Transcriptional mechanisms directing terminal differentiation of B lineage cells

Abstract

The interaction between the B cell receptor (surface immunoglobulin) and its ligand (antigen) can result in extensive cell growth and induction of differentiation. The terminally differentiated B cell fulfills the ultimate purpose of B cell existence: to secret antibodies in order to protect the organism against invading pathogens.

A feature inherent to the immune system is the ability to discriminate between structures that are perceived as foreign and structures that are not. Since the B cell does not possess the necessary attributes to evaluate the pathogenic status of the substrate, it is dependent on additional external signals, often provided by helper T cells, to ensure responses that are of benefit to the organism. Appropriately, these supplementary signals are subsequently interpreted at the genetic level by transcription factors to either implement, or prohibit, the preset program of cellular differentiation.

Through genetic gain, and loss of function studies in rodents, transcription factors critical for the development of B lineage cells have been identified. Some of these factors, Pax5, PU. 1, NF-kappaB and Oct-2, are instrumental in the work presented in this thesis.

The immunoglobulin heavy chain 3 enhancer (HS1,2) that is regulated by these factors, was appended to a heterologous reporter gene and used as a model to investigate transcriptional suppression and activation. The first part of this thesis shows that NF-kappaB together with additional activators is an important regulator of HS1,2 function. In subsequent studies the transcriptional corepressor Groucho related gene 4 (Grg4) was found to play a regulatory role in modulating the function of Pax5, PU.1 and Oct-2. Further characterization and identification of likely molecular prerequisites led to the postulation of a co-recruitment model for Grg4-mediated repression.

My observations suggested a possible explanation as to how a limited set of transcription factors can serve as interpreters of signals that not only initiate terminal B cell differentiation but can also inhibit its developmental program.