-/odontogenic differentiation of hDPSCs. Importantly, PD-L1/PD-1 signaling has been shown to negatively regulate the osteo-/odontogenic differentiation of hDPSCs. Mechanistically, depletion of either PD-L1 or PD-1 expression increased ERK and AKT phosphorylation levels through the upregulation of Ras enzyme activity, which plays a pivotal role during hDPSCs osteo-/odontogenic differentiation. Treatment with nivolumab (a human anti-PD-1 monoclonal antibody), which targets PD-1 to prevent PD-L1 binding, successfully enhanced osteo-/odontogenic differentiation of hDPSCs through enhanced Ras activity-mediated phosphorylation of ERK and AKT. Our findings underscore that downregulation of PD-L1 expression accompanies during osteo-/odontogenic differentiation, and hDPSCs-intrinsic PD-1 signaling inhibits osteo-/odontogenic differentiation. These findings provide a significant basis that PD-1 blockade could be effective immunotherapeutic strategies in hDPSCs-mediated dental pulp regeneration.

Emerging Role of PD-1 in the Central Nervous System and Brain Diseases

Programmed cell death protein 1 (PD-1) is an immune checkpoint modulator and a major target of immunotherapy as anti-PD-1 monoclonal antibodies have demonstrated remarkable efficacy in cancer treatment. Accumulating evidence suggests an important role of PD-1 in the central nervous system (CNS). PD-1 has been implicated in CNS disorders such as brain tumors, Alzheimer's disease, ischemic stroke, spinal cord injury, multiple sclerosis, cognitive function, and pain. PD-1 signaling suppresses the CNS immune response via resident microglia and infiltrating peripheral immune cells. Notably, PD-1 is also widely expressed in neurons and suppresses neuronal activity via downstream Src homology 2 domain-containing protein tyrosine phosphatase 1 and modulation of ion channel function. An improved understanding of PD-1 signaling in the cross-talk between glial cells, neurons, and peripheral immune cells in the CNS will shed light on immunomodulation, neuromodulation, and novel strategies for treating brain diseases.

Pathophysiology of ocular toxoplasmosis: Facts and open questions

Infections with the protozoan parasite Toxoplasma gondii are frequent, but one of its main consequences, ocular toxoplasmosis (OT), remains poorly understood. While its clinical description has recently attracted more attention and publications, the underlying pathophysiological mechanisms are only sparsely elucidated, which is partly due to the inherent difficulties to establish relevant animal models. Furthermore, the particularities of the ocular environment explain why the abundant knowledge on systemic toxoplasmosis cannot be just transferred to the ocular situation. However, studies undertaken in mouse models have revealed a central role of interferon gamma (IFNγ) and, more surprisingly, interleukin 17 (IL17), in ocular pathology and parasite control. These studies also show the importance of the genetic background of the infective Toxoplasma strain. Indeed, infections due to exotic strains show a completely different pathophysiology, which translates in a different clinical outcome. These elements should lead to more individualized therapy. Furthermore, the recent advance in understanding the immune response during OT paved the way to new research leads, involving immune pathways poorly studied in this particular setting, such as type I and type III interferons. In any case, deeper knowledge of the mechanisms of this pathology is needed to establish new, more targeted treatment schemes.

Exploring the role of programmed cell death protein 1 and its ligand 1 in eye diseases

Programmed death receptor-1 (PD-1) and its ligand, PD-L1, as negative co-stimulatory molecules, are indispensable for regulating both physiological and pathological immune responses. The PD-1/PD-L1-mediated signaling pathway has been studied extensively in cancer research and has become a hotspot for biopharmaceuticals and immunotherapy. Furthermore, monoclonal antibodies to PD-1 have just been approved by the US Food and Drug Administration to treat certain types of malignancies. Recent research has unveiled a close association between the PD-1/PD-L1 system and eye diseases. This review describes the expression and physiological functions of PD-1 and its ligand in ocular tissues and summarizes the pathogenic, regulatory, and therapeutic roles of PD-1/PD-L1 system in eye diseases, including uveal melanoma, autoimmune uveitis, autoimmune dry eye, sympathetic ophthalmia, Graves' ophthalmopathy, diabetic retinopathy, herpes simplex keratitis, and trachoma, with the intent of highlighting the potential of PD-1/PD-L1 as novel therapeutic targets or biomarkers for these ocular diseases.

In tumor cells, thyroid hormone analogues non-immunologically regulate PD-L1 and PD-1 accumulation that is anti-apoptotic

The PD-1/PD-L1 immune checkpoint involving tumor cells and host immune defense lymphocytes is a well-studied therapeutic target in oncology. That PD-1 and PD-L1 may have additional functions within tumor cells that are independent of the checkpoint is indicated by actions of a thyroid hormone analogue, L-thyroxine (T₄), on these checkpoint components. Acting at a cell surface receptor on plasma membrane integrin αvβ3, T₄ stimulates intracellular accumulation of PD-L1 in cancer cells. In these thyroid hormone-treated cells, T₄-induced PD-L1 is non-immunologically anti-apoptotic, blocking activation of p53. Several laboratories have also described accumulation of PD-1 in a variety of cancer cells, not just immune defense lymphocytes and macrophages. Preliminary observations indicate that T₄ stimulates intracellular accumulation of PD-1 in tumor cells, suggesting that, like PD-L1, PD-1 has non-immunologic roles in the setting of cancer. Where such roles are anti-apoptotic, thyroid hormone-directed cancer cell accumulation of PD-1 and PD-L1 may limit effectiveness of immunologic therapy directed at the immune checkpoint.

