Immunogenic duplex nucleic acids are nuclease resistant

Several derivatives of the synthetic duplex DNA poly(d(GC] were prepared. These analogs contained ribo- and phosphorothioate substitutions, namely, poly(r(GC], poly(rGdC), poly(d(sGC], poly(d(GsC], and poly(d(sGsC]. The nucleic acids were complexed to methylated BSA and injected into C57BL/6 mice. Poly(d(GC] was not immunogenic, whereas the analogs gave a strong response. Analysis of the sera revealed that antibody populations were present, which recognized A, B, and Z form nucleic acids. In particular, immunization with poly(d(sGC] and poly(d(GsC] produced antibodies that bound poly(d(GC] and poly(d(IC]. The nuclease sensitivity of these polymers also was investigated. Poly(d(GC] was rapidly degraded by pancreatic DNase I, whereas the phosphorothioate derivatives were resistant, and the ribo-substituted polymers were resistant to both RNases and DNase I. Similarly, the nucleases present in mouse serum digested calf thymus DNA and ribosomal rRNA while both types of analog persisted indefinitely. These results suggest the hypothesis that duplex DNAs are generally not immunogenic because they are rapidly degraded by serum nucleases. Therefore, they escape immune detection.