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B cells can be activated by T cell-dependent stimuli, such as CD40 ligation and cytokines, which induce extensive proliferation, class switch recombination and somatic hypermutation. Epstein-Barr virus (EBV) can also induce B cell activation by mimicking T cell help through its main oncoprotein, latent membrane protein 1 (LMP-1). It is regulated by another EBV-encoded protein, EBV nuclear antigen 2 (EBNA-2), which is absent in Hodgkin and Burkitt lymphomas. We have studied LMP-1 induction by cytokines in vitro and shown that LMP-1 is induced through the transcription factor signal transducer and activator of transcription (STAT6) and a newly defined high-affinity STAT6-binding site. When IL-4 is added together with lipopolysaccharide (LPS) or α-CD40 to B cells, it induces homotypic round and tight aggregates in vitro, whereas LPS alone does not induce such morphological changes. I describe here attempts to identify the molecules that regulate these responses. I have shown that the Rho GTPase Cdc42 controls the spreading of B cells, whereas two other molecules in the same family, Rac1 and Rac2, control homotypic adhesion. Further, I have shown by conditional deletion of Cdc42 in B cells that it is important in the humoral immune response.  Dock10 is a guanosine nucleotide exchange factor (GEF) for Cdc42. It is expressed through all differentiation stages of B cell development. However, targeted deletion of Dock10 in B cells does not result in an aberrant phenotype. Furthermore, by studying conditional knockout mice for Dock10, Cdc42, Rac1 and Rac2, I have elucidated the mechanism of cytoskeletal changes during B cell activation, leading to adhesion and motility. My results may lead to a better understanding of normal B cell activation and of EBV infection, which is associated with many human tumours and may help to understand cancer development and progression in B cells. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript.</p>