Correspondence to: B.J. Fowlkes, Bldg. 4, Rm. 111, Laboratory of Cellular and Molecular Immunology, National Institutes of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892-0420. Tel:301-496-5530 Fax:301-402-4891 E-mail:bfowlkes{at}nih.gov.
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Abstract |
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The T cell receptor (TCR) and the pre-TCR promote survival and maturation of early thymocyte precursors. Whether these receptors also influence
versus
ß lineage determination is less clear. We show here that TCR
gene rearrangements are suppressed in TCR
ß transgenic mice when the TCR
ß is expressed early in T cell development. This situation offers the opportunity to examine the outcome of
versus
ß T lineage commitment when only the TCR
ß is expressed. We find that precursor thymocytes expressing TCR
ß not only mature in the
ß pathway as expected, but also as CD4-CD8- T cells with properties of
lineage cells. In TCR
ß transgenic mice, in which the transgenic receptor is expressed relatively late, TCR
rearrangements occur normally such that TCR
ß+CD4-CD8- cells co-express TCR
. The results support the notion that TCR
ß can substitute for TCR
to permit a
lineage choice and maturation in the
lineage. The findings could fit a model in which lineage commitment is determined before or independent of TCR gene rearrangement. However, these results could be compatible with a model in which distinct signals bias lineage choice and these signaling differences are not absolute or intrinsic to the specific TCR structure.
Key Words: lineage commitment, TCR transgenic mice, thymus, differentiation, positive selection
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Introduction |
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The thymus is able to generate distinct types of mature T cells that are differentiated for specific TCR recognition and effector functions. Early in development, precursor thymocytes rearrange and express the genes encoding TCRs and mature as either ß or
lineage T cells (for reviews, see references (1), 2). The first T cells are
lineages that arise only in the fetal thymus. Each of these bears a unique, canonical TCR and colonizes distinct epithelial tissues of the periphery. The
T cells that populate the lymphoid organs have more diverse receptors and develop in both the fetal and adult thymus. Lymphoid
T cells and precursors to the
ß T cell lineage (bearing the pre-TCR) appear roughly around the same time in the adult thymus and are thought to derive from a common CD4-CD8- precursor. The productive rearrangement and expression of the TCR
or of the pre-TCR (a heterodimer of TCRß with invariant pT
) is critical for survival and further differentiation of these early thymocytes (3). Of major interest is whether these receptors play a role in
ß versus
lineage determination or only in the progression of already committed precursors (4) (5).
The pathways of ß and
T cell development are quite distinct. Although discrete stages of
development have not been identified, most
lineage T cells never express the CD4 or CD8
ß coreceptors and have no requirement for MHC for maturation (6) (7). In contrast, precursor CD4-CD8- thymocytes expressing the pre-TCR proliferate, upregulate TCR
rearrangement, and progress to a CD4+CD8+ intermediate stage (3). If rearrangement of TCR
is productive, TCR
replaces pT
to form the mature TCR
ß. Recognition of MHC by TCR
ß is required for the development of mature
ß lineage T cells, expressing either CD4 or CD8. The development of an additional subset of
ß T cells, the so-called NK T cells, is ß2-microglobulin (ß2m) dependent (8) (9). This minor population of T cells expresses either CD4 or no coreceptor, a restricted TCR repertoire, and is not detected until after birth. Although the lineage relationship of NK T cells to conventional
ß T cells is somewhat controversial, NK T cells have characteristic phenotypic and functional properties that clearly distinguish them from other T cell subsets (9).
With the advent of TCRß transgenic mice, a novel population of TCR
ß+CD4-CD8- (TCR
ßDN)1 T cells was observed (10) (11) (12) (13). These cells appear early in the fetal thymus, colonize both epithelial and lymphoid tissues, and are especially prominent in TCR
ß transgenic mice undergoing strong negative selection. Naturally, questions arose as to their origin and lineage relationship to other T cells. There was speculation that these cells could be related to the TCR
ß1CD4-CD8- cells of wild-type mice (NK T cells) or to the abnormal TCR
ß1CD4-CD8- cells observed in lpr mutant mice (14). Others suggested that they derive from conventional
ß T cells after the downregulation of CD4 or CD8 (14) (15) or that they mature in the
ß lineage without ever expressing the CD4/CD8 coreceptors (16).
