Background: Mature T cell lymphomas are aggressive, treatment resistant cancers that are associated with poor prognosis. Clinical application of immunotherapeutic approaches has been limited by a lack of target antigens that discriminate malignant from healthy T cells. Unlike B cell depletion, pan T cell aplasia is prohibitively toxic. The mutually exclusive expression of TCR β chains (TRBC) 1 or 2 allows targeting the malignant T cell population while preserving T cell function. Notably, a two amino acid inversion is the only exposed and antibody-accessible feature differentiating the two isoforms, making the development of therapeutics targeting TRBC1 and TRBC2 challenging. We have previously described a TRBC1 specific antibody which has been incorporated into a Chimeric Antigen Receptor (CAR) (Maciocia et al, Nat Med, 2017). Here we describe the derivation and characterization of a TRBC2 specific binder and CAR. In addition, we investigate use of these selective binders as Antibody Drug Conjugates (ADCs) for the treatment of HTLV-1 associated leukemia/lymphoma for which CAR T cell therapy may not be well suited.

Methods: Anti TRBC2 antibodies were derived through crystallography and structural engineering of the previously described TRBC1 specific antibody. Briefly, TRBC2 binders were obtained by screening of small libraries of the TRBC1 binder with randomizations in key residues identified by crystallographic data and in silico design. TRBC2 binder candidates were first optimized for CAR function. Subsequently, TRBC2 specific CAR T cells were evaluated in vitro and in vivo for anti-tumor activity and selectivity. TRBC1/TRBC2 targeting antibodies, were further characterized as ADCs for biophysical properties, antibody internalization and cytotoxic function.

Results: In vitro testing of TRBC2 CARs showed comparable efficacy to the previously described TRBC1 CAR. TRBC2 CAR T cells were effective at killing TRBC2 target cells while sparing TRBC1 positive cells. In vivo mouse models demonstrated that both TRBC1 and TRBC2 directed CARs could target their respective antigens with a high degree of specificity. TRBC1 and TRBC2 specific antibodies were used to generate ADC molecules as a proof of concept. Anti-TRBC1/TRBC2 antibodies were internalized and showed potential as ADCs. Interestingly, we demonstrate that an optimal affinity window facilitates improved antibody uptake and have further engineered both our TRBC1 and TRBC2 antibodies to take advantage of this particularity.

Conclusions: Following on from structural and library-based approaches to generate CAR T cells capable of specifically targeting TRBC2, we have further characterized TRBC2 specific CAR T cells in vitro and in vivo. We have shown that highly specific antibodies, engineered as targeting moieties for TRBC1 and TRBC2 CAR T cells, show promising characteristics for utility potentially also as ADCs, offering another modality through which this targeting paradigm can be exploited for the treatment of peripheral T cell lymphomas.

Citation Format: Mathieu Ferrari, Vania Baldan, Priyanka Ghongane, Alex Nicholson, Reyisa Bughda, Zulaikha Akbar, Patrycja Wawrzyniecka, Paul Maciocia, Shaun Cordoba, Simon Thomas, Shimobi Onuoha, Martin Pule. Targeting TRBC1 and 2 for the treatment of T cell lymphomas [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2183.