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

Identical expression of CD45R isoforms by CD45RC+ 'revertant' memory and CD45RC+ naive CD4 T cells

Naive and memory CD4 T cells are frequently defined by exon-specific monoclonal antibodies (mAb) which stain (or not) high- or low-molecular-weight (MW) isoforms of the leucocyte common antigen CD45. The link between isoform and the naive/memory designation is complicated by the fact that CD4 T cells with a 'memory' phenotype (CD45RA-, RB-, RC-, or CD45RO+) may revert ('revertants') and re-express the high mw isoform (CD45RA+, RB+, RC+). Isoform expression also changes during normal T-cell development. Furthermore, the picture may be incomplete since an exon-specific mAb will not detect all possible isoforms on a cell. We have used molecular techniques to determine whether revertant CD4 memory T cells were different from naive T cells with respect to CD45R isoform expression. Using the anti-CD45RC mAb OX22 to purify rat lymphocyte subsets, CD45R isoform expression was examined at the mRNA level in CD4 T cells at different stages of development and compared with that of B cells and unseparated lymphocytes. B cells contained abundant message for the highest MW 3-exon isoform ABC, the 2-exon isoforms AB and BC, and the null isoform O. Both immature CD45RC- (i.e. CD4+8- 'single positive' thymocytes, and peripheral Thy-1+ recent thymic emigrants) and mature CD45RC- 'antigen-experienced' CD4 T cells had message for single-exons B, possibly C and for the O exon. In contrast, CD45RC+ CD4 T cells contained mRNA coding for ABC (low level), AB, BC, B, C (low level) and O (low level). Importantly, there was no difference between CD45RC+ T cells that had not seen antigen ('truly native') and CD45RC+ antigen-experienced revertant memory T cells. This observation has implications for understanding long-term immunological memory.

A function of Ly-5 (CD-45) in the generation of lymphokine-activated killer cells defined by Ly-5 anti-sense oligodeoxyribonucleotides and Ly-5 monoclonal antibody

The unique feature of the Ly-5 system is that it is a major cell surface glycoprotein, representing up to 10% of the total cell surface complement, confined to the hematopoietic cells as a family of isoforms generated by alternative splicing of a single Ly-5 gene. The cytoplasmic domain of Ly-5 has protein tyrosine phosphatase activity suggesting that Ly-5 is involved in signal transduction. We used Ly-5 anti-sense oligodeoxyribonucleotides (oligo) and Ly-5 monoclonal antibody (mAb) to study the functional role of Ly-5 in the concanavalin A mitogenesis response by spleen cells, as well as in the generation of lymphokine-activated killer cells and the proliferative response by spleen cells induced by recombinant human interleukin-2 (rhIL-2). Our results indicate that the Ly-5 mAb could enhance these activities whereas the anti-sense oligo was inhibitory. These data clearly suggest that Ly-5 is involved in the IL-2 and IL-2 receptor responsive circuit.

Will the transgenic mouse serve as a Rosetta Stone to glycoconjugate function?

The overwhelming diversity of oligosaccharide structures on glycoproteins and glycolipids is both the most fascinating and the most frustrating aspect of glycobiology. Moreover, a single protein may be variably glycosylated and thereby represented by multiple glycoforms. As envisioned, many modifications may serve no useful function while others are likely to be essential [1]; hence, experimental approaches to understand the biological basis for such complexity can be difficult to formulate. In a recent comprehensive review on oligosaccharide function [2], Varki concludes that oligosaccharides carry out a large number of biological roles and that 'while all theories are correct, exceptions to each can be found'. Although a common theme to oligosaccharide function may never appear, crucial biological information can be observed to reside within various glycoforms. Examples include the glycoform-dependent mechanism of selectin function in mediating haemopoietic cell extravasation during inflammatory responses [3] and the clearance of particular glycoforms from serum by various glycoform-specific receptors [4-6]. Together, studies of glycosyltransferase biochemistry, naturally-occurring and experimentally-induced glycoform mutations, and the genetic basis for the production of glycoform complexity have allowed crucial steps in the biosynthesis of specific glycan structures to be reconstructed as they appear to occur in the endoplasmic reticulum and Golgi apparatus of intact cells [7]. With a significant foundation of biochemical knowledge achieved, genetic approaches are under way further to decipher the physiological roles encoded within the diverse and dynamic mammalian oligosaccharide repertoire.

