The phosphoinositide 3-kinases (PI3Ks) are a family of intracellular signaling enzymes that are essential for many functions of B lymphocytes, including cell migration and cell:cell interactions. When activated by receptors such as chemokine receptors, PI3Ks generate several distinct phosphoinositide (PI) products which can bind intracellular proteins that regulate various cellular functions. The PI3K pathway has been found to have essential roles in pathogenesis of B cell malignancies such as chronic lymphocytic leukemia (CLL). As described in this thesis, we have carried out studies on various molecular components of the PI3K signalling pathways to determine their functions in the context of malignant B cell biology, including different PI3K enzyme isoforms, a PI phosphatase and PI binding proteins. In Chapter 2, I investigated the role of the PI3K product PI(3,4)P2 in malignant B cell migration, using the PI(3,4)P2-specific lipid phosphatases INNP4A/B. We demonstrated that the decreased PI(3,4)P2 level caused by overexpression of INPP4A impaired both the speed and directionality of malignant B cell migration. Two PI(3,4)P2 binding proteins TAPP2 and lamellipodin (Lpd) were also investigated and knockdown of TAPP2 or Lpd led to impaired cell migration. The intracellular localization of these proteins in migrating B cells, as well as their co-localization with other molecules, implicated PI(3,4)P2 and its binding proteins in orchestrating cytoskeletal reorganization required for effective cell migration. In Chapter 3, I investigated the distinct roles of the PI3K γ and δ isoforms in malignant B cell migration. Although the PI3Kδ-specific inhibitor Idelalisib impairs CLL cell migration and shows efficacy in treatment of CLL and other B cell malignancies, the function of PI3Kγ, which functions downstream of chemokine receptors has not been extensively studied in B cells. We observed that pharmaceutical inhibition of PI3Kγ significantly impaired CLL cell migration, whereas the dual PI3Kδ/γ inhibitor duvelisib had a greater impact than single isoform-selective inhibitors. Microscopic tracking of cell migration within chemokine gradients revealed that PI3Kγ functions in gradient sensing and impacts cell morphology and F-actin polarization. This study provided the first evidence for a role of PI3Kγ in malignant B cell migration. In Chapter 4, I investigated the cell-cell interaction mechanisms involved in CLL pathogenesis. I found that blocking the PI3K pathway using γ or δ isoform-specific inhibitors can disrupt the stable adhesion between CLL cells and stromal cells and that PI3Kδ inhibitors can act in part via direct effects on stromal cells. I further investigated the potential role of follicular helper T cells (TFH) in CLL. I found evidence that the TFH type 1 subset is expanded in CLL patients in association with disease burden, and that increased TFH1 cells are found in the blood as well as in lymph nodes and bone marrow. Recently-approved breakthrough therapies for CLL include inhibitors of Bruton’s tyrosine kinase (Btk), which is a phosphoinositide-binding protein regulated by the PI3K pathway. I found that the Btk inhibitor Ibrutinib could decrease the activation and expansion of the TFH population in vivo and impair T cell migration in vitro. These studies provided new insights into the cell-cell interactions mechanisms supporting CLL growth within the lymphoid tissue microenvironment. Together the work reported in this thesis significantly advance our understanding of how different components of the PI3K pathway contribute to B cell migration and cell-cell interactions. We have confirmed the importance of PI3Kδ in these processes and provided the first evidence that PI3Kγ, PI(3,4)P2 and its binding proteins TAPP2 and Lpd have an important role in the context of B cell malignancy. Finally, I have identified type I TFH cells as cell type abnormally expanded in CLL and found preliminary evidence that PI3K pathway inhibition may target these cells.