Identification and Characterization of Canine Dendritic Cells Generated In Vivo

Allostimulatory potential of unsorted peripheral blood leukocytes obtained from dogs before (pre-FL) and after (post-FL) completion of 10 days of Flt3-ligand (FL) treatment. Different numbers of stimulators cells (5-20 × 10⁴) obtained from dogs pre-FL and post-FL (100 μg/kg/day given subcutaneously for 10 days) were incubated with constant numbers (1 × 10⁵) of allogeneic, third-party responder cells as described in Materials and Methods. Shown is one representative experiment of 5 experiments. Compared to untreated control dogs, FL treatment increased the MLC allostimulatory potential of unsorted peripheral blood leukocytes by a median of 150% (range: 36%-900%).

Fluorescence activated cell sorting of phenotypically distinct cell populations from peripheral blood of FL-treated dogs. FSC/SSC characteristics of peripheral blood leukocytes before (A) and after (B) 10 days of FL treatment (100 μg/kg/day). Phenotypically distinct cell populations were sorted from post-FL peripheral blood as described in Materials and Methods. The following gates were combined with Gate 1, which was established according to FSC/SSC characteristics: Gate 2 for B lymphocytes (C); Gates 3 and 4 for monocytes and granulocytes, respectively (D); and Gates 5 and 6 for CD4 and CD8 T cells, respectively (E). Purity of sorted cell populations was ≥97%. CD34⁺ progenitor cells were not used as stimulator cells in MLC (F).

Phenotypic characterization of putative dendritic cells (DC). Putative canine DC were sorted from post-FL blood using a 3-step approach. First, cell debris was excluded by establishing a gate according to FSC/SSC characteristics (Gate 1) (A). Second, Gate 2 was chosen to exclude all cells that were CD14⁺ (monocytes) and DM5⁺ (granulocytes) (B). Third, HLA-DR⁺/CD11c⁺ cells (R5) were collected (C). Thus, putative DC were defined as live gated cells that were HLA-DR⁺/CD11c⁺/DM5⁻/CD14⁻.

Functional characterization of sorted cell populations in mixed lymphocyte cultures. Phenotypically distinct cells populations were sorted from FL-treated dogs as described in Materials and Methods. Varying numbers (5-20 × 10³) of irradiated stimulator cells were placed into mixed lymphocyte cultures using constant numbers (1 × 10⁵) of allogeneic, third-party responder cells. Shown is 1 of 6 representative experiments.

Transmission electron microscopy of sorted putative dendritic cells. CD11c⁺/HLA-DR⁺/DM5⁻/CD14⁻ cells were sorted and examined by transmission electron microscopy as described in Materials and Methods. The purity of the sorted population was 97%. In contrast to macrophages, which frequently displayed large and dark phagocytic vacuoles (A; closed arrowhead), putative DC were characterized by long cytoplasmic processes (B,C), only small, if any, lysosomal organelles (B,C), and an abundant Golgi apparatus and endoplasmic reticulum (D). Multivesicular bodies (A and C; open arrowheads).