Selection of an antibody library identifies a pathway to induce immunity by targeting CD36 on steady state CD8α+ dendritic cells

Due to their prominent role in the orchestration of a broad range of immune responses, dendritic cells (DCs) have emerged in the past decade as central target for cancer immunotherapy. Recent advance in the knowledge of DCs functions and subset specialization led to the design of novel immunotherapeutic approaches based on the possibility to target DCs directly in vivo, thus avoiding their ex vivo manipulation. A promising strategy is to use antibodies to target antigens to cell-surface molecules expressed by DCs, in order to increase T-cell mediated immune responses. The studies performed so far, have revealed that the efficacy of in vivo DC vaccination depend on several factors including the specific DC subset targeted, their maturation status and the nature and biological properties of the receptor targeted. Therefore, the identification of the most appropriate ligand/receptor pairing it is a requisite to improve the modality of delivery tumor Ags to DCs. To this aim, we screened a library of Ab fragments on mouse DCs to isolate new potential antibodies capable of targeting DCs in vivo and able to induce T-cell mediated immune responses against specific antigens. In this study, we provide the proof of principle that the phage display technology can be successfully used to isolate internalizing antibodies on mouse DCs. We further develop such technology by engineering the selected molecules to create antigen fusion proteins to use in vaccination protocols. In particular, we focus on a high affinity Ab against CD36, a multiligand scavenger receptor primarily expressed by the CD8α+ subset of conventional DCs. We characterize the antigen presenting properties of this receptor which help to delineate a novel function of CD36 in adaptive immunity. We show that targeting CD36 on DCs results in the delivery of exogenous Ags to both the MHC class-I and MHC class-II processing pathways. In addition, immunization with the recombinant anti CD36-Ag fusion Ab induces the robust activation of naïve CD4+ and CD8+ Ag specific T lymphocytes and the differentiation of primed CD8+ T cells into long term effector CTLs. Finally, we demonstrate that in vivo targeting of CD8α+ DCs with anti-CD36 Ab elicits humoral and cell mediated protection from the growth of an Ag specific tumor. Collectively, these results identify CD36 as an appropriate receptor to better elucidate the properties of the lymphoid organ resident CD8+ DCs and indicate it as a novel potential target for cancer immunotherapy.