Evidence that the TCRßDN T cells mature in a lineage separate from conventional
ß T cells came from studies of transgenic HY TCR mice. In contrast to the CD8 T cells of these mice, the TCR
ßDN cells do not express endogenous TCR
genes, their TCR
gene segments are not deleted (17), and they do not develop in mice deficient for the common cytokine receptor
chain (18). TCR
ßDN cells mature in the absence of the selecting MHC and, most noteworthy, in HY TCR mice with a pT
null mutation (pT
-/-), a few TCR
ßDN cells coexpress endogenous TCR
and the transgenic TCR
ß (17). Given these characteristics, it was proposed that TCR
ßDN cells of TCR
ß transgenic mice belong to the
lineage. In this model, the transgenic TCR
ß replaces TCR
while still allowing
lineage development. This model was contested, however, in an additional report using DO11.10 TCR transgenic mice (16). Since TCR
ßDN cells required specific MHC for development, the authors hypothesized that these cells were
ß lineage T cells that mature without passing through the CD4+CD8+ intermediate stage of development.
In previous studies, there was only limited characterization of TCRßDN cells of TCR
ß transgenic mice, making it difficult to determine their relationship to conventional T cell subsets. As no single marker can distinguish
lineage T cells (with the exception of the TCR itself), we examined TCR
ßDN cells using a number of criteria (phenotype, function, development, and localization). An analysis of several strains of TCR
ß transgenic mice reveals that TCR
ßDN cells clearly exhibit characteristics of
lineage T cells. The MHC requirements for maturation and the regulation of TCR gene rearrangement are distinctly different in TCR
ßDN cells than in conventional
ß lineage T cells. The results indicate that the premature expression of TCR
ß allows thymocyte precursors to mature in the
lineage. These findings have implications for models of
/
ß lineage determination.
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Materials and Methods |
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Mice.
C57BL/6 (B6), C57BL/10 (B10), B10.A, B10.Q, and B10.D2 mice were obtained from a National Institutes of Allergy and Infectious Diseases contract to Taconic Farms, Inc., and B10.BR and BALB/c, from The Jackson Laboratory. TCRß transgenic mice were backcrossed, intercrossed, and selected as described previously (19) to obtain H-2b, H-2k, H-2d, H-2q, H-2b recombination activating gene (RAG)-2-/-, H-2q RAG-2-/-, or H-2b MHC class II+/-CD4+/- AND TCR mice (20) (21) (22) (23); H-2d and H-2b class II-/- DO11.10 TCR mice (24); and H-2d HA TCR mice (25). H-2b and H-2d HY TCR mice (26) were obtained by backcrossing 12 times to B10 and then to B10.D2; H-2b and H-2k 5CC7 TCR mice, by crossing B6 5CC7 TCR mice (27) to B10 or B10.A; and H-2b and H-2b class I-/- P14 TCR mice (28), by backcrossing 10 times to B6 and then to ß2m-/- (29). Except where noted, all TCR
ß transgenic mice were on the positive-selecting MHC background: AND TCR (H-2b or H-2k), 5CC7 TCR (H-2k), DO11.10 TCR (H-2d), HY TCR (H-2b), and P14 TCR (H-2b). TCR
transgenic mice included the G8 TCR mice (H-2b ß2m-/-) crossed and selected as described (7), or H-2b TG78 TCR mice (30), backcrossed eight times to B6.
Fetal mice were obtained from timed matings. The day of finding a vaginal plug was designated as day 0 of embryonic development. Mice were bred and maintained in a National Institutes of Allergy and Infectious Diseases Research Animal Facility or on a National Institutes of Allergy and Infectious Diseases contract to Taconic Farms, Inc., according to American Association of Accreditation of Laboratory Animal Care specifications. All protocols for animal studies were approved by the National Institutes of Allergy and Infectious Diseases Animal Care and Use Committee.
Cell Preparation, Antibodies, and Flow Cytometry.
