Genes, structures and function of T lymphocyte antigen receptors

Our knowledge of the genes encoding the T-cell antigen receptors has expanded dramatically in the last year. Much of the new data was presented at a recent EMBO workshop. The participants attempted to integrate the structural and genetic information with some of the immunological properties of T lymphocytes.

Expression of the T-cell-specific gamma gene is unnecessary in T cells recognizing class II MHC determinants

Subtractive complementary DNA cloning combined with partial protein sequencing has allowed identification of the genes encoding the alpha and beta subunits of T-cell receptors. The subtractive cDNA library prepared from the cytotoxic T lymphocyte (Tc) clone 2C has been found to contain a third type of clone encoding the gamma chain. The gamma gene shares several features with the alpha and beta genes: (1) assembly from gene segments resembling immunoglobulin V, J and C (respectively variable, joining and constant region) DNA segments; (2) rearrangement and expression in T cells and not in B cells; (3) sequences reminiscent of transmembrane and intracytoplasmic regions of integral membrane proteins; (4) a cysteine residue at the position expected for an interchain disulphide bond. The alpha and beta genes are expressed at equivalent levels in both Tc cells and helper T cells (TH). The gamma gene, obtained from 2C, has been found to be expressed in all Tc cells studied. Here we present evidence that strongly suggests that TH cells do not require gamma gene expression.

A novel serine esterase expressed by cytotoxic T lymphocytes

Cytotoxic T (Tc) lymphocytes recognize and lyse target cells and are thought to serve as an important defence against viral infections and possibly against neoplasms. The nature of the receptors responsible for antigen recognition by these cells is becoming clearer, but the molecular mechanisms responsible for their cytolytic activity remain largely unknown. The possibility that proteases are involved in this process has been suggested by the effects of certain inhibitors. Here we demonstrate that clones of murine Tc cells possess considerable trypsin-like esterase activity when assayed by a sensitive colorimetric assay. This activity was blocked completely by two serine esterase inhibitors, diisopropylfluorophosphoridate (DFP) and phenylmethylsulphonyl fluoride (PMSF), but not by N alpha-tosyl lysyl chloromethyl ketone (TLCK). The use of 3H-DFP as an affinity-labelling reagent demonstrated that the esterase activity resides in a protein of relative molecular mass (Mr) 28,000 (28K). A wide variety of other lymphocytes, including those from thymus, spleen and lymph node, established lines of B cells and noncytotoxic T cells, and clones of T helper cells, had about 300-fold less esterase activity than the Tc-cell clones and far smaller amounts of the DFP-reactive 28K protein. However, in thymocytes the esterase activity increased 20-50-fold and the 28K protein became more prominent 4 days after these cells had been stimulated in vitro to generate Tc cells.

Limited diversity of the rearranged T-cell gamma gene

The immunoglobulin-related, T-cell specific gamma gene is rearranged in a wide variety of murine T lymphocytes. We detected gamma-gene transcripts in all cloned cytotoxic T lymphocytes examined but in only 1 of 11 T-helper cell lines or hybridomas. Although in cytotoxic T cells, the rearranged gamma gene seems to have been assembled from the same germ-line variable and joining gene segments, the transcribed gene exhibited distinct sequence diversity near the junction between these segments.

Chromosomal locations of the murine T-cell receptor alpha-chain gene and the T-cell gamma gene

Two independent methods were used to identify the mouse chromosomes on which are located two families of immunoglobulin (Ig)-like genes that are rearranged and expressed in T lymphocytes. The genes coding for the alpha subunit of T-cell receptors are on chromosome 14 and the gamma genes, whose function is yet to be determined, are on chromosome 13. Since genes for the T-cell receptor beta chain were previously shown to be on mouse chromosome 6, all three of the Ig-like multigene families expressed and rearranged in T cells are located on different chromosomes, just as are the B-cell multigene families for the Ig heavy chain, and the Ig kappa and lambda light chains. The findings do not support earlier contentions that genes for T-cell receptors are linked to the Ig heavy chain locus (mouse chromosome 12) or to the major histocompatibility complex (mouse chromosome 17).

