- Email: [email protected]
Light chain surrogacy
ASHOK
VENKITARAMAN
LYMPHOCYTE
DEVELOPMENT
Light chain surrogacy Mature B cells develop only if progenitors successfully rearrange their immunoglobulin genes. Recent results reveal the mechanisms that regulate this process. The immune system recognizes foreign antigens through specific receptors expressed on the surface of T and B lymphocytes. These T- and B-cell antigen receptors are heterodimers of heavy and light chain polypeptides encoded by the T-cell receptor and immunoglobulin (Ig) genes respectively. These genes are unusual in that they include several segments (V, variable; D, diversity and J, joining), which are separated in the germline and brought together by a series of DNA rearrangements during lymphocyte development. The rearranged V-D-J segments encode the amino-terminal antigen-binding domain of the receptor proteins, and are linked to constant region exons that specify the other domains (Fig.l). Given that there are hundreds of V segments as well as tens of D and J segments, a large number of different receptor specificities can be generated combinatorially by the rearrangement process. Moreover, the junctions between the joined segments are imprecise and may also contain nucleotides added or deleted at the time of rearrangement, making the potential repertoire of the T- and B-cell receptors specificities truly enormous. For this diversification strategy to work, the rearrangement process must be tightly controlled. The imprecision inherent in joining means that most products will encode non-functional or aberrant proteins; strict 'quality control' is therefore necessary. Anti-self or other deleterious receptor specificities may be generated and the cells carrying them must be eliminated during development, just as lymphocytes with relevant specificities must later be expanded during an immune response. Selective elimination and expansion are possible only if each lymphocyte bears antigen receptors of a single specificity; therefore only one functional rearrangement can be allowed in each receptor chain. In B lymphocytes, much of the control to achieve these requirements is exerted during rearrangement of the Ig heavy (H) chain gen~s (reviewed in [1]) and a simple explanatory model has taken shape over the years to
explain this. Rearrangement begins in pre-B cells, at first on both alleles with D H joining to JH and later VH joining to DH/H. A decision point is reached when 'productive' rearrangement of any one allele results in an in-frame VHD~¢IH directing the synthesis of a functional IgH chain protein of the IgM (I.t) class. At this point fur tiler changes on the second allele are inhibited (allelic exclusion), and the cell proceeds to Ig light (L) chain gene rearrangement. A series of recent studies [2-5] have confirmed some of the main predictions of this model but have also revealed unexpectedly intricate new twists. A central feature of this model is that pre-B cells must 'sense' successful IgH rearrangements by detecting a functional btH chain protein. How is this accomplished? The pH chain protein exists in secretory (tis) and membrane-bound (~trn) forms, which differ at their extreme carboxyl termini by the presence or absence of a transmembrane anchor. Earlier experiments (reviewed in [1] ), in cell lines and transgenic mice, showed that only l.tm causes IgH allelic exclusion, implying dependence on the transmission of a transmembrane signal. The subsequent observation that Pm is expressed on the surface of pre-B cells that have not yet rearranged their L-chain genes supported a signalling role for this molecule, but also raised a puzzling question because L chains are essential for transport of btm to the surface o f mature B cells. This puzzle was resolved by the identification in B cell progenitors of two mo!ecules, Vpre~B and X5, which together function as a surrogate for the conventional light chains found in mature cells [6,7]. Ypre-B and X5 associate with Pm to form a complex that resembles the IgH/L dimer. X5, which corresponds in sequence to the L-chain constant region, is covalently bound to btm, whereas Vpre_B, with sequence homology to the Lchain variable region, associates non-covalently [7]. In mature B cells, membrane-bound Ig molecules of all classes are expressed at the cell surface with at least ~s
]
I
I V H segments (>500)
I DH
I
L__I
segments JH segments (> 10) (4-5)
I / ~m j
L bt constant region exons
Fig. 1. Germline organization of the immunoglobulin I.t heavy-chain locus.
