Expansion of a circulating Ki67-positive effector T-cell population following combined PD-1 and CTLA-4 blockade for melanoma is predictive of treatment response.

Background: Despite the success of combined cytotoxic T-lymphocyte associated protein 4 (CTLA-4) and programmed cell death protein-1 (PD-1) immune checkpoint blockade (cICB), the majority of patients with melanoma fail to respond or experience severe treatment-related toxicity. Currently, there are no reliable biomarkers available to predict these events and guide treatment choices. We here evaluated the peripheral immune compartment to identify features associated with cICB outcome and toxicity.

Methods: Blood samples were collected from 51 patients with advanced melanoma prior to commencing and after one cycle of cICB. Patients were classified as responders or non-responders based on radiographic best overall response to treatment, and grouped by the occurrence of severe toxicity. Absolute immune cell counts were obtained and peripheral blood mononuclear cells were cryopreserved prior to spectral flow-cytometric T-cell immunophenotyping.

Results: 20 patients (39%) failed to respond to treatment, and 29 (57%) experienced severe toxicity. Pre-treatment, patients had fewer T cells than age-matched healthy controls (median 892 vs 1297 cells/µL, p=0.0004), mostly due to reduced naive CD4⁺ (p=0.0038) and CD8⁺ (p=0.0031) T cells. One cycle of cICB restored patient T cells to levels equivalent to healthy controls through expansion and activation of CD4⁺ and CD8⁺ memory and regulatory, but not naive subsets, and skewed the T-cell compartment towards an activated phenotype. This T-cell expansion correlated strongly with pre-treatment PD-1 (r=0.88, p=0.0003) but not CTLA-4 (r=0.32, p=0.34) expression levels, and was accompanied by upregulation of molecules including Ki67, inducible co-stimulator of T cells (ICOS), T-cell immunoglobulin and mucin-domain containing-3 (TIM-3), and T cell immunoreceptor with Ig and ITIM domains (TIGIT) on effector CD4⁺ and CD8⁺ T cells. Greater upregulation of Ki67 in CD4⁺ central memory cells significantly differentiated responders and non-responders after one cycle of treatment (p=0.0086, area under the curve (AUC)=0.74, 95% CI 0.59 to 0.88), while higher on-treatment TIM-3 frequency within CD8⁺ T cells differentiated patients who experienced severe toxicity (p=0.0086, AUC=0.74, 95% CI 0.59 to 0.88).

Conclusions: We here show that response and toxicity to cICB in advanced melanoma are driven by distinct immune features evident after only one cycle of treatment. These could serve as prognostic biomarkers upon validation in larger cohorts.