Pretransplant Recipient Blood CD14+ preDC Levels Correlate with Increased Acute GVHD after Allogeneic PBSC Transplantation.

Host dendritic cells (DC) are thought to be essential in the induction of acute GVHD after allogeneic HSCT. Human peripheral blood contains various circulating DC precursors including CD11c+ myeloid preDC (mDC), CD14+ monocytic DC precursors (CD14+ preDC) and plasmacytoid preDC (pDC). In this study we employed flow cytometry to enumerate both mDC (lin-, HLA-DR+ and CD11c+), mono-DC (CD14+, CD45 bright) and pDC (lin-, HLA-DR+ and CD123+) numbers in the blood of patients receiving an allogeneic HSCT. Fifty consecutive patients undergoing HSCT from HLA-matched either related (n=28) or unrelated (n=22) donors were enrolled in the study. The stem cell source was bone marrow in all unrelated donors, and G-CSF mobilized PBSC in related donors. Indications to transplant were AML (n=12), ALL (n=11), MM (n=10), CML (n=7), NHL (n=6) and HD (n=4). 13 patients (26%) received reduced dose conditioning regimens (RIC). All patients received CsA and MTX as GVHD prophylaxis. Moreover, 26 patients (52%) received ATG before transplant. mDC and pDC PB counts were significantly lower in patients as compared to 28 age-matched healthy controls [8.8 cells/microL (25^th to 75^th percentile 3.5–14.5) mDC, and 2.8 (1.3–5.5) pDC, vs 15.5 (12.1–25.1) and 8.6 (5.6–13.1), respectively] (p<0.001). However the mDC/pDC ratio was significantly higher in the patient group [3.5 (1.6–6.2) vs 1.7 (1.3–2.6), p=0.002], indicating a preferential decrease of pDC. CD14+ preDC counts were not significantly different. Among 46 patients evaluable, 12 (26%) developed acute GVHD grade II–IV. Risk factors significantly associated with acute GVHD were older age (p=0.01), PBSC transplants (p=0.02) and the absence of ATG in the conditioning regimen (p=0.01). Patients with acute GVHD had significantly higher pre-transplant mDC:pDC ratio [5.7 (3.3–16.4) vs 3.1 (1.6–5.5)] (p=0.03) and CD14+ preDC counts [395 (326–625) vs 284 (187–395] (p=0.02). A subset analysis was performed in PBSC patients, only 3 of whom had received ATG before transplant. Among 26 evaluable patients, 10 (38%) developed acute GVHD grade II–IV. Besides older age (p=0.02), the only risk factors significantly associated with acute GVHD in PBSC patients were the pre-transplant mDC:pDC ratio [5.7 (4.1–13.1) vs 1.7 (1.2–2.9)](p=0.008) and CD14+ preDC counts [395 (352–710) vs 259 (199–314)](p=0.004). In multivariate analysis, only older age (p=0.04) and pretransplant circulating CD14+ preDC numbers (p=0.04) were significantly associated with acute GVHD in PBSC transplants. Patients with pretransplant CD14+ preDC counts > 310/mL (median value) had a higher actuarial probability of developing acute GVHD grade II–IV than patients with lower CD14+ preDC counts (61% vs 15%). No correlation was observed between pretransplant recipient PB mDC, pDC or CD14+ preDC counts and chronic GVHD. These findings demonstrate that blood levels of DC precursors may represent an important biomarker correlating with a higher risk of developing aGVHD. Based on these results, future studies will exploit clinical strategies aimed at depleting or inactivating host preDC before allotransplant as a means of GVHD prophylaxis.