Volume 2
N u m b e r 10
1992
559
two other proteins - - the products of the mb-1 and B29 genes - - w h i c h function as the signal-transducing components of the complex [8]..The mb-I and B29 genes are switched on very early in B-cell development and their products may associate with p.m-surrogate light chains and other molecules in pre-B cells [9] to signal success in IgH rearrangement.
rearrangements are normal in number but, unexpectedly, some newly mature cells that have successfully completed both H- and L-chain gene rearrangements are found, al though far fewer than in normal animals. Thus, the blockade in development is 'leaky' and lets a small number of B lymphocytes trickle through to maturity. In k e e p ing with these findings, the number of mature B cells in the periphery is drastically decreased in young (less than 6 weeks) X5-deficient mice, but gradually reaches near-normal levels in 5 to 6 months. The behaviour of a subset of B cells which expresses the marker CD5 is of particular interest because it has been speculated that this population is self-renewing. Indeed, although these cells are as infrequent as conventional B cells in 2 week old mutant mice, they are remarkably quick to recover with normal numbers present at 5 to 7 weeks.
The stringency of the mechanism that screens for /.tm synthesis is revealed dramatically by targeted disruption of the exons encoding the/.t m transmembrane segment [2]. Mice homozygous for this mutation completely lack mature B cells in the periphery. Primitive B-lineage committed precursors are found in the bone marrow but preB cells expressing Izm at the surface are not, suggesting that development is blocked at or during the stage of IgH rearrangement. The more primitive precursors do not accumulate in the bone marrow, and so may be destined to die unless 'rescued' by successful I.tm synthesis.
That I.tm is essential for allelic exclusion has been established by elegant experiments with heterozygous mice carrying one disrupted I.tm allele (which can encode IZs but not Pm) and one wild-type allele [4]. The two alleles are of different allotypes and their protein prod ucts can be distinguished by specific monoclonal antibodies. B cells develop in normal numbers in these heterozygous mice. However, a significant proportion (10%) of mature B cells in the bone marrow and periphery express membrane-bound IgM of the wild-
If I.tm exerts its effect through the complex it forms with the surrogate light chain, Vpre_B/~,5, targeted disruption of the X5 gene should create an identical phenotype. Surprisingly, it does not, the major difference being that B-lymphocyte development is impaired but not completely abrogated [3]. In the bone marrow of X5-deficient mice, primitive precursors that have not yet started IgH Precursor cell in bone marrow, committed to the B-cell lineage; both heavy-chain alleles and both light-chain alleles in germline configuration (only one of each type of gene segment shown for simplicity).
Heavy-chain gene rearrangement begins with DHLJH jolnmg,
Committed precursor cell
VH-DH JHjoining of one heavychain allele allows celbsufface expressionof p-mchains together with the 'surrogate' light chains X5 and Vp~.B;signalsfrom the #Ecomplex preventfurther heavy-chaingene rearrangement (alle!ic exclusion). Pre-B cell
Productive rearrangement of a light-chain allele allows cell-surface expression of functional immunoglobin m0lecu[es in what are now mature B cells that can leave the bone marrow. Mature B cell
i!!:::t¸
,ain
~ p r e - B X5 Allelic exclusioq, " ~ < : ~ growth, up-regulation " ~ of IgL rearrangement lam
Allele 1 Heavy-chain genes VH Allele 2 VH
DH
JH
CH
DH JH
CH
VH DHJH CH
>
VHDHJH CH
VH DHJH CH (or germline)
Allele 1 Light-chain genes VL
>
~L[m VHDHJH
VH DHJH CH
VH DHJH CH (or germline)
JL
CL
VL
JL
CL
VL
JL
eL
JL
CL
VL
JL
eL
VL
JL
CL
Clt
(or germline) VLJL
CL
JL
CL
Allele 2
VL
VL
Fig. 2. Steps in the maturation of B cells, showing the role of surrogate light chains in allowing cell surface expression of the Iz heavy chain prior to rearrangement of the light-chain gene and the postulated signalling activity of the Iz heavy chain complex leading to allelic exclusion. (Rarely, premature rearrangement of the light-chain gene may bypass the need for surrogate light chains.) 560
@ 1992 Current Biology
