Thymic Lineage Commitment Rather Than
Selection Causes Genetic Variations in Size of
CD4 and CD8 Compartments
This information is current as
of June 15, 2017.
Joost P. M. van Meerwijk, Teresa Bianchi, Samuel Marguerat and
H. Robson MacDonald
J Immunol 1998; 160:3649-3654; ;
http://www.jimmunol.org/content/160/8/3649
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References
Thymic Lineage Commitment Rather Than Selection Causes
Genetic Variations in Size of CD4 and CD8 Compartments
Joost P. M. van Meerwijk,1 Teresa Bianchi, Samuel Marguerat,2 and H. Robson MacDonald3
During their development, immature CD41CD81 thymocytes become committed to either the CD4 or CD8 lineage. Subsequent
complete maturation of CD41 and CD81 cells requires a molecular match of the expressed coreceptor and the MHC specificity
of the TCR. The final size of the mature CD41 and CD81 thymic compartments is therefore determined by a combination of
lineage commitment and TCR-mediated selection. In humans and mice, the relative size of CD41 and CD81 peripheral T cell
compartments shows marked genetic variability. We show here that genetic variations in thymic lineage commitment, rather than
TCR-mediated selection processes, are responsible for the distinct CD4/CD8 ratios observed in common inbred mouse strains.
Genetic variations in the regulation of lineage commitment open new ways to analyze this process and to identify the molecules
involved. The Journal of Immunology, 1998, 160: 3649 –3654.
Ludwig Institute for Cancer Research, Lausanne Branch, University of Lausanne,
1066 Epalinges, Switzerland
Received for publication October 27, 1997. Accepted for publication December
8, 1997.
The costs of publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked advertisement in accordance
with 18 U.S.C. Section 1734 solely to indicate this fact.
1
Current address: National Institute of Health and Medical Research (INSERM)
U395, Purpan Hospital, and University “Paul Sabatier,” Toulouse, France.
2
Current address: Department of Genetics and Microbiology, University of Geneva
Medical School, Geneva, Switzerland.
3
Address correspondence and reprint requests to Dr. H. Robson MacDonald, Ludwig
Institute for Cancer Research, Lausanne Branch, University of Lausanne, 1066 Epalinges, Switzerland.
Copyright © 1998 by The American Association of Immunologists
tory mouse strains C57BL/6 and DBA/2. These strains were chosen because of the availability of relatively large numbers of
C57BL/6 3 DBA/2 recombinant inbred (BXD RI)4 strains as well
as congenic strains expressing different MHC and TCR alleles.
Our results indicate that the distinct relative sizes of the CD4 and
CD8 compartments in these mice are determined by genetic variations in the process of thymic lineage commitment rather than by
TCR-mediated positive or negative selection.
Materials and Methods
Mice
DBA/2, C57BL/6, C57BL/10, B10.D2, and (C57BL/6 3 DBA/2)F1 mice
were obtained from Harlan Sprague Dawley, Zeist, Netherlands. Recombinant inbred C57BL/6 3 DBA/2 mice (BXD RI) were purchased from
The Jackson Laboratory, Bar Harbor, ME. C57BL/6 Vba and DBA/2 Vba
mice were provided by H. Bour and P. Brawand and originally obtained
from Dr. A. Livingstone, Basel Institute for Immunology, Basel, Switzerland. These mice carry the TCR Vba locus (containing a deletion of several
Vb regions as well as point mutations in others (15, 16)) derived from
C57L mice and were backcrossed for 15 generations.
Cytofluorometric analysis
Three- and four-color flow cytometric analyses of thymocytes were performed
using the following mAbs: anti-TCRb-FITC (H57-597 (PharMingen, San Diego, CA); anti-CD8b-FITC (H35-17.2) (17); anti-CD5.1-FITC (H11-86.1,
PharMingen); anti-HSA/CD24-FITC (M1/69) (18); anti-CD69-FITC
(H1.2F3) (19); anti-CD4-PE (H129.19, Boehringer Mannheim, Mannheim,
Germany); anti-TCRb-PE (H57-597, PharMingen); anti-CD4-Red613
(H129.19, Life Technologies, Gaithersburg, MD); anti-CD8-Red613 (53-6.7,
Life Technologies); anti-CD8-APC (53.6.7, PharMingen).
Cell cycle analysis was performed on electronically sorted
CD41CD82TCRhigh and CD42CD81TCRhigh cells as well as on unpurified thymocytes. Cells (0.5–10 3 104) were permeabilized in 0.2% Nonidet
P-40 and DNA stained with 50 mg/ml propidium iodide. During analysis,
doublets were excluded using pulse processing.
