Russell Garland

CAR-T Cells: The Importance of Cell Persistence

Improving infused cell persistence and how it impacts clinical outcome

A couple of years ago, I authored a blog introducing CAR-T-cells and highlighting some of the challenges to developing them. In the intervening time, whilst much of the world’s attention has been drawn to the global pandemic, progress in the other fields, including CAR-T, has continued.

In this summary, I would like to discuss two themes from scientific articles in 2020: firstly a clinical trial involving the administration of tandem CD19/CD20 CAR, introducing the association between CAR-T cell persistence and clinical responses and, secondly, a review of the aspects of CAR-T which are could be manipulated so as to improve this persistence.

CD19-targeted CAR-T cells, which have been engineered to recognize the CD19 cell surface molecule of malignant B cells, show remarkable efficacy in patients with B cell acute lymphoblastic leukaemia. However, some patients relapse due to the CAR-T therapy not persisting in the body, leading to antigen loss or tumour escape. To overcome this researchers generated autologous Tandem CARs (TanCARs) that express bi-specific receptors to target both CD19 and CD20 at the same time.

These TanCARs showed potent anti-tumour activity in vitro and showed an overall response rate of 79% in 28 patients with refractory/relapsed B cell (non-Hodgkin) lymphoma. The response rate continued for a median of 19 months, although 50% of the patients suffered the common CAR-T side effect of cytokine release syndrome.

My main interest is how exploratory, pharmacodynamic (PD) biomarkers can be included in clinical trials to add value. Whilst the primary aim of early phase 1/2a clinical studies is to determine the safety and tolerability of the cellular product, exploratory biomarker endpoints are increasingly being included at this stage. In this study, infused cells were detected in the blood after infusion by flow cytometry and qPCR. The authors concluded that ‘if CAR-T cell therapy is to be used as a definitive therapy, rather than a bridging therapy, there should be considerable effort to extend their half-life in vivo to ensure durable efficacy’.

Building on this theme, this article recognizes that the persistence and functionality ofCAR-T cells play important roles in determining the outcome of cancer immunotherapy and reviews strategies that can be employed to improve these.

Nicely summarized in Figure 3 in that article, a range of features of CAR-T cells and their production can be manipulated with a view to improving persistence. Ex vivo culture conditions, including human serum and cytokine supplements, can influence the final T cell product, for example by driving a fully differentiated T cell population predisposed to activation induced cell death (AICD) and poor persistence after infusion. An understanding of the impact of growth conditions on the desired features of anti-tumour activity, resistance to AICD and ability to migrate to secondary lymphoid organs is important. As well as the presence of cytokines in the growth medium, some CAR-T are employed which are engineered to contain transgenes expressing cytokines to enhance the desired features.

Pharmacological inhibitors of T cell transduction pathways (MAPK and PI3K/AKT/mTOR) have been investigated for their ability to modulate the differentiation state and proliferative ability of T cells. Manipulation of the telomerase (hTERT) content of the infused cell product may also help to prevent the telomere shortening associated with cell division which results in replicative senescence.

Two important subsets of T cell in immunotherapies are memory (including stem cell memory, central memory and effector memory) and naïve T cells. Considering the kinetics, longevity and persistence of different T cell subsets, standardization of cell transduction and expansion protocols may be required, potentially including selection of the desired T cell subsets before modification.

We must be mindful that external (non- CAR-T-related) factors, such as the tumour microenvironment (including antigen escape leading to relapse) and the patient conditioning regime are also relevant. It is common in oncology settings for lymphodepletion or conditioning therapies to be employed in preparation for a therapeutic intervention. The composition of these pre-treatments and the persistence of their effects can influence the expansion and survival of the infused cells. For example, Fludarabine containing regimens have been associated with improved persistence of CAR-T cells. 

Another approach is to eradicate existing immunosuppressive cells (such as Treg) and anti-transgene rejection responses mediated by host cells directed towards the infused cell product; sometimes by simplifying the CAR antigen binding domain to exclude murine sequences.

The primary barrier to clinical efficacy of CAR-T cells is limited persistence. A number of features of CAR-T and the tumour microenvironment can be manipulated to the benefit of CAR-T persistence and, ultimately, improve clinical responses. I strongly support the inclusion of exploratory endpoints to improve our understanding of the desirable features associated with infused cell persistence and, ultimately, clinical remission.