How TopBP1 Controls Dendritic Cell Maturation for Anti-Tumor Immunity
Conventional dendritic cells (cDCs) activate killer T cells against tumors, but how internal signals drive their maturation from bone‑marrow precursors was uncertain. TopBP1 participates in DNA double-strand break repair during V(D)J recombination in lymphocytes, though its role in dendritic cells had not been studied. Given clinical use of Flt3 ligand (Flt3L) to expand cDCs for cancer immunotherapy, researchers investigated whether this protein also acts as a transcriptional co‑factor in dendritic cell development.
Previous work identified PU.1 as essential for total cDC formation, partnering with IRF8 to drive cDC1 specialization, while other factors influence cDC2 and plasmacytoid dendritic cell (pDC) paths . Yet it remained unclear whether a scaffold protein like TopBP1 could physically connect these transcription factors to their genomic targets—a possibility hinted by TopBP1’s role in bridging partners elsewhere in the nucleus.
First, the team deleted TopBP1 specifically in dendritic cells using a CD11c‑cre driver, generating TopBP1cKO mice. They implanted subcutaneous tumors and monitored growth and immune infiltrates ; TopBP1cKO mice showed significantly accelerated tumor progression compared to wild‑type controls, alongside reduced spleen‑resident cDCs and impaired CD8+ T‑cell function. Testing whether the defect arose during development or survival, they mixed wild‑type and TopBP1cKO bone marrow, transferred it into irradiated hosts, and traced donor‑derived dendritic cells ([Figure 3c,d]). TopBP1cKO precursors yielded far fewer mature cDCs while accumulating as pre‑DCs, indicating an intrinsic block in differentiation—not proliferation or death. Parallel in vitro cultures confirmed that wild‑type feeder cells could not compensate for the TopBP1 deficit in precursors ([Figure 3e]), establishing a cell‑autonomous requirement.
Examining Flt3L, a cytokine used clinically to boost dendritic cells for immunotherapy, revealed that in wild‑type mice, Flt3L increased emergency progenitor (EP; CD11c+CD117+) numbers and drove their maturation into XCR1+CD24+ cDC1s—the subset specializing in cross‑presenting tumor antigens to CD8+ T cells ([Figure 4a,b,c]). Despite comparable EP pools in TopBP1cKO animals, Flt3L failed to raise cDC1 output; instead, XCR1−CD24+ progenitors accumulated ([Figure 5b,c,e,f]). Bulk RNA‑sequencing of these progenitors uncovered significant downregulation of genes typically activated by PU.1, IRF8, or IRF4 (false discovery rate <0.05, fold change <0.5) ([Figure 6c,d]), while transcription factor protein levels persisted (IRF8 was even elevated in TopBP1cKO progenitors; see Supplementary Fig. 9d). Biochemical assays showed TopBP1 directly bound PU.1 and IRF8 from nuclear extracts, and microscopy verified the three proteins co‑localized in dendritic cells ([Figure 7c,d,e]). Human HEK293 cells mirrored this interaction ([Figure 7f]). Crucially, PU.1 and IRF8 formed a complex without TopBP1 ([Figure 7g]) but could not activate target genes—meaning TopBP1 enables the complex’s transcriptional function without assembling it.
- While TopBP1 supports both cDC1 and cDC2 development under steady‑state conditions, its role in cDC2 appears indirect, likely through sustaining overall cDC numbers rather than direct lineage instruction—as seen by unchanged cDC2 ratios in Flt3L settings despite reduced absolute counts (see Supplementary Fig. 7d). Its involvement in BRCA1 C‑terminal (BRCT) domains, which mediate protein‑protein interactions, awaits mapping to PU.1 and IRF8 engagement. Future steps include testing whether restoring TopBP1 function selectively in dendritic cells rescues immunotherapy resistance in TopBP1cKO mice, connecting molecular mechanism to physiological outcome.
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Tumor growth and immune cell infiltration in wild‑type versus TopBP1cKO mice.
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cDC and pDC populations in lymphoid and nonlymphoid organs of wild‑type versus TopBP1cKO mice.
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Adoptive transfer and in vitro differentiation of pre‑DCs demonstrating TopBP1’s intrinsic role in pre‑DC to cDC conversion.
Figure 3 — from the original paper -
Efficacy of Flt3L immunotherapy in wild‑type, TopBP1cKO, and Batf3KO mice.
Figure 4 — from the original paper -
Differentiation of XCR1−CD24+ emergency progenitors into cDC1s, showing TopBP1 dependence.
Figure 5 — from the original paper -
Transcriptomic analysis of transcription factor target genes downregulated in TopBP1cKO emergency progenitors.
Figure 6 — from the original paper -
Physical interaction between TopBP1 and PU.1/IRF8, including co‑immunoprecipitation and confocal microscopy data.