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B Lymphopoiesis in the Thymus
William H. Carr and Irving L. Weissman
Koichi Akashi, Lauren I. Richie, Toshihiro Miyamoto,
2000; 164:5221-5226; ;
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Copyright © 2000 by The American Association of
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The Journal of Immunology
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B Lymphopoiesis in the Thymus1
Koichi Akashi,2,3Lauren I. Richie,3Toshihiro Miyamoto, William H. Carr, and
Irving L. Weissman
The thymus has been regarded as the major site of T cell differentiation. We find that in addition to ?? and ?? T cells, a significant
number (?3 ? 104per day) of B220?IgM?mature B cells are exported from the thymus of C57BL/6 mice. Of these emigrating
B cells, we estimate that at least ?2 ? 104per day are cells which developed intrathymically, whereas a maximum of ?0.8 ? 104
per day are cells which circulated through the thymus from the periphery. The thymus possesses a significant number of pro-B
and pre-B cells that express CD19, VpreB, ?5, and pax-5. These B cell progenitors were found in the thymic cortex, whereas
increasingly mature B cells were found in the corticomedullar and medullary regions. Other lymphoid cells, including NK cells
and lymphoid dendritic cells, are not exported from the thymus at detectable levels. Thus, the thymus contributes to the formation
of peripheral pools of B cells as well as of ?? and ?? T cells. The Journal of Immunology, 2000, 164: 5221–5226.
cess resulting in the export of self-tolerant TCR?? T cells and
TCR?? T cells. During their tenure in the thymus, maturing thy-
mocytes interact with the thymic microenvironment, which
provides the appropriate cellular and/or soluble factors to enable
thymocyte proliferation, development, and selection. This micro-
environment includes several types of thymic epithelial cells, bone
marrow-derived dendritic cells, macrophages, bone marrow de-
rived B cells, and several mesenchymal elements. Thymic epithe-
lial cells produce a number of cytokines, including steel factor and
IL-7, both of which play important roles in progenitor prolifera-
tion, survival, and commitment to the T cell lineages (1, 2). Both
steel factor and IL-7 are also critical for the maturation of B cells
in the bone marrow (3–5).
In early mouse fetal life, the first seeded thymic progenitors
collectively include cells capable of T or B cell maturation (6–8).
In adults, the thymus is continuously seeded at a low rate. These
bone marrow-derived precursors have not yet been fully charac-
terized as to their phenotype or function. One candidate is the
recently identified common lymphoid progenitor (CLP),4which
has lymphoid-restricted differentiation potential into T, B, and NK
cells (9), found in adult bone marrow. The CD4lowCD44highc-Kit?
earliest thymic precursor population (10) has been shown to be
capable of differentiating not only into T cells but also into NK
hymocyte progenitors migrate from hematopoietic tis-
sues, such as the bone marrow, to the thymus where they
proliferate, mature, and undergo a stringent selection pro-
cells, B cells, and lymphoid dendritic cells at low frequencies (10–
12). Therefore, one might expect to find some B cells, NK cells,
and dendritic cells maturing within and emigrating from the adult
To test this hypothesis, we analyzed recent thymic emigrants
(RTEs) to peripheral lymphoid organs. FITC was injected intra-
thymically to label cells in the thymus, and at various time points
after labeling, the spleen and lymph nodes were analyzed for the
presence of RTE. We found that, in addition to ?? and ?? T cells,
a significant number (3 ? 104per day) of non-T cells were ex-
ported from the thymus; these were B220?IgM?B cells. We also
demonstrated that the majority of these cells were generated from
precursors in the thymus, whereas a minority could have been
derived from circulating B cells that had entered the thymus.
Materials and Methods
The congenic strains of mice, C57BL6-Ly5.2 or C57BL6-Ly5.1 mice,
were used. The C57BL6-Ly5.1/Ly5.2 mice were made by crossing
C57BL6-Ly5.2 with C57BL6-Ly5.1 (F1). The strains differed only at the
Ly5 allele, and this difference made it possible to detect donor-derived
cells. C57BL6-TCR?-deficient mice (13) were purchased from The Jack-
son Laboratory (Bar Harbor, ME). Mice were bred and maintained in the
animal care facility at Stanford University School of Medicine and were
used at 4–8 wk of age.
Labeling of thymocytes and flow cytometric analysis
The technique for in vivo intrathymic labeling of thymocytes with FITC
has been previously described (14). Briefly, 10 ?l of FITC (1 mg/ml;
Sigma, St. Louis, MO) was injected into both thymic lobes of 5- to 8-wk-
old C57BL/6 mice. Single-cell suspensions were made from the spleen and
the lymph nodes, including cervical, axillary, submandibular, inguinal, bra-
chial, and mesenteric lymph nodes. Cells were stained with PE or allophy-
cocyanin-conjugated anti-IgM (clone 331; obtained from Dr. L. A. Her-
zenberg, Stanford University, Stanford, CA), Mac-1 (M1/70), Gr-1 (8C5),
anti-CD3 (KT31.1), anti-CD4 (GK1.5), anti-CD5 (53-7.8), and/or anti-
CD8 (53-6.7) Abs. Anti-IAb, anti-CD11c, anti-??TCR, and anti-??TCR
Abs were purchased from PharMingen (San Diego, CA). In analyzing
CD4?CD8?RTEs, cells were additionally stained with Texas Red-conju-
gated anti-CD4 and anti-CD8 Abs. For thymic B cell progenitor analysis,
thymocytes were stained with FITC-conjugated anti-CD43 (S7) and anti-
mouse IgD (clone 11-26; obtained from Dr. L. A. Herzenberg); PE; or
Cy5-PE-conjugated anti-mouse IgM, PE-conjugated anti-CD19, allophy-
cocyanin-conjugated B220, and Texas Red-conjugated anti-CD4 and anti-
CD8 Abs (PharMingen). Cells were analyzed by a highly modified five-
color FACS Vantage (Becton Dickinson, Mountain View, CA) (9). Dead
Departments of Pathology and Developmental Biology, Stanford University School of
Medicine, Stanford, CA
Received for publication August 30, 1999. Accepted for publication February
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.
1This work was supported by U.S. Public Health Service Grant CA42551 (to I.L.W.)
and by a grant from the Jose Carreras International Leukemia Society (to K.A.). L.I.R.
is a Howard Hughes Medical Institute predoctoral fellow, and a Stanford Graduate
2Address correspondence and reprint requests to Dr. Koichi Akashi at the current
address: Department of Cancer Immunology and AIDS, Dana-Farber Cancer
Institute, Sm 770B, 44 Binney Street, Boston, MA 02115. E-mail address:
3K.A. and L.I.R. contributed equally to the research described in this paper.
4Abbreviations used in this paper: CLP, common lymphoid progenitor; RTE, recent
Copyright © 2000 by The American Association of Immunologists0022-1767/00/$02.00
by guest on June 13, 2013
cells were excluded by positive staining with propidium iodide and were
detected in a Cy5-PE channel.
Estimation of mature T and B cell immigrants into the thymus
Single spleen cell suspension was obtained from C57BL6-Ly5.1/Ly5.2
mice. Myeloid spleen cells were removed by incubation with Gr-1, Mac-1,
and Ter119 Abs before negative selection using anti-rat IgG-conjugated
immunomagnetic beads (Dynal, Oslo, Norway). The purified 1 ? 108
splenic lymphocytes from adult (Ly5.1 ? Ly5.2)F1mice were injected into
1.5- to 3-wk-old Ly5.1 mice. The percentages of donor-derived mature ??
T cells and B cells in the secondary lymphoid organs were analyzed 7, 18,
and 24 days postinjection, when the injected Ly5.1 mice reached the age of
Sections of the thymus were cut from frozen samples at 4 microns and were
fixed with acetone for 10 min (15). Samples were treated with the Vector
Avidin/Biotin Blocking Kit (Vector Laboratories, Burlingame, CA) and
stained with biotinylated anti-IgM, B220, or MD2 Abs (16). They were
incubated with Streptavidin HRP (Caltag, South San Francisco, CA), vi-
sualized with 3-amino-9-ethylcarbazole as the chromagen for 20 min, and
counterstained with Gill’s hematoxylin (Medical Chemical, Fairfield, NJ).
