Downstream CH Research Articles

Loop extrusion promotes an alternate pathway for isotype switching.

SUMMARYClass-switch recombination (CSR) involves replacement of the Cμ constant region with another downstream CH region. CSR is initiated by activation-induced cytidine deaminase (AID)-mediated DNA breaks that are targeted to transcriptionally active switch (S) regions. S region promoters (Prs) direct synapsis by associating with the Eμ and 3′Eα enhancers that jointly anchor a chromatin loop. We report that asymmetric loop extrusion allows 3′Eα to track along the locus and form Pr-Pr-E interactions that mediate CSR between downstream S regions, followed by switching to donor Sμ. This alternative pathway bypasses sequential switching and creates immunoglobulin (Ig)E+ B cells in the absence of IgG1 expression. Based on the analysis of diagnostic CSR products in B cell subsets, we identify a BCR-negative cell intermediate that is pivotal to efficient CSR.

Orientation-specific joining of AID-initiated DNA breaks promotes antibody class switching.

During B cell development, RAG endonuclease cleaves immunoglobulin heavy chain (IgH) V, D, and J gene segments and orchestrates their fusion as deletional events that assemble a V(D)J exon in the same transcriptional orientation as adjacent Cμ constant region exons1,2. In mice, six additional sets of constant region exons (CHs) lie 100-200 kb downstream in the same transcriptional orientation as V(D)J and Cμ exons2. Long repetitive switch (S) regions precede Cμ and downstream CHs. In mature B cells, class switch recombination (CSR) generates different antibody classes by replacing Cμ with a downstream CH2. Activation-Induced Cytidine Deaminase (AID) initiates CSR by promoting deamination lesions within Sμ and a downstream acceptor S region2,3; these lesions are converted into DNA double-strand breaks (DSBs) by general DNA repair factors3. Productive CSR must occur in a deletional orientation by joining the upstream end of an Sμ DSB to the downstream end of an acceptor S region DSB (Fig. 1a). However, the relative frequency of deletional to inversional CSR junctions had not been measured. Thus, whether orientation-specific joining is a programmed mechanistic feature of CSR as it is for V(D)J recombination and, if so, how this is achieved was unknown. To address this question, we adapted high-throughput genome-wide translocation sequencing (HTGTS)4 into a highly sensitive DSB end-joining assay and applied it to endogenous AID-initiated S region DSBs. We find that CSR indeed is programmed to occur in a productive deletional orientation and does so via an unprecedented mechanism that involves in cis IgH organizational features in combination with frequent S region DSBs initiated by AID. We further implicate ATM-dependent DSB response (DSBR) factors in enforcing this mechanism and provide a solution to the enigma of why CSR is so reliant on the 53BP1 DSBR factor.

3′RR targeting in lymphomagenesis: a promising strategy?

3′RR targeting in lymphomagenesis: a promising strategy?

Molecular Mechanisms of IgE Class Switch Recombination.

Immunoglobulin (Ig) E is the most tightly regulated of all Ig heavy chain (IgH) isotypes and plays a key role in atopic disease. The gene encoding for IgH in mature B cells consists of a variable region exon-assembled from component gene segments via V(D)J recombination during early B cell development-upstream of a set of IgH constant region CH exons. Upon activation by antigen in peripheral lymphoid organs, B cells can undergo IgH class switch recombination (CSR), a process in which the initially expressed IgH μ constant region exons (Cμ) are deleted and replaced by one of several sets of downstream CH exons (e.g., Cγ, Cε, and Cα). Activation of the IL-4 receptor on B cells, together with other signals, can lead to the replacement of Cμ with Cε resulting in CSR to IgE through a series of molecular events involving irreversible remodeling of the IgH locus. Here, we discuss the molecular mechanisms of CSR and the unique features surrounding the generation of IgE-producing B cells.

AIDing Chromatin and Transcription-Coupled Orchestration of Immunoglobulin Class-Switch Recombination.

