Differential impact of the CD45 juxtamembrane wedge on central and peripheral T cell receptor responses

Abstract

The cooperative activity of protein tyrosine kinases and phosphatases plays a central role in regulation of T cell receptor (TCR) signal strength. Perturbing this balance, and thus the threshold for TCR signals, has profound impacts on T cell development and function. We previously generated mice containing a point mutation in the juxtamembrane wedge of the receptor-like protein tyrosine phosphatase CD45. Demonstrating the critical negative regulatory function of the wedge, the CD45 E613R (WEDGE) mutation led to a lymphoproliferative disorder (LPD) and a lupus-like autoimmune syndrome. Using genetic, cellular, and biochemical approaches, we now demonstrate that the CD45 wedge influences T cell development and function. Consistent with increased TCR signal strength, WEDGE mice have augmented positive selection and enhanced sensitivity to the CD4-mediated disease experimental autoimmune encephalitis (EAE). These correspond with hyperresponsive calcium and pERK responses to TCR stimulation in thymocytes, but surprisingly, not in peripheral T cells, where these responses are actually depressed. Together, the data support a role for the CD45 wedge in regulation of T cell responses in vivo and suggest that its effects depend on cellular context. autoimmunity ͉ tyrosine phosphatase ͉ tyrosine kinase ͉ thymocyte development T cell receptor (TCR) signal strength is influenced by the integration of multiple inputs including affinity for antigen, presence and activity of costimulatory molecules, and duration of the interaction with antigen-presenting cells (APCs) (1). Alterations in TCR signal strength impact many aspects of T cell biology including CD4 versus CD8 lineage commitment (1), effector and memory cell generation (2), and autoimmunity (3, 4). Currently, the factors defining signal strength and its functional outcome are incompletely understood.CD45, a receptor-like protein tyrosine phosphatase (RPTP) expressed on all nucleated hematopoietic cells, plays a critical positive regulatory role in antigen receptor signaling. Its absence in both mice and humans results in severe combined immunodeficiency (5, 6). CD45 mediates its effects, at least in part, by modulating the activation state of Src family protein tyrosine kinases (SFKs) (5, 7). Phosphorylation of the SFK C-terminal tyrosine by Csk inhibits the kinase while autophosphorylation of the catalytic domain tyrosine results in full activity. CD45 opposes Csk by dephosphorylating the negative regulatory tyrosine, generating a pool of primed SFKs capable of rapid activation upon receptor stimulation. In some cell types, CD45 can also dephosphorylate the catalytic tyrosine and negatively regulate SFKs.Regulation of CD45 itself is complex. Alternative splicing of the extracellular domain, regulated in a cell-and activation-specific manner, generates multiple isoforms differing in size and charge (5). Interestingly, CD45 polymorphisms influencing its alternative splicing, and thus isoform expression, are associated with several human autoimmune diseases (8). We initi...

The receptor protein tyrosine phosphatase LAR promotes R7 photoreceptor axon targeting by a phosphatase-independent signaling mechanism

“…Consistent with an essential role for dimerization, a peptide homologous to the wedge specifically inhibits the biological functions of LAR in vertebrate cells (45). Interestingly, a wedge mutation in the mouse RPTP CD45 has opposite effects on T-cell response in central and peripheral T cells (46). This suggests that RPTPs other than LAR may also signal in both catalytic and non-catalytic modes that are affected differently by changes in the wedge domain.…”

TCR Signal Transduction: Opening the Black Box

“…This is a consequence of the negative regulatory site of the tyrosine phosphorylation of Src family kinases, including Lck and Fyn in T cells, being a proximal substrate of CD45 catalytic function. Recent work by previous lab members Ravi Majeti, Michelle Hermiston, and Zheng Xu has focused on the regulation of CD45 by dimerization, which seems to inhibit CD45 function (54,(57)(58)(59). Julie Zikherman, a postdoc fellow in the lab, presented work at this AAI meeting addressing why so much CD45 is expressed on the cell surface.…”

Unbiased Modifier Screen Reveals That Signal Strength Determines the Regulatory Role Murine TLR9 Plays in Autoantibody Production

“…Despite hyper-responsive BCR and TCR signaling responses, CD45E613R mice fail to break tolerance and develop ANA or any stigmata of SLE on an F10 B6 genetic background (8–11). However, this mutation can cooperate with other lupus-prone alleles or genetic backgrounds to cause a fulminant autoimmune phenotype on the B6 genetic background, clearly demonstrating a role for genetic modifiers in this model (7, 9, 10, 12).…”

“…αβ T cells are also implicated in SLE pathogenesis (1) and dysregulated in CD45E613R mice (11). To examine whether CD45E613R αβ T cells were required for autoantibody production, we generated chimeras using congenically marked CD45WT and CD45E613R TCRα +/+ or TCRα −/− marrow at a 2:3 ratio to achieve approximately 50% CD45E613R CD19 + B cells in the peripheral blood 8 weeks post transfer (Fig.…”

“…Mirroring the variable presentation of human SLE, the phenotype of CD45E613R mice is extremely sensitive to genetic context. Despite hyper-responsive antigen receptor signaling, CD45E613R mice fully backcrossed to B6 or 129/Sv genetic backgrounds fail to develop autoantibodies or end organ damage (8–11). However, true B6×129/Sv CD45E613R F1 mice recapitulate the original lupus phenotype with 100% penetrance (12).…”