Isotype-matched rat IgG was used as a negative control.
Total RNA was isolated from 1000 purified thymic pro-B and pre-B cells.
cDNA was analyzed for the presence of pax-5, VpreB, or ?5 by amplifi-
cation of 36, 32, or 32 cycles, respectively. The following primers were
used: pax-5-forward, CTA CAG GCT CCG TGA CGC AG; pax-5-reverse,
TCT CGG CCT GTG ACA ATA GG (Tanneal, 65°C; expected length, 439
bp; Ref. 17); VpreB-forward, GTC TGA ATT CCT CCA GAG CCT AAG
ATC CC; VpreB-reverse, CAG GTC TAG AGC CAT GGC CTG GAC
GTC TG (Tanneal, 60°C; expected length, 400 bp; Ref. 18); ?5-forward,
GGG TCT AGT GGA TGG TGT CC; and ?5-reverse, CAA AAC TGG
GGC TTA GAT GG (Tanneal, 60°C; expected length, 205 bp; Ref. 19).
Recent thymic emigrants include B cells as well as ?? and ?? T
FITC was injected intrathymically to label cells in the thymus, and
the spleen and lymph nodes were later analyzed for the presence of
RTE. After intrathymic FITC injection, the percentage of labeled
cells in the thymus was followed over time (Fig. 1A). Six hours
postinjection, 95% of the thymocytes were FITC?(Fig. 1A) in
comparison with noninjected controls. From 6 to 48 h postinjec-
tion, the percentage of FITC-labeled cells decreased in a nearly
linear fashion. One of the main causes of the loss of FITC might
be cell division, resulting in a 50% decrease in surface labeling per
cell division (20), although it is possible that the loss of FITC label
could be due to normal turnover of membrane proteins in the ab-
sence of division (21). As shown in Fig. 1B, the intensity of FITC
signals at 6 h postinjection was strong enough to detect more than
85% of FITC-labeled cells after one or two cell divisions and
?50% of cells after four cell divisions. Because the estimated
cycling time for thymocytes is ?8–12 h (22), the majority of prog-
eny from cycling FITC-labeled thymocytes might maintain detect-
able levels of FITC at least 24 h after injection. At 24 h postin-
jection, ?84% of thymocytes retained detectable levels of FITC,
indicating that the bulk of thymocytes can be estimated to turn
over in ?6 days. This estimate is in accordance with turnover rates
of thymocytes in previous reports (23). Because only ?1% of thy-
mocytes emigrate per day (14), the major loss of labeled cells is
likely due to apoptosis of thymocytes that either failed positive
selection or were deleted by negative selection (22, 24).
The phenotype of FITC-labeled thymic emigrants in the spleen
is shown in Fig. 2. Approximately 95% of FITC?recent thymic
emigrants were CD4?CD8?or CD4?CD8?single-positive ??T
cells (Fig. 2A). The remaining ?5% of FITC?RTEs were
double-negative cells that were composed of
CD3?TCR???T cells, B220?IgM?mature B cells, and some
CD3?TCR???T cells (Fig. 2C). The majority of B220?IgM?
mature B cells expressed IgD, but only a minority were CD5?
(Fig. 2C). The rate of appearance of FITC-labeled RTEs is almost
linear in the periphery up to 24 h postinjection (Fig. 3). The pro-
files of RTE in the lymph nodes were similar to those in the spleen
(data not shown). Because most lymphocytes collect in the spleen
injection of FITC. A, Decrease in the number of FITC?thymocytes after
intrathymic injection of FITC. The vertical bars represent SD in each time
point of analysis (each point represents five mice). B, Levels of FITC in
thymocytes 6 h after FITC injection. ƒ, Limit of FITC intensity detectable
on FACS. Each vertical broken line represents a scale for a two-fold dif-
ference in FITC intensity. Therefore, the number on top of each vertical
line indicates the time of cell division that allows the cells on the lines to
remain detectable by FACS.
