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CUTTING EDGE |
Harrison Department of Surgical Research, Department of Surgery, University of Pennsylvania Medical Center, Philadelphia, PA 19104
| Abstract |
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| Introduction |
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Promiscuous recognition of allogeneic MHC with peptide ligands is thought to provoke potent T cell responses against major histocompatibility Ags, represented in vitro by vigorous primary MLR 6, 9, 10, 11 . On the other hand, the study of alloimmune responses against minor histocompatibility Ags has been limited by the unavailability of assays that can detect the response of a T cell repertoire to minor Ags. Minor histocompatibility Ags fail to induce detectable T cell proliferation in vitro despite the fact that transplanted tissues mismatched for minor Ags are rejected almost as rapidly as MHC incompatible allografts 12 . Therefore, comparison of the cumulative response of a T cell population against major and multiminor histocompatibility Ags has not been feasible. In this study, we use a bioassay that permits characterization of the in vivo kinetics of CD4+ T cell division in response to major as well as minor histocompatibility Ags. In addition, we demonstrate distinct requirements for CD28 costimulation in CD4+ T cell responses against major and minor histocompatibility Ags.
| Materials and Methods |
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C57BL/6 (H-2b), C57BL/6-Cd28tmlMak (H-2b), BALB/c (H-2d), C3H/HeJ (H-2k), and C3H.SW-H2b/SnJ (H-2b) mice were purchased from Jackson Laboratories (Bar Harbor, ME). C57BL/6TacfBR-[KO]Abß mice were purchased from Taconic (Germantown, NY). C57BL/6-Cd28tmlMak and C57BL/6TacfBR-[KO]Abß mice are homozygous for targeted disruption of the CD28 gene (CD28-/-) and Aß(H-2b) gene (MHC class II-/-), respectively 13, 14 . C3H.SW-H2b/SnJ mice are congenic for the H-2b allele 15 .
In vivo MLR
Wild-type and CD28-/- C57BL/6 mice were designated as responder strains and C3H/HeJ and C3H.SW mice as stimulator strains. In addition, we wished to use stimulator mice that were deficient in MHC class II. Because of the unavailability of MHC class II-/- C3H mice, MHC class II-/- C57BL/6 mice were used as stimulators with BALB/c responders. Lymphocytes were isolated from peripheral lymph nodes and spleens of 8- to 10-wk-old responder strain mice, labeled with carboxyfluorescein diacetate succinimidyl ester (CFSE)3 (Molecular Probes, Eugene, OR) as previously described, and transferred via tail vein into whole-body irradiated (1800 rad) stimulator mice (3040 million lymphocytes/stimulator) 16 . Responder cells were recovered from stimulator spleens to assure sufficient numbers of lymphocytes for flow cytometric analysis at serial time points.
Flow cytometry
Harvested lymphocytes were stained with phycoerythrin-conjugated anti-CD4 mAb (GK1.5; PharMingen, San Diego, CA) and analyzed on a FACScan (Becton Dickinson Immunocytometry Systems, San Jose, CA). CD4+ T cells of responder strain origin were identified in the CFSE+CD4+ gate.
Calculation of alloreactive precursor frequencies
Precursor frequencies of alloreactive CD4+ T cells were calculated as previously described 17 . Briefly, discrete generations of alloreactive CD4+ T cells were identified by their CFSE fluorescence. The number of alloreactive CD4+ T cells in each daughter generation was counted using Cell Quest software (Becton Dickinson Immunocytometry Systems), and the number of precursors for each generation was extrapolated using the formula a/2n (a = number of cells in generation, n = generation). For example, 8 daughter cells counted in the third daughter generation would be extrapolated to be the progeny of 1 precursor that divided three times (8/23 = 1). The frequency of alloreactive CD4+ cells in the responder population was then calculated by dividing the total number of precursors from all daughter cells that proliferated beyond background levels by the sum of all precursors and undivided cells.
| Results and Discussion |
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Major histocompatibility Ags are known to yield vigorous primary
MLR; however, these assays typically measure incorporation of tritiated
thymidine, which only gives a measure of DNA synthesis in vitro 18 .
To characterize the kinetics of alloreactive CD4+ T cell
division in vivo, we used the fluorescein-based dye CFSE. Labeling of
lymphocytes with CFSE yields a homogenously stained population that
fluoresces at wavelengths characteristic of the fluorescein moiety of
CFSE 19 . CFSE-labeled C57BL/6 (H-2b) responder
lymphocytes were transferred into whole-body irradiated C57BL/6,
C3H/HeJ (H-2k), and C3H.SW (H-2b) stimulator
mice. Responder lymphocytes were retrieved from the spleens of
stimulator mice and analyzed for CFSE fluorescence by flow cytometry.
In Fig. 1
, histograms depict CFSE
fluorescence of responder CD4+ T cells at serial time
points after transfer. Because of the established property of CFSE
fluorescence of labeled cells to halve successively with cell division,
in each histogram the undivided parent generation is represented by the
peak of highest CFSE fluorescence and daughter generations of
alloreactive CD4+ T cells are represented by peaks of
diminishing fluorescence on the log scale.
