How B cell receptors and Toll-like receptors collaborate in shaping B cell responses

• Main Author: Eckl-Dorna, J.
• Published: University College London (University of London) 2009
• Subjects: 616
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.625233

Antigen recognition by B cells results in their activation followed by specific antibody production. These events are initiated by antigen binding to their surface B cell receptors (BCR) which triggers both signalling and internalization of the receptor bound antigen to the endosome. However B cells also express features of the innate immune system such as Toll like receptors (TLRs), that can be located either on the surface of the cell or intracellularly where they recognize bacterial and viral nucleic acids. Engagement of these receptors within B cells is associated with enhancement of humoral responses. The aim of my PhD project was to investigate how endosomal TLR ligands in a particulate form could gain access to their intracellular receptors in the B cell and which impact the subsequent TLR engagement had on B cell fate. To achieve this, I directly linked both antigen and TLR9 ligand to particulates. Immunisation of mice with those particulates resulted in enhanced specific antibody titers compared to stimulation with particulate antigen alone. To dissect the underlying mechanism, I employed transgenic B cells bearing BCR specificity for the same antigen and stimulated them with particulate antigen-TLR9 ligand conjugates. Particulate TLR9 ligand could not gain access to its receptor within B cells via unspecific macropinocytosis and instead depended on BCRmediated internalization. Subsequent engagement of intracellular TLR9 by its ligand present in the conjugates resulted in B cell activation and proliferation, followed by differentiation into plasma cells and antigen specific antibody secretion. The uptake of the antigen-TLR9 ligand particulates both in vitro and in vivo depended on the affinity of the antigen once a defined threshold required for internalization was surpassed. The extent of plasma cell differentiation however could be modulated by the amount of TLR9 ligand present on the particulates. Thus I observed that direct linking of antigen and TLR ligand resulted in PC differentiation through antigen specific BCR mediated internalization and subsequent TLR engagement. This reveals a mechanism that may operate during the initiation of a primary immune response.