From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
IMMUNOBIOLOGY
Dendritic cells drive memory CD8 T-cell homeostasis via IL-15 transpresentation
Spencer W. Stonier,1 Lisa J. Ma,1 Eliseo F. Castillo,1 and Kimberly S. Schluns1
1Department
of Immunology, University of Texas M. D. Anderson Cancer Center, Houston
Interleukin-15 (IL-15) is crucial for the
development of naive and memory CD8
T cells and is delivered through a mechanism called transpresentation. Previous
studies showed that memory CD8 T cells
require IL-15 transpresentation by an as
yet unknown cell of hematopoietic origin.
We hypothesized that dendritic cells (DCs)
transpresent IL-15 to CD8 T cells, and we
examined this by developing a transgenic
model that limits IL-15 transpresentation
to DCs. In this study, IL-15 transpresenta-
tion by DCs had little effect on restoring
naive CD8 T cells but contributed to the
development of memory-phenotype CD8
T cells. The generation of virus-specific,
memory CD8 T cells was partially supported by IL-15R␣ⴙ DCs through the preferential enhancement of a subset of
KLRG-1ⴙCD27ⴚ CD8 T cells. In contrast,
these DCs were largely sufficient in driving normal homeostatic proliferation of
established memory CD8 T cells, suggesting that memory CD8 T cells grow more
dependent on IL-15 transpresentation by
DCs. Overall, our study clearly supports a
role for DCs in memory CD8 T-cell homeostasis but also provides evidence
that other hematopoietic cells are involved in this function. The identification
of DCs fulfilling this role will enable future
studies to better focus on mechanisms
regulating T-cell homeostasis. (Blood.
2008;112:4546-4554)
Introduction
The generation and maintenance of memory CD8 T cells is
regulated by multiple mechanisms that involve both early
programming of memory T-cell precursors as well as a continuous supply of external signals.1,2 Whereas the events and signals
that program memory precursors are not well understood,
interleukin-15 (IL-15) has clearly been shown to drive the
generation and maintenance of memory CD8 T cells.3,4 The
recent evidence that IL-15 is delivered through the mechanism
of transpresentation via IL-15R␣ by cell-cell contact5 opens up
many questions in the field of memory CD8 T-cell homeostasis,
the foremost being the identity of the cell transpresenting IL-15
to memory CD8 T cells.
To better understand how IL-15 transpresentation is regulated, the identity of the cell type mediating IL-15 transpresentation must be known. Studies attempting to address this have
found that either parenchymal or hematopoietic cells mediate
transpresentation, depending on the responding lymphocyte.6,7
This finding is seminal as it shows that cells can be in an
IL-15R␣⫹ environment and remain ignorant to IL-15. Considering that IL-15R␣ is ubiquitously expressed, it is surprising that
the responding cell has such strict requirements for a specific
cell to transpresent IL-15.8 In contrast to IL-15R␣ expression,
IL-15 protein is not believed to be expressed by many IL-15R␣–
expressing cells (ie, T cells); however, expression of IL-15 has
been difficult to determine as reagents detecting IL-15 protein
have been limited. As such, multiple studies using functional
readouts have found that coexpression of IL-15 and IL-15R␣ is
integral for a cell to transpresent IL-15.9-11 These observations
emphasize that the identity of an IL-15 transpresenting cell may
not simply be implied by the expression of IL-15R␣ but rather is
better determined through functional analysis.
For memory CD8 T cells, in vivo studies found that IL-15R␣–
expressing hematopoietic cells are the dominant cell types
driving homeostatic proliferation.7,12 The hematopoietic cells
providing the IL-15 signal are RAG-1–independent, indicating
that IL-15 transpresentation is not mediated by either T or
B lymphocytes.9 As DCs, monocytes, and macrophages have
been shown to express IL-15R␣ protein and are capable of
inducing IL-15,13,14 these cells are potential mediators of IL-15
transpresentation. Indeed, bone marrow (BM)–derived DCs and
some monocytic cell lines can transpresent IL-15 in vitro5;
however, whether their analogous in vivo counterparts transpresent IL-15 to CD8 T cells is not known. Since IL-15 transpresentation requires cell-cell interactions and DCs have a natural
propensity for T-cell interactions, DCs are a prime candidate to
transpresent IL-15 to memory CD8 T cells. Therefore, the goal
of this study was to examine the contribution of DCs in
transpresentation of IL-15 to CD8 T cells.
In this study, we demonstrate that DCs transpresent IL-15 to
CD8 T cells driving specific functions during differentiation.
Although IL-15 is important for development of naive CD8
T cells and intraepithelial lymphocytes (IELs),15,16 exclusive
IL-15 transpresentation by DCs failed to restore these populations. DCs did, however, preferentially promote the survival of
KLRG-1⫹CD27⫺ cells during the development of virus-specific
memory CD8 T cells. Furthermore, DCs were highly efficient in
driving homeostatic proliferation of memory CD8 T cells.
Collectively, our data identify a previously unknown role of
DCs in the differentiation and homeostasis of memory CD8
T cells; however, the inability of DCs to restore all IL-15
functions indicates that other cell types participate in transpresenting IL-15 to memory CD8 T cells.
Submitted May 8, 2008; accepted September 2, 2008. Prepublished online as
Blood First Edition paper, September 23, 2008; DOI 10.1182/blood-2008-05156307.
payment. Therefore, and solely to indicate this fact, this article is hereby
marked ‘‘advertisement’’ in accordance with 18 USC section 1734.
The publication costs of this article were defrayed in part by page charge
© 2008 by The American Society of Hematology
4546
BLOOD, 1 DECEMBER 2008 䡠 VOLUME 112, NUMBER 12
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
BLOOD, 1 DECEMBER 2008 䡠 VOLUME 112, NUMBER 12
Methods
Mice
C57BL/6J (CD45.2) and C57BL/6J (Thy1.1⫹) were purchased from The
Jackson Laboratory (Bar Harbor, ME) and C57BL/6J (CD45.1) mice were
purchased from the National Cancer Institute (NCI; Frederick, MD).
