Isoforms of CD44 are differentially modified by the glycosaminoglycans (GAGs) chondroitin sulfate (CS), heparan sulfate (HS), and keratan sulfate. GAG assembly occurs at serines followed by glycines (SG), but not all SG are utilized. Seven SG motifs are distributed in five CD44 exons, and in this paper we identify the HS and CS assembly sites that are utilized in CD44. Not all the CD44 SG sites are modified. The SGSG motif in CD44 exon V3 is the only HS assembly site; this site is also modified with CS. HS and CS attachment at that site was eliminated by mutation of the serines in the V3 motif to alanine (AGAG). Exon E5 is the only other CD44 exon that supports GAG assembly and is modified with CS. Using a number of recombinant CD44 protein fragments we show herein that the eight amino acids located downstream of the SGSG site in V3 are responsible for the specific addition of HS to this site. If the eight amino acids located downstream from the first SG site in CD44 exon E5 are exchanged with those located downstream of the SGSG site in exon V3, the SG site in E5 becomes modified with HS and CS. Likewise if the eight amino acids found downstream from the first SG in E5 are placed downstream from the SGSG in V3, this site is modified with CS but not HS. We also show that these sequences cannot direct the modification of CD44 with HS from a distance. Constructs containing CD44 exon V3 in which the SGSG motif was mutated to AGAG were not modified with HS even though they contained other SG motifs. Thus, a number of sequence and structural requirements that dictate GAG synthesis on CD44 have been identified.

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The influence of valence and heavy chain on antibody activity was investigated using transfectoma-derived, class-switched IgG1 and IgM human monoclonal antibodies (MAbs) reactive with the bacterial pathogens Escherichia coli K1 and group B Streptococcus species. IgG-IgM pairs were compared in vitro for antigen binding and opsonic activities and in vivo for protective efficacy in neonatal rats. For the anti-E. coli pair, the IgM MAb was 1000-fold more potent in all assay formats. Importantly, the 50% protection dose (PD50) of the IgM MAb was 10-20 ng/rat, while 100 micrograms of the IgG MAb was only minimally protective. For the group B streptococcal MAbs, the IgM was 100- and 4500-fold more potent in binding and opsonization assays, respectively. However, while 20 micrograms of IgM protected neonatal rats, 100 micrograms of IgG MAb was partly protective. These experiments demonstrate the utility of recombinant DNA technology for creating a panel of antibodies that may aid in selecting potential immunotherapeutic candidates.

Two ribosome-inactivating proteins (RIPs) were isolated and characterized from the roots of Bryonia dioica. One of these was a novel 27-kDa protein termed bryodin 2 (BD2), while the second was a previously reported RIP, referred to here as bryodin 1 (BD1). The amino-terminal sequence obtained for BD2 was similar, but distinct from BD1, ricin A chain, trichosanthin, and momorcharin. BD2-specific monoclonal antibodies were generated and found not to react with BD1 or ricin A chain. Purified BD1 and BD2 RIP inhibited protein synthesis in a cell-free in vitro translation assay at EC50 values of 7 and 9 pM, respectively. Intravenous administration of BD1 was less toxic to mice than BD2, with LD50 values of > 40 for BD1 and 10-12 mg/kg for BD2. Primary human endothelial cells were 5-8-fold less sensitive to BD1 and BD2 than compared to ricin A chain. BD1 and BD2 were constructed as immunoconjugates with the chimeric form of BR96 (chiBR96), a carcinoma-reactive, internalizing antibody. ChiBR96-BD1 and chiBR96-BD2 were found to bind to and kill BR96 antigen-positive carcinoma cells while not killing antigen-negative carcinoma cells. Bryodins represent RIPs that may be useful in constructing immunotoxin conjugates with reduced toxicity and vascular sensitivity, as compared to ricin A chain immunotoxins.

Group B streptococcal (GBS) infections cause significant mortality and morbidity among infants. Passive antibody immunotherapy has been proposed as treatment for infected infants. To this end, two human mAb-secreting cell lines were produced by EBV immortalization of human B cells. The mAbs were specific for the group B polysaccharide and bound to strains of all five serotypes as demonstrated by ELISA and crossed immunoelectrophoresis. The mAbs reacted and opsonized 100% (132/132) of the clinical isolates tested which represented all four capsule types. Both prophylactic and therapeutic protection with these mAbs were demonstrated in neonatal rats given lethal infections of types Ia and III human clinical isolates. These data indicate that a single human mAb directed against the group B carbohydrate can protect against GBS infections caused by the different serotypes. This antibody may be useful in the passive immunotherapy of infants infected with GBS.