Structure, organization, and somatic rearrangement of T cell gamma genes

We present the initial characterization of a novel family of genes that rearrange in T cells, but do not encode either of the defined (alpha/beta) subunits of the clone-specific heterodimer of the T cell receptor. The family comprises at least three variable (V) gene segments, three constant (C) gene segments, and three junction (J) gene segments. In a cloned cytolytic T lymphocyte, 2C, one of each of these fragments has productively rearranged to yield an expressed VJC transcription unit, which shows no evidence for somatic mutation. Short sequences similar to those implicated in immunoglobulin gene and T cell receptor beta chain gene rearrangement flank the V and J segments of this family. The linkage of two of the three V gene segments has been determined: the segments lie approximately 2.5 kb apart, and are arranged head-to-head. The inverted arrangement may cast light upon the mechanisms utilized by lymphocytes for gene rearrangement.

Attachment of an anti-receptor antibody to non-target cells renders them susceptible to lysis by a clone of cytotoxic T lymphocytes

The molecular basis for the dependence of antigen recognition by T cells on products of the major histocompatibility complex (MHC) is unknown, and the antigenic structures that are actually bound by T-cell receptors are ill-defined. In this study, we asked whether a monoclonal antibody (mAb) that reacts with the T-cell receptor of a clone of murine cytotoxic T lymphocytes (CTL) and not with the receptors of other CTL clones can substitute for that clone's natural ligand in specific cytolytic reactions. To answer the question, a mAb (1B2) to the receptor of a CTL clone (2C) was attached covalently to 51Cr-labeled cells that were not otherwise susceptible to lysis by clone 2C, and the cells thus modified were then tested as targets for clone 2C and other CTL clones of similar specificity. All labeled cells modified in this way, including a murine cell line that expresses no cell-surface MHC class I molecules and a human cell line, were lysed by clone 2C but not by other CTL clones. If, however, instead of attaching the mAb to the receptor of clone 2C, the cells were modified by attaching to them mAbs to other surface antigens on CTL [lymphocyte function-associated antigen (LFA-1), Thy-1.2], they were not lysed. In cytolytic titrations, the cells that had been converted by attachment of mAb 1B2 into specific targets for clone 2C were just as susceptible to lysis by that clone as the clone's natural H-2d targets (e.g., P815 cells). However, some accessory surface molecules (LFA-1, Lyt-2) that are required for clone 2C to lyse its natural H-2d targets seemed not to be required for this clone to lyse the mAb-converted target cells. By demonstrating that a variety of different cell types can be thus converted into target cells for CTL, the approach described in this study may provide opportunities to analyze further the mechanisms by which CTL destroy target cells.

A third rearranged and expressed gene in a clone of cytotoxic T lymphocytes

In addition to the two previously identified genes rearranged and expressed in a cytotoxic T-lymphocyte clone, we have identified a third gene that is also rearranged and expressed in the same clone. This new gene shows clonal diversity, codes for a polypeptide chain that contains immunoglobulin-like variable and constant domains, carries potential N-glycosylation sites and is a particularly attractive candidate for the gene that encodes the alpha-subunit of the heterodimeric antigen receptor of this T-cell clone.