IL-15R␣⫺/⫺ mice16 were generously provided by A. Ma (University of
California at San Francisco) and backcrossed to C57BL/6 mice
15 generations, and IL-15⫺/⫺ mice15 were purchased from Taconic Farms
(Germantown, NY). The full-length murine IL-15R␣ cDNA was cloned by
polymerase chain reaction (PCR) from cDNA generated from a C57BL/6
spleen RNA using TA Topo cloning kit (Invitrogen, Carlsbad, CA). The
IL-15R␣ cDNA was then subcloned downstream of the CD11c promoter by
ligating into the pCD11c-pDOI-5 vector17 (graciously provided by Thomas
Brocker, Institute for Immunology, Ludwig-Maximilian University, Munich, Germany) at the EcoRI site. The final 7.4-kb CD11c-IL-15R␣
fragment was removed from the vector backbone by XhoI and NotI
digestion and used for microinjection into C57BL/6 pronuclei for generation of transgenic mice. Microinjection was performed by the Genetically
Engineered Mouse Facility at the University of Texas M. D. Anderson
Cancer Center. Tg-positive mice were identified by PCR of tail DNA and
bred to IL-15R␣⫺/⫺ background. All mice were maintained under specific
pathogen–free conditions at the M. D. Anderson Cancer Center in accordance with its Institutional Animal Care and Use Committee guidelines.
Bone marrow chimeras were generated as previously described.7
Flow cytometric analysis
Lymphocytes were isolated from various tissues as previously described.6,7
IL-15R␣ and IL-15 were detected with goat anti–IL-15R␣-biotin (R&D
Systems, Minneapolis, MN) and rabbit anti–IL-15 biotin (Peprotech, Rocky
Hill, NJ), respectively, followed by streptavidin-APC (Jackson ImmunoResearch Laboratories, West Grove, PA). Background staining was determined by staining analogous populations from either IL-15R␣⫺/⫺ or
IL-15⫺/⫺ mice or with a biotinylated Ig control (Jackson ImmunoResearch
Laboratories). Prior to staining for IL-15 and IL-15R␣, cells were
preincubated with Fc block (BD Biosciences, San Jose, CA). The following
monoclonal antibodies (mAbs) were purchased from either BD Biosciences, eBioscience (San Diego, CA), or BioLegend (San Diego, CA):
CD44, CD62L, CD8␣, CD19, CD3, DX5, CD4, CD11b, CD11c, CD127,
Thy1.1, KLRG-1, CD27, and MHC class II (I-Ab). For stimulation with
lipopolysaccharide (LPS), cells were isolated from spleen and cultured at
10 ⫻ 106/mL in RPMI containing 2.5 mM HEPES, 5.5 ⫻ 10⫺5 M 2-ME,
100 U/mL penicillin, 100 ␮g/mL streptomycin, 5 mM glutamine, and 10%
fetal bovine serum (FBS; complete medium [CM]) at 37°C with 5% CO2
either in the presence or absence of 1 ␮g/mL LPS (Sigma-Aldrich, St Louis,
MO) for 24 hours. For studies of a viral immune response, mice were
infected intravenously with 105 pfu (vesicular stomatitis virus [VSV],
Indiana strain) and VSV-specific CD8 T cells were detected using an H-2Kb
tetramer (National Institutes of Health [NIH] tetramer facility) loaded with
the VSV nucleoprotein epitope (RGYVYQGL) as previously described.7
Flow cytometric data were acquired with an LSRII (BD Biosciences) or
FACSCalibur (BD Biosciences) and analyzed with FlowJo software
(Treestar, Ashland, OR). Lymphocyte percentages and total cell numbers
were calculated and evaluated using the Student t test. Values of P less than
.05 were considered statistically significant.
Analysis of homeostatic proliferation
Proliferation of endogenous cells was assessed by BrdU incorporation.
VSV-infected mice were given BrdU in their drinking water (0.8 mg/mL)
for 5 to 6 weeks. Cells were stained for surface markers as usual followed
by staining using the BrdU Flow Kit (BD Biosciences) according to
manufacturer’s instructions. For transfer studies, C57BL/6 (Thy1.1) mice
were infected intravenously with 105 pfu VSV and boosted at least 30 days
after infection with VSV-New Jersey. Spleens were harvested at least
30 days after most recent infection. Splenic CD8 T cells were negatively
DC TRANSPRESENTATION OF IL-15 TO CD8 T CELLS
4547
enriched using the Dynal CD8 Enrichment Kit (Dynal Biotech, Oslo,
Norway) according to the manufacturer’s instructions, labeled with 2 mM
carboxyfluorescein succinimidyl ester (CFSE), and injected intravenously
into various recipients (approximately 10 ⫻ 106 cells/mouse). Enriched
cells were found to be 75% to 85% CD8⫹, 4% to 8% NK1.1, and less than
1% to 2% CD11c, B220, CD4.
Results
CD11c promoter drives IL-15R␣ expression preferentially by
DCs
Since DCs are highly specialized for T-cell interactions and
stimulation as well as express IL-15 and IL-15R␣,14,18,19 we
examined the contribution of IL-15 transpresentation by DCs on
CD8 T-cell development and the generation of memory CD8
T cells. To target IL-15R␣ to DCs, a transgenic mouse model was
generated in which IL-15R␣ expression is driven by the murine
CD11c promoter, a well-described promoter with a high specificity
to DCs.17 We chose to control IL-15R␣ protein expression rather
than IL-15 to address specifically the role of IL-15 transpresentation. To eliminate IL-15R␣ expression from all other cells, the
CD11c-IL-15R␣ Tg mice were bred to the IL-15R␣⫺/⫺ background. Once backcrossed, CD11c-specific expression of cell
surface IL-15R␣ protein was assessed using immunofluorescence
staining and flow cytometry.
Using this approach, 2 founders were identified that expressed
IL-15R␣ by CD11c⫹ cells: one founder line with IL-15R␣
restricted to CD11c⫹ cells and a second founder line with
expression spilling over onto CD11c⫺ hematopoietic cells. Overall,
the CD11c-driven expression of IL-15R␣ closely correlated with
the endogenous expression of CD11c, resulting in a model where
DCs are the dominant cell type expressing IL-15R␣ (ie, Tg line 1).
Analysis of transgenic IL-15R␣ expression showed that IL-15R␣
was highly expressed by lineage-negative, CD19, CD3, DX5/
CD11b⫹ CD11chi, and CD11b⫺CD8⫹CD11chi DCs and was moderate in lin⫺CD11clo (putative DC precursors and pDCs) in both Tg
founder lines (Figure 1A). On CD11c⫺ cells, which include
monocytes (CD11b⫹SSClo), granulocytes (CD11b⫹SSChi), and
T cells, Tg-1 had no expression of IL-15R␣ whereas Tg-2 had
levels similar to that of wild-type (Wt) cells (Figure 1A,B). As
natural killer (NK) cells and IELs express CD11c, each of these cell
types expressed IL-15R␣ in both Tg founder lines at varying levels
(Figure 1B). Whereas previous studies have shown that IL-15R␣ is
expressed at a low level on CD11c⫹ cells,20 a comparison of
IL-15R␣ expression by DC subsets directly ex vivo had not been
described before. Interestingly, this analysis demonstrates that
IL-15R␣ expression is relatively low and homogenous among DC
subsets and other myeloid cells (Figure 1A).
