Abstract
A major challenge for both the immune system as well as anti-tumor drugs is the specific killing of tumor cells. The challenge lies in distinguishing “dangerous self” cells from “non-dangerous self” cells. One sophisticated approach to specifically kill tumor cells is to exploit synthetic lethal interactions as demonstrated by PARP inhibitors. These inhibitors target PARP1, a ubiquitous protein that is crucial for the early detection of DNA damage through ADP-ribosylation of target proteins and specifically induce tumor cell death in cancer cells with mutations in other DNA damage repair-associated proteins, such as BRCA1/2. PARP inhibitors have demonstrated beneficial effects in cancer patients regardless of their BRCA1/2 status, and, intriguingly, both PARP inhibitors and PARP1 have also emerged as immune regulators. This suggests that the role of this abundant nuclear protein extends beyond the orchestration of DNA damage repair. Indeed, PARP1 knockout animals are resistant to injection of a lethal dose of LPS. The increased survival is accompanied by a decrease in pro-inflammatory cytokines, including IFN-gamma. Notably, macrophage activation by LPS results in the expression of a pro- inflammatory gene signature that is dependent on PARP1 but not its enzymatic function. Conversely, several reports indicate that the treatment with PARP inhibitors alters cytokine expression, raising the question of which cell type is affected by PARP1-mediated ADP-ribosylation. Here, we identify PARP1 and PARP1-dependent ADP-ribosylation as critical immune regulators in NK cells and highlight their importance in controlling T cell polarization and infiltration. In vitro stimulation of NK-specific PARP1 knockout splenocytes or the PARP1 knockout human NK cell line Nishi demonstrated PARP1-dependent transcription of IFN-gamma. Furthermore, the absence of PARP1 in Nishi cells correlated with a loss of factors involved in T cell differentiation. Interestingly, in vivo immune cell stimulation of WT and NK-specific PARP1 knockout animals by intraperitoneal LPS injection showed a significant reduction of IFN-gamma+ T cells compared to control animals after LPS treatment. Moreover, the control of T cell susceptible MC38 tumors but not of NK susceptible RMA-S tumors, depended on functional PARP1 in NK cells. In subcutaneously injected MC38 tumors, T cell infiltration correlated with PARP1-dependent expression of T cell chemokines in NK cells. Taken together, these findings suggest that PARP1 controls the transcription of specific genes in NK cells and emphasizes the importance of an effective intercellular communication for a functional immune response. In a second study, we analyzed ADP-ribosylhydrolase 3, which acts as an “eraser” of PARP1-dependent ADP-ribosylation. We show a strong increase in histone-ADP-ribosylation upon ARH3 knockdown. Immune cell type-specific investigations revealed a robust histone-ADP-ribosylation signal with comparable levels between the types analyzed, suggesting that ARH3 affects ADP-ribosylation in the spleen, but in a cell type independent manner.
Böhi, Flurina