CD34+ SSC^loALDH^br – a New Marker for Improved Characterization of Poor Myeloid Regeneration in the Autologous Hematopoietic Transplantation Setting?,

An increasing body of evidence in different organ systems suggests that the presence of the cytosolic enzyme Aldehyde Dehydrogenase (ALDH) correlates to stemcellness. Thus, in mobilized peripheral blood stem cells (PBSCs), the side scatter low ALDH bright (SSC^loALDH^br) population seems to be highly enriched for HSC as determined e.g. by Fallon et al¹. However, these interesting preliminary findings need confirmation. Problem formulation: The generally accepted indirect relationship between CD34 content on the one hand, and successful hematopoietic regeneration after transplant on the other, is at least to a certain extent marred by the observation that, despite receiving sufficient weight-correlated amounts of CD34 positive HSC, some patients experience prolonged time to regeneration. Hypothesis: We hypothesized that the number of CD34+SSC^loALDH^br is predictive of time to short and long-term regeneration following autologous bone marrow transplantation compared to the number of CD34+ cells alone. Results: PBSCs from 30 patients with refractory or relapsed diffuse large cell B-lymphoma referred for autologous stem cell transplantation after having failed at least three cytoreductive regimens were analyzed for ALDH expression. Laboratory results were registered on day 14 and day 100 post transplantation. Time to absolute neutrophil count (ANC) above 0,5 × 10⁶/mL on two consecutive days was registered for each of the 30 patients. While a trend was noted for number of reinfused viable CD34+ cells (10⁶/kg) to be correlated to ANC > 0,5 × 10⁶/mL r=0.33, P = 0.07), only the CD34+SSC^loALDH^br population correlated significantly to time to ANC > 0,5 × 10⁶/mL (r=0.41, P =0.024). Moreover, while a positive correlation was observed between both of the analyzed subpopulations and CFU-GM proliferation, this correlation was strongest for the number of reinfused CD34+SSC^loALDH^br/kg (r= 0.84, P <0.001). While neither of the analyzed subpopulations could be correlated to time to short term platelet recovery (defined as platelets > 20 × 10⁶/mL on two consecutive days) an interesting finding emerged, when we considered day 100 platelet recoveries. Here, 11 patients were grouped as poor mobilizers (PM) having not attained a platelet count of 100 × 10⁶/mL at day 100, and 16 patients were grouped as good mobilizers (GM) while in three cases no data were available. While mere CD34 enumeration was once more not informative, we observed a trend towards low numbers of CD34+SSC^loALDH^brcells in the grafts in PM (p=0.06). With regard to long-term erythroid engraftment (defined as Hgb<100 g/L at day 100) 6 were PM (21 GM and data missing in 3) a significantly lower fraction of reinfused CD34+SSC^loALDH^brwere administered to PM compared to GM (p=0.01). Once more, no significant difference in number of infused viable CD34+ cells between the groups was observed. Conclusion: In the autologous transplant setting the addition of SSC^loALDH^brto standard CD34 enumeration seems to constitute a valuable marker for the identification of both time to ANC > 0,5 × 10⁶/mL and inferior day +100 hematopoietic regeneration. While the assay does entail extra lab efforts, this layout is probably amply repaid by the possibility to institute pre-emptive therapy in PM patients. Moreover, application of the assay at the onset of leukapheresis sessions could provide a window of opportunity to administer alternative mobilization regimens, i.e. containing Plerixafor.

No relevant conflicts of interest to declare.