IL-12 in antitumor immunotherapy: to target or not to target?

Lentiviral vectors (LVs) represent powerful vaccines for antitumor immunotherapy since they can deliver tumor-associated antigens together with intrinsic stimulatory signals to antigen-presenting cells (APCs) in situ. Mostly broad tropism LVs are used, which are able to infect both APCs as well as non-APCs. This creates two complaints. On the one hand infection of different APC-subtypes could lead to a dispersed immune response. On the other hand, the transduction of long-lived non-APCs increases the risk for insertional mutagenesis. Therefore we propose to target LVs to specific APC-subtypes as we assume that this could increase both their safety and efficacy profile. To generate APC-subtype targeted LVs, we developed the Nanobody display technology, which allows the inclusion of a fusogenic but bindingdefective form of VSV.G together with an APC-specific Nanobody in the LVs’ envelope. We specified their tranduction profile after in vivo or ex vivo intranodal injection of murine and human lymph nodes respectively. Furthermore, their potential to stimulate an ovalbumin specific immune response was evaluated using several immunological assays. First, we demonstrated specific transduction of murine and human short-lived APC-subtypes, which was also reflected in their short transgene expression period compared to the broad tropism LVs. Next we evaluated their immune stimulatory potential and observed that targeting doesn’t improve their potential to induce functional T cells, which was reflected in their reduced ability to induce a therapeutic benefit. Moreover, when we added the IL-12 encoding gene to the LV-construct in order to improve their immune stimulatory potential, this had only a major impact on the therapeutic potential of the broad tropism LVs. In conclusion we show that the Nanobody display technology allows the generation of APC-subtype specific LVs with a safer kinetic profile. However, we also showed that the transduction of non-APCs is of paramount importance to induce an adequate therapeutic response.