type allotype as well as secretory IgM of the mutant allotype, showing that the mutant I.tm locus is incapable of signalling to prevent rearrangement of the wild type allele. It is not clear why these 'double-expressors' are less than expected (50 %) if each allele is" equally likely to be the frst to successfully rearrange perhaps the targeted allele is defective in some way. Allelic exclusion is hardly affected in mice homozygous for the X5 disruption [3]; most mature B cells are perfectly allelically excluded and only 0.3 % express IgM from both alleles. Why is the arrest in B-cell development and allelic exclusion leaky in X5 deficient mice? One explanation is that conventional L chains are prematurely rearranged and take over the role of Vpr e B/;L5 in a few maturing B cells, which then escape the developmental blockade. IgH rearrangement is not a necessary pre-requisite for this: 'spontaneous' L chain gene rearrangements have been observed to precede H-chain rearrangements in mouse and human pre-B cell lines and have now been found to occur at a low frequency in the bone marrow of even the btm-deficient mice [4]. Another facet to the developmental regulation of IgH rearrangement has also surfaced in these mutant mice. Of the three possible translational reading frames (rf) of DH, which could be used in productive VErDI_F-JHjoins, one of them (rf3) contains stop codons in most DH segments and is therefore infrequently seen in the sequences of antibody variable domains in mature B lymphocytes. Of the two remaining, preferential usage of rfl is observed in antibody sequences, whereas rf2 is highly under-represented. This bias is unexpected because the rearrangement mechanism itself is imprecise, suggesting that rfl usage is in some way actively selected for during development. The evolutionary advantage of such selection may lie in the fact that rfl, unlike rf2, encodes multiple tyrosine and tryptophan residues in the DH-JH segment. Aromatic residues in this region are often used in antigen binding. Analysis of ~tm-deficient mice shows that one impor tant component of rf selection occurs through the btm-surrogate light chain complex [5]. After DH-JH joining but before VH-DHIH joining, rf2 (but not rfl) in "almost all D~/s can encode a truncated version of the I.tm protein (DI.tm), which includes an amino-terminal leader peptide and could, in theory, be exported. There is evidence to support this possibility: DI.tm can be expressed in transfected pre-B cells, where it associates with VpreB/~,5 and ig transported to the cell surface [10]. It has been postulated tha~ the D~tm surrogate light chain complex signals prematurely to stop further VH to DvJH rearrangements, aborting further development of D~tm-expressing cells. This would then account for the observed under-representation of rf2 usage in mature calls. Disruption of the I.tm gene should impede this by preventing Dbtm expression in pre-B cells using rf2. This is precisely what is observed: B lymphocytes that carry one disrupted and one wild-type btm allele display an equal frequency of rfl and rf2 usage in DH-JH joins on the mutant allele [5]. Elegant though this demonstration
Volume2
Number 10
1992
is, further analysis will be necessary before the general significance of rf selection becomes clear. The studies summarized here permit a more precise definition of the functions of the btm-surrogate light chain complex (Fig. 2) They confirm that a transmembrane signal, transmitted by the complex, is required for allelic exclusion, selection of DH reading frame usage and for the survival or expansion of progenitors undergoing IgH rearrangement. Surprisingly, IgL rearrangement occurs at a low frequency even in the absence of I~m and so the complex probably signals to upregulate rather than to initiate this process. The production of both CD5-expressing and conventional B cells is affected in a very similar way by the disruption of btm or )v 5: divergence, if any, of their respective developmental pathways must occur relatively late. Several puzzles remain. How does the ~tm-surrogate light chain complex transmit a signal? How is IgH rearrangement stopped? How is IgL rearrangement induced? The btm- and X5-deficient mice are powerful new tools for the analysis of these questions.
References 1.
SCHATZ D, OETITNGER M, SCHLISSEL M: V(D)J recombination: molecular biology and regulation. A n n u Rev i m m u n o l 1992, 10:359-383.
2.
