Influence of the oncolytic parvovirus H-1, CTLA-4 antibody tremelimumab and cytostatic drugs on the human immune system in a human in vitro model of colorectal cancer cells.

Introduction: Tumor-directed and immune-system-stimulating therapies are of special interest in cancer treatment. Here, we demonstrate the potential of parvovirus H-1 (H-1PV) to efficiently kill colorectal cancer cells and induce immunogenicity of colorectal tumors by inducing maturation of dendritic cells (DCs) alone and also in combination with cytostatic drugs in vitro. Using our cell culture model, we have additionally investigated the effects of anti-CTLA-4 (cytotoxic T-lymphocyte-associated antigen 4) receptor antibody tremelimumab on this process.

Materials And Methods: Colon carcinoma cell lines were treated with different concentrations of cytostatic drugs or tremelimumab or were infected with H-1PV in different multiplicities of infection (MOIs), and viability was determined using MTT assays. Expression of CTLA-4 in colon carcinoma cell lines was measured by FACScan™. For the coculture model, we isolated monocytes using adherence, and differentiation into immature DCs (iDCs) was stimulated using interleukin-4 and granulocyte-macrophage colony-stimulating factor. Maturation of iDCs into mature DCs (mDCs) was induced by a cytokine cocktail. SW480 colon carcinoma cells were infected with H-1PV or treated with cytostatic drugs. Drug treated and H-1PV-infected SW480 colon carcinoma cells were cocultured with iDCs and expression of maturation markers was measured using FACScan™. Cytokine measurements were performed using enzyme-linked immunosorbent assay.

Results: Colon carcinoma cells SW480 were potently infected and killed by H-1PV. CTLA-4 expression in SW480 cells increased after infection with H-1PV and also after treatment with cytostatic drugs. Tremelimumab had no influence on viability of the colon carcinoma cell line. There was no maturation of iDCs after coculture with SW480; instead, H-1PV-infected or drug pretreated SW480 induced maturation. Cytokine production was higher for H-1PV-infected cells but was not significantly enhanced by tremelimumab treatment alone or in combination. Addition of tremelimumab did not interfere with the maturation process as measured by markers of maturation as well as by determination of cytokine levels.

Conclusion: By enhancing both cell death and immunogenicity of tumors, H-1PV is of special interest for tumor-directed therapy. These features make it a promising candidate for clinical application in human colorectal cancer. As tremelimumab does not significantly interfere with this process, an interesting therapeutic combination of active enhancement of tumor immunogenicity and independent masking of the CTLA-4 silencing process on tumor cells is highlighted.