IRF8 controls T cell development and survival to regulate T cell antitumor activity

Interferon Regulatory Factor 8 (IRF8, or ICSBP1) is a member of the Interferon Regulatory transcription factor family, and functions as a key hematopoietic transcription factor. Loss of IRF8 leads to defective antigen-presenting cell activity, perturbations in B cell development and, in mouse models, an accumulation of CD11b + Gr1 + immature myeloid cells. However, the role of IRF8 in T cell development and antitumor activity remains unclear. Whole body and chimeric IRF8-knockout mice (IRF8-KO) demonstrate increased susceptibility to both allogenic transplant and carcinogen-induced tumor models. T cell function is crucial for the immune system9s endogenous antitumor response. Analysis of the T cell compartment of IRF8-KO mice demonstrated a deficiency in both naive T cell percentages and total number. Despite this peripheral decrease in T cell numbers, early T cell progenitors in both the bone marrow and thymus were significantly increased in IRF8-KO mice compared to wild-type. To further investigate the role of IRF8 in T cell development and survival, IRF8-KO:WT mixed chimera mice were generated by lethal irradiation of CD45.1 + CD45.2 + recipient mice, followed by transfer of CD45.2 + IRF8-KO and CD45.1 + WT bone marrow (BM). Surprisingly, analysis of blood obtained from reconstituted mice demonstrated preferential engraftment and survival of T cells derived from WT, rather than IRF8-KO BM. This imbalanced phenotype was not rescued by increasing the proportion of IRF8-KO BM administered to mice, suggesting the effect was not due to failure of IRF8-KO BM engraftment. Furthermore, analysis of T cell populations in both primary (thymus) and secondary (spleen) lymphoid organs showed a progressive loss of IRF8-KO T cells during their maturation and development process, while WT T cells remained unaltered. Given that IRF8 has been shown in tumor cells to regulate a variety of pro- and anti-apoptotic molecules, we hypothesized that IRF8 controls the peripheral survival of T cells. To test this hypothesis, resting T cells were isolated from the spleen of mixed chimera mice and viability was measured by Annexin V/PI staining. Resting IRF8-KO cells, but not WT, demonstrated a pro-apoptotic phenotype, as shown by increased Annexin V staining. Accordingly, upon in vitro stimulation and activation, IRF8-KO T cells demonstrated increased apoptosis. Our data determine that IRF8 controls both T cell development and peripheral survival, and that loss of IRF8 impairs the T cell antitumor immune response. Citation Format: John D. Klement, Amy V. Paschall, Mary A. Zimmerman, Mohammed L. Ibrahim, Priscilla S. Redd, Chunwan Lu, Hussein Sultan, Esteban Celis, Keiko Ozato, Kebin Liu. IRF8 controls T cell development and survival to regulate T cell antitumor activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4682.