Cultured cell lines used for these studies included: DN7.3 (TCRV2/V
5), a mouse CD4-CD8- T cell/BW5147 hybridoma, and DCEK, a mouse L cell fibroblast line transfected with E
Ebk. Thymocytes, LNs, and LN T cells were prepared in single cell suspensions as described previously (31). For enrichment of heat stable antigen (HSA)lo (CD24lo) thymocytes, a culture supernatant of anti-HSA (J11d) antibody was used with a 1:10 dilution of Lo-ToxM rabbit complement (Cedarlane) and DNase (106 U/ml; Calbiochem). For magnetic bead isolation of CD4-CD8- thymocytes or LN T cells, 107 cells were reacted with 250 µl of purified H129.19 and 53-6.7 (and RA3-6B2, for LN T cells) antibodies (30 min, 4°C). CD4+CD8+ cells were removed by treatment with sheep antirat IgG-coated magnetic beads (30 min, 4°C) at a 5:1 bead to cell ratio, using an MPC-1 magnetic particle concentrator (Dynal). This process was repeated at a 10:1 bead to cell ratio. Epidermal lymphocytes were isolated and prepared in a single cell suspension as described (32). Trypsinized surface antigens were resynthesized in overnight culture with 20 U/ml recombinant IL-2 (Genzyme). To enrich for viable cells, harvested cells were incubated with biotin-labeled goat antihamster IgG (Caltag) (30 min, 4°C), washed twice, and bound to streptavidin-coated magnetic beads (Miltenyi Biotech) (30 min, 4°C). Cells were passed over a MACS column (Miltenyi Biotech) and the nonadherent fraction was collected.
Antibodies and staining reagents included: antiTCRßFITC, PE or allophycocyanin (APC) (H57-597), anti TCR-FITC, -PE, or unlabeled (GL3), anti-CD4FITC, PE, APC, or CyChrome (RM4-5), anti-CD8
CyChrome or unlabeled (53-6.7), anti-CD8ß.2FITC (53-5.8), antiIL-2RßFITC (TM-ß1), anti-NK1.1PE (PK136), anti-CD5FITC (53-7.3), anti-V
11 TCRFITC or unlabeled (RR8-1), anti-V
2FITC or PE (B20.1), anti-AbFITC or PE (AF6-120.1), anti-EkFITC (14-4 (45)), antiH-2KdFITC (SF1-1.1), antiH-2KkFITC (36-7 (5)), antiH-2KbFITC (AF6-88.5), antiH-2KqFITC (KH-114), and anti-CD45R/B220FITC, PE, or unlabeled (RA3-6B2), all obtained from BD PharMingen; anti-CD8
FITC, PE, or biotin (CT-CD8a), anti-CD4biotin (YTS 191.1), Thy 1.2FITC or PE (5a-8), streptavidin-APC or -TriColor, goat antimouse IgG1PE, goat antimouse IgG2aFITC, all obtained from Caltag; goat antirat IgGFITC (Kirkegaard & Perry); rat antimouse IgG1FITC and streptavidin-FITC (Zymed Laboratories); and anti-CD24 (J11d), anti-HY TCR (T3.70), anti-HA TCR (6.5), and anti-DO11.10 TCR (KJ-126) culture supernatants.
Cells were stained and analyzed by flow cytometry and/or electronically sorted using standard protocols (33). For some analyses, cells were pretreated with an unlabeled anti-FcR culture supernatant (24G2) to block Fc receptor binding of the labeled antibodies. Multicolor flow cytometry was performed on a FACS® 440, FACSCaliburTM, FACStarPlusTM, or FACS VantageTM (Becton Dickinson). Dead cells were excluded by light scatter and propidium iodide gating. 150,000 events were collected for three- and four-color analyses. For live-gated samples, 10,00020,000 CD4-CD8- events were collected. Isolation of thymocyte and LN T cell subsets by electronic cell sorting was performed on a FACStarPlusTM (Becton Dickinson) or an EPICS 753 (Beckman Coulter).
For typing of transgenic or mutant mice, peripheral blood lymphocytes were stained with labeled antibody to the appropriate surface antigen, counterstained with Thy1.2 or B220 (used for live gating for T or B cells, respectively). After staining, samples were depleted of red blood cells with ACK lysing buffer (pH 7.4) and analyzed by flow cytometry.
In Vitro TCR Stimulation for Proliferation, Induction of CD8 Expression, and IL-4 Secretion.
For TCR stimulation, cells were added to U-bottomed 96-well plates coated with anti-TCR antibodies as described (31). Proliferation was determined on day 3 of culture, measuring [3H]thymidine incorporation (1 µCi/ml pulse for 18 h). Coexpression of CD8 and CD8ß was assessed on day 4 of culture by flow cytometry. IL-4 production was assayed by specific ELISA (34) using 100 µl of supernatant collected at day 3 of culture and stored at -20°C.
Radiation Bone Marrow Chimeras.
