Sepsis is a major cause of death in intensive care units worldwide. The acute phase of sepsis is often accompanied by sepsis-associated encephalopathy, which is highly associated with increased mortality. Moreover, in the chronic phase, more than 50% of surviving patients suffer from severe and long-term cognitive deficits compromising their daily quality of life and placing an immense burden on primary caregivers. Due to a growing number of sepsis survivors, these long-lasting deficits are increasingly relevant. Despite the high incidence and clinical relevance, the pathomechanisms of acute and chronic stages in sepsis-associated encephalopathy are only incompletely understood, and no specific therapeutic options are yet available. Here, we review the emergence of sepsis-associated encephalopathy from initial clinical presentation to long-term cognitive impairment in sepsis survivors and summarize pathomechanisms potentially contributing to the development of sepsis-associated encephalopathy.

Sepsis-associated encephalopathy (SAE) is a severe and frequent complication of sepsis causing delirium, coma, and long-term cognitive dysfunction. We identified microglia and C1q complement activation in hippocampal autopsy tissue of patients with sepsis and increased C1q-mediated synaptic pruning in a murine polymicrobial sepsis model. Unbiased transcriptomics of hippocampal tissue and isolated microglia derived from septic mice revealed an involvement of the innate immune system, complement activation, and up-regulation of lysosomal pathways during SAE in parallel to neuronal and synaptic damage. Microglial engulfment of C1q-tagged synapses could be prevented by stereotactic intrahippocampal injection of a specific C1q-blocking antibody. Pharmacologically targeting microglia by PLX5622, a CSF1-R inhibitor, reduced C1q levels and the number of C1q-tagged synapses, protected from neuronal damage and synapse loss, and improved neurocognitive outcome. Thus, we identified complement-dependent synaptic pruning by microglia as a crucial pathomechanism for the development of neuronal defects during SAE.

Background: Autoimmune encephalitis is a new spectrum of autoimmune disorders of the central nervous system (CNS), which are characterized by pathogenic autoantibodies against neuronal surface antigens. Clinical presentations range from acute to subacute encephalopathy with neurological and psychiatric symptoms, and life-threatening autonomic dysfunction in severe cases. There exist no approved therapies nor is data available from controlled clinical trials. Patients are usually treated with diverse combinations of immunotherapy. However, effect of immunotherapy on antibody-producing cells and thus on levels of pathogenic autoantibodies is insufficient. Therefore, therapeutic response is sometimes prolonged with necessity of long-time intensive care treatment and also irreversible deficits occur in severe cases. This trial will investigate the efficacy and safety of bortezomib, a proteasome inhibitor known to selectively deplete plasma cells, in patients with severe autoimmune encephalitis who have been treated with rituximab with insufficient response. Methods: Generate-Boost is an investigator-initiated, multicenter, double-blinded, randomized controlled phase II trial which will be conducted in specialized neurological hospitals within the GENERATE (GErman NEtwork for Research on AuToimmune Encephalitis) network in Germany. Adult patients with severe autoimmune encephalitis (modified Rankin scale, mRS ≥ 3), autoantibodies against neuronal surface antigens, and pretreatment with rituximab are eligible for study participation. Fifty patients will be randomized 1:1 and undergo up to 3 cycles (each 21 days with 4 s. c. applications) of bortezomib or placebo. All patients will receive concomitant medication with dexamethasone, acyclovir and co-trimoxazole. The primary efficacy endpoint is the mRS score 17 weeks after first treatment application. Secondary endpoints are neurocognitive function, antibody titers, markers of neuronal cell damage, length of ICU/hospital stay, and mRS and Glasgow coma scale scores throughout the trial up to week 17. General and bortezomib-specific adverse events are monitored continuously. Discussion: The expected outcome of the study is to obtain first reliable data on a hypothesis-driven therapeutic option in severe and difficult-to-treat autoimmune encephalitis. If treatment with bortezomib is beneficial in these cases, this will be the basis for implementation in the current guidelines. Trial registration: Clinicaltrials.gov, NCT03993262. Registered June 20, 2019; German Clinical Trials Register, DRKS00017497.

Sepsis is the most common cause of admission to intensive care units worldwide. Sepsis patients frequently suffer from sepsis-associated encephalopathy (SAE) reflecting acute brain dysfunction. SAE may result in increased mortality, extended length of hospital stay, and long-term cognitive dysfunction. The diagnosis of SAE is based on clinical assessments, but a valid biomarker to identify and confirm SAE and to assess SAE severity is missing. Several blood-based biomarkers indicating neuronal injury have been evaluated in sepsis and their potential role as early diagnosis and prognostic markers has been studied. Among those, the neuroaxonal injury marker neurofilament light chain (NfL) was identified to potentially serve as a prognostic biomarker for SAE and to predict long-term cognitive impairment. In this review, we summarize the current knowledge of biomarkers, especially NfL, in SAE and discuss a possible future clinical application considering existing limitations.

Background: Sepsis-associated encephalopathy (SAE) is characterized by symptoms of delirium including hallucinations, impaired concentration, agitation, or coma and is associated with poor outcome in the early phase of sepsis. In addition, sepsis survivors often suffer from persisting memory deficits and impaired executive functions. Recent studies provide evidence that microglia are involved in the pathophysiology of SAE. Methods: Here, we investigated whether pharmacological depletion of microglia using PLX5622 (1200 ppm or 300 ppm) in the acute phase of sepsis is able to prevent long-term neurocognitive decline in a male mouse model of polymicrobial sepsis or lipopolysaccharide-induced sterile neuroinflammation. Therefore, we performed the novel object recognition test at different time points after sepsis to address hippocampus-dependent learning. To further assess synapse engulfment in microglia, colocalization analysis was performed using high-resolution 3D Airyscan imaging of Iba1 and Homer1. We also investigated the effect of PLX5622 on acute astrocyte and chronic microglia proliferation in the hippocampus after sepsis induction using immunofluorescence staining. Results: High-dose application of the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 (1200 ppm) seven days prior to sepsis induction lead to 70–80% microglia reduction but resulted in fatal outcome of bacterial sepsis or LPS induced inflammation. This is likely caused by severely compromised host immune response upon PLX5622-induced depletion of peripheral monocytes and macrophages. We therefore tested partial microglia depletion using a low-dose of PLX5622 (300 ppm) for seven days prior to sepsis which resulted in an increased survival in comparison to littermates subjected to high-dose CSF1R inhibiton and to a stable microglia reduction of ~ 40%. This partial microglia depletion in the acute stage of sepsis largely prevented the engulfment and microglia-induced stripping of postsynaptic terminals. In addition, PLX5622 low-dose microglia depletion attenuated acute astrogliosis as well as long-term microgliosis and prevented long-term neurocognitive decline after experimental sepsis. Conclusions: We conclude that partial microglia depletion before the induction of sepsis may be sufficient to attenuate long-term neurocognitive dysfunction. Application of PLX5622 (300 ppm) acts by reducing microglia-induced synaptic attachement/engulfment and preventing chronic microgliosis.

Rationale: Anti-IgLON5 disease is a complex neurological illness which is characterized by progressive sleep and movement disorders and defined by specific autoantibodies to IgLON5. We here describe the first case of a patient with coexisting anti-IgLON5 as well as anti-γ-aminobutyric acid B (GABA_B)-receptor antibodies and predominant clinical features of anti-IgLON5 disease. Patient concerns: The patient initially presented with subacute symptoms of severe sleep disorder, gait stability, dysarthria, cognitive impairment, depressive episode and hallucinations. Diagnoses: The patient was diagnosed with autoimmune encephalitis, based on clinical features and positive anti-IgLON5 antibodies in serum as well as in cerebrospinal fluid and anti-GABA_B-receptor antibodies in serum only. Interventions: Initially, the patient was treated with high dosages of methylprednisolone and subsequently with plasmapheresis. Due to the lack of clinical improvement immunosuppressive treatment with intravenous cyclophosphamide was initiated. Outcomes: Following the first year of cyclophosphamide treatment, neurological examination revealed an improvement in gait instability, visual and acoustic hallucinations and sleep disorder. Lessons: The case report demonstrates that anti-IgLON5 and anti-GABA_B-receptor antibodies can coexist in the same patient whereas clinical leading symptoms are determined by those antibodies that were tested positive in cerebrospinal fluid.