Secondary diversification of the antibody repertoire upon antigenic challenge, in the form of immunoglobulin heavy chain (IgH) class-switch recombination (CSR) endows mature, naïve B cells in peripheral lymphoid organs with a limitless ability to mount an optimal humoral immune response, thus expediting pathogen elimination. CSR replaces the default constant (CH) region exons (Cμ) of IgH with any of the downstream CH exons (Cγ, Cε, or Cα), thereby altering effector functions of the antibody molecule. This process depends on, and is orchestrated by, activation-induced deaminase (AID), a DNA cytidine deaminase that acts on single-stranded DNA exposed during transcription of switch (S) region sequences at the IgH locus. DNA lesions thus generated are processed by components of several general DNA repair pathways to drive CSR. Given that AID can instigate DNA lesions and genomic instability, stringent checks are imposed that constrain and restrict its mutagenic potential. In this review, we will discuss how AID expression and substrate specificity and activity is rigorously enforced at the transcriptional, post-transcriptional, post-translational, and epigenetic levels, and how the DNA-damage response is choreographed with precision to permit targeted activity while limiting bystander catastrophe.

Immature B cells preferentially switch to IgE with increased direct Sμ to Sε recombination

Immunoglobulin heavy chain (IgH) class-switch recombination (CSR) replaces initially expressed Cμ (IgM) constant regions (C(H)) exons with downstream C(H) exons. Stimulation of B cells with anti-CD40 plus interleukin-4 induces CSR from Cμ to Cγ1 (IgG1) and Cε (IgE), the latter of which contributes to the pathogenesis of atopic diseases. Although Cε CSR can occur directly from Cμ, most mature peripheral B cells undergo CSR to Cε indirectly, namely from Cμ to Cγ1, and subsequently to Cε. Physiological mechanisms that influence CSR to Cγ1 versus Cε are incompletely understood. In this study, we report a role for B cell developmental maturity in IgE CSR. Based in part on a novel flow cytometric IgE CSR assay, we show that immature B cells preferentially switch to IgE versus IgG1 through a mechanism involving increased direct CSR from Cμ to Cε. Our findings suggest that IgE dysregulation in certain immunodeficiencies may be related to impaired B cell maturation.

Endonuclease G plays a role in immunoglobulin class switch DNA recombination by introducing double-strand breaks in switch regions

Immunoglobulin (Ig) class switch DNA recombination (CSR) is the crucial mechanism diversifying the biological effector functions of antibodies. Generation of double-strand DNA breaks (DSBs), particularly staggered DSBs, in switch (S) regions of the upstream and downstream CH genes involved in the specific recombination process is an absolute requirement for CSR. Staggered DSBs would be generated through deamination of dCs on opposite DNA strands by activation-induced cytidine deaminase (AID), subsequent dU deglycosylation by uracil DNA glycosylase (Ung) and abasic site nicking by apurinic/apyrimidic endonuclease. However, consistent with the findings that significant amounts of DSBs can be detected in the IgH locus in the absence of AID or Ung, we have shown in human and mouse B cells that AID generates staggered DSBs not only by cleaving intact double-strand DNA, but also by processing blunt DSB ends generated in an AID-independent fashion. How these AID-independent DSBs are generated is still unclear. It is possible that S region DNA may undergo AID-independent cleavage by structure-specific nucleases, such as endonuclease G (EndoG). EndoG is an abundant nuclease in eukaryotic cells. It cleaves single and double-strand DNA, primarily at dG/dC residues, the preferential sites of DSBs in S region DNA. We show here that EndoG can localize to the nucleus of B cells undergoing CSR and binds to S region DNA, as shown by specific chromatin immunoprecipitation assays. Using knockout EndoG−/− mice and EndoG−/− B cells, we found that EndoG deficiency resulted in a two-fold reduction in CSR in vivo and in vitro, as demonstrated by reduced cell surface IgG1, IgG2a, IgG3 and IgA, reduced secreted IgG1, reduced circle Iγ1-Cμ, Iγ3-Cμ, Iɛ-Cμ, Iα-Cμ transcripts, post-recombination Iμ-Cγ1, Iμ-Cγ3, Iμ-Cɛ and Iμ-Cα transcripts. In addition to reduced CSR, EndoG−/− mice showed a significantly altered spectrum of mutations in IgH JH-iEμ DNA. Impaired CSR in EndoG−/− B cells did not stem from altered B cell proliferation or apoptosis. Rather, it was associated with significantly reduced frequency of DSBs. Thus, our findings determine a role for EndoG in the generation of S region DSBs and CSR.