A similar technique was used to measure IL-15 on the cell
surface using a recently available antibody. On lineage-negative
myeloid cells, cell-surface IL-15 was detected on CD11b⫹CD11c⫹
and CD8⫹CD11b⫺CD11c⫹ DCs as well as on CD11cloCD11b⫹
cells at similar levels in both Wt, Tg-1, and Tg-2 cells, although
was not present on CD11cloCD11b⫺ pDCs (Figure 1C). In Wt and
Tg-2 but not Tg-1 mice, IL-15 expression was observed at a lower
level on CD11b⫹CD11c⫺ cells (monocytes and granulocytes;
Figure 1C) displaying the potential for cells other than DCs to
transpresent IL-15. Among lymphocytes, neither CD4⫹TCR␤⫹,
CD8⫹TCR␤⫹, NK1.1⫹TCR␤⫺, or CD19⫹ cells isolated from Wt
or either CD11c-IL-15R␣ Tg mice expressed IL-15 on the cell
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
4548
STONIER et al
BLOOD, 1 DECEMBER 2008 䡠 VOLUME 112, NUMBER 12
Figure 1. Expression of IL-15 and IL-15R␣ in CD11c-IL15R␣ Tg mice. (A) Histograms show IL-15R␣ expression
on myeloid subsets in Wt, CD11c-IL-15R␣ Tg-1 and Tg-2,
and IL-15R␣⫺/⫺ mice after gating on lineage-negative
(CD3, CD19, DX5) splenocytes. Filled histograms indicate IL-15R␣ staining on indicated populations, open
histograms represent staining of the same population
from IL-15R␣⫺/⫺ mice. (B) Histograms show IL-15R␣
staining on the respective lymphoid populations: top
3 panels are CD4⫹TCR␣␤⫹, CD8⫹TCR␣␤⫹, NK1.1⫹TCR␣␤⫺ cells from the spleen; bottom 3 panels show
respective IEL populations. (C,D) Splenocytes (107) from
Wt, CD11c/IL-15R␣ Tg-1, Tg-2, and IL-15⫺/⫺ mice were
cultured overnight in CM prior to staining. Filled histogram
indicates IL-15 staining, the open histogram represents
staining of the same cells from IL-15⫺/⫺ mice. Histograms
in panel C were first gated on lineage-negative cells as in
panel A. (E) Splenocytes were treated as in panel C, in
the absence or presence of 1 ␮g/mL LPS. IL-15 staining
was determined after gating on CD11chi CD11b⫹ cells.
surface (Figure 1D, data not shown). In response to LPS stimulation, the amount of IL-15 up-regulated was similar in DCs of both
Wt and Tg mice (Figure 1E) suggesting that, in spite of elevated
IL-15R␣ on Tg DCs, IL-15 expression is not coordinately enhanced. Altogether, DCs, putative DC precursors, and monocytes/
granulocytes express cell-surface IL-15 in Wt mice; however, since
IL-15 transpresentation requires both IL-15 and IL-15R␣ expression, DCs are likely the only cell transpresenting IL-15 in the
CD11c-IL-15R␣ Tg-1 mice.
Naive and memory-phenotype CD8 T cells are partially
recovered in the presence of IL-15R␣ⴙ DCs
IL-15R␣⫺/⫺ mice have reduced numbers of peripheral CD8 T cells,
with the deficiency in memory-phenotype CD8 T cells affected
more than the naive CD8 T cells.16 Decreased numbers of naive
CD8 T cells in the absence of IL-15R␣ are due predominantly to
the decreased survival of naive CD8 T cells, as only minor effects
on the thymic CD8 T cells are observed.21 To determine if
CD11c-driven IL-15R␣ affects thymic T-cell development, thymocytes were analyzed from the CD11c-IL-15R␣ Tg mice and
compared with IL-15R␣⫺/⫺ and Wt mice. The proportions of the
major thymocyte subsets (CD4⫺CD8⫺, CD4⫹CD8⫹, CD4⫹, and
CD8⫹ thymocytes) were similar in all groups of mice (Figure 2A
and data not shown), indicating that transgenic expression of
IL-15R␣ does not cause major disruptions in thymic T-cell
development.
Upon extending our analysis to peripheral tissues, differences in the CD8 T cells between the different groups of mice
were apparent. As previously reported, the percentage of CD8
T cells in the IL-15R␣⫺/⫺ mice is decreased by approximately
50% compared with Wt mice,16 a trend observed not only in the
spleen but also in the lymph node, lung, and bone marrow
(Figure 2A,B). In both CD11c-IL-15R␣ Tg lines, the percentages of splenic CD8 T cells were increased compared with those
observed in the IL-15R␣⫺/⫺ mice (⬃9% and 11% compared
with only ⬃6%), but were lower than in the Wt mice (⬃12%;
Figure 2A,B). This trend was seen consistently in the other
tissues analyzed. Since the total CD8 T-cell population is
comprised of both naive and memory-phenotype CD8 T cells
and IL-15 preferentially affects the memory CD8 T cells,19 the
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
BLOOD, 1 DECEMBER 2008 䡠 VOLUME 112, NUMBER 12
DC TRANSPRESENTATION OF IL-15 TO CD8 T CELLS
4549
Figure 2. Phenotype of CD8 T cells as a function of CD11c-driven IL-15R␣ expression. Lymphocytes from thymus, spleen (SP), lymph node (LN), lung (LG), and bone
marrow (BM) were isolated from the mice indicated (6-8 weeks old) and stained for cell-surface markers to identify differences in T-cell populations. (A) Representative flow
cytometry plots from thymus and spleen. (B) The percentage of CD8 T cells along with the proportion of naive (CD44low) and memory-phenotype (CD44hi) in various tissues.
