Inducing immunity to haematological malignancies with DNA vaccines

Our aim is to develop DNA vaccines against haematological malignancies, with a focus on multiple myeloma (MM), a plasma cell tumour with no known cure. B cell tumours display an idiotype (Id) derived from immunoglobulin (Ig) variable (V) region. We have shown previously that assembly of V genes as a single chain variable fragment (scFv) format in a DNA vaccine induced weak immune responses. Fusing scFv to a pathogen-derived sequence fragment C (FrC) from tetanus toxin however, promoted Id-specific antibody and T-cell responses and led to protection from a lethal tumour challenge in lymphoma and MM animal models. Validation of this scFv DNA fusion vaccine design has led to current phase I/II clinical trials. Since MM cells are MHC class I positive, there is also an interest in activating cytotoxic CD8+ T cells (CTL). A novel DNA vaccine has emerged in our laboratory, fusing the first domain of FrC (p.DOM) to a MHC class I epitope, which is able to generate potent CTL responses. This design opens up the possibility of attack on intracellular antigens, likely to be presented by MHC class I. For MM, the cancer testis antigens (CTA) have emerged as tumour-specific intracellular targets. To model DNA fusion vaccine design against CTA, we examined the murine P815 mastocytoma, which expresses the P1A gene analogous to human CTA. P1A encodes a well-defined MHC class I H2-Ld motif (AB). A p.DOM-P1A/AB vaccine was constructed and a single vaccination led to detection of activated epitope-specific CTL ex vivo, which could be expanded on re-stimulation in vitro. These CTL were able to kill P815 tumour cells in an epitope-specific manner. Importantly, in protection experiments approximately 40% of vaccinated mice were protected from tumour challenge. However in some cases there was evidence for immune pressure leading to the growth of antigen loss escape variant tumour cells, highlighting the need to target additional antigen targets. We also investigated mucin 1 (MUC1), an over-expressed glycoprotein antigen in MM. Early work in our laboratory had shown that vaccination with a DNA vaccine encoding full length human MUC1, either alone or fused to FrC or DOM, could provide low level protection against tumour challenge in vivo using wild type mice, with no role for anti-MUC1 antibodies. The protective immunity generated by these vaccines has been re-assessed by lymphocyte depletion, revealing a prominent role for both CD4+ T cells and CD8+ T cells. To improve activation of CTL, two p.DOM.epitope vaccines were constructed incorporating known MUC1 MHC class I epitopes.