Mechanistic rationale for CD19-targeted chimeric antigen receptor T-cell therapy in systemic sclerosis

The application of cellular therapy in SSc stems from the fundamental role of B lymphocytes in the initiation and perpetuation of the disease process, which is confirmed by numerous studies on SSc pathogenesis [7, 10]. B lymphocytes in SSc exhibit features of constitutive hyperreactivity, which manifests, among other things, as approximately 20% increased expression of the signaling molecule CD19 compared to healthy individuals. This CD19 overexpression lowers the activation threshold of B cells, leading to excessive stimulation and disruption of homeostasis, as manifested by the expansion of naive B lymphocytes alongside a decrease in the number of memory cells [10]. A consequence of this dysregulation is the differentiation of B lymphocytes into plasma cells that produce autoantibodies strictly correlated with the clinical phenotype, such as anti-Scl-70 and anti-RNA polymerase III (ARPA), whose titers are linked to disease activity and prognosis [7, 8]. Importantly, the pathogenicity of B lymphocytes in SSc extends beyond the production of classic nuclear autoantibodies; functional agonist autoantibodies have also been identified, directed, for example, against the platelet-derived growth factor receptor (PDGF) or the angiotensin II type 1 receptor. These antibodies can directly stimulate fibroblasts to produce collagen and promote vascular changes, thus forming a direct link between autoimmunity and fibrosis [10]. Furthermore, activated B lymphocytes perform crucial effector functions, secreting pro-inflammatory and pro-fibrotic cytokines, such as IL-6 and TGF-β, and acting as antigen-presenting cells, which stimulate T lymphocytes and drive chronic inflammation [7, 9].

Although B-lymphocyte depletion strategies using anti-CD20 monoclonal antibodies, such as rituximab (RTX), have demonstrated some clinical efficacy in improving skin conditions, their potential in SSc is limited by the incomplete elimination of pathogenic clones [6, 10, 11]. Rituximab often fails to achieve complete B-lymphocyte depletion in deep tissue compartments, such as fibrotic skin or lymph nodes, where these cells can survive and perpetuate the disease process [1, 6]. CAR-T anti-CD19 therapy offers a mechanistic advantage due to its broader target range and distinct mode of action. The CD19 antigen is expressed on a broader spectrum of the B-lymphocyte lineage than CD20, covering stages from pro-B lymphocytes through mature cells up to plasmablasts and early plasma cells [1, 12]. Plasmablasts, a key source of autoantibodies, often lack CD20 expression, rendering them resistant to RTX treatment, but they remain susceptible to anti-CD19 CAR-T therapy [1, 6, 10]. Furthermore, as a “living drug,” CAR-T cells are capable of active migration from the bloodstream into inflamed tissues, allowing for effective elimination of residual B lymphocytes inaccessible to passively acting antibodies [1, 10] (Fig. 1).

Fig. 1

Production and administration process of autologous anti-CD19 CAR-T cell therapy.

Anti-CD19 – anti-cluster of differentiation 19, CAR-T – chimeric antigen receptor T cells.

Support for this mechanism comes from histopathological studies showing extensive elimination of B lymphocytes from the skin of SSc patients following CAR-T cell infusion [10]. The complete depletion of B lymphocytes at both the peripheral and tissue levels by CAR-T cells may promote immune system reconstitution, a phenomenon commonly referred to as an “immunological reset” [1, 7]. Available clinical observations indicate that after a period of B-lymphocyte aplasia, lasting on average approximately 112 days, the B-cell population undergoes reconstitution characterized predominantly by naïve cells with a newly formed receptor repertoire lacking autoreactive clones [1, 10]. This process may interrupt pathological immunological memory and contribute to treatment-free remission in selected cases [1, 5]. However, the mechanism of action of CAR-T in SSc extends beyond the adaptive immune response, also affecting innate immunity. It has been demonstrated that the elimination of autoantibodies and circulating immune complexes after CAR-T therapy leads to the phenotypic “rejuvenation” of NK cells. In the active phase of the disease, immune complexes activate NK cells via the CD16 receptor, contributing to tissue damage; the cessation of this stimulus after CAR-T therapy restores the normal, less activated NK cell phenotype [11]. The potential clinical reflection of these molecular processes may be the regression of fibrotic changes observed in some studies, which were previously considered irreversible [1, 7, 8]. In patients treated with both autologous and allogeneic CAR-T products, stabilization as well as improvement in imaging parameters, including lung computed tomography and cardiac magnetic resonance imaging, were observed. In addition, a significant reduction in the modified Rodnan skin score (mRSS) was noted [7, 8, 10]. This suggests that the precise removal of the driving factor, which is autoreactive B lymphocytes, allows for the activation of endogenous tissue repair mechanisms even in advanced stages of the disease, although this hypothesis remains to be substantiated in larger prospective studies [7, 10, 13].

