Development of the primary antibody repertoire

Stable propagation of the active transcriptional state of an immunoglobulin mu gene requires continuous enhancer function

Enhancer sequences control the expression of the immunoglobulin mu heavy chain gene. In this study we examined whether the active transcriptional state of the mu gene can be propagated from mother to daughter cells after deletion of its enhancer. A mu gene that contains the enhancer flanked by D and J recombination signals was stably introduced into pre-B cells, resulting in mu gene transcription. Subsequent deletion of the enhancer from the transfected gene by D-to-J joining reproducibly resulted in switching off of mu expression. Moreover, transcriptional inactivation occurred although the mu gene was found to be hypomethylated prior to deletion of the enhancer. We conclude that the enhancer does not confer upon the mu gene "memory" of its active transcriptional state. Thus, the enhancer seems to be required both for establishment and for maintenance of mu gene expression.

Genomic multipotentiality of differentiated somatic cells

Nuclear transplantations from several differentiated somatic cell types into amphibian oocytes and eggs revealed that their genome contains the genes required for the development of prefeeding tadpoles. In addition, erythrocyte nuclei directed the formation of feeding tadpoles (independent organisms) that advanced to larval stages with hind limb buds. Thus, the genome of several differentiated somatic cell types can undergo widespread activation and specify a multiplicity of cell types. Although evidence for the genetic totipotency of differentiated somatic cells is lacking, we speculate that the genetic totipotency of at least some differentiated somatic cell types still remains a tenable hypothesis.

A T cell clone expresses two T cell receptor alpha genes but uses one alpha beta heterodimer for allorecognition and self MHC-restricted antigen recognition

All of the T cell receptor alpha- and beta-chain rearrangements present in a dual reactive T cell clone were characterized. This clone exhibits allelic exclusion of its beta-chain genes in that only one of the two alleles is productively rearranged. Unexpectedly, it displays two productive V alpha-gene rearrangements, which are both transcribed into 1.5 kb mRNA. The contribution of each of the two productive alpha genes to the dual recognition was analyzed by gene transfer. To this end, each of the two alpha genes was separately transfected with the single productively rearranged beta gene. Transfer of only one of the two alpha beta combinations restored both allogeneic MHC recognition and self MHC-restricted antigen recognition. Thus, T cell dual recognition results from the cross-reactive recognition of an allo-MHC product by a single antigen-specific and MHC-restricted alpha beta T cell receptor. Furthermore, the presence of two productively rearranged alpha-chain genes in a T cell clone raises questions concerning the level at which allelic exclusion operates in T cells.

A recombinase from Drosophila melanogaster embryos

We have partially purified a DNA strand-exchange activity (recombinase) from nuclear extracts of Drosophila melanogaster embryos. The protein fraction forms a joint molecule between a circular single-strand DNA and a homologous linear duplex DNA that is resolved from the substrates by agarose gel electrophoresis. A strand-exchange activity can be obtained from nuclear extracts from embryos as old as 24 hr. The activity is similar to that partially purified from human cells [Hsieh, P., Meyn, S.M. & Camerini-Otero, R.D. (1986) Cell 44, 885-894]. It is homology-dependent, requires Mg2+, appears to be directional in that it prefers to displace the 3' end of the noncomplementary strand, and does not require exogenous ATP. Forty nanograms of protein in the partially purified DNA strand-exchange fraction from D. melanogaster embryos can completely convert 50 ng of substrate single-strand DNA into joint molecules in 10 min. In the electron microscope, joint molecules are seen to consist of a circular single-strand DNA molecule attached to only one end of a linear duplex DNA molecule; a displaced strand is also seen. The region of heteroduplex formation can be as long as 600 base pairs. The demonstration of a strand-exchange activity from wild-type D. melanogaster embryos invites analysis of recombination-defective mutants to explore the role of DNA strand exchange in homologous recombination.

