Influence of host irradiation on long-term engraftment by CD34-deficient hematopoietic stem cells

To the editor:

Hematopoietic stem cell (HSC) transplantation and evaluation of long-term repopulation (LTR) is the gold standard for assessing HSC function. Although myeloablative irradiation is typically used in animal models to enhance host engraftment,¹  a frequently overlooked concern is that this severely damages bone marrow (BM) architecture and may therefore mask defects in HSC trafficking.²  To illustrate this concept, we evaluated the importance of a commonly used HSC marker, the antiadhesin CD34, in engraftment of irradiated and nonirradiated recipients.

W/W^v mice were used as recipients since a lack of the functional stem cell factor (SCF) receptor, c-kit, renders them highly receptive to donor engraftment in the absence of lethal irradiation.^3,4  A 1:1 ratio of wt and cd34^−/− embryonic day 15 (E15) fetal liver cells (FTLs) was transplanted into lethally or sublethally irradiated W/W^v recipients, and donor engraftment was assessed, as outlined in Figure 1A. Strikingly, we found that while cd34^−/− and wt HSCs exhibited similar abilities to reconstitute W/W^v mice pretreated with high-dose irradiation (Figure 1B), cd34^−/− cells performed very poorly (5-fold less engraftment) in sublethally irradiated recipients (Figure 1C).

To confirm that these results were not a W/W^v-related artifact, we also injected wt or cd34^−/− (CD45.2) cells into nonirradiated wt (CD45.1) recipients and assessed the frequency of donor-derived cells in peripheral blood 12 weeks after transplantation. Since donor cells have no advantage over endogenous cells in this system, reconstitution levels were predictably low but, as with W/W^v experiments, wt cells were considerably more effective at LTR than cd34^−/− cells (Figure 1D). Taken together, our results demonstrate that in 2 independent systems, although cd34^−/− and wt cells show similar abilities to engraft lethally irradiated mice, cd34^−/− cells are profoundly impaired in engraftment of nonirradiated or sublethally irradiated recipients.

What then is the function of CD34 in BM engraftment? Previous studies suggest that sialomucins, like CD34, tend to block cell adhesion through their bulky, negatively charged extracellular domains.⁵  For example, CD34-null mast cells aggregate in vitro, while ectopic expression decreases cell adhesion.⁶  Likewise, overexpression of the CD34 relative, podocalyxin, also serves to decrease cell adhesion.^7,8  Thus, CD34 expression on migrating hematopoietic cells and most vascular endothelial cells would normally prevent inappropriate adhesion and enhance mobility.

We therefore propose that our results reflect an impaired ability of the more adhesive cd34^−/− cells to cross intact endothelial barriers en route to BM stem cell niches (Figure 1E). Conversely, irradiation-induced vascular permeability facilitates migration of cd34^−/− HSCs into subvascular spaces, thereby explaining their favorable competition with wt cells for engraftment in preconditioned recipients. These data serve to highlight the importance of evaluating the ability of mutant and wild-type HSCs to engraft both irradiated and nonirradiated recipients, particularly when a mutation may influence the mobility or trafficking of stem cells.

Approval for these studies was obtained from the University of British Columbia Animal Care Committee.