Dawn of CAR-T cell therapy in autoimmune diseases

SLE

SLE is a systemic autoimmune disorder characterized by the presence of various autoantibodies that form immune complexes, which flow in the bloodstream and deposit in tissues to activate the complement system and recruit cytokines, ultimately inducing systemic damage.^[16] Current treatments include glucocorticoids, immunosuppressants, and belimumab, but they often fall short in achieving drug-free remission. To address this, anti-CD19 CAR-T cell therapy has been proposed as a potential treatment. In animal studies, anti-CD19 CAR-T therapy showed promising results in alleviating lupus manifestations. Mice treated with anti-CD19 CAR-T cells exhibited reductions in plasma antibodies and improvements in longevity, proteinuria, splenomegaly, and skin damage. The CAR-T cells demonstrated sustained viability and functionality over time.^[11] Another study compared anti-CD19 CAR-T cells with a mAb and found that CAR-T cells cleared more CD19⁺ B cells and prolonged the lifespan of mice.^[17] CAR-natural killer (NK) cells, another type of cellular therapy, also showed potential in treating SLE. Anti-programmed cell death-ligand 1 (PD-L1) CAR-NK92 cells can recognize and kill follicular helper T cells characterized by elevated PD-1 expression on the surface, consequently reducing memory B cells proliferation and differentiation, immunoglobulin secretion, as well as ameliorating splenomegaly.^[18] In summary, both CAR-T and CAR-NK cell therapies demonstrated efficacy in alleviating SLE manifestations in animal models, priming the ground for promising clinical trials.

In a pioneering clinical trial, autologous anti-CD19 CAR-T cells were administered to a patient with severe/refractory SLE who had failed to respond to other treatments. The therapy resulted in the depletion of circulating B cells, decreased antibody levels, and improvements in proteinuria and SLEDAI scores.^[14] The same team then extended their investigation to five patients with refractory SLE, infusing 1 × 10⁶/kg anti-CD19 CAR-T cells. The results indicated that anti-CD19 CAR-T therapy was well tolerated and could induce rapid remission in severe refractory SLE.^[19] Besides, infusion of anti-CD19/BCMA CAR-T cells exhibited double benefits in a female with SLE and diffuse large B-cell lymphoma.^[20] To date, 20 clinical trials of CAR-T therapy treating R/R SLE are underway, employing anti-CD19/BCMA CAR-T cells or anti-CD19 CAR-T cells [Supplementary Table 3, http://links.lww.com/CM9/B986]. In conclusion, both animal and clinical trials highlight the potential of CAR-T therapy for R/R SLE, but long-term, large-scale clinical trials are imperative to firmly establish its efficacy and safety.

RA

RA is an autoimmune disorder that affects joints and connective tissues (e.g., blood vessel, lung), significantly diminishes patients’ quality of life, with an approximately 4-fold increase in the incidence in woman vs. man.^[21] In order to develop precise and radical remedies for RA, researchers are focused on targeting the anticitrullinated protein antibody, which is responsible for causing damage to multiple organs by binding to citrullinated residues within self-proteins (e.g., vimentin, α-enolase, fibronectin, fibrinogen, type II collagen, etc.).^[22] One study by Zhang et al^[23] involved the development of CAR-T cells that specifically targeted overreactive B cells, guided by citrullinated type II collagen (COII) and fluorescein isothiocyanate. In the presence of antigenic peptides, these CAR-T cells demonstrated proficiency in eliminating hybridoma cells generated through antigenic peptide immunization, alongside B cells from mice with COII-induced arthritis and RA patients. Notably, the CAR-T cells showed minimal off-target effects in killing macrophages, monocytes, and dendritic cells that express FcγR. Another approach involved the use of HLA-DR1-based CAR-T incorporating COII into its molecular architecture to target pathogenic CD4⁺ T cells that recognize RA-related autoantigens presented by HLA-DR alleles. In vitro, anti-DR1-COII CAR-T cells recognized and depleted CD4⁺ T cells; in COII-induced mice, these CAR-T cells significantly reduced the incidence, delayed the onset, and mitigated the severity of arthritis. Additionally, serologic markers such as autoantibodies and T cell proliferative responses were markedly reduced.^[24] In summary, preclinical studies have demonstrated the efficacy and safety of CAR-T therapy for RA. However, the translation to clinical trials is essential to validate the feasibility of CAR-T therapy in human settings, encompassing considerations of safety, efficacy, durability, and cost-effectiveness.^[25]

Type 1 diabetes mellitus (T1DM)

