Selectively Baiting Autoantibody producing B Cells
A proof of concept study paves the way for selective B cell depletion using CAART
Neutralizing autoantibodies against interferon-γ (nAIGAs) pose a significant health challenge, leading to adult-onset immunodeficiency and increasing susceptibility to severe infections, including disseminated nontuberculous mycobacterial infection (dNTM) and Talaromyces marneffei infections. The immune system's ability to fight these intracellular pathogens is impaired because nAIGAs block the function of IFN-γ, a crucial cytokine for immunity.
Current clinical management often involves continuous antimicrobial therapy, but this frequently results in recurrent infections in a substantial portion of patients. A large proportion of patients with AIGA positivity (42.6%) have experienced recurrence and/or persistent infection even with long-term and intensive antimicrobial treatments. Broad B cell depletion using agents like rituximab has shown some promise in reducing nAIGA titers and ameliorating associated infections. Rituximab therapy has been shown to diminish the nAIGA titer and ameliorate an associated infection in some cases. It was used in 4 patients with high-titer anti–IFN-γ autoantibodies and progressive refractory nontuberculous mycobacterial disease, showing clinical and laboratory evidence of therapeutic response. This response included clearance of infection, resolution of inflammation, reduction of anti–IFN-γ autoantibody levels, and improved IFN-γ signaling. Anti–IFN-γ autoantibody titers decreased by between 1 and 2 logs from baseline in all subjects treated with rituximab. Plasma inhibition of IFN-γ signaling, measured by pSTAT-1 production and IFN-γ–induced gene transcription, was also partially or progressively alleviated after rituximab treatment.
However, a clear therapeutic effect of rituximab hasn't been consistently observed in larger studies. Moreover, relapses of infection and recovery of nAIGA levels have been observed, possibly due to insufficient clearance of autoreactive B cells in tissues. Rituximab also increases the susceptibility to infectious diseases, which is particularly hazardous for patients with nAIGAs who already experience various pathogen infections. This highlights a critical unmet medical need for more effective strategies that can achieve long-term remission by specifically targeting the B cells producing these harmful autoantibodies.
Chimeric Autoantibody Receptor (CAAR) T Cells: A Targeted Approach
Adoptive T cell therapy, particularly the use of Chimeric Antigen Receptor (CAR) T cells, has shown promising outcomes in treating B cell malignancies and is now being applied to treat autoimmune diseases. Building on this success, Chimeric Autoantibody Receptor (CAAR) T cells represent an advanced CAR design specifically engineered to target autoreactive B cells. Instead of recognizing an antigen on a malignant cell, CAAR T cells use an autoantigen as a "bait" on their surface to bind to the B cell receptor (BCR) on autoreactive B cells. This interaction directs the cytotoxic activity of the T cell specifically against the B cell producing the autoantibody. This strategy has been explored for conditions like pemphigus vulgaris, myasthenia gravis, and NMDAR encephalitis.
The Challenge of Targeting Cytokine Autoantibodies
Applying the CAAR T cell concept to diseases caused by autoantibodies against cytokines, like nAIGAs, presented a unique challenge. Cytokines are biologically active molecules with receptors often expressed on various cell types throughout the body. Designing a cytokine-mimicking bait for a CAAR runs the risk of the CAAR inadvertently interacting with the native cytokine receptor, leading to potentially widespread and dangerous off-target toxicity. For instance, the IFN-γ receptor (IFN-γR) is ubiquitously expressed in multiple cell types, making IFN-γR cross-reactive toxicity a serious safety concern for an IFN-γ-based CAAR. An IFN-γ CAAR would have difficulty distinguishing between the nAIGA BCR on B cells and the IFN-γR, as their affinities for IFN-γ are comparable.
An Innovative Solution: A Mutated, Truncated IFN-γ Bait
This research (link) successfully overcame the challenge of developing CAAR T cells for an anticytokine autoantibody disease by employing an innovative design: IFN-γ CAAR T cells were engineered using a mutated, truncated variant of human IFN-γ as the bait.