Characteristics of patients treated with CAR-T cell therapy for SSc are shown in Table I [7–9, 14, 15].

Primary and secondary endpoints

The primary objective of most studies was to evaluate the safety of CAR-T cell therapy in SSc. Key secondary outcomes included changes in the mRSS (with higher scores indicating worse skin fibrosis), imaging (which is a component of the assessment of lung fibrosis), anti-nuclear antibody (ANA) levels, ARPA levels (specifically the RP11 and RP155 subunits), and anti-DNA topoisomerase I (ATA) levels (which were assessed by immunoblot and ELISA). Changes in the lungs were assessed using forced vital capacity (FVC) and diffusing capacity of the lung for carbon monoxide (DLCO), whereas heart function was assessed by measuring left ventricular ejection fraction, pulmonary artery systolic pressure (PASP), and N-terminal pro-B-type natriuretic peptide (NT-proBNP). Other outcomes included changes in the European Scleroderma Trials and Research Group Activity Index (EUSTAR-AI) and the American College of Rheumatology Composite Response Index in Systemic Sclerosis (ACR-CRISS) score, which is a weighted score that includes the following 5 core set measures: mRSS, FVC as a percentage of the predicted value, the Health Assessment Questionnaire-Disability Index (HAQ-DI), and patient and clinician global assessments [16].

Efficacy of chimeric antigen receptor T-cell therapy in systemic sclerosis

The median age among the 21 SSc patients who underwent CAR-T therapy was 43 years. Women accounted for 12 of 21 (57%), and men for 9 of 21 (43%); however, the sex distribution varied across studies [7–9, 14, 15]. In the study by Pecher et al. [9], women accounted for 4 out of 5 (80%), whereas in other case series, the population consisted exclusively of men. The average disease duration was 3.2 years, and the follow-up period was 11.2 months. Nevertheless, the follow-up period was very short in some cases, so the results should be interpreted with appropriate caution. Auth et al. [7] observed a tenfold decrease in the average ANA titer after 3 months. In one patient, ARPA was the predominant autoantibody type, and it was completely eliminated following treatment. However, starting on day 618, ARPA became detectable again. In contrast, ATA concentrations decreased steadily over the observation period, reaching a maximum decrease of more than 90% relative to baseline. Müller et al. [15] also observed a 66% decrease in ATA levels after 6 months.

Lung function, as assessed by FVC and DLCO, improved significantly in most cases [7–9, 14, 15]. Among 9 patients in the CASTLE study, FVC increased significantly from a median of 2.83 to 3.23 l (+14.1%), while DLCO increased significantly from a median of 4.23 mmol/ml/kPa to 5.48 mmol/ml/kPa (+29.5%) [15]. Despite treatment, 3 patients showed progression of pulmonary fibrosis following the diagnosis of interstitial lung disease (ILD) associated with SSc. A reduction in ground-glass opacities, which are associated with ILD activity related to SSc, was observed in the lungs. Meanwhile, the severity of the reticular pattern, which reflects visible fibrous tissue remodeling, remained stable. Cardiac function was assessed only by Auth et al. [7]. Left ventricular ejection fraction remained stable in all patients during follow-up (55–60%). One patient had an NT-proBNP level of 7.202 pg/ml, which normalized during CAR-T therapy.

Skin lesions showed significant improvement, with an average reduction of 11.2 points on the mRSS scale across all patients. Müller et al. [15] reported a reduction in median mRSS scores from 26 to 12 points, whereas Auth et al. [7] observed improved skin condition in all patients, with a median decrease of 8 points over 100 days. This improvement was either maintained or further reduced during the 12-month follow-up.

The average improvement in the EUSTAR-AI index was 4.2 points [7–9, 14, 15]. According to a study by Auth et al. [7], the mean EUSTAR-AI score was 5.7 before the start of CAR-T therapy and decreased by an average of 3.4 points in 4 patients at the last follow-up. In their case report, Albach et al. [14] observed a 5-point improvement on the EUSTAR-AI scale. The remaining studies used the ACR-CRISS criteria, except for 5 patients described by Pecher et al. [9], who did not use EUSTAR-AI or ACR-CRISS. In the CASTLE trial, all patients met the ACR CRISS 25 response criteria; 8 out of 9 met the CRISS 50 (2 out of 5 items) criteria, and 7 out of 9 met the CRISS 50 (3 out of 5 items) criteria.

Assessment of skin changes, general activity, and adverse events (AEs) during CAR-T cell therapy are shown in Table II [7–9, 14, 15].

