The Peptidyl-Prolyl isomerase, Pin1, associates with Protein Kinase C θ a critical Phospho-Thr-Pro motif in the V3 regulatory domain.

Protein kinase C-θ (PKCθ) is a member of the novel PKC subfamily known for its selective and predominant expression in T lymphocytes where it regulates essential functions required for T cell activation and proliferation. Our previous studies provided a mechanistic explanation for the recruitment of PKCθ to the center of the immunological synapse (IS) by demonstrating that a proline-rich (PR) motif within the V3 region in the regulatory domain of PKCθ is necessary and sufficient for PKCθ IS localization and function. Herein, we highlight the importance of Thr-Pro residue in the PR motif, the phosphorylation of which is key in the activation of PKCθ and its subsequent IS localization.

Termination of TCR-mediated activation signals is regulated by CrkII-dependent Cbl-mediated ubiquitination and degradation of C3G.

Crk adaptor proteins are key players in signal transduction from multiple cell surface receptors, including the T cell antigen receptor (TCR). The involvement of CrkII in the early stages of T cell activation is well documented, but little is known about its role during the termination of the activation response. We substantiated findings showing that CrkII utilizes its SH3N and SH2 domains to constitutively associate with C3G and transiently with Cbl in resting and TCR/CD3-stimulated T cells, respectively.

Cyclophilin A associates with and regulates the activity of ZAP70 in TCR/CD3-stimulated T cells.

The ZAP70 protein tyrosine kinase (PTK) couples stimulated T cell antigen receptors (TCRs) to their downstream signal transduction pathways and is sine qua non for T cell activation and differentiation. TCR engagement leads to activation-induced post-translational modifications of ZAP70, predominantly by kinases, which modulate its conformation, leading to activation of its catalytic domain. Here, we demonstrate that ZAP70 in TCR/CD3-activated mouse spleen and thymus cells, as well as human Jurkat T cells, is regulated by the peptidyl-prolyl cis-trans isomerase (PPIase), cyclophilin A (CypA) and that this regulation is abrogated by cyclosporin A (CsA), a CypA inhibitor.

Natural Killer Cell Recruitment and Activation Are Regulated by CD47 Expression in the Tumor Microenvironment.

Elevated CD47 expression in some cancers is associated with decreased survival and limited clearance by phagocytes expressing the CD47 counterreceptor SIRPα. In contrast, elevated CD47 mRNA expression in human melanomas was associated with improved survival. Gene-expression data were analyzed to determine a potential mechanism for this apparent protective function and suggested that high CD47 expression increases recruitment of natural killer (NK) cells into the tumor microenvironment.

CD47 Expression in Natural Killer Cells Regulates Homeostasis and Modulates Immune Response to Lymphocytic Choriomeningitis Virus.

CD47 is a ubiquitous cell surface receptor that directly regulates T cell immunity by interacting with its inhibitory ligand thrombospondin-1 and limits clearance of cells by phagocytes that express its counter-receptor signal-regulatory protein-α. Murine natural killer (NK) cells express higher levels of CD47 than other lymphocytes, but the role of CD47 in regulating NK cell homeostasis and immune function remains unclear. mice exhibited depletion of NK precursors in bone marrow, consistent with the antiphagocytic function of CD47.

TCR crosslinking promotes Crk adaptor protein binding to tyrosine-phosphorylated CD3ζ chain.

T cell antigen receptor (TCR) binding of a peptide antigen presented by antigen-presenting cells (APCs) in the context of surface MHC molecules initiates signaling events that regulate T cell activation, proliferation and differentiation. A key event in the activation process is the phosphorylation of the conserved tyrosine residues within the CD3 chain immunoreceptor tyrosine-based activation motifs (ITAMs), which operate as docking sites for SH2 domain-containing effector proteins. Phosphorylation of the CD3ζ ITAMs renders the CD3 chain capable of binding the ζ-chain associated protein 70 kDa (ZAP70), a protein tyrosine kinase that is essential for T cell activation.

Crk adaptor proteins regulate CD3ζ chain phosphorylation and TCR/CD3 down-modulation in activated T cells.

T cell receptor (TCR) recognition of a peptide antigen in the context of MHC molecules initiates positive and negative cascades that regulate T cell activation, proliferation and differentiation, and culminate in the acquisition of effector T cell functions. These processes are a prerequisite for the induction of specific T cell-mediated adaptive immune responses. A key event in the activation of TCR-coupled signaling pathways is the phosphorylation of tyrosine residues within the cytoplasmic tails of the CD3 subunits, predominantly CD3ζ.

In vivo regulation of human CrkII by cyclophilin A and FK506-binding protein.

Members of the Crk family of adaptor proteins are key players in signal transduction from a variety of cell surface receptors. CrkI and CrkII are two alternative-spliced forms of a single gene which possess an N-terminal SH2 domain and an SH3 domain that mediate interaction with other proteins. CrkII possesses an additional C-terminal linker region plus an extra SH3 domain, which does not interact with other proteins, but operates as regulatory moiety.

Insights into peptidyl-prolyl cis-trans isomerase structure and function in immunocytes.

Peptidyl-prolyl isomerase (PPIase) catalyzes the interconversion of a specific Pro-imide bond between the cis and trans conformations. Two families of PPIases, cyclophilins and FKBPs, have been extensively studied because of their high affinity for immunosuppressive drugs in particular cyclosporine A and FK506. Despite apparent differences, these protein families share conserved amino acid sequences in their catalytic domains and impose similar enzymatic functions to their substrates.

Immunophilins control T lymphocyte adhesion and migration by regulating CrkII binding to C3G.

Crk adaptor proteins are key players in signal transduction from a variety of cell surface receptors. CrkI and CrkII, the two alternative spliced forms of CRK, possess an N-terminal Src homology 2 domain, followed by a Src homology 3 (SH3) domain, whereas CrkII possesses in addition a C-terminal linker region plus a SH3 domain, which operate as regulatory moieties. In this study, we investigated the ability of immunophilins, which function as peptