Abstract 3385: Expression of CD22, a late B lymphoid antigen, does not distinguish leukemia stem cells from the bulk population in acute lymphoblastic leukemia cases

Introduction: Pediatric ALL is still fatal in ∼20% of cases, motivating development and clinical trials of novel antineoplastic agents in recurrent ALL such as CD22 monoclonal antibody and immunotoxins. CD22 is expressed on most ALL cases, but not every cell in every ALL case is detectably CD22+. In normal B lymphocyte development, CD22 is first expressed on the cell membrane at the early B to proB cell stage, at the time of D-J gene rearrangement. Taken together, these 2 facts beg the question of whether most or all of the presumed earliest cells in ALL cases, the leukemia stem cells, might be CD22-; if so, CD22 monoclonal antibody treatment might miss ALL stem cells. We therefore investigated CD22 expression on pediatric ALL stem cells. Methods: Studies were performed using secondary xenotransplants of five cases of pediatric precursor-B ALL. Cells (>90% human leukemia cells from spleens of immunodeficient mice) were stained with monoclonal antibodies and fluorescence-activated cell sorted (FACS). Titered doses of human ALL cells (100 -100000 cells/mouse; n=5 mice/group) from (1) unsorted, (2) mock sorted, (3) CD22high, and (4) CD22low populations, excluding non-viable and mouse cells, were separately transplanted by intravenous injection into NSG (NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ) mice. NSG mice lack functional T, B cells, and NK cells, and are also deficient in innate immunity. Time to leukemia was determined from injection to onset of clinical signs of leukemia. Leukemia was confirmed by autopsy (splenomegaly) and FACS-analysis for human ALL. Pre- and post-sorted and pre- and post-transplanted cells were evaluated via multicolor FACS-analysis, using CD22, a viability dye, lymphoid (CD10, CD19), myeloid (CD13, CD33), stem-progenitor (CD34), and mouse cell (mouse CD45) markers. Results: Time to leukemia decreased in an inverse transplanted cell dose dependent manner in all 4 experimental groups (CD22high, CD22low, unsorted and mock sorted populations of each case), but was not statistically different among the four groups in any of the tested ALL cases. LSC quantitation is ongoing, based on frequency of mice with leukemias at the lowest transplanted cell doses. The heterogeneity of antigen expression in the leukemia cell populations was not different in cells analyzed before versus after NSG transplantation. Discussion: In these five cases of pediatric precursor-B ALL, the level of expression of CD22 did not, in evidence obtained to date, distinguish leukemia stem cells (as defined by NSG mouse-engrafting capacity) from the bulk population of ALL cells. Specifically, there is no suggestion from our data that leukemia stem cells were enriched in the CD22low populations, which provides further evidence against a hierarchical model of ALL stemness. Furthermore, targeting CD22 would not be predicted to differentially spare leukemia stem cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3385. doi:1538-7445.AM2012-3385