Bone marrow chimeras were made as described by reconstituting irradiated recipients (1,000 rads, Cs source) with T-depleted bone marrow (19). For the cyclosporine A (CsA) experiments, reconstituted mice received daily intraperitoneal injections of 0.4 or 0.6 mg SandimmuneTM CsA (Sandoz) in 100 µl olive oil (Bertolli Classico) or of 100 µl olive oil only, starting on day 3 after reconstitution.
Quantitative PCR.
T cell subsets were isolated by electronic cell sorting. 105 sorted cells were digested using 1x PCR Buffer (PerkinElmer), 2.5 mM MgCl2, 20 mg/ml proteinase K, 0.05% Tween 20, and 20% InstaGene Matrix (Bio-Rad Laboratories) at 56°C (2 h), followed by boiling (10 min). PCR was performed using a reaction mixture containing 1x PCR Buffer (PerkinElmer), 2.5 mM MgCl2, 200 µM each dNTP, 12.5 pmol each primer, and 0.25 U native Taq polymerase (PerkinElmer) bound to anti-Taq (CLONTECH Laboratories, Inc.). The total reaction volume was 50 µl with 5 µl of DNA. Samples were incubated at 95°C (5 min); amplified for 40 cycles at 94°C (30 s), 56°C (1 min), and 72°C (1.5 min), and incubated at 72°C (10 min) using a 96-well plate in a PTC-100 thermocycler (MJ Research, Inc.). Aliquots of 5 µl were removed every three cycles beginning at cycle 18.
The following primers and probes were used: V2, TGTCCTTGCAACCCCTACCC; J
1, TGTTCCTTCTGCAAATACCTTG; V
2 probe, GAGGAAGAAGACGAAGCTATC; 5' C
1, TTACAGACAAAAGGCTTGAGTC; 3' C
1, GTTCTCATGTTTGACAATACATCTG; and C
1 probe, CTGAAGACTAACGACACATAC.
Quantitation was performed using a modified ELISA as described (35). In brief, one primer for each gene was labeled with a 5' biotin moiety allowing capture of the PCR product on an avidin-coated plate. The second strand was denatured with 0.1 M NaOH and an FITC-labeled probe was bound to the captured strand. Bound probe was detected with an anti-FITC labeled with alkaline phosphatase in the presence of substrate, CSPD (Tropix). Chemiluminescence was measured using a luminometer (Dynatech).
To estimate the relative frequency of V2J
1 rearrangements in the experimental populations, a standard curve generated by titrating DN7.3 cells (containing three V
2J
1 rearrangements per cell (36)) with DCEK fibroblast cells and amplifying the serially diluted samples in the same PCR. For each sample of 105 cells, a PCR ELISA was performed and the quantity of PCR product (in light units) was determined as a function of cycle number (1839 cycles). Primers and probes specific for C
1 were used to normalize the amount of DNA present. Data from the luminometer were fit to a logistic equation, and the parameters were used to calculate the cycle value at half-maximum (C50) of amplification (35). C50 values were plotted against the corresponding log10 cell number of DN7.3 cells in each input sample and a best-fit line was generated. C50 values for experimental samples obtained in the same assay could be matched to this best-fit line to estimate the relative frequency of V
2J
1 rearrangements.
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Results |
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CD4-CD8- T Cells of TCRß Transgenic Mice Have Properties of
Lineage T Cells.
Wild-type mice bear two CD4-CD8- subpopulations of mature T cells, one bearing TCRß (referred to as NK T cells) and the other, TCR
. In contrast, an analysis for TCR on CD4-CD8- T cells of HY TCR (TCR
ß) transgenic mice reveals no TCR
+ and a larger than usual population of TCR
ß+ cells (37). Also in contrast to CD4 or CD8
ß lineage T cells, the CD4-CD8- T cells of HY TCR and 2B4 TCR transgenic mice express only the transgenic TCR
and no endogenous TCR
(17) (38). Because of these unusual features, we further characterized the TCR
ßDN subset of AND TCR and other TCR transgenic mice to assess lineage properties relative to normal T cell subsets.
TCRßDN cells were analyzed for phenotype and function and compared with the NK T,
T, and the major CD4 and CD8
ß T cell subsets of wild-type mice, as well as CD4-CD8-TCR
+ cells of TCR
transgenic mice (TG78) (30). As shown previously (8), freshly isolated NK T cells of B6 mice (TCR
ßDN) express IL-2R (CD122) and NK 1.1, and produce high amounts of IL-4 in response to in vitro TCR stimulation (Fig 1A and Fig B, and Fig 2). In contrast, the TCR
ßDN population of AND TCR mice expresses lower levels of these markers and produces no IL-4 ((39); Fig 1A and Fig B, and Fig 2). The TCR
ßDN cells also express relatively lower levels of CD5, delineating this subset from mature CD4 and CD8 T cells, but not from TCR
+ T cells (Fig 1 C). Together these phenotypic and functional properties distinguish TCR
ßDN cells from NK T and the major
ß lineage, but not from
lineage T cells.
