The Novel Bispecific Diabody αCD40/αCD28 Strengthens Leukemic Dendritic Cell Induced T Cell Reactivity.

Dendritic cell (DC)-based immunotherapy faces new challenges since efficacy of DC vaccines in clinical trials has been inconsistent. Strategies to improve immune responses induced by DC are currently being explored. We have recently shown the feasibility of generating fully functional DC from Acute Myeloid Leukemic (AML) blasts, but with varying expression levels of the important costimulatory molecule CD86. To overcome this variability, we developed a novel bispecific diabody (BsDb) simultaneously and agonistically targeting CD40 on AML-DC and CD28 on naïve T cells. Beside optimization of CD28-mediated signaling, the resulting cellular cross-linking was also hypothesized to increase the strength and duration of T cell/AML-DC interactions, thus increasing T cell responsiveness to AML antigens. Indeed the αCD40/αCD28-bispecific diabody provokes increased T cell-DC cluster formation as assessed by light microscopy. Significant increased cluster formation was observed when T cells and AML-DC were cocultured in presence of the BsDb as compared to T cells incubated with a control protein (46%±2 versus 22%±1 respectively, p<0.05). Prior incubation of T cells and/or AML-DC with CD28 or CD40, respectively, completely prevented cluster formation in presence of the BsDb indicating specific binding of the BsDb to CD40 and CD28. The αCD40/αCD28 BsDb significantly increases T cell proliferation induced by AML-DC as compared to the unstimulated cocultures, in a dose dependent manner, as evaluated by mixed lymphocyte reactions (fold increased T cell proliferation of cocultures stimulated with BsDb as compared to unstimulated cocultures:170%±12, p<0.05). In addition, BsDb is capable of DC maturation induction as shown by significant increased mean fluorescence index (MFI) of the maturation markers CD80 (MFI of AML-DC cultured in presence of control protein vs AML-DC cultured in presence of BsDb: 22±5 vs 12±3, p<0.05) and CD83 (4±1 vs 1.5±0.5, p<0.05). In order to determine the effect of aCD40/aCD28-bispecific diabody-mediated cross-linking of AML-derived DC and CD8⁺ T cells on the induction efficiency of tumor-specific CTL, AML-DC derived from the HLA-A2⁺ AML cell line MUTZ-3 were pre-incubated with the aCD40/aCD28-bispecific diabody, loaded with the heteroclitic variant of the aa988 epitope of hTERT, and used as stimulator cells in an HLA-A2-matched allogeneic in vitro CTL induction protocol. In total nine parallel bulk cultures, were stimulated twice with peptide-loaded MUTZ-3 DC, either pulsed with control protein or the aCD40/aCD28-bispecific diabody. hTERT_988Y-specific CD8⁺ T cells could be detected in 5/9 individual cultures when stimulated with DC pulsed with the aCD40/aCD28-bispecific diabody, whereas in only 1/9 individual cultures hTERT_988Y-specific CD8⁺ T cells could be detected when stimulated with DC pulsed with the control protein. Thus, priming efficacy of tumor-specific cytotoxic T cells can also be improved by cross-linking AML-DC and T cells with the αCD40/αCD28 diabody. We propose that the αCD40/αCD28-bispecific diabody can serve as a potent therapeutic tool to effectively augment anti-tumor T cell responses elicited by AML-DC.