Mobilization of Plasma Cells in Healthy Individuals Treated with G-CSF for Hematopoietic Stem Collection.
Authors: Caraux Anouk , Larroque Marion , Duperray Christophe , Lu Zhao-Yang , Rossi Jean-François , Klein Bernard 
Published: October 2019

Abstract
Abstract 3679
Poster Board III-615
In patients with advanced hematologic malignancies, allogenic transplantation with hematopoietic stem cell (HSC) can improve survival. Granulocyte Colony-Stimulating Factor (G-CSF) makes it possible the mobilization of bone marrow HSCs mobilized cells into peripheral blood (PB) and the harvesting of large HSC numbers. G-CSF stimulates BM neutrophils to produce enzymes that cleave adhesion molecules and chemokines inducing HSC exit from the BM environment. Tokoyoda et al. have shown that plasma cells (PC) use similar BM niches as HSC, which involve CXCR4 ligand - CXCL12 - producing cells. Evidences have been obtained that G-CSF mobilization process may impart CXCR12/CXCR4 signaling. In the present study, we have monitored the PC mobilization after G-CSF treatment of healthy donors.
Every day hematopoietic niches release a few HSCs in the peripheral circulation (2 HSC/μL) - in the PB of healthy individuals, 2 CD38++ PC circulate per μL. Twenty four healthy donors treated with G-CSF in order to mobilize and harvest HSCs were analyzed by multiparametric flow cytometry. After 5 day mobilization by subcutaneous administration of 5-10 μg/kg/day of G-CSF, a median value of 110 CD34+ HSC/μL and 18 CD38++ PC/μL were detected in PB. A median value of 1510 CD34+ HSC/μL and 140 CD38++ PC/μL were collected in one leukapheresis and patients were allografted with 5.9 106 HSC/kg and 0.5 106 CD38++PC/kg. These mobilized PCs have a slightly different phenotype than that from steady state blood-borne PCs. Among these mobilized PCs, two sub-populations CD138− and CD138+ were identified, respectively 63.3% and 36.7%. Compared to CD138− PCs, CD138+ PCs present a more mature phenotype: lower CD45 expression, strong cytoplasmic Igs, as well as CD38 and CD27 expression. As opposed to B cells, both PC populations are cell cycling (KI-67+), and express CD43. Compared to steady state circulating CD38++ PCs, mobilized PCs lose CD62L expression, and are less cycling. Besides, it has previously been shown that a large part of CXCR4 protein is sequestered intracellularly in CD34+ HSC. In leukapheresis, the majority of CD34+ HSC and CD38++ PC do not expressed CXCR4 on the cell surface while it is sequestered intracellularly.
In this study, we demonstrated that as HSCs, PCs are mobilized by G-CSF (less efficiently, respectively 55 fold and 9 fold increase), and CXCR4 intracellular sequestration is observed in mobilized cells. The CXCR4 sequestration mechanism may be an important control mechanism for HSC and PC mobilization. Steady state circulating PCs do not express CXCR4 on the cell surface but CXCR4 is also sequestered intracellularly. This data seems to indicate that this HSC and PC mobilization mechanism is likely a physiological mechanism. How G-CSF mobilization affect CXCR4 expression on cell surface remains undefined.
This study raises several open questions. When donor PCs are injected to a patient, do they also confer on the patient the donor's humoral immunity? Moreover, where do the donor mobilized PCs migrate after transplantation in the recipient?

Disclosures:
No relevant conflicts of interest to declare.