Lymphocytes in autoimmune encephalitis: Pathogenesis and therapeutic target

Autoimmune encephalitis (AE) is an inflammatory disease of the central nervous system characterized by the production of various autoimmune antibodies targeting neuronal proteins. The pathogenesis of AE remains elusive. Accumulating evidence suggests that lymphocytes, particularly B and T lymphocytes, play an integral role in the development of AE. In the last two decades, autoimmune neural antibodies have taken center stage in diagnosing AE. Recently, increasing evidence has highlighted the importance of T lymphocytes in the onset of AE. CD4 + T cells are thought to influence disease progression by secreting associated cytokines, whereas CD8 + T cells exert a cytotoxic role, causing irreversible damage to neurons mainly in patients with paraneoplastic AE. Conventionally, the first-line treatments for AE include intravenous steroids, intravenous immunoglobulin, and plasma exchange to remove pathogenic autoantibodies. However, a minority of patients are insensitive to conventional first-line treatment protocols and suffer from disease relapse, a condition referred to as refractory AE. In recent years, new treatments, such as rituximab or CAAR-T, which target pathogenic lymphocytes in patients with AE, have offered new therapeutic options for refractory AE. This review aims to describe the current knowledge about the function of B and T lymphocytes in the pathophysiology of AE and to summarize and update the immunotherapy options for treating this disease.


Introduction
Autoimmune encephalitis (AE) is a noninfectious but immunemediated inflammatory disease of the central nervous system (CNS) that results from the interaction between the immune system and brain parenchyma (Graus et al., 2016).The involvement of autoimmune antibodies in AE was first described by Dalmau et al. in 2007 in a study mainly involving young women with ovarian teratomas (Dalmau et al., 2008).Advances in diagnostic assays have led to the identification of a wide range of disease-associated autoantibodies (Sabatino Jr. et al., 2019).More than 10 AE subtypes have been identified according to the autoantibodies (Dalmau and Graus, 2018).AE can occur as a paraneoplastic manifestation of a tumor expressing neuronal proteins.Memory B cells can migrate into the CNS and are reactivated upon encountering the same antigen (Jain and Yong, 2022).Besides autoimmune antibodies, the role of T lymphocytes in AE should not be overlooked (Voltz et al., 1998).CD4 + T lymphocytes can differentiate into Th cells or regulatory T cells (Tregs) influenced by the micro-environment.Th cells mainly exert their pro-inflammatory role by secreting cytokines, whereas Tregs may have a protective function.In addition, CD8 + T cells are involved when the antigen is located intracellularly, as observed in patients with paraneoplastic AE.These cells exert cytotoxic effects by producing granzyme B and perforin (van de Griend et al., 1987).
Currently, the first-line therapy options include intravenous steroids, intravenous immunoglobulin (IVIG), and plasma exchange (PLEX) (Abboud et al., 2021a).These treatments reduce serum antibody titers; Abbreviations: AAV, antineutrophil cytoplasmic antibody-associated vasculitides; ADCC, antibody-dependent cellular cytotoxicity; AE, auto-immune encephalitis; APC, antigen-presenting cells; BBB, blood-brain barrier; BGE, basal ganglia encephalitis; CDC, complement-dependent cytotoxicity; CNS, central nervous system; CSF, cerebrospinal fluid; EAE, experimental autoimmune encephalomyelitis; GC, germinal center; IL-6, interleukin-6; IL-2, interleukin-2; IVIG, intravenous immunoglobulin; LFA-1, lymphocyte function-associated antigen-1; LLPCs, long-lived plasma cells; MHC, major histocompatibility complex; MM, multiple myeloma; MS, multiple sclerosis; PLEX, plasma exchange; RA, rheumatoid arthritis; SLE, systemic lupus erythematosus; SLPBs, short-lived plasmablasts; Tfh, follicular helper T cells; TNF, tumor necrosis factor; Treg, regulatory T cells.however, a minority of patients have persistently high cerebrospinal fluid (CSF) antibody titers.They are insensitive to traditional treatment and suffer from disease relapse (Yang and Liu, 2021).With a better understanding of the pathogenesis of AE, numerous therapeutic approaches have been proposed and applied to its treatment.This review aims to examine the complicated role of B and T lymphocytes in the pathogenesis of AE and provide insight into the mechanisms underlying novel AE treatments in recent years.