Changes in the FITC level of thymocytes after intrathymic
emigrants in the spleen 24 h after intrathymic in-
jection of FITC. A, FITC?spleen cells contain
CD4?CD8?cells as well as CD4?CD8?and
CD4?CD8?single-positive mature T cells. B,
Gate for the FITC?CD4?CD8?cells. C, Surface
phenotypes of the FITC?CD4?CD8?cells.
Analysis of FITC?recent thymic
5222THYMIC B LYMPHOPOIESIS
by guest on June 13, 2013
and lymph nodes, it can be estimated that the thymus exports at
least 1.2 ? 106?? T cells, ?3 ? 104?? T cells, and ?3 ? 104
B cells per day to the periphery (Fig. 3). The estimated numbers of
?? and ?? T cell emigrants are compatible with previous reports
(14, 25). On the other hand, Mac-1?Gr-1?granulocytes/macro-
phages, CD3?NK1.1?NK cells, and CD8??/?CD11c?MHC-
class II?dendritic cells were not detectable in the RTE (FITC?)
fractions throughout these experiments (Fig. 2C). The lack of
FITC?granulocytes/macrophages indicates that the FITC label
did not leak into the circulation, and therefore, unincorporated
FITC does not persist in a form that would be available for labeling
newly immigrating or circulating cells.
A significant fraction of B cells that are exported from the
thymus developed in the thymus
It is important to know whether the export of B cells from thymus
reflects B cell production in the thymus or results from FITC la-
beling of B cells that have migrated into the thymus from the
periphery. We tested whether or not peripheral B cells could re-
enter the thymus and contribute to the thymic B cell pool. We
injected i.v. 1 ? 108splenic lymphocytes from adult (Ly5.1 ?
Ly5.2)F1mice into Ly5.1 mice and evaluated the percentages of
donor-derived mature ?? T cells and B cells in the secondary
lymphoid organs 7, 18, and 24 days postinjection. The peak chi-
merism for donor-derived cells was seen at 18 days postinjection.
Approximately 4% of ?? T cells in peripheral organs were of
donor origin, whereas only 0.01% of mature ?? T cells in the
thymus were of donor origin (Table I). The rare reentry of these T
cells into the thymus is compatible with previous reports (26, 27).
On the other hand, ?5–7% of B cells in peripheral organs were of
donor origin, whereas only ?0.6% of thymic mature B cells were
of donor origin (Table I and Fig. 4). If 100% of peripheral B cells
could be replaced by donor-derived B cells, then 0.6 ? (100/
5?7) ? 12?15% of thymic B cells could be replaced by B cells
of donor origin. Therefore, a maximum of (12?15%) ? (?5 ?
104) ? 0.6?0.8 ? 104thymic B cells could have migrated from
the periphery. Even if all such B cell immigrants could leave the
thymus in 1 day, a maximum of 3 ? 104? 0.8 ? 104? ?2 ? 104
thymic B cell emigrants that developed intrathymically would be
exported each day.
B cell progenitors in the thymus
It has been reported that B cell progenitors are present in the thy-
mus and that isolated thymic B cell progenitors can intrathymically
differentiate into mature B cells after reinjection into the thymus
(28). The phenotype of thymic B cell progenitors is similar to that
of bone marrow B cell progenitors (29). In the C57BL/6 strain, the thy-
mus contains significant numbers of immature B220?CD43?IgM?
pre-B cells (?1.8 ? 104/thymus) and B220?CD43?IgM?pro-B cells
(?1.2 ? 104/thymus) (Fig. 5A and Table II). These thymic B220?
B cell progenitors coexpressed CD19 but not NK1.1 (Fig. 5B). The
thymic IgM?B cells were composed of IgD?and IgD?B cells as
in bone marrow B cells. Although thymic B cells are reported to
express a broad range of CD5 in the C3H mouse strain (28, 30, 31),
organs 18 days after injection of mature splenic lymphocytes into 2-wk-old
mice (Ly5.1?Ly5.2?). Percentages of donor-type B220?IgM?B cells
(Ly5.1?Ly5.2?) are 5–7% in lymph nodes, blood, and spleen but only
0.6% in the thymus. Data from a representative mouse are shown (see
Donor-derived mature B cells (Ly5.1?Ly5.2?) in lymphoid
Table I. Chimerism of mature T and B cells 18 days after injection of
Percentage of Donor-Derived Cells
Mature B cells 5.3 ? 1.2
Mature T cells
6.7 ? 1.6
3.7 ? 1.6
5.1 ? 1.8
3.9 ? 0.9
0.6 ? 0.2
0.01 ? 0.0024.1 ? 1.3
aA total of 1 ? 108myeloid cell-depleted (Mac-1?Gr-1?Ter119?) splenocytes
from C57BL/6 (Ly5.1/5.2) mice were injected i.v. into C57BL/6 (Ly5.2) mice.