Results are averages with n ⫽ 10 mice/group for Tg-1. For Tg-2, n ⫽ 8 for spleen and n ⫽ 2 for other tissues. § represents a significant difference between naive T cells in Tg
and Wt (P ⬍ .05), whereas * indicates differences between memory phenotype in Tg and Wt (P ⬍ .05). (C) The number of each IEL population recovered from small intestine of
the indicated mice. Error bars represent standard deviation (SD) from a representative experiment; values from Tg mice are not significantly different than those from
IL-15R␣⫺/⫺ mice. (D) Total numbers of myeloid populations in the spleen of the indicated mice. Error bars represent SD, n ⫽ 2, for one representative experiment.
effects of the restricted IL-15R␣ expression on these 2 subpopulations was measured. For the naive CD8 T cells (CD44low), the
presence of IL-15R␣⫹ DCs in Tg-1 increased the percentage of
naive CD8 T cells from approximately 50%, as observed in
IL-15R␣⫺/⫺ mice, to between 62% and 71% of Wt values,
depending on the tissue (Figure 2B); however, this effect was
more dramatic in the Tg-2 line where the percentages of naive
T cells were almost similar to levels seen in Wt mice. Among
total CD8 T cells, percentages of memory-phenotype (CD44hi)
CD8 T cells in both CD11c-IL-15R␣ Tg mice approached parity
with Wt levels in all tissues examined, ranging from 90% to
100% of Wt levels (Figure 2B). Although the percentage of
memory CD8 T cells is almost recovered, a deficiency in the
total levels of memory CD8 T cells still exists as a function of
the deficit in naive CD8 T cells in CD11c-IL-15R␣ Tg mice
(Figure 2B). Altogether, these findings show that the CD11c-IL15R␣ transgenic environment partially restored the deficiencies
in total CD8 T cells observed in the IL-15R␣ mice with memoryphenotype CD8 T cells affected more than naive CD8 T cells.
Intestinal IELs, specifically TCR␥␦ and CD8␣␣TCR␣␤ cells,
depend heavily on IL-15 for their normal development.16 To
determine whether CD11c-driven expression of IL-15R␣⫹ affects
IEL development, IELs were isolated from each group of mice and
the respective populations were compared. Similar to IL-15R␣⫺/⫺
mice, CD11c-IL-15R␣ Tg-1 and Tg-2 mice were similarly deficient
in TCR␥␦ and CD8␣␣TCR␣␤ cells whereas conventional
CD8␣␤TCR␣␤ cells were present in normal numbers (Figure 2C).
While there are clearly IL-15R␣⫹ cells present in the intestines, the
inability of these cells to recover the defects in IEL development
exemplifies the importance of cell specificity in responsiveness to
transpresented IL-15.
Since IL-15R␣ expression by DCs could indirectly affect CD8
T-cell development by altering the development and normal
function of DCs, general characteristics of DCs were examined.
Among lineage-negative cells, no differences in the number or
distribution of various DC subsets were observed in spleen (Figure
2D) among the 4 groups of mice. In addition, MHC class II
expression by CD11chiDCs was not different (data not shown).
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
4550
STONIER et al
Altogether, elevated IL-15R␣ expression did not alter the development or maturation of DCs.
IL-15R␣ expression by DCs contributes to the generation of
virus-specific memory CD8 T cells
We next measured the generation and maintenance of antigenspecific memory CD8 T cells in response to an acute viral infection
in our transgenic system. In separate experiments, Wt, CD11c-IL15R␣ Tg-1, Tg-2, and IL-15R␣⫺/⫺ mice were infected with 105 pfu
VSV and the levels of N-tetramer⫹ CD8 T cells in the peripheral
blood were measured at various times after infection. During the
expansion phase, no detectable differences in the percent of
VSV-specific T cells were observed between the groups of mice
(Figure 3A), which is consistent with previous studies showing
IL-15R␣ is not required for normal Ag-specific CD8 T-cell
expansion.3,4 Following the contraction phase (30 days after
infection), the percentage of VSV-specific CD8 T cells present in
the blood was increased in the CD11c-IL-15R␣Tg-1 mice compared with IL-15R␣⫺/⫺ mice, but was lower than that of Wt mice
(Figure 3A); this was in contrast to Tg-2 mice, which had levels
similar to Wt mice (Figure 3B). During the memory phase, both
CD11c-IL-15R␣ Tg mice were able to maintain levels of VSVspecific CD8 T cells similar to Wt mice (Figure 3A,B). The
discrepancy between the 2 Tg mice is best explained by the leaky
expression of IL-15R␣ expression onto CD11c⫺ cells, therefore
suggesting that transpresentation by cells other than DCs provide
IL-15 to CD8 T cells during the contraction phase.
To determine whether tissue-specific effects were evident, the
level of VSV-specific CD8 T cells was measured in the spleen,
lymph nodes, and lung. In all the tissues analyzed, the percentage
of VSV-specific memory CD8 T cells in CD11c-IL-15R␣ Tg-1 was
greater than in the IL-15R␣⫺/⫺ mice, but less than in the Wt mice
(Figure 3C). When IL-15R␣ is present on both CD11c⫹ and
CD11c⫺ cells (Tg-2), the percentage of viral-specific CD8 T cells
in the tissues was similar to that of Wt mice (Figure 3D). As
expected, due to preexisting defects in total CD8 T cells, total
numbers of Ag-specific CD8 T cells were also deficient (Figure 3E
and data not shown). Overall, the effect of transpresentation by
DCs in different tissues was commensurate with the trends
observed in the blood, indicating no tissue-specific effects.
CD27, KLRG-1, IL-7R␣, and CD62L are cell-surface markers
that have been used to define subpopulations of effector and
memory CD8 T cells.22-24 During the transition from effector to
memory T cell, CD27⫹ KLRG-1⫺, IL-7R␣hi CD8 T cells have
been identified as those having a greater potential to differentiate
into memory T cells. In contrast, KLRG-1⫹CD27⫺IL-7R␣⫺ cells
are thought to represent short-lived effectors that rely temporarily
on IL-15 for their survival.22 Therefore, to determine whether
IL-15R␣⫹ DCs alter the differentiation of these subsets, expression
of CD27, KLRG-1, and IL-7R␣ by antigen-specific CD8 T cells
was analyzed at various times after infection in the CD11c-IL15R␣ Tg-1 mice. We chose to use the combination of CD27 and
KLRG-1, as CD27⫺KLRG-1⫹ cells were more easily defined than
IL-7R␣⫺KLRG-1⫹ cells. At the peak of the expansion (7 days after
infection), no differences in the proportion of VSV-specific CD8
T cells defined by KRLG-1 and CD27 were observed between the
different groups (Figure 3F). Interestingly, from 19 to 60 days after
infection, the VSV-specific CD8 T cells in the IL-15R␣–deficient
mice were predominantly CD27⫹KLRG-1⫺, whereas both
CD27⫹KLRG-1⫺ and CD27⫺KLRG-1⫹ populations were abundant at similar levels in both the CD11c-IL-15R␣ and Wt mice
(Figure 3F). Considering that all memory CD8 T cells use IL-15,
BLOOD, 1 DECEMBER 2008 䡠 VOLUME 112, NUMBER 12
our findings suggest that IL-15R␣⫹ DCs preferentially drive the
survival of short-lived CD27⫺ KLRG-1⫹ memory T cells, whereas
other IL-15R␣⫹ cells may promote development of long-lived
memory CD8 T cells.
