Immune Checkpoint Inhibitor Associated Autoimmune Encephalitis, Rare and Novel Topic of Neuroimmunology: A Case Report and Review of the Literature

Immune checkpoint inhibitors (ICIs) are being used in patients with various advanced malignancies, and patient outcomes have improved considerably. Although ICIs can effectively treat tumors, 30–60% of patients experience immune-related adverse events (irAEs). Autoimmune encephalitis (AE) is a rare irAE that has become a novel topic in neuroimmunology and has received increasing attention in recent years. Herein, we report a rare case of GAD65-antibody–associated AE after metastatic small cell lung cancer treatment with pembrolizumab. The patient received IVIg therapy for AE and continuous pembrolizumab therapy without suspension of tumor treatment. At 1 year follow-up, both the patient’s AE symptoms and tumors were stable. We consider that the treatment of ICI-associated AE should be more individualized with prudent decision-making and should balance the tumor progression and AE treatment. In addition, we have also comprehensively reviewed the literature of ICI-associated AE, and summarized the clinical features, treatment, and prognosis of AE caused by ICI, thus broadening our understanding of the neurological complications caused by ICI.


Introduction
Immune checkpoint inhibitors (ICIs) are a class of monoclonal antibodies that target regulatory immune checkpoint molecules that inhibit T cell activation. ICIs can enhance T cell-mediated anti-tumor immunity and promote immune-mediated tumor cell clearance by blocking co-inhibitory signaling pathways [1][2][3]. Their antitumor effects have been recognized in clinical trials and have been approved by the FDA for the treatment of malignant tumors, such as melanoma, non-small cell lung cancer, colorectal cancer, and hepatocellular carcinoma [4].
ICIs can be divided into monoclonal antibodies against CTLA-4 and monoclonal antibodies against PD-1/PD-L1 depending on the pathways they act on [5]. Monoclonal Brain Sci. 2022, 12, 773 2 of 12 antibodies against PD-1/PD-L1 are the favorable method for modern immunotherapy of solid tumors [6][7][8]. ICIs can enhance the anti-tumor immune response by removing the inhibitory effect of PD-1 or PD-L1 immune checkpoints on the activation and proliferation of T cells, and reduce the number and/or inhibitory activity of Treg cells [9]. By doing so, the killing function of T cells against the tumor is restored, leading to the inhibition of tumor development [1]. However, when ICIs induce activation of the immune system, they also nonspecifically lead to the destruction of the immune homeostasis of non-tumor tissues, resulting in severe immune and inflammatory reactions. These include clinical immune-related adverse events (irAEs) [10,11]. At present, the specific mechanism of irAE generation is not clear, and it is generally believed to be related to the disorder of immune homeostasis caused by ICI treatment [12,13]. There are large amounts of ICs on the surface of non-tumor cells; ICI combined with them can lead to the activation of complement in the body and increase the level of inflammation, thereby disrupting immune homeostasis [14].
Common irAEs include skin rash, itching, colitis, hepatitis, and all types of endocrine diseases [15]. Nervous system irAEs are relatively rare, which include central nervous system (CNS) irAEs and peripheral nervous system (PNS) irAEs. PNS irAEs mainly include myasthenia gravis, Guillain-Barre syndrome, and peripheral sensory motor neuropathy [16]. CNS irAEs are much rarer than PNS irAEs. ICI-associated autoimmune encephalitis (AE) is an uncommon complication that has rarely been reported [17,18], and thus not completely understood. Herein, we report the first case of pembrolizumabassociated GAD65 antibody AE with a favorable short-term prognosis after treatment. Furthermore, in order to better understand the CNS complications caused by ICI, we comprehensively reviewed ICI-associated AE.

Materials and Methods
References for the review were identified by searching English literature in the PubMed database published from 2016 to 2022, using the search terms (alone or in logical combinations): "immune checkpoint inhibitor", "autoimmune encephalitis", "immune-related adverse events", "anti PD1", "anti PDL1", and "anti CTLA4." The inclusion criteria are as follows: patients with (1) encephalitis symptoms identical to classical AE, regardless of whether neuronal autoantibodies were detected; (2) any AE symptoms associated with classic tumor neuronal autoantibodies; or (3) symptoms associated with autoantibodies against synaptic receptors or other neuron cell surface proteins after ICI treatment. Patients were excluded if neuronal autoantibodies were detected before ICI treatment [19].