Genomic deletion of the whole IgH 3′ regulatory region (hs3a, hs1,2, hs3b, and hs4) dramatically affects class switch recombination and Ig secretion to all isotypes

AbstractThe immunoglobulin heavy chain locus (IgH) undergoes multiple changes along B-cell differentiation. In progenitor B cells, V(D)J assembly allows expression of µ heavy chains. In mature B cells, class switch recombination may replace the expressed constant (C)µ gene with a downstream CH gene. Finally, plasma cell differentiation strongly boosts IgH transcription. How the multiple IgH transcriptional enhancers tune these changes is unclear. Here we demonstrate that deletion of the whole IgH 3′ regulatory region (3′RR) allows normal maturation until the stage of IgM/IgD expressing lymphocytes, but nearly abrogates class switch recombination to all CH genes. Although plasma cell numbers are unaffected, we reveal the role of the 3′RR into the transcriptional burst normally associated with plasma cell differentiation. Our study shows that transcriptional changes and recombinations occurring after antigen-encounter appear mainly controlled by the 3′RR working as a single functional unit.

Switch Region Identity Plays an Important Role in Ig Class Switch Recombination

Ig class switch recombination (CSR) is regulated through long-range intrachromosomal interactions between germline transcript promoters and enhancers to initiate transcription and create chromatin accessible to activation-induced deaminase attack. CSR occurs between switch (S) regions that flank Cmu and downstream C(H) regions and functions via an intrachromosomal deletional event between the donor Smicro region and a downstream S region. It is unclear to what extent S region primary sequence influences differential targeting of CSR to specific isotypes. We address this issue in this study by generating mutant mice in which the endogenous Sgamma3 region was replaced with size-matched Sgamma1 sequence. B cell activation conditions are established that support robust gamma3 and gamma1 germline transcript expression and stimulate IgG1 switching but suppress IgG3 CSR. We found that the Sgamma1 replacement allele engages in micro-->gamma3 CSR, whereas the intact allele is repressed. We conclude that S region identity makes a significant contribution to CSR. We propose that the Sgamma1 region is selectively targeted for CSR following the induction of an isotype-specific factor that targets the S region and recruits CSR machinery.

Downstream class switching leads to IgE antibody production by B lymphocytes lacking IgM switch regions

Ig heavy chain (IgH) class-switch recombination (CSR) replaces the IgH C mu constant region exons with one of several sets of downstream IgH constant region exons (e.g., C gamma, C epsilon, or C alpha), which affects switching from IgM to another IgH class (e.g., IgG, IgE, or IgA). Activation-induced cytidine deaminase (AID) initiates CSR by promoting DNA double-strand breaks (DSBs) within switch (S) regions flanking the donor C mu (S mu) and a downstream acceptor C(H) (e.g., S gamma, S epsilon, S alpha) that are then joined to complete CSR. DSBs generated in S mu frequently are joined within S mu to form internal switch region deletions (ISD). AID-induced ISD and mutations have been considered rare in downstream S regions, suggesting that AID targeting to these S regions requires its prior recruitment to S mu. We have now assayed for CSR and ISD in B cells lacking S mu (S mu(-/-) B cells). In S mu(-/-) B cells activated for CSR to IgG1 and IgE, CSR to IgG1 was greatly reduced; but, surprisingly, CSR to IgE occurred at nearly normal levels. Moreover, normal B cells had substantial S gamma1 ISD and increased mutations in and near S gamma1, and levels of both were greatly increased in S mu(-/-) B cells. Finally, S mu(-/-) B cells underwent downstream CSR between S gamma1 and S epsilon on alleles that lacked S mu CSR to these sequences. Our findings show that AID targets downstream S regions independently of CSR with Smu and implicate an alternative pathway for IgE class switching that involves generation and joining of DSBs within two different downstream S regions before S mu joining.

