The immunomodulatory effect of microglia on ECM neuroinflammation via the PD-1/PD-L1 pathway

Unmasking a Paradox: Roles of the PD-1/PD-L1 Axis in Alzheimer's Disease-Associated Neuroinflammation

Alzheimer's disease (AD) represents the most prevalent form of dementia, characterized by progressive cognitive impairment and chronic neuroinflammation. Immune checkpoint inhibitors (ICIs), including anti-programmed cell death (PD)-1 and anti-PD-L1, signify a revolutionary advancement in cancer treatment by preventing T-cell exhaustion; however, their therapeutic application in AD presents a conundrum. Hypothesis: Recent preclinical studies indicate that PD-1 inhibition in AD mouse models induces an interferon-gamma (IFN-γ)-mediated response, leading to increased recruitment of monocyte-derived macrophages into the brain, enhanced clearance of amyloid-beta (Aβ) plaques, and improved cognitive performance. Nonetheless, this therapeutic effect is counterbalanced by the potential for exacerbated neuroinflammation, as PD-1/PD-L1 blockade may potentiate pro-inflammatory T helper (Th)1 and Th17 responses. In this review, we critically discuss the pertinent pro-inflammatory and neuroprotective facets of T cell biology in the pathogenesis of AD, emphasizing the potential for modulation of the PD-1/PD-L1 axis to influence both Aβ clearance and the dynamics of neuroinflammatory processes. In summary, we determine that ICIs are promising tools for reducing AD pathology and improving cognition. However, it is essential to refine treatment protocols and carefully select patients to optimize neuroprotective effects while adequately considering inflammatory risks.

Neurons upregulate PD-L1 via IFN/STAT1/IRF1 to alleviate damage by CD8+ T cells in cerebral malaria

BACKGROUND

Cerebral malaria (CM) is the most lethal complication of malaria, and survivors usually endure neurological sequelae. Notably, the cytotoxic effect of infiltrating Plasmodium-activated CD8+ T cells on cerebral microvasculature endothelial cells is a prominent feature of the experimental CM (ECM) model with blood-brain barrier disruption. However, the damage effect of CD8+ T cells infiltrating the brain parenchyma on neurons remains unclear. Based on the immunosuppressive effect of the PD-1/PD-L1 pathway on T cells, our previous study demonstrated that the systemic upregulation of PD-L1 to inhibit CD8+ T cell function could effectively alleviate the symptoms of ECM mice. However, it has not been reported whether neurons can suppress the pathogenic effect of CD8+ T cells through the PD-1/PD-L1 negative immunomodulatory pathway. As the important inflammatory factor of CM, interferons can induce the expression of PD-L1 via different molecular mechanisms according to the neuro-immune microenvironment. Therefore, this study aimed to investigate the direct interaction between CD8+ T cells and neurons, as well as the mechanism of neurons to alleviate the pathogenic effect of CD8+ T cells through up-regulating PD-L1 induced by IFNs.

METHODS

Using the ECM model of C57BL/6J mice infected with Plasmodium berghei ANKA (PbA), morphological observations were conducted in vivo by electron microscope and IF staining. The interaction between the ECM CD8+ T cells (immune magnetic bead sorting from spleen of ECM mice) and primary cultured cortical neurons in vitro was observed by IF staining and time-lapse photography. RNA-seq was performed to analyze the signaling pathway of PD-L1 upregulation in neurons induced by IFNβ or IFNγ, and verified through q-PCR, WB, IF staining, and flow cytometry both in vitro and in vivo using IFNAR or IFNGR gene knockout mice. The protective effect of adenovirus-mediated PD-L1 IgGFc fusion protein expression was verified in ECM mice with brain stereotaxic injection in vivo and in primary cultured neurons via viral infection in vitro.

RESULTS

In vivo, ECM mice showed infiltration of activated CD8+ T cells and neuronal injury in the brain parenchyma. In vitro, ECM CD8+ T cells were in direct contact with neurons and induced axonal damage, as an active behavior. The PD-L1 protein level was elevated in neurons of ECM mice and in primary cultured neurons induced by IFNβ, IFNγ, or ECM CD8+ T cells in vitro. Furthermore, the IFNβ or IFNγ induced neuronal expression of PD-L1 was mediated by increasing STAT1/IRF1 pathway via IFN receptors. The increase of PD-L1 expression in neurons during PbA infection was weakened after deleting the IFNAR or IFNGR. Increased PD-L1 expression by adenovirus partially protected neurons from CD8+ T cell-mediated damage both in vitro and in vivo.

CONCLUSION

Our study demonstrates that both type I and type II IFNs can induce neurons to upregulate PD-L1 via the STAT1/IRF1 pathway mediated by IFN receptors to protect against activated CD8+ T cell-mediated damage, providing a targeted pathway to alleviate neuroinflammation during ECM.

CD8+ T cell infiltration and proliferation in the brainstem during experimental cerebral malaria

INTRODUCTION

Cerebral malaria (CM) is a lethal neuroinflammatory disease caused by Plasmodium infection. Immune cells and brain parenchyma cells contribute to the pathogenesis of CM. However, a systematic examination of the changes that occur in the brain parenchyma region during CM at the single-cell resolution is still poorly studied.

AIMS

To explore cell composition and CD8+ T cell infiltration, single-cell RNA sequencing (scRNA-seq) was performed on the brainstems of healthy and experimental cerebral malaria (ECM) mice. Then CD8+ T cell infiltration was confirmed by flow cytometry and immunofluorescence assays. Subsequently, the characteristics of the brain-infiltrated CD8+ T cells were analyzed. Finally, the interactions between parenchyma cells and brain-infiltrated CD8+ T cells were studied with an astrocytes-CD8+ T cell cocultured model.