Cytofluorometric analysis was performed using FACScan and FACStarPlus flow cytometers (Becton Dickinson, San Jose, CA).
Bone marrow chimeras
Mixed bone marrow chimeras were produced as follows: lethally irradiated
(900 rad g irradiation, Cs137 source) DBA/2 and B10.D2 hosts were reconstituted the next day by i.v. injection of a mixture of 1 to 2 3 107
B10.D2 and DBA/2 bone marrow cells (1:1 ratio) that were T cell depleted
using anti-Thy1 IgM AT83 (20) and complement (Saxon Europe, Suffolk,
4
Abbreviations used in this paper: BXD RI, C57BL/6 3 DBA/2 recombinant inbred
strain; HSA, heat-stable Ag.
0022-1767/98/$02.00
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T
he peripheral T lymphocyte repertoire is shaped by a variety of TCR-dependent and -independent mechanisms. In
the thymus, only immature thymocytes expressing a TCR
that is capable of recognition of self MHC/peptide complexes survive the process of positive selection (reviewed in Refs. 1 and 2).
This mechanism is thought to select useful T lymphocytes from an
immature pool of precursors with an estimated frequency of MHC
reactivity of only 5% (3). Potentially autoreactive thymocytes are
physically deleted by induction of apoptosis upon activation by
thymic professional APCs (reviewed in Refs. 4 and 5). The latter
process (negative selection) eliminates approximately half of the
positively selectable thymocytes (6, 7). Finally, in peripheral lymphoid organs, T lymphocytes need to interact with MHC molecules
to survive (8 –10).
While complete maturation of thymocytes depends on a molecular match of the expressed coreceptor and MHC class specificity
of the TCR, recent data on transgenic and mutant mice and on in
vitro differentiation systems indicate that the initial CD4 vs CD8
lineage choice by CD41CD81 precursors probably is independent
of the MHC class specificity of the TCR (reviewed in Ref. 1).
Lineage commitment appears, therefore, to be of a stochastic nature rather than instructed by TCR-MHC interactions.
The relative size of the peripheral CD4 and CD8 T cell compartments is known to be genetically determined in mice and man
(11–14). The contribution of TCR-mediated selection and/or lineage commitment to this genetic variation is unknown. We report
here a detailed analysis of the mechanism(s) responsible for the
distinct CD4/CD8 ratios observed in the commonly used labora-
3650
GENETIC VARIATIONS IN THYMOCYTE CD4/CD8 LINEAGE COMMITMENT
cytes from C57BL/6 and DBA/2 mice. Again, we observed a
slightly increased percentage of mature CD41CD82TCRhigh cells
in DBA/2 mice as compared with C57BL/6 animals, while
CD42CD81TCRhigh thymocytes were present at a lower frequency (Fig. 1). The CD4/CD8 ratio was approximately 1.5-fold
higher in DBA/2 as compared with C57BL/6 mice (Fig. 1). Therefore, the distinct CD4/CD8 ratios observed in peripheral T lymphocytes in these mouse strains originate in the thymus.
The difference in CD4/CD8 ratios is not due to proliferation or
thymic retention
U.K.). Mice were kept on antibiotic containing water (0.2% Bactrim,
Roche, Basel, Switzerland) until analysis at 6 wk postengraftment.
Analysis of BXD RI mice was performed using hemopoietic chimeras
produced by reconstituting lethally irradiated (C57BL/6 3 DBA/2)F1 hosts
with T cell-depleted bone marrow derived from BXD RI (as well as control
C57BL/6 and DBA/2) mice. Per bone marrow origin, three hosts were
injected.
Analysis of recombinant inbred strains
Thymocytes from BXD RI hemopoietic chimeras were analyzed 6 wk after
engraftment. Ratios of CD41CD82TCRhigh to CD42CD81TCRhigh thymocytes and their mean and SD were determined. Assignment of the observed CD4/CD8 ratio phenotype to that of DBA/2 or C57BL/6 was determined using Student’s t test. The allelic distributions of the TCRa and
CD8a genes were retrieved from the Mouse Genome Database (MGD),
Mouse Genome Informatics, The Jackson Laboratory (World Wide Web
URL: http://www.informatics.jax.org, June, 1997).