Whereas KLRG-1, CD27, and IL-7R␣ identify T-cell subsets
during the transition from the effector phase to the memory phase,
high CD62L expression on a subset of memory T cells is associated
with the ability to migrate to lymphoid tissues and an enhanced
self-renewal known as central memory.25,26 To determine whether
CD11c-directed expression of IL-15R␣ affects the generation of
central and effector memory subsets, CD62L expression on VSVspecific memory CD8 T cells was examined. Depending on the
tissue site of isolation, VSV-specific memory CD8 T cells had a
similar pattern of CD62L expression among all groups of mice
(Figure 3G and data not shown). This suggests that IL-15R␣ did
not impact the proportion of memory CD8 T-cell subsets, as
defined by CD62L expression, that arise during an acute viral
infection.
IL-15R␣ⴙ DCs drive homeostatic proliferation of memory CD8
T cells
Whereas we demonstrated that IL-15R␣⫹ DCs alone were not
completely sufficient in generating normal levels of memory CD8
T cells, it was unclear whether IL-15R␣⫹ DCs could still drive
normal homeostatic proliferation. To examine the homeostatic
proliferation of endogenous Ag-specific memory CD8 T cells,
VSV-infected mice from each group (Wt, CD11c-IL-15R␣ Tg-1,
and IL-15R␣⫺/⫺) were given BrdU in their drinking water at
40 days after infection for 4 weeks. After this course of treatment,
BrdU incorporation of the VSV-specific memory CD8 T cells in
various tissues was measured. In all the tissues analyzed, VSVspecific CD8 T cells incorporated similar levels of BrdU in
CD11c-IL-15R␣ Tg-1 and Wt mice (Figure 4A). In contrast,
VSV-specific memory CD8 T cells had little BrdU incorporation in
the IL-15R␣⫺/⫺ mice. Although the extent of BrdU incorporation
was slightly different among tissues, the effect of the transgenic
IL-15R␣⫹ by DCs drove almost normal levels of cell division. The
similar BrdU incorporation in VSV-specific memory CD8 T cells
between Wt and CD11c-IL-15R␣ Tg-1 mice is evidence that
IL-15R␣⫹ DCs are pivotal in mediating homeostatic proliferation
of memory CD8 T cells.
The possibility exists that the ability of memory CD8 T cells to
undergo homeostatic proliferation is affected earlier in the immune
response by the transgenic expression of IL-15R␣. We therefore
examined the proliferation of VSV-specific memory CD8 T cells
generated in a normal environment. CD8 T cells were enriched
from congenic Wt mice (CD45.1⫹) previously infected with VSV,
CFSE-labeled, and adoptively transferred into CD11c-IL-15R␣
Tg-1, Wt, and IL-15R␣⫺/⫺ mice (all CD45.2⫹). Approximately
2 months later, CFSE dilution of the donor CD8 T cells was
measured. In both Tg and Wt mice, the memory-phenotype CD8
T cells divided 2 to 3 times in the spleen, lung, and BM in a similar
manner (Figure 4B). Similar results were observed with the
CD11c-IL-15R␣ Tg-2 line (data not shown). In contrast, the
IL-15R␣⫺/⫺ host was unable to support division of the memoryphenotype CD8 T cells (Figure 4B). Among the VSV-specific
memory CD8 T cells and memory CD8 T cells in the BM, both the
Wt and Tg mice were able to induce cell division; however, cell
division was greater in the Wt than in the Tg hosts (Figure 4B).
Overall, the exclusive expression of IL-15R␣ to DCs provides a
sufficient signal for memory CD8 T cells to proliferate under
homeostatic conditions.
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
BLOOD, 1 DECEMBER 2008 䡠 VOLUME 112, NUMBER 12
DC TRANSPRESENTATION OF IL-15 TO CD8 T CELLS
4551
Figure 3. Role of IL-15R␣ⴙ DCs in a viral immune response. The kinetics of an antiviral CD8 T-cell response was compared among different groups of mice by measuring
the percent of VSV-specific CD8 T cells present in the peripheral blood as determined by N-tetramer staining: (A) CD11c-IL-15R␣ Tg-1, (B) CD11c-IL-15R␣ Tg-2. (C,D) Dot
plots show the percent of N-tetramer⫹ cells versus CD44 expression after gating on CD8⫹ cells present in SP, LN, and LG 25 weeks after infection in Tg-1 (C) and Tg-2 (D).
(E) Graphs show total numbers of N-tetramer⫹ cells recovered from the various tissues in Wt, Tg-1, and IL-15R␣⫺/⫺ mice. Error bars represent SD, n ⫽ 2. Numbers above the
bars represent percent of Wt mice. (F) KLRG-1 and CD27 expression on N-tetramer⫹ CD8 T cells in peripheral blood at indicated times after infection. (G) Histograms show
CD62L expression by N-tetramer⫹ CD8 T cells in various tissues 25 weeks after infection.
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
4552
STONIER et al
BLOOD, 1 DECEMBER 2008 䡠 VOLUME 112, NUMBER 12
Figure 4. Homeostatic proliferation of memory CD8
T cells. (A) Histograms show the percentage of VSV-N–
specific memory CD8 T cells that have incorporated
BrdU. (B) Spleen cells from normal, CD45.1⫹, VSVinfected mice were enriched for CD8 T cells, labeled with
CFSE, and transferred to various CD45.2⫹ hosts. Two
months after transfer, lymphocytes from SP, LG, and BM
were analyzed for CFSE intensity. Top panels show
CFSE dilution in CD45.1⫹ CD44hi CD8⫹ T cells (memoryphenotype) in various tissues. Bottom row depicts CFSE
dilution in splenic N-tetramer⫹ cells.