KITAMURAD, ROES J, KOHN R, RAJEWSKYK: A B cell-deficient m o u s e by targeted disruption of the m e m b r a n e e x o n of t h e immunnglobulin g chain gene. Nature 1991, 350:423-426.
3.
KITAMURAD, KUDO A, SCHAA S, MOLTERW, MELCHERSF, RAJEWSKY K; A critical role of X5 protein in B-cell development. Cell 1992, 69:823431.
4,
I~TAMURAD, RAJEWSKYK: Targeted disruption of g chain m e m brane e x o n causes loss of heavy-chain allelic exclusion. Nature 1992, 356:154-156.
5.
GU H, KITAMURAD, RAJEWSKYK: B-cell development regulated by gene rearrangement: arrest of maturation by m e m b r a n e b o u n d Dbt protein and selection of D H e l e m e n t reading frames. Cell 1991, 65:47-54.
6.
PILLAI S, BALTIMORED: Formation of disulphide-linked p.2c02 tetramers in pre-B cells by t h e 18K c0-immunoglobulin light chain. Nature 1987, 329:172-174.
7.
KARASUYAMAH, KUDO A, MELCHERS F: The proteins e n c o d e d by the Vpre. B and %5 pre-B cell specific genes can associate with each other and with bt heavy chain. J Exp M e d 1990, 172:969-972.
8.
VENK1TARAMANA, WILLIAMSG, DARIAVACHP, NEUBERGERM: The B-cell antigen receptor of t h e five immunoglobulin classes. Nature 1991, 352:777-781.
9.
TAKEMORIT, ET AL.: Two types of ~t chain c o m p l e x e s are e x p r e s s e d during differentiation from pre-B to mature cells. EMBO J 1990, 9:2493-2498.
10.
TSUBATAT, TSUBATAR, RETH M: Cell surface expression of t h e short immunoglobulin bt chain (Dp. protein) in murine pre-B cells is differently regulated from that of the intact bt chain. Eur J I m m u n o l 1991, 21:13591363.
Ashok Venkitaraman, Medical Research Council Laboratow of Molecular Biology, Cambridge CB2 2QH.
S61
VENKITARAMAN
LYMPHOCYTE
DEVELOPMENT
Light chain surrogacy Mature B cells develop only if progenitors successfully rearrange their immunoglobulin genes. Recent results reveal the mechanisms that regulate this process. The immune system recognizes foreign antigens through specific receptors expressed on the surface of T and B lymphocytes. These T- and B-cell antigen receptors are heterodimers of heavy and light chain polypeptides encoded by the T-cell receptor and immunoglobulin (Ig) genes respectively. These genes are unusual in that they include several segments (V, variable; D, diversity and J, joining), which are separated in the germline and brought together by a series of DNA rearrangements during lymphocyte development. The rearranged V-D-J segments encode the amino-terminal antigen-binding domain of the receptor proteins, and are linked to constant region exons that specify the other domains (Fig.l). Given that there are hundreds of V segments as well as tens of D and J segments, a large number of different receptor specificities can be generated combinatorially by the rearrangement process. Moreover, the junctions between the joined segments are imprecise and may also contain nucleotides added or deleted at the time of rearrangement, making the potential repertoire of the T- and B-cell receptors specificities truly enormous. For this diversification strategy to work, the rearrangement process must be tightly controlled. The imprecision inherent in joining means that most products will encode non-functional or aberrant proteins; strict 'quality control' is therefore necessary. Anti-self or other deleterious receptor specificities may be generated and the cells carrying them must be eliminated during development, just as lymphocytes with relevant specificities must later be expanded during an immune response. Selective elimination and expansion are possible only if each lymphocyte bears antigen receptors of a single specificity; therefore only one functional rearrangement can be allowed in each receptor chain. In B lymphocytes, much of the control to achieve these requirements is exerted during rearrangement of the Ig heavy (H) chain gen~s (reviewed in [1]) and a simple explanatory model has taken shape over the years to
