Combinatorial antigen recognition with balanced signaling promotes selective tumor eradication by engineered T cells

Current T-cell engineering approaches redirect patient T cells to tumors by transducing them with antigen-specific T-cell receptors (TCRs) or chimeric antigen receptors (CARs) that target a single antigen. However, few truly tumor-specific antigens have been identified, and healthy tissues that express the targeted antigen may undergo T cell–mediated damage. Here we present a strategy to render T cells specific for a tumor in the absence of a truly tumor-restricted antigen. T cells are transduced with both a CAR that provides suboptimal activation upon binding of one antigen and a chimeric costimulatory receptor (CCR) that recognizes a second antigen. Using the prostate tumor antigens PSMA and PSCA, we show that co-transduced T cells destroy tumors that express both antigens but do not affect tumors expressing either antigen alone. This 'tumor-sensing' strategy may help broaden the applicability and avoid some of the side effects of targeted T-cell therapies.

Adoptive cell therapies using genetically modified autologous T cells have shown efficacy for melanoma and indolent B-cell malignancies. However, their broad applicability is limited by the paucity of truly tumor-specific target antigens. Extra-tumoral antigen expression may indeed result in “on-target, off-tumor” effects. These effects can be acceptable, as is the case with CD19, an antigen expressed in B-cell malignancies and normal B-lineage cells, resulting in B-cell aplasia. In other instances, targeting, for example, carbonic anhydrase IX or human epidermal growth factor receptor 2 (HER2), the side effects may be intolerable and potentially life-threatening.

Here we present an approach to render engineered T cells specific for a tumor even in the absence of a truly tumor-restricted antigen. T-cell activation requires TCR- or CAR-mediated recognition of one antigen, which in this work is either CD19 or prostate stem cell antigen (PSCA). T-cell costimulation must be independently mediated by a CCR specific for a second antigen, here being prostate-specific membrane antigen (PSMA). This dual-targeting approach facilitates augmented T-cell reactivity against tumors positive for two antigens compared to single-positive tumors, that is, those positive for only one antigen. However, this approach alone fails to prevent T-cell reactivity to single-positive tumors, as we show here. To achieve tumor selectivity, we diminished the efficiency of T-cell activation to a level where it is ineffective in the absence of simultaneous CCR recognition of the second antigen. We hypothesized, and demonstrate below, that T cells expressing suboptimal activation receptors are functionally rescued at the tumor site by a CCR engaging a co-expressed tumor antigen.

To demonstrate that both T-cell activation and costimulation signals can be supplied in vivo using two distinct antigen-specific receptors, we initially evaluated the combination of a CAR that provides a CD3ζ-mediated activation signal upon recognition of the B-cell marker CD19 (19z1) and a CCR specific for PSMA. Based on results showing synergy between CD28 and 4-1BB costimulation, including through their cytoplasmic domains arranged in tandem, we added the 4-1BB cytoplasmic domain to the PSMA-specific CCR P28 ), as described, to generate P28BB . Primary human peripheral blood T cells were retrovirally transduced with 19z1 and/or P28BB, typically yielding expression of both receptors in 45–70% of T cells . Four groups of T cells were analyzed in all subsequent studies, expressing 19z1, P28BB, 19z1+P28BB or neither (mock).

We first measured the in vitro cytotoxic and proliferative responses of transduced T cells exposed to mouse lymphoma cell line EL4 target cells expressing CD19 and/or PSMA. Cytotoxicity against CD19-expressing target cells was, as expected, imparted by 19z1 expression and was unaltered in the presence of PSMA in all T-cell groups . A quantitative comparison after normalizing to the fraction of 19z1-transduced T cells for the 19z1 and 19z1+P28BB groups and the P28BB-transduced fraction for the P28BB group showed that 19z1 and 19z1+P28BB T cells specifically lysed 40–47% CD19-expressing target cells at the 50:1 effector/target ratio, whereas the P28BB-transduced T cells did not lyse PSMA-expressing targets . We next examined the proliferation of T cells repeatedly exposed to artificial antigen-presenting cells (AAPCs) expressing CD19 and/or PSMA in the absence of exogenous cytokine. Over 4 weeks, only the 19z1+P28BB T cells underwent robust proliferation (58-fold expansion) when co-cultured on AAPCs expressing both antigens. In contrast, 19z1 or P28BB T cells underwent modest expansion over the first 14 d, as did the 19z1+P28BB T cells exposed to CD19⁺PSMA⁻ AAPCs . Further evidence of stronger T-cell activation in the presence of both antigens was provided by quantitative assessment of cytokine production and the induction of the anti-apoptotic molecule Bcl_XL, which were maximal in 19z1+P28BB T cells .

Figure 1: Dual chimeric receptor-mediated activation and costimulation of human T cells facilitates robust cytotoxicity, proliferation and tumor eradication.

(a,b) T cells were mock transduced or were transduced with retroviruses encoding a CD19-specific CAR (19z1) and/or a PSMA-specific CCR (P28BB). (a) T cells were incubated at indicated effector/target ratios with ⁵¹Cr-loaded target cells expressing CD19 and/or PSMA, and target cell lysis (chromium release) was measured. Plots represent at least four experiments, with error bars representing s.d. of the mean of three replicates. (b) T cells were co-cultured with PC3 tumor cell lines expressing CD19 and/or PSMA (arrows indicate restimulation of T cells using freshly irradiated tumor cells). T-cell numbers were measured at indicated time intervals. Plots represent at least four experiments with error bars representing s.d. of the mean of three replicates. (c) T cells (1.0 × 10⁶) described in a and b were injected intravenously into NSG mice bearing green fluorescent protein/firefly-luciferase fusion protein (GFP/Luc) expressing CD19⁺PSMA⁺PC3 human prostate tumors. Tumor burden was measured weekly by bioluminescent imaging. Top, images of two representative mice from each group at each time point, with the pixel intensity represented in color. Bottom, average tumor burden as quantified by luminescence of the tumors using units of photons per second per square centimeter per steradian (p/sec/cm²/sr); error bars represent s.d. from the mean. Values were generated from six mice per group. (d) 1 × 10⁶ CD19⁺PSMA⁻, 1 × 10⁶ CD19⁻PSMA⁺ and 1 × 10⁶ CD19⁺PSMA⁺ PC3 cells were injected subcutaneously into the left flank, right flank and back, respectively, of NSG mice. T cells expressing 19z1 and/or P28BB were infused intravenously 7 d later. Top, representative images of two mice per time point per group as analyzed by bioluminescent imaging. Graphs, tumors were quantitatively measured using calipers and tumor volumes were plotted versus time for each tumor. Error bars represent s.d. from the mean of six mice. Statistical significance was determined using two-tailed unpaired t-tests to compare values obtained from 19z1 T cells and 19z1 + P28BB T cells; *P < 0.05, **P < 0.01.