Annu. Rev. Immuno/. Copyright ©
1990. 8:531-56
1990 by Annual
Reviews Inc. All rights reserved
ANNUAL REVIEWS
Further
Quick links to online content
DEVELOPMENTAL BIOLOGY OF T CELLS IN T CELL
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
RECEPTOR TRANSGENIC MICE Harald von Boehmer Basel Institute for Immunology, 487 Grenzacherstrasse, CH-4005 Switzerland KEY WORDS:
positive selection, negative selection, deletion, tolerance, T cell receptor transgenic mice.
T CELL REPERTOIRE SELECTION-THE LAST TEN YEARS
Experiments concerned with selection of the T cell repertoire have a long history. This topic received an exciting twist when it was discovered that the specificity of T cells related both to foreign antigen molecules and to self-major histocompatibility complex (MHC)-encoded molecules (1-3). Since then, several important details of antigen recognition by T cells have been elucidated: It became clear that the T cell receptor (TCR) complex consisted of polymorphic or variable, disulfide-linked, rxfJ or yb TCR chain subunits noncovalently associated with a number of invariant CD3 polypeptides implicated in signal transduction (4-6). Gene transfer of rx and fJ TCR genes was sufficient to transfer the dual specificity for antigen and MHC molecules from one T cell into another (7, 8). A series of studies revealed that the ligands for the rxfJ TCR were peptides bound and presented by MHC molecules (9-11). Again, gene transfection experiments showed that CD4 and CD8 coreceptors assisted the interaction of T cells with other cells (12, 13) by binding to nonpolymorphic residues of class II and class-I MHC molecules, respectively (14, 15). The issue of T cell repertoire selection, as compared to the issue of T cell antigen recognition, remained much more controversial during the last decade. This stemmed in part from imprecisely formulated concepts as well as inconclusive experimentation. In our mind, the crucial question 531 0732-0582/90/0410-0531$02.00
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
532
VON BOEHMER
was whether the specificity of the TCR controlled early T-cell development at stages preceding that of a newly formed resting T cell, which latter can be induced by antigen to proliferate and to develop effector functions. A good example for such developmental control is contained in the hypoth esis of Lederberg (16) who argues that prior to the stage of a mature lymphocyte, there is an immature stage where binding of a ligand by the antigen receptor results in cell death (suicide) rather than immune effector function. This concept thus couples negative selection (deletion) to an immature stage of T-cell development. We had likewise coupled positive selection of T cells to an immature stage of T-cell development by postu lating that the generation of mature T cells required a binding of the TCR on immature T cells to thymic MHC antigens (17, 18) .. These concepts differed from those arguing that negative and positive selection represented "skewing," "bending," or "more or less intentional priming" of the rep ertoire of already matured T cells (19-21). We wanted to address the question, whether in addition to "skewing," "bending," and "priming " of mature T cells, there were other mechanisms of repertoire selection. The question of repertoire selection among immature T cells was, however, precisely the one that was difficult to address by the experiments conducted during the last decade. This was so because the readout of T-cell specificity required T-cell activation and T-cell expansion. Therefore, selective events taking place during activation and expansion could have been misinterpreted as events controlling early T-cell development. Other complicating factors were the degeneracy of T-cell specificity (22, 23) and the often impaired health of the experimental animals. Because of the complexity of the experimental models,it was also impossible to determine whether apparent selective events were the direct consequence of receptor ligand interaction or were mediated indirectly through regulatory T cells (21). Developmental repertoire selection, therefore, had to be studied by analyzing receptors of known specificity on developing T cells in the absence of putative regulatory T cells. We and others chose to do this in TCR transgenic mice where a large proportion of T cells expressing a receptor defined by specificity as well as by monoclonal antibodies could be followed through development in different environments. With regard to the question of putative regulatory T cells, we thought it proper to aim at a mouse with a quasi-monoclonal immune system. Thus we introduced productively rearranged TCR genes into mice with defective TCR rearrangement. These mice were suffering from severe combined immune deficiency (Scid mice) (24). It turned out that using this approach, we learned not only about the control of late T-cell development by specific receptor-ligand interactions but also about the control of early T-cell