Role of lymphocytes in AE pathogenesis
The involvement of B and T lymphocytes was proposed in patients with anti-NMDAR encephalitis.B cells and CD4 + T cells are found mainly in the perivascular spaces of the CNS, whereas CD8 + T lymphocytes are located predominantly in the parenchyma close to neurons (Malviya et al., 2017).Clinical studies showed that CD4 + and CD8 + T cells in CSF were higher in patients with anti-GAD65 encephalitis and those with anti-GABA B R encephalitis than in healthy controls.In contrast, the numbers of these cells did not differ in the peripheral blood (Golombeck et al., 2016).This observation suggested that CD138 + B lymphocytes and CD4 + and CD8 + T cells existed behind the blood-brain barrier (BBB) in AE (Titulaer et al., 2013).However, the mechanism by which these cells infiltrate the barrier to access the CNS still needs further investigation (Engelhardt et al., 2017).

Role of B lymphocytes and associated antibodies
AE is conceived as a group of diseases predominantly caused by autoantibodies, with B cells playing a significant role (Sabatino Jr. et al., 2020).However, B cells are not located around AE lesions.The functions of B cells within or outside the CNS in AE have not been elucidated.
Naïve B cells generally differentiate into plasma cells after antigen stimulation (Zografou et al., 2021).This process occurs in the early stage after antigen stimulation and is responsible for rapid clearance, known as the extra-follicular pathway, ultimately leading to the generation of short-lived plasmablasts (SLPBs).However, the follicular pathway occurs within the germinal centers (GCs) of lymphatic follicles (Zografou et al., 2021).With the help of follicular helper T (Tfh) cells, mature B cells undergo an antibody class switch and form high-affinity plasma cells and memory cells (Thaler et al., 2019).Some of these cells enter a survival niche after exiting the GC and become CD138 + long-lived plasma cells (LLPCs) (Zografou et al., 2021).Notably, LLPCs do not express CD20.
In AE, the antibodies against neuronal cell surface receptors or synaptic proteins are predominantly IgG1 and IgG4.IgG4 production depends on the influence of interleukin (IL)-4, IL-10, and IL-13 secreted by Th2 cells (Zografou et al., 2021).The antibodies' titer is higher in CSF than in peripheral blood, suggesting that antibody production may also occur behind the BBB (Dalmau and Graus, 2018).The clonal proliferation of CD138 + plasma cells within the CNS has been detected in patients with anti-NMDAR AE (Camdessanche et al., 2011).Researchers induced a B-cell-mediated anti-NMDAR encephalitis in mice via subcutaneous injection of GluN1 359-378 peptide.This model featured a B-cell response without parenchyma infiltration or contribution from T cells.The precise mechanisms responsible for B-cell migration toward the brain in this model remain to be investigated (Wagnon et al., 2020).In vitro, adhesion molecules expressed on endothelial cells, such as intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1), are associated with the transmigration of B cells (Alter et al., 2003).In addition, chemokine receptors such as CXCR3, CXCR5, CCR5, and leukocyte very late antigen-4 (VLA-4) expressed on B cells help them migrate into the CNS (Dalmau et al., 2017).

Cytokines related to B cells in AE
B cells can secrete various cytokines, including lymphotoxin-α, interleukin-6 (IL-6), and tumor necrosis factor (TNF).These cytokines contribute to the activation and proliferation of Th1 and Th17 cells, which exhibit pro-inflammatory effects in AE (Planaguma et al., 2015).
IL-6 can be secreted by different cell sets, including B lymphocytes, stimulating the survival of plasma blast cells and enhancing antibody secretion.Thus, the production of intrathecal pathogenic autoantibodies in patients with anti-NMDAR encephalitis may be associated with increased CSF IL-6 levels (Kang et al., 2015).In addition, IL-6 can facilitate Th17 cell proliferation and ultimately inhibit the formation of Tregs, thereby contributing to inflammation related to AE (Hunter and Jones, 2015).A clinical cohort of 40 AE patients revealed that the serum IL-6 concentration persistently increased after appropriate immunotherapy targeting the adaptive immune system.Besides the adaptive arm of the immune system, CD14 + CD16 + monocytes of the innate immune system also secrete IL-6 (Wesselingh et al., 2023).

