Vaccine based immunotherapy as a strategy to bypass drug resistance in multiple myeloma

Clonal evolution under therapy leads to drug resistance and relapse in multiple myeloma (MM), requiring new treatment mo- dalities to control disease. Immunotherapy and specifically vacci- nation to induce anti-tumor CD8+ cytotoxic T-cells (CTLs) provides one such strategy, but susceptibility of drug resistant MM cells to CTL attack is as yet not defined. To examine this, a first requirement is identification of tumour-associated antigens in drug refractory MM. To this end, we had previously specifically evaluated cancer testis antigens (CTAs) in MM for retention at relapse post- therapy, and identified SPAG9 and MAGEC1 as frontline antigen targets for vaccines. Of these, SPAG9 is generic to relapse MM (expressed in w100% of cases) and MAGEC1 is essentially tumour-specific (expressed w60% of cases). An optimal strategy for vaccination as therapy is in disease remission to attack re-emerging MM cells that escape drug therapy. A second requirement is to model drug resistance in MM, and accordingly we have now generated derivatives of the XG2 MM cell line that are resistant to melphalan (XG2 M) or Bortezomib (XG2 600R3); the RPMI8226-derivative R5 line with multiple drug resistance was also available. A third requirement is to show effective lysis of drug resistant MM cells by antigen specific CTLs. To generate antigen-specific CTLs, we utilized pDOM-CTAepitope DNA vaccine delivery in HLA-A2 transgenic HHD mice. We examined two epitopes in respective vaccines, pDOM-SPAG9p1 and pDOM-MAGEC1p2. Both vac- cines elicited epitope specific T-cell responses in HHD measured by IFN release assays following prime and boost with electroporation. Significantly, using MM cell lines made chimeric for HHD MHC class I as targets, both vaccines induced CTLs in HHD mice that were able to effectively lyse primary (XG2 and others) and drug resistant MM cell lines (XG2M, XG2 600R3, R5) effectively. These data demonstrate that drug resistant MM cells are susceptible to CTL killing, substantiating a role for vaccination in MM to over- come drug resistance and hence prevent relapse