T1DM, also known as insulin-dependent diabetes mellitus, usually begins in childhood or adolescence. Due to autoantibodies such as glutamic acid decarboxylase antibodies and islet cell antibodies attacking pancreatic β cells, there is an absolute insulin deficiency, rendering blood glucose levels more volatile than in type II diabetes mellitus. Long-term insulin therapy is necessary for T1DM management, but patient adherence can be challenging. CAR-T therapy has emerged as a potential cure for T1DM by addressing this issue. In one study conducted by Zhang et al^[26], they utilized the specific targeting ability of mAb287, which binds to the I-Ag7-B:9-23(R3) complex, to delay or prevent T1DM onset in non-obese diabetes (NOD) mice. They designed anti-mAb287 CAR-T cells, which could both kill antigen-presenting cells expressing this complex in vitro, and significantly delay the onset of hyperglycemia in mouse models of T1DM. However, over time CAR-T cells became progressively exhausted, there was no discernible difference in T1DM incidence among groups by week 30. On the other hand, another study explored the use of 5MCAR-T cells with five structure modules, which demonstrated lasting effects for up to 1 year in vivo. These CAR-T cells prevented T1DM onset, attenuated islet inflammatory response, and alleviated symptoms by specifically recognizing and eliminating CD4⁺ T cells expressing the BDC 2.5 T cell receptor.^[27] In addition to CAR-T cells, researchers have investigated CAR-Tregs for T1DM. For instance, anti-HPi2 (clone HIC1-2B4.2B) CAR-Tregs demonstrated functional activity, however, their rapid exhaustion due to the multiple expression of HPi2 on all CD4⁺ T cells created a constant activation signal that hindered sustained in vivo function.^[28] Similarly, insulin-specific CAR-Tregs lasted up to 4 months in vivo but failed to prevent T1DM development.^[29] Additionally, tetraspanin 7, a membrane protein highly expressed on the surface of pancreatic β cells, was found to be inadequate recognized by CARs.^[30] These setbacks may stem from inappropriate target antigens. Continued research endeavors are indispensable to refine and optimize these approaches, ensuring a tailored and effective therapeutic strategy for T1DM.

MS

MS is an autoimmune-mediated inflammatory demyelinating disease of the central nervous system (CNS). The primary targets of the autoimmune attack are myelin oligodendrocyte glycoprotein (MOG) and myelin basic protein (MBP).^[31] Currently, studies exploring CAR-T therapy for MS largely rely on preclinical investigations using EAE mouse models.^[32] Fransson et al^[33] initially designed anti-MOG CAR-Tregs for EAE, demonstrating significant in vitro immunosuppressive effects. In vivo, these cells successfully reversed the disease state and prevented relapse. Moreover, the overall astrocyte proliferation and myelin production were more vigorous in the anti-MOG CAR-Treg treatment group, suggesting a broader influence. Similarly, another preclinical study showed that anti-MOG/MBP CAR-Tregs alleviated EAE disease scores and delayed disease progression.^[34] In addition to the immune-regulatory effects of CAR-Tregs, anti-CD19 CAR-T cells have also promise in ameliorating EAE by targeting B cells, which contribute to autoantibody production. Gupta et al^[35] reported that anti-CD19 CAR-T cells significantly reduced EAE scores, delayed disease onset, and maintained the absence of CD19⁺ B cells for over 25 weeks, surpassing the efficacy of CD20 antibodies. However, contrasting findings were observed in another experiment where injection of anti-CD19 CAR-T cells resulted in a worsened form of EAE after 2 weeks, with a broader range of pathological demyelination and axonal loss, despite helping clear meningeal B-cell deposits implicated in the immunopathology of MS.^[36] This may be attributed to the diverse autoimmune mechanisms involved in EAE. As for the CNS autoimmune diseases such as EAE, what makes CAR-related therapy outperform mAbs is its ability to migrate to the CNS to exert therapeutic effects. For example, B cells were thoroughly depleted in the periphery and within the CNS by anti-CD19 CAR-T cells, indirectly proving their penetration into the CNS.^[35] And anti-MOG CAR-Treg cells were also shown to localize in the soma of several brain structures such as whiter matter of cerebellum and relieve EAE score.^[33] This characteristic of CAR-T cells makes them favorable candidates for the treatment of CNS autoimmune diseases, while the differing experimental outcomes necessitate comprehensive animal studies to underscore CAR therapy’s efficacy and safety profile in MS.