Role of T cells in AE pathogenesis
Besides B cells, the role of T cells in the development of AE should not be overlooked.Active immunization with NMDAR holoproteins induces a disease state in mice that recapitulates the core features of human anti-NMDAR encephalitis.In this study, CD4 + T-cell infiltration was an early feature, and both mature T and B cells were required for disease induction (Jones et al., 2019).

CD4 + T cells
CD4 + T cells, important regulatory cells of the adaptive immune system, can differentiate into Th cells or Tregs.Different Th cells can secrete one or more lineage-specific cytokines to exert their functions (Zhu and Paul, 2008).In healthy individuals, few tissue-resident T or B cells exist within the parenchyma.Studies have demonstrated the presence of CD4 + T lymphocytes in the perivascular space of the brain in patients with AE (Colony and Malamud, 1956).In addition, a more significant proportion of effector and memory T cells have been found in the CSF of patients with AE than in that of healthy controls (Dik et al., 2021).In experimental autoimmune encephalomyelitis (EAE) models, T cells can enter the CNS with the help of the gut microbiota (Engelhardt et al., 2017).Upon entering the CNS, they must recognize the cognate antigen from APCs in the CNS perivascular or leptomeningeal spaces.Failure to do so may result in apoptosis or even lead to cell departure from the CNS (Engelhardt et al., 2017).
The function of CD4 + T cells in AE is multifaceted because Th cells or Tregs can activate or dampen the function of other immune cells.Antigen presentation, co-stimulatory molecules, and the cytokine milieu can activate and modulate the naïve CD4 + T-cell response in peripheral lymph nodes (Sallusto, 2016).As a result, T cells differentiate into distinct Th cell subtypes, such as Th1, Th2, Th9, Th17, Tfh, and Treg cells.Specific Th cells play a crucial role in the pathogenesis of AE, as discussed next.
2.2.1.1.Th1 cells.Th1 cells were first defined in the 1980s and are characterized by the expression of the transcription factor T-bet.These cells specifically secrete IFN-γ and TNF-α, which have been demonstrated to exert detrimental effects on AE (Lee et al., 2014).In 2017, Lutz et al. revealed in an EAE mouse model that Th1 cells migrated into the CNS via caveolar transcytosis (Lutz et al., 2017).In another study, Th1 cells disrupted tight junctions and induced adhesion molecule expression on endothelial cells, thereby facilitating CD4 + T lymphocyte migration into the parenchyma (Sonar et al., 2017).Evidence has shown that microglia are the target of Th1 cells within the CNS.Th1 cells help trigger the differentiation of microglia and sustain local inflammation by secreting cytokines.TNF-α has been shown to induce the production of metalloproteinases and disrupt the astrocyte-foot structure to recruit myeloid cells (Engelhardt et al., 2017).