Case Report
A 51-year-old man was admitted with an unsteady gait and slurred speech, which persisted for 20 days. He had undergone pulmonary nodule resection in April 2019. Frozen section pathology revealed small-cell carcinoma in the anterior segment of the right upper lobe. He received multiple doses of VP-16, carboplatin, chemotherapy, and radiotherapy from May 2019 to November 2020. In January 2021, positron emission tomography-computed tomography (PET-CT) revealed two round and low-density lesions with increased fluorodeoxyglucose (FDG) metabolism on the right liver lobe, enlarged lymph nodes with FDG hypermetabolism in the retroperitoneal area and the space between the right kidney and pancreas. The size of lesions on the right lobe of the liver were 1.3 × 1.1 cm and 1.34 × 1.44 cm. Considering the liver and lymph node metastases, on the 3rd and 25th of February 2021, the patient received pembrolizumab therapy. On 15 April 2021, he developed slurred speech and an unsteady gait. The patient was again treated with pembrolizumab on the 29 of April 2021, and his symptoms progressed significantly and his modified Rankin Scale (mRS) score deteriorated to 4. Neurological examination revealed dysmetria on the finger-to-nose test, positive Romberg's sign, ataxic gait, and nystagmus. The cranial nerves were intact and there was no nuchal rigidity. The patient had normal muscle strength, muscle tone, and tendon reflexes. Cerebrospinal fluid (CSF) analysis showed normal opening pressure (120 mmH 2 O), nucleated cell count 2 × 10 6 /L, protein 514.45 mg/L, chloride 128 mmol/L, and sugar 2.96 mmol/L. Cranial magnetic resonance imaging (MRI) showed evidence of mild cerebellar atrophy ( Figure 1A,B). 18 F-FDG-PET-CT indicated decreased metabolism in both cerebellar hemispheres.
(CTCAE-5.0) [20]. The patient was given intravenous immunoglobulin (IVIg) at 0.4 g/kg/day for 5 days, and his speech and gait improved significantly. His mRS improved to 1. Considering that excessive immunosuppressive therapy might lead to tumor recurrence, this patient did not receive glucocorticoid or immunosuppressive therapy. Furthermore, pembrolizumab treatment was continued for the tumor. He received IVIg therapy 3 months later to prevent the recurrence of AE symptoms. At the 1-year follow-up, his AE symptoms had largely recovered, and the tumor was stable. The titer of GAD65 antibodies in serum decreased to 1:1. CT showed that the size of the metastases in the right lobe of the liver were 0.3 × 0.51 cm and 0.67 × 0.88 cm. The retroperitoneal lymph nodes and lymph nodes in the right renal space between the pancreas were also reduced ( Figure 1C, timeline for this patient).  The autoimmune encephalitis antibody panel revealed that anti-GAD65 and anti-SOX1 antibodies were present in both serum and CSF, while other antibodies were absent. The titers of GAD65 and SOX1 antibodies in the serum were 1:30 and 1:10, respectively; and while they were 1:100 and 1:10, respectively, in the CSF. Therefore, the patient was diagnosed with pembrolizumab-associated AE. The irAE grade was 3 according to the National Cancer Institute Common Terminology Criteria for Adverse Events version 5.0 (CTCAE-5.0) [20]. The patient was given intravenous immunoglobulin (IVIg) at 0.4 g/kg/day for 5 days, and his speech and gait improved significantly. His mRS improved to 1. Considering that excessive immunosuppressive therapy might lead to tumor recurrence, this patient did not receive glucocorticoid or immunosuppressive therapy. Furthermore, pembrolizumab treatment was continued for the tumor. He received IVIg therapy 3 months later to prevent the recurrence of AE symptoms. At the 1-year follow-up, his AE symptoms had largely recovered, and the tumor was stable. The titer of GAD65 antibodies in serum decreased to 1:1. CT showed that the size of the metastases in the right lobe of the liver were 0.3 × 0.51 cm and 0.67 × 0.88 cm. The retroperitoneal lymph nodes and lymph nodes in the right renal space between the pancreas were also reduced ( Figure 1C, timeline for this patient).