PD-1 is required to maintain stem cell properties in human dental pulp stem cells

Programmed cell death-1 (PD-1) belongs to an inhibitory signaling pathway capable of maintaining central and peripheral immune tolerance. Blockage of PD-1 has been identified as a promising immunotherapeutic approach for cancer and chronic infectious diseases. However, it is unknown whether PD-1 pathway regulates stem cell function. It is generally believed that mesenchymal stem cells (MSCs) produce PD-1 ligand, but fail to express PD-1. In this study, we show that neural crest-derived MSCs from dental pulp (MSC-DP), but not MSCs from bone marrow, expressed PD-1. Knocking down PD-1 expression in MSC-DP results in a significantly reduced capacity for cell proliferation and accelerated multipotential differentiation. Mechanistically, we show that PD-1 regulates a SHP2/ERK/Notch cascade to maintain proliferation and a SHP2/ERK/β-catenin cascade to inhibit osteo-/odontogenic differentiation. This study indicates that PD-1 is a key surface molecule controlling cell proliferation and multipotential differentiation of MSC-DP. Through regulating PD-1/SHP2/ERK signaling, we can significantly improve the quality and quantity of culture-expanded MSC-DP for potential clinical therapies.

Astrocyte-derived lipoxins A4 and B4 promote neuroprotection from acute and chronic injury

Astrocytes perform critical non-cell autonomous roles following CNS injury that involve either neurotoxic or neuroprotective effects. Yet the nature of potential prosurvival cues has remained unclear. In the current study, we utilized the close interaction between astrocytes and retinal ganglion cells (RGCs) in the eye to characterize a secreted neuroprotective signal present in retinal astrocyte conditioned medium (ACM). Rather than a conventional peptide neurotrophic factor, we identified a prominent lipid component of the neuroprotective signal through metabolomics screening. The lipoxins LXA4 and LXB4 are small lipid mediators that act locally to dampen inflammation, but they have not been linked directly to neuronal actions. Here, we determined that LXA4 and LXB4 are synthesized in the inner retina, but their levels are reduced following injury. Injection of either lipoxin was sufficient for neuroprotection following acute injury, while inhibition of key lipoxin pathway components exacerbated injury-induced damage. Although LXA4 signaling has been extensively investigated, LXB4, the less studied lipoxin, emerged to be more potent in protection. Moreover, LXB4 neuroprotection was different from that of established LXA4 signaling, and therapeutic LXB4 treatment was efficacious in a chronic model of the common neurodegenerative disease glaucoma. Together, these results identify a potential paracrine mechanism that coordinates neuronal homeostasis and inflammation in the CNS.

Pharmacologic inhibition of reactive gliosis blocks TNF-α-mediated neuronal apoptosis

Reactive gliosis is an early pathological feature common to most neurodegenerative diseases, yet its regulation and impact remain poorly understood. Normally astrocytes maintain a critical homeostatic balance. After stress or injury they undergo rapid parainflammatory activation, characterized by hypertrophy, and increased polymerization of type III intermediate filaments (IFs), particularly glial fibrillary acidic protein and vimentin. However, the consequences of IF dynamics in the adult CNS remains unclear, and no pharmacologic tools have been available to target this mechanism in vivo. The mammalian retina is an accessible model to study the regulation of astrocyte stress responses, and their influence on retinal neuronal homeostasis. In particular, our work and others have implicated p38 mitogen-activated protein kinase (MAPK) signaling as a key regulator of glutamate recycling, antioxidant activity and cytokine secretion by astrocytes and related Müller glia, with potent influences on neighboring neurons. Here we report experiments with the small molecule inhibitor, withaferin A (WFA), to specifically block type III IF dynamics in vivo. WFA was administered in a model of metabolic retinal injury induced by kainic acid, and in combination with a recent model of debridement-induced astrocyte reactivity. We show that WFA specifically targets IFs and reduces astrocyte and Müller glial reactivity in vivo. Inhibition of glial IF polymerization blocked p38 MAPK-dependent secretion of TNF-α, resulting in markedly reduced neuronal apoptosis. To our knowledge this is the first study to demonstrate that pharmacologic inhibition of IF dynamics in reactive glia protects neurons in vivo.

