Post-transcriptional control of T-cell cytokine production: Implications for cancer therapy

Abstract

As part of the adaptive immune system, T cells are vital for the eradication of infected and malignantly transformed cells. To perform their protective function, T cells produce effector molecules that are either directly cytotoxic, such as granzymes, perforin, interferon-γ and tumour necrosis factor α, or attract and stimulate (immune) cells, such as interleukin-2. As these molecules can also induce immunopathology, tight control of their production is required. Indeed, inflammatory cytokine production is regulated on multiple levels. Firstly, locus accessibility and transcription factor availability and activity determine the amount of mRNA produced. Secondly, post-transcriptional mechanisms, influencing mRNA splicing/codon usage, stability, decay, localization and translation rate subsequently determine the amount of protein that is produced. In the immune suppressive environments of tumours, T cells gradually lose the capacity to produce effector molecules, resulting in tumour immune escape. Recently, the role of post-transcriptional regulation in fine-tuning T-cell effector function has become more appreciated. Furthermore, several groups have shown that exhausted or dysfunctional T cells from cancer patients or murine models possess mRNA for inflammatory mediators, but fail to produce effector molecules, hinting that post-transcriptional events also play a role in hampering tumour-infiltrating lymphocyte effector function. Here, the post-transcriptional regulatory events governing T-cell cytokine production are reviewed, with a specific focus on the importance of post-transcriptional regulation in anti-tumour responses. Furthermore, potential approaches to circumvent tumour-mediated dampening of T-cell effector function through the (dis)engagement of post-transcriptional events are explored, such as CRISPR/Cas9-mediated genome editing or chimeric antigen receptors.

Keywords: AU-rich elements; T cells; cancer; effector function; post-transcriptional regulation.

FIGURE 1

Transcriptional and post‐transcriptional regulatory events together shape protein production. Firstly, transcription factor availability and activation status, locus accessibility and cellular transcription rates determine the amount of pre‐mRNA that is produced. Secondly, post‐transcriptional splicing affects the inclusion of alternative exons and determines the usage of alternative 3’UTRs. Thirdly, post‐transcriptional regulatory events, mediated by mRNA modifications, and binding by (long) non‐coding RNAs, miRNAs and RBPs influence mRNA decay, localization, stability and translational rate, and thus determine the amount of protein that is produced from a single mRNA molecule

FIGURE 2

Splicing of pre‐mRNA and the impact of alternative splicing on IL‐2 production. (a) Intronic splicing relies on the recognition of the internal splicing site (ISS) by SR proteins. Subsequently, splicing factors U1 and U2 bind the 5’ and 3’ splicing sites, forming spliceosome complex A. U5 and U4‐U6 are recruited, together forming spliceosome complex B. Through splicing factor rearrangement, U1 and U4 are released, and the active spliceosome is formed, which is catalytically active and able to excise the intron. (b) Alternative splicing of MALT1 under the influence of several RBPs, including hnRNP U, results in the expression of MALT1 isoforms MALT1A and MALT1B. MALT1A is capable of cleaving Regnase‐1, an RNAse involved in the translation‐dependent breakdown of IL2 mRNA. As a result, cells expressing MALT1A are superior IL‐2‐producing cells

FIGURE 3

Post‐transcriptional regulatory processes governing T‐cell cytokine production. Left, antigen‐experienced T cells possess IFNG and TNFA mRNA that is kept translationally silent through the binding of their respective 3’UTR AREs by ZFP36L2. Middle, upon T‐cell activation, ZFP36L2 dissociates from IFNG and TNFA mRNA, enabling translation. HuR transiently binds the 3’UTR AREs in IFNG and TNFA mRNA, stabilizing the transcripts. De novo transcription of IFNG and IL2 mRNA results in increasing mRNA levels. IL2 mRNA is stabilized through the binding of Nucleolin, YB‐1 and NF‐90. Right, upon resolution of the stimulatory signal, cytokine mRNA is destabilized and degraded through the binding of multiple RBPs (IFNG: ZFP36, Roquin1/2; IL2: ZC3H12A, ZC3H12D, ZFP36; TNFA: Roquin1/2, TIA‐1, TIAR, ZC3H12D)