Minimal Requirements for in vitro Activity of Chemically Defined Synthetic Antigens as Immunogenic Carrier Molecules

A primary antibody response to 2,4-dinitrophenyl (Dnp)-oligolysines of defined chain length was induced in vitro. A molecular size of at least eight lysine residues was critical for the induction of antibody response to the hapten, but the location of the hapten on the carrier did not seem to play a significant role in this regard. Shorter peptides were nonimmunogenic, but did not paralyze the response to other, noncrossreacting antigens. The electric charge of the carrier molecule was found to have an effect on immunogenicity in vitro. A synthetic copolymer with a positive charge was immunogenic, whereas a similar molecule carrying a negative charge was inactive. On the other hand, changing the negative charge of carriers such as polyglutamic acid was not sufficient to render them immunogenic. Furthermore, neutralization of the negative charge of the surface of the spleen tissue by preincubation with positive polymers did not enhance the response to conjugates of the hapten with negatively charged carriers. These observations are interpreted on the basis of higher affinity of positively charged molecules for the negatively charged cell surface. Accordingly, the specific binding of the antigen to the cell surface is made more stable, and this ensures stimulation.