Cytokine release syndrome

Cytokine release syndrome is characterized as a systemic inflammatory response resulting from the rapid activation of CAR-T cells and subsequent cytokine release, particularly IL-6. The primary associated symptom is fever, which may be accompanied by hypotension, tachycardia, and an altered emotional state [17, 18]. Cytokine release syndrome is the most prevalent AE associated with CAR-T cell therapy. The severity of CRS is typically assessed using the American Society for Transplantation and Cellular Therapy (ASTCT) consensus criteria. These criteria are based on established guidelines and include factors such as fever and the need for supplemental oxygen or vasopressor support [17]. In cases of mild CRS, the recommended approach is threefold: observation, symptomatic treatment, and exclusion of infection. In contrast, the prevailing therapeutic approach for CRS ≥ 2 requires prompt administration of tocilizumab, often in combination with intravenous glucocorticoids (GCs) if the patient’s condition does not improve [18]. Refractory or severe CRS necessitates the administration of GCs, most commonly dexamethasone [19].

Immune effector cell-associated neurotoxicity syndrome

Immune effector cell-associated neurotoxicity syndrome (ICANS) is presented as a manifestation of neurological complications following chimeric antigen receptor CAR-T cell therapy. The pathophysiology of ICANS is based on neuroinflammatory mechanisms and damage to the blood-brain barrier [20]. As outlined in the clinical findings, the manifestation of symptoms includes convulsive seizures, ataxia, and confusion. Conversely, cerebral edema may be a consequence of severe ICANS [19]. The standard treatment for ICANS entails the intravenous administration of GCs (particularly dexamethasone) and anticonvulsants. Anakinra, an IL-1 receptor antagonist, is recommended for treating GC-refractory ICANS [21]. Moreover, in instances where toxic effects on the nervous system coexist with significant CRS, tocilizumab is the therapeutic agent of choice in such circumstances [18].

Cytokine release syndrome and immune effector cell-associated neurotoxicity syndrome during chimeric antigen receptor T-cell therapy for the treatment of systemic sclerosis

Recent scientific reports have focused on evaluating the efficacy and safety of CD19 CAR-T therapy in the treatment of SSc. Grade 1 CRS was observed in 12 of 21 patients (57%), while grade 2 CRS occurred in 2 of 21 patients (10%) who received CAR-T therapy. No cases of CRS above grade 2 were reported [7–9, 14, 15]. In the CASTLE study, the median time to onset of CRS was 1 day, and the median duration of CRS was 2 days [15]. In most cases of CAR-T therapy use in SSc, conservative treatment of grade 1 CRS proved sufficient. Müller et al. [15] administered tocilizumab to 5 of 9 patients (56%) and GCs to 1 patient. Current scientific reports on SSc suggest a favorable early neurological profile for ICANS treatment in this patient cohort. This assertion is bolstered by the absence of clinically significant ICANS among SSc patients who underwent the treatment model presented [7–9, 14, 15]. However, the current sample size of treated patients is insufficient to rule out the possibility of rare or delayed neurotoxicity. Compared with hematologic malignancies, SSc appears to be associated with a lower incidence and reduced severity of treatment-related toxicity. This phenomenon can be attributed to the lower degree of “burden” on target cells in patients with autoimmune diseases [17, 18, 22, 23]. Criteria for bone marrow suppression associated with lymphodepletion therapy are shown in Figure 2.

Fig. 2

Criteria for bone marrow suppression associated with lymphodepletion therapy.

ANC – absolute neutrophil count, GC – glucocorticosteroid, ICAHT – immune effector cell-associated hematotoxicity, LICATS – local immune effector cell-associated toxicity.

Other adverse reactions

Potential AEs of CAR-T therapy in SSc, in addition to the previously mentioned CRS and ICANS, encompass prolonged cytopenia [24]. Müller et al. [15] attempted to standardize criteria for reporting myelotoxicity associated with lymphodepletion therapy using immune effector cell-associated hematotoxicity (ICAHT) and local immune effector cell-associated toxicity (LICATS). Late ICATS occurred in 6/9 (66%) patients, early ICAHT in 3/9 (33%), and late ICAHT in 1/9 (11%) [15]. These criteria require wider dissemination and further validation, but they may prove useful for reporting this type of AE. Dahunsi et al. [25] posited that the administration of granulocyte-colony stimulating factor and blood transfusions is a recommended course of action. Furthermore, in cases of refractory thrombocytopenia, thrombopoietin agonists are recommended. As demonstrated in the existing body of research, delayed infections are among the most significant late complications, necessitating antiviral prophylaxis and immunoglobulins. The pathomechanism of this phenomenon has been determined to result from prolonged B-cell aplasia and hypogammaglobulinemia. In addition, a potential AE associated with the treatment is the risk of secondary malignancies, particularly myeloid malignancies, although the incidence remains low [26]. Pecher et al. [9] reported a case of a patient who developed fatal secondary hemophagocytic lymphohistiocytosis, which was probably caused by a herpes simplex virus infection.

Having considered both the clinical efficacy outcomes and the distinct toxicity profiles of CD19-targeted CAR-T cell therapy, a comprehensive synthesis of the advantages and disadvantages of this emerging modality compared to the current gold standard, autologous HSCT, is provided in Table III [27, 28].