Heterogeneous pro-inflammatory response to BRAFV600E-induced thyroid tumor development

Background The tumor immune microenvironment likely plays a central role in progression of thyroid cancer. As for most other solid tumors, it is unknown if immune dysregulation contributes to earlier, subclinical stages of thyroid tumor development, or whether thyroid tumor heterogeneity might involve differential expression of pro-inflammatory mediators. Methods The time course of tumor-associated inflammation was studied in Tg-CreER T2 ;Braf CA/+ mice representing a model of BRAF V600E -driven papillary thyroid carcinoma (PTC). Tumor growth was estimated by histological examination and magnetic resonance imaging. Cytokine expression was monitored by quantitative RT-PCR, RNAScope and Western blot analyses. Results Based on spontaneous Braf CA activation due to leaky Cre activity in a minority of targeted cells tumors developed within a preserved thyroid tissue architecture to multifocal papillary thyroid carcinoma (PTC) over a period of 12 months. Tumorigenesis was accompanied by a gradually increased mRNA and protein expression of interleukin-1beta (IL-1β), interleukin-6 and tumor necrosis factor-alpha (TNF-α) starting already before Braf mutant cells commenced neoplastic growth. RNAScope revealed that both follicular cells and stromal cells expressed Il1b whereas Il6 and Tnfa transcripts were mostly confined to neoplastic epithelia. Early cytokine expression was associated with oncogene-induced senescence, whereas during tumor development (3-6 months) and in advanced tumor stages (at 12 months) the cytokine expression pattern differed among glands and tumor foci of the same gland accompanied by a highly variable locoregional lymphocytic infiltration. Oral treatment of mutant mice for 1 month with PLX4720, a vemurafenib prodrug, partially reduced cytokine expression along with inhibited tumor growth and redifferentiation of thyroid function. The magnitude of reduced cytokine expression differed much between glands and among mice of both sexes. Conclusions These findings indicate that oncogenic BRAF V600E targeted to the thyroid both stimulates endogenous production of IL-1β, IL-6 and TNF-α and recruits inflammatory cells to foci of early tumor development. PTCs of different clonal origin are distinguished by differential expression of pro-inflammatory cytokines. The anti-inflammatory effect of mutant Braf kinase inhibition varies presumably related to heterogeneous tumor development, which evolves from stochastic Braf CA activation suggesting there are clonally different probabilities of acquiring drug resistance among Braf mutant thyroid follicular cells.