Th2 cells.
Th2 cells, which express the transcription factor GATA3 and secrete the cytokine IL-4, mainly defend against parasites (Bettelli et al., 2006).However, the function of Th2 cells in AE has not been fully elucidated due to limited research, especially in animal models.In an EAE mouse model, heparanase alleviated inflammation by upregulating the levels of cytokines secreted by Th2 cells, such as IL-4 and IL-10 ( Bitan et al., 2010).IL-4 and IL-13 secreted by Th2 cells might be associated with the presence of pathogenic IgG4 antibodies (Wagner et al., 2020).However, in a clinical study, the serum IL-4 and IL-10 levels were not significantly different between patients with AE and controls (Wesselingh et al., 2023).
2.2.1.4.Th17 cells.Th17 cells, defined by the expression of the transcription factor nuclear receptor RORγt and secretion of the cytokine IL-17, have been implicated in basal ganglia encephalitis, a condition precipitated by recurrent Group A Streptococcus infections in children (Platt et al., 2020).Th17 cells have been shown to induce BBB leakage by releasing pro-inflammatory cytokines such as IL-17 and IFN-γ.Th17 lymphocytes enter the CNS prior to Th1 cells via tight junction remodeling (Lutz et al., 2017;Daneman and Engelhardt, 2017).The expression of CCR6 on Th17 cells also helps them cross the BBB by interacting with CCL20 on the choroid plexus (Reboldi et al., 2009).Furthermore, Th17 cells can stimulate microglial activation, leading to the secretion of cytokines such as CCL2, which facilitates caveolin-1-mediated transcytosis in CNS endothelial cells, promoting the deposition of IgG (Stamatovic et al., 2009).In mice, the lack of Th17 lymphocytes leads to reduced BBB leakage, diminished antibody infiltration into the CNS, and decreased microglial activation (Platt et al., 2020).
2.2.1.5.Th22 cells.Th22 cells constitute a cell subset related to Th17 cells and are characterized by the expression of IL-22 and the master transcription factor aryl hydrocarbon receptor.They were initially found to be involved in the immunopathology of skin diseases (Duhen et al., 2009).However, whether Th22 cells play a pro-or antiinflammatory role in AE is unknown.
2.2.1.6.Tfh cells.Tfh cells express the transcription factor B-cell lymphoma 6 (BCL6) and form GCs together with B cells.They are required for antibody isotype switching in germinal center B cells (Crotty, 2019).These compounds facilitate the formation of LLPCs, contributing to refractory AE (Jain and Yong, 2022).In addition, Tfh cells are involved in CD8 + T RM cell proliferation and cytotoxic function (Frieser et al., 2022).
2.2.1.7.Tregs.Tregs, characterized by the expression of the transcription factor forkhead box protein P3 (FOXP3), CD25, and CTLA4 in humans, suppress the inflammatory response (Roncarolo and Gregori, 2008).A clinical study suggested that the number of Tregs and the mRS score were significantly negatively correlated in patients with AE.In addition, the Treg frequency was greater in patients with paraneoplastic asthma than in patients with synaptic antibodies or seronegative patients (Byun et al., 2021).In a mouse EAE model, the use of an antiinflammatory peptide increased the number of Tregs in the brain, thereby reducing symptoms (Shapira et al., 2010).

CD8 + T cells
CD8 + T lymphocytes can recognize brain cells that express major histocompatibility complex I (MHCI) and can differentiate into cytotoxic T cells to exert cytotoxic effects on neurons with intracellular selfantigen by secreting granzyme B and perforin (Bien et al., 2012).MHCI in neural cells is not constitutively expressed but is induced under certain conditions (Neumann et al., 2002).
In patients with paraneoplastic or anti-GAD65 limbic encephalitis, the infiltrating T cells cause neuronophagia, granzyme B neurotoxicity, neuronal loss, and gliosis (Bien et al., 2012).The loss of NeuN + neurons in the hippocampus is strongly associated with the infiltration of CD8 + T cells (Dik et al., 2021).A previous study revealed that an increasing number of CD8 + T cells in CSF was associated with memory impairment in patients with AE (Hansen et al., 2022).Another clinical study found that the low CD4/CD8 + T-cell proportions in CSF were associated with blood-CSF barrier dysfunction and inflammation in patients with LE (Hansen et al., 2020a).It challenged the CD4/CD8 + T-cell ratio in peripheral blood as a biomarker for LE.In experimental studies, chronic inflammation and neuron loss were associated with tissue-resident CD8 + memory T cells rather than with recirculating CD8 + T cells.Their proliferation and cytotoxic function are under the control of CD4 + T cells, especially Tfh cells (Frieser et al., 2022) (Fig. 2).