Expression of structurally diverse Qa-2-encoded molecules on the surface of cloned cytotoxic T lymphocytes

Extracts of 125I-labeled cloned murine cytotoxic T lymphocytes (CTL) were immunoprecipitated with alloantisera to the cloned CTL and rabbit antisera to beta-2 microglobulin. Polyacrylamide gel electrophoresis (PAGE) of the specific precipitates revealed, as expected, 125I-labeled components that corresponded to products of class I genes of the major histocompatibility complex (MHC). However, additional class I gene products of relatively low apparent molecular weight (Mr) were also observed. Similar analyses of spleen cells from a variety of MHC-congenic mouse strains suggested that the class I molecules of relatively low Mr are encoded in the Qa-2 region of the MHC, and this was confirmed by immunoprecipitation with a monoclonal antibody to Qa-2. Surprisingly, however, the cell surface Qa-2 molecules of different CTL clones differed in Mr, in isoelectric focusing (IEF) pattern, and in the number of distinguishable molecules expressed per clone: some clones seemed to express only a single Qa-2-encoded molecule while others expressed two distinct ones. Treatment of the immunoprecipitated Qa-2 with endoglycosidase F (Endo F) resulted in a decrease in Mr of approximately 5,000-6,000, corresponding to the expected loss of N-linked oligosaccharides, but the decrease did not eliminate structural variability among the clones. Structural diversity of the Qa-2-encoded molecules expressed on CTL could arise because CTL clones differ (a) in the particular Qa-2 genes they express, (b) in the way they splice Qa-2 gene transcripts or, perhaps, (c) in Endo F-resistant oligosaccharides on their Qa-2 molecules.

Amino acid sequence diversity in mouse lambda 2 variable regions

The lambda-chains of immunoglobulins from BALB/c mice constitute the simplest system presently available for studying patterns of variable-region diversity. The limited number of V lambda and J lambda germ-line gene segments facilitates comparison of expressed and germ-line sequences. We report here the complete amino acid sequence of the variable regions of three lambda 2 chains and of one chain representing a V lambda 2----J lambda 3 rearrangement. Together with the previously determined sequence of the lambda 2 chain from myeloma MOPC-315, the results illustrate the following types of variable-region diversification: expression of a single V gene segment with more than one J segment, variability at the V-J junction, and presumably, somatic mutation in V and in J. The extent of somatic diversification in these lambda 2 chains is limited, consistent with results obtained previously with lambda 1 chains.

Amino acid sequence diversity in mouse lambda 2 variable regions

The lambda-chains of immunoglobulins from BALB/c mice constitute the simplest system presently available for studying patterns of variable-region diversity. The limited number of V lambda and J lambda germ-line gene segments facilitates comparison of expressed and germ-line sequences. We report here the complete amino acid sequence of the variable regions of three lambda 2 chains and of one chain representing a V lambda 2----J lambda 3 rearrangement. Together with the previously determined sequence of the lambda 2 chain from myeloma MOPC-315, the results illustrate the following types of variable-region diversification: expression of a single V gene segment with more than one J segment, variability at the V-J junction, and presumably, somatic mutation in V and in J. The extent of somatic diversification in these lambda 2 chains is limited, consistent with results obtained previously with lambda 1 chains.

Diversity at the variable-joining region boundary of lambda light chains has a pronounced effect on immunoglobulin ligand-binding activity

By recombining lambda light (L) chains having known variable (V) region amino acid or nucleotide sequences with a heavy (H) chain from a myeloma protein or a monoclonal antibody, we obtained reconstituted Igs that differed from each other in sequence by only one or a few amino acid substitutions at known L chain positions. Differences in affinity of the reconstituted Igs for 2,4-dinitrophenyl (DNP) ligands revealed a pronounced effect on Ig binding activity of amino acids at the V-J boundary of the lambda chains. In one instance, two reconstituted Igs that differed about 1000-fold in affinity for epsilon-DNP-aminocaproate differed in primary structure by only a single tyrosine-phenylalanine substitution at the V-J junction (position 98) of their lambda 2 chains--i.e., by only one out of approximately 660 amino acid residues (L + H chains). By focusing on affinity changes, chains with unusual V lambda-J lambda junctional residues were identified. It is possible that because of a critical effect on tertiary structure junctional amino acid variations arising from gene segment assembly (V/J and perhaps V/D/J) constitute an important source of ligand-binding diversity of antibodies.