DEVELOPMENTAL BIOLOGY OF T CELLS
533
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
development by the rxf3TCR, irrespective of its specificity. In the following, the various experimental models are described (including some that do not involve TCR transgenic mice), and the results and their relevance to models of T-cell development are summarized. Before doing so, it may be helpful for the less experienced reader to become familiar with the major thymocyte subsets and their precursor product relationship (reviewed previously-IS); these are outlined briefly in the following paragraph. THE MAJOR THYMOCYTE SUBSETS AND THEIR POSSIBLE PRECURSOR-PRODUCT RELATIONSHIP
The thymus is continuously colonized by hemopoietic cells which express neither CD4 nor CDS coreceptors. After immigration TCR gene segments begin to rearrange in pro T cells. Most of these cells are Jlld or Ml /69 positive, and they act as precursors for all other thymocyte subsets. After birth however, CD4-S-, Jl ld or M1/69 negative cells are also detected; a significant proportion of these cells express rxf3 TCRs. The immediate precursors of this latter subset are unknown. Some of the CD4-S- Jl l d+ and CD4 -8- JIId- cells express yb receptors, and these cells can be induced to effector function by lectins or antigens. In the rxf3 TCR lineage, the immediate products of CD4-S- precursors are CD4-S+ large thymo cytes, most of which express no or low levels of receptors and which become CD4+8+ within hours. The CD4+8+ immature thymocytes, of which about 50% express low levels of rxf3 TCR on the cell surface, rep resent the most common thymocyte (SO%), consisting of dividing pre cursors and non-dividing progeny. These cells cannot be activated by antigen or lectins to develop effector functions. It is currently believed that some of these cells are the precursors of CD4+S - and CD4 -S+ progeny which contain the precursors of T helper and T killer cells, respectively. CD4+8+ thymocytes are produced in vast excess over CD4+S- and CD4- 8+ mature T cells, and it is suspected that only some CD4+S+ cells are selected for further development (18) (Figure I). In the following we consider the expression of TCRs on these subsets in various experimental animals. EXPRESSION OF T CELL RECEPTOR TRANSGENES
Different Constructs Initial attempts to express TCR transgenes were unsuccessful until it was realized that expression of genomic DNA required a regulatory element
534
VON BOEHMER
CURRENT DOGMA OF T CELL DEVELOPMENT
a(3
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
a(3
1
a(3
Figure I
The product relationship of thymocyte subsets and their possible precursors. The
box represents the thymus in which TCR rearrangement occurs in CD4-S- and CD4+S+ cells leading to the expression of yo and rxfJ TCRs. Some of the CD4+S+ thymocytes will differentiate into CD4-8+ and CD4+8- progeny which will eventually migrate from the thymus.
located 5 kb downstream of Cf32 (25). With other f3 TCR constructs, lacking the natural enhancer, expression could be achieved by insertion of an immunoglobulin (Ig) enhancer (60). The expression of the a TCR gene, at least in cell lines, required an enhancer 3 kb downstream of Ox (27). While this enhancer was sufficient to achieve expression at the level of cell surface proteins in the majority of transfeeted T-cell lines,it was insufficient to achieve cell surface expression in the majority of T cells in TCR trans genic mice: Several independent transgenic mice containing the a construct showed variable surface expression ranging from 1 to 20% of the T cells. This was also true when a TCR transgenic mice were crossed with f3 TCR transgenic mice, and F I hybrids containing both transgenes were analyzed. In one particular TCR transgenic line produced by coinjection of a and f3 TCR transgenes, however, we observed a TCR expression on the majority of T cells (28). This high expression of the a TeR transgene may have been caused by tandem integration of f3 and a TCR DNA such that the expression of the a TCR DNA was enhanced by the regulatory element located 5 kb downstream of Cf32. Another way to achieve high expression of the a TCR transgene is the insertion of an Ig enhancer (26).
