Constitutive expression of IκBζ promotes tumor growth and immunotherapy resistance in melanoma [mouse RNA-seq]

Background: IκBζ, a rather unknown co-regulator of NF-κB, is mostly inducibly expressed and can either activate or repress a specific subset of NF-κB target genes. While its role as a transcriptional regulator of various cytokines and chemokines in immune cells has been revealed, IκBζ’s function in solid cancer remains unclear. Methods: We investigated IκBζ expression in melanoma, and assessed its impact on target gene expression, tumor growth, and response to immunotherapy in melanoma cell lines, mouse models, and patient samples. Results: Unlike in other cell types, IκBζ protein was found to be constitutively expressed in a subfraction of melanoma cell lines, and around 35% of melanoma cases. This atypical expression pattern of IκBζ did not correlate with its mRNA levels or known driver mutations, but instead seemed to result from changes in its post-transcriptional or post-translational regulation. Deleting constitutively expressed IκBζ abrogated the activity and chromatin association of STAT3 and p65, leading to reduced expression of the pro-proliferative cytokines IL-1β and IL-6 in melanoma cells. Consequently, loss of tumor-derived IκBζ suppressed self-sustained melanoma cell growth both in vitro and in vivo. Additionally, constitutive IκBζ expression suppressed the induction of the chemokines CXCL9, CXCL10, and CCL5, which impaired the recruitment of NK and CD8+ T-cells to the tumor, causing resistance to α-PD-1 immunotherapy in mice. Furthermore, the expression of tumor-derived IκBζ also correlated with the absence of CD8+ T-cells in human melanoma samples and progressive disease during immunotherapy. Conclusion: We propose that tumor-derived IκBζ could serve as a new therapeutic target and prognostic marker that characterizes melanoma with high tumor cell proliferation, cytotoxic T- and NK-cell exclusion, and unfavorable immunotherapy responses. Targeting IκBζ expression might open up a new therapy option to re-establish the recruitment of cytotoxic cells, thereby resensitizing for immunotherapy.