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to values obtained after infusion of control Th2 cells that expressed a T
EM
phenotype
[81]
. The favorable effect of adoptive transfer of T
CM
cells (in
terms of prolonged engraftment of the memory T-cell pool) has also been
demonstrated in nonhuman primates
[82]
. Such observations have impor-
tant translational implications for Th1/Th2 cell immuno-gene therapy as
applied in the context of autologous or allogeneic transplantation: that is,
how does one attain both a less differentiated T-cell status and an enrich-
ment for antigen specificity, which requires multiple rounds of cell division
during antigen-driven clonal expansion?
First, with advances in T-cell receptor and chimeric antigen receptor gene
transfer (see Chapters 3 and 7), it may be possible to genetically enforce anti-
gen specificity into sorted, highly purified naïve T cells; however, it should
be noted that naïve T cells are relatively rare in the adult population
[83]
.
Alternatively, it is possible that pharmacologic maneuvers might promote
effector T-cell expression of genes associated with less differentiated cells;
for example, GSK3 inhibition and subsequent Wnt signaling promoted a
T-cell transcriptional program with stem-like characteristics that associ-
ated with enhanced anti-tumor immunity after adoptive T-cell transfer
[84]
.
Ex vivo
incorporation of rapamycin represents another pharmaco-
logic method for favorable modulation of T-cell differentiation status. That
is, inhibition of T-cell mTOR signaling enhances T-cell expression of the
T
CM
markers CD62 ligand and CCR7, in part through upregulation of the
KLF2 transcription factor
[84]
. Consistent with this mechanistic information,
in murine studies, we found that
ex vivo
rapamycin directly promoted Th2
cell expression of CD62L and CCR7 independent of Th2 cell division
[78]
.
233
In addition to T-cell differentiation status, we have also found that the
T-cell apoptotic threshold represents a critical determinant of the
in vivo
efficacy of adoptively transferred T cells. In initial studies, we determined
that
ex vivo
rapamycin, in addition to promoting T
CM
differentiation mark-
ers, yielded T cells with a multifaceted antiapoptotic phenotype
[85]
. In this
study, we determined that: (1) the antiapoptotic phenotype could be mani-
fested in both Th1- and Th2-type T cells; (2) rapamycin-exposed T cells
exhibited initial caspase activation but had reduced activation of distal cas-
pases, thereby indicating modulation of the intrinsic apoptotic pathway; (3)
rapamycin-exposed T cells preferentially expressed antiapoptotic members
of the Bcl-2 family of genes relative to proapoptotic gene members; and (4)
such modulation correlated with enhanced T-cell capacity to accumulate
in vivo
at advanced numbers of proliferative cycles.
Most recently
[55]
, we have identified that the antiapoptotic effect of
ex vivo
rapamycin is due in part to a cellular process known as autophagy
(see
Figure 11.2
; electron micrograph of human CD4
+
T cell undergoing
autophagy upon rapamycin exposure). During conditions of increased cel-
lular stress or signals of nutrient deprivation (such as mTOR inhibition via
rapamycin), T cells may become autophagic as a means to self-digest cellu-
lar organelles such as mitochondria
[86]
as a fuel source and as an attempt
to downsize cellular energetics to achieve cell survival
[87]
. In our experi-
ments
[55]
, human T cells generated in rapamycin underwent autoph-
agy, as indicated by: (1) reduction in mitochondrial mass with associated
improvement in mitochondrial membrane stability; and (2) alteration of
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