Results and Discussion
Distinct CD4/CD8 ratios in C57BL/6 and DBA/2 mice originate
in the thymus
Differences in the numeric ratios of CD41 to CD81 T lymphocytes
in the lymph nodes of different mouse strains have been described
previously (13, 14). We investigated whether the difference in
CD4/CD8 ratio between C57BL/6 and DBA/2 mice is due to extrathymic or intrathymic mechanisms. As shown in Figure 1, the
percentage of CD41 cells among T lymphocytes in the blood of
DBA/2 mice was slightly higher than that in C57BL/6 mice. In
contrast, fewer CD81 T cells were found among PBL of DBA/2
than of C57BL/6 mice. Accordingly, the ratio of CD41 to CD81
T lymphocytes was significantly (;2-fold) higher in DBA/2 mice
than in C57BL/6 animals (Fig. 1). This difference in CD4/CD8
ratios could be due to peripheral and/or intrathymic mechanisms.
To distinguish between these possibilities, we analyzed thymo-
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FIGURE 1. Distinct C57BL/6 and DBA/2 CD4/CD8 ratios in peripheral blood T lymphocytes originate in the thymus. Peripheral blood and
thymocytes of C57BL/6 and DBA/2 mice were incubated with anti-TCR,
anti-CD4, and anti-CD8 Abs, and percentages of mature CD41 and CD81
cells as well as CD4/CD8 ratios were determined by cytofluorometry. In
the analysis of PBL, 3 animals were used per mouse strain, while the data
for thymocytes are compiled from four experiments with a total of 13 mice
per strain. The difference in thymocyte CD4/CD8 ratios between C57BL/6
and DBA/2 is highly significant (Student’s t test, p , ,0.0001).
Differences in steady state representations of mature thymocyte
populations may be caused by variations in the generation, thymic
retention, or proliferation of those cells. To distinguish between
these possibilities, we examined cell cycle status as well as phenotype of mature thymocytes using markers in which expression
levels change during final maturation.
The mature CD41 and CD81 thymocyte populations are known
to contain dividing cells (21, 22). We investigated whether this
intrathymic proliferation could account for the difference in CD4/
CD8 ratios observed in C57BL/6 and DBA/2 mice. Electronically
sorted CD41CD82TCRhigh and CD42CD81TCRhigh thymocytes
from the two mouse strains were detergent permeabilized and their
DNA content analyzed by flow cytometry (Fig. 2A). The proportion of cycling mature CD41 and CD81 thymocytes was similar in
C57BL/6 and DBA/2 mice. Therefore, differences in proliferation
of mature thymocytes in C57BL/6 and DBA/2 mice cannot account for the differences in CD4/CD8 ratio.
CD69 is an early lymphocyte activation marker that is transiently
up-regulated during thymic positive selection (23–25). CD69 expression is gradually extinguished on CD41CD82TCRhigh and
CD42CD81TCRhigh thymocytes. No gross differences in the proportion of CD69low cells among mature CD41 or CD81 thymocytes in
DBA/2 vs C57BL/6 mice were observed (Fig. 2B). Moreover, when
considering only the least mature CD69high CD41CD82TCRhigh and
CD42CD81TCRhigh cells, a relative difference in the CD4/CD8 ratios of C57BL/6 vs DBA/2 mice was found that was similar to that
obtained using CD4, CD8, and TCR as criteria (Fig. 2B), suggesting
that this difference is not caused by increased retention of mature
thymocytes.
The expression of heat-stable Ag (HSA, CD24), a marker
highly expressed on immature thymocytes, is also down-modulated during final maturation of CD41CD82TCRhigh and
CD42CD81TCRhigh thymocytes (26, 27). Moreover, in contrast to the more mature HSAlow thymocytes, immature
TCRhighHSAhigh cells are known to respond very poorly to mitogenic stimuli. The proportion of HSAlow cells among
CD41CD82TCRhigh and CD42CD81TCRhigh thymocytes was
indistinguishable in C57BL/6 vs DBA/2 mice (Fig. 2C). Again,
the ratio of the least mature HSAhigh CD41CD82TCRhigh to
CD42CD81TCRhigh cells was significantly higher in DBA/2
than in C57BL/6 mice (Fig. 2C), and the relative difference in
the ratios between these mouse strains was similar to that obtained using thymocytes gated on CD4, CD8, and TCR only
(Fig. 1). Collectively, these data indicate that the difference in
the CD4/CD8 ratio between C57BL/6 and DBA/2 mice is not
caused by increased retention or proliferation of mature thymocytes but rather by variations in the differentiation of those
cells, i.e., thymocyte CD4/CD8 lineage choice and/or TCR-mediated selection processes.