DEVELOPMENTAL BIOLOGY OF T CELLS
535
Surface Expression of TCR Transgenes in {3 and li{3 TCR Transgenic Mice In f3 TCR transgenic mice the vast majority of thymocytes and T cells showed surface expression of the transgenic f3 TCR chain. This included immature CD4+g+ as well as CD4-g- thymocytes (29, 30). The f3 TCR protein could be detected as early as day 14 of gestation on the surface of CD4 8 thymocytes, as a disulfide-linked dimer with an unknown partner chain (64). This dimer was not associated with CD3 molecules as cells with this phenotype did not stain with CD3 antibodies and could not be induced with CD3 or f3 TCR chain antibodies to proliferate in media containing interleukin 2 (IL-2) (Table I). These results differed from those obtained with CD4 -g - cells from af3 transgenic mice where both the transgenic ix and f3 TCR chains could be detected on early CD4 -g - thymocytes and where the rxf3TCR heterodimer was expressed at a higher density than the dimer in f3TCR transgenic mice (Figure 2). The af3 dimer was associated with CD3 molecules on CD4-g thymocytes, and cells of this phenotype could be induced to proliferate by CD3 - and f3-TCR-chain antibodies to proliferate in media containing interleukin 2. In both types of mice, both JIld+ and Jlld- CD4-g thymocytes expressed TCR dimers. These data indicate that rearranged TCR rx and f3receptor genes can be expressed on the surface of early CD4-g - thymocytes which act as pre cursors for CD4+g + thymocytes (31). It appears that the f3TCR chain can be expressed in immature T cells in the absence of CD3 molecules and other TCR chains encoded by rearranging TCR loci (see below). Most CD4-g- thymocytes, however, appear to synthesize CD3 proteins, because the introduction of both f3and rx transgenes leads to the expression of af3 TCR heterodimers associated with CD3 molecules in this cell popu lation. In normal mice these cells are awaiting productive rx and f3 TCR rearrangements, and this is why most of these cells in normal mice show only low levels of expression of TCR proteins. The early expression of the TCR rx chain in !Xf3 TCR transgenic mice may in addition be achieved by the neighboring f3 TCR enhancer.
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
-
-
Table 1 Differential susceptibility to CD3 anti bodies of CD4-8- M l/69+ thymocytes from f3 transgenic and a{3 transgenic mice
Thymocytes f3 Transgenic af3 Transgenic
IL2
aCD3+IL2
230
270
310
8.250
536
VON BOEHMER
CD4-a- ap TRANSGENIC MALE THYMDCYTES
" I,
f� ./ \...
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
CELL NUMBER
F23.1
T3.70
CDS
Jlld
Figure 2 The expression of transgenic fJ and a: TCR chains ( -- control, - antibody) recognized by the F23. l and T3.70 monoclonal antibody, respectivel y, on CD4-S-, thy mocytes composed of Jlld + and Jll d - cells. -
TCR Transgene Expression and Rearrangement of Endogenous TCR Genes The early expression of both f3 and a TCR trans genes had different effects on the expression of endogenous f3 and a TCR genes: In both f3 and af3 transgenic mice, productive and nonproductive rearrangement of endoge nous Vf3 genes was almost completely inhibited (29, 32), while there was a lot of rearrangement of endogenous Va gene segments in cells of rxf3TCR transgenic mice. In fact, several T-cell clones from rxf3TCR transgenic mice expressed two different (transgenic and endogenous) rx TCR chains on the cell surface, both paired with the transgenic f3 chain (Figure 3). The expression of endogenous Vrx genes in rxf3 TCR transgenic mice was also evident because both CD4+S- and CD4-S+ T cells gave significant responses to alloantigens (32). The different extent of inhibition of rearrangement of endogenous Vf3 and Va gene segments may in part be due to the fact that V f3rearrangements are preceded by DJ rearrangements (Dp rearrangements are incompletely inhibited in the TCR transgenic mice) whereas the a locus lacks D elements. It is also conceivable that a different feedback control works for the f3 and a TCR locus in such a way that the expression of a f3 protein (possibly as a dimer in CD4-S- thymocytes)
537
DEVELOPMENTAL BIOLOGY OF T CELLS
Hybridomas Preclearing c
«rVO
1990. 8:531-56
1990 by Annual
Reviews Inc. All rights reserved
ANNUAL REVIEWS
Further
Quick links to online content
DEVELOPMENTAL BIOLOGY OF T CELLS IN T CELL
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
RECEPTOR TRANSGENIC MICE Harald von Boehmer Basel Institute for Immunology, 487 Grenzacherstrasse, CH-4005 Switzerland KEY WORDS:
positive selection, negative selection, deletion, tolerance, T cell receptor transgenic mice.
