IL-12 From Endogenous cDC1, and Not Vaccine DC, Is Required for Th1 Induction

In this study, we generated a chimeric situation by injection of different gene-modified BM-DCs into different strains of gene-modified recipient mice. This allowed us to identify the separate functional contributions of injected versus endogenous DCs for Th1 polarization. We identified the cellular source of IL-12p70 production after subcutaneous BM-DC vaccination as endogenous migratory XCR1+ bystander DCs in the skin draining lymph nodes. DC-DC and DCT cell interaction studies revealed a time course of Th0 priming by injected BM-DCs, followed by interactions of BM-DC with the IL-12+ XCR1+ bystander DCs, and finally IL-12+ XCR1+ bystander DC interactions for Th1 induction. Transcriptional profiling of the bystander DCs underscores their Th1 polarization potential. Together, this study shows that DC-vaccination requires the bystander activation of endogenous DCs for Th1 priming. Our data also challenge the general concept of Th1 priming by a single DC providing all signals 1, 2 and 3 to T cells for Th1 polarization. Overall design: To study the transcriptional changes occurring in the endogenous migratory bystander DCs, we sorted the migratory MHC II high CD11c+ lymph node DC subsets before (naive mice) and 48h after BM-DC injection (bystander activation). Thereby we could compare migratory XCR1+ CD11b- dDCs (cDC1s), CD11b+ XCR1- dDCs (cDC2s), and CD11b- XCR1- DCs (DN) at steady-state and 48h after LPS.BM-DC injection. Also, the appearance of CD11b+ CD64+ Ly6Clow inflammation-induced MoDCs was identified and these cells were also sorted at 48h.