We developed a new monoclonal antibody. B-B4, which specifically identifies human plasma cells. It strongly reacts with all multiple myeloma cell lines and with malignant plasma cells of all tumour samples of the multiple myeloma patients tested. B-B4 does not react with any peripheral blood, bone marrow or tonsil cells. Cloning of the B-B4 antigen reveals that the monoclonal antibody recognizes syndecan-1. It appears that the monoclonal antibody B-B4 is a suitable marker for human plasmocyte identification among haemopoietic cells and a useful probe for the diagnosis of haematological malignancies. Furthermore, this monoclonal antibody can be used for depletions prior to CD34 grafting.

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Using a phage display peptide library, we characterized the epitope of two monoclonal antibodies reacting with syndecan-1: B-B2 and B-B4. The identified epitopes QDIT, for B-B2, and LPEV, for B-B4, were found to align with residues 36^39 and 90^93 of the mature protein, respectively. In contrast to B-B4, the B-B2 epitope is close to a potential glycosaminoglycan attachment site. Since syndecan-1 is heavily glycosylated and post-translational modifications are cell type specific, these results might explain the differences observed in the reactivity pattern of B-B2 and B-B4 and suggest that these monoclonal antibodies are useful probes to study cell surface exposed syndecan-1.z 1998 Federation of European Biochemical Societies.

SUMMARYCompetition between RNA3 from alfalfa mosaic virus (AIMV) strain S (RNA3-S), strain B (RNA3-B) and strain 425L (RNA3-L) was studied. The identification of the RNA3 species multiplying in infected leaves was possible since the RNA3 5' noncoding leader sequences in strains S, B and 425L differ in length. RNA3 present in total RNA from infected tobacco leaves was detected, and strains were identified from the length of the cDNA reverse-transcribed from RNA primed with a specific oligonucleotide. In competition experiments the inoculum, containing known amounts of RNA1, 2, 3 and 4 of one strain, was complemented with various amounts of heterologous RNA3 and inoculated to a systemic host. It is shown that RNA3-S was better replicated in vivo by the A1MV replicase of strain B than was RNA3-B itself, and to a lesser extent better replicated by the AIMV replicase of strain L than was RNA3-L. Comparison of genetic information carried by the RNA3 species present in the inoculum suggests that the more efficient multiplication of RNA3-S is related to the structure of the leader sequence of RNA3-S.The genome of alfalfa mosaic virus (A1MV) consists of three single-stranded RNA species: RNA1, RNA2 and RNA3. RNA1 and 2 encode proteins implicated in viral RNA replication (Nassuth & Bol, 1983) and RNA3, a bicistronic RNA, codes for a non-structural protein (P3) thought to be involved in cell-to-cell spread (Godefroy-Colburn et al., 1986) and for the viral coat protein (P4). The coat protein is expressed from a subgenomic RNA4. Genomic RNAs and RNA4 (or its translation product) are needed to initiate infection of plants (Bol et al., 1971).The four viral RNAs are separately encapsidated in four distinct particles (Bol et al., 1971) and their relative amounts in virus preparations depend on the strain of virus when multiplication is carried out in the same host under standard conditions. In the three strains used here, A1MV-425 isolate L (A1MV-L) (Bol et al., 1971), A1MV-S and AIMV-B (Walter et al., 1985), the relative proportions of viral RNA were as shown in Fig. 1. There was a clear difference between the three strains in the amounts of RNA3 and RNA4; in particles of strain L there was little RNA3 but much RNA4, this ratio was reversed in strain S, and in particles of strain B both RNAs were equally abundant as judged by the intensity of staining with toluidine blue.The lengths of the leader sequences of RNA3 of the three strains of A1MV, i.e. the sequences from the 5' end to the first AUG initiation codon, were 240 nucleotides for strain B (Ravelonandro et al., 1983), 314 nucleotides for strain S and 345 nucleotides for strain 425L (Langereis et al., 1986). These leader sequences show characteristic features: two stretches of 56 and 76 nucleotides are duplicated in RNA3-S and RNA3-L respectively, a 27 to 30 nucleotide region is repeated four times in RNA3-S and RNA3-L and three times in RNA3-B, and the RNA3-L contains an A-rich region of 40 nucleotides (Langereis et al., 1986; Dore & Pinck, 1988). Based on the RNA3 seque...