Amino acid and nucleotide sequences of variable regions of mouse immunoglobulin light chains of the lambda 3-subtype

To learn about the V lambda gene segments that are expressed in lambda 3 light chains, the most recently identified lambda-chain subtype in inbred mice, we determined partial amino acid sequences of the V regions of two of these chains, L5-8 and Lc49 . The partial sequences were extended by establishing the complete V region sequence of cDNA for the lambda-chain mRNA from the hybridoma (RZ 5-8) and the myeloma ( CBPC -49) that produce these chains. The primer extension method used to sequence the cDNA is described in detail, because the same primer (a synthetic heptadecadeoxynucleotide ) can be used for sequencing cDNA for lambda-chains of all three subtypes of inbred mice and probably for lambda-chains from some other vertebrate species as well. The results confirm earlier preliminary findings that for both chains the V region is encoded by the V lambda 1 and J lambda 3 gene segments. The unmutated germ-line sequences of these gene segments are present in both chains, but the two chains, nevertheless, differ at codon 97, the V lambda-J lambda boundary. A T/G difference in the third position of this codon resulted in a codon for histidine (CAT) in one chain (L5-8) and for glutamine (CAG) in the other chain ( Lc49 ). This difference can be accounted for by variation in the site of V lambda 1-J lambda 3 recombination. Though the V region amino acid sequences of L5-8 and Lc49 differ only by the His/Gln substitution at position 97, the two chains have been shown (manuscript in preparation) to differ in their ability to form an effective combining site for the 2,4-dinitrophenyl group.

Frequency of lambda light chain subtypes in mouse antibodies to the 2,4-dinitrophenyl (DNP) group

Of the three lambda chain subtypes made by inbred mice, chains of the lambda 1 subtype are much more frequent than those of the other subtypes (lambda 2,lambda 3) in antibodies (Ab) to those few antigenic structures that are known to elicit responses, in which lambda chains are the predominant type of light chain [(4-hydroxy-3-nitrophenyl)acetyl (NP) and dextran]. The reason for the frequency differences are not understood, and the large difference between the lambda 1 and lambda 3 frequencies is particularly puzzling, because in nearly all (about 95%) chains of these subtypes the N-terminal 97 or 98 amino acids are endoded by the same V lambda-gene segment. In an effort to identify an Ab response that has different lambda subtype frequencies, we analyzed the light chains of the Ab made by BALB/c and B6 mice in response to 2,4-dinitrophenylated chicken gamma globulin (DNP-CGG). We found that approximately 40% of the elicited anti-DNP molecules had lambda chains and of these approximately 40% were of the lambda 2 or lambda 3 subtype. Polyacrylamide gel electrophoresis indicated that the lambda 2 and lambda 3 chains were about equally abundant. Similar lambda subtype frequencies were found in the anti-DNP Ab produced by the hybridoma made with spleen cells from the same immunized mice. In the anti-DNP Ab elicited by DNP-CGG and in the anti-NP Ab elicited by NP-CGG the different lambda subtype frequencies (lambda 1/lambda 2 + lambda 3 = ca. 1.0-1.5 in anti-DNP and ca. 30 in anti-NP) were unaffected by immunizing mice with each of these antigens alone or with a mixture of the two. This finding, though preliminary, suggests that isotype-specific regulatory T cells are not responsible for the markedly different lambda subtype frequencies in anti-DNP and anti-NP Ab.