Pembrolizumab-Associated GAD65 Antibody AE, a Very Rare Complication of ICI
The incidence of irAEs in the nervous system after ICI treatment is 2% to 6% [21,22], of which the incidence of encephalitis is 0.05% [16]. As far as we know, ICI-associated AE is currently limited to a single case report or a few small case series [18,23]. Cases of ICI-associated AE with autoantibodies are very rare. This is the first case of pembrolizumabassociated GAD65 antibody-positive AE. We noticed that this patient experienced a reduction in tumor size and progression of ataxia with GAD65 and SOX1 autoantibodies after pembrolizumab treatment, which indicated a diagnosis of pembrolizumab-associated AE [19]. In this patient, pembrolizumab was very effective in the treatment of tumors; both the tumor size in the lung and tumor metastasis were well controlled after the treatment. In addition, after five days of IVIg treatment, the patient's AE symptoms improved significantly, and we resumed the pembrolizumab treatment. At 1-year follow-up, both the patient's AE symptoms and tumors were stable. In previous reports, steroids rather than IVIg were generally chosen for ICI-associated AE [24], and ICI treatment was usually discontinued. However, in our patient, timely and effective immunotherapy and continued treatment with pembrolizumab resulted in significant reduction of both AE and tumors. Our case may provide new directions for future treatment of ICI-associated AE.

Literature Review of ICI-Associated AE
By reviewing the literature, we found 50 cases of ICI-associated AE; of which, 28 and This table summarized the reported cases of ICI-associated AE (abbreviations: NA = not available, FLAIR = fluid-attenuated inversion recovery, MTL = mesial temporal lobe, OCB = oligoclonal band). We noticed that among the reported cases, only seven cases were related to the GAD65 antibody [51], ranging in age from 33 to 64 years. All patients developed neuropsychiatric symptoms after using nivolumab alone or in combination with ipilimumab; none have been reported for pembrolizumab with the GAD65 antibody. Among the seven patients, four had poor prognosis or died. Six patients suspended ICI immediately after the onset of AE symptoms. Piepgras et al. reported a patient whose treatment with nivolumab was continued after controlling symptoms with steroids and infliximab but died after the encephalitis symptoms recurred [36]. In contrast to the previously reported patient, our patient had a good prognosis at the 1-year follow-up. The prognosis of ICI-associated AE varied after treatment.
The mechanisms of ICI-associated AE may be due to the following reasons: (1) The immune response induced by ICI may cross-react with CNS autoantigens; (2) AE is closely associated with tumors [52]. ICI may simultaneously enhance the immune response of the tumor and the immune response to CNS; and (3) ICI may recognize innate immune molecules on the surface of neurons, and thus directly kill neurons through the complement system or cytotoxicity, leading to the release of intracellular antigens [53].

Current Dilemma: The Balance between ICI Therapy, Tumor Progression, and AE Treatment
Immune-related adverse events are classified into five grades [20]. Due to the presence of pathogenic autoantibodies, the treatment of ICI-associated AE includes withdrawal of ICI, IVIg, plasma exchange, and immunosuppressive therapies. As discussed above, most scholars support stopping ICI therapy and initiating immunosuppressive therapy [58]. According to the principle of toxicity management, only Grade 1 irAEs could continue ICI. Grades 2-4 irAEs should withhold ICI and receive stronger immunosuppressive therapy [59]. However, discontinuation of ICI therapy may lead to tumor progression. In addition, strong immunosuppression is associated with tumor recurrence. Being able to balance the treatments may prove difficult. Based on our experience, the treatment of ICI-associated AE should be more individualized and prudent. We advocate that IVIg or plasma exchange may be the first choices for Grade 2 and Grade 3 patients, and ICI may not be discontinued at first. The patients need more close monitoring. If patients do not respond to first-line treatment or symptoms progression, ICI treatment needs to be suspended and stronger immunosuppressants can be used [60]. In the literature we reviewed, we found two Grade 2 patients who continued using ICI or were switched to another type of ICI, and had good prognosis [24]. However, because ICI-associated AE is a relatively new topic in neuroimmunology, we need more cases and multi-center research in the future.

Conclusions
This is the first case of pembrolizumab-associated AE with GAD65 and SOX1 antibodies, characterized by progressive cerebellar ataxia. It should be noted that this patient received IVIg therapy for AE and continuous pembrolizumab therapy without suspension of tumor treatment. At present, the treatment of ICI-associated AE needs to be more individualized with close monitoring, evaluation, and prudent decision-making. Finally, the mechanisms, clinical features, and treatment of ICI-associated AE are a new concept in the field of neuroimmunology, and further studies are needed.