TopBP1 orchestrates PU.1–IRF8 transcriptional programming of dendritic cell differentiation and Flt3L-driven tumor immunity

www.nature.com/emm ARTICLE OPEN TopBP1 orchestrates PU.1–IRF8 transcriptional programming of dendritic cell differentiation and Flt3L-driven tumor immunity Min-Suk Cha1,2, Myeong-Ho Kang1,2, Jinjoo Lee1,2, JungHyub Hong1,2, Yu Sun Jeong1,2, Yoe-Sik Bae ✉ ✉ Seok-Hee Park 1,2 and Yong-Soo Bae1,2 1,2,3 , Ho Lee 4 , 1234567890();,: © The Author(s) 2026 DNA topoisomerase II-binding protein 1 (TopBP1) plays a critical role in V(D)J recombination and DNA damage repair during B and T cell development. However, its role in the development of conventional dendritic cells (cDCs) remains unexplored. Mice with DCspecific depletion of TopBP1 (TopBP1cKO) exhibited accelerated tumor progression due to impaired anti-tumor immunity, which was characterized by cDC deficiency and pre-DC accumulation. The cDC deficiency observed in TopBP1cKO mice was not attributable to cell death resulting from accumulated DNA damage during DC development. Notably, Flt3 ligand (Flt3L)-mediated tumor immunotherapy was ineffective in TopBP1cKO tumor-bearing mice. Here we demonstrate that TopBP1 is required not only for the steady-state differentiation of total cDCs, including both cDC1 and cDC2, but also for the terminal differentiation of XCR1−CD24⁺ emergency progenitors (CD11c⁺cKit⁺) into XCR1⁺CD24⁺ cDC1s in response to Flt3L. Furthermore, TopBP1 was found to be essential for the function of the PU.1–IRF8 heterodimeric transcription factor complex, which is critical for cDC lineage specification. TopBP1 directly binds to this complex and facilitates the transcription of downstream target genes required for cDC development. These findings establish TopBP1 as a pivotal regulator of both steady-state and Flt3L-driven emergency cDC differentiation, particularly in guiding emergency progenitors into functional cDC1s. Our study highlights the previously unrecognized role of TopBP1 as a co-regulator of lineage-defining transcription factors and as a determinant of Flt3L-mediated antitumor efficacy. Experimental & Molecular Medicine; https://doi.org/10.1038/s12276-026-01715-1 INTRODUCTION Most cells undergo multiple stages of proliferation and differentiation to reach their final cell types. Substantial DNA damage occurs during these processes, which must be properly repaired to prevent cell death. Topoisomerase II binding protein 1 (TopBP1) is one of the key regulator of DNA damage repair1–4 and is also involved in cell cycle checkpoint regulation5–7, DNA replication initiation8,9, transcriptional regulation10,11 and cell survival12,13. TopBP1 is induced during the G1/S transition, where it interacts with Treslin and CDC45 to initiate DNA replication9,14. In response to DNA damage, TopBP1 interacts with ATR15 and ATM1,16, facilitating DNA repair and enabling cell cycle progression. Akt phosphorylates TopBP1 at Ser1159, inducing its oligomerization10,11. Oligomerized TopBP1 inhibits its association with chromatin and ATR, reducing its function in activation of ATR. Instead, oligomerized TopBP1 is associated with E2F1 and inhibits E2F1-dependent cell apoptosis11. In addition, TopBP1 is crucial for cell survival12, and its complete knockout causes embryonic lethality and neurogenesis defects17,18. In immunological studies, TopBP1 is essential for V(D)J recombination and development of B, T and iNKT cells through double-strand break (DSB) damage repair19. Dendritic cells (DCs) are professional antigen-presenting cells that bridge innate and adaptive immunity by presenting antigens to specific T cells. DCs can be broadly categorized into conventional DCs (cDC) and plasmacytoid DCs (pDC)20. pDCs play an important role in anti-viral immunity by secreting large amounts of type I interferon, whereas cDCs initiate and induce adaptive immune responses against foreign invaders and cancers. Under steady-state conditions, hematopoietic stem cells give rise to common DC progenitors through several intermediate progenitor stages in the bone marrow (BM). These common DC progenitors possess the capacity to differentiate into pDCs and pre-CDCs (pre-DCs; Lin−CD135⁺MHCII−CD11c⁺CD115−CD117−/int) within the BM. Pre-DCs migrate through the blood to the respective organs where they further differentiate into cDC subsets (XCR1+cDC1, CD11b+cDC2) depending on the microenvironment21–24. The Flt3 ligand (Flt3L), a growth factor necessary for pre-DCs and cDC1 differentiation, promotes cDC1-mediated antitumor immunity and is widely used in cancer immunotherapy25. It has been reported that Flt3L increases the level of emergency progenitors (EPs; CD11c+cKit+ population), which are normally present in very small numbers in BM and that the EPs expanded by Flt3L ultimately differentiate into IRF8-dependent cDC1s26. 1 Department of Biological Sciences, Sungkyunkwan University, Suwon, Republic of Korea. 2Center for Immune Research on Non-Lymphoid Organs, Sungkyunkwan University, Suwon, Republic of Korea. 3Department of Health Sciences and Technology, Samsung Advanced Institute for Health Sciences and Technology, Sungkyunkwan University, Seoul, Republic of Korea. 4Division of Convergence Technology, Research Institute, National Cancer Center, Goyang, Republic of Korea. ✉email: ; Received: 16 July 2025 Revised: 18 December 2025 Accepted: 16 February 2026 M.-S. Cha et al. 2 Meanwhile, GM-CSF promotes cDC2 and monocyte-derived DC differentiation. Many transcription factors (TFs) have been identified as essential for DC subset specification. For instance, E2-2 is critical for the differentiation and maintenance of pDCs27–29; IRF830,31, ID232,33, Batf334 and Nfil332,33,35 are indispensable for cDC1 development, whereas IRF436,37, Notch238 and Klf439,40 are important for cDC2 differentiation39,41. However, despite this extensive characterization of subset-specific TFs, the transcriptional regulation of total cDC differentiation, including both cDC1 and cDC2, remains poorly understood. So far, the Ets-family protein PU.1, encoded by Spi1, is the only well-established TF essential for the overall differentiation and function of total cDCs including cDC1 and cDC242,43. PU.1 is highly expressed in cDCs, where it maintains identity and promotes formation, particularly of cDC1s, by inducing the transcriptional regulator DC-SCRIPT but inhibits the development of pDCs43. PU.1 is also required for differentiation of other immune cells44,45. TopBP1 has not previously been reported to be involved in DC biology. To explore a potential connection between TopBP1 and DC-specific TFs, we examined known TopBP1-interacting partners and found that TopBP1 interacts with Miz-1 (Myc-associated zincfinger protein). Miz-1 is a TF known to interact with MYC and regulate the transcription of cell cycle inhibitory genes (for example, p15INK4b). Miz-1 also stabilizes TopBP1 by forming a complex with HectH9, which inhibits TopBP1 ubiquitination and protects it from proteasomal degradation, thereby maintaining its role in cell cycle checkpoint control during DNA damage responses46. (...truncated)