Amino acid and nucleotide sequences of variable regions of mouse immunoglobulin light chains of the lambda 3-subtype

To learn about the V lambda gene segments that are expressed in lambda 3 light chains, the most recently identified lambda-chain subtype in inbred mice, we determined partial amino acid sequences of the V regions of two of these chains, L5-8 and Lc49 . The partial sequences were extended by establishing the complete V region sequence of cDNA for the lambda-chain mRNA from the hybridoma (RZ 5-8) and the myeloma ( CBPC -49) that produce these chains. The primer extension method used to sequence the cDNA is described in detail, because the same primer (a synthetic heptadecadeoxynucleotide ) can be used for sequencing cDNA for lambda-chains of all three subtypes of inbred mice and probably for lambda-chains from some other vertebrate species as well. The results confirm earlier preliminary findings that for both chains the V region is encoded by the V lambda 1 and J lambda 3 gene segments. The unmutated germ-line sequences of these gene segments are present in both chains, but the two chains, nevertheless, differ at codon 97, the V lambda-J lambda boundary. A T/G difference in the third position of this codon resulted in a codon for histidine (CAT) in one chain (L5-8) and for glutamine (CAG) in the other chain ( Lc49 ). This difference can be accounted for by variation in the site of V lambda 1-J lambda 3 recombination. Though the V region amino acid sequences of L5-8 and Lc49 differ only by the His/Gln substitution at position 97, the two chains have been shown (manuscript in preparation) to differ in their ability to form an effective combining site for the 2,4-dinitrophenyl group.

Complete primary structure of a heterodimeric T-cell receptor deduced from cDNA sequences

Two related, but distinct, cDNA clones have been isolated and sequenced from a functional murine cytotoxic T-lymphocyte clone. The genes corresponding to these cDNA are expressed and rearranged specifically in T cells and both have similarities to immunoglobulin variable and constant region genes. It is concluded that these genes code for the two subunits of the heterodimeric antigen receptor on the surface of the T cell; its complete deduced primary structure is presented.

Restricted association of V and J-C gene segments for mouse lambda chains

The frequencies of diverse rearrangements of variable (V)lambda to joining (J)lambda gene segments were examined by Southern blot hybridization in 30 murine B-cell lines, each producing an immunoglobulin lambda light chain of known subtype (lambda 1, lambda 2, or lambda 3). For 11 out of 12 lambda 1 chains, the rearrangement was V lambda 1----J lambda 1; for 9 out of 9 lambda 2 chains, it was V lambda 2----J lambda 2; and for 8 out of 9 lambda 3 chains, it was V lambda 1----J lambda 3. Similar results were obtained by considering the partial or complete sequences at the amino acid or cDNA level of 44 other lambda chains (24 previously described): for 43 of these chains the rearranged V-J gene segments were evidently V lambda 1-J lambda 1 for 28 lambda 1 chains, V lambda 2-J lambda 2 for 10 lambda 2 chains, and V lambda 1-J lambda 3 for 5 lambda 3 chains. Of the combined total of 74 chains there were 3 with unusual V lambda rearrangements, all involving the V lambda 2 gene segment: for 2 of these unusual chains, the encoding segments were V lambda 2-J lambda 1-C lambda 1 and for one they were V lambda 2-J lambda 3-C lambda 3. Thus, the results for all 74 lambda chains show that, in contrast to the apparently unrestricted V kappa----J kappa rearrangements for kappa chains, for each of the 3 murine lambda-chain subtypes V-J recombination is severely restricted: the V lambda gene segment expressed in lambda 1 and lambda 3 chains was nearly always V lambda 1 (95% and 93%, respectively), whereas in lambda 2 chains it was without exception V lambda 2 (19 out of 19 chains). Therefore V lambda-J lambda combinatorial variation is not a significant source of amino acid sequence diversity of lambda chains of inbred mice. If the order of the lambda gene segments is 5' V lambda 2-J lambda 2C lambda 2J lambda 4C lambda 4-V lambda 1-J lambda 3C lambda 3J lambda 1C lambda 1 3', as suggested previously and by the present findings, it appears that (i) when a V lambda gene segment rearranges in a developing B cell it ordinarily recombines with a J lambda gene segment in the nearest downstream (3') cluster of J lambda C lambda segments, and (ii) V lambda rearrangement to the upstream (5') cluster is very rare and possibly may not take place at all.

