Biomedical Engineering Reference
In-Depth Information
which are either plated out in vitro or inoculated in vivo (Sieburg et al. 2002). In
LTC-IC experiments, at the end of the culture period, the proportion of positive
wells (contain at least 1 CFU) is determined and plotted against the number of
input cells to the LTC-IC assay. The Poisson statistic is then used to assess the
frequency of LTC-IC in the input cell culture, and the absolute number of LTC-
IC and mean CFU per LTC-IC are calculated. Limiting dilution analysis is used
in a similar manner in murine in vivo HSCmodels to determine the frequency of
SRC and SL-IC (Wang et al. 1997).
2 Leukemia Stem Cells
2.1 Myeloid Leukemias
Following the identification and characterization of the HSC, comparisons
were drawn with the behavior of cancer cells, in particular leukemias, with the
first evidence for the 'cancer stem cell' being described in acute myeloid leuke-
mia (AML) (Blair et al. 1997; Bonnet and Dick 1997). Bonnet and Dick
demonstrated that the SL-IC, which was capable of causing human AML in
NOD-SCID mice, possessed the potential for self-renewal and capacity for
differentiation and proliferation predicted for a LSC. Furthermore, the SL-
ICs from all subsets of AML, regardless of morphological heterogeneity in
maturation of the leukemic blasts, were exclusively CD34
+
38
, analogous to
normal SRC. This suggested that normal primitive progenitor cells and not
committed progenitor cells were the target for leukemic transformation. Impor-
tantly, SL-ICs were capable of differentiating in vivo into leukemic blasts,
providing the first evidence that the leukemic clone, like normal hematopoiesis,
was organized as a hierarchy. Further support for this hypothesis came from
Hope et al. (2004), who were able to track individual human LSCs, using NOD-
SCIDmice serially transplanted with human AML cells. They showed that, like
normal hematopoiesis, the LSC compartment comprised a hierarchy with dis-
tinct LSC fates decided by heterogeneous self-renewal potential and that nor-
mal development pathways were not entirely eradicated by leukemogenesis.
The cell surface phenotype of LSCs was further elucidated by Blair et al.,
who demonstrated that, at diagnosis, the majority of AML blasts lacked
expression of Thy-1 (CD90), CD71, HLA-DR, and CD117 which differentiated
primitive AML progenitor cells from normal hematopoietic progenitor cells
(Blair et al. 1997, 1998; Blair and Sutherland 2000), and Jordan et al. who
showed that the interleukin-3 receptor a chain (IL-3a) is a unique marker for
human AML stem cells (Jordan et al. 2000). Very recently, CD96 has been
identified as a surface marker on the majority of CD34
+
38
AML cells, with
minimal expression on normal CD34
+
38
cells (Hosen et al. 2007). Further-
more, in murine transplantation experiments, only CD96
+
CD34
+
38
cells
showed significant bone marrow engraftment in recipient mice, indicating
