Primary T-cell and activated macrophage response associated with tumor protection using peptide/poly-N-acetyl glucosamine vaccination

The mode of peptide-based cancer vaccine administration critically affects the ability to achieve a clinically relevant tumor-specific response. We ha ve previously shown (Cole et at, Clin, Cancer Res., 3: 867-873, 1997) that a specific formulation of the polysaccharide poly-N-acetyl glucosamine (p-G lcNAc, designated as F2 gel) is an effective vehicle for sustained cytokine and peptide delivery in vitro. The purpose of this study was to evaluate t he efficacy of F2 gel/peptide vaccination in the murine EG.7-OVA tumor mode l end to elucidate potential mechanisms involved in the observed cell-media ted response. C57BL/6 mice were given injections of 200 mu l in the base of tail/footpad using either F2 gel alone or 200 mu g of: SIINFEKL minimal pe ptide (OVA) in PBS, OVA peptide/endoplasmic reticulum insertion signal sequ ence fusion (ESOVA) in PBS, OVA in F2 gel, or ESOVA in F2 gel. Splenocytes were tested 10 days later for a secondary response using a Cr-51 assay as w ell as a primary CTL response using the lactate dehydrogenase cytotoxicity assay, Splenocytes from immunized mice were harvested at specific time poin ts and assayed for cell surface and intracellular markers. On day 10 postva ccination, animals were challenged with EG.7-OVA murine thymoma cells. Tumo r size and appearance were recorded, Vaccination with F2 gel/peptide (eithe r OVA or ESOVA) resulted in a primary T-cell response (up to 25% tumor cell -specific lysis) and no tumor growth in 69% of the mice, By 48 h, the propo rtion of splenic T cells had increased 4-fold compared with B cells. Presen ce of an increased Th1 CD4 helper population was demonstrated by IFN-gamma production. CD4 cells were activated at 24 and 48 h as shown by IL-2 recept or alpha chain expression (from 2% basal expression to 15.4% at 48 h), Acti vated splenic macrophages increased from 3 to 8% within 10 h, and their lev el of B7-2 expression doubled. Depletion of macrophages before vaccine inje ction abolished any tumor-specific primary CTL response, F2 gel/peptide tum or vaccine can prime the immune system in an antigen-specific manner by gen erating a measurable primary T-cell response with minimal peptide; this pro cess involves macrophage presence and activation as well as induction of Th 1 CD4 cells. This is the first demonstration of a primary CTL response gene rated with minimal peptide vaccination using a noninfectious delivery syste m. These results justify additional studies to better define the mechanisms involved in F2 gel/peptide vaccination in preparation for clinical trials.