Short-term Flt3L treatment effectively mobilizes functional macaque dendritic cells

Abstract: In vivo administration of soluble Flt3L increases dendritic cell (DC) numbers to favor im-proved DC targeting of vaccine antigens, augment-ing vaccine efficiency. In addition to confirming the effectiveness of human Flt3L in macaques, we strove to determine the optimal regimen to elevate numbers of functional DCs. Circulating DCs were identified within lineage–human leukocyte antigen-DR cells, which comprised CD11c–CD123 plas-macytoid DCs (PDCs) and CD123 – cells including CD11cCD123 – myeloid DCs as well as CD11c– CD123 – cells. Traditionally, DCs have been mon-itored 1–2 days after 10- to 14-day treatments with Flt3L (100 g/kg/day). We demonstrate that al-though standard treatment increased macaque DC percentages, as little as 5–7 days of treatment was sufficient, if not more effective at mobilizing DCs. Moreover, DC frequency continued to escalate over the ensuing days, peaking at 4 days post 7 days of treatment and ultimately decreasing there-after. As expected, there was a more pronounced increase in the percentages and actual numbers of CD123 – cells (CD11c and CD11c – subsets) com-pared with PDCs. Flt3L-mobilized DCs exhibited slightly increased CD80/CD86 expression but typ-ically still that of immature DCs and were resilient to freeze-thawing. Overnight culture activated the cells, up-regulating CD80/CD86 expression as well as interleukin-12 release, typically being boosted by CD40L. This was even more apparent for en-riched DC cultures. These data verify that peak mobilization of large numbers of functional ma-caque DCs occurs a few days, not immediately, after short-term Flt3L dosing. This has important implications for improved DC-targeting vaccine strategies to prevent infection with human immu-nodeficiency virus and other pathogens. J. Leukoc