The scid defect affects the final step of the immunoglobulin VDJ recombinase mechanism

Abelson murine leukemia virus-transformed precursor B lymphocytes from scid (severe combined immunodeficient) mice, like A-MuLV transformants from normal mice, actively rearrange segments of their Ig heavy chain variable region gene locus during growth in culture. Targeting of recombination to appropriate segments appears normal in these lines as evidenced by initial rearrangement of sequences from within the D and JH locus to form aberrant "DJH" rearrangements and secondary rearrangement of sequences from within the VH locus to the aberrant "DJH" intermediates. A detailed analysis of the joints in these rearrangements indicates that the VDJ recombinase in scid pre-B cells can correctly recognize heptamernonamer signal sequences and perform precise endonucleolytic scissions at these sequences. We propose that the scid defect involves the inability of scid precursor lymphocytes to join correctly the cleaved ends of the coding strands of variable region gene segments.

Duplications and deletions of Vh genes in inbred strains of mice

The evolution of variable region (Vh) gene family copy number and polymorphism was investigated by the analysis of the immunoglobulin heavy chain variable region (Igh-V) locus in 74 inbred strains and substrains of mice. Several strains were found to have slight differences from Igh-V haplotypes previously identified, usually involving the gain or loss of one or a few members of a single Vh gene family. These results indicate that the evolution of copy number in the mouse Igh-V locus proceeds largely by the accumulation of incremental changes, reflecting the clustered organization of the mouse Igh-V locus. We have found no evidence of very large or frequent duplication or deletion events indicative of rapid expansion or contraction processes. The existence of one or more particularly large Vh gene families most likely reflects random copy number variation, rather than selection for the amplification of their members. The identification of strains with recombinant Vh gene arrays demonstrates that recombination, both within and between haplotypes, appears to be the predominant mechanism generating the high restriction fragment length polymorphism in the Igh-V locus.

A human monoclonal antibody to cytokeratin intermediate filament antigens derived from a tumor draining lymph node

Human lymphocytes derived from a lymph node draining a primary breast adenocarcinoma were fused with the mouse myeloma P3X63Ag8.653 to generate human-mouse hybridomas secreting human monoclonal antibodies (MAbs) to tumor associated antigens (TAAs). One of the resulting human MAbs, YBB 190 (IgM) is described. Enzyme-linked immunosorbent assays (ELISA) employing membrane and cytosol fractions of human tissues demonstrated YBB 190 reactivity against cytosol but not membrane components of malignant and normal epithelial tissues. When tested by an indirect immunoperoxidase staining method against fresh frozen human tissue sections, YBB 190 reacted with malignant cells in 26 of 28 epithelial cancers and with normal epithelia in 11 different benign tissues. Preliminary western blot antigen characterization indicated that YBB 190 recognizes cytokeratin intermediate filaments, or a protein that is closely associated with cytokeratins. These data indicate that B cells with specificity for intermediate filaments are present in tumor draining lymph nodes. Our findings provide insights into the nature of potential autoimmune responses in cancer patients and suggest that improved tumor directed sensitization procedures may be required to more effectively utilize lymphocytes from tumor draining lymph nodes to generate therapeutically useful human MAbs to TAAs.

Comparison of the fetal and adult functional B cell repertoires by analysis of VH gene family expression

The functional B cell repertoire in BALB/c mice was assessed at various stages in ontogeny. This was done by analyzing VH gene family expression using the sensitive technique of in situ hybridization. The B cell repertoire was probed with the mitogen, LPS, and the antigen DNP. DNP was chosen because B cells responsive to this hapten appear very early in ontogeny. The APCs that developed after stimulation with LPS or DNP were analyzed for VH gene expression by in situ hybridization of individual cells using radiolabeled VH gene family probes. The results indicated that VH gene expression in fetal B cells after stimulation was distinct from adult B cells in that there was a biased expression of D proximal families. The results indicated that this bias was associated with developmental age and not a given differentiation stage in the B cell lineage. In addition, stimulation of fetal B cells with DNP resulted in a large increase in expression of member(s) of VH 36-60, suggesting that the early appearance of DNP-responsive B cells is not strictly correlated with preferential rearrangement of D proximal families, VH 7183 and VH Q52. However, the results suggested that a large proportion of pre-B cells that preferentially rearrange D proximal families early in ontogeny become part of the functional developing repertoire.

