The Mechanism Of Ly108-SAP Regulating T Cell – Antigen Presenting Cell Interactions

The germinal center is a specialized structure of the B cell follicle in secondary lymphoid organs. It comprises germinal center B cells and some specialized stromal cells. It’s the place of BCR hypermutation and isotype class switch so as to produce high affinity antibodies. The germinal center response requires cooperation between Ag-specific T and B lymphocytes, which takes the form of long-lasting cell–cell conjugation in vivo. Signaling lymphocytic activation molecule(SLAM)–associated protein(SAP) is coded by SH2D1 A gene and is an intracellular adaptor protein. It only consists of an SH2 domain and a C-terminal tail of about 20 amino acids. It is highly conservative in mammals and binds to the intracellular region of the SLAM family members to transducer signal. It is required for stable cognate T–B cell conjugation. SLAM family members are a series of transmembrane(TM) receptors, which are highly expressed on T, B and dendritic cells. They homotypic interact with each other, and the family member Ly108 may negatively regulate T-B interaction. Through cell conjugate assay, bone marrow transplantation, immunoprecipitation, intravital two photon imaging, structural illumination microscopy and fluorescence resonance energy transfer imaging, etc., we show that, other than phosphotyrosine-binding, SAP does not harbor motifs that recruit additional signaling intermediates to stabilize T–B adhesion. Ly108 dampens T cell adhesion to not only Ag-presenting B cells, but also dendritic cells by inhibiting CD3? phosphorylation through two levels of regulated Ly108–CD3? interactions. Constitutively associated with Src homology 2 domain–containing tyrosine phosphatase-1 even in SAP-competent cells, Ly108 is codistributed with the CD3 complex within a length scale of 100–200 nm on quiescent cells and can reduce CD3? phosphorylation in the absence of overt TCR stimulation or Ly108 ligation. When Ly108 is engaged in trans during cell–cell interactions, Ly108–CD3? interactions are promoted in a manner that uniquely depends on Ly108 TM domain, leading to more efficient CD3? dephosphorylation. Whereas replacement of the Ly108 TM domain still allows the constitutive, colocalization-dependent inhibition of CD3? phosphorylation, it abrogates the ligation-dependent Ly108–CD3? interactions and CD3? dephosphorylation, and it abolishes the suppression on Ag-triggered T–B adhesion. These results offer new insights into how SAP and Ly108 antagonistically modulate the strength of proximal TCR signaling and thereby control cognate T cell–APC interactions.