The Journal of Immunology
3651
Distinct CD4/CD8 ratios develop independently of MHC
haplotype
The processes of thymic positive and negative selection depend on
interactions between the clonotypic TCR and its MHC/peptide ligand. The different MHC haplotypes expressed by DBA/2 (H-2d)
and C57BL/6 (H-2b) mice may therefore cause variations in positive and/or negative selection and may thus explain the different
CD4/CD8 ratios. To investigate this possibility, we analyzed
C57BL/10 congenic mice expressing products of the DBA/2-derived MHC locus (B10.D2). C57BL/10 (B10) mice are genetically
very similar to C57BL/6 mice and had a thymic CD4/CD8 ratio
similar to that of C57BL/6 animals (Figs. 1 and 3). The CD4/CD8
ratio in B10.D2 mice was similar to that of B10 rather than DBA/2
animals. Therefore, the different CD4/CD8 ratios observed in
DBA/2 and C57BL/10 mice are dictated by genetic background
rather than by MHC haplotype.
from the two types of precursor cells (Fig. 4). The CD4/CD8 ratio
of B10.D2 and DBA/2 thymocytes developing in DBA/2 hosts was
similar to that of thymocytes from normal B10.D2 and DBA/2
mice, respectively (Fig. 4). Moreover, the relative difference in the
CD4/CD8 ratios between B10.D2 and DBA/2 cells was also conserved in B10.D2 hosts, although individual CD4/CD8 ratio values
were slightly higher (Fig. 4). Therefore, thymocyte-intrinsic factors rather than variations in thymic microenvironment clearly determine the difference in CD4/CD8 ratio between B10.D2 and
DBA/2 mice.
Difference in CD4/CD8 ratio is determined by thymocyteintrinsic factors
Antigenic peptides and superantigens presented by MHC molecules are also known to be involved in thymic positive and negative selection processes (reviewed in Refs. 2, 28, and 29). Moreover, during development thymocytes interact with a variety of
APC and thymic stromal cells (reviewed in Ref. 30). Since both
factors probably vary between the C57BL/6 and DBA/2 mouse
strains, we sought to investigate their involvement in the generation of the different CD4/CD8 ratios.
Radiation bone marrow chimeras were produced in which
B10.D2 and DBA/2 thymocytes developed in an identical microenvironment. Lethally irradiated B10.D2 and DBA/2 hosts were
reconstituted with a mixture of DBA/2 and B10.D2 bone marrow
cells at a ratio of 1:1. The allelic difference in CD5 between B10
and DBA/2 mice (31) was used to distinguish thymocytes derived
FIGURE 3. MHC haplotype does not influence CD4/CD8 ratio. Thymocytes from C57BL/10, DBA/2, and B10.D2 mice were analyzed by flow
cytometry using anti-TCR, anti-CD8, and anti-CD4 Abs. Percentages of
CD41CD82TCRhigh and CD42CD81TCRhigh thymocytes and CD4/CD8
ratios were determined. Depicted are mean values 6 SD (n 5 5). The
CD4/CD8 ratio in DBA/2 mice is significantly higher than in C57BL/10
( p , 0.001) and B10.D2 ( p , 0.001) mice.
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FIGURE 2. Differentiation rather than proliferation or retention of thymocytes causes variation in CD4/CD8 ratio. A, Total thymocytes and
electronically sorted CD41CD82 TCRhigh and
CD42CD81TCRhigh cells (purity . 95%) were
detergent permeabilized, and DNA content was
assessed by propidium iodide staining and flow
cytometry. Statistics indicate mean percentage
(6SD, n 5 5) of cells within depicted gates (S 1
G2/M). Thymocytes were labeled with anti-CD4,
anti-CD8, anti-TCR, and anti-CD69 (B) or antiHSA (C) Abs. CD69 and HSA fluorescence histograms of electronically gated CD41CD82
TCRhigh and CD42CD81TCRhigh thymocytes
are shown. Numbers indicate mean percentage
(6SD, n 5 5) of events within depicted CD69 or
HSA gates among total, CD41CD82 TCRhigh,
and CD42CD81 TCRhigh thymocytes. Bar
graphs indicate mean ratios (6SD) of percentage
of CD69highCD41CD82TCRhigh to CD69high
CD42CD81TCRhigh (B) and HSAhighCD41
CD82TCRhigh to HSAhigh CD42CD81TCRhigh
thymocytes (C). CD4/CD8 ratios of DBA/2 cells
are significantly higher than those of C57BL/6
thymocytes shown in B ( p , ,0.001) and C
( p , ,0.001).
3652
GENETIC VARIATIONS IN THYMOCYTE CD4/CD8 LINEAGE COMMITMENT
FIGURE 4. Distinct CD4/CD8 ratios are determined by a thymocyteintrinsic mechanism. Lethally irradiated B10.D2 and DBA/2 hosts were
reconstituted with a mixture of bone marrow cells from the two strains at
a ratio of 1:1. Six weeks later, thymocytes were analyzed by four-color
flow cytometry using anti-CD5.1, anti-TCR, anti-CD4, and anti-CD8 Abs.
CD4/CD8 contour plots of CD5.12 (B10.D2-derived) and CD5.11 (DBA/
2-derived) thymocytes are depicted with the CD41CD82 and CD42CD81
gates used to analyze TCR expression (not shown). Numbers indicate percentages (6SD, n 5 5) of CD41CD82TCRhigh and CD42CD81TCRhigh
cells among CD5.12 or CD5.11 thymocytes. Bar graphs depict CD4/CD8
ratios (6SD) calculated for each individual chimera. The CD4/CD8 ratio
of B10.D2 thymocytes is significantly lower that that of DBA/2 cells in
B10.D2 ( p , 0.01) and DBA/2 ( p , ,0.001) hosts.
Difference in CD4/CD8 ratio is TCRab independent
Since the difference in CD4/CD8 ratio between C57BL/6 and
DBA/2 mice seems to be determined by a thymocyte intrinsic
mechanism(s), we subsequently analyzed surface molecules
known to be involved in TCR-dependent clonotypic selection
mechanisms. The major thymocyte surface molecule implicated in
positive and negative selection is the clonotypic TCRab heterodimer. To our knowledge, no differences between the products
of the TCRb loci of DBA/2 and C57BL/6 mice have been reported. The b-chain-encoding segments of the two loci have not
been completely sequenced, however, and differences may have
remained undetected. To investigate whether the TCRb locus influences the development of different CD4/CD8 ratios in the two
strains of mice, we analyzed DBA/2 and C57BL/6 congenic mice
expressing the products of an identical, C57L-derived TCRb locus
(Vba) containing a deletion of several Vb segments as well as
point mutations in others (15, 16). A similar difference in thymic
CD4/CD8 ratios was observed in C57BL/6 Vba vs DBA/2 Vba
and C57BL/6 Vbb vs DBA/2 mice Vbb (Fig. 5). Moreover,
C57BL/6 Vba and DBA/2 Vba-derived thymocytes had CD4/CD8
ratios indistinguishable from those of C57BL/6 and DBA/2 mice
(Fig. 5). Therefore, the difference in CD4/CD8 ratio between
C57BL/6 and DBA/2 mice cannot be explained by variations in
expressed TCR Vb segments. Interestingly, despite the very significant (approximately twofold) difference in the number of Vb
segments encoded by the Vba and Vbb alleles (16), similar CD4/
CD8 ratios develop, reinforcing the concept that CD4/CD8 ratios
develop in a selection independent manner.
Inasmuch as allelic differences between the DBA/2 and
C57BL/6 TCR Ca and Va loci have been reported (32, 33), we
also investigated whether this variation causes different CD4/CD8
ratios in C57BL/6 and DBA/2 mice using BXD RI strains. To
synchronize thymocytes developing in the several BXD RI strains
and to limit our analysis to thymocyte-intrinsic factors, we produced hemopoietic chimeras reconstituting (C57BL/6 3 DBA/
2)F1 hosts with BXD RI-derived hemopoietic cells. Chimeras with
CD4/CD8 ratios that were assignable to the C57BL/6 or DBA/2
parental strains (11 of 22 total) are shown in Figure 6. The distribution of CD4/CD8 ratios among the BXD RI chimeric thymocytes did not correlate with that of the TCRa alleles. Therefore, the
observed difference in CD4/CD8 ratio is not caused by allelic differences in the DBA/2 and C57BL/6 TCRa loci. Taken together,
the data indicate that variations in the expressed TCR are not responsible for the development of different CD4/CD8 ratios in
C57BL/6 and DBA/2 mice.