Neuromyelitis optica spectrum disorder (NMOSD)

NMOSD is a group of autoimmune-mediated CNS primary inflammatory demyelinating diseases that mainly involve the optic nerves and the spinal cord, with autoantibodies targeting aquaporin (AQP)-4. Binding of AQP4-IgG produced by plasma cells to AQP4 on the surface of astrocytes recruits granulocytes and activates the complement system,^[37] resulting in optic nerve and spinal cord injury. Qin et al^[38] used anti-BCMA CAR-T cells in treating R/R AQP4-IgG-positive NMOSD. The primary outcome was safety, which was mainly reflected in hematologic toxicity, including leukopenia (100%), neutropenia (100%), anemia (50%), and thrombocytopenia (25%), and cytokine release syndrome (CRS) (grades 1–2). Notably, grade 3 and higher infections occurred mainly in patients over 60 years old with long-standing autoimmune dysfunction and use of immunosuppressants for more than 20 years. In terms of clinical manifestations, 11 (92%) patients achieved drug-free remission during a median follow-up period of 5.5 months, and showed varying degrees of improvement in scales, vision, ambulation, and voluntary urination and defecation; and serologic markers such as AQP4-IgG was also reduced, affirming anti-BCMA CAR-T cells’ efficacy and safety for AQP4-IgG-positive NMOSD patients. Moreover, the role of AQP4-IgG suggests the feasibility of constructing anti-AQP4 CAAR-T cells, specifically eliminating autoantibody-producing plasma cells, which aligns with the precise treat-to-target goal.

Myasthenia gravis (MG)

MG is an autoimmune disease caused by neuromuscular junction synaptic lesions.^[39] It mainly manifests as morbid fatigue of the affected muscles, with severe cases impinging on respiratory muscles, leading to dyspnea. Current treatments for MG include cholinesterase inhibitors, immunosuppressants, and glucocorticoids. However, there are still a subset of patients showing resistance to these modalities. As a result, precise immunotherapy is emerging as a promising approach for MG treatment. For example, anti-muscle tyrosine kinase (MuSK) CAAR-T cells have been designed and have shown specific cytotoxicity against anti-MuSK B cell receptor (BCR)-expressing B cell in preclinical studies,^[40] laying the groundwork for the clinical application of CAR therapy for MG. Apart from targeting autoantibodies like MuSK, anti-BCMA CAR-T cells also hold the potential for MG treatment. Recently, CAR-T cells engineered with RNA (rCAR-T) targeting BCMA were used to treat 14 patients with generalized MG. Remarkably, these rCAR-T cells displayed significant reductions in MG severity scales for up to 9 months of follow-up, with no observed adverse reactions such as dose-limiting toxicity, CRS or neurotoxicity. The use of temporary and non-replicable mRNA in these CAR-T cells reduces signal amplification, leading to decreased side effects and enabling outpatient infusions.^[41] Currently, there are four ongoing clinical trials of CAR-T cell therapy targeting BCMA and CD19, as well as CAAR-T cell therapy targeting MuSK for MG treatment [Supplementary Table 3, http://links.lww.com/CM9/B986].

PV

PV is an autoimmune skin disorder, characterized by IgG that attacks desmoglein-1 (Dsg1) and Dsg3 depositing at the surface of keratinocytes.^[42] Treating PV with glucocorticoids presents challenges, including increased infection risk and relapse after drug withdrawal.^[43] To address these challenges, Ellebrecht et al^[12] engineered anti-Dsg3 CAAR-T cells to specifically eliminate self-reactive B cells. This experiment verified that these cells can recognize and kill anti-Dsg B cells in vitro, as well as significantly improve gross and serologic performance in vivo. Notably, anti-Dsg3 CAAR-Ts demonstrated efficacy akin to anti-CD19 CAR-T cells in eliminating CD19⁺ Nalm-6 B cells expressing anti-Dsg3 BCRs from PV patients. Furthermore, the presence of anti-Dsg antibody in the blood of patients was simulated, proving that the antibody does not neutralize, but rather bolsters the efficacy and persistence of CAAR-T. Another experiment demonstrated that anti-Dsg3 CAAR-T cells can decrease serum and tissue-bound autoantibodies and target cell burden.^[44] Overall, the preclinical findings strongly support the efficacy of CAAR-T cell therapy for PV. Accordingly, a phase I clinical trial of anti-Dsg3 CAAR-T cell therapy for mucosa-dominated PV is currently underway [Supplementary Table 3, http://links.lww.com/CM9/B986].

Other autoimmune diseases

In addition to these well-studied autoimmune diseases, CAR-T therapy is being expanded to diverse conditions.

