Structural features of Ly-5 glycoproteins of the mouse and counterparts in other mammals

Donor marker infidelity in transgenic hematopoietic stem cells

Transgenic marking approaches are increasingly used to evaluate the developmental potential of stem cells. However, cell fate mapping studies using different transgenic marking systems have produced conflicting results. These disparate findings may be due in part to the infidelity of donor marker gene expression. Analysis of hematopoietic stem cells (c-Kit+, Sca-1+, lineage marker- [KSL]) from a transgenic mouse (1Osb) engineered to ubiquitously express the enhanced green fluorescent protein (EGFP) reveals two distinct populations. Forty percent of KSL cells demonstrate intermediate levels of EGFP fluorescence and differentiate into subpopulations of B cells, T cells, and myeloid cells that do not express EGFP. By contrast, progeny of the remaining 60% of KSL cells are almost exclusively EGFP bright. Long-term multilineage hematopoietic reconstitution and serial transplantation experiments show that these differences in EGFP are a property of self-renewing stem cells. Furthermore, both the transgene integration site and the activation status of a cell are important determinants of EGFP expression. These results indicate that a combination of donor cell markers is required to reliably track the full differentiation potential of transgenic stem cells.

Glycosylation of CD45: carbohydrate composition and its role in acquisition of CD45R0 and CD45RB T cell maturation-related antigen specificities during biosynthesis

The contribution of oligosaccharides to the biochemical composition and antigen heterogeneity of the phosphotyrosine phosphatase CD45 glycoproteins has been studied on the K-562 erythroleukemic cell line. Treatment of immunoprecipitated CD45 glycoproteins with distinct exo- and endoglycosidases revealed the presence of highly sialylated O- and N-linked complex carbohydrates in the composition of mature CD45 glycoproteins. Incubation of K-562 cells with the N-glycosylation inhibitor tunicamycin blocked carbohydrate processing during biosynthesis of CD45 proteins, generating unglycosylated polypeptides similar in size to those resulting from digestion of CD45 proteins with a mixture of both N- and O-glycanases. Epitopes defining the T cell maturation related CD45R0 and CD45RB antigen specificities were present on the mature 180- and 190-kDa K-562 CD45 proteins, respectively. However, the CD45R0 and CD45RB epitopes were not detected on the high mannose biosynthetic CD45 precursors. Furthermore, treatment of CD45 proteins with O-glycanase or neuraminidase resulted in the loss of both CD45R0 and CD45RB epitopes, although reactivity of the anti-CD45R0 and anti-CD45RB mAb was not affected by mAb preincubation with either sialic acids or sialyllactose in solution. From these results we conclude that the blockade of early steps of N-glycosylation during carbohydrate processing resulted in the inhibition of subsequent incorporation of O-linked sugars on CD45 polypeptides, thus preventing the late acquisition of the CD45R0 and CD45RB determinants on the 180- and 190-kDa CD45 polypeptides.

Further data on the selective expression of Ly-5 isoforms

Ly-5 (CD45) glycoproteins of the mouse, expressed by all or most hematopoietic cell lineages and specified by a single Ly-5 gene, range in size from isoform T200 of T cells (the smallest), in which exons 4, 5, and 6 are not represented, to isoform B220 of B cells (the largest), in which all three of these optional exons are represented. The main purpose of the present study, utilizing the polymerase chain reaction (PCR), was to ascertain whether known isoforms of intermediate size are generated by single or dual usage of optional exons 4, 5, and 6. Transcripts representing all eight isoforms predictable from varied use of three exons were observed among a diverse panel of nine B-cell tumors in culture, but there was no evident concordance with known contrasting differential features that distinguish members of the B-cell tumor panel. No two B tumors exhibited the same variety of transcripts and the relative quantities of transcripts expressed varied greatly from tumor to tumor. Cloning of B-cell tumors did not alter their distinctive transcript patterns. Separation methods (sodium dodecyl sulfate polyacrylamide gel electrophoresis; SDS-PAGE) did not suffice to segregate all corresponding expressed isoforms but did establish that transcripts representing usage of a single optional exon and of two optional exons were actually translated, which supports a provisional inference that all eight isoforms exist. The considerable diversity of B-cell transcript phenotypes was not seen among seven T-cell leukemias, two cytolytic T-cell lines, and three Th 1 helper T-cell lines, all of which displayed a uniform phenotype comprising major expression of the T200 transcript (no optional exon) and minor expression of a transcript employing exon 5. However, a panel of five cloned Th2 T-cell lines, which represent a second and functionally different branch of the helper/inducer T-cell category, exhibited a characteristic transcript pattern which distinguished them from a panel of three Th1 T-cell lines. The major transcript in the Th2 lines was also T200, but the Th2 lines showed higher representation of transcripts containing optional exons. A single Th2 clone expressed an unusual transcript suggesting a potential isoform not compounded simply by varied inclusion of the three identified optional exons. After activation of the helper T-cell lines with concanavalin A (Con A), expression of transcripts containing optional exons appeared to decrease.

The extra segments of sequence in rat leucocyte common antigen (L-CA) are derived by alternative splicing of only three exons and show extensive O-linked glycosylation

The leucocyte common antigen (L-CA, CD45, or T200) consists of a family of heavily glycosylated glycoproteins of apparent Mr 180,000-240,000 found at the surface of leucocytes but not other cell types. Populations of lymphocytes express forms that differ in antigenicity, apparent Mr, and glycosylation. Some of this heterogeneity is due to polypeptide differences caused by the insertion of up to three different segments of sequence near the NH2-terminus. We report the complete sequence of the region of the rat L-CA gene encoding the extra segments. Analysis of this sequence showed that each segment was encoded by an exon but no further exons could be identified, implying that the polypeptide heterogeneity is solely due to selection from these three exons by alternative splicing. Amino acid sequencing of glycopeptides prepared from the largest forms of L-CA indicated extensive O-linked glycosylation in at least one of the extra segments.