Conventional therapy
The conventional treatments for AE consist of intravenous steroids, IVIG, and PLEX (Yang and Liu, 2021).Recently, a large-cohort study analyzed the data of 335 patients and 2669 apheresis cycles.It reflected that apheresis treatment improved the functional disability measured via the mRS in patients with AE.Besides, these patients might also benefit from later B-cell depletion therapy (Fousse et al., 2024).In addition, alkylating agents such as cyclophosphamide and azathioprine are often used as complementary therapies for treating AEs (Brummaier et al., 2013).However, 19%-33% of patients respond poorly to the treatments.Hence, emerging monoclonal antibodies (mAbs), known as third-line therapy, offer hope for patients with refractory AE (Wright and Wood, 2020).

Rituximab
Rituximab, a genetically engineered chimeric mAb, was initially applied to treat non-Hodgkin's lymphoma (NHL) (Sabroe et al., 2000).It targets the transmembrane protein CD20 expressed on immature, mature, and memory B cells, except for B-cell precursors and LLPCs (Zografou et al., 2021).Consequently, rituximab can reduce the number of CD20 + SLPBs through antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), phagocytosis, and apoptosis (Klein et al., 2013;Abulayha et al., 2014).Besides B cells, a small number of CD3 + T cells that express CD20 can also induce apoptosis (Graves et al., 2014).An observational cohort study including 577 patients with anti-NMDAR encephalitis demonstrated the effectiveness of rituximab when first-line therapies failed (Titulaer et al., 2013).In addition, rituximab can also serve as a long-term immunosuppressant in patients with recurrent AEs and lower the chance of relapse (Abboud et al., 2021b).Patients with anti-CASPR2 and anti-LGI1 encephalitis seemed to respond well to rituximab, whereas the effectiveness of rituximab in patients with anti-GAD65 encephalitis was less satisfactory (Thaler et al., 2021).
The limitation of the application of rituximab may originate from its inability to reduce the number of CD20 − LLPCs and overcome the BBB (Gresa-Arribas et al., 2014).The intrathecal administration of rituximab has been reported to alleviate clinical symptoms in refractory patients (Casares et al., 2019).The main side effects were neutropenia and infusion-related incidents in a retrospective study of patients with AE receiving rituximab treatment (Lee et al., 2016a).

Ocrelizumab
Ocrelizumab, another humanized anti-CD20 mAb used to reduce the immunogenicity of rituximab, was developed for application in adults with relapsing-remitting multiple sclerosis in March 2017 in the United States (Lamb, 2022).The target epitope was different but structurally similar to that of rituximab.In vivo, ocrelizumab had a more favorable benefit-to-risk profile than rituximab because it caused an increased incidence of ADCC and a reduced CDC effect (Sellebjerg et al., 2020).A randomized controlled trial evaluated the efficacy of ocrelizumab for treating AEs.However, the study failed to meet its main objective due to the small sample size (Blackburn et al., 2022).However, additional multicenter clinical trials and robust therapy plans are needed to achieve adequate enrollment in evaluating the effectiveness of ocrelizumab for treating AEs.

Inebilizumab
Inebilizumab is a humanized IgG1 antibody that targets CD19 expressed on B cells, particularly on CD20 − LLPCs (Hoshino et al., 2020).It was developed for treating a range of autoimmune diseases and was first approved for treating NMOSD (Frampton, 2020).An observational study revealed that the proportion of CD19 + B cells in the peripheral blood and CSF was greater in patients with serum antibodynegative LE than in those with autoimmune antibodies to the CNS (Hansen et al., 2020b).In mice, inebilizumab has a superior ability to deplete B cells (Chen et al., 2016).The use of inebilizumab for treating patients with anti-NMDAR antibodies was investigated in the ExTIN-GUISH trial, which started in 2020 (NCT04372615).As inebilizumab can deplete CD20 − plasma blasts and plasma cells, it potentially has an advantage over rituximab.