RESULTS

The brainstem is the most severely damaged site during ECM. ScRNA-seq revealed a large number of CD8+ T cells infiltrating into the brainstem in ECM mice. Brain-infiltrated CD8+ T cells were highly activated according to scRNA-seq, immunofluorescence, and flow cytometry assays. Further analysis found a subset of ki-67+ CD8+ T cells that have a higher transcriptional level of genes related to T cell function, activation, and proliferation, suggesting that they were exposed to specific antigens presented by brain parenchyma cells. Brain-infiltrated CD8+ T cells were the only prominent source of IFN-γ in this single-cell analysis. Astrocytes, which have a high interferon response, act as cross-presenting cells to recruit and re-activate brain-infiltrated CD8+ T cells. We also found that brain-infiltrated CD8+ T cells were highly expressed immune checkpoint molecule PD-1, while parenchyma cells showed up-regulation of PD-L1 after infection.

CONCLUSIONS

These findings reveal a novel interaction between brain-infiltrated CD8+ T cells and parenchyma cells in the ECM brainstem, suggesting that the PD-1/PD-L1 signal pathway is a promising adjunctive therapeutic strategy for ECM targeting over-activated CD8+ T cells.

Pathogenetic mechanisms and treatment targets in cerebral malaria

Malaria, the most prevalent mosquito-borne infectious disease worldwide, has accompanied humanity for millennia and remains an important public health issue despite advances in its prevention and treatment. Most infections are asymptomatic, but a small percentage of individuals with a heavy parasite burden develop severe malaria, a group of clinical syndromes attributable to organ dysfunction. Cerebral malaria is an infrequent but life-threatening complication of severe malaria that presents as an acute cerebrovascular encephalopathy characterized by unarousable coma. Despite effective antiparasite drug treatment, 20% of patients with cerebral malaria die from this disease, and many survivors of cerebral malaria have neurocognitive impairment. Thus, an important unmet clinical need is to rapidly identify people with malaria who are at risk of developing cerebral malaria and to develop preventive, adjunctive and neuroprotective treatments for cerebral malaria. This Review describes important advances in the understanding of cerebral malaria over the past two decades and discusses how these mechanistic insights could be translated into new therapies.

Ferroptosis participates in neuron damage in experimental cerebral malaria and is partially induced by activated CD8+ T cells

Cerebral malaria is the most serious complication of malaria infection, with 26% of surviving children having neurological sequelae, which may be caused by neuron damage, but the mechanism is not clear. Ferroptosis has been reported to play an important role in neuron damage in several nervous system diseases. However, the occurrence of ferroptosis in experimental cerebral malaria (ECM) pathogenesis is still unknown. In this study, we firstly detected increased levels of malondialdehyde (MDA) and iron, which are indicators of ferroptosis, in the cerebrum of ECM mice. Some important regulators of ferroptosis, including upregulated expression of transferrin receptor 1 (TfR1) and acyl-CoA synthetase long-chain family member 4 (ACSL4), and downregulation of glutathione peroxidase 4 (GPX4) levels, were also confirmed in ECM mice. Consistently, neuron damage, which was detected in the cerebrum of ECM mice, was positively correlated with reduced GPX4 expression and furtherly rescued by administration of the ferroptosis inhibitor ferrostatin-1 (Fer-1). In addition, primary neurons were damaged by activated CD8⁺ T cells, an effect that was also partially rescued by Fer-1 on amyloid precursor protein expression and mitochondrial membrane potential levels in vitro. Activated CD8⁺ T cells were also shown to infiltrate the cerebrum of ECM mice and upregulate TfR1 expression in primary neurons, which may be an important event for inducing ferroptosis in ECM. Altogether, we show that ferroptosis contributes to neuron damage in ECM pathogenesis, and activated CD8⁺ T cells may be important inducers of neuronal ferroptosis. Hence, targeting ferroptosis may be a promising adjuvant therapeutic strategy for neurological sequelae in patients with cerebral malaria.

A Glitch in the Matrix: The Role of Extracellular Matrix Remodeling in Opioid Use Disorder

Opioid use disorder (OUD) and deaths from drug overdoses have reached unprecedented levels. Given the enormous impact of the opioid crisis on public health, a more thorough, in-depth understanding of the consequences of opioids on the brain is required to develop novel interventions and pharmacological therapeutics. In the brain, the effects of opioids are far reaching, from genes to cells, synapses, circuits, and ultimately behavior. Accumulating evidence implicates a primary role for the extracellular matrix (ECM) in opioid-induced plasticity of synapses and circuits, and the development of dependence and addiction to opioids. As a network of proteins and polysaccharides, including cell adhesion molecules, proteases, and perineuronal nets, the ECM is intimately involved in both the formation and structural support of synapses. In the human brain, recent findings support an association between altered ECM signaling and OUD, particularly within the cortical and striatal circuits involved in cognition, reward, and craving. Furthermore, the ECM signaling proteins, including matrix metalloproteinases and proteoglycans, are directly involved in opioid seeking, craving, and relapse behaviors in rodent opioid models. Both the impact of opioids on the ECM and the role of ECM signaling proteins in opioid use disorder, may, in part, depend on biological sex. Here, we highlight the current evidence supporting sex-specific roles for ECM signaling proteins in the brain and their associations with OUD. We emphasize knowledge gaps and future directions to further investigate the potential of the ECM as a therapeutic target for the treatment of OUD.