טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentShafir Alon Shani
SubjectCD19 Regulates Expression of Recombination Activating Genes
and AICD in Transformed B Cells
DepartmentDepartment of Medicine
Supervisor Professor Doron Melamed
Full Thesis textFull thesis text - English Version


Abstract

B cells are instructed continuously by B cell receptor (BCR) signals to make crucial cell fate decisions at several checkpoints during their development. The B cell specific surface protein, CD19, functions to balance these signals as a positive regulator, thus its appropriate expression is critical for development maturation and activation of B lineage cells. Thetyrosine phosphorylation of CD19 upon mIgM crosslinking, leads to the binding of PI3K to CD19 and sets one of the initiation points for the PI3K signaling cascade in B cells.

The PI3K activation can promote different fates including proliferation, survival, migration and differentiation, depending on the B cell differentiation stage.

In normal immature B cells, tonic signaling by the mature surface IgM activates PI3K to extinguish Ragexpression and promotes developmental progression. The suppression of Rag expression prevents aberrant V(D)J recombination which may result in genomic instability and mutations in proliferating B cells, that eventually leads to tumor formation. Studies have shown that in some B cell malignancies, apoptosis resistance is strongly related to constitutive PI3K activation.

The present study aims to determine the role of CD19/PI3K in regulating intracellular signals that affects B cell fate decisions in a ligand dependent (signals generated upon BCR ligation) and ligand independent manner (tonic signals). 

By using the shRNA system to CD19 in immature 38C-13 B cell lymphoma, we managed to increase their sensitivity to BCR induced apoptosis. The knock down of CD19 enhanced the degradation of c-Myc after BCR ligation leading to increase in apoptosis rates. The results we show implicate the enhanced turn-over of c-Myc in these cells with impaired PI3K signaling and reduced activation of Glycogen Synthase kinase 3 (GSK3). The same experimental system had been utilized tolink between CD19 expression and Rag's regulation through the PI3K pathway. Here we show that in the absence of the CD19 inhibitory signals Rag's transcription increases, with a direct correlation between CD19 expression level and Rag's transcripts. This transcription is mediated through the binding of Foxo1 transcription factor to the Rag enhancer regions.

In summary, CD19 regulates both the expression of recombination activating genes and AICD in transformed B cells, through the PI3K pathway thus suggesting that inhibition of this molecule will be useful for changing fate decisions of B cells and/or for human disease therapy.