Browsing Graduate Student Dissertations & Theses by Subject "GENESIS"
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GENESIS AND MAINTENANCE OF LONG-TERM IgM+ T-BET+ B CELLSIgM memory cells are recognized as an important component of B cell memory, based on several studies both in mice and humans. Our studies of B cells elicited in response to ehrlichial infection identified a population of CD11c/T-bet-positive IgM memory cells and an IgM T-bet-positive bone marrow antibody-secreting cell population (ASCs). The origin of these populations was unknown, although an early T-independent spleen CD11c-and T-bet-positive IgM plasmablast population precedes both, suggesting a linear relationship. The majority of IgM memory cells detected after day 30 post-infection had undergone somatic hypermutation, indicating that they expressed activation-induced cytidine deaminase (AID). Therefore, to identify early AID-expressing precursor cells, we infected an AID-regulated tamoxifen-inducible Cre-recombinase-EYFP reporter strain. Tamoxifen administration led to labeling of both the IgM memory cells and bone marrow ASCs on day 30 and later post-infection. High frequencies of labeled cells were identified on day 30 post-infection,following tamoxifen administration on day 10 post-infection. Both IgM memory cells and IgM bone marrow ASCs were labeled when tamoxifen was administered as early as day 4 post-infection. We also identified mechanisms involved in maintenance of the IgM bone marrow ASCs and IgM+ memory cells, namely proliferation and FcγRIIb respectively. BrdU studies revealed that the bone marrow IgM ASCs were maintained by proliferation, unlike the IgM memory cells. RNAseq analysis revealed a 2-fold higher expression of inhibitory Fc receptor, FcγRIIb. Because FcγRIIb inhibits B cell activation, we hypothesized that FcγRIIb negatively regulates IgM+ memory B cells by binding immune complexes present during low-level chronic infection. E. murisinfection of FcγRIIb-deficientmice revealed a 3-fold expansion of the IgM+ memory 30 days post-infection. We further demonstrated that the expansion of the IgM+ memory cells was not due to increased proliferation, but a decrease of apoptosis, due to a lack of Fas expression in FcγRIIb-deficient mice. This result was mimicked in AID-deficient mice, which lack the ability to class switch to IgG and make immune complexes, revealing a role for immune complexes in regulating IgM+ memory. Altogether, these studies demonstrate a novel germinal center-independent pathway for the generation of two distinct long-term IgM-positive B cell populations.
ROLE OF BMP AND ACTIVIN/NODALSIGNALING REPRESSION IN RETINALPROGENITOR CELL SPECIFICATION ANDCONE PHOTORECEPTOR GENESISEye formation begins with the specification of the eye field from the anteriorneural plate during the transition from gastrulation to neuralization. The morphogeneticgradients formed by BMP and Activin/Nodal ligands organize the body axis andregulate intracellular signals to bias the cell lineages.Therefore, to understand theextracellular signaling mechanisms governing eye field specification, we looked closerat the mechanisms responsible for neuralization. This project takes advantage of thedevelopmental accessibility of theXenopus laevisembryo to investigate the role ofBMP and Activin/Nodal signaling pathways during early eye development.Nogginis a secreted BMP antagonist and is sufficient to induce formation of ectopic retinaltissue. My results presented in Chapter 2 show that intracellular repression of BMP-Smad1/5/8 and Activin/Nodal-Smad2/3 signaling was sufficient to replicate the retina-promoting activity of Noggin and efficiently drive pluripotentXenopusprimitive ectodermto a retinal progenitor cell fate. Furthermore, my work presented in Chapter 3 suggeststhat repression of BMP and Activin/Nodal in the anterior neural plate allows the anteriorneural patterning transcription factor, Otx2, to directly activate expression of the earliesteye field transcription factor,tbx3. This study elucidates the initial step by which theanterior neural plate is patterned to specify the eye field. We next translated our findings to mouse embryonic stem cell culture (Chapter 4). We observed that mouse embryonic stemcells can be efficiently directed towards retinal progenitor cells by first converting themto an early primitive ectoderm-like state, followed by simultaneous repression of BMPand Activin/Nodal and activation of FGF signaling. After differentiation, we observedthat photoreceptor genesis is synchronized, resulting in 96% of aggregates expressingmarkers for cone photoreceptors after 9 days. This differentiation protocol is more efficientand quicker than previously published mouse stem cell protocols. Optimizing treatmentconditions resulted in up to 62% of cells expressing the cone/rod photoreceptor marker,CRX. This study was the first to show that photoreceptor-like cells can be generated frommouse pluripotent stem cells as efficiently as published human pluripotent stem cell reports,but with a shorter culture period. These studies will provide the basis of future work todetermine the underlying molecular mechanism driving retinal differentiation.