Long-range oncogenic activation of Igh-c-myc translocations by the Igh 3' regulatory region.

B cell malignancies, such as human Burkitt’s lymphoma (BL), often harbor translocations that link c-myc or other proto-oncogenes to the immunoglobulin heavy chain locus (IgH)1. The nature of elements that activate oncogenes within such translocations has been a longstanding question. Translocations within IgH involve DNA double strand breaks (DSBs) initiated either by the RAG1/2 endonuclease during V(D)J recombination or by activation induced cytidine deaminase (AID) during class switch recombination (CSR)2-4. V(D)J recombination in progenitor B (pro-B) cells assembles IgH variable region exons upstream of μ constant region (Cμ) exons, which are the first of several sets of CH exons (“CH genes”) within a CH locus that spans several hundred kilobases5,6. In mature B cells, CSR deletes Cμ and replaces it with a downstream CH gene6. An enhancer (iEμ) between the variable region exons and Cμ promotes V(D)J recombination in developing B cells7. In addition, the IgH 3’ regulatory region (IgH3’RR) lies downstream of the CH locus and modulates CSR by long-range transcriptional enhancement of CH genes8-10. Transgenic mice bearing iEμ or IgH3’RR sequences fused to c-myc are predisposed to B lymphomas, demonstrating such elements can confer oncogenic c-myc expression11-16. However, in many B cell lymphomas, IgH/c-myc translocations delete iEμ and place c-myc up to 200kb upstream of the IgH3’RR1. We now address the oncogenic role of the IgH3’RR by inactivating it in two distinct mouse models for B cell lymphoma with IgH/c-myc translocations. The IgH3’RR is dispensable for pro-B lymphomas with V(D)J recombination-initiated translocations, but required for peripheral B cell lymphomas with CSR-associated translocations. As the IgH3’RR is not required for CSR-associated IgH breaks or IgH/c-myc translocations in peripheral B cell lymphoma progenitors, we conclude this regulatory region confers oncogenic activity via long-range and developmental stage-specific activation of translocated c-myc genes.

Viral Double-Stranded RNA Triggers Ig Class Switching by Activating Upper Respiratory Mucosa B Cells through an Innate TLR3 Pathway Involving BAFF

Class switch DNA recombination (CSR) from IgM to IgG and IgA is crucial for antiviral immunity. Follicular B cells undergo CSR upon engagement of CD40 by CD40 ligand on CD4+ T cells. This T cell-dependent pathway requires 5-7 days, which is too much of a delay to block quickly replicating pathogens. To compensate for this limitation, extrafollicular B cells rapidly undergo CSR through a T cell-independent pathway that involves innate Ag receptors of the TLR family. We found that a subset of upper respiratory mucosa B cells expressed TLR3 and responded to viral dsRNA, a cognate TLR3 ligand. In the presence of dsRNA, mucosal B cells activated NF-kappaB, a transcription factor critical for CSR. Activation of NF-kappaB required TRIF (Toll/IL-1R domain-containing protein inducing IFN-beta), a canonical TLR3 adapter protein, and caused germline transcription of downstream CH genes as well as expression of AID (activation-induced cytidine deaminase), a DNA-editing enzyme essential for CSR. Subsequent IgG and IgA production was enhanced by BAFF (B cell-activating factor of the TNF family), an innate mediator released by TLR3-expressing mucosal dendritic cells. Indeed, these innate immune cells triggered IgG and IgA responses upon exposure to dsRNA. By showing active TLR3 signaling and ongoing CSR in upper respiratory mucosa B cells from patients with CD40 signaling defects, our findings indicate that viral dsRNA may initiate frontline IgG and IgA responses through an innate TLR3-dependent pathway involving BAFF.

