gms | German Medical Science

Diffusely infiltrating tumors such as gliomas require adjuvant therapies which can cope with this aggressive biological behaviour. Since vaccination represents one such approach we investigated whether DNA vaccination against a model antigen (E. coli lacZ) prior to tumor cell implantation prevents tumor formation in a syngeneic rat gliosarcoma model.

Fisher 344 rats were vaccinated thrice in weekly intervals via the intramuscular route with an expression plasmid encoding lacZ or an empty control plasmid, and plasmids encoding putative adjuvants were added in some animals. Following vaccination, 9LlacZ cells were implanted into the striatum. After three weeks brains were removed after splencectomy and frozen in liquid nitrogen for assessment of tumor size and immunohistochemical studies.

Vaccination without plasmid or with the control plasmid resulted in the formation of large tumors (183.7 mm³ ±99.2; mean ±standard deviation). In lacZ vaccinated animals tumors were significantly smaller (18.9 mm³ ±13.3). Co-injection of expression plasmids encoding interleukin-12 (59.2 mm³ ±24.6) or flt-3 ligand (69.2 mm³ ±40.6) did not have an adjuvant effect but resulted in the formation of larger tumors. All tumors were heavily infiltrated with CD8+ cells and less with NK cells, but only a minority expressed activation markers (CD25+ and perforin, respectively). Tumors were also infiltrated with non-actived microglial cells (ED-1 negative). Cytotoxic T cells generated from restimulated spleen preparations (CTL assay) revealed target cell lysis of >50% in some lacZ vaccinated animals which was specific for 9LlacZ as well as wild-type 9L target cells whereas another syngeneic cell line (MADB 106) was not lysed. In control vaccinated animals, lysis rates remained below 25%. This is consistent with higher IFN-γ production in lymphocytes of lacZ vaccinated animals compared to control vaccinated animals as determined by the ELISPOT assay. However, both in vitro-assays did not correlate with tumor size.

Effective but incomplete inhibition of tumor formation following DNA vaccination has been demonstrated in a syngeneic rat gliosarcoma model. This model is suited for further investigations related to more effective adjuvants and the immunological mechanisms involved.