Repeated administration of cytosine-phosphorothiolated guanine-containing oligonucleotides together with peptide/protein immunization results in enhanced CTL responses with anti-tumor activity

The development of therapeutic anti-cancer vaccines designed to elicit CTL responses with anti-tumor activity has become a reality thanks to the ident ification of several tumor-associated Ags and their corresponding peptide T cell epitopes, However, peptide-based vaccines, in general, fail to elicit sufficiently strong CTL responses capable of producing therapeutic anti-tu mor effects (i.e., prolongation of survival, tumor reduction). Here we repo rt that repeated administration of synthetic oligonucleotides containing fo reign cytosine-phosphorothiolated guanine (CpG) motifs increased 10- to 100 -fold the CTL response to immunization with various synthetic peptides corr esponding to well-known T cell epitopes, Moreover, repeated CpG administrat ion allowed the induction of CTL to soluble protein even in the absence of additional adjuvant, Our results indicate that the potentiating effect of C pG in CTL responses required the participation of Th lymphocytes. Repeated CpG administration resulted in overt splenomegaly and lymphadenopathy with a significant increase in the numbers of CTL precursors and dendritic cells . Protein vaccination in combination with repeated CpG therapy was effectiv e in delaying tumor cell growth and extending survival in mice bearing mela noma tumors, These findings support the contention that repeated administra tion of CpG-oligonucleotides enhances the effect of peptide and protein vac cines leading to potent anti-tumor responses, presumably through the induct ion of Th1 and dendritic cells, which are essential for optimal CTL respons es. The immunostimulatory properties of CpG motifs may be key in inducing a consistent long term immunity to tumor-associated Ags when using peptides or proteins as T cell-inducing vaccines.