It’s high TIM-3 for armored CAR-T therapy for B-ALL Free

In this issue of Blood, Falgàs et al¹ acknowledge that despite huge advances in CD19 chimeric antigen receptor T-cell (CAR-T) therapy for B-cell acute lymphoblastic leukemia (B-ALL), CD19⁺ relapse remains depressingly common, accounting for around two-thirds of all relapses after CAR-T therapy.² The failure of this mode of treatment is often ascribed to impaired patient T-cell “fitness” or the deleterious effect of the immunosuppressive B-ALL tumor microenvironment (TME) on CAR-T function in vivo. To address this, several groups have tested B-ALL TME modulation via programmed death receptor-1 (PD-1) blockade, but clinical results to date have been disappointing.³ 

In an attempt to understand how the B-ALL TME promotes therapy resistance, Falgàs et al performed a detailed characterization of immune checkpoint receptor (and ligand) expression in leukemic blasts, T cells, and mesenchymal stromal cells derived from patient bone marrow samples at diagnosis (n = 47) and relapse (n = 38). Here, the most striking finding was not overexpression of PD-1, but rather the significant parallel upregulation of the checkpoint receptor TIM-3 on CD8⁺ T cells and of its ligand, galectin-9 (Gal-9), on relapsed tumor samples. TIM-3 is commonly expressed on CAR-Ts in the context of “exhaustion,” and binding of TIM-3 to Gal-9 can lead to reduced T-cell function and apoptosis. Falgàs et al neatly show that the Gal-9–TIM-3 axis is upregulated in B-ALL, that it appears to portend worse clinical outcomes, and that culturing CD19 CAR-T in vitro with Gal-9 leads to impaired CAR-T survival and cytotoxicity against antigen-positive targets (see figure).

Taking this one step further, Falgàs et al describe a novel and elegant CAR-T armoring strategy to overcome TIM-3–Gal-9–mediated CAR-T exhaustion. They engineered CD19 CAR-T with a module encoding a secreted TIM-3 decoy receptor that is stabilized by a silent Fc receptor. The purpose of the TIM-3–Fc decoy is to saturate Gal-9 in the B-ALL TME, preventing binding of Gal-9 to TIM-3, and thus protecting CAR-Ts from Gal-9–TIM-3–mediated exhaustion and apoptosis. Testing this in immunodeficient murine models engrafted with patient-derived B-ALL xenografts, TIM-3–Fc decoy re ceptor improved both antileukemic activity and CAR-T engraftment and persistence, irrespective of whether it was delivered in trans or in cis with CD19 CAR-T (ie, coadministration, or 2-product approach, vs bicistronic cassette design). Notably, the potential benefits of TIM-3–Fc decoy receptor are not limited to CAR-Ts, but by virtue of payload secretion in the TME, activation of bystander effector cells may also be achieved, with the potential to further enhance tumor rejection.

Although armoring CAR-T with engineered TIM-3–Fc to overcome the inhospitable B-ALL TME is a compelling strategy, other approaches have also been explored and are worthy of note. Agarwal et al demonstrated that deletion of the inhibitory checkpoint receptor CTLA-4 from CAR-Ts improves functionality in preclinical models.⁴ A broader approach to limit transmission of immune inhibitory signaling into T cells can be achieved via truncation of SHP1 or SHP2, key proteins in the transmission of signals from diverse inhibitory receptors.⁵ It is possible that TIM-3–Fc in combination with the other engineered TME-evasion strategies mentioned here may be complementary and could further improve CAR-T function in B-ALL.

In summary, this study nicely shows that blocking the TIM-3–Gal-9 axis in B-ALL can potentiate CAR-T function and promote tumor rejection. Further, the TIM-3–Gal-9 axis has been implicated in other hematologic malignancies⁶ and solid tumors,⁷ suggesting that a TIM-3–Fc decoy receptor strategy could have broader application and potential benefit in cancers beyond B-ALL.

Conflict-of-interest disclosure: The authors declare no competing financial interests.