The key to this novel approach was the careful modification of the IFN-γ protein used in the CAAR. The researchers introduced specific amino acid substitutions (A23Q and H111D) known to be critical for native IFN-γ receptor binding and signaling. These mutations, combined with truncation of the IFN-γ protein, effectively abrogated the interaction of the CAAR bait with the native IFN-γ receptor. This clever design was crucial for minimizing the risk of off-target toxicity associated with the widespread expression of IFN-γR.
The researchers developed two versions of the IFN-γ CAAR T cells, designated AF and 1-to-137, based on different epitopes recognized by nAIGAs.
Key Findings Highlight Efficacy and Safety:
The study provided compelling evidence for the potential of these engineered CAAR T cells:
Effective Elimination of Target Cells: Both AF and 1-to-137 IFN-γ CAAR T cells demonstrated the ability to kill engineered target cells expressing nAIGA BCRs in laboratory tests and showed potent depletion of these target cells in mouse models, significantly reducing the level of circulating nAIGAs secreted by the target cells. The two CAAR versions exhibited preferential activity towards B cells expressing nAIGAs that bind to different epitopes on IFN-γ (AF for site II/III, 1-to-137 for site I).
Specificity Maintained Despite Soluble Autoantibodies: While the presence of soluble nAIGAs in circulation could potentially reduce the cytotoxicity of IFN-γ CAAR T cells, soluble AIGAs did not dampen their specificity, and the cells retained their ability to specifically target B cells expressing the nAIGA BCR.
Remarkable Absence of Off-Target Toxicity: Crucially, the designed IFN-γ CAAR T cells demonstrated a favorable safety profile. Thanks to the mutated bait design, they showed no significant cross-reactivity or cytotoxicity against cells expressing physiological levels of the IFN-γ receptor or other immune cells like NK cells and monocytes in vitro and in vivo. The in vivo models specifically assessed IFN-γR cross-reactive toxicity and Fc-redirected toxicity, finding neither to be a significant issue.
Eliminating Patient-Derived B Cells: Perhaps most excitingly, the IFN-γ CAAR T cells were shown to effectively eliminate autoreactive B cell clones derived from the peripheral blood of patients with nAIGAs in ex vivo cultures. AF IFN-γ CAAR T cells depleted most nAIGA B cell clones, while 1-to-137 IFN-γ CAAR T cells could eliminate all AIGA B cell clones from the four tested patients, suggesting the latter may cover a broader spectrum of nAIGA clone diversity. These results directly validate their potential to target the actual source of the disease in patients.
Broader Implications and Future Potential:
This research represents a significant step forward in treating autoantibody-mediated diseases.
It offers a potential therapeutic strategy for patients suffering from nAIGA-associated immunodeficiency, holding the promise of more sustained remission compared to current treatments. This is particularly relevant given that existing immunotherapies like rituximab or cyclophosphamide, while sometimes achieving remission, lack large-scale data for evaluation and carry risks of complications and new infections.
By selectively targeting autoreactive B cells, this approach may cause less damage to the overall host immune system compared to broad B cell depletion therapies like anti-CD20 treatment, which can increase susceptibility to infections – a particular concern for nAIGA patients.
The successful development of a CAAR targeting autoantibodies against a cytokine provides a crucial proof-of-concept model. This opens up the possibility of applying similar CAAR T cell strategies to treat other serious conditions caused by autoantibodies against different cytokines, such as GM-CSF or type I interferons, which are implicated in other life-threatening infections and autoimmune diseases.
While challenges such as targeting long-lived plasma cells (which may contribute to nAIGA production and may not be recognized by CAAR T cells) and managing extremely high autoantibody titers (which could potentially reduce efficacy, although in vitro results suggest retention of function at high concentrations) still require further investigation, this research presents a novel and highly promising therapeutic path forward for addressing autoantibody-mediated diseases, starting with nAIGA-associated immunodeficiency. Further clinical examination and confirmation will be needed to fully assess the implications for safety in patients with AIGAs.