Isolation and partial characterization of concanavalin A receptors on cloned cytotoxic T lymphocytes

A small set of concanavalin A (Con A)-binding glycoproteins was isolated from the surface membrane of cloned cytotoxic T lymphocytes (CTL) and partly identified using monoclonal antibodies. The binding of Con A by these glycoproteins on the CTL surface results in the secretion of gamma-interferon and in blocking the effector functions of the cells-namely, antigen-specific and lectin-dependent cytotoxicity. The Con A is evidently bound tightly to some surface structures ("Con A-receptors") that are required for the activation and cytotoxic activity of CTL. To isolate and identify these receptors, antibodies to Con A were used. After Con A was allowed to bind to radiolabeled cloned CTL (labeled with 125I or [35S]methionine or 3H-labeled amino acids), the cells were washed thoroughly, lysed in detergents and anti-Con A antibodies were added to bind to the Con A-receptor complexes. The resulting aggregates were adsorbed with protein A-bearing Staphylococci and the receptors were then specifically released from the pelleted bacteria by alpha-methyl-D-mannoside and analyzed by polyacrylamide gel electrophoresis under reducing conditions. Eight to nine labeled components were seen by autoradiography and with the aid of monoclonal antibodies to known T-cell surface molecules, four were identified as T200, lymphocyte function-associated antigen (LFA)-1, alpha- and beta-chains, and (on some clones) Lyt-2. Other components with Mr congruent to 160,000, 120,000, 46,000, 42,000, and 23,000 have not been identified. The procedures described here may have general application in the studies of the functional properties of other cell surface molecules.

Immunoprecipitation of cell surface structures of cloned cytotoxic T lymphocytes by clone-specific antisera

Clones of cytotoxic T lymphocytes (CTL) differ in their specific reactivity with diverse target cell antigens. To learn about the uniqueness of individual CTL clones we injected rats and mice with cloned CTL in an effort to prepare clone-specific antisera and to analyze the CTL surface molecules that were immunoprecipitated by these antisera. Three clones were studied. They were all derived from BALB.B mice and were specific for antigens encoded by the major histocompatibility complex of the H-2d haplotype. Antisera raised in rats against individual clones contained antibodies to lymphocyte function-associated antigen type 1 (LFA-1) and inhibited the cytotoxic activity of all of the clones. In contrast, BALB/c and BALB.K mice injected with individual clones consistently yielded alloantisera that were clone specific in their ability to inhibit CTL-mediated lysis of target cells (P815). In addition, these alloantisera immunoprecipitated from extracts of 125I-radiolabeled CTL a disulfide-bonded dimer consisting of approximately equal to 45-kilodalton subunits. This dimer resembles the putative T-cell antigen-recognition receptor recently identified in several laboratories. The alloantisera also immunoprecipitated CTL surface molecules that were associated with beta 2-microglobulin and that differed in apparent molecular mass (37-38 kilodaltons) in different clones.

The site of action of N-alpha-tosyl-L-lysyl-chloromethyl-ketone (TLCK) on cloned cytotoxic T lymphocytes

N-alpha-tosyl-L-lysyl-chloromethyl-ketone (TLCK), an irreversible inhibitor of trypsin-like serine proteases, is a potent, nontoxic inhibitor of cytotoxic T lymphocyte (CTL) activity with half-maximal inhibition of an alloreactive CTL clone occurring at [TLCK] = 30 microM. We have utilized TLCK as an affinity probe for functionally important CTL surface molecules by raising rabbit antibodies specific for the tosyl group and employing them as immunoprecipitating reagents. When 125I-labeled cloned CTL were treated with TLCK, immunoprecipitation with rabbit anti-tosyl antibodies and analysis by polyacrylamide gel electrophoresis revealed a small number of TLCK-binding proteins. Prior alkylation of radiolabeled CTL with iodoacetamide inhibited TLCK binding only slightly, suggesting that TLCK binding did not occur via free sulfhydryl groups. Thymocytes and a second CTL clone both had very similar patterns of TLCK-binding proteins; in contrast the TLCK-binding proteins of B cells differed greatly. Sequential immunoprecipitation experiments identified the predominant CTL TLCK-binding protein as T200. Lymphocyte function-associated antigen-1 also reacted with TLCK but to a lesser extent. The inhibitory role of cell-surface bound TLCK (vs intracellular TLCK) was demonstrated by protection experiments using Concanavalin A, a reversible ligand of the CTL cell surface. These experiments suggest that T200 may be required for cytotoxic activity of CTL.

