Host Plasmacytoid or Conventional Dendritic Cells Alone Are Sufficient To Initiate Graft-Versus-Host Disease.

Graft-versus-host disease (GVHD) is a major complication after allogeneic hematopoietic stem cell transplantation. Alloantigen expression on host dendritic cells (DCs) is critical to initiate GVHD. DCs can be divided into two main subpopulations; conventional DCs (cDCs) and plasmacytoid DCs (pDCs), however, the contribution of each DC subset to elicit GVHD remains unclear. We examined the ability of cDCs and pDCs to initiate GVHD. pDCs, cDCs and B cells were isolated from C57BL/6 (B6: H–2^b) mice treated with Flt3 ligand in order to expand DCs. pDCs were enriched from bone marrow by depleting CD3⁺, CD19⁺, CD11b⁺, and CD49b⁺ cells, followed by a FACS sorting of CD11c^int B220⁺ cells. cDCs and B cells were sorted from splenocytes as CD11c^hi B220⁻ cells and CD11c⁻ B220⁺ cells, respectively. Isolated pDCs showed plasmacytoid morphology, produced IFN-α in response to CpG oligonucleotide. Although pDCs stimulated allogeneic T cells far less potently than cDCs, stimulation with CpG enhanced their allostimulatory capacity as potent as cDCs. We compared the ability of each DC subset to initiate GVHD by an add-back study of MHC class II-expressing DCs into MHC class II-deficient (II^−/−) mice that were resistant to CD4-dependent GVHD. Lethally irradiated II^−/− B6 mice were injected with 2 × 10⁶ pDCs, cDCs or B cells from wild-type (II^+/+) B6 mice on day -1 and injected with 2 × 10⁶ CD4⁺ T cell from BALB/c (H–2^d) mice on day 0. A flow cytometric analysis of the mesenteric lymph nodes on day +5 demonstrated significantly greater expansion of donor CD4⁺ T cells in recipients of pDCs or cDCs than those of B cells (Table). While injection of B cells did not cause any sign of GVHD, injection of pDCs or cDCs alone was sufficient to produce clinical and pathological GVHD (Table), thus breaking GVHD resistance of II^−/− mice. We next examined the ability of pDCs to induce CD8-dependent GVHD in MHC-matched transplant using mice deficient in functional MHC class I expression (β2m^−/−). Again, injection of pDCs or cDCs alone was sufficient to cause expansion of donor CD8⁺ T cells (p<0.05). We next asked whether signaling through Toll-like receptors (TLRs) could be required for pDCs to initiate GVHD. However, injection of pDCs isolated from MyD88/TRIF-double deficient mice was able to initiate GVHD as potent as wild-type pDCs, thus demonstrating that pDCs initiate GVHD in a TLR signaling independent manner. These results provide important information for developing strategies aimed at inactivating host DCs to prevent GVHD.