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T cells appear early in adult T cell development, before the
ß lineage T cells (40) (41) (42). To assess when the TCR
ßDN cells arise in thymic development, we generated hematopoietic stem cell chimeras using bone marrow from AND TCR mice to reconstitute irradiated recipients. Between days 10 and 15 after reconstitution, we observed a population of V
11+CD4-CD8- followed by V
11+ CD4+CD8+ thymocytes. By days 1820, mature CD4+ CD8- thymocytes develop (data not shown). On day 15, transgenic TCR+ (V
11) CD4-CD8-, and CD4+CD8+ thymocytes were sorted and stimulated in vitro using anti-V
11 antibody (Fig 3, a and b). The V
11+CD4-CD8- thymocytes are competent to incorporate [3H]thymidine in response to anti-V
11 cross-linking while V
11-bearing CD4+CD8+ thymocytes are not. Therefore, like
T cells, TCR
ßDN T cells appear well before the CD4+ CD8- thymocytes and much earlier than NK T cells that arise after the CD4+CD8- and CD4-CD8+ thymocytes in wild-type mice (8).
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Previous studies (43) (44) (45) have indicated that the development of the major ß T cell subsets (CD4/CD8), as well as the minor TCR
ß+CD4-CD8- (NK T) subset of wild-type mice, are inhibited by CsA.
lineage T cells are relatively less sensitive. To further assess lineage properties, TCR
ßDN cells of AND TCR mice were tested for sensitivity to CsA, administered over the course of adult T cell development. Irradiated recipients reconstituted with AND TCR bone marrow were treated daily with CsA for 5 wk, after reconstitution. As shown in Table 1, the development of V
11+CD4-CD8- TCR+ is up to 75-fold less sensitive to CsA than are V
11+CD4+CD8- thymocytes. These data indicate that TCR
ß+DN cells are relatively resistant to CsA administered during development, as are
lineage T cells.
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Since CD4-CD8- TCR+ thymocytes of wild-type mice can be induced to express CD8
after in vitro activation ((46); Fig 4 c), we tested the ability of mature TCR
ßDN cells to make this response. As shown in Fig 4, a and b, V
11+CD4-CD8-, but not V
11+CD4+ T cells, are induced to express CD8
in response to anti-TCR stimulation. Similar responses have been obtained from TCR
ßDN splenocytes of TCR
transgenic mice (39). Thus, by all of the criteria we examined, TCR
ßDN cells are clearly distinguished from conventional
ß lineage and NK T cells, and most resemble
lineage T cells.
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TCRß DN Cells of TCR
ß Transgenic Mice Do Not Require MHC for Development.
One of the hallmarks of ß T cell development is the requirement for MHC-specific positive selection (47). In contrast,
T cells fully mature in the absence of MHC (6) (7). Since there are conflicting reports on the selection requirements of TCR
ßDN cells (10) (12) (16), we tested several strains of TCR
ß transgenic mice, bearing MHC class I or class IIspecific TCRs. As shown in Fig 5, the TCR
ßDN cells of five different strains of TCR
ß mice develop equally well in the positively selecting or in the neutral (nonselecting) MHC background. Development is comparable both in percentage (Fig 5) and in absolute number (data not shown). Thus, TCR
ßDN cells show no MHC dependence for development, in clear contrast to mainstream
ß lineage T cells (CD4-CD8+ or CD4+CD8-) of the same mice that show an absolute requirement for specific MHC. These findings argue against the view that TCR
ßDN cells derive from conventional CD4 or CD8 T cells by the downregulation of a coreceptor.
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TCRßDN Cells of Some Strains Coexpress Endogenous TCR
and Transgenic TCR
ß.
