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Dendritic cells (DCs) comprise a distinct lineage of potent antigen-presenting mononuclear phagocytes that serve as both mediators of innate immune responses and key facilitators of the adaptive immune response. DCs play both immunogenic and tolerogenic roles through their dual ability to elicit pathogen-specific T cell immunity as well as induce regulatory T cell (Treg) responses to promote tolerance in the steady state. The aim of the work presented here is to examine the normal regulatory mechanisms of DC development and function, starting with the dissection of mechanisms behind an aberrantly activated developmental pathway, followed by the exploration of new mechanisms governed by two candidate transcription factors. The first chapter of the thesis focuses on the growth factor receptor Flt3, an essential regulator of normal DC development in both mice and humans, and concurrently one of the most commonly mutated proteins found in acute myeloid leukemia (AML). We investigated the effect of its most common activating mutation in AML, the Flt3 internal tandem duplication (Flt3-ITD), and found that this mutation caused a significant cell-intrinsic expansion of all DC populations. This effect was associated with an expansion of Tregs and the ability to dampen self-reactivity, with an inability to control autoimmunity in the absence of Tregs. Thus, we describe a potential mechanism by which leukemia can modulate T cell responses and support Treg expansion indirectly through DCs, which may compromise immunosurveillance and promote leukemogenesis. The subsequent chapters explore the basic molecular mechanisms of DC development by using Flt3 expression as a guide to uncover new candidates involved in the DC transcriptional program. We show that Myc family transcription factor, Mycl1, is largely dispensable for DC development and function, contrary to recent published findings that propose a role in proliferation and T cell priming. On the other hand, we find that conditional deletion of our second candidate gene, an Ets family transcription factor, has diverse effects on DC development, monocyte homeostasis, and cytokine production. Overall, our studies highlight an unexpected molecular link between DC development and leukemogenesis, and elucidate novel mechanisms controlling DC differentiation and function.

The AKR strain of mice have a genetic trait for spontaneous development of lymphocytic leukemia. In this study, leukemic mice were found to have significantly larger (p<0.01) thymuses and spleens than preleukemic mice. The enlarged leukemic tissues were densely packed with a light staining cell, with a hollow-appearing nucleus. Tissues from preleukemic mice were observed to be infiltrated with a smaller, darker-staining lymphocyte. Fluorescent antibody staining was done on preleukemic and leukemic tissues, using three antisera against murine lymphocyte theta antigen, and an antiserum against murine IgG. Significantly brighter fluorescence, (p <0.05) with theta-specific antisera, was found in leukemic thymuses,spleens, and kidneys than in the same preleukemic tissues. Leukemic tissues had significantly brighter fluorescence (p <0.05) than preleukemic tissues with IgG antiserum.

Indiana University-Purdue University Indianapolis (IUPUI) / Juvenile Myelomonocytic Leukemia (JMML) is rare, fatal myeloproliferative disease (MPD) affecting young children, and is characterized by expansion of monocyte lineage cells and hypersensitivity to Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) stimulation. JMML is frequently associated with gain-of-function mutations in the PTPN11 gene, which encodes the protein tyrosine phosphatase, Shp2. Activating Shp2 mutations are known to promote hyperactivation of the Ras-Erk signaling pathway, but Akt is also observed to have enhanced phosphorylation, suggesting a potential role for Phosphatidylinositol-3-Kinase (PI3K)-Akt signaling in mutant Shp2-induced GM-CSF hypersensitivity and leukemogenesis. Having demonstrated that Class IA PI3K is hyperactivated in the presence of mutant Shp2 and contributes to GM-CSF hypersensitivity, I hypothesized the hematopoietic-specific Class IA PI3K catalytic subunit p110δ is a crucial mediator of mutant Shp2-induced PI3K hyperactivation and GM-CSF hypersensitivity in vitro and MPD development in vivo. I crossed gain-of-function mutant Shp2 D61Y inducible knockin mice, which develop fatal MPD, with mice expressing kinase-dead mutant p110δ D910A to evaluate p110δ’s role in mutant Shp2-induced GM-CSF hypersensitivity in vitro and MPD development in vivo. As a comparison, I also crossed Shp2 D61Y inducible knockin mice with mice bearing inducible knockout of the ubiquitously expressed Class IA PI3K catalytic subunit, p110α. I found that genetic interruption of p110δ, but not p110α, significantly reduced GM-CSF-stimulated hyperactivation of both the Ras-Erk and PI3K-Akt signaling pathways, and as a consequence, resulted in reduced GM-CSF-stimulated hyper-proliferation in vitro. Furthermore, I found that mice bearing genetic disruption of p110δ, but not p110α, in the presence of gain-of-function mutant Shp2 D61Y, had on average, smaller spleen sizes, suggesting that loss of p110δ activity reduced MPD severity in vivo. I also investigated the effects of three PI3K inhibitors with high specificity for p110δ, IC87114, GDC-0941, and GS-9820 (formerly known as CAL-120), on mutant Shp2-induced GM-CSF hypersensitivity. These inhibitors with high specificity for p110δ significantly reduced GM-CSF-stimulated hyperactivation of PI3K-Akt and Ras-Erk signaling and reduced GM-CSF-stimulated hyperproliferation in cells expressing gain-of-function Shp2 mutants. Collectively, these findings show that p110δ-dependent PI3K hyperactivation contributes to mutant Shp2-induced GM-CSF hypersensitivity and MPD development, and that p110δ represents a potential novel therapeutic target for JMML.