Replication-defective recombinant Semliki Forest virus encoding GM-CSF as a vector system for rapid and facile generation of autologous human tumor cell vaccines

Treatment of rapidly growing K-BALB and CT26 mouse tumours using Semliki Forest virus and its derived vector

To assess the potential of immune stimulation in combination with apoptosis induction by Semliki Forest virus (SFV) and its derived vector for tumour treatment, we have utilized the poorly immunogenic and rapidly growing K-BALB and CT26 murine tumour models. Both cell lines underwent apoptosis and expressed viral antigen when infected with the SFV4 strain of SFV, or recombinant SFV (rSFV) virus-like particles (VLPs) encoding the p62-6k viral structural proteins. VLPs were used to immunize groups of BALB/c and BALB/c nu/nu mice prior to subcutaneous tumour induction and treatment. Direct intratumoral injection of VLPs or SFV4 resulted in an immediate and intense inflammatory reaction in immunized groups that was not observed in naive groups until day 5 of treatment, and was not observed in nu/nu groups. A significantly higher level of tumour growth inhibition was observed in immunocompetent groups than in athymic mice. For K-BALB tumours, SFV4 treated groups showed greater inhibition than that observed in VLP-treated groups, with immunization prior to treatment enhancing the overall antitumour effect and immune response. No significant difference was observed in CT26 tumours between VLP and SFV4-treated groups, but prior immunization considerably enhanced the antitumoural response. It is concluded that use of the inherent apoptosis-inducing capability of SFV or its vector, by perfusion in combination with immune stimulation, may have potential for the treatment of rapidly growing tumours.

Superior Therapeutic Efficacy of Alphavirus-Mediated Immunization against Human Papilloma Virus Type 16 Antigens in a Murine Tumour Model: Effects of the Route of Immunization

In our efforts to develop a strong, effective immune response against cervical carcinoma and premalignant disease, we study the use of recombinant Semliki Forest virus (SFV) encoding the oncoproteins E6 and E7 from high-risk human papilloma viruses (HPVs). Optimal immunization conditions are required for immunotherapeutic treatment of cervical cancer as it has been postulated that cervical cancer patients are immune-suppressed and/or immunologically tolerant for HPV. We previously generated an optimized construct encoding a fusion protein of HPV16 E6 and E7 and a translational enhancer (enhE6,7). Immunization of mice with SFV-enhE6,7 was shown to induce cytoxic T cell (CTL) responses and resulted in the eradication of established tumours. We now demonstrate, using HPV16-specific MHC class I tetramers, that high pCTL frequencies can be induced. However, this induction is strongly influenced by the route of immunization applied. Whilst in bulk CTL assays, requiring in vitro restimulation, CTL activity can be observed upon sc, ip, iv and im immunization, detectable pCTL frequencies, without in vitro restimulation, are only induced upon im and iv immunization. The route of immunization also strongly influences the dose of viral vector needed to induce CTLs and tumour therapy. As few as 5x104 SFV-enhE6,7, primed and boosted iv, are needed to eradicate tumours in six out of seven mice treated. Furthermore, exponentially growing tumours of approximately 500 mm3 in size were seen to completely resolve and even tumours as large as 1500 mm3 decreased to one-third of their size. Apart from this potency, SFV vectors can safely be used for the expression of oncoproteins such as E6 and E7, since the viral RNA is not integrated in the host genome. Thus SFV-enhE6,7 meets with the criteria that a vaccine against cervical cancer should be safe and induce a very strong, long-lasting CTL response, strong enough to eradicate existing tumours.