The analyses above indicate that TCRßDN cells have
lineage properties. Therefore, CD4-CD8- T cells of several strains of TCR
ß transgenic mice were analyzed for expression of TCR
. An obvious population of CD4-CD8- thymocytes and peripheral T cells bearing only the transgenic TCR
ß was apparent in all of the mice analyzed. In some strains, however, there existed a second subset of CD4-CD8- T cells coexpressing the transgenic TCR
ß and endogenous TCR
(Fig 6 and Table 2). This latter subset bearing both TCRs was most prominent in the P14 TCR mice. It is noteworthy that like the TCR
ßDN subset of AND TCR mice, both TCR
ßDNbearing subsets of P14 TCR mice exhibited properties of
lineage T cells (Fig 1 and data not shown). It was previously reported that TCR
+ cells develop in P14 TCR mice (48); however, it was not appreciated that these T cells coexpress the transgenic TCR
ß.
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These different patterns of TCR expression prompted us to investigate the timing of transgenic TCRß expression during fetal thymic ontogeny, using the AND and P14 TCR mice as prototypes. As shown in Fig 7, AND TCR is expressed early on a majority of E14 thymocytes. In contrast, the P14 TCR is first detected around E1516, and then only on a minor subset of fetal thymocytes. These data, considered together with the data from adult thymocytes in Fig 6, suggest that very early expression of the transgenic TCR
ß inhibits endogenous TCR
gene rearrangement and/or expression.
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Endogenous TCR Gene Rearrangements Are Suppressed in TCR
ßDN Cells of AND, but Not in TCR
ßDN Cells of P14 TCR Mice.
To determine the basis for differences in TCR expression in TCRßDN cells of AND and P14 TCR mice, TCR
gene rearrangements were examined using a quantitative PCR assay. Since TCRV
2 is commonly used by lymphoid
T cells (49), the frequency of TCRV
2
J
1 rearrangement was determined in mature T cell subsets (Fig 8). The analyses indicate that this gene rearrangement is much more suppressed in TCR+CD4- CD8- (TCR
ßDN) cells of AND TCR than of P14 TCR mice. Similar differences between AND TCR and P14 TCR mice were observed with other V
and V
gene segments, although the rearrangement frequencies were much lower (data not shown). Of note, the occurrence of V
2
J
1 rearrangement in TCR+CD4-CD8- T cells of P14 TCR mice is equivalent to those of TCR
+ cells of G8 TCR
(V
2+) transgenic mice and of B6 wild-type mice (Fig 8 a). Thus, in the P14 TCR mice that express the transgenic receptor relatively late, TCR
rearrangement is uninhibited and TCR
ßDN cells bearing TCR
are observed (Fig 6 Fig 7 Fig 8).
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Interestingly, an analysis of the frequency of V2
J
1 rearrangements in CD4 T cells of AND TCR mice is increased in the semiselecting (class II+/-, CD4+/-) or nonselecting (H-2q) MHC background in comparison over the frequency in the selecting MHC (H-2b) background (Fig 8 b). These results fit with the notion that MHC engagement terminates RAG expression during
ß development (50). In contrast, the TCR
ßDN cells developing in the CD4-CD8- (
) pathway follow different rules since rearrangement frequency is independent of MHC (Fig 8 a). These findings suggest that TCR gene rearrangement is differentially regulated in the
and
ß lineages.
TCR Is Expressed by Skin Lymphocytes of AND TCR Mice.
TCR gene rearrangements in thymocyte precursors that localize to skin epithelium occur much earlier in fetal development than those destined for migration and residence in the lymphoid tissues (2). Therefore, there was the possibility that the dendritic epidermal T lymphocytes of AND TCR mice would express TCR
since some of their thymic precursors may have rearranged TCR
before transgenic TCR
ß expression. In contrast to the lymphoid CD4-CD8- T cells that fail to express TCR
(Fig 6), skin lymphocytes express two subsets of T cells (Fig 9), one expressing TCR
ß alone and the second expressing both TCR
ß and TCR
. Thus, when TCR
ß transgene expression occurs after endogenous TCR
rearrangements, rearrangement is not suppressed, and TCR
and TCR
ß can be expressed by the same cells. We determined in parallel analyses that these cells are CD4-CD8-, V
11+, Vß3+, and V
3+.
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In the periphery of normal mice, the canonical V3+ TCR is expressed exclusively on skin lymphocytes (2). The finding that T cells, bearing the AND TCR and coexpressing the expected TCR
, can home at the right time to what is normally a
-specific site, provides additional evidence that TCR
ßDN cells are
lineage T cells. Presumably, the skin lymphocytes expressing only the transgenic TCR
ß have an out of frame TCR
or, alternatively, some cells express the transgenic TCR early enough to suppress endogenous TCR
rearrangements. In any case, the finding that even the TCR
ß+TCR
- subpopulation is able to traffic to this traditionally
-specific site demonstrates that skin homing is not dependent on the canonical TCR. These data, like those above, reveal that when the TCR is expressed early (regardless of whether it is TCR
or TCR
ß), the receptor allows
lineage commitment and maturation in the
lineage.
