Chemotherapy-induced bystander effect

Abstract: Donor cell Leukaemia (DCL) is a rare complication following haematopoietic stem cell transplantation (HSCT) whereby the patients develop a new type of cancer from the cells that have come from the donor months/years after transplantation. The mechanism by which this occurs remains unknown as majority of the donors remain healthy. We proposed that pre-transplant chemotherapy can induce a bystander effect from mesenchymal stem cells, which are in direct interaction with haematopoietic stem cells in the BM microenvironment, to the incoming cells thereby inducing donor cell leukaemia. HS-5 Stromal cell line was treated with clinically relevant doses of chlorambucil (40µM), carmustine (10µg/ml), etoposide (10µM) and mitoxantrone (500ng/ml) for 24 hours, washed off and were then co-cultured with TK6 lymphoblast cell line for 24 hours to ascertain if they transfer effect of the chemo drugs to the TK6. Damage in the TK6 cells were defined by cytotoxicity and genotoxicity using trypan blue and micronucleus assays respectively. Results show that therapeutic concentrations of mitoxantrone (p < 0.01), chlorambucil and carmustine decreased the number of viable cells relative to the control whilst exposure to etoposide-treated HS5 cells increased the number of viable TK6 cells. Furthermore, all drug groups exhibited an increased number of micronuclei in the cells relative to the control, however, only mitoxantrone group was statistically significant (p < 0.05). Collectively, our findings suggest that MSC within the BM microenvironment induces a bystander effect in neighbouring cells following treatment with clinically relevant doses of chemotherapeutic drugs, which may account for the damage found in donor cells during donor cell leukaemia.