Higher HHV-6 Reactivation After Cord Blood Allo-SCT Is Not Related to HHV-6 Cell Receptor CD46 Expression

Cord Blood (CB) are increasingly used as an alternative stem cells source in adults for allogeneic Stem Cell Transplantation (allo-SCT). The risk of human herpes virus (HHV-6) reactivation is significantly higher after CB transplant vs unrelated peripheral blood stem cells (PBSC) allo-SCT (Chevallier et al, BMT 2010). Higher HHV-6 cell receptor CD46 expression on progenitor cells in CB may explain this difference (Thulke et al, Virol J 2006).

We have prospectively compared the HHV-6 cell receptor CD46 expression on various cell subsets of three freshly harvested blood sources on one hand and of three graft sources on the other hand. 52 samples were used for the purpose of this study. They were issued from peripheral blood (PB, n=10), G-CSF mobilised PB (GCSF-PB, n=10), cord blood (CB, n=10), unmanipulated bone marrow (uBM, n=5), leukapheresis product (LP, n=10) and thawed CB graft (n=7). CD46 expression was assessed by FACS analysis using a FACS CANTO II (BD Biosciences, San Jose, CA, USA) on total lymphocytes, monocytes, NK cells, T and B cells subsets, plasmacytoid (pDCs) dendritic cells and stem cells.

As all cell subsets were found CD46 positive, CD46 mean fluorescence intensity (MFI) was then considered for comparison. When considering the three blood sources, CD46 MFI were found similar on T cells, CD4-/CD8+ and CD4-/CD8- T cells, NKT cells, Tregs, memory B lymphocytes, pDCs and CD34+ stem cells. CD46 MFI was significantly lower on CD4+/CD8- and CD4+/CD8+ T cells, transitional B cells, total and naïve B lymphocytes, and NK cells in CB while higher on monocytes. The highest CD46 MFI was observed on monocytes in CB and on CD4+/CD8+ T cells in GCSF-PB and PB. Also, highest CD46 MFI was detected on T cells compared to B lymphocytes and NK cells in all blood sources while CD46 MFI was higher on CD4+/CD8- T cells compared to CD8+/CD4- T cells.

When considering the three graft sources, CD46 MFI was similar on CD4-/CD8- T cells and NKT cells. CD46 MFI was found significantly lower on all other sub-populations in thawed CB graft, except monocytes. The highest CD46 MFI was observed on monocytes in CB graft, on CD4+/CD8+ T cells in LP and on monocytes and on CD4+/CD8- T cells in uBM. Also, highest CD46 MFI was detected on T cells compared to B lymphocytes and NK cells in all graft sources while CD46 MFI was higher on CD4+/CD8- T cells compared to CD8+/CD4- T cells.

This original study shows strong differences in term of quantitative CD46 expression between several blood and grafts samples. Our results suggest that other factors (such as another HHV-6 cell surface receptor) than the qualitative CD46 expression play a role in the higher HHV-6 reactivation observed after CB transplant in adults.

No relevant conflicts of interest to declare.