Programmed cell death-1 is expressed in large retinal ganglion cells and is upregulated after optic nerve crush

Programmed cell death-1 (PD-1) is a key negative receptor inducibly expressed on T cells, B cells and dendritic cells. It was discovered on T cells undergoing classical programmed cell death. Studies showed that PD-1 ligation promotes retinal ganglion cell (RGC) death during retinal development. The purpose of this present study is to characterize PD-1 regulation in the retina after optic nerve crush (ONC). C57BL/6 mice were subjected to ONC and RGC loss was monitored by immunolabelling with RNA-binding protein with multiple splicing (Rbpms). Time course of PD-1 mRNA expression was determined by real-time PCR. PD-1 expression was detected with anti-PD-1 antibody on whole mount retinas. PD-1 staining intensity was quantitated. Colocalization of PD-1 and cleaved-caspase-3 after ONC was analyzed. Real-time PCR results demonstrated that PD-1 gene expression was significantly upregulated at day 1, 3, 7, 10 and 14 after ONC. Immunofluorescent staining revealed a dramatic increase of PD-1 expression following ONC. In both control and injured retinas, PD-1 tended to be up-expressed in a subtype of RGCs, whose somata size were significantly larger than others. Compared to control, PD-1 intensity in large RGCs was increased by 82% in the injured retina. None of the large RGCs expressed cleaved-caspase-3 at day 5 after ONC. Our work presents the first evidence of PD-1 induction in RGCs after ONC. This observation supports further investigation into the role of PD-1 expression during RGC death or survival following injury.

Mechanisms of immune privilege in the posterior eye

Immune privilege protects vital organs and their functions from the destructive interference of inflammation. Because the eye is easily accessible for surgical manipulation and for assessing and imaging the outcomes, the eye has been a major tissue for the study of immune privilege. Here, we focus on the immune regulatory mechanisms in the posterior eye, in part, because loss of immune privilege may contribute to development of certain retinal diseases in the aging population. We begin with a background in immune privilege and then focus on the select regulatory mechanisms that have been studied in the posterior eye. The review includes a description of the immunosuppressive environment, regulatory surface molecules expressed by cells in the eye, types of cells that participate in immune regulation and finally, discusses animal models of retinal laser injury in the context of mechanisms that overcome immune privilege.

Neuronal programmed cell death-1 ligand expression regulates retinal ganglion cell number in neonatal and adult mice

OBJECTIVES

During mouse retina maturation, the final number of retinal ganglion cells (RGCs) is determined by highly regulated programmed cell death. Previous studies demonstrated that the immunoregulatory receptor programmed cell death-1 (PD-1) promotes developmental RGC death. To identify the functional signaling partner(s) for PD-1, we identified retinal expression of PD-1 ligands and examined the effect of PD-1 ligand expression on RGC number. We also explored the hypothesis that PD-1 signaling promotes the development of functional visual circuitry.

METHODS

Characterization of retinal and brain programmed cell death-1 ligand 1 (PD-L1) expression were examined by immunofluorescence on tissue sections. The contribution of PD-ligands, PD-L1, and programmed cell death-1 ligand 2 (PD-L2) to RGC number was examined in PD-ligand knockout mice lacking 1 or both ligands. Retinal architecture was assessed by spectral-domain optical coherence tomography, and retinal function was analyzed by electroretinography in wild-type and PD-L1/L2 double-deficient mice.

RESULTS

PD-L1 expression is found throughout the neonatal retina and persists in adult RGCs, bipolar interneurons, and Müller glia. In the absence of both PD-ligands, there is a significant numerical increase in RGCs (34% at postnatal day 2 [P2] and 18% in adult), as compared to wild type, and PD-ligands have redundant function in this process. Despite the increased RGC number, adult PD-L1/L2 double-knockout mice have normal retinal architecture and outer retina function.

CONCLUSION

This study demonstrates that PD-L1 and PD-L2 together impact the final number of RGCs in adult mice and supports a novel role for active promotion of neuronal cell death through PD-1 receptor-ligand engagement.

Immunization with different viral antigens alters the pattern of T cell exhaustion and latency in herpes simplex virus type 1-infected mice

We have shown previously that immunization with herpes simplex virus type 1 (HSV-1) glycoprotein K (gK) exacerbated corneal scarring (CS) in ocularly infected mice. In this study, we investigated whether higher levels of CS were correlated with higher levels of latency and T cell exhaustion in gK-immunized mice. BALB/c mice were vaccinated with baculovirus-expressed gK or gD or mock immunized. Twenty-one days after the third immunization, mice were ocularly infected with 2 × 10(4) PFU/eye of virulent HSV-1 strain McKrae. On day 5 postinfection, virus replication in the eye was measured, and on day 30 postinfection, infiltration of the trigeminal ganglia (TG) by CD4, CD8, programmed death 1 (PD-1), and T cell immunoglobulin and mucin domain-containing protein 3 (Tim-3) was monitored by immunohistochemistry and quantitative real-time PCR (qRT-PCR). This study demonstrated that higher levels of CS were correlated with higher levels of latency, and this was associated with the presence of significantly higher numbers of CD4(+)PD-1(+) and CD8(+)PD-1(+) cells in the TG of the gK-immunized group than in both the gD- and mock-immunized groups. Levels of exhaustion associated with Tim-3 were the same among gK- and mock-vaccinated groups but higher than levels in the gD-vaccinated group. In this study, we have shown for the first time that both PD-1 and Tim-3 contribute to T cell exhaustion and an increase of latency in the TG of latently infected mice.