IL-15R␣ⴙ DCs are not the only hematopoietic cells participating
in homeostatic proliferation of memory CD8 T cells
The homeostatic proliferation of memory CD8 T cells was almost
completely recovered in mice that express IL-15R␣ exclusively by
CD11c⫹ cells (Figure 4B), indicating that other cell types transpresent IL-15 to memory CD8 T cells. Since our previous studies
showed that IL-15R␣ by parenchymal cells could play a minor role
in homeostatic proliferation,7 we wanted to determine if a host
containing IL-15R␣⫹ DCs along with IL-15R␣⫹ parenchymal cells
would be sufficient to completely restore the homeostatic proliferation of memory CD8 T cells. Therefore, 3 groups of BM chimeras
were generated with each group containing different combinations
of IL-15R␣ on either DCs and/or parenchymal cells: (1) Wt BM in
Wt hosts (IL-15R␣ expressed by all cells); (2) CD11c-IL-15RTg-1
in Wt hosts (IL-15R␣ expressed by DCs and parenchymal cells);
(3) IL-15R␣⫺/⫺ BM in Wt hosts (IL-15R␣ expressed only by
parenchymal cells). After reconstitution, CD8 T cells (⬃10 ⫻ 106)
from a cohort of VSV-infected Thy1.1⫹ mice (at least 40 days after
infection) were isolated, labeled with CFSE, and transferred into
the various groups of chimeras. Six weeks later, homeostatic
proliferation of the donor memory CD8 T cells was assessed by
measuring CFSE dilution. Memory CD8 T cells divided 1 to
3 times in the Wt chimeras (Wt into Wt), representing the normal
level of homeostatic proliferation over this time period (Figure 5).
In the chimeras containing IL-15R␣⫹ DCs along with IL-15R␣⫹
Figure 5. Contribution of CD11cⴚ hematopoietic cells in memory CD8 T-cell
homeostasis. BM cells were isolated from IL-15R␣⫺/⫺, CD11c-IL-15R␣Tg-1, and Wt
mice (all CD45.2⫹) and transferred into lethally irradiated Wt hosts (CD45.1⫹). CD8
T cells were isolated from VSV-infected Thy1.1⫹ cells, labeled with CFSE, and
transferred into the various BM chimeras after lymphocyte reconstitution (between
8 and 12 weeks after BM transfer). Six weeks after transfer, CFSE dilution of donor
cells was measured.
parenchymal cells (CD11c-IL-15R␣Tg-1 into Wt), the memory
CD8 T cells divided but not to same extent as in the Wt chimeras
(Figure 5). Little cell division of memory CD8 T cells was
observed in the chimeras containing IL-15R␣ only in the parenchyma (IL-15R␣⫺/⫺ into Wt; Figure 5). These differences indicate
that homeostatic proliferation of memory CD8 T cells is not
completely restored in the presence of IL-15R␣⫹ DCs and parenchymal cells and suggest that IL-15R␣⫹ hematopoietic cells other
than DCs participate in the response.
Discussion
Generating a model with DC-restricted IL-15R␣ expression has
allowed us to demonstrate that DCs are not only important for
presenting antigen for T-cell stimulation but also for presenting
IL-15 during later stages of CD8 T-cell differentiation. This
function of DCs in memory CD8 T-cell homeostasis had been
speculated but never demonstrated. Our study has also demonstrated that IL-15 transpresentation by DCs has specific roles
during CD8 T-cell differentiation in that DCs were most effective at
inducing homeostatic proliferation of established memory CD8
T cells, while having less of a role in generating memory CD8
T cells and the homeostasis of naive CD8 T cells. In contrast,
transpresentation by DCs had no effect on IEL development. In
concurrent analysis of our model, we have also found that IL-15
transpresentation by DCs heavily influenced NK-cell development
while only minimally affecting NK T-cell development (E.F.C. and
K.S.S., manuscript in preparation). Altogether, our study has shown
that cell specificity is pivotal in dictating IL-15 responsiveness by
transpresentation and that DCs are an integral cell type transpresenting IL-15 to memory CD8 T cells.
The enhanced ability of DCs to transpresent IL-15 as differentiation progresses could be an indication that developing memory
CD8 T cells become more accessible to DCs and preferentially
receive IL-15 signals in a DC-enriched environment. This is
contradictory to studies touting BM as the predominant site for
memory CD8 T homeostasis, as BM is a site with a very low
frequency of DCs. The importance of BM in providing homeostatic
signals is based on the presence of memory CD8 T cells undergoing a higher rate of cell division,27-29 which could be explained by
preferential migration of actively dividing central memory CD8
T cells. If BM is the predominant site for memory CD8 T homeostasis, we would predict an exaggerated defect in homeostatic
proliferation of memory CD8 T cells in the BM of our Tg model,
where only DCs transpresent IL-15. On the contrary, homeostatic
proliferation of memory CD8 T cells was restored rather efficiently
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
BLOOD, 1 DECEMBER 2008 䡠 VOLUME 112, NUMBER 12
DC TRANSPRESENTATION OF IL-15 TO CD8 T CELLS
in the BM of the CD11c-IL-15R␣ Tg mice, suggesting that
memory CD8 T cells received adequate IL-15 signals elsewhere
and then preferentially migrated to the BM. Moreover, memory
CD8 T cells that are sequestered in the spleen and lymph node with
FTY720 treatment undergo normal homeostatic proliferation despite this tissue restriction.30 This, along with our findings, suggests
that memory CD8 T cells efficiently receive homeostatic signals in
many tissues besides the BM.
In addition to the specific effects on the homeostatic proliferation of established memory CD8 T cells, DCs appeared to have a
distinct role in generating memory CD8 T cells. During the
contraction phase, IL-15 begins to become important by mediating
survival of Ag-specific CD8 T cells, as these cells are not proliferating.31 Specifically, a subpopulation of short-lived effectors (SLECs),
defined as KLRG-1⫹, are predominantly affected by the lack of
IL-15.32 In our model, IL-15 transpresentation by DCs limited the
contraction of the Ag-specific memory CD8 T cells by increasing the
proportion of SLEC-like cells. Considering all memory CD8 T cells use
IL-15, our findings yield 2 possible scenarios: SLECs may be more
responsive or more accessible to DCs providing IL-15.
