Cancer immunotherapy, particularly monoclonal antibodies against immune checkpoint inhibitors, has shown surprising efficacy in several types of advanced incurable tumors, including malignant melanoma. Tregs, a subset of lymphocytes involved in immune-surveillance and self-tolerance, are usually increased in melanoma patients. Lymphocytes are particularly rich in FKBP51, 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, generated the splicing of FKBP5 gene inducing a lower molecular weight form (FKBP51s), in both melanoma and lymphocyte. Aim of this study is to assess the role of Treg FKBP51s+ as potential biomarker of response to anti-PD1 drugs.
Treg FKBP51s+ were measured in peripheral blood by flow cytometry. To date, we have outcomes of 11 patients. For 6 patients, we have collected from 4 up to 16 blood samples, before each anti-PD1 administration, with a total of 80 sample analysis. iTregs were generated by purified CD4+ T lymphocytes from normal donor, stimulated with CD3+CD28+beads. The suppressive capacity was assessed according to the parameters CD25high, Ki67high and p70S6khigh.
In 5 responder patients, Treg FKBP51s+ was 1.2-4.8%; in 5 non-responders, the count was 0.04-0.8%. Interestingly, a patient with count 0.72% developed autoimmune side effects that led to drug discontinuation. Resolution of side effects was accompanied by an increase in Treg FKBP51s+ value to 9.9%. In vitro iTreg generation suggested that FKBP51s was induced in Treg CD25high Ki67high p70S6khigh, corresponding to a highly metabolically active profile associated with strong suppressive capability. Use of a siRNA for FKBP51s silencing resulted in reduction of this subtype of iTreg.
Our data reinforce the hypothesis that melanoma patients that benefit from immunotherapy are recognizable by an expansion of a Treg subset which plays a central role in tumor immune evasion. This Treg subset is marked by FKBP51s, a splicing protein isoform generated by triggering of surface antigens (PD-L1, PD1) abundantly expressed on highly suppressive Tregs.
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Legal entity responsible for the study
Università degli Studi della Campania Luigi Vanvitelli.
Has not received any funding.
All authors have declared no conflicts of interest.