Medulloblastoma development: tumor biology informs treatment decisions

Medulloblastoma is the most common malignant pediatric brain tumor. Current treatments including surgery, craniospinal radiation and high-dose chemotherapy have led to improvement in survival. However, the risk for recurrence as well as significant long-term neurocognitive and endocrine sequelae associated with current treatment modalities underscore the urgent need for novel tumor-specific, normal brain-sparing therapies. It has also provided the impetus for research focused on providing a better understanding of medulloblastoma biology. The expectation is that such studies will lead to the identification of new therapeutic targets and eventually to an increase in personalized treatment approaches.

Defective Flt3L driven DC development in the absence of Ikaros (111.35)

Ikaros null (Ik-) mice have impaired dendritic cell (DC) development. This defect is recapitulated when Ik- bone marrow (BM) is cultured with Flt3L, a DC growth factor. Ik- BM cultures generate significantly fewer DCs than their wild type (WT) counterparts. This phenotype is specific to Flt3L-driven DC developments as Ik- GM-CSF cultures generate normal numbers of DCs. Time course gene expression analyses of Flt3L-cultures show that genes important for DC development such as Irf4, Irf8 and Spib are variably under-expressed in both Ik- progenitors and mature DCs, with Spib being the most affected. These data demonstrate that low expression of DC genes is not solely due to decreased DC differentiation and suggest that Ikaros may regulate their expression, which we are now exploring. The DCs that develop in Ik- Flt3L cultures are also qualitatively different from their WT counterparts. They have an activated phenotype, implicating Ikaros in regulation of DC function as well. Enforced expression of full-length Ikaros (Ik1) in Ik- BM cannot restore Flt3L-driven DC development, suggesting a different Ikaros isoform may be needed for DC rescue. Studies now underway will define Ikaros isoform expression during DC development and determine if enforced expression of discrete Ikaros isoforms or the Flt3 receptor itself can restore Flt3L-driven DC development in Ik- BM. These studies will place Ikaros relative to Flt3 signals during DC development and define its molecular role.

Ikaros null mice demonstrate defects in dendritic cell development. (36.17)

Ikaros is a transcription factor that is expressed almost exclusively in cells of the hematopoietic lineage. Ikaros null mice lack several hematopoietic cell lineages including some dendritic cell (DC) subsets. Additionally, several Ikaros null bone marrow (BM) progenitor populations have a decreased expression of the receptor tyrosine kinase, Flt3. DCs can be generated in vitro by culture of BM with either GM-CSF or Flt3L (the ligand for Flt3). Our studies find that loss of Ikaros leads to a specific defect in the ability of BM precursors to generate DCs in response to culture with Flt3L but not in response to culture with GM-CSF. Those DCs that do develop from the culture of Ikaros null BM with Flt3L have an activated phenotype with higher expression of MHC class II and the costimulatory molecules CD40, CD80 and CD86 than their wild type counterparts. An examination of gene expression in Ikaros null BM progenitors cultured with Flt3L shows an inappropriately low expression of Irf4, Irf8 and Spib, genes encoding transcription factors known to be important for DC development. Studies are currently underway to determine whether enforced expression of Ikaros, Flt3 or one of the above-mentioned transcription factors can rescue DC development upon culture of Ikaros null BM with Flt3L. These studies will provide a potential molecular mechanism for the impaired development of DCs in Ikaros null mice.

Cutting Edge: Ikaros null thymocytes mature into the CD4 lineage with reduced TCR signal: A study using CD3{zeta} immunoreceptor tyrosine-based activation motif transgenic mice

Positive selection is a critical T cell developmental checkpoint that is driven by TCR signals. Enhanced positive selection toward the CD4 lineage occurs in the absence of Ikaros. One explanation for this phenotype is that Ikaros establishes the TCR signaling threshold that must be overcome for positive selection to occur. In the current study, this possibility is explored through the use of CD3zeta ITAM transgenic mice that express a CD3 zeta-chain with zero, one, or three ITAMs and an MHC class II (DO11.10)- or MHC class I (H-Y)-restricted TCR transgene. Using this system, we demonstrate that in the absence of Ikaros, thymocytes are able to mature into the CD4 lineage with reduced TCR signaling potential compared with that required to drive the maturation of wild-type thymocytes. We also demonstrate that maturation into the CD8 lineage is enhanced under conditions of reduced TCR signaling potential in the absence of Ikaros.