The limited requirements for DCs in transpresenting IL-15 to
naive and differentiating memory CD8 T cells highlights the idea
that cells other than DCs are providing IL-15 to CD8 T cells during
these earlier phases. This is not surprising, as previous studies
using BM chimeras indicated that IL-15R␣ expression by parenchymal cells is important for generation of naive and memory CD8
T cells.7,12 Our current studies provide further evidence for this, as
IL-15 transpresentation by DCs cannot assume this role of the
parenchyma. When IL-15R␣ expression was restricted to DCs, the
contraction of the Ag-specific CD8 T cells was more exaggerated,
resulting in less memory CD8 T cells generated; however, when
IL-15R␣ expression was present on DCs along with other cell
types, the contraction and generation of memory CD8 T cells was
normal. Therefore, our study provides evidence that cells other than
DCs, which could include CD11c⫺ hematopoietic cells and parenchymal cells, can also participate in the generation of memory CD8
T cells in vivo.
The incomplete abilities of DCs to transpresent IL-15 to CD8
T cells was not only present during the generation of memory
CD8 T cells, but could also be observed to a small degree during
the homeostasis of memory CD8 T cells. While parenchymal
cells can have a minor contribution in homeostatic proliferation,7 the combination of IL-15R␣ on both DCs and parenchyma
did not completely restore normal levels of homeostatic proliferation, providing evidence that hematopoietic cells other than
DCs transpresent IL-15 to memory CD8 T cells during the
memory phase.
In our model, the sheer effectiveness of DCs in transpresenting
IL-15 to CD8 T cells could indicate that DCs are superior at
transpresentation; however, this could be due to the imprecise
specificity of the transgenic promoter. Whereas high expression of
CD11c is specific to DCs, low levels of CD11c are expressed on
non-DCs, such as activated T cells, NK cells, and IELs.33 In our Tg
models, even when some IL-15R␣ is expressed on these cells, it
was not accompanied by IL-15 expression. As IL-15 is not
4553
transferred from other transpresenting cells even under inflammatory circumstances,11 this IL-15R␣ does not appear to be transpresenting IL-15. Furthermore, since previous studies have shown that
IL-15R␣ expression by T cells is dispensable for generation and
maintenance of memory CD8 T cells and the IL-15–dependent
development of IELs,6,7,12 the IL-15R␣ expression that is expressed
by T cells is likely irrelevant.
Another caveat of our model are the supraphysiologic levels of
IL-15R␣ present on DCs that could make the DCs more efficient.
Indeed, recent reports demonstrate that the presence of IL-15R␣
can stabilize IL-15 protein and increase bioactivity34; however, our
studies do not support the idea that higher IL-15R␣ translates to
more IL-15 transpresentation, as IL-15R␣hi DCs did not have
higher levels of IL-15 than IL-15R␣lo DCs. Altogether, our
observations suggest that IL-15 expression appears to be endogenously controlled and is the limiting factor in transpresentation.
In summary, we have developed an in vivo model whereby DCs
are the only cell able to transpresent IL-15. Using this model, DCs
are shown to effectively mediate the generation and maintenance of
memory CD8 T cells via IL-15 transpresentation, thus identifying a
novel function of DCs. Lastly, the inability of IL-15 transpresented
by DCs to recover all of IL-15 functions in CD8 T cells is a sterling
example of how cell specificity dictates responsiveness to transpresented IL-15. In general, these findings will enable future studies to
better focus on mechanisms regulating T-cell homeostasis.
Acknowledgments
We thank Bhavin Shah and Luis Acero for technical assistance, the
Genetically Engineered Mouse Facility at the University of Texas
M. D. Anderson Cancer Center, Thomas Brocker (Institute for
Immunology, Ludwig-Maximilian University, Munich, Germany)
for generously providing the CD11c promoter construct, and the
NIH tetramer facility for providing tetramer. We also thank Willem
Overwijk and Loredana Frasca (M. D. Anderson Cancer Center)
for critical reading of the manuscript.
This research was supported by National Institutes of Health
grant AI070910 and the M. D. Anderson Trust Fellowship (K.S.).
Authorship
Contribution: S.W.S. performed a majority of the experiments,
analyzed and interpreted data, and cowrote the manuscript; L.J.M.
and E.F.C. performed some experiments and analyzed data; and
K.S.S. designed, analyzed, and interpreted data and cowrote the
manuscript.
Conflict-of-interest disclosure: The authors declare no competing financial interests.
Correspondence: Kimberly S. Schluns, Department of Immunology, M/C 901, UT MDACC, PO Box 301429, Houston, TX 77030;
e-mail: [email protected].
References
1.
Van Stipdonk MJ, Hardenberg G, Bijker MS, et al.
Dynamic programming of CD8⫹ T lymphocyte
responses. Nat Immunol. 2003;4:361-365.
2.
Masopust D, Kaech SM, Wherry EJ, Ahmed R.
The role of programming in memory T-cell development. Curr Opin Immunol. 2004;16:217-225.
3.
4.
Schluns KS, Williams K, Ma A, Zheng XX, Lefrancois
L. Cutting edge: requirement for IL-15 in the generation of primary and memory antigen-specific CD8
T cells. J Immunol. 2002;168:4827-4831.
Becker TC, Wherry EJ, Boone D, et al. Interleukin
15 is required for proliferative renewal of virus-
specific memory CD8 T cells. J Exp Med. 2002;
195:1541-1548.
5.
Dubois S, Mariner J, Waldmann TA, Tagaya Y.
IL-15Ralpha recycles and presents IL-15 In
trans to neighboring cells. Immunity. 2002;17:
537-547.
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
4554
6.
7.
8.
9.
STONIER et al
Schluns KS, Nowak EC, Cabrera-Hernandez A,
et al. Distinct cell types control lymphoid subset
development by means of IL-15 and IL-15 receptor alpha expression. Proc Natl Acad Sci U S A.
2004;101:5616-5621.
Schluns KS, Klonowski KD, Lefrancois L. Transregulation of memory CD8 T-cell proliferation by
IL-15R alpha(⫹) bone marrow-derived cells.
Blood. 2004;103:988-994.
Anderson DM, Kumaki S, Ahdieh M, et al. Functional characterization of the human interleukin-15 receptor alpha chain and close linkage of
IL15RA and IL2RA genes. J Biol Chem. 1995;
270:29862-29869.
Burkett PR, Koka R, Chien M, et al. Coordinate
expression and trans presentation of interleukin
(IL)-15R{alpha} and IL-15 supports natural killer
cell and memory CD8⫹ T cell homeostasis. J Exp
Med. 2004;200:825-834.
10. Sandau MM, Schluns KS, Lefrancois L, Jameson
SC. Cutting edge: transpresentation of IL-15 by
bone marrow-derived cells necessitates expression of IL-15 and IL-15R alpha by the same cells.
