Clonal Expansion of Tet2 ^+/-hematopoiesis Is Driven By Inflamm-Ageing Associated IL-1 Increase in Mice

Clonal Hematopoiesis of Indeterminate Potential (CHIP) is defined as the presence of an expanded somatic blood cell clone carrying a mutation in genes that are drivers of hematologic malignancy including DNMT3A, TET2, ASXL1, TP53, JAK2, and SF3B1, at a variant allele frequency (VAF) of at least 2% in the absence of other hematological abnormalities. CHIP has a prevalence of about 10% in the 70-80 year old population, further increases with ageing and associates with an increased risk of hematological malignancies, cardiovascular disease and all-cause mortality (Genovese et al. NEJM 2014; Jaiswal, S. et al. NEJM 2014). Recent studies indicate that higher pre-malignant clonal size and mutational burden increase the chances of malignant transformation in individuals carrying CHIP (Desai, P. Nat. Med. et al., 2018; Abelson, S. et al. Nature, 2018). While age is the best predictor of CHIP development and correlates directly with pre-malignant clonal size, the specific cellular-extrinsic factors promoting CHIP clonal expansion in the context of physiological aging are still unclear. We hypothesized that ageing associated low-grade inflammation (termed "inflamm-ageing") is a driver of CHIP clonal expansion.

We used standard bone marrow (BM) chimera models and developed a novel, irradiation independent, hematopoietic specific and tamoxifen inducible genetic mosaicism mouse model of Tet2 ^+/- driven CHIP (HSC-Scl-Cre-ER^T; Tet2^+/flox; R26 ^{+/floxstop-EGFP} triple transgenic mice) to determine the contribution of inflamm-aging factors to Tet2 ^+/- hematopoieticclonal expansion. Using these complementary models, we observe that peripheral Tet2 ^+/- clonal expansion rates increase with age (evident in erythroid, myeloid, lymphoid B and T lineages), which is paralleled by a significant expansion of Tet2 ^+/- hematopoietic stem and progenitor cell (HSPCs) populations in aged mice (12-14 months old). Importantly, Tet2 ^+/- clonal expansion associates with increased levels of inflammatory cytokine IL-1 in aged mice, which derives partially from Tet2 ^+/-mutant mature hematopoietic cells. To test the contribution of IL-1 to Tet2 ^+/- clonal expansion, we administered IL-1 (0.5ug/day for 14 days) to young CHIP carrying mice (2-4 months of age) and observed an IL-1R1-dependent expansion of Tet2 ^+/-hematopoietic mature lineages and HSPCs. Dissection of the cellular mechanisms operating downstream of IL-1/IL-1R1 revealed that Tet2 ^+/-clonal expansion results from increased multilineage differentiation and associates with increased HSPC cell-cycle progression (while not depending on IL-1-mediated effects on HSPC viability). Moreover, Tet2 ^+/- HSPCs show a higher in vitro and in vivo repopulation capacity in response to prolonged IL-1 exposure compared to their WT counterparts. Finally, to directly test the contribution of IL-1 to drive Tet2 ^+/- clonal expansion in the context of physiological aging, we set up genetic (BM chimeras using donor BM from Tet2 ^+/-; Il-1r1-/- compound mutants) or pharmacological inhibited IL-1 signaling (Anakinra, hIL-1ra) during mouse ageing. Strikingly, both approaches prevented ageing-dependent Tet2 ^+/- clonal expansion, thus confirming IL-1 as key "inflamm-ageing" driver of Tet2 ^+/- clonal expansion. Overall, our data provide proof-of-concept that IL-1 production derived from aged BM cells is a relevant and targetable driver of Tet2 ^+/- clonal expansion in aged mice.