B220?IgM?B cells and TCR??highT cells were analyzed. Data are shown as the
mean ? SD in four chimeric mice.
T (A), ?? T (B), and B cell (C) emi-
grants in the spleen and lymph nodes.
The vertical bars represent SD in each
time point of analysis (each point rep-
resents five mice).
Numbers of FITC???
5223The Journal of Immunology
by guest on June 13, 2013
CD5 expression was almost limited to the pro-B cell fraction in the
C57BL/6 strain thymi (Fig. 5B).
The thymic pro-B and pre-B cells were purified and analyzed for
expression of early B cell-related molecules such as VpreB and ?5
as well as a transcription factor, pax-5. All of these were detectable
in the thymic pro-B cells and were increasingly expressed in pre-B
cells (Fig. 6). The expression pattern of these molecules was con-
sistent with that in bone marrow pro-B and pre-B cells (data not
Thymic B lymphopoiesis was significantly enhanced in C57BL/
6-TCR?-deficient mice in which thymic T cell development is
impaired. The TCR?-deficient thymi had more than a 100-fold
increase in the frequencies (Fig. 5A) and more than a 10-fold in-
crease in absolute numbers of both mature B cells and B cell pro-
genitors in comparison to wild-type thymi (Table II).
In immunohistochemical stainings of normal and TCR?-defi-
cient thymi, IgM?B cells reside mainly in the corticomedullar
junction and in the medulla, whereas B220?cells were found
throughout the thymus, indicating that immature B220?IgM?B
cells mainly reside in the cortex through the corticomedullar junc-
tion (Fig. 7). Accordingly, thymic B lymphopoiesis might occur
concomitantly with B cell progenitor migration from the cortex to
the medulla of the thymus, mirroring T lymphopoiesis (32).
We demonstrate that the adult C57BL mouse thymus physiologi-
cally generates and exports a significant number of B cells into the
peripheral B cell pool. The findings of B cell development in the
thymus and the recently described ?? T cell development in
the bone marrow (33, 34) challenge the paradigm of lymphocyte
classification into bone marrow-derived B cells and thymus-de-
rived T cells. Our study also suggests that the thymus may not
actively export NK cells and dendritic cells into the periphery or
that the level of export may be below the level of detection in this
The thymus possesses B cell progenitors of each stage that are
similar to those in bone marrow. This suggests that the thymic
microenvironment can fully support B as well as T cell maturation.
Adult thymi have been shown to be capable of generating B cells
after intrathymic injection of hematopoietic stem cells or CLPs (9,
35). Although it has not been shown that the CLPs themselves
migrate from the bone marrow to the thymus, it is possible that
CLPs or one of their immediate offspring could migrate to and seed
the thymus. We have previously reported that IL-7, which is pre-
sumably secreted from thymic epithelial cells, promotes survival
of thymocytes undergoing positive selection through the up-regu-
lation of Bcl-2 (1, 4). IL-7 is essential also for Ig gene rearrange-
ment in developing B cells (5). Because IL-7 is known to exist in
the thymic milieu, IL-7 would be available for the intrathymic
development of B cells as well as T cells (36). Therefore, the
thymic microenvironment should be able to physiologically sup-
port B lymphopoiesis as well as T lymphopoiesis.