Variation in the CD4/CD8 ratio develops independently of CD4
and CD8
Coreceptors (CD4 and CD8) are known to influence positive and
negative selection processes. Thus, differences in expression level
of coreceptors change the balance between no selection and positive or negative selection (34 –38). Since expression levels of coreceptors are similar in C57BL/6 and DBA/2 mice (data not
shown), we analyzed whether allelic differences in the encoding
genes might be responsible for the different thymic CD4/CD8 ratios. To our knowledge, no allelic differences in the products of the
CD4 and CD8b genes of these two mouse strains have been reported. However, DBA/2 and C57BL/6 mice express different
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FIGURE 5. TCR Vb repertoire does not influence CD4/CD8 ratio. Thymocytes from C57BL/6 and DBA/2 congenic mice expressing TCR Vba or
Vbb alleles were incubated with anti-CD4, anti-CD8, and anti-TCR Abs
and the ratio of CD41CD82TCRhigh to CD42CD81TCRhigh cells determined by flow cytometry. Graphs depict mean values 6 SD (C57BL/6
Vba/Vbb, n 5 5; DBA/2 Vba/Vbb, n 5 3).
The Journal of Immunology
3653
exact distribution of the C57BL/6 and DBA/2 alleles of Notch1 in
the BXD RI strains is not known, it has been reported that three
recombinations occurred between Notch1 and Ass1 (40). Since the
CD4/CD8 ratio differs in at least eight BXD RI strains from the
inherited Ass1 allele (data not shown), the Notch1 molecule itself
does not seem to determine CD4/CD8 ratio. Nevertheless, it remains possible that other molecules involved in Notch1 signaling
(e.g., Numb (41) and Jagged (42)) are involved.
Analysis of thymocyte CD4/CD8 lineage commitment has been
performed in vivo using mice with targeted mutations of several
genes or expression of a variety of transgenes and in vitro using
thymocyte differentiation assays (reviewed in Ref. 1). Our data
indicating genetic variations in CD4/CD8 lineage choice in common laboratory mouse strains open new avenues to analyzing this
process and to identifying the molecules involved.
CD8a alleles (CD8a.1 and CD8a.2, respectively) (31), and the
distribution of the two alleles in the BXD RI strains is known. As
shown in Figure 6, the distribution of the CD4/CD8 ratios among
BXD RI thymocytes in hemopoietic chimeras does not correlate
with that of the CD8a alleles. Moreover, BALB/c mice, which
express the same CD8a and CD8b alleles as C57BL/6 animals
(31), have a CD4/CD8 ratio at least as high as that in DBA/2 mice
(data not shown and Refs. 13 and 14). Therefore, the difference in
CD4/CD8 ratios observed in C57BL/6 vs DBA/2 mice appears not
to be due to differences in expression level or expressed alleles of
CD4 and/or CD8.
Collectively, our data indicate that the distinct relative sizes of
the CD4 and CD8 compartments observed in C57BL/6 and DBA/2
mice are not due to differences in positive and/or negative selection, since the involvement of TCR ligands, thymic microenvironment, and the TCR complex itself has been excluded. Indeed, large
variations in expression of MHC and TCR genes do not significantly alter the CD4/CD8 ratio, indicating that it develops independently of thymic selection processes.
The distinct CD4/CD8 ratios observed in C57BL/6 and DBA/2
mice are most readily explained by the variation in the process of
CD4 vs CD8 lineage commitment, although other possibilities cannot formally be excluded. Recently, extensive analysis of the
mechanism underlying this process has revealed that the MHC
class specificity of the TCR expressed by thymocytes probably
does not determine their CD4 vs CD8 lineage choice (reviewed in
Ref. 1). Rather, the correlation between the MHC class specificity
of the TCR and the lineage in which the thymocyte can successfully develop is proposed to be determined by TCR specificitydependent selection mechanisms following lineage commitment.
Our data indicating that a TCR selection-independent mechanism
determines CD4/CD8 ratio in normal, unmanipulated mice supports current stochastic models of lineage commitment.
It has recently been suggested that the CD4/CD8 lineage choice
may be determined by the activity of Notch1 (39). Although the
Acknowledgments
We thank Dr. Alexandra Livingstone (Basel Institute for Immunology,
Basel, Switzerland) for generously providing C57BL/6 Vba and DBA/2
Vba congenic mice. These mice were bred in our facilities and provided by
Hélène Bour and Pierre Brawand. Pierre Zaech is acknowledged for expert
flow cytometry.
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FIGURE 6. TCRa and CD8a do not determine CD4/CD8 ratio. Lethally irradiated (C57BL/6 3 DBA/2)F1 hosts were reconstituted with
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CD8a is included for comparison.
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