Adverse effects

Persistence

The persistence of CAR-T therapy hinges on its ability to recognize target antigens and execute cytotoxic actions. However, there is a phenomenon known as B-cell recurrence, where CAR-T cells lose their functionality in vivo. This may result in the loss of self-immune system protection and potentially trigger disease relapse.^[73] Interestingly, a clinical trial of anti-CD19 CAR-T for the treatment of SLE observed B-cell reconstitution without associated SLE relapse, and the reconstituted B cells were mostly CD21⁺CD27^– naïve cells, which can be deemed as a reset of immune system for patients with autoimmune diseases.^[19] Similarly, none of the patients experienced a relapse of NMOSD during a median follow-up of 5.5 months, except one patient who had a suspected episode of decreased visual acuity in the left eye at month 14. It is worth noting that anti-BCMA CAR-T cells were detectable in only 17% of patients at month 6.^[38] This suggests that the persistence of functional CAR-T might not be the sole determinant of patients’ clinical outcomes. Furthermore, it was found that of the same CAR-T cells exhibited shorter persistence in patients with NMOSD compared to patients with MM, which may be attributed to the lower burden of BCMA target antigens in patients with NMOSD and the poor quality of intrinsic T cells resulting from the prolonged immunosuppressive therapy prior to CAR-T therapy in most patients with relapsed/refractory NMOSD.^[38] Whether the relatively short survival of CAR-T cells could lead to autoimmune diseases relapse needs longer follow-up and further monitoring. As for the long survival of CAR-T cells in vivo, it may bring additional benefits to patients or lead to long-term deficiency of B cells that increases the risk of infection. Whether the long persistence of CAR-T cells is necessary in autoimmune diseases requires further improvement of CAR-T cell engineering or adopting CAAR-T cells to specifically eliminate autoreactive B cells.

Resistance

While CAR-T therapy holds promise for refractory patients, there are instances where patients exhibit poor response. For example, in a clinical trial of anti-CD19 CAR-T for relapsed/refractory SLE, patient 3 encountered transient proteinuria at month 4 post-infusion. And his SLEDAI-2K score was 2, not decreasing to 0.^[19] In addition, patient 1 subjected to anti-BCMA CAR-T therapy experienced a suspected decrease in visual acuity 14 months after infusion.^[38] The early onset of poor response may be related to cumulative organ damage prior to treatment, which is consistent with the patient 3 with stage IV lupus kidney impairment at baseline. While later symptomatic relapses after CAR-T treatment could be attributed to target antigen loss or CAR-T cell exhaustion. In a flow cytometry analysis after anti-CD19 CAR-T treatment of R/R B-cell acute lymphoblastic leukemia, it was observed that 12 out of 17 patients progressed to CD19-negative disease, with genomic sequencing revealing CD19 mutations in all 12 patients,^[74] demonstrating that target loss due to mutation is one of the mechanisms of triggering relapse. Besides, CAR-T cells may fail to generate an immune response when the target antigen density falls below the threshold.^[75] And the level of AQP4-IgG in the serum of patient 1 with decreased visual acuity showed an earlier rebound, suggesting CAR-T cell exhaustion.

CAR-T structure innovation

Combination therapy

In cases of CAR-T therapy relapse, salvage therapy emerges as a potential approach to extend life expectancy. Van Oekelen et al^[89] recruited patients with MM who had relapsed following anti-BCMA CAR-T treatment. The results showed that these patients gained approximately 3.5 months of PFS after 2 months (median) of salvage regimen; gained a median of 9.1 months of PFS by administering T-cell related therapies (e.g., CAR-T and bispecific antibodies) immediately after relapse; and patients who received allogeneic/autologous hematopoietic stem cell transplantation as salvage therapy, achieved a median overall survival of 23.2 months. Although autoimmune disease relapse after CAR-T therapy has not yet been reported, the insights gained from salvage regimens provide valuable knowledge for addressing potential relapse scenarios. In addition to salvage therapy, combining two antigens as a target works as well. A clinical trial combined anti-BCMA CAR-T cells and anti-CD19 CAR-T cells, indicating an impressive overall response rate of 92%, with complete remission or better observed in 60% of MM patients, and the median PFS extended to 18.3 months. Therefore, compared to CAR-T therapy alone, combination therapy appears to induce more durable responses and prolong PFS,^[90] potentially shaping the optimization of treatment approaches for relapsed/refractory autoimmune diseases.

In conclusion, CAR-T cell therapy has shown considerable efficacy and safety in relapsed/refractory autoimmune diseases. It offers precise targeting and drug-free remission, distinguishing it from traditional therapies. Autoimmune diseases generally have a lower burden of target antigens compared to hematological tumors, reducing the required CAR-T dosage and the likelihood of adverse reactions. Despite these promising advancements, the realm of CAR-related therapy for autoimmune diseases remains relatively limited in scope. To further advance this field, additional experiments should be conducted with larger sample sizes, longer experimental timelines, and rigorous long-term safety monitoring. By building upon the knowledge gained from CAR-T treatment for hematological tumors, there is an opportunity to optimize the clinical application of CAR-related therapy for R/R autoimmune diseases. Continued innovation and research will contribute to the maturation of this therapy and its potential to provide improved outcomes for patients with autoimmune conditions.