Recent studies have shown that, in CD3-? transgenic mice (37)
and Notch1-deficient mice (38), the number of thymic B cells sig-
nificantly increases in association with severe impairment of thy-
mic T cell development. It is possible that alterations of these
genes could skew the commitment of immature thymic progenitors
toward the B cell lineage (37, 38). However, we demonstrate in
cell compartments in either C57BL/6- or C57BL/
6-TCR?-deficient mice. A, B220/IgM profiles of
CD4?CD8?thymocytes (upper panels) and
B220/CD43 profiles of CD4?CD8?IgM?thy-
mocytes (lower panels). B, Additional surface
phenotypic analysis of thymic B cell progenitors:
the CD19/CD43 (a) and CD5/CD43 (b) profiles
of B220?IgM?cells, the B220/NK1.1 profile of
CD4?CD8?cells, and the IgM/IgD profile of
Phenotypic analysis of thymic B
sorted CD4?CD8?IgM?B220?CD43?pro-B cells and CD4?CD8?IgM?-
B220?CD43?pre-B cells increasingly expressed pax-5, VpreB, and ?5.
RT-PCR analysis of sorted thymic B cell progenitors. The
Table II. Numbers and frequencies of B cell compartments in the thymus
Absolute Numbers/Thymus (? 104)a
Pro-BPre-B Mature B
1.20 ? 0.10 (0.006)b
10.60 ? 2.77 (0.530)
1.81 ? 0.14 (0.010)
25.20 ? 3.93 (1.260)
4.96 ? 0.14 (0.028)
84.12 ? 6.15 (4.206)
aData are shown as the mean ? SD in four mice in each strain.
bPercentage of total cells.
5224THYMIC B LYMPHOPOIESIS
by guest on June 13, 2013
this study that the disruption of T cell development simply by a
TCR? gene knockout also results in a relative and absolute in-
crease in thymic B cell compartments. Accordingly, these data
collectively suggest that in normal thymi, rapid expansion of T
cells might take up most of the microenvironmental niches, pre-
venting efficient thymic B cell maturation.
The presence of B cell development in and export from the
thymus has potential implications for the immune system. For ex-
ample, B cells that develop in the thymus may have a different
repertoire of Ig receptors than do bone marrow-derived B cells,
resulting from local Ags and stimuli mediating their positive and
negative selection. This may allow a more diverse set of Ig recep-
tors in the periphery. It is also possible that the thymic B cells may
contribute to the cellular interactions that mediate positive and
negative selection of maturing T cells (39), including Ig isotype
and idiotype as selecting elements (40). Thus, it is important to
clarify the role of these ectopically but physiologically developed
thymic B cells in the immune system.
We thank L. Jerabek for capable laboratory management, L. Hidalgo and B.
Lavarro for animal care, V. Braunstein for Ab preparation, and D. Dalma-
Weiszhausz for critically reviewing the manuscript.
1. Akashi, K., M. Kondo, U. von Freeden-Jeffry, R. Murray, and I. L. Weissman.
1997. Bcl-2 rescues T lymphopoiesis in interleukin-7 receptor-deficient mice.
2. Maraskovsky, E., L. A. O’Reilly, M. Teepe, L. M. Corcoran, J. J. Peschon,
andA. Strasser. 1997. Bcl-2 can rescue T lymphocyte development in interleu-
kin-7 receptor-deficient mice but not in mutant rag-1?/?mice. Cell 89:1011.
3. Corcoran, A. E., F. M. Smart, R. J. Cowling, T. Crompton, M. J. Owen, and
A. R. Venkitaraman. 1996. The interleukin-7 receptor ? chain transmits distinct
signals for proliferation and differentiation during B lymphopoiesis. EMBO J.
4. Kondo, M., K. Akashi, J. Domen, K. Sugamura, and I. L. Weissman. 1997. Bcl-2
rescues T lymphopoiesis, but not B or NK cell development, in common ? chain-
deficient mice. Immunity 7:155.
5. Corcoran, A. E., A. Riddell, D. Krooshoop, and A. R. Venkitaraman. 1998. Im-
paired immunoglobulin gene rearrangement in mice lacking the IL-7 receptor.
6. Peault, B., I. Khazaal, and I. L. Weissman. 1994. In vitro development of B cells
and macrophages from early mouse fetal thymocytes. Eur. J. Immunol. 24:781.
7. Kawamoto, H., K. Ohmura, and Y. Katsura. 1998. Presence of progenitors re-
stricted to T, B, or myeloid lineage, but absence of multipotent stem cells, in the
murine fetal thymus. J. Immunol. 161:3799.
