Microphthalmia-associated transcription factor (MITF) is a key player in phenotype switching in melanoma. We showed previously that MITF regulates phenotypic melanoma heterogeneity through changing extracellular matrix (ECM) composition and tumour microarchitecture. We now hypothesize that MITF controls immune cell infiltration through changed ECM structure in our 3D melanoma spheroid model.
Our results demonstrate that MITF can impact immune cell infiltration, yet this is cell-line dependent and dependent on additional factors, including chemokines and cytokines. Notably, peripheral blood mononuclear cells (PBMCs) preferably killed proliferating cells in the G1 phase of the cell cycle, whereas G1-arrested cells escaped from immune cell surveillance. However, this phenomenon appears to be cell-line dependent as we did not find a decrease in G1 cells in another melanoma cell line tested. Nevertheless, Natural Killer (NK) cells are likely to be responsible for this G1 cell elimination, as the short duration of this experiment does not allow for T cell activation. To study the role of the Rho-ROCK-myosin-contractility axis on phenotypic heterogeneity and tumour microarchitecture, we treated melanoma spheroids with a ROCK inhibitor, which indeed mimicked the effect of MITF-overexpression and thus might enhance immune cell penetration.
Taken together, we report for the first time that PBMCs may prefer proliferative cells in the G1 phase. This is a major challenge for current clinical treatments, as dormant cells are often responsible for melanoma relapse. However, this finding is significant beyond melanoma, as phenotypic heterogeneity occurs in a range of solid tumours in addition to melanoma, including prostate, breast, ovarian and lung cancer. Furthermore, ROCK inhibition enhances homogeneous cell proliferation and contributes to a less compact spheroid morphology, potentially promoting PBMC infiltration and melanoma cell death, substantially enhancing immune checkpoint inhibitor therapy.