SAP-controlled T-B cell interactions underlie germinal centre formation

Generation of long-term antibody-mediated immunity depends on the germinal centre (GC) reaction, which requires cooperation between antigen-specific T and B lymphocytes. In the human X-linked lymphoproliferative disease and its gene-targeted mouse model, loss-of-function mutations in signalling lymphocyte activation molecule-associated protein (SAP, encoded by SH2D1a) cause a profound defect in GC formation by an as yet unknown mechanism. Using two-photon intravital imaging, here we show that SAP deficiency selectively impairs the ability of CD4(+) T cells to stably interact with cognate B cells but not antigen-presenting dendritic cells. This selective defect results in a failure of antigen-specific B cells to receive adequate levels of contact-dependent T cell help to expand normally, despite sap(−/−) T cells exhibiting the known characteristics of otherwise competent helper T cells. Furthermore, lack of stable interactions with B cells renders sap(−/−) T cells unable to be efficiently recruited to and retained in a nascent GC to sustain the GC reaction. These results offer a compelling explanation for the GC defect due to SAP deficiency and provide novel insights into the bi-directional communication between cognate T and B cells in vivo.