IgH class switching and translocations use a robust non-classical end-joining pathway

Immunoglobulin variable region exons are assembled in developing B cells by V(D)J recombination. Once mature, these cells undergo class-switch recombination (CSR) when activated by antigen. CSR changes the heavy chain constant region exons (Ch) expressed with a given variable region exon from Cmu to a downstream Ch (for example, Cgamma, Cepsilon or Calpha), thereby switching expression from IgM to IgG, IgE or IgA. Both V(D)J recombination and CSR involve the introduction of DNA double-strand breaks and their repair by means of end joining. For CSR, double-strand breaks are introduced into switch regions that flank Cmu and a downstream Ch, followed by fusion of the broken switch regions. In mammalian cells, the 'classical' non-homologous end joining (C-NHEJ) pathway repairs both general DNA double-strand breaks and programmed double-strand breaks generated by V(D)J recombination. C-NHEJ, as observed during V(D)J recombination, joins ends that lack homology to form 'direct' joins, and also joins ends with several base-pair homologies to form microhomology joins. CSR joins also display direct and microhomology joins, and CSR has been suggested to use C-NHEJ. Xrcc4 and DNA ligase IV (Lig4), which cooperatively catalyse the ligation step of C-NHEJ, are the most specific C-NHEJ factors; they are absolutely required for V(D)J recombination and have no known functions other than C-NHEJ. Here we assess whether C-NHEJ is also critical for CSR by assaying CSR in Xrcc4- or Lig4-deficient mouse B cells. C-NHEJ indeed catalyses CSR joins, because C-NHEJ-deficient B cells had decreased CSR and substantial levels of IgH locus (immunoglobulin heavy chain, encoded by Igh) chromosomal breaks. However, an alternative end-joining pathway, which is markedly biased towards microhomology joins, supports CSR at unexpectedly robust levels in C-NHEJ-deficient B cells. In the absence of C-NHEJ, this alternative end-joining pathway also frequently joins Igh locus breaks to other chromosomes to generate translocations.

AID is required to initiate Nbs1/gamma-H2AX focus formation and mutations at sites of class switching.

Class switch recombination (CSR) is a region-specific DNA recombination reaction that replaces one immunoglobulin heavy-chain constant region (Ch) gene with another. This enables a single variable (V) region gene to be used in conjunction with different downstream Ch genes, each having a unique biological activity. The molecular mechanisms that mediate CSR have not been defined, but activation-induced cytidine deaminase (AID), a putative RNA-editing enzyme, is required for this reaction. Here we report that the Nijmegen breakage syndrome protein (Nbs1) and phosphorylated H2A histone family member X (gamma-H2AX, also known as gamma-H2afx), which facilitate DNA double-strand break (DSB) repair, form nuclear foci at the Ch region in the G1 phase of the cell cycle in cells undergoing CSR, and that switching is impaired in H2AX-/- mice. Localization of Nbs1 and gamma-H2AX to the Igh locus during CSR is dependent on AID. In addition, AID is required for induction of switch region (S mu)-specific DNA lesions that precede CSR. These results place AID function upstream of the DNA modifications that initiate CSR.

Molecular regulation of class switch recombination to IgE through epsilon germline transcription.

Molecular regulation of class switch recombination to IgE through epsilon germline transcription.