T CELL REPERTOIRE SELECTION-THE LAST TEN YEARS
Experiments concerned with selection of the T cell repertoire have a long history. This topic received an exciting twist when it was discovered that the specificity of T cells related both to foreign antigen molecules and to self-major histocompatibility complex (MHC)-encoded molecules (1-3). Since then, several important details of antigen recognition by T cells have been elucidated: It became clear that the T cell receptor (TCR) complex consisted of polymorphic or variable, disulfide-linked, rxfJ or yb TCR chain subunits noncovalently associated with a number of invariant CD3 polypeptides implicated in signal transduction (4-6). Gene transfer of rx and fJ TCR genes was sufficient to transfer the dual specificity for antigen and MHC molecules from one T cell into another (7, 8). A series of studies revealed that the ligands for the rxfJ TCR were peptides bound and presented by MHC molecules (9-11). Again, gene transfection experiments showed that CD4 and CD8 coreceptors assisted the interaction of T cells with other cells (12, 13) by binding to nonpolymorphic residues of class II and class-I MHC molecules, respectively (14, 15). The issue of T cell repertoire selection, as compared to the issue of T cell antigen recognition, remained much more controversial during the last decade. This stemmed in part from imprecisely formulated concepts as well as inconclusive experimentation. In our mind, the crucial question 531 0732-0582/90/0410-0531$02.00
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
532
VON BOEHMER
was whether the specificity of the TCR controlled early T-cell development at stages preceding that of a newly formed resting T cell, which latter can be induced by antigen to proliferate and to develop effector functions. A good example for such developmental control is contained in the hypoth esis of Lederberg (16) who argues that prior to the stage of a mature lymphocyte, there is an immature stage where binding of a ligand by the antigen receptor results in cell death (suicide) rather than immune effector function. This concept thus couples negative selection (deletion) to an immature stage of T-cell development. We had likewise coupled positive selection of T cells to an immature stage of T-cell development by postu lating that the generation of mature T cells required a binding of the TCR on immature T cells to thymic MHC antigens (17, 18) .. These concepts differed from those arguing that negative and positive selection represented "skewing," "bending," or "more or less intentional priming" of the rep ertoire of already matured T cells (19-21). We wanted to address the question, whether in addition to "skewing," "bending," and "priming " of mature T cells, there were other mechanisms of repertoire selection. The question of repertoire selection among immature T cells was, however, precisely the one that was difficult to address by the experiments conducted during the last decade. This was so because the readout of T-cell specificity required T-cell activation and T-cell expansion. Therefore, selective events taking place during activation and expansion could have been misinterpreted as events controlling early T-cell development. Other complicating factors were the degeneracy of T-cell specificity (22, 23) and the often impaired health of the experimental animals. Because of the complexity of the experimental models,it was also impossible to determine whether apparent selective events were the direct consequence of receptor ligand interaction or were mediated indirectly through regulatory T cells (21). Developmental repertoire selection, therefore, had to be studied by analyzing receptors of known specificity on developing T cells in the absence of putative regulatory T cells. We and others chose to do this in TCR transgenic mice where a large proportion of T cells expressing a receptor defined by specificity as well as by monoclonal antibodies could be followed through development in different environments. With regard to the question of putative regulatory T cells, we thought it proper to aim at a mouse with a quasi-monoclonal immune system. Thus we introduced productively rearranged TCR genes into mice with defective TCR rearrangement. These mice were suffering from severe combined immune deficiency (Scid mice) (24). It turned out that using this approach, we learned not only about the control of late T-cell development by specific receptor-ligand interactions but also about the control of early T-cell