8. McKenna, H. J., and P. J. Morrissey. 1998. Flt3 ligand plus IL-7 supports the
expansion of murine thymic B cell progenitors that can mature intrathymically.
J. Immunol. 160:4801.
9. Kondo, M., I. L. Weissman, and K. Akashi. 1997. Identification of clonogenic
common lymphoid progenitors in mouse bone marrow. Cell 91:661.
10. Wu, L., M. Antica, G. R. Johnson, R. Scollay, and K. Shortman. 1991. Devel-
opmental potential of the earliest precursor cells from the adult mouse thymus.
J. Exp. Med. 174:1617.
11. Matsuzaki, Y., J. Gyotoku, M. Ogawa, S. Nishikawa, Y. Katsura, G. Gachelin,
and H. Nakauchi. 1993. Characterization of c-kit positive intrathymic stem cells
that are restricted to lymphoid differentiation. J. Exp. Med. 178:1283.
12. Wu, L., C. L. Li, and K. Shortman. 1996. Thymic dendritic cell precursors:
relationship to the T lymphocyte lineage and phenotype of the dendritic cell
progeny. J. Exp. Med. 184:903.
13. Mombaerts, P., A. R. Clarke, M. A. Rudnicki, J. Iacomini, S. Itohara,
J. J. Lafaille, L. Wang, Y. Ichikawa, R. Jaenisch, M. L. Hooper, et al. 1992.
Mutations in T-cell antigen receptor genes ? and ? block thymocyte development
at different stages. Nature 360:225.
14. Scollay, R. G., E. C. Butcher, and I. L. Weissman. 1980. Thymus cell migration:
quantitative aspects of cellular traffic from the thymus to the periphery in mice.
Eur. J. Immunol. 10:210.
15. Gutman, G. A., and I. L. Weissman. 1972. Lymphoid tissue architecture: exper-
imental analysis of the origin and distribution of T-cells and B-cells. Immunology
16. Small, M., W. Van Ewijk, A. M. Gown, and R. V. Rouse. 1989. Identification of
subpopulations of mouse thymic epithelial cells in culture. Immunology 68:371.
17. Adams, B., P. Dorfler, A. Aguzzi, Z. Kozmik, P. Urbanek, I. Maurer-Fogy, and
M. Busslinger. 1992. Pax-5 encodes the transcription factor BSAP and is ex-
pressed in B lymphocytes, the developing CNS, and adult testis. Genes Dev.
18. Rolink, A., E. ten Boekel, F. Melchers, D. T. Fearon, I. Krop, and J. Andersson.
1996. A subpopulation of B220?cells in murine bone marrow does not express
CD19 and contains natural killer cell progenitors. J. Exp. Med. 183:187.
19. Delassus, S., I. Titley, and T. Enver. 1999. Functional and molecular analysis of
hematopoietic progenitors derived from the aorta-gonad-mesonephros region of
the mouse embryo. Blood 94:1495.
20. Akashi, K., M. Kondo, and I. L. Weissman. 1998. Two distinct pathways of
positive selection for thymocytes. Proc. Natl. Acad. Sci. USA 95:2486.
21. Butcher, E. C., and I. L. Weissman. 1980. Direct fluorescent labeling of cells with
fluorescein or rhodamine isothiocyanate. I. Technical aspects. J. Immunol. Meth-
22. Akashi, K., and I. L. Weissman. 1996. The c-kit?maturation pathway in mouse
thymic T cell development: lineages and selection. Immunity 5:147.
23. Scollay, R., and D. I. Godfrey. 1995. Thymic emigration: conveyor belts or lucky
dips? Immunol. Today 16:268.
24. Surh, C. D., and J. Sprent. 1994. T-cell apoptosis detected in situ during positive
and negative selection in the thymus. Nature 372:100.
25. Kelly, K. A., M. Pearse, L. Lefrancois, and R. Scollay. 1993. Emigration of
selected subsets of ???T cells from the adult murine thymus. Int. Immunol.
