Linking GATA2 Deficiency to Dysregulated Innate Immune Signaling

Cell type-specific transcription factors governing hematopoietic stem and progenitor cell transitions establish networks containing hundreds of genes and proteins. Network complexity renders it challenging to discover essential versus modulatory or redundant components. This scenario is exemplified by GATA2 mechanisms that control hematopoiesis during embryogenesis. Loss of a far upstream Gata2 enhancer (-77) disrupts the GATA2-dependent genetic network governing hematopoietic progenitor cell differentiation (Johnson KD. et al., Sci. Adv., 2015). The aberrant network includes the transcription factor Interferon Regulatory Factor-8 and a host of innate immune regulators, including Toll-like receptors (TLRs) (Johnson KD. et al., J. Exp. Med., 2020). Mutant embryonic progenitors lose the capacity to balance production of diverse hematopoietic progeny and generate excessive monocytic progeny. As IRF8 is vitally important for monocytic and dendritic cell differentiation (Yanez A. and Goodridge H., Curr. Opin. Hematol., 2016), we asked whether IRF8 is essential, contributory, or inconsequential. Using a double-mutant genetic rescue in vivo system, we demonstrated that reducing Irf8, in the context of the -77 mutant allele, reversed granulocytic deficiencies and the excessive accumulation of dendritic cell-committed progenitors. In -77 ^-/- E14.5 fetal livers, monocyte progenitors (MPs) increased 2.3-fold (P = 0.006), granulocyte progenitors (GPs) decreased 2.2-fold (P = 0.003) and common dendritic cell progenitors (CDPs) increased 10.2-fold (P = 0.021) relative to wildtype littermates. Ablating Irf8 in -77 mutants (-77 ^-/-; Irf8^-/-) restored MPs and CDPs to wildtype levels and reversed the GP deficiency; further increasing GPs 4.2-fold relative to wildtype (P = 0.0009). Despite many dysregulated components that control vital transcriptional, signaling and immune processes, the aberrant elevation of a single transcription factor deconstructed the embryonic hematopoiesis program. We analyzed the mechanistic and biological implications of IRF8 dysregulation concomitant with ectopic upregulation of other innate immune genes (including Toll-like receptors (TLRs) in GATA2-deficient embryonic progenitors. In principle, such genes might function upstream, downstream, or in parallel with IRF8. Based on TLR upregulation and TLR roles in progenitor mechanisms (Nagai Y. et al., Immunity, 2006; Schuettpelz L. et al., Leukemia, 2014; Caiado F. et al., J. Exp. Med., 2021), we tested whether GATA2 deficiency in embryonic progenitors impacts cellular responsiveness to TLR ligands. Wild type and -77 enhancer-mutant progenitors were treated with increasing concentrations of the TLR1/2 ligand Pam ₃CSK ₄. The mutant progenitors were hypersensitive to Pam ₃CSK ₄, which resulted in supra-physiological induction of Tnf expression (2.8-fold at 34 nM, P = 0.004; 3.2-fold at 68 nM, P = 0.0003). Quantitative analyses indicated that hypersensitivity reflected increased Pam ₃CSK ₄ efficacy, but not potency. GATA2 re-expression in the mutant progenitors attenuated the elevated IRF8 expression and TLR signaling, normalizing Tnf and Ccl3 expression to a level comparable to that of wild type progenitors. In GATA2-rescued mutant progenitors, Tnf and Ccl3 expression decreased 3.9-fold (P = 0.005) and 2.5-fold (P = 0.047), respectively. Thus, GATA2 suppresses TLR signaling in embryonic progenitors. Ongoing studies are elucidating the mechanistic interconnections between IRF8- and TLR-dependent inflammatory networks in GATA2 deficiency during embryonic and adult hematopoiesis in cell populations and single cells, relationships between murine and human mechanisms, and the impact of targeted interventions that modulate these mechanisms.