Abstract

The Generation of Tumor-Associated T Regulatory Cells Critically Depends on Calcineurin

Francesco Marangoni, Instructor in Medicine, Harvard Medical School

CD4⁺FoxP3⁺ T regulatory cells (T_regs) play a key role maintaining immune homeostasis, yet they are also improperly co-opted by tumors where they blunt anti-tumor immunity and facilitate progression.  Treg function is maintained in vivo by continuous triggering of the T cell receptor (TCR), yet it is unknown how interactions between T_regs and APCs in the context of tumor development dynamically trigger the TCR and the numerous signaling pathways downstream of it. Among these, the role of the calcineurin / NFAT pathway in T_reg biology is controversial. We thus investigated the role of calcineurin / NFAT signaling in supporting tumor-associated T_reg (T_uT_reg) generation and function by using mice with Treg-specific deletion of calcineurin B (CnB) and intravital imaging of NFAT activation.  In tumor-draining lymph nodes, T_reg activation and the expression of chemokine receptors promoting tumor homing was completely dependent on calcineurin.  Consistently, CnB-deficient T_regs failed to infiltrate the tumor mass, whose growth was controlled. In contrast, T_reg-specific deletion of the costimulatory molecule CD28 allowed a sizeable population of T_regs to infiltrate the tumor, whose progression was only partially limited.
Within the tumor mass, polyclonal T_uT_regs efficiently activated the calcineurin / NFAT pathway during even transient interactions with APCs, whereas polyclonal effector CD4⁺ cells failed to do so.  This was due to selection of a tumor-reactive TCR repertoire, because T_uT_regs and effector T cells bearing an identical TCR recognizing a tumor-associated antigen got comparably activated in vivo.  The functional relevance of calcineurin / NFAT activation in T_uT_regs is currently under investigation. Our data indicate a key role of