DEVELOPMENTAL BIOLOGY OF T CELLS
533
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
development by the rxf3TCR, irrespective of its specificity. In the following, the various experimental models are described (including some that do not involve TCR transgenic mice), and the results and their relevance to models of T-cell development are summarized. Before doing so, it may be helpful for the less experienced reader to become familiar with the major thymocyte subsets and their precursor product relationship (reviewed previously-IS); these are outlined briefly in the following paragraph. THE MAJOR THYMOCYTE SUBSETS AND THEIR POSSIBLE PRECURSOR-PRODUCT RELATIONSHIP
The thymus is continuously colonized by hemopoietic cells which express neither CD4 nor CDS coreceptors. After immigration TCR gene segments begin to rearrange in pro T cells. Most of these cells are Jlld or Ml /69 positive, and they act as precursors for all other thymocyte subsets. After birth however, CD4-S-, Jl ld or M1/69 negative cells are also detected; a significant proportion of these cells express rxf3 TCRs. The immediate precursors of this latter subset are unknown. Some of the CD4-S- Jl l d+ and CD4 -8- JIId- cells express yb receptors, and these cells can be induced to effector function by lectins or antigens. In the rxf3 TCR lineage, the immediate products of CD4-S- precursors are CD4-S+ large thymo cytes, most of which express no or low levels of receptors and which become CD4+8+ within hours. The CD4+8+ immature thymocytes, of which about 50% express low levels of rxf3 TCR on the cell surface, rep resent the most common thymocyte (SO%), consisting of dividing pre cursors and non-dividing progeny. These cells cannot be activated by antigen or lectins to develop effector functions. It is currently believed that some of these cells are the precursors of CD4+S - and CD4 -S+ progeny which contain the precursors of T helper and T killer cells, respectively. CD4+8+ thymocytes are produced in vast excess over CD4+S- and CD4- 8+ mature T cells, and it is suspected that only some CD4+S+ cells are selected for further development (18) (Figure I). In the following we consider the expression of TCRs on these subsets in various experimental animals. EXPRESSION OF T CELL RECEPTOR TRANSGENES
Different Constructs Initial attempts to express TCR transgenes were unsuccessful until it was realized that expression of genomic DNA required a regulatory element
534
VON BOEHMER
CURRENT DOGMA OF T CELL DEVELOPMENT
a(3
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
a(3
1
a(3
Figure I
The product relationship of thymocyte subsets and their possible precursors. The
box represents the thymus in which TCR rearrangement occurs in CD4-S- and CD4+S+ cells leading to the expression of yo and rxfJ TCRs. Some of the CD4+S+ thymocytes will differentiate into CD4-8+ and CD4+8- progeny which will eventually migrate from the thymus.
located 5 kb downstream of Cf32 (25). With other f3 TCR constructs, lacking the natural enhancer, expression could be achieved by insertion of an immunoglobulin (Ig) enhancer (60). The expression of the a TCR gene, at least in cell lines, required an enhancer 3 kb downstream of Ox (27). While this enhancer was sufficient to achieve expression at the level of cell surface proteins in the majority of transfeeted T-cell lines,it was insufficient to achieve cell surface expression in the majority of T cells in TCR trans genic mice: Several independent transgenic mice containing the a construct showed variable surface expression ranging from 1 to 20% of the T cells. This was also true when a TCR transgenic mice were crossed with f3 TCR transgenic mice, and F I hybrids containing both transgenes were analyzed. In one particular TCR transgenic line produced by coinjection of a and f3 TCR transgenes, however, we observed a TCR expression on the majority of T cells (28). This high expression of the a TeR transgene may have been caused by tandem integration of f3 and a TCR DNA such that the expression of the a TCR DNA was enhanced by the regulatory element located 5 kb downstream of Cf32. Another way to achieve high expression of the a TCR transgene is the insertion of an Ig enhancer (26).
