Cancer Immune Therapy Edited by G. Stuhler and P. Walden ...

184 9 Dendritic Cells and Cancer: Prospects for Cancer Vaccination
less effectively with Flt-3L [85] and Flt-3L/G-CSF (D Ashley personal communication)
in cancer patients.
Further studies in cancer patients are clearly necessary to establish what DC populations
are available to be harvested for immunotherapy. Studies in multiple myeloma
and low-grade NHL suggest sufficient numbers of blood DCs for immunotherapy
may be available without mobilization (Vuckovic et al., submitted).
9.5
DC Preparations for Immunotherapy
In vivo targeting of DCs for immunotherapy has yet to be explored, but there have
been attempts to mobilize DCs into tumors or the skin for vaccination purposes using
granulocyte macrophage colony stimulating factor (GM-CSF)/IL-4 [86] or GM-CSF/
TNF-a [87]. Flt-3L proved efficacious in generating anticancer responses in mice, perhaps
as a result of NK activation, but does not seem to achieve a clinical effect on its
own apart from DC mobilization. Thus, most investigators take the approach of removing
DCs from the patient and loading them with TAAs, whilst activating them in
the test tube. The down side to this, of course, is that the complexity of normal DC
function may be compromised in this process. In broad terms, at least three types of
DCs preparations are being tested for immunotherapy as described below.
The availability of CD34 + mobilized and immunoselected stem cells has encouraged
a few groups to differentiate DCs from these for therapeutic purposes [88±90]. Whilst
some cell expansion occurs and a range of different growth factor/cytokine cocktails
have been reported to produce reasonable yields of DCs, it seems a biologically difficult
process to control and the end result may be something akin to Mo-DCs preparations.
Others have used different protocols and carefully characterized two potential
DCs populations ± CD1 a hi and CD1 a lo [91].
The majority of attention has focused on Mo-DCs using multiple myeloma variations
of the original protocol described by Sallusto and Lanzavecchia [45, 92, 93].
GM-CSF and IL-4 are used routinely in varying concentrations to develop a so-called
ªimmatureº Mo-DCs after 5±7 days in vitro culture. These have been exposed to antigen
and then injected using the argument that a natural maturation process would
take place in vivo. Another argument to justify this approach is that the cells might
retain normal migratory capacity and indeed a small proportion do [94, 95]. GM-
CSF/IL-4-generated Mo-DCs are not stable. Upon cytokine removal the re-adhere,
convert to macrophages and lose allostimulatory function. Adding macrophage colony
stimulating factor (M-CSF) hastens CD14 re-expression and the proportion of
adherent spindle-shaped macrophages in vitro [96]. Immature Mo-DCs are also sensitive
to the effects of IL±10 [97]. The clinical imperative has bypassed some basic
studies. Recent data [98] would suggest that the differentiated Mo-DC is an effective
antigen-presenting cell (APC) but suggests caution in the use of the undifferentiated
Mo-DC: in two patients tested, reduced specific immune responses were documented
[99]. A semimature variant of Mo-DCs with higher expression of co-stimulatory
molecules is obtained in the presence of fetal calf serum. An alternative approach