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Compared with the response against major histocompatibility Ags, the
CD4+ T cell response against minor Ags encoded by C3H.SW
stimulators was delayed and the frequency of alloreactive cells was
decreased (Fig. 1
C). Division to the third daughter
generation by a subset of alloreactive cells was detected after 48
h. At subsequent time points, alloreactive cell division continued to
trail the response to major Ags such that seventh and eighth generation
progeny accumulated in stimulator spleens only after 84 h. The
lower frequency of CD4+ T cells dividing after transfer
into minor histocompatibility-mismatched stimulators was apparent
because of the small size of daughter generation peaks relative to the
undivided parent generation peak.
Direct recognition of allogeneic MHC class II by responder cells
We hypothesized that the proliferative responses of alloreactive
responder cells resulted from direct recognition of allogeneic MHC
class II on the surface of APCs of stimulator origin. To evaluate this,
we used stimulator mice deficient in MHC class II. In contrast to the
vigorous proliferation by alloreactive CD4+ T cells after
transfer into wild-type stimulators, no proliferation was detected
60 h after transfer into MHC class II-/- stimulators
(Fig. 2
). This finding confirms that in
this experimental system alloreactive CD4+ T cell
proliferation is dependent upon direct recognition of allogeneic MHC
class II. Indirect recognition of histocompatibility Ags processed and
presented in the context of self-MHC class II by APCs of responder
strain origin does not contribute significantly to the observed
proliferative responses. The practical implication of this finding is
that direct allorecognition per se can be studied in this system
without the potentially confounding presence of the indirect pathway.
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To assess the role of CD28 costimulation in allorecognition of
major and minor histocompatibility Ags, we transferred CFSE-labeled
responder cells from CD28-/- C57BL/6 mice into C3H/HeJ
and C3H.SW stimulators. CD28-/- responder
CD4+ T cells divided less rapidly than wild-type responder
cells in response to major histocompatibility Ags. Although a subset of
alloreactive CD28-/- CD4+ T cells divided to
the sixth daughter generation after 60 h (Fig. 3
A), the proportion of
alloreactive cells dividing maximally at this time point was lower than
that of wild-type responder cells (Fig. 3
B). Despite this
slowing in division kinetics, alloreactive CD28-/-
CD4+ T cells did retain the capacity to divide successively
such that the majority of alloreactive progeny accumulated in the
seventh and eighth daughter generations after 84 h (data not
shown). In contrast, proliferation of responder CD4+ T
cells in response to minor Ags demonstrated marked dependence on CD28
costimulation. Minimal division by CD28-/-
CD4+ T cells was detected 84 h after transfer into
minor histocompatibility-mismatched stimulators (Fig. 4
A), in contrast to division
to seventh and eighth generations by wild-type CD4+ T cells
(Fig. 4
B).
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The precursor frequency values calculated by these means are uniformly higher than previous estimates of 110% for CD4+ T cells alloreactive to major histocompatibility Ags 6, 21, 22 . We attribute this discrepancy to sequestration and enrichment of alloreactive cells in the lymphoid organs of stimulator mice, as has been previously reported 20, 23, 24 . In vitro stimulation of CFSE-labeled responder cells allowed unbiased calculation of alloreactive precursor frequencies for major histocompatibility-mismatched strain combinations that fell within the previously reported range of 110% (data not shown).
Findings from this study highlight the unique signaling processes that lead to recognition of major histocompatibility Ags. T cell activation has been shown to result from the engagement of a threshold number of TCR-Ag interactions, with CD28 costimulatory signals lowering the threshold for activation 25 . Here, recognition of C3H-encoded peptides in the context of allo-MHC rather than self-MHC led to more rapid division by alloreactive CD4+ T cells with decreased requirement for CD28 costimulation. The potency of direct TCR-allo-MHC interactions allowed many CD4+ T cells to achieve their activation threshold without CD28 costimulation. In contrast, TCR-self-MHC interactions failed to achieve activation threshold for the majority of CD4+ T cells specific for minor Ag unless CD28 costimulation was provided.
Blockade of costimulatory pathways has been used to prolong or induce permanent survival of transplanted organs in a number of experimental models, although precisely how these regimens work remains unclear 26, 27, 28, 29 . Our findings indicate that direct recognition of allogeneic MHC is relatively resistant to the blockade of CD28 costimulation, whereas recognition of histocompatibility Ags in the context of self-MHC may be more susceptible to such interventions. Therefore, prolongation of allograft survival by CD28 costimulatory blockade suggests that indirect presentation of histocompatibility Ags may be the dominant pathway of Ag presentation leading to rejection in these transplantation models. Future studies using this experimental system may define how other signaling pathways help set the threshold for allorecognition and allow efficient testing of strategies aimed at abrogating these responses.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. Ali Naji, Department of Surgery, 4th Floor Silverstein Pavilion, Hospital of the University of Pennsylvania, 3400 Spruce Street, Philadelphia, PA 19104. ![]()
3 Abbreviation used in this paper: CFSE, carboxyfluorescein diacetate succinimidyl ester. ![]()
Received for publication December 2, 1998. Accepted for publication December 31, 1998.
| References |
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