DEVELOPMENTAL BIOLOGY OF T CELLS
535
Surface Expression of TCR Transgenes in {3 and li{3 TCR Transgenic Mice In f3 TCR transgenic mice the vast majority of thymocytes and T cells showed surface expression of the transgenic f3 TCR chain. This included immature CD4+g+ as well as CD4-g- thymocytes (29, 30). The f3 TCR protein could be detected as early as day 14 of gestation on the surface of CD4 8 thymocytes, as a disulfide-linked dimer with an unknown partner chain (64). This dimer was not associated with CD3 molecules as cells with this phenotype did not stain with CD3 antibodies and could not be induced with CD3 or f3 TCR chain antibodies to proliferate in media containing interleukin 2 (IL-2) (Table I). These results differed from those obtained with CD4 -g - cells from af3 transgenic mice where both the transgenic ix and f3 TCR chains could be detected on early CD4 -g - thymocytes and where the rxf3TCR heterodimer was expressed at a higher density than the dimer in f3TCR transgenic mice (Figure 2). The af3 dimer was associated with CD3 molecules on CD4-g thymocytes, and cells of this phenotype could be induced to proliferate by CD3 - and f3-TCR-chain antibodies to proliferate in media containing interleukin 2. In both types of mice, both JIld+ and Jlld- CD4-g thymocytes expressed TCR dimers. These data indicate that rearranged TCR rx and f3receptor genes can be expressed on the surface of early CD4-g - thymocytes which act as pre cursors for CD4+g + thymocytes (31). It appears that the f3TCR chain can be expressed in immature T cells in the absence of CD3 molecules and other TCR chains encoded by rearranging TCR loci (see below). Most CD4-g- thymocytes, however, appear to synthesize CD3 proteins, because the introduction of both f3and rx transgenes leads to the expression of af3 TCR heterodimers associated with CD3 molecules in this cell popu lation. In normal mice these cells are awaiting productive rx and f3 TCR rearrangements, and this is why most of these cells in normal mice show only low levels of expression of TCR proteins. The early expression of the TCR rx chain in !Xf3 TCR transgenic mice may in addition be achieved by the neighboring f3 TCR enhancer.
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
-
-
Table 1 Differential susceptibility to CD3 anti bodies of CD4-8- M l/69+ thymocytes from f3 transgenic and a{3 transgenic mice
Thymocytes f3 Transgenic af3 Transgenic
IL2
aCD3+IL2
230
270
310
8.250
536
VON BOEHMER
CD4-a- ap TRANSGENIC MALE THYMDCYTES
" I,
f� ./ \...
Annu. Rev. Immunol. 1990.8:531-556. Downloaded from www.annualreviews.org by University of Missouri - Columbia on 04/18/13. For personal use only.
CELL NUMBER
F23.1
T3.70
CDS
Jlld
Figure 2 The expression of transgenic fJ and a: TCR chains ( -- control, - antibody) recognized by the F23. l and T3.70 monoclonal antibody, respectivel y, on CD4-S-, thy mocytes composed of Jlld + and Jll d - cells. -
TCR Transgene Expression and Rearrangement of Endogenous TCR Genes The early expression of both f3 and a TCR trans genes had different effects on the expression of endogenous f3 and a TCR genes: In both f3 and af3 transgenic mice, productive and nonproductive rearrangement of endoge nous Vf3 genes was almost completely inhibited (29, 32), while there was a lot of rearrangement of endogenous Va gene segments in cells of rxf3TCR transgenic mice. In fact, several T-cell clones from rxf3TCR transgenic mice expressed two different (transgenic and endogenous) rx TCR chains on the cell surface, both paired with the transgenic f3 chain (Figure 3). The expression of endogenous Vrx genes in rxf3 TCR transgenic mice was also evident because both CD4+S- and CD4-S+ T cells gave significant responses to alloantigens (32). The different extent of inhibition of rearrangement of endogenous Vf3 and Va gene segments may in part be due to the fact that V f3rearrangements are preceded by DJ rearrangements (Dp rearrangements are incompletely inhibited in the TCR transgenic mice) whereas the a locus lacks D elements. It is also conceivable that a different feedback control works for the f3 and a TCR locus in such a way that the expression of a f3 protein (possibly as a dimer in CD4-S- thymocytes)
537
DEVELOPMENTAL BIOLOGY OF T CELLS
Hybridomas Preclearing c
«rVO