Daratumumab
Daratumumab is a human mAb that targets CD38, a transmembrane glycoprotein expressed in plasma cells and T cells (Nooka et al., 2019).Daratumumab was initially used for treating multiple myeloma, depleting plasma cells, and modulating T-cell function.In a case report of a 60-year-old patient with anti-CASPR2 encephalitis, the application of daratumumab ameliorated clinical symptoms and reduced antibody titers in the CSF; however, the patient died of G − septicemia (Scheibe et al., 2020).Daratumumab could reduce the autoimmune antibody titers in the serum and CSF of refractory patients with anti-NMDAR AE (Ratuszny et al., 2020).However, these positive effects could not be attributed to daratumumab because the effect of previously used immunosuppressive therapies cannot be ignored.In 2022, a singlecenter retrospective case series of seven autoimmune patients showed substantial clinical improvement after the application of daratumumab (Scheibe et al., 2022).Current data are insufficient to make appropriate recommendations for the use of daratumumab for treating AEs.

Bortezomib
Bortezomib is an inhibitor of the 26S proteasome that predisposes both short-and long-lived plasma cells to apoptotic death, reducing autoantibody titers.This is because it reversibly inhibits the function of the ubiquitin-proteasome system, preventing the degradation of abnormal proteins in cells (Lazzarin et al., 2020).As expected, bortezomib has been shown to deplete antibody-secreting cells insensitive to rituximab (Yimer et al., 2019).Initially used to treat multiple myeloma (Ito, 2020), bortezomib has been reported to be a promising escalation agent for patients with refractory anti-NMDAR AE (Schroeder et al., 2018;Sveinsson et al., 2017;Wang et al., 2021), but side effects such as infections and neuropathy cannot be ignored (Dinoto et al., 2021).Current evidence suggests that bortezomib is an effective treatment for refractory anti-NMDAR encephalitis, but caution is warranted because more than one-third of patients experience side effects.

mAbs targeting IL-6
Tocilizumab, developed by Japanese scientists in 1993, is a humanized mAb that targets IL-6R, blocking IL-6-mediated signaling pathways.Consequently, it inhibits the stimulatory effect of IL-6 on both B and T cells (Mihara et al., 2005).
A patient with anti-CASPR2 syndrome was successfully treated with tocilizumab (Krogias et al., 2013).An institutional cohort study and a series of pediatric cases revealed the clinical benefits of using tocilizumab compared with first-line treatments; tocilizumab therapy was found to be significantly beneficial in both adults and children.In addition, a majority of responders to tocilizumab showed clinical improvements in the early treatment phase (Lee et al., 2016b;Randell et al., 2018).A T-SIRT study of patients with anti-NMDAR encephalitis revealed that the use of tocilizumab provided a better clinical outcome than traditional first-line therapy.Delayed application of combined immunotherapy was less effective (Lee et al., 2021).
However, the side effect of tocilizumab may be that it can significantly reduce C-reactive protein levels, which can sometimes disguise an underlying infection (Rossi et al., 2015).Therefore, clinicians should consider infections when using tocilizumab, especially in patients receiving multiple immunotherapy agents.
Satralizumab is another humanized mAb that targets IL-6R.It was designed based on tocilizumab using a novel antibody recirculation technology.It dissociates from receptors at an acidic pH within endosomes and is returned to circulation to prolong its cycling duration (Levy et al., 2021).In a randomized, double-blind, multicenter study involving patients with NMOSD, satralizumab was found to effectively reduce the rate of NMOSD relapse (Yamamura et al., 2019).However, no clear evidence supports its efficacy in treating AE.

NMDAR-CAAR-T cells to selectively eliminate anti-NMDAR B cells
CAR-T therapy has been a novel leukemia treatment since 2017 (Wang et al., 2017).As anti-NMDAR encephalitis is the most common AE, Reincke et al. engineered chimeric autoantibody receptor T cells expressing the NMDAR antigen subunit.In mice, this novel approach could reduce NMDAR autoantibody levels in the serum and eliminate autoreactive B cells (Reincke et al., 2023).However, further clinical trials are needed to validate its efficacy and safety.

