BRAF/MEK inhibition following immune checkpoint inhibitors promotes antitumor immunity associated with expansion of progenitor-exhausted CD8⁺ T cells

Background: Targeted BRAF and MEK therapy (TT) and immune-checkpoint inhibitors (ICI: anti-CTLA4/anti-PD1), have revolutionised treatment for cutaneous melanoma, however both have their own limitations.To increase the frequency of durable disease control while avoiding the high-grade toxicities associated with the combination of ICI and TT treatments, clinical strategies have shifted toward optimising sequencing of these therapies.Current standard of care favours initiating treatment with ICI, followed by TT upon progression.Notably, emerging trial data suggests that switching from TT to ICI while patients are still responding can achieve outcomes comparable to ICI to TT given sequentially, underscoring the need to investigate how treatment timing and sequencing influences therapeutic efficacy and tumour immune dynamics.Method: We evaluated sequencing strategies in a mouse melanoma model involving upfront combination of TT and ICI (TT+ICI) and a short term first-line therapy of either TT followed by ICI (TTICI) or ICI followed by TT (ICITT).Results: No significant differences in survival were observed across treatment strategies; however, both TT+ICI and ICITT schedules were associated with delayed tumour growth.Analysis of the tumour immune microenvironment at the time of treatment switch revealed a more immunosuppressive landscape in response to first-line TT, marked by an increase in regulatory T cells (Treg) compared to ICI.In contrast, administering TT after ICI led to an enriched CD8 + T cell pool with a progenitor-exhausted, stem-like phenotype.Conclusion: These findings suggest that ICI as a first-line therapy effectively primes the tumour immune microenvironment, enhancing the immune stimulating effects of second-line TT.This priming limits the accumulation of suppressive immune cells, ultimately leading to improved progression-free survival.