Chimeric antigen receptor (CAR)-T cell therapies have shown promise against blood cancers, but have proven far less effective against solid tumors. This is partly because the cells enter a state known as T-cell exhaustion, which renders therapy ineffective. Studies by scientists at the La Jolla Institute of Immunology (LJI) now suggest that CAR-T cells engineered to lack a family of three transcription factors don’t develop T-cell exhaustion and so are better at fighting cancer. Initial in vivo studies showed that tumor-bearing mice treated using Nr4a transcription factor-deficient CAR-T cells developed smaller tumors and lived longer than animals treated using CAR-T cells that hadn’t been modified.
The researchers acknowledge that there is a long way to go before such treatment might be translated into the clinic. Nevertheless, Joyce Chen, a graduate student in the laboratory of research head Anjana Rao, PhD, suggests that these early results could help point scientists in the right direction for developing approaches that circumvent the T-cell exhaustion issue. “There are a lot of steps between our experiments and the clinic, but every new bit of information we discover adds to our knowledge base and may contribute to improving cancer immunotherapies,” noted Chen, who is first author of the team’s published paper in Nature, which is titled, “NR4A transcription factors limit CAR-T cell function in solid tumors.”
CAR-T cells targeting human CD19 (hCD19) have been effective against B cell cancers, but when tested against solid tumors CAR-T cell therapy has been less effective, due in part to T-cell exhaustion. This occurs when the cells enter a hyper-responsive—exhausted, or dysfunctional—state that is triggered by chronic antigen stimulation, the authors explained. T-cell exhaustion is characterized by the upregulation of inhibitor receptors and loss of effector function.
Previous studies in the laboratory of Rao, who is a professor in LJI’s division of signaling and gene expression, had found that the NFAT family of transcription factors switched on Nr4a proteins in tumor-infiltrating T cells. In 2017, research by the Rao lab in collaboration with LJI researcher Patrick Hogan, PhD, found that NFAT and Nr4a protein contributed to exhaustion in cancer-fighting T cells in a mouse model of melanoma.
Previous work in Rao’s lab had separately demonstrated elevated levels of Nr4a transcription factors in T cells that faced chronic viral infections. This work indicated that, similar to T cells that are exposed to cancer antigens for long periods, T cells that are constantly exposed to and stimulated by viral antigens also develop exhaustion, and become ineffective.
The researchers’ latest studies have now built on these findings. They engineered human CD19-targeting anticancer CAR-T cells to lack three Nr4a transcription factor molecules. They then transferred the modified CAR-T cells into experimental mice bearing human CD19-expressing tumors. The effects of treatment were compared with therapy using CAR-T cells in which the Nr4a transcription factors were retained.
The team found that most of the mice receiving the Nr4a-deleted CAR-T cells survived, and their tumors shrank over the course of the 90-day study period. In contrast, nearly all the mice that received CAR-T cells that retained their Nr4a transcription factors died by day 35 as a result of their tumors. “We have shown that the NFAT–NR4A axis controls the expression of multiple inhibitory receptors, and that treatment of tumor-bearing mice with CAR-T cells lacking all three NR4A transcription factors resulted in tumor regression and prolonged survival,” the authors reported. “We identify NR4A transcription factors as having an important role in the cell-intrinsic program of T-cell hyporesponsiveness and point to NR4A inhibition as a promising strategy for cancer immunotherapy.”
“It is really encouraging to see that we can identify and demonstrate the function of the transcription factors that have a strong role in T-cell exhaustion,” commented Chen.
Interestingly, knocking down the Nr4a transcription factors in CAR-T cells also blocked the inhibitor receptors TIM3, and PD-1, which is itself a target for anticancer immunotherapies. “Because NR4A deficiency results in downregulation of PD-1 and TIM3, the effect of NR4A deficiency is functionally similar to that of PD-1 blockade, but NR4A deficiency affects a wider range of regulatory elements than PD-1 blockade alone,” they stated. This is of particular relevance because while immune cell therapies do offer “considerable promise” for treating cancer, the use of antibodies that block individual targets, such as PD-1 and CTLA4, only rarely result in complete cures. As an alternative, the team suggested, “inhibiting the function of NR4A family members in tumor-infiltrating T cells could be a promising strategy in cancer immunotherapy as it would be expected to mimic combination therapies with blocking antibodies against multiple inhibitory receptors.”