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Discussion |
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These studies examine T cell development in transgenic mice with premature expression of TCRß. An interesting feature of the mice is a population of mature CD4-CD8- thymocytes and peripheral T cells, expressing only the transgenic TCR
ß (TCR
ßDN). To determine whether TCR
ßDN cells belong to the
ß or
lineage, we analyzed these cells in several TCR transgenic strains and compared them to the T cell subsets of normal mice. By all criteria examined, the TCR
ßDN cells clearly exhibit characteristics of
lineage T cells. The lack of a coreceptor, the level of CD5, and the early maturation delineate TCR
ßDN cells from the major TCR
ß+ CD4 and CD8 T cell subsets. TCR
ßDN cells do not express NK1.1 or IL-2Rß (CD122) or produce IL-4, distinguishing them from the NK T cells of wild-type mice. In contrast, TCR
ßDN cells are similar to
T cells since their development is early, is relatively insensitive to CsA, and is MHC independent. Also, like
lineage cells, TCR
ßDN cells can be induced to express CD8
homodimers in response to anti-TCR stimulation. Most notable, in TCR
ß strains where the transgenic receptor is expressed later in development, CD4-CD8- T cells arise coexpressing the transgenic TCR
ß and endogenous TCR
(Table 2, and Fig 6 and Fig 7). TCR
ßDN cells with both receptors exhibit the same phenotype and properties as those lacking TCR
expression. These findings provide the most direct evidence that TCR
ßDN cells are
lineage T cells.
The different patterns of TCR expression in CD4-CD8- T cells of TCRß mice appear to be related to the timing of TCR
ß transgene expression with respect to endogenous TCR
gene rearrangement. As modeled in Fig 10, the early expression of transgenic TCR
ß in precursor thymocytes of AND TCR mice causes suppression of endogenous TCR
gene rearrangement; nevertheless, the transgenic receptor allows continued maturation in the CD4-CD8- (
) pathway. In P14 TCR mice, the transgenic receptor is expressed later such that TCR
gene rearrangements occur normally. If rearrangements are productive, mature CD4-CD8- T cells emerge coexpressing the TCR
ß (P14 TCR) and endogenous TCR
(Fig 6 Fig 7 Fig 8, and Table 2). The different TCR expression patterns in skin versus lymph node CD4-CD8- T cells of AND TCR mice also can be explained by this model. A subset of epidermal lymphocytes coexpresses the transgenic TCR
ß and endogenous TCR
(Fig 9), but lymph node T cells bear only the transgenic TCR
ß (Fig 6). Thus, the rearrangements of genes encoding the lymphoid type TCR
are suppressed by AND TCR expression, whereas rearrangements that occur early in the fetal thymus, encoding the TCR
of skin lymphocytes, are not suppressed. Of significance, either TCR expression pattern allows development in the CD4-CD8- (
) pathway.
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We have considered these and previous results for understanding the role of the TCR in ß versus
lineage determination. Evidence exists for an instructional model in which successful rearrangement of TCR
or TCRß genes biases the decision of a precursor to become a
or
ß lineage T cell. Of note,
ß lineage T cells are depleted of productive TCR
and -
rearrangements, suggesting that the production of a functional TCR
favors a
lineage decision (51) (52) (53). In addition, mice deficient for the pT
component of the pre-TCR show an increase in the number of
lineage T cells, implying that normally pre-TCR signals inhibit
lineage development (54). Other studies, however, have prompted speculation that
/
ß lineage determination may occur before or independent of TCR gene rearrangement (55) (56) (57) (58). Of relevance, a few CD4+CD8+ thymocytes arise in TCRß-/- null mutant mice (59), and these cells are enriched for in-frame TCR
rearrangements (60) (61), indicating that TCR
, in some circumstances, can promote
ß development. Moreover, CD4+CD8+ cells develop, although inefficiently, in TCR
transgenic mice when endogenous TCRß recombination is diminished or suppressed (56) (62). Even in normal mice, a minor population of TCR
-bearing CD4+CD8+ cells has been observed (63). Complicating the issue further are reports that the majority of TCRß rearrangements are productive in TCR
+ T cells (51) (64). Others disagree, finding that these rearrangements are predominantly out of frame (65). Clearly, the data on this question are mixed and the issue is unresolved.
