SYNTHESIS AND IMMUNOLOGICAL PROPERTIES OF VI AND DI-O-ACETYL PECTIN PROTEIN CONJUGATES WITH ADIPIC ACID DIHYDRAZIDE AS THE LINKER

The Vi capsular polysaccharide of Salmonella typhi, a licensed vaccine for typhoid fever in individuals greater than or equal to 5 Sears old , induces low and short-lived antibodies in children, and reinjection does not elicit booster responses at any age, Its immunogenicity was i mproved by binding Vi to proteins by using N-succinimidyl-3-(2-pyridyl dithio)propionate (SPDP) as a linker, Similar findings were observed w ith the structurally related, di-O-acetyl derivative of pectin [poly-a lpha(1-->4)-D-GalpA] designated OAcP. Protein conjugates of Vi and OAc P were synthesized by carbodiimide-mediated synthesis with adipic acid dihydrazide (ADI-I) as the linker, Hydrazide groups were introduced i nto proteins (bovine serum albumin or recombinant Pseudomonas aerugino sa exoprotein A) by treatment with ADH and 1-ethyl-3(3-dimethylaminopr opyl carbodiimide (EDC), The resultant adipic acid hydrazide derivativ es (AH-proteins), containing 2.3 to 3.4% AH, had antigenic and physico chemical properties similar to those of the native proteins, The AN-pr oteins were bound to Vi and OAcP by treatment with EDC, The immunogeni city of Vi or OAcP, alone or as protein conjugates, was evaluated in y oung outbred mice and guinea pigs by subcutaneous injection of 2.5 and 5.0 mu g, respectively, of polysaccharide, and antibodies were measur ed by enzyme-linked immunosorbent assay, All conjugates were significa ntly more immunogenic than Vi or OAcP alone and induced booster respon ses with 5- to 25-foId increases of antibodies, Vi conjugates were sig nificantly more immunogenic than their OAcP analogs, A carboxymethyl d erivative of yeast beta-glucan enhanced the anti-Vi response elicited by an OAcP conjugate but had no effect on the immunogenicity of Vi or of OAcP alone, Vi and OAcP conjugates synthesized by this scheme will be evaluated clinically.