We also thank the members of the Hara laboratory for helpful discussion during the preparation of this manuscript. of SMAD3. Thus, deletion of and reduces CX3CR1 expression, thereby inhibiting Mo-MDSC accumulation in tumours expressing CX3CL1 and suppressing the tumour progression in mice. Notably, blockade of the CX3CL1/CX3CR1 axis suppresses tumour growth, whereas Captopril inactivation of CDKs elicits the opposite effect. These findings reveal an unexpected function of and and indicate that regulation of Mo-MDSCs chemotaxis is usually a valuable potential strategy for control of tumour development. Introduction The and genes, which encode cyclin-dependent kinase (CDK) inhibitors, are upregulated in cultured mammalian primary cells upon detection of various potentially oncogenic stimuli1,2. This unique feature of p16Ink4a and p21Waf1/Cip1, together with their ability to induce irreversible cell cycle arrest (termed cellular senescence), suggests that these genes act as AIbZIP a safeguard against neoplasia3C5. Indeed, mice lacking and/or exhibit early onset of cancer6C9, illustrating the importance of p16Ink4a and p21Waf1/Cip1 in tumour suppression in vivo. To observe the physiological roles of p16Ink4a and p21Waf1/Cip1 during tumour formation, we previously generated transgenic mice lines expressing firefly luciferase under the control of the or reporter mice (mice), in which the coding sequence was replaced with cDNA encoding firefly luciferase12. Notably, in addition to ageing and de novo tumorigenesis, p16Ink4a expression was strikingly induced in the stroma of developing neoplasia. Lethal irradiation coupled with bone marrow (BM) transplantation from syngeneic mice indicated the presence of and in mice results in a substantial decrease in infiltration of Mo-MDSCs into tumours and causes slower growth of tumour allografts. Conversely, inactivation of CDKs by chemical inhibitors increases the expression of CX3CR1 in Mo-MDSCs, resulting in accumulation of Mo-MDSCs in tumours and consequent acceleration of tumour growth in allograft mouse models. These results uncover a novel function of p16Ink4a and p21Waf1/Cip1 in MDSC chemotaxis, and provide valuable new insight into how to bypass this undesirable side effect of CDK inhibitors. Results p16 and p21 are expressed in MDSCs in tumour-bearing mice We previously performed in vivo imaging of p16Ink4a or p21Cip1/Waf1 expression in mice and elucidated the dynamics Captopril of their expression during the development of skin cancer, using p16-luc or p21-luc mice9C11. This approach, together with the analysis of and/or and mRNA levels were examined by quantitative real-time reverse transcription (qRT-) PCR (Fig.?1g, h). Interestingly, although was expressed in both PMN-MDSCs and Mo-MDSCs, was only expressed in Mo-MDSCs. As p16Ink4a and p21Cip1/Waf1 CDK inhibitors have established roles in cellular senescence, we tested if p16Ink4a- and/or p21Cip1/Waf1-expressing MDSCs exhibit senescence-like phenotypes. Consistent with a previous report23, BM?Mo-MDSCs are proliferative and the percentage of Mo-MDSCs in the S phase increases in mice lacking both and (p16/p21-DKO mice), compared to in wild-type (WT) mice (Supplementary Fig.?1a). On the other hand, in either splenic or intratumoural MDSCs, there is no difference in cell cycle phase distribution between MDSCs from WT mice and those from p16/p21-DKO mice (Supplementary Fig.?1a). Notably, although proliferation of MDSCs isolated from spleen was rarely detected by a 5-ethynyl-2-deoxyuridine (EdU) incorporation assay in vivo (Supplementary Fig.?1b), a Captopril carboxyfluoroscein diacetate succinimidyl ester (CFSE) dilution analysis indicated that a substantial amount of these MDSCs (Mo-MDSCs >20%, PMN-MDSCs >60%) resumed proliferation upon stimulation with GM-CSF in vitro (Supplementary Fig.?1c). Moreover, other senescence-associated phenotypic characteristics, such as accumulation of H2AX foci and 53BP1 foci (signs of DNA damage), reduced amount of lamin B1 manifestation24, and induction of IL-6 manifestation25, weren’t seen in these MDSCs (Supplementary Fig.?1dCg). These total results, using the observations these MDSCs had been resistant to ABT-263 collectively, a senolytic medication that kills senescent cells26, in both in vitro and.