New Bone Marrow Culture System Generates Large Numbers of Functional cDC1 Cells

Conventional type 1 dendritic cells (cDC1 cells) help launch immune responses against tumors and intracellular pathogens. They are particularly important because of their ability to cross-present cell-associated antigens to CD8-positive T cells. Because cDC1 cells are scarce in vivo, though, it has been hard to collect enough of them, in uniform enough populations, for mechanistic and preclinical work. 

To address this, a team at the National Cancer Institute created a culture system that uses mouse bone marrow to generate large numbers of highly enriched CD103-positive cDC1 cells. Their study, published in Myeloid Cells, refers to the system as induced cDC1, or iDC1, cultures.

The 23-day protocol combines three cytokines: FLT3L, low-dose GM-CSF, and Kit ligand. With it, the team produced an estimated 1.5 billion CD103-positive cDC1 cells from a single mouse’s bone marrow, and the target cells made up more than 95% of the final culture. This yield was at least 75 times greater than what earlier recombinant cytokine-based methods could achieve.

The researchers went beyond surface markers to characterize the cultures. Other bone marrow-derived dendritic cell methods produced more non-target cells, including cDC2 cells, plasmacytoid dendritic cells and macrophages, than iDC1 cultures did. In transcriptomic analyses and reference comparisons, iDC1 cells aligned most closely with the cDC1 lineage and were clearly separate from macrophages.

The team also tested whether the cells kept key cDC1 functions. After innate immune stimulation, iDC1 cells increased their expression of co-stimulatory molecules and secreted IL-12p40 along with several chemokines. Functional experiments showed that the cells could cross-present cell-associated antigen to CD8-positive T cells. When transferred into cDC1-deficient mice, iDC1 cells helped restore this response in two of the four recipients. This points to activity in vivo, although more research is needed to improve these responses. 

Mechanistic experiments showed that each culture factor plays a distinct role. Early in cDC1 generation, Kit ligand and GM-CSF worked together, and in later stages, GM-CSF supported cell survival and proliferation. The +32 kb Irf8 enhancer, STAT5 and BRD4-associated regulatory programs also contributed to efficient iDC1 generation.

The iDC1 system offers a reproducible supply of relatively uniform cDC1 cells for research on antigen processing, dendritic cell biology, vaccine research, tumor immunology, and other preclinical applications. Because the method was developed with mouse bone marrow cells, further work will need to determine whether similar approaches can be established with human cells and whether they could support translational or clinical applications.