Since a transgenic TCRß permits both
and
ß development, our results and those of others (17) (38) (39) could fit a model in which
/
ß fate is predetermined, before or independent of TCR rearrangement/expression (4) (66). In this scenario, the TCR plays no role in lineage commitment but is needed only for survival and/or lineage progression. While this model would not always couple the appropriate TCR with lineage commitment, it is noteworthy that additional mechanisms operate to correct TCR expression in the wrong lineage. In the
ß lineage, TCR
is downregulated at the CD4+CD8+ stage (67) and TCR
rearrangement results in the deletion of the TCR
locus. In the
pathway, pT
is turned off (68) and TCR
rearrangement is not upregulated (69).
At first glance, the finding that premature expression of TCRß can permit both a
and
ß cell fate appears to be inconsistent with an instructional mechanism for lineage commitment. However, one version of an instructional model proposes that TCR
and pre-TCR signals influence lineage commitment, but does not necessarily imply that signaling differences are absolute or inherent in the TCR structure. Thus, quantitative differences in TCR
and pre-TCR signaling could bias lineage choice. Perhaps signals generated by the prematurely expressed transgenic TCR
ß quantitatively mimic TCR
signals. An additional possibility is that the timing of TCR expression influences the lineage decision. Recent evidence indicates that TCR
rearrangements occur slightly ahead of TCRß in adult thymopoiesis (41) (42). Conceivably, these ordered rearrangements could be coordinated with developmentally regulated changes in TCR signal transduction such that the earliest TCR signals promote a
fate, whereas later TCR signals favor an
ß fate. Our data could fit with such a sequential model since distinct TCR signals regulating lineage choice would be generated as a function of time, irrespective of TCR substitutions. In some sense, this sequential model can be seen as both predetermined and instructional: predetermined, since changes in intracellular TCR signals over time are developmentally preprogrammed, and instructional, since distinct signals mediate lineage commitment. However, such signals are not inherent to the TCR structure. In any case, the previous results demonstrating that
ß T cells are depleted of in-frame TCR
rearrangements (51) (52) (53) and the low frequency of productive TCRß rearrangements in
T cells (65) support a sequential model.
TCRß transgenic mice are widely used to study antigen-specific immune responses in vivo. The studies reported here should send a note of caution regarding the use of such mice for this purpose. If, as we conclude, the transgenic TCR
ß receptor can substitute for the TCR
in
lineage T cells, cells that would normally be immunologically silent can now participate in an antigen-specific response. Because
T cells have unique developmental, functional, and homing properties, they could contribute to the response in nonphysiological ways. Thus, difficulties with these mice could be related to the large number of mature T cells expressing a single TCR, but also because
lineage cells (bearing transgenic TCR
ß) contribute to the antigenic response in unpredictable ways. Even sorting for CD4+ cells may not help, since a few
T cells express CD4 (57). A new generation of TCR transgenic mice, with delayed TCR
ß expression, may provide a solution to this problem.
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Footnotes |
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1 Abbreviations used in this paper: APC, allophycocyanin; ß2m, ß2-microglobulin; B6, C57BL/6; B10, C57BL/10; CsA, cyclosporine A; HSA, heat stable antigen; RAG, recombination activating gene; TCRßDN, TCR
ß+CD4-CD8-.
K. Terrence and C.P. Pavlovich contributed equally to this work.
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Acknowledgements |
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The authors thank S. Hedrick, H. von Boehmer, D. Loh, F. Alt, L. Glimcher, B. Koller, H. Pircher, M. Davis, J. Bluestone, and G. Sims for gifts of transgenic or gene-targeted mutant mice, and A. Kruisbeek and Ron Germain for cell lines. We thank our colleagues R. Swofford and C. Eigsti for flow cytometry and sorting, and A. Bendelac, S. Gurunathan, E. Schweighoffer, E. Robey, and Juan Zuniga-Pflucker for advice, assistance, and/or critical comments on the manuscript.
K. Terrence and C. Pavlovich performed this work as Howard Hughes Medical InstituteNational Institutes of Health Research Scholars.
Submitted: 7 April 2000
Revised: 23 May 2000
Accepted: 25 May 2000
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References |
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