explain this. Rearrangement begins in pre-B cells, at first on both alleles with D H joining to JH and later VH joining to DH/H. A decision point is reached when 'productive' rearrangement of any one allele results in an in-frame VHD~¢IH directing the synthesis of a functional IgH chain protein of the IgM (I.t) class. At this point fur tiler changes on the second allele are inhibited (allelic exclusion), and the cell proceeds to Ig light (L) chain gene rearrangement. A series of recent studies [2-5] have confirmed some of the main predictions of this model but have also revealed unexpectedly intricate new twists. A central feature of this model is that pre-B cells must 'sense' successful IgH rearrangements by detecting a functional btH chain protein. How is this accomplished? The pH chain protein exists in secretory (tis) and membrane-bound (~trn) forms, which differ at their extreme carboxyl termini by the presence or absence of a transmembrane anchor. Earlier experiments (reviewed in [1] ), in cell lines and transgenic mice, showed that only l.tm causes IgH allelic exclusion, implying dependence on the transmission of a transmembrane signal. The subsequent observation that Pm is expressed on the surface of pre-B cells that have not yet rearranged their L-chain genes supported a signalling role for this molecule, but also raised a puzzling question because L chains are essential for transport of btm to the surface o f mature B cells. This puzzle was resolved by the identification in B cell progenitors of two mo!ecules, Vpre~B and X5, which together function as a surrogate for the conventional light chains found in mature cells [6,7]. Ypre-B and X5 associate with Pm to form a complex that resembles the IgH/L dimer. X5, which corresponds in sequence to the L-chain constant region, is covalently bound to btm, whereas Vpre_B, with sequence homology to the Lchain variable region, associates non-covalently [7]. In mature B cells, membrane-bound Ig molecules of all classes are expressed at the cell surface with at least ~s
]
I
I V H segments (>500)
I DH
I
L__I
segments JH segments (> 10) (4-5)
I / ~m j
L bt constant region exons
Fig. 1. Germline organization of the immunoglobulin I.t heavy-chain locus.
Volume 2
N u m b e r 10
1992
559
two other proteins - - the products of the mb-1 and B29 genes - - w h i c h function as the signal-transducing components of the complex [8]..The mb-I and B29 genes are switched on very early in B-cell development and their products may associate with p.m-surrogate light chains and other molecules in pre-B cells [9] to signal success in IgH rearrangement.
rearrangements are normal in number but, unexpectedly, some newly mature cells that have successfully completed both H- and L-chain gene rearrangements are found, al though far fewer than in normal animals. Thus, the blockade in development is 'leaky' and lets a small number of B lymphocytes trickle through to maturity. In k e e p ing with these findings, the number of mature B cells in the periphery is drastically decreased in young (less than 6 weeks) X5-deficient mice, but gradually reaches near-normal levels in 5 to 6 months. The behaviour of a subset of B cells which expresses the marker CD5 is of particular interest because it has been speculated that this population is self-renewing. Indeed, although these cells are as infrequent as conventional B cells in 2 week old mutant mice, they are remarkably quick to recover with normal numbers present at 5 to 7 weeks.
The stringency of the mechanism that screens for /.tm synthesis is revealed dramatically by targeted disruption of the exons encoding the/.t m transmembrane segment [2]. Mice homozygous for this mutation completely lack mature B cells in the periphery. Primitive B-lineage committed precursors are found in the bone marrow but preB cells expressing Izm at the surface are not, suggesting that development is blocked at or during the stage of IgH rearrangement. The more primitive precursors do not accumulate in the bone marrow, and so may be destined to die unless 'rescued' by successful I.tm synthesis.
That I.tm is essential for allelic exclusion has been established by elegant experiments with heterozygous mice carrying one disrupted I.tm allele (which can encode IZs but not Pm) and one wild-type allele [4]. The two alleles are of different allotypes and their protein prod ucts can be distinguished by specific monoclonal antibodies. B cells develop in normal numbers in these heterozygous mice. However, a significant proportion (10%) of mature B cells in the bone marrow and periphery express membrane-bound IgM of the wild-
If I.tm exerts its effect through the complex it forms with the surrogate light chain, Vpre_B/~,5, targeted disruption of the X5 gene should create an identical phenotype. Surprisingly, it does not, the major difference being that B-lymphocyte development is impaired but not completely abrogated [3]. In the bone marrow of X5-deficient mice, primitive precursors that have not yet started IgH Precursor cell in bone marrow, committed to the B-cell lineage; both heavy-chain alleles and both light-chain alleles in germline configuration (only one of each type of gene segment shown for simplicity).
