The Spectrum of CAR Cellular Effectors: Modes of Action in Anti-Tumor Immunity

Abstract

Chimeric antigen receptor-T cells have spearheaded the field of adoptive cell therapy and have shown remarkable results in treating hematological neoplasia. Because of the different biology of solid tumors compared to hematological tumors, response rates of CAR-T cells could not be transferred to solid entities yet. CAR engineering has added co-stimulatory domains, transgenic cytokines and switch receptors to improve performance and persistence in a hostile tumor microenvironment, but because of the inherent cell type limitations of CAR-T cells, including HLA incompatibility, toxicities (cytokine release syndrome, neurotoxicity) and high costs due to the logistically challenging preparation process for autologous cells, the use of alternative immune cells is gaining traction. NK cells and γδ T cells that do not need HLA compatibility or macrophages and dendritic cells with additional properties such as phagocytosis or antigen presentation are increasingly seen as cellular vehicles with potential for application. As these cells possess distinct properties, clinicians and researchers need a thorough understanding of their peculiarities and commonalities. This review will compare these different cell types and their specific modes of action seen upon CAR activation.

Keywords: adoptive cell therapy; chemokines; chimeric antigen receptor; cytokines; dendritic cells; gamma-delta T cells; killing mechanism; kinetics; macrophages; mode of action; natural killer cells; persistence.

Figure 1

Intracellular signaling pathways in NK cells after CAR activation with different signaling domains. CAR stimulation can lead to two main pathways depending on the signaling domain. CD3ζ, DAP12, and FcRγ signal through ITAM. DAP10 signals through YxxM and is ITAM-independent. Pathways are color-coded. Created with BioRender.com.

Figure 2

The persistence of different cell effectors in vitro and in vivo. Depicted in weeks. In vivo data contain murine or human data. Data include cell effectors from cell lines, primary cells and stem cells. NK cells in light blue, γδ T cells in dark blue, macrophages in orange, and dendritic cells in purple. In vitro data shown with the lined pattern, murine data shown with the chequered pattern. Human data shown without a pattern [24,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69]. Persistence data present data from confirmed cell detection at the specified time point. Persistence beyond this time point is possible but speculative.

Figure 3

CAR-NK cell effector function against tumor cells. Depiction of the immunological synapse and soluble and membrane-bound effector molecules. CAR activation induces cell activation with the release of cytolytic granules and cytokines/chemokines. Additional apoptosis induction with FASL/TRAIL expression and ADCC via FcγRIII receptors. Created with BioRender.com.

Figure 4

Intracellular signaling pathways in macrophages after CAR activation with different signaling domains. CAR stimulation can lead to two main pathways depending on the signaling domain. CD3ζ, Megf10, FcRγ, Bai and MerTK pathways converge on Rac1 and induce actin polymerization and phagocytosis. TIR mainly induces iNOS, chemokines, cytokines, and survival and proliferation signals. Pathways are color-coded. Created with BioRender.com.

Figure 5

CAR-M cell effector functions in relation to tumor cells. The depiction of soluble and membrane-bound effector molecules and their functions. The main functions of CAR-M are phagocytosis induced by phagocytosis/efferocytosis receptors, antigen presentation via MHC II, ECM degradation by MMP, ROS production, apoptosis, necroptosis, pyroptosis, cytokine and chemokine release. Created with BioRender.com.

Figure 6

Effector functions of CAR γδ T and dendritic cells in relation to tumor cells. Depiction of soluble and membrane-bound effector molecules. Main functions of CAR γδ T cells: killing capacity with cytolytic granules after CAR activation, innate killing capacity with FCγRIII via ADCC and NK cell receptors, antigen presentation and cytokine and chemokine release. Main functions of dendritic cells: endocytosis, antigen (cross-) presentation, cytokine and chemokine release. Intracellular signalling pathways are not depicted due to insufficient published data and information. Created with BioRender.com.