The present invention provides highly active cyclic-di-nucleotide (CDN) immune stimulators that activate DCs via a recently discovered cytoplasmic receptor known as STING (Stimulator of Interferon Genes). In particular, the CDNs of the present invention are provided in the form of a composition comprising one or more cyclic purine dinucleotides induce human STING-dependent type I interferon production, wherein the cyclic purine dinuclotides present in the composition are 2’-fluoro substituted, bis-3’,5’ CDNs, and most preferably one or more 2’,2”-diF-Rp,Rp, bis-3’,5’CDNs.本發明提供高活性環狀二核苷酸(CDN)免疫刺激劑,其經由新近發現的稱為STING(干擾素基因刺激因子)之細胞質受體活化DC。特定言之,本發明之CDN係以包含一或多種環狀嘌呤二核苷酸之組合物形式提供,其誘導人類STING依賴性I型干擾素產生,其中該組合物中存在之環狀嘌呤二核苷酸為經2'-氟取代的雙-3',5' CDN,且最佳為一或多種2',2"-二氟-Rp,Rp雙3',5' CDN。