Heavy-chain gene rearrangement begins with DHLJH jolnmg,
Committed precursor cell
VH-DH JHjoining of one heavychain allele allows celbsufface expressionof p-mchains together with the 'surrogate' light chains X5 and Vp~.B;signalsfrom the #Ecomplex preventfurther heavy-chaingene rearrangement (alle!ic exclusion). Pre-B cell
Productive rearrangement of a light-chain allele allows cell-surface expression of functional immunoglobin m0lecu[es in what are now mature B cells that can leave the bone marrow. Mature B cell
i!!:::t¸
,ain
~ p r e - B X5 Allelic exclusioq, " ~ < : ~ growth, up-regulation " ~ of IgL rearrangement lam
Allele 1 Heavy-chain genes VH Allele 2 VH
DH
JH
CH
DH JH
CH
VH DHJH CH
>
VHDHJH CH
VH DHJH CH (or germline)
Allele 1 Light-chain genes VL
>
~L[m VHDHJH
VH DHJH CH
VH DHJH CH (or germline)
JL
CL
VL
JL
CL
VL
JL
eL
JL
CL
VL
JL
eL
VL
JL
CL
Clt
(or germline) VLJL
CL
JL
CL
Allele 2
VL
VL
Fig. 2. Steps in the maturation of B cells, showing the role of surrogate light chains in allowing cell surface expression of the Iz heavy chain prior to rearrangement of the light-chain gene and the postulated signalling activity of the Iz heavy chain complex leading to allelic exclusion. (Rarely, premature rearrangement of the light-chain gene may bypass the need for surrogate light chains.) 560
@ 1992 Current Biology
type allotype as well as secretory IgM of the mutant allotype, showing that the mutant I.tm locus is incapable of signalling to prevent rearrangement of the wild type allele. It is not clear why these 'double-expressors' are less than expected (50 %) if each allele is" equally likely to be the frst to successfully rearrange perhaps the targeted allele is defective in some way. Allelic exclusion is hardly affected in mice homozygous for the X5 disruption [3]; most mature B cells are perfectly allelically excluded and only 0.3 % express IgM from both alleles. Why is the arrest in B-cell development and allelic exclusion leaky in X5 deficient mice? One explanation is that conventional L chains are prematurely rearranged and take over the role of Vpr e B/;L5 in a few maturing B cells, which then escape the developmental blockade. IgH rearrangement is not a necessary pre-requisite for this: 'spontaneous' L chain gene rearrangements have been observed to precede H-chain rearrangements in mouse and human pre-B cell lines and have now been found to occur at a low frequency in the bone marrow of even the btm-deficient mice [4]. Another facet to the developmental regulation of IgH rearrangement has also surfaced in these mutant mice. Of the three possible translational reading frames (rf) of DH, which could be used in productive VErDI_F-JHjoins, one of them (rf3) contains stop codons in most DH segments and is therefore infrequently seen in the sequences of antibody variable domains in mature B lymphocytes. Of the two remaining, preferential usage of rfl is observed in antibody sequences, whereas rf2 is highly under-represented. This bias is unexpected because the rearrangement mechanism itself is imprecise, suggesting that rfl usage is in some way actively selected for during development. The evolutionary advantage of such selection may lie in the fact that rfl, unlike rf2, encodes multiple tyrosine and tryptophan residues in the DH-JH segment. Aromatic residues in this region are often used in antigen binding. Analysis of ~tm-deficient mice shows that one impor tant component of rf selection occurs through the btm-surrogate light chain complex [5]. After DH-JH joining but before VH-DHIH joining, rf2 (but not rfl) in "almost all D~/s can encode a truncated version of the I.tm protein (DI.tm), which includes an amino-terminal leader peptide and could, in theory, be