Dendritic Cells Recovery and Chimerism after Reduced Intensity Hematopoietic Stem Cell Transplantation.
Authors: Tabrizi Reza , Belloc Francis , Lafarge Xavier , Perreau Virginie , Bouabdallah Krimo , Marit Gerald , Bijou Fontanet , Leguay Thibaut , Lacombe Francis , Pigneux Arnaud , Melot Cyril , Comeau François , Boiron Jean-Michel 
Published: April 2020

Abstract
The circulating dendritic cells (DC) are known to have an immunoregulatory role after allogeneic HSC transplantation, and recipient DC have been shown to be important in the development of GVHD in animal model. We studied the DC chimerism of 21 patients (pts) transplanted with reduced intensity conditioning regimen between January 2004 and August 2005. The blood was sampled at days -1, 15, 28 and 56 after transplantation. A series of 17 control normal bloods were also analyzed. DC were identified as ILT3-expressing cells negative for CD14. These cells were sorted by flow cytometry and chimerism was analyzed by PCR of Short Tandem Repeat motifs. Preliminary experiments showed that at least 500 sorted cells were necessary to perform chimerism analysis. Eight females and 13 males (median of age: 54 yrs; 25–61) were enrolled in the study. Diagnoses were 6 AML, 2 sAML, 1 MDS, 3 ALL, 6 MM, 2 NHL and 1 CML. Fifteen pts had high-risk disease. As conditioning regimen, all but 3 pts received cumulative dose of ATG (Thymoglobulin, Genzyme, Lyon, France) (2.5 mg/kg for sibling and 7.5 mg/kg for MUD), in addition to Busulfan 8 mg/kg and Fludarabine 150mg/m2. Eight pts received stem cells from a 10/10 MUD, 2 pts from 9/10 MUD, and 11 pts from sibling donor. For all but one patient, the stem cell source was blood. CsA alone was used for 11 pts, CsA with methotrexate for 8 pts and CsA with MMF for 2 pts. In the absence of aGVHD, the immunosuppressive therapy was tapered within 4 weeks (after day 28 in sibling donor and after day 90 for MUD). The kinetics of the absolute number of DC showed significantly lower count of circulating DC than in control samples at day -1, and a rapid increase, reaching normal values at day 15 post-transplant while the other leukocytes remained at a low value. To determine the origin of post-transplant blood DC, chimerism was analyzed on sorted DC. From 20 pts DC chimerism at day 15 was of full donor origin for 8 pts, mixed in 10 pts. Two pts had no detectable DC. At day 28 from 18 pts, only 4 pts had mixed chimerism. Of these 4 pts, 3 presented at day 56 a full donor chimerism and one patient died from relapse. For T cells at day 15, only one/17 pt had full donor chimerism, and one had no detectable circulating T cells. At day 28, 7/20 pts had full donor chimerism and one without detectable T cells. Only 2/17 had still mixed chimerism at 3 months. Six out of 21 pts relapsed and 3 died from relapse. Among these 6 pts, all but one reached full donor T cells, 3 had a full donor DC at day 28. Six pts from 21 had grade ≥ 2 aGVHD and 3 died from aGVHD. 7/17 evaluable pts had cGVHD. We didn’t found any correlation between DC chimerism and engraftment or relapse. At day 15, the median percentage of recipient DC was lower in pts who developed cGVHD (P&lt;0.017) while it was higher in those with aGVHD (but p not significant). In conclusion, this study demonstrates that the circulating DC pool is rapidly reconstituted from both donor and recipient origins. Thereafter at day 28, donor engraftment of DC became predominant. The median of recipient DC was significantly higher in pts without cGVHD. An analysis on a larger series would be useful to determine if the chimerism in DC could be predictive for cGVHD.