The site of action of N-alpha-tosyl-L-lysyl-chloromethyl-ketone (TLCK) on cloned cytotoxic T lymphocytes

N-alpha-tosyl-L-lysyl-chloromethyl-ketone (TLCK), an irreversible inhibitor of trypsin-like serine proteases, is a potent, nontoxic inhibitor of cytotoxic T lymphocyte (CTL) activity with half-maximal inhibition of an alloreactive CTL clone occurring at [TLCK] = 30 microM. We have utilized TLCK as an affinity probe for functionally important CTL surface molecules by raising rabbit antibodies specific for the tosyl group and employing them as immunoprecipitating reagents. When 125I-labeled cloned CTL were treated with TLCK, immunoprecipitation with rabbit anti-tosyl antibodies and analysis by polyacrylamide gel electrophoresis revealed a small number of TLCK-binding proteins. Prior alkylation of radiolabeled CTL with iodoacetamide inhibited TLCK binding only slightly, suggesting that TLCK binding did not occur via free sulfhydryl groups. Thymocytes and a second CTL clone both had very similar patterns of TLCK-binding proteins; in contrast the TLCK-binding proteins of B cells differed greatly. Sequential immunoprecipitation experiments identified the predominant CTL TLCK-binding protein as T200. Lymphocyte function-associated antigen-1 also reacted with TLCK but to a lesser extent. The inhibitory role of cell-surface bound TLCK (vs intracellular TLCK) was demonstrated by protection experiments using Concanavalin A, a reversible ligand of the CTL cell surface. These experiments suggest that T200 may be required for cytotoxic activity of CTL.

Synthesis of lambda light chain subtypes by stimulated and unstimulated mouse B cells

Inbred mouse make 3 lambda chain subtypes. The lambda 1 and lambda 3 chains have similar variable (V) regions (in both the same V gene segment [V lambda 1] is used), whereas lambda 2 and lambda 3 have similar constant (C) regions. Despite the lambda 1 and lambda 3 V region similarity, lambda 1 occurs much more frequently than lambda 3 (and lambda 2) in the serum immunoglobulins and antibody responses of most inbred strains of mice. To explore the basis for the lambda 1 predominance, we compared the rates of synthesis of the 3 subtypes and the frequencies of the B cells that synthesize them, focussing on "resting" (i.e., unstimulated) and on polyclonally stimulated B cells from spleens of unimmunized BALB/c mice. In resting cells the relative rates of synthesis and the relative frequencies of the respective B cells were in accord, indicating that the rate of lambda chain synthesis is approximately the same per resting B cell, regardless of the lambda subtype it produces. However, in the polyclonally stimulated cells, lambda 1 was made 7 times faster than lambda 2 and 10 times faster than lambda 3; normalizing these rates by the frequencies of the respective stimulated cells suggests that in stimulated B cells lambda 1 chains are made 5 times faster per cell than lambda 2 or lambda 3, while the latter are made at about the same rate per cell. In view of the marked structural homology between lambda 2 and lambda 3 genes in segments other than the V-gene segment, we suggest that the pronounced differences among polyclonally stimulated B cells in expression of the genes for the various lambda subtypes may be due to the presence of less potent enhancer-like sequences in the lambda 2 and lambda 3 genes than in the lambda 1 gene.