exported. There is evidence to support this possibility: DI.tm can be expressed in transfected pre-B cells, where it associates with VpreB/~,5 and ig transported to the cell surface [10]. It has been postulated tha~ the D~tm surrogate light chain complex signals prematurely to stop further VH to DvJH rearrangements, aborting further development of D~tm-expressing cells. This would then account for the observed under-representation of rf2 usage in mature calls. Disruption of the I.tm gene should impede this by preventing Dbtm expression in pre-B cells using rf2. This is precisely what is observed: B lymphocytes that carry one disrupted and one wild-type btm allele display an equal frequency of rfl and rf2 usage in DH-JH joins on the mutant allele [5]. Elegant though this demonstration
Volume2
Number 10
1992
is, further analysis will be necessary before the general significance of rf selection becomes clear. The studies summarized here permit a more precise definition of the functions of the btm-surrogate light chain complex (Fig. 2) They confirm that a transmembrane signal, transmitted by the complex, is required for allelic exclusion, selection of DH reading frame usage and for the survival or expansion of progenitors undergoing IgH rearrangement. Surprisingly, IgL rearrangement occurs at a low frequency even in the absence of I~m and so the complex probably signals to upregulate rather than to initiate this process. The production of both CD5-expressing and conventional B cells is affected in a very similar way by the disruption of btm or )v 5: divergence, if any, of their respective developmental pathways must occur relatively late. Several puzzles remain. How does the ~tm-surrogate light chain complex transmit a signal? How is IgH rearrangement stopped? How is IgL rearrangement induced? The btm- and X5-deficient mice are powerful new tools for the analysis of these questions.
References 1.
SCHATZ D, OETITNGER M, SCHLISSEL M: V(D)J recombination: molecular biology and regulation. A n n u Rev i m m u n o l 1992, 10:359-383.
2.
KITAMURAD, ROES J, KOHN R, RAJEWSKYK: A B cell-deficient m o u s e by targeted disruption of the m e m b r a n e e x o n of t h e immunnglobulin g chain gene. Nature 1991, 350:423-426.
3.
KITAMURAD, KUDO A, SCHAA S, MOLTERW, MELCHERSF, RAJEWSKY K; A critical role of X5 protein in B-cell development. Cell 1992, 69:823431.
4,
I~TAMURAD, RAJEWSKYK: Targeted disruption of g chain m e m brane e x o n causes loss of heavy-chain allelic exclusion. Nature 1992, 356:154-156.
5.
GU H, KITAMURAD, RAJEWSKYK: B-cell development regulated by gene rearrangement: arrest of maturation by m e m b r a n e b o u n d Dbt protein and selection of D H e l e m e n t reading frames. Cell 1991, 65:47-54.
6.
PILLAI S, BALTIMORED: Formation of disulphide-linked p.2c02 tetramers in pre-B cells by t h e 18K c0-immunoglobulin light chain. Nature 1987, 329:172-174.
7.
KARASUYAMAH, KUDO A, MELCHERS F: The proteins e n c o d e d by the Vpre. B and %5 pre-B cell specific genes can associate with each other and with bt heavy chain. J Exp M e d 1990, 172:969-972.
8.
VENK1TARAMANA, WILLIAMSG, DARIAVACHP, NEUBERGERM: The B-cell antigen receptor of t h e five immunoglobulin classes. Nature 1991, 352:777-781.
9.
TAKEMORIT, ET AL.: Two types of ~t chain c o m p l e x e s are e x p r e s s e d during differentiation from pre-B to mature cells. EMBO J 1990, 9:2493-2498.
10.
TSUBATAT, TSUBATAR, RETH M: Cell surface expression of t h e short immunoglobulin bt chain (Dp. protein) in murine pre-B cells is differently regulated from that of the intact bt chain. Eur J I m m u n o l 1991, 21:13591363.
Ashok Venkitaraman, Medical Research Council Laboratow of Molecular Biology, Cambridge CB2 2QH.
S61