Co-inhibitory immune signaling generates the splicing of an immunophilin which marks a Tregs subset associated with immunotherapy response of melanoma patients

Cancer immunotherapy has shown surprising efficacy in several types of advanced and incurable tumors, particularly, malignant melanoma. There are several immunotherapeutic strategies aimed at enhancing immunological defenses against tumor. Among these, monoclonal antibodies against the so-called “immune checkpoint inhibitors”, that counteract tumor-induced immune-disarming pathways, have shown the best outcomes. Regulatory T lymphocytes or Tregs are a subset of lymphocytes involved in immune-surveillance and maintenance of self-tolerance. Tumor often exploits Tregs to allow tolerance to its own antigens and avoid immune system attack. Tregs are usually increased in melanoma patients. It is noticeable that Tregs is a heterogeneous population with respect to their immunosuppressive capability. Lymphocytes are particularly rich in FKBP51 (FKBP5 gene), an immunophilin better known as the intracellular receptor for FK506 and rapamycin. Melanoma aberrantly expresses this immunophilin, which supports cancer resistance and invasion. Recently, our group has shown that melanoma interaction with immune cells, through PD-L1/PD1, bidirectionally generated the splicing of FKBP5 gene inducing a lower molecular weight form of FKBP51, termed FKBP51s, in both melanoma and lymphocyte. A study performed on PBMC of 64 patients with advanced melanoma (stage III/IV) showed that FKBP51s marks a Treg subset which was correlated, as an independent variable, to anti-CTLA4 (ipilimumab) response. More precisely, a low frequency of Treg FKBP51spos (<1% of total CD3/CD4 lymphocytes) was associated with unresponsiveness to ipilimumab (Chi-square=9.916, p=0.002). In vitro iTreg generation suggested that FKBP51s was associated with CD25high, Ki67high and p70S6khigh iTregs, corresponding to a highly metabolically active profile associated with strong suppressive capability. FKBP51s silencing by siRNA attenuated the suppressive phenotype of such iTregs. A study performed on a different cohort of patients receiving anti-PD1 reinforced the hypothesis that melanoma patients that benefit from immune checkpoint targeted therapy are recognizable by an expansion of FKBP51s+Treg subset which may be involved in de-activation of stimulatory co-signalling pathways, in support of tumor immune evasion.