Unlinked regulation of cell growth and differentiation in immature B cell lines

Isolation of a cDNA clone encoding a novel membrane protein expressed in lymphocytes

By subtractive hybridization using single-stranded phagemids with directional inserts, we isolated a mouse cDNA clone, LSM-1, from temperature-sensitive Abelson virus-transformed immature B cells whose differentiation was being induced after the shift from the permissive (35 degrees C) to the non-permissive temperature (39 degrees C). LSM-1, which encodes an as yet unknown peptide of 197 amino acids, has a putative signal sequence and a trans-membrane region, and is expressed in B- and T-cell lines, in spleen, thymus, and bone marrow of adult mice, and in embryos.

Cyclic regulation of B220 antigen expression in immature B cell lines

Three independent immature B cell lines transformed with temperature-sensitive mutants of Abelson murine leukaemia virus (A-MuLV) persistently expressed B220 antigen detected by both of the anti-B220 antibodies RA3-6B2 and 14.8 during culture at the permissive temperature (35 degrees C). RA3-6B2 recognizes the B220 eptiope and that 14.8 is a CD45RA antibody. However, when the culture temperature was shifted up to the non-permissive temperature (39 degrees C) and the culture was continued for 2-3 weeks, a population of the cells (RA3-6B2-14.8+ cells) that lost RA3-6B2 antigen appeared in all three cell lines. From these populations, RA3-6B2-14.8+ subclones were independently isolated by limiting dilution and the phenotype was stable during culture at the permissive temperature. When the culture temperature of the RA3-6B2-14.8+ subclones was shifted up to the non-permissive temperature and the culture was continued for 2-3 weeks, RA3-6B2 antigen was re-expressed in a subpopulation of the RA3-6B2-14.8+ cells. These results demonstrated cyclic regulation of B220 antigen expression during proliferation and/or differentiation of immature B cell lines.

Nucleotide deletion of T cell receptor V gamma 2 and J gamma 2 coding sequences at V gamma 2-J gamma 2 junctions in immature B cell lines

Immature B cell lines transformed with a temperature-sensitive mutant of Abelson murine leukaemia virus undergo preferentially V gamma 2 to J gamma 2 joinings during culture at a non-permissive temperature (39 degrees C). Here we examined nucleotide addition and deletion at the V gamma 2-J gamma 2 junctions in these V gamma 2 to J gamma 2 joinings, in comparison with the V gamma 2-J gamma 2 junctional sequences reported previously in T cells. Forty-eight V gamma 2-J gamma 2 junctions were PCR-amplified and sequenced. Only three of 48 V gamma 2-J gamma 2 junctions had nucleotide addition. The average nucleotide deletion of V gamma 2 coding sequences at the V gamma 2-J gamma 2 junctions was 6.9 nucleotides in immature B cells and 5.1 nucleotides in T cells (p less than 0.05). Also, the average nucleotide deletion of J gamma 2 coding sequences at the V gamma 2-J gamma 2 junctions was 3.1 nucleotides in immature B cells and 1.9 nucleotides in T cells (p less than 0.01). The average of total number of the deleted nucleotides at the V gamma 2-J gamma 2 junctions was 10.0 nucleotides in immature B cells and 7.0 nucleotides in T cells (p less than 0.01). No correlation was found between the extent of the nucleotide deletion of the V gamma 2 coding sequence and that of the J gamma 2 coding sequence at each V gamma 2-J gamma 2 junction in immature B and T cells. These results demonstrated that nucleotide deletion at the V gamma 2-J gamma 2 junctions was significantly wider in immature B cells than in T cells.

