Assessing CD97 and CD99 As Markers of Leukaemia Initiating Cells in Paediatric ALL

Abstract 1882

Overall survival rates in paediatric acute lymphoblastic leukaemia (ALL) have dramatically improved but around 20% do not respond to current therapies and subsequently relapse. Leukaemia initiating cells (LIC) are the topic of much investigation, as these cells can self-renew and may have the potential to cause relapse. It has been shown that multiple subpopulations of ALL cells have the ability to initiate the disease in immune deficient mouse models. Therefore, treatment should be targeted at all cells with this capacity, if the disease is to be eradicated. Minimal residual disease (MRD) detection is an invaluable tracking tool to assess early treatment response and recent studies have highlighted potential markers that may improve the sensitivity of MRD detection by flow cytometry. CD97 and CD99 are two markers which were over expressed in paediatric ALL. Incorporating these markers into investigations of LIC may allow discrimination of leukaemia cells from normal haemopoietic stem cells (HSC). In this study we evaluated the expression of CD34 in combination with CD97 in B cell precursor (BCP) ALL cases and CD99 in T-ALL cases and subsequently assessed the functional capacity of the sorted subpopulations in vitro and in vivo. Ten ALL samples (6 B-ALL & 4 T-ALL) with a median age 7 years (range 2–15 years) were studied. One B-ALL case and 3 T-ALL cases were considered high risk by molecular assessment of MRD at day 28 of treatment. Flow cytometric analyses of the ALL samples and 8 normal haemopoietic cell samples demonstrated that both CD97 and CD99 were over expressed in ALL patients (78.9±14.8% & 76.4±32.8%, respectively) when compared to normal haemopoietic cells (14.1±25.4%; p=0.001, 47.1±10%; p=0.03, respectively). Cells were sorted for expression/lack of expression of these markers and proliferation of the sorted cells was assessed in suspension culture over a 6 week period. In the B-ALL patients the CD34⁺/CD97⁺ subpopulation represented the bulk of leukaemia cells (65.2±32.1%), the CD34⁻/CD97⁺ the smallest fraction (3.3±2.4%) with the CD34⁺/CD97⁻ and CD34⁻/CD97⁻ subpopulations representing 21.1±31.5% and 10.5±5.8% of cells, respectively. When the functional capacity of these subpopulations was assessed in vitro greatest expansion was observed in cells derived from CD34⁺/CD97⁻ subpopulation (2–173 fold) from 9.4×10³ at initiation up to 1.5×10⁶ cells at week 6. Expansion was also observed, to a lesser extent in the CD34⁻/CD97⁻ subpopulation (3.4–28 fold) from 8×10³ up to 1.4×10⁶ cells. No expansion was observed in cultures of CD34⁺/CD97⁺ and CD34⁻/CD97⁻ subpopulations but cells were maintained throughout the culture period. These sorted subpopulations were also inoculated into NOD/LtSz-SCID IL-2Rγ^c null (NSG) mice to evaluate repopulating capacity. To date, engraftment has been achieved with 3 subpopulations; CD34⁺/CD97⁺ (3–28.8% CD45⁺), CD34⁺/CD97⁻ (0.5–25.5% CD45⁺) and CD34⁻/CD97⁺ (23.8% CD45⁺) cells. When the functional capacity of T-ALL cases was assessed the CD34⁺/CD99⁺ subpopulation represented the bulk of cells at sorting (51.87±47.2%), the CD34⁺/CD99^- subpopulation was the smallest (0.9±0.8%) and the CD34⁻/CD99⁺ and CD34⁻/CD99⁻ subpopulations represented 32.1±38.9% and 27.2±33.4% of cells, respectively. Greatest expansion was observed in cultures of CD34⁺/CD99^- cells (4.6–1798 fold) from 7.5×10³ up to 2.6×10⁶ cells at week 6. The other 3 subpopulations expanded to a lesser extent (1.3–216 fold) from 5×10³ up to 1.8×10⁶ cells. When the functional capacity of these cells was assessed in NSG mice, engraftment was achieved in all subpopulations; CD34⁺/CD99⁺ (87–90.5% CD45⁺), CD34⁺/CD99⁻ (1.5–84.9% CD45⁺), CD34⁻/CD99⁺ (31.3–98.6% CD45⁺) and CD34⁻/CD99⁻ (3–92.9% CD45⁺). In some cases, cells recovered from BM of NSG inoculated with CD99⁻ cells had high expression of CD99, typical of the patient samples at diagnosis, indicating that the inoculated CD99⁻ cells had differentiated in vivo. Studies are ongoing to assess the self-renewal capacity of these subpopulations by serial transplantation. The findings to date indicate that targeting CD97 and CD99, either alone or in combination with CD34 would not eliminate all cells with the capacity to initiate and maintain B-ALL and T-ALL, respectively. Further developments in therapy may require targeting leukaemogenic pathways, rather than only cell surface markers to improve survival outcome in paediatric ALL.