Preclinical Animal Models for CAR-T Cell Therapy

Chimeric antigen receptor (CAR)-T cell therapy is one of the leading immunotherapies in oncology. Although many clinical trials have produced encouraging results, further development of this therapeutic approach remains challenging. CAR-T cell therapy has not been effective in treating solid tumours. It can also cause side effects such as off-tumour toxicity, cytokine release syndrome (CRS) and neurotoxicity. 

Preclinical animal models are often used in evaluating CAR-T treatment. The challenge is accessing the relevant animal models. Therefore, animal model choice should be carefully considered based on the primary goals of the study to maximise the success of each study. 

In vivo evaluation models

Syngeneic (Immuno-competent Allograft) Mouse Models

Syngeneic tumour mouse models are allografts of mouse tumour cell lines, which are then engrafted back into host mice with the same genetic background and immune capabilities, creating an immunocompetent model for immunotherapy assessment. With the intact host immune system, it allows the study of off-tumour effects. They are also often used in combination studies, including immunotherapy with standard treatments or with immune checkpoint inhibitors.

Syngeneic mouse tumour cell lines are widely used to recapitulate human cancer in mice. Syngeneic cell lines can be easily cultured and expanded. BALB/c is widely used in cancer immunotherapy research, while C57BL/6, the most popular inbred strain in the scientific community, has been used extensively for making gene edited models for varied applications in genetics, oncology, and other studies. BALB/c and C57BL/6 have different immune characteristics, such as varying expression levels of CD14, toll-like receptor, TGF beta, and MDSC as well as different responses to some viral infections.

Human Xenograft Mouse Models 

Human xenograft mouse models, are mouse models injected with either patient-derived xenografts (PDX) or human cell line-derived xenograft (CDX) in immunocompromised mice. These immunocompromised mice are deficient in their immune system, and therefore cannot be used to study off-tumour effects. However, it allows for the test of complex CAR designs and as proof-of-concept efficacy studies. 

Currently, the most immunocompromised model to date is the NCG (NOA CRISPR Prkdc Il2r Gamma) model, a triple immunodeficient mouse model, that lacks functional/mature T, B, and NK cells, and has reduced macrophage and dendritic cell function. It has the highest rate of engraftment and is best suited for human tumour engraftments. Next-Gen NCG strains, such as the NCG-SGM3 mice can be used to validate adverse reactions such as cytokine storms. 

Humanized Transgenic Mouse Models 

Humanized transgenic mouse models are typically mouse models with the murine protein replaced by a human protein. Like the syngeneic models, transgenic models have intact murine immune system, so they can also be used to study the effect of CAR T-cell therapy on the host immune system; and like in the xenograft models, it allows for the study of human CAR targets.

Some of the humanized transgenic mouse models used for evaluation of toxicity of CAR-T cells include – CD19, CEA, and Her2.

Humanized mice with reconstituted immune system

Humanized mice reconstituted with immune system are immunocompromised mice implanted with human immune cells. This is a mouse model to bridge the gap between syngeneic and xenograft models, where the mice are able to engraft with human tumours yet with aspects of a human immune system. This is an excellent model to evaluate CAR-T cells in an intact immune system. 

Immune system reconstituted humanized mice are generated by implanting CD34+ hematopoietic stem cells (huHSC) in NCG mice. The humanized mice are then transplanted with tumour cells prior to CAR-T treatment. These humanized models provide valuable information such as the off-target potential and cytokine release syndrome (CRS) and other adverse events.