The B‐cell system of human mucosae and exocrine glands

Summary: The mucosae and exocrine glands harbour the largest activated B‐cell system of the body, amounting to some 80–90% of all immunoglobulins (Ig)‐producing cells. The major product of these immunocytes is polymeric (p)IgA (mainly dimers) with associated J chain. Both pIgA and pentameric IgM contain a binding site for the polymeric Ig receptor (pIgR), or secretory component (SC), which is a requirement for their active external transport through secretory epithelia. The pIgR/SC binding site depends on covalent incorporation of the J chain into the quaternary structure of the polymers when they are produced by the local immunocytes. This important differentiation characteristic appears to be sufficient functional justification for the J chain to be expressed also by most B cells terminating at secretory effector sites with IgD or IgG production; they probably represent a ‘spin‐off’ from sequential downstream CH switching on its way to pIgA expression, thus apparently reflecting a maturational stage of effector B‐cell clones compatible with homing to these sites. Observations in IgA‐deficient individuals suggest that the magnitude of this homing is fairly well maintained even when the differentiation pathway to IgA is blocked. Certain microenvironmental elements such as specific cytokines and dendritic cells appear to be required for induction of IgA synthesis, but it remains virtually unknown why this isotype normally is such a dominating product of local immunocytes and why they have such a high level of J chain expression. Also, despite the recent identification of some important requirements in terms of adhesion molecules (e.g. integrin α4β7 and MAdCAM‐1) that explain the “gut‐seeking” properties of enterically induced B cells, the origin of regionalized homing of B cells to secretory effector sites outside the gut remains elusive. Moreover, little is known about immune regulation underlying the striking disparity of both the class (IgD, IgM) and subclass (IgA1, IgA2, IgGI, IgG2) production patterns shown by local iinmttnocytes in various regions of the body, although the topical microbiota and other environmental stimuli might be important. Rational design of local vaccines will depend on better knowledge of both inductive and migratory properties of human mucosal B cells.

Expression of I mu-C gamma hybrid germline transcripts subsequent to immunoglobulin heavy chain class switching.

Germline CH transcripts initiate from a non-coding I exon and terminate downstream of the associated CH exons. Ig heavy chain class switch recombination from the VDJ-C mu gene to particular downstream CH genes appears to be regulated by a process that involves mitogen and/or cytokine induction of germline CH transcripts from the downstream genes. We have examined the expression of germline C mu transcripts (I mu-C mu transcripts) in splenic B cells and pre-B cell lines after cytokine and mitogen stimulation. In contrast to the expression of the germline transcripts from downstream CH genes, expression of germline C mu transcripts was constitutive and unaffected by mitogen and cytokine treatment. After a primary switch recombination event, the germline I mu promoter, which is now associated with a downstream CH gene, continues to be active--leading to the generation of a novel germline transcript consisting of the I mu exon spliced to the CH exons of the switched CH gene. We discuss the potential role of the expression of hybrid I mu-containing transcripts in the class switch process. We also describe a novel and sensitive assay, based on the detection of the hybrid I mu-containing transcripts, that allows detection of class switch recombination events even in heterogeneous populations of cells.

IL-4-induced expression of germline gamma 1 transcripts in B cells following cognate interactions with T helper cells.

T cell-dependent B cell activation and the induction of isotype switching require antigen and direct contact with helper T (Th) cells. During activation, B cells can switch from the expression of IgM to that of IgG, IgE or IgA, depending on the lymphokines secreted by the Th cell with which they interact. Studies of lipopolysaccharide (LPS)-activated B cells have suggested that lymphokines regulate isotype switching via a transcriptional mechanism that increases the accessibility of downstream CH genes to a switch recombinase(s). To assess the roles of T cell contact and lymphokines in isotype switching, we have examined the accessibility model for the regulation of isotype switching to IgG1 in the context of cognate interactions between Th cells and normal B cells. We demonstrate that Th2 cells that secrete IL-4 can induce expression of germline gamma 1 transcripts in B cells. The steady-state level of germline gamma 1 transcripts induced by Th2 cells is enhanced as compared with the level induced by IL-4 alone or Il-4 and LPS also alters the relative usage of the germline gamma 1 transcription initiation sites. Enhanced expression of germline gamma 1 transcripts requires direct contact between T and B cells suggesting a role for T cell contact-mediated signals in regulating the accessibility of switch regions.

S alpha BP/BSAP/NF-S mu B1, a murine and human B cell stage specific nuclear factor with DNA binding specificity implying roles in switch-recombination and transcription.

Immunoglobulin genes undergo a programmed series of DNA rearrangements in B lymphoid cells. Following assembly of a functional variable (V) region gene, an independent series of recombinations determine the heavy chain isotype of the expressed antibody (IgM, IgG, IgE and IgA). Heavy chain class switches result in the deletion and replacement of the Cμ gene with a downstream CH gene (Cℓ3, Cℓ1, Cℓ2b, Cℓ2a, C∈ or Cα) (reviewed in Marcu, 1982; Shimizu and Honjo, 1984; Radbruch et al., 1986; Gritzmacher, 1989). These recombinations are mediated by switch (S) regions which are positioned about 2 Kb 5’ of every CH gene segment with the exception of Cδ. Differential regulation of CH class switching is believed to be manifested by the accessibility of S segments to a B cell specific switch-recombinase activity (Stavnezer and Sirlin, 1986). Switch-recombination occurs at widely spaced positions within Sμ and other downstream S regions and only limited sequence homology between S sequences may be present near the site of recombination (Gritzmacher, 1989; Ott and Marcu, 1989). S regions consist of short tandemly repetitive sequences of varying length and homology which are believed to mediate switch recombination by an illegitimate mechanism. GAGCT and GGGGT sequences are found in all S regions in addition to three other commonly observed pentamers: ACCAG, GCAGC and TGAGC (reviewed in Shimizu and Honjo, 1984; Gritzmacher, 1989). A heptamer consensus motif, YAGGTTG (Y=pyrimidine), has also been found nearby a number of switch recombination sites in plasma cell tumors and hybridoma lines and is also repeated in various S segments (Marcu et al., 1982). The 5’ portion of the Sμ region is non-repetitive but does contain interspersed GAGCT, GGGGT and YAGGTTG like sequences while the 3’ portion is composed of a simple repetitive block of (GAGCT)nGGGGT motifs where it ranges from one to seven (Nikaido et al., 1981). The Sμ, S∈ and Sα sequences display considerable homology particularly in areas containing repeats of GAGCT sequences while the Sℓ regions contain a lower density of these pentamers but are typified by 49mer or 52mer (Sℓ2a) repeats and TGGGG, GCAGC and ACCAG motifs (Kataoka et al., 1981). The overall homology of the Sℓ regions correlates with their distance from SSμ (e.g., SSℓ3> Sℓ1> Sℓ2b > Sℓ2a) (shimizu et al., 1982; Stanton and Marcu 1982).KeywordsCell StageImmunoglobulin Heavy ChainSwitch RegionPlasma Cell TumorHeavy Chain ClassThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The switch region associated with immunoglobulin C mu genes is DNase I hypersensitive in T lymphocytes.

It is not known whether the antigen-specific receptor of T lymphocytes is encoded by conventional immunoglobulin genes. Several T-cell lines, as well as thymus cells, have been found to contain aberrant IgM mRNAs, but other T-cell lines are apparently negative1–4. Rearrangement of D and J gene segments which code for portions of immunoglobulin heavy (H) chain variable (V) regions has been observed in some T cells, but no complete V–D–J rearrangements, as required for H gene expression in B cells, have been found5. To investigate the chromatin structure around the Cμ gene in T lymphocytes, we have probed the regions flanking the gene by mild digestion with DNase I. We report here that T-lymphocyte chromatin exhibits sites hypersensitive to DNase I in a region 5′ of Cμ. In B cells, this region has been proposed to be responsible for switching to downstream CH genes.