FCγ Chimeric Receptor-Engineered T Cells: Methodology, Advantages, Limitations, and Clinical Relevance

Abstract

For many years, disappointing results have been generated by many investigations, which have utilized a variety of immunologic strategies to enhance the ability of a patient's immune system to recognize and eliminate malignant cells. However, in recent years, immunotherapy has been used successfully for the treatment of hematologic and solid malignancies. The impressive clinical responses observed in many types of cancer have convinced even the most skeptical clinical oncologists that a patient's immune system can recognize and reject his tumor if appropriate strategies are implemented. The success immunotherapy is due to the development of at least three therapeutic strategies. They include tumor-associated antigen (TAA)-specific monoclonal antibodies (mAbs), T cell checkpoint blockade, and TAA-specific chimeric antigen receptors (CARs) T cell-based immunotherapy. However, the full realization of the therapeutic potential of these approaches requires the development of strategies to counteract and overcome some limitations. They include off-target toxicity and mechanisms of cancer immune evasion, which obstacle the successful clinical application of mAbs and CAR T cell-based immunotherapies. Thus, we and others have developed the Fc gamma chimeric receptors (Fcγ-CRs)-based strategy. Like CARs, Fcγ-CRs are composed of an intracellular tail resulting from the fusion of a co-stimulatory molecule with the T cell receptor ζ chain. In contrast, the extracellular CAR single-chain variable fragment (scFv), which recognizes the targeted TAA, has been replaced with the extracellular portion of the FcγRIIIA (CD16). Fcγ-CR T cells have a few intriguing features. First, given in combination with mAbs, Fcγ-CR T cells mediate anticancer activity in vitro and in vivo by an antibody-mediated cellular cytotoxicity mechanism. Second, CD16-CR T cells can target multiple cancer types provided that TAA-specific mAbs with the appropriate specificity are available. Third, the off-target effect of CD16-CR T cells may be controlled by withdrawing the mAb administration. The goal of this manuscript was threefold. First, we review the current state-of-the-art of preclinical CD16-CR T cell technology. Second, we describe its in vitro and in vivo antitumor activity. Finally, we compare the advantages and limitations of the CD16-CR T cell technology with those of CAR T cell methodology.

Keywords: CD16-CR T cells; CRC; Fc gamma chimeric receptor; antitumor activity; chimeric antigen receptor T cells; hematologic malignancies; immunotherapy; solid tumor.

Figure 1

Schematic representation of CD16-CR and classical chimeric antigen receptor molecular structures. The first generation of CR has the extracellular domain linked to the intracellular signaling motif of CD3ζ chain while the second generation of CR has an additional co-stimulatory endodomain derived from CD28 or 4-1BB linked to the N-terminal of CD3ζ chain.

Figure 2

Therapeutic monoclonal antibodies (mAbs) confer to CD16-CR T cells multiple target specificities. (A) The ability of chimeric antigen receptor (CAR) to redirect T cells toward tumor cells is restricted to a single tumor-associated antigen (TAA) expressed on tumor cell surface. (B) The availability of therapeutic mAbs allows CD16-CR to redirect T cells virtually against all TAA expressed on a variety tumor types including hematological and epithelial malignancies.

Figure 3

Mechanisms of CD16-CR T cell-mediated tumor cell elimination. CD16-CR T cells acquire the specificity of a monoclonal antibody (mAb) recognizing a tumor-associated antigen (TAA) on the cell surface of tumor cells through the binding of the CD16 with the mAb Fc fragment leading to the activation of T cell-mediated cytotoxicity. In this context, engineered T cells kill tumor cells by using two mechanisms. The first involves the activation of T cell killing machinery by the release of cytotoxic granules (upper panel). The second arises from the induction of CD16-CR dependent FAS expression on the cell surface of tumor cells that allows FAS ligand positive-engineered T cells to kill tumor cells by a granule independent cellular cytotoxicity (lower panel). However, the latter mechanism is just a hypothetical model based on a single study employing transformed MD45 mouse T cells. Then, the confirmation of the existence of a role for granule independent cytotoxicity requires additional studies.