Deciphering mechanisms of transcriptional activation and repression in B lymphocytes

Abstract

During the course of evolution, our immune system has developed, to defend us against infections, a diverse number of specialized cell types. Immunoglobulin producing B lymphocytes are produced by the differentiation of hematopoietic stem cells (HSC) into gradually more lineage-restricted progenitors. This process is undertaken in an environment of secreted cytokines, chemokines and cell surface expressed ligands. In addition, the hematopoietic cells themselves independently express a large array of lineage specific transcription factors. Together, both independently and in combination, these different modulators allow for B cell production and ultimately terminal differentiation to either memory or plasma cells.

Transcription factors not only activate or repress genes, but act as integrators between signalling pathways and the physiological template of DNA, chromatin. However, a puzzling aspect of genetic control is how a limited set of DNA binding proteins can regulate a much larger number of expressed genes. One solution is the selective recruitment of cofactors which are either conducive or suppressive to transcriptional activation.

This thesis is largely concerned with the role of cofactors and their utilization by B lymphocyte specific transcription factors. Pax5, Pu.1, NF-kappaB and Oct proteins perform divergent and often vital functions during the life and death of B lymphocytes. Not only this, they also seemingly discriminate between transcriptional targets that must be either up or down regulated. Oct1 and Oct2, by altering their conformation upon binding to different cis sequences can differentially associate with the transcriptional co-repressor Grg4. A second method of complex formation with Grg4 is witnessed in the 3' enhancer of the immunoglobulin heavy chain. This is repressed by Pax5, Pu.1 and NF-kappaB as they form an enhance some surface which attracts the binding of Grg4. Seemingly, this repression is released by the extinction of Grg4 during the terminal differentiation of B cells, thus facilitating activation.

This implies that expression of Pax5 in later stages of B cell development would not be sufficient for repression of its genetic targets. The creation of an adenoviral expression system in B lymphocytes demonstrated normal plasma cell development despite continued Pax5 expression. From this evidence it would seem that a combination of conformation, cooperative and temporal expression patterns governs certain aspects of transcriptional control in B lymphocytes.