Natalizumab
Natalizumab is a humanized antibody against VLA-4.VLA-4, also known as α4β1, is expressed on both T and B cells.It interacts with VCAM-1 on endothelial cells to transmigrate through the BBB (Alon et al., 1995).It is used in treating AE, mainly paraneoplastic neurological syndrome (PNS), which is more likely to be caused by T cells than Hu-Ab.Autopsy studies of patients with PNS have shown cytotoxic T-cell infiltration around neurons with neuronal deficits (Sillevis Smitt et al., 1995).
In mice, natalizumab injection prevents the development of autoimmune inflammation (Yednock et al., 1992).Hottinger et al. reported that the administration of natalizumab helped decrease CNS inflammation in a female patient with small-cell lung cancer (Hottinger et al., 2018).A male patient with lung adenocarcinoma developed severe mental confusion after receiving four cycles of pembrolizumab and was diagnosed with meningoencephalomyelitis related to the use of pembrolizumab.He was successfully treated after receiving a natalizumab injection (Basin et al., 2021).However, in a phase II trial, natalizumab improved the course of Hu-PNS but was not superior to other reported immunosuppressive agents (Bastiaansen et al., 2021).More effective treatments are urgently needed.

Low-dose IL-2
IL-2 can influence the differentiation and survival of both effector cells and Tregs (Abbas et al., 2018).However, Tregs seem more sensitive than other T cells and have a lower threshold for IL-2 as effector T cells.
A retrospective clinical study of 10 patients suggested that low-dose IL-2 was a viable treatment for refractory AE because the mRS scores improved significantly after low-dose IL-2 treatment (Lim et al., 2016) (Fig. 3).

Rapamycin
Rapamycin, acts as an inhibitor of mTOR by forming a complex after binding to FK506-binding protein 12, which can suppress the proliferation of T cells (Canivet et al., 2015).Studies demonstrated that rapamycin exerted therapeutic and protective effects on the injured CNS by inhibiting the inflammatory response in patients with stroke.In a retrospective study of 17 patients with anti-Hu encephalitis treated with rapamycin, the dysfunction and neuronal damage improved in more than half of the patients (de Jongste et al., 2015).However, the outcome of this T-cell-targeted therapy was not superior to that of other immunotherapies for Hu-PNS.In a mouse model of anti-NMDAR encephalitis, rapamycin could alleviate mitochondrial dysfunction and behavioral disorders (Kong et al., 2024).

Other potential therapeutic approaches related to B and T cells
Until recently, the major treatment applications for AE have been associated with the depletion of B cells.Besides rituximab and ocrelizumab, ofatumumab is another mAb that targets CD20 and has been approved for the treatment of multiple sclerosis (MS) in 2020 (Cencioni et al., 2021).In addition, atacicept, which is already used to treat systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), affects only the function of naïve and antibody-secreting cells (Tak et al., 2008).The inhibitor of Bruton tyrosine kinase (BTK), which is involved in B cell activation and B cell receptor signaling, has been tested in MS (Montalban et al., 2019).However, detailed information about their use in treating AE is still lacking.
The balance between effector Th17 cells and Tregs is an important factor for maintaining a healthy immune environment (Knochelmann et al., 2018).Besides low-dose interleukin-2 (IL-2), other immunomodulatory treatments, such as sphingosine receptor modulators and cladribine, have regulatory effects on MS.More evidence is needed to support their development for application in treating AE (Moser et al., 2020).In addition, RORγt plays a central role in the development of Th17 cells.Therefore, RORγt can be another therapeutic target (Han et al., 2015).The inhibitors, including digoxin, TM920, and GSK805, seem to be selective and potent.However, the evidence mainly comes from animal models, and additional clinical trials are needed to support the effectiveness of these treatments (Xie et al., 2014).In patients with severe AE refractory to conventional therapy, autologous hematopoietic stem cell transplantation, which enables the establishment of a new, tolerant immune system, might be a promising treatment option (Muraro et al., 2017) (Tables 1 and 2).

