University of Helsinki
Supervisor: Mauno Vihinen
Crosslinking of B cell antigen receptor (BCR) results in the activation of several cellular processes, and further to differentiation of B leukocyte. B cell differentiation has been divided into early, antigen-independent, and late, antigen-dependent, steps. The former stages include pro-B, pre-B and mIg+ B cell stages and the latter the subsequent stages leading to the plasma cell. All these stages including successive rearrangements of Ig genes in B cells arise from organized activation and repression of genes.
BCR is multimeric antigen receptor complex. BCR comprises of the surface immunoglobulin (sIg) and a heterodimer of Ig-alpha (CD79a) and Ig-beta (CD79b). The receptor has in its cytoplasmic region motifs designated ITAMs (immunoreceptor tyrosine-based activation motifs), that are phosphorylated in tyrosine residues when stimulated. Phosphorylation of the ITAMs triggers multitude of effects where numerous tyrosine kinases including Src family members Blk, Fyn, Hck, Lck and Lyn, Tec family members Bmx, Btk, Itk and Tec as well as Syk are activated. These enzymes further activate cascades that lead into proliferation and differentiation of cells.
In the cDNA microarray technology, genome expression of hundreds of genes can be assayed simultaneously, which facilitates monitoring differential expression of a large number of activated or suppressed genes. The relative intensities of the obtained expression patterns can be used to estimate the extent of alteration in expression levels.
To investigate the multiple mechanisms governing differentiation of lymphocytes we have analysed the expression of more than 500 tightly transcriptionally controlled genes in stimulated and nonstimulated B and T cells by using Ramos and Jurkat cell lines. Since numerous factors are recruited upon stimulation, simultaneous assay of the affected genes elucidates multiple induction related mechanisms.
During the B cell development different transcription factors are activated depending on the cell stage. This leads to expression of genes which are specific and essential for different B cell stages. Anti-IgM stimulated B cells first activate the expression of early response genes which are mostly transcription factors and function as 'third messengers' in signal transduction. This happens in first couple of hours after which this gene products activate genes needed for proliferation, differentiation or apoptosis of cells. This depends of the signal cells get and different cell lines. In the future we will focus on these genes. We will study gene expression in (differently) stimulated B cells using different time scales and in B cell which represent different cell stages.
Vihinen, M. and Smith, C.I.E. (1996). Structural aspects of signal transduction in B-cells.
Crit. Rev. Immunol. 16:251-75
Chalifour, L.E., Fahmy, R., Holder, E.L., Hutchinson, E.W., Osterland, C.K., Schipper, H.M. and Wang, E. (1994). A method for analysis of gene expression patterns. Anal. Biochem. 216:299-304.
Ollila, J. and Vihinen, M. (1998). Stimulation of B and T cells activates expression of transcription and differentiation factors. Biochem. Biophys. Res. Commun. 249:475-80.