Glycosylation sites identified by detection of glycosylated amino acids released from Edman degradation: the identification of Xaa-Pro-Xaa-Xaa as a motif for Thr-O-glycosylation

Here we report the use of automated Edman degradation of covalently linked glycopeptides to identify positively the sites of O- and N-glycosylation. The O-glycosidic linkage of carbohydrate to the hydroxy amino acids Ser and Thr is a major form of post-translational modification. However, unlike Asn-linked glycosylation, which is identified by the consensus sequence Asn-Xaa-Thr/Ser, no simple motif conferring O-linkage to Thr and Ser has been described. After sequencing glycopeptides derived from two cell surface glycoproteins, a Thr-O-glycosylation motif of Xaa-Pro-Xaa-Xaa, where at least one Xaa = Thr(Sac), has been defined. This motif predicts the site(s) of Pro- associated Thr-O-glycosylation in O-glycosylated proteins, although it is clear that there are also other forms of Thr-O-glycosylation not associated with Pro.

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.

Identification of an additional member of the protein-tyrosine-phosphatase family: evidence for alternative splicing in the tyrosine phosphatase domain

Protein-tyrosine-phosphatases (protein-tyrosine-phosphate phosphohydrolase, EC 3.13.48) have been implicated in the regulation of cell growth; however, to date few tyrosine phosphatases have been characterized. To identify additional family members, the cDNA for the human tyrosine phosphatase leukocyte common antigen (LCA; CD45) was used to screen, under low stringency, a mouse pre-B-cell cDNA library. Two cDNA clones were isolated and sequence analysis predicts a protein sequence of 793 amino acids. We have named the molecule LRP (LCA-related phosphatase). RNA transfer analysis indicates that the cDNAs were derived from a 3.2-kilobase mRNA. The LRP mRNA is transcribed in a wide variety of tissues. The predicted protein structure can be divided into the following structural features: a short 19-amino acid leader sequence, an exterior domain of 123 amino acids that is predicted to be highly glycosylated, a 24-amino acid membrane-spanning region, and a 627-amino acid cytoplasmic region. The cytoplasmic region contains two approximately 260-amino acid domains, each with homology to the tyrosine phosphatase family. One of the cDNA clones differed in that it had a 108-base-pair insertion that, while preserving the reading frame, would disrupt the first protein-tyrosine-phosphatase domain. Analysis of genomic DNA indicates that the insertion is due to an alternatively spliced exon. LRP appears to be evolutionarily conserved as a putative homologue has been identified in the invertebrate Styela plicata.

Characterization of the MRC OX40 antigen of activated CD4 positive T lymphocytes--a molecule related to nerve growth factor receptor

The antigen recognized by the monoclonal antibody (mAb) MRC OX40 is present on activated rat CD4 positive T lymphocytes but not other cells. cDNA clones were isolated from an expression library using the MRC OX40 mAb and the protein sequence for the OX40 antigen deduced. It contains a typical signal sequence and a single putative transmembrane sequence of 25 predominantly hydrophobic amino acids giving an extracellular domain of 191 amino acids and a cytoplasmic domain of 36 amino acids. The sequence of the extracellular domain includes a cysteine-rich region with sequence similarities with the low affinity nerve growth factor receptor (NGFR) of neurons and the CD40 antigen present on human B cells. Within this region three cysteine-rich motifs can be recognized in OX40 compared with four similar motifs in both NGFR and CD40. OX40, CD40 and NGFR constitute a new superfamily of molecules with expression including lymphoid cells (OX40, CD40) and neuronal cells (NGFR). This is reminiscent of the immunoglobulin superfamily whose molecules are variously found at the surface of lymphoid or brain cells or both.

Expression of CD45 alters phosphorylation of the lck-encoded tyrosine protein kinase in murine lymphoma T-cell lines

CD45 is a family of high molecular weight leukocyte cell surface glycoproteins. Recently, two related subregions of the cytoplasmic domain of CD45 have been shown to have 30-40% amino acid identity with a human placental protein phosphotyrosine phosphatase, and CD45 isolated from human spleen was found to exhibit intrinsic protein phosphotyrosine phosphatase (EC 3.1.3.48) activity. In the present studies, we demonstrate that each of the known isoforms of murine CD45 has an equivalent basal level of protein phosphotyrosine phosphatase activity and establish that this enzymatic activity is associated with the cytoplasmic domain of the glycoprotein. Studies with three independent sets of well-characterized parental CD45+, mutant CD45-, and revertant CD45+ lymphoma cell lines indicate that loss of CD45 increases the phosphorylation of the src-related leukocyte-specific tyrosine protein kinase p56lck on tyrosine-505, a putative negative regulatory site. This suggests that CD45 may play a role in leukocyte growth regulation by altering the kinase activity of p56lck.