Knowledge gap and future perspectives
AE is an inflammatory disease of the CNS usually accompanied by the production of autoantibodies.The diagnosis and treatment of AE are currently hampered by an incomplete understanding of its pathogenesis.The clinical diagnosis of AE often relies on the detection of antibodies in serum and CSF.Additionally, the treatment option for antibodynegative patients is limited by the lack of a homogeneous and comparable patient population.The cause of AE is still unclear and may be related to a systemic tumor.
Although the symptoms of AE seem to be severe, prompt diagnosis and immunotherapy can significantly ameliorate or even cure the disorder.Third-line treatment approaches have emerged, but many  Terminally differentiated B cells that produce large amounts of antibodies and are short-lived unless they find a survival niche allowing long-term maintenance.
Daratumumab Bortezomib (Cordani et al., 2020) questions are still unresolved.The most prominent issue is the lack of evidence from prospective or controlled studies.However, further studies are needed to explore these approaches for prompt diagnosis and scientific treatment.

Fig. 1 .
Fig. 1.Role of B lymphocytes and several associated antibodies against neuronal cell surface antigens in AE.(a) In AE, the activation of naïve B lymphocytes occurs in the deep cervical lymph nodes.In the germinal center, naïve B cells engage with antigens and differentiate into plasma and memory B cells with the help of T follicular helper cells.Memory B cells can be reactivated and secrete autoantibodies upon encountering the same antigens.(b) B cells cross the BBB at the level of post-capillary venules.B lymphocytes must overcome the tight junctions of specialized BBB endothelial cells and the basement membrane formed by pericytes and glia limitans to reach the parenchyma.Although the mechanism underlying this phenomenon has not been fully elucidated, the function of chemokines cannot be ignored.ICAM-1 and VCAM-1 on the blood endothelium can interact with adhesion molecules on B lymphocytes to facilitate B-cell recruitment into the CNS.(c) In patients with anti-NMDAR AE, autoantibodies can cause cross-linking and internalization, leading to the depletion of NMDARs.(d) In patients with anti-GABA B R AE, autoantibodies bind to the extracellular domain of the GABA-B1 subunit, blocking the function of target receptors.(e) In patients with anti-LGI1 IgA encephalitis, autoantibodies interfere with protein-protein interactions, disrupting the normal function of the receptor.LGI1, leucinerich glioma-inactivated 1.

Fig. 2 .
Fig. 2. Role of Th1, Th17, and CD8 + T cells in AE pathogenesis and three mAbs associated with the BBB.In patients with AE, the BBB is a significant barrier for lymphocytes and antibodies to overcome.(a) Th17 lymphocytes enter the CNS prior to Th1 cells through tight junction remodeling, whereas caveolin-1 facilitates Th1 cell migration.Once within the CNS, both Th1 and Th17 cells can secrete cytokines, such as IFN-γ, TNF-α, and IL-17, which play pro-inflammatory roles and activate the function of microglia.(b) CD8 + T lymphocytes migrate into the CNS via the interaction between CLA-4 and VCAM-1.CD8 + T cells exert cytotoxic effects on antigens within neurons by secreting granzyme B and perforin, which induces apoptosis.(c) Natalizumab inhibits the migration of CD8 + T cells and Th1 cells by interfering with VLA-4 and VCAM-1 expression.The limitation of rituximab is that it cannot cross the BBB, whereas tofacitinib can cross the BBB.

Fig. 3 .
Fig. 3. Third-line therapeutic strategies involving monoclonal antibodies and other approaches.(a) Associated therapeutic strategies targeting B, T, and plasma cells.Bortezomib, tocilizumab, and daratumumab address the limitation of rituximab not targeting plasma cells.Tofacitinib overcomes the issue of rituximab not crossing the BBB.(b) Potential therapeutic targets involve an immune balance between effector T and regulatory T lymphocytes.Low-dose IL-2 and rapamycin can alleviate clinical symptoms by modulating the immune balance between Tregs and effector T cells.

Table 1
Summary of the novel therapy strategies associated with AE.

Table 2
Summary of the detection of B and T lymphocytes' phenotype involved in AE.IgM hi represent a crucial link between immature B cells in the bone marrow and mature peripheral B cells.CD20 + CD27 + IgD − A subset of B cells that is antigen experienced and can participate in secondary immune responses CD20 + CD27 + CD38 + The precursor to plasma cells that has begun to produce antibodies and is still proliferating