26. Michie, S. A., E. A. Kirkpatrick, and R. V. Rouse. 1988. Rare peripheral T cells
migrate to and persist in normal mouse thymus. J. Exp. Med. 168:1929.
27. Agus, D. B., C. D. Surh, and J. Sprent. 1991. Reentry of T cells to the adult
thymus is restricted to activated T cells. J. Exp. Med. 173:1039.
28. Mori, S., M. Inaba, A. Sugihara, S. Taketani, H. Doi, Y. Fukuba, Y. Yamamoto,
Y. Adachi, K. Inaba, S. Fukuhara, and S. Ikehara. 1997. Presence of B cell
progenitors in the thymus. J. Immunol. 158:4193.
29. Hardy, R. R., C. E. Carmack, S. A. Shinton, J. D. Kemp, and K. Hayakawa. 1991.
Resolution and characterization of pro-B and pre-pro-B cell stages in normal
mouse bone marrow. J. Exp. Med. 173:1213.
cal staining of a TCR?-deficient
thymus. The thymus from a TCR?-
deficient mouse was stained with
MD-2 Abs that react with medullar
epithelial cells (A), B220 Abs (B),
and anti-IgM Abs (C). Cx, Cortex;
5225 The Journal of Immunology
by guest on June 13, 2013
30. Miyama-Inaba, M., S. Kuma, K. Inaba, H. Ogata, H. Iwai, R. Yasumizu, Download full-text
S. Muramatsu, R. M. Steinman, and S. Ikehara. 1988. Unusual phenotype of B
cells in the thymus of normal mice. J. Exp. Med. 168:811.
31. Inaba, M., K. Inaba, Y. Adachi, K. Nango, H. Ogata, S. Muramatsu, and
S. Ikehara. 1990. Functional analyses of thymic CD5?B cells: responsiveness to
major histocompatibility complex class II-restricted T blasts but not to lipopoly-
saccharide or anti-IgM plus interleukin 4. J. Exp. Med. 171:321.
32. Weissman, I. L. 1973. Thymus cell maturation: studies on the origin of cortisone-
resistant thymic lymphocytes. J. Exp. Med. 137:504.
33. Dejbakhsh-Jones, S., L. Jerabek, I. L. Weissman, and S. Strober. 1995. Ex-
trathymic maturation of ?? T cells from hemopoietic stem cells. J. Immunol.
34. Garcia-Ojeda, M. E., S. Dejbakhsh-Jones, I. L. Weissman, and S. Strober. 1998.
An alternate pathway for T cell development supported by the bone marrow
microenvironment: recapitulation of thymic maturation. J. Exp. Med. 187:1813.
35. Spangrude, G. J., and R. Scollay. 1990. Differentiation of hematopoietic stem
cells in irradiated mouse thymic lobes: kinetics and phenotype of progeny. J. Im-
36. Montecino-Rodriguez, E., A. Johnson, and K. Dorshkind. 1996. Thymic stromal
cells can support B cell differentiation from intrathymic precursors. J. Immunol.
37. Tokoro, Y., T. Sugawara, H. Yaginuma, H. Nakauchi, C. Terhorst, B. Wang, and
Y. Takahama. 1998. A mouse carrying genetic defect in the choice between T and
B lymphocytes. J. Immunol. 161:4591.
38. Radtke, F., A. Wilson, G. Stark, M. Bauer, J. van Meerwijk, H. R. MacDonald,
and M. Aguet. 1999. Deficient T cell fate specification in mice with an induced
inactivation of Notch1. Immunity 10:547.
39. Inaba, M., K. Inaba, M. Hosono, T. Kumamoto, T. Ishida, S. Muramatsu,
T. Masuda, and S. Ikehara. 1991. Distinct mechanisms of neonatal tolerance
induced by dendritic cells and thymic B cells. J. Exp. Med. 173:549.
40. Avery, A. C., Z. S. Zhao, A. Rodriguez, E. K. Bikoff, M. Soheilian, C. S. Foster,
and H. Cantor. 1995. Resistance to herpes stromal keratitis conferred by an
IgG2a-derived peptide. Nature 376:431.
5226 THYMIC B LYMPHOPOIESIS
by guest on June 13, 2013