April 15, 2021 -- During a plenary session at the American Association for Cancer Research (AACR) 2021 virtual meeting held April 9-14, speakers described impressive work that researchers have done to modify and target T cells as a means of treating cancers.
Dr. Nina Bhardwaj, PhD, director of immunotherapy at the Tisch Cancer Institute Icahn School of Medicine at Mount Sinai in New York City, opened the plenary session with an overview of the activation of T-cell responses. Here, T cells are primed (initial activation), leading to T-cell expansion and differentiation into effector CD8+ T cells. These cells can take two states, either as memory T cells, which provide antigen clearing, or exhausted T cells, which allow antigen persistence.
Bhardwaj explained that T-cell immunity can be improved by modulating T cells, focusing on designs that "overcome immune suppression and exhaustion to enhance T-cell effector function."
The rest of the AACR session focused on just a few ways that scientists are working to modulate T cells to develop cancer immunotherapies.
Combating the exhausted T-cell state by targeting transcription factors
For example, Anjana Rao, PhD, professor at the La Jolla Institute for Immunology in La Jolla, CA, detailed her journey learning about and overcoming challenges of exhaustive T cells in chimeric antigen receptor (CAR) therapies. She explained how she first learned about the exhaustive T-cell state by investigating the effect on T-cell activation of the nuclear factor of activated T cells (NFAT), a family of transcription factors.
NFAT is more stable when it is bound to activator protein 1 (AP-1), another transcription factor. Its structure is a heterodimer composed of proteins belonging to the Fos and Jun families. During T-cell responses, T-cell receptor activation and co-stimulatory receptors lead to the activation of Fos3 and Jun (AP-1), which work in concert with NFAT for T-cell activation.
Nearly 30 years ago, Rao worked with colleagues to develop an NFAT mutant that did not bind to AP-1 and showed that T cells derived from this activation scheme were hyporesponsive to T-cell receptor stimulation and adapted the "exhaustion" state.
Hallmarks of exhausted T cells include expression of inhibitory surface receptors such as programmed cell death 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), decreased cytokine production (interleukin-2, tumor necrosis factor, and interferon gamma), and altered expression and use of key transcription factors.
Importantly, Rao detailed how some cancer immunotherapies work to counteract the exhaustive state. Checkpoint inhibitors such as anti-PD1/antiprogrammed cell death ligand 1 (anti-PD-L1) and anti-CTLA4 work to convert exhausted T cells into effector T cells. And adoptive T-cell therapies, like CAR T cells, modify T cells to prevent them from adopting the exhaustive state.
Approaching the question from the adoptive cell therapy side of the equation, Rao asked if elimination of the key transcription factors -- nuclear receptor transcription factors (NR4) and TOX High Mobility Group Box Family Member 2 (TOX2), which mediate the transcription program of exhaustion -- would prevent T cells from adopting the exhaustive state.
CAR tumor infiltrating lymphocytes (TILs) with NR4 or TOX2 knockout resulted in a reduction in tumor size and increased survival in mouse models. The CAR TILs showed molecular characteristics of effector T cells rather than exhaustive T cells. Conclusively, Rao explained that blocking the expression of transcription factors for NR4A and TOX led to increased effector function.
At a higher level, NR4 and TOX2 are controlled by transcription factor families, the basic leucine zipper domain (bZIP) (which include Fos and Jun) and nuclear factor kappa-light-chain-enhancer of activated B cells (NFkB). Therefore, Rao posed the question of whether bZIP or NFkB transcription factor repression in wildtype effector T cells can prevent exhaustion in T cells.
In a series of experiments, Rao's team demonstrated that basic leucine zipper ATF-like transcription factor (BATF) (a member of the dZIP family), is highly overexpressed in CAR TILs. BATF-transduced CAR T cells demonstrated a massive expansion of CAR T cells with reduced hallmarks of exhaustive phenotype were seen via RNA sequencing. Moreover, these cells protected mice against tumor rechallenge (presence of CD27, CD127, TCF1 in CAR T cells were proof of memory function).
To investigate how these TILs are more effective, Rao's team explored how BATF-overexpressing TILs effect T-cell receptor signaling. They found that once the BATF-CAR TILs enter the nucleus, they become inaccessible and block T-cell receptor binding. This effectively prevents the TILs from adopting the exhaustive state.
Further teasing out this mechanism, the researchers recently found that BATF overexpression promotes tumor clearance and antitumor memory by recruiting its cooperating transcription factor interferon regulatory factor 4 (IRF4) in a specific motif to promote survival and effector function of TILs. This ultimately leads to blunting T-cell receptor signaling to exhaustion-related genes.
This impressive set of comprehensive studies demonstrates the power of targeting transcriptional programs to counteract T-cell exhaustion and promote effective T-cell responses against tumors.
Other speakers in the session explored how different immunotherapies, such as engineered monoclonal antibodies and newer versions of CARs could be used to target the root causes of cancer and stop them in their tracks.
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