(P016) The Cell Cycle Inhibitor P21 Regulates Langerhans Cell Radiation Resistance and PromotesT Regulatory Cell Induction Upon Exposure to Radiotherapy

Jeremy G. Price, Juliana Idoyaga, Brandon Hogstad, Helene Salmon, Marylene Leboeuf, Miriam Merad; Icahn School of Medicine at Mount Sinai; Stanford University

Several studies have revealed that exposure to ionizing irradiation (IR) may lead to increased accumulation of tumor-infiltrating T regulatory cells (Tregs), which in turn promotes tumor resistance to radiation therapy. Here, we questioned the contribution of tissue-resident antigen-presenting cells (APCs) to the induction of Tregs upon exposure to IR. Specifically, we focused on Langerhans cells (LCs), the resident APCs of the epidermis, because of their unique ability to resist depletion by high-dose IR. Therefore, LC IR resistance may inform us of the underlying IR resistance mechanisms utilized by other progenitor cells. However, a comprehensive study of the molecular and cellular mechanisms conferring LC IR resistance has never been undertaken.

We found that LCs do not undergo apoptosis following IR as do other dendritic cell (DC) subsets but instead persist and migrate to skin-draining lymph nodes. Subsequent analysis in migration-deficient CCR7−/− mice revealed a constant number of epidermal LCs, indicating that changes in LC number following IR are due solely to migration and not to cell death. Moreover, we show for the first time that LCs are resistant to the formation of DNA damage, as measured by induction of H2AX foci and COMET assay. In contrast, other members of the myeloid lineage, notably lymphoid tissue-resident DCs, are exquisitely sensitive to IR-induced DNA damage. In addition, both steady-state and postlethal IR LCs express a unique repertoire of prosurvival and stress-related proteins and diminished levels of proapoptotic molecules by microarray analysis. Additionally, we found that the cell cycle regulator p21 is overexpressed in LCs at rest and that in contrast to WT LCs, p21−/− LCs undergo apoptosis, accumulate significant DNA damage, and fail to experience cell cycle arrest following IR. Strikingly, upon skin exposure to IR, WT LCs lead to significant expansion of Treg, whereas p21−/− LCs fail to do so. In a cutaneous tumor model, we show that p21−/− LCs cannot promote tumor-infiltrating Tregs and correlate with smaller tumor volumes.

Moving forward, these data suggest a novel means by which targeting LC IR resistance can be used to increase the response to radiotherapy of cutaneous tumors. LCs uniquely express an IR resistance module of genes that permits them to persist and repopulate the epidermal niche following IR. Future analyses will seek to elaborate the functional implications of LCs and IR-sensitive DC IR-exposure as it relates to the priming and development of graft-versus-host disease (GVHD) and local antitumor immunity following radiation therapy.

Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org