J Immunol. 2004;173:6537-6541.
11. Mortier E, Woo T, Advincula R, Gozalo S, Ma A.
IL-15Ralpha chaperones IL-15 to stable dendritic
cell membrane complexes that activate NK cells
via trans presentation. J Exp Med. 2008;205:
1213-1225.
12. Burkett PR, Koka R, Chien M, et al. IL-15R alpha
expression on CD8⫹ T cells is dispensable for
T cell memory. Proc Natl Acad Sci U S A. 2003;
100:4724-4729.
13. Musso T, Calosso L, Zucca M, et al. Human
monocytes constitutively express membranebound, biologically active, and interferon-gammaupregulated interleukin-15. Blood. 1999;93:35313539.
14. Ruckert R, Brandt K, Bulanova E, et al. Dendritic
cell-derived IL-15 controls the induction of CD8
T cell immune responses. Eur J Immunol. 2003;
33:3493-3503.
BLOOD, 1 DECEMBER 2008 䡠 VOLUME 112, NUMBER 12
15. Kennedy MK, Glaccum M, Brown SN, et al. Reversible defects in natural killer and memory CD8
T cell lineages in interleukin 15-deficient mice. J
Exp Med. 2000;191:771-780.
16. Lodolce JP, Boone DL, Chai S, et al. IL-15 receptor maintains lymphoid homeostasis by supporting lymphocyte homing and proliferation. Immunity. 1998;9:669-676.
17. Jung S, Unutmaz D, Wong P, et al. In vivo depletion of CD11c(⫹) dendritic cells abrogates priming of CD8(⫹) T cells by exogenous cell-associated antigens. Immunity. 2002;17:211-220.
18. Mattei F, Schiavoni G, Belardelli F, Tough DF.
IL-15 is expressed by dendritic cells in response
to type I IFN, double-stranded RNA, or lipopolysaccharide and promotes dendritic cell activation.
J Immunol. 2001;167:1179-1187.
19. Zhang X, Sun S, Hwang I, Tough DF, Sprent J.
Potent and selective stimulation of memory-phenotype CD8⫹ T cells in vivo by IL-15. Immunity.
1998;8:591-599.
20. Lucas M, Schachterle W, Oberle K, Aichele P,
Diefenbach A. Dendritic cells prime natural killer
cells by trans-presenting interleukin 15. Immunity.
2007;26:503-517.
21. Berard M, Brandt K, Bulfone-Paus S, Tough DF.
IL-15 promotes the survival of naive and memory
phenotype CD8⫹ T cells. J Immunol. 2003;170:
5018-5026.
22. Joshi NS, Cui W, Chandele A, et al. Inflammation
directs memory precursor and short-lived effector
CD8(⫹) T cell fates via the graded expression of
T-bet transcription factor. Immunity. 2007;27:281295.
23. Hikono H, Kohlmeier JE, Takamura S, et al. Activation phenotype, rather than central- or effectormemory phenotype, predicts the recall efficacy of
memory CD8⫹ T cells. J Exp Med. 2007;204:
1625-1636.
24. Kaech SM, Tan JT, Wherry EJ, et al. Selective
expression of the interleukin 7 receptor identifies
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
effector CD8 T cells that give rise to long-lived
memory cells. Nat Immunol. 2003;4:1191-1198.
Sallusto F, Lenig D, Forster R, Lipp M,
Lanzavecchia A. Two subsets of memory T lymphocytes with distinct homing potentials and effector functions. Nature. 1999;401:708-712.
Masopust D, Vezys V, Marzo AL, Lefrancois L.
Preferential localization of effector memory cells
in nonlymphoid tissue. Science. 2001;291:24132417.
Mazo IB, Honczarenko M, Leung H, et al. Bone
marrow is a major reservoir and site of recruitment for central memory CD8⫹ T cells. Immunity.
2005;22:259-270.
Becker TC, Coley SM, Wherry EJ, Ahmed R.
Bone marrow is a preferred site for homeostatic
proliferation of memory CD8 T cells. J Immunol.
2005;174:1269-1273.
Parretta E, Cassese G, Barba P, et al. CD8 cell
division maintaining cytotoxic memory occurs
predominantly in the bone marrow. J Immunol.
2005;174:7654-7664.
Moyron-Quiroz JE, Rangel-Moreno J, Hartson L,
et al. Persistence and responsiveness of immunologic memory in the absence of secondary lymphoid organs. Immunity. 2006;25:643-654.
Yajima T, Yoshihara K, Nakazato K, et al. IL-15
regulates CD8⫹ T cell contraction during primary
infection. J Immunol. 2006;176:507-515.
Kaech SM, Ahmed R. Memory CD8⫹ T cell differentiation: initial antigen encounter triggers a developmental program in naive cells. Nat Immunol.
2001;2:415-422.
Huleatt JW, Lefrancois L. Antigen-driven induction of CD11c on intestinal intraepithelial lymphocytes and CD8⫹ T cells in vivo. J Immunol. 1995;
154:5684-5693.
Bergamaschi C, Rosati M, Jalah R, et al. Intracellular interaction of interleukin-15 with its receptor
{alpha} during production leads to mutual stabilization and increased bioactivity. J Biol Chem.
2008;283:4189-4199.
From www.bloodjournal.org by guest on June 17, 2017. For personal use only.
2008 112: 4546-4554
doi:10.1182/blood-2008-05-156307 originally published
online September 23, 2008
Dendritic cells drive memory CD8 T-cell homeostasis via IL-15
transpresentation
Spencer W. Stonier, Lisa J. Ma, Eliseo F. Castillo and Kimberly S. Schluns
Updated information and services can be found at:
http://www.bloodjournal.org/content/112/12/4546.full.html
Articles on similar topics can be found in the following Blood collections
Immunobiology (5489 articles)
Information about reproducing this article in parts or in its entirety may be found online at:
http://www.bloodjournal.org/site/misc/rights.xhtml#repub_requests
Information about ordering reprints may be found online at:
http://www.bloodjournal.org/site/misc/rights.xhtml#reprints
Information about subscriptions and ASH membership may be found online at:
http://www.bloodjournal.org/site/subscriptions/index.xhtml
Blood (print ISSN 0006-4971, online ISSN 1528-0020), is published weekly by the American Society
of Hematology, 2021 L St, NW, Suite 900, Washington DC 20036.
Copyright 2011 by The American Society of Hematology; all rights reserved.