A map of VH genes located next to the DH region in the Igh locus of two congenic Igh-recombinant mouse strains

A new congenic mouse strain (C57BL/6-Igh-Vb-Ca) with a recombinant chromosome 12 is described. It carries the Igh-1a allele, but shows the serological characteristics of C57BL/6 when analyzed for idiotype expression with respect to the antigens dextran and (4-hydroxy-5-iodo-3-nitrophenyl)acetyl (NIP). We analyzed liver DNA from one animal for restriction fragment length polymorphism by hybridization to probes detecting members of nine VH gene families and DH segments, and compared it to DNA from animals carrying the nonrecombinant haplotypes Igha and Ighb, respectively. The breakpoint of recombination maps to the region carrying members of VH gene families VGAM3.8, PC7183 and Q52. The CB8KN strain which according to the serological analysis carries a recombinant Igh locus (Igh-Va-Cb) on BALB/c background was also analyzed. In this strain the breakpoint of recombination again maps to the region carrying members of VH gene families VGAM3.8, PC7183 and Q52. Our results show that the VH genes of families PC7183 and Q52 are interspersed and map to the region next to the DH locus. At least one gene from the VGAM3.8 family also maps to this region in the Igha and the Ighb haplotype.

Developmentally regulated and strain-specific expression of murine VH gene families

We have devised a simple assay that provides an instantaneous representation of VH family usage in primary and peripheral lymphoid tissues. This assay lacks complex manipulations out of the animal and thus minimizes the risk of in vitro artifacts. We have used this assay to demonstrate a dramatic preference for utilization of the most JH-proximal VH segments in the newborn liver of BALB/c and C57BL/6 mice. Furthermore, we find that VH segments from across the entire VH locus are utilized early in development, but at frequencies directly related to their JH proximity. A major shift away from the position-dependent VH repertoire of the neonate is seen in unprimed or polyclonally-activated adult spleen cells, in which relative utilization of the various VH families is related to family size. We also report consistent strain-specific differences in the expression of certain VH families. Our data indicate that a position-dependent VH repertoire is generated in differentiating pre-B lymphocytes (probably reflecting constraints imposed by the immunoglobulin gene assembly process), and that mechanisms that operate subsequent to rearrangement then randomize this position-dependent repertoire in a strain-specific manner.

Mitogen- and IL-4-regulated expression of germ-line Ig gamma 2b transcripts: evidence for directed heavy chain class switching

Treatment of murine B cells with bacterial lipopolysaccharide (LPS) in the presence or absence of different lymphokines results in cell populations that differentially express particular immunoglobulin heavy chain constant region (CH) genes. This class switch involves recombination between switch regions located upstream of the germ-line CH genes. We have treated Abelson murine leukemia virus-transformed pre-B cells and normal splenic B cells with LPS or LPS plus the lymphokine IL-4 and examined the effect on the germ-line gamma 2b locus and gamma 2b class switching. In both cell types, LPS induces transcription specifically through the germ-line gamma 2b locus before gamma 2b class switching. Furthermore, IL-4 inhibits LPS induction of germ-line gamma 2b transcripts in spleen cells and correspondingly abrogates switching to this CH gene. Thus treatment with mitogens and lymphokines can alter transcription of germ-line CH genes in B lineage cells and thereby directly regulate class switching in the context of a recombinase accessibility mechanism.

Nuclear matrix bound terminal deoxynucleotidyl transferase in rat thymus nuclei. II. Effect of ATP on free and matrix bound TdT

Endogenous nuclease digestion of thymus nuclei from 3-4 week old rats followed by a step wise extraction with low salt, 0.5 M salt and 1 M salt removed approximately 70-85% of total nuclear terminal deoxynucleotidyl transferase (TdT) whereas approximately 15-30% of the enzyme remained tightly bound to the residual nuclear matrix. The cytoplasmic TdT as well as the bulk of nuclear TdT extracted in low salt and 0.5 M salt was found to be strongly inhibited at low concentration of ATP whereas matrix bound TdT and a significant portion of the enzyme in 1 M salt extract was completely insensitive to this nucleotide. The ATP resistant enzyme in the 1 M salt extract was unstable and slowly converted to ATP sensitive form upon prolonged preincubation on ice whereas under similar conditions it remained unaffected in the matrix bound form. These observations lead us to suggest that ATP resistant matrix bound TdT being capable of discriminating unnatural rNTPs against the natural dNTP substrates, may be the functionally organized form of the enzyme and that free TdT having lost the capability to distinguish between dNTP and rNTP may be the nonfunctional form of the enzyme in the thymus gland.

The Gm system. Anti-Gm's: characteristics in rheumatoid arthritis; experimental induction without resort to allotype; frequent occurrence in mononucleosis

Gm system: Some RFs specifically detect Mendelian markers of human Ig, originally proving that Ig production is gene controlled. The genetic marker systems of human Ig with 17 Gm, 2 Am, 3 Km and 1 Em markers are briefly described. Examples of the usefulness of the Gm system in medicine, immunology and molecular biology are mentioned.

CONCLUSIONS

1. Some RFs have been essential tools in elucidating genetic control mechanisms in Ig production. 2. Knowledge of the Gm system is extensive. Anti-Gm's: High-titered anti-Gm's are common in R.A. The anti-allotypes in R.A. are markedly restricted as to their specificity. They are usually not directed against the individuals own Gm markers. Anti-human-immunoglobulins were observed in 12 of 18 sera from patients with mononucleosis. The majority of these anti-immunoglobulins were inhibitable by native human Ig and showed restricted specificity.

CONCLUSIONS

1. There is a strong stimulus for production of particular anti-Gm's in a majority of R.A. cases. 2. Notions of an autoimmune origin for many anti-Gm's in R.A. are not in obvious agreement with experimental observations. 3. Anti-human-Ig's with restricted specificity are commonly induced in mononucleosis.

Nuclear matrix bound terminal deoxynucleotidyl transferase in rat thymus nuclei. I. A possible site for TdT mediated function

Approximately 80% of the terminal deoxynucleotidyl transferase (TdT) in thymus glands from 3-4 week old rats was found to be localized in the nucleus and the remaining 20% in the cytosol. Following endogenous nuclease digestion of the thymus nuclei, 70-85% of the nuclear TdT could be removed by low salt and high salt extractions, whereas 15-30% of the enzyme remained tightly bound to the residual nuclear matrix. Low salt and high salt extracts of the nuclei contained a mixture of 58, 56, 45 and 44 kDa species of TdT whereas only 58 kDa species of the enzyme was found to be associated with the matrix. In addition to TdT, 20-25% of the nuclear DNA polymerase alpha was also tightly bound to the isolated nuclear matrix. These observations lead us to propose that besides being the site of DNA replication via-matrix bound replicational complexes [Van der Velden H.M.W. & Wanka F., Molecular Biology Reports 12 (1987): 69], nuclear matrix may also be the site of TdT mediated function and that matrix bound TdT and free TdT could be the functional and nonfunctional forms of the enzyme, respectively, in the thymus gland.

Evolutionary and somatic selection of the antibody repertoire in the mouse

The repertoire of antibody variable (V) regions has been subject to evolutionary selection, affecting both the diversity of V region genes in the germline and their expression in the B lymphocyte population and its subsets. In ontogeny, contact with an antigen leads to the expansion of B cells expressing antibodies complementary to it. In a defined phase of B cell differentiation, new sets of V regions are generated from the existing repertoire through somatic hypermutation. Cells carrying advantageous antibody mutants are selected into the memory compartment and produce a stable secondary response upon reexposure to the antigen.