Downstream Targets of PKCβII in Dendritic Cell Differentiation,

Abstract 3245

An important component of antitumor immunity is the antigen presenting dendritic cell (DC), which initiates innate and adaptive immune responses. It is therefore beneficial for cancer to inhibit the function of these DCs. Tumors often depress DC activity by inhibiting their differentiation, which results in an accumulation of immature DCs which are immunosuppressive and aid in tumor progression and immunologic evasion. Tumor-derived factors prevent hematopoietic progenitor cells (HPCs) from fully completing differentiation to a mature DC, but the intracellular mechanisms by which tumors do this remain largely unknown.

Previous studies have shown that the serine-threonine kinase, protein kinase C βII (PKCβII), is required for DC differentiation from HPCs. It has also been shown that the interferon regulatory factor 8 (IRF8) is necessary for DC differentiation from HPCs. Loss of either PKCβII or IRF8 results in an accumulation of immunosuppressive immature DCs, incapable of inducing immune responses, and capable of promoting tumor maintenance and progression. The signaling pathways both of these molecules have independently been shown to play critical roles in DC differentiation, and to be downregulated in tumor presence to promote survival. However, whether a relationship exists between the two molecules in this process has yet to be established.

In human and murine models, we have determined that inhibition of PKβCII with a specific molecular inhibitor, CGP 53353, prevented differentiation of lineage negative progenitor cells to DCs. The cells treated with CGP 53353 failed to upregulate DC markers analyzed by flow cytometry, and failed to induce allogeneic T-cell proliferation compared to progenitor cells treated with DC-inducing cytokines. We also found through q-PCR that the PKC agonist phorbol 12-myristate 13-acetate (PMA) upregulated IRF8 expression, and in turn PKCβII inhibition lead to a loss in IRF8 expression. Therefore we have demonstrated a role for PKCβII in DC differentiation in mice (which was previously uncharacterized), as well as identified a potential relationship between IRF8 and PKCβII signaling during differentiation. Identifying such a relationship would provide a signaling model for DC differentiation, allowing a better understanding of how DC differentiation is initiated and impaired. It may also provide signaling components to target with antitumor therapeutics in order to induce DC differentiation in a tumor environment.