Regulation of Thy-1 gene expression by the methylation of the 5' region of Thy-1 gene and intracellular regulatory factors in immature B cells

In c mu- B220+ Thy-1- FL2-52-2, an immature B cell line transformed with a temperature-sensitive mutant of Abelson murine leukemia virus(ts OS-59), Thy-1 antigen expression was induced after the shift of the culture temperature from a permissive(35 degrees C) to a non-permissive temperature(39 degrees C), and a Thy-1+ subclone, FL2-52-2-1 was isolated by limiting dilution. Furthermore, since a population of Thy-1+FL2-52-2-1 lost Thy-1 antigen expression during culture at a non-permissive temperature, Thy-1- FL2-52-2-1-1 was isolated from the cultured cells. Methylation analysis by Southern blotting experiments showed that 5' region of the Thy-1 gene was methylated in Thy-1- FL2-52-2 but demethylated in Thy-1+ FL2-52-2-1 and Thy-1-FL2-52-2-1-1. To determine whether or not there exist intracellular regulatory factors responsible for Thy-1 gene expression, Thy-1a allele was transfected into Thy-1(Thy-1.2-) FL2-52-2 and Thy-1- FL2-52-2-1-1. The transfected Thy-1a allele was expressed in Thy-1- FL2-52-2, but not in Thy-1- FL2-52-2-1-1, indicating the presence of the intracellular regulatory factors requisite for Thy-1 gene expression in Thy-1- FL2-52-2. It appeared that in Thy-1- FL2-52-2-1-1, Thy-1 gene was not expressed because of the absence of the intracellular regulatory factors although the 5' region of the Thy-1 gene was demethylated. These results indicated the existence of at least two regulatory mechanisms of Thy-1 gene expression in immature B cells: methylation of the 5' region of the Thy-1 gene and intracellular regulatory factors.

Continuing Vγ2 to Jγ2 rearrangements of murine T cell receptor gamma genes in a B-committed immature cell line

In SPL2-1-2, a murine B-committed immature cell line transformed with a temperature-sensitive mutant of Abelson virus, T cell receptor (TCR) gamma gene rearrangements, together with IgH gene rearrangements, were induced during culture at a non-permissive temperature (39 degrees C). During 11-12 months of culture, TCR gamma gene rearrangements occurred in all cells. In contrast to TCR gamma genes, neither TCR beta or TCR delta were detected even after 11-12 months of culture at a non-permissive temperature. The majority of the TCR gamma gene rearrangements observed here were V gamma 2 to J gamma 2 joinings and the remaining rearrangements were J gamma 1-linked. V gamma 1 to J gamma 4 and V gamma 3 to J gamma 3 joinings were not detected. Approximately 70% of cells with TCR gamma gene rearrangements produced normal-sized transcripts from the rearranged TCR gamma genes. Cloning and sequencing analysis of a cDNA from the transcripts demonstrated that the whole structure of the cDNA was similar to that of T-lineage cells. These results showed that TCR gene rearrangements were restricted to the gamma genes and that V gamma 2 to J gamma 2 joinings occurred preferentially in this B-committed immature cell line. Furthermore, TCR gamma gene rearrangements also occurred in intracytoplasmic mu-chain producing cells. This indicated that the existence of intracytoplasmic mu-chains did not prevent TCR gamma gene rearrangements, although the existence of mu-chains is known to inhibit further productive IgH gene rearrangements, (allelic exclusion). These results should provide many implications for the mechanism of TCR gene rearrangements, especially that of cross-lineage rearrangements.

Transitory expression of Thy-1 antigen in immature B cell lines

In cmu- B220+ Thy-1- murine immature B cells transformed with a temperature-sensitive mutant of Abelson murine leukemia virus, a low level of Thy-1 antigen was transitorily expressed after the shift of the culture temperature from the permissive (35 degrees C) to the non-permissive (39 degrees C) temperature. On the other hand, B220 antigen was persistently expressed regardless of whether the cells were cultured at the permissive or non-permissive temperature. No other T-lineage-specific antigens, CD3, CD4, and CD8 were induced during the culture at the non-permissive temperature. Expression of Thy-1 antigen was confirmed by the detection of a low level of Thy-1-specific mRNA. These results clearly showed that immature B cells could express Thy-1 antigen during proliferation and/or differentiation. The results imply a role for Thy-1 antigen in B cell proliferation and/or differentiation.