The large ectodomains of CD45 and CD148 regulate their segregation from and inhibition of ligated T-cell receptor

Abstract

• The large extracellular domains of the tyrosine phosphatases CD45 and CD148 prevent them from inhibiting T-cell receptor triggering.• These domains are required for optimal segregation from the engaged T-cell receptor, supporting the kineticsegregation model of triggering.T-cell receptor (TCR) triggering results in a cascade of intracellular tyrosine phosphorylation events that ultimately leads to T-cell activation. It is dependent on changes in the relative activities of membrane-associated tyrosine kinases and phosphatases near the engaged TCR. CD45 and CD148 are transmembrane tyrosine phosphatases with large ectodomains that have activatory and inhibitory effects on TCR triggering. This study investigates whether and how the ectodomains of CD45 and CD148 modulate their inhibitory effect on TCR signaling. Expression in T cells of forms of these phosphatases with truncated ectodomains inhibited TCR triggering. In contrast, when these phosphatases were expressed with large ectodomains, they had no inhibitory effect. Imaging studies revealed that truncation of the ectodomains enhanced colocalization of these phosphatases with ligated TCR at the immunological synapse. Our results suggest that the large ectodomains of CD45 and CD148 modulate their inhibitory effect by enabling their passive, size-based segregation from ligated TCR, supporting the kinetic-segregation model of TCR triggering. (Blood. 2013;121(21):4295-4302) Introduction T cells are stimulated through the T-cell receptor (TCR) when it binds cognate peptide presented by a major histocompatibility complex molecule (pMHC) on another cell. As a consequence of ligation, immunoreceptor tyrosine-based activation motifs (ITAMs) in the cytoplasmic domains of the TCR/CD3 complex are phosphorylated by lymphocyte-specific protein tyrosine kinase (Lck). These phosphorylated ITAMs recruit z-chain-associated protein tyrosine kinase 70 to the membrane, and Zap-70 phosphorylates substrates such as linker of activated T cells (LAT). 1Despite the extensive research in this field, the mechanism by which the binding of TCR to pMHC leads to phosphorylation of TCR/CD3 ITAMs is still contested, and several models have been proposed. 2One common feature of some of these models is that TCR triggering is initiated by changes in the relative concentrations of membrane tyrosine kinases and phosphatases in the vicinity of ligated TCR. 2The principal membrane tyrosine phosphatases involved in regulating TCR-induced tyrosine phosphorylation are CD45 and CD148. 3 The importance of this dynamic equilibrium between kinase and phosphatase activity in TCR triggering was highlighted in studies that use phosphatase inhibitors such as pervanadate. [4][5][6] Treatment of T cells with these inhibitors alone, in the absence of any TCR ligand, was sufficient to induce full activation of TCR signaling pathways, ranging from early events such as phosphorylation of TCR ITAMs, Zap-70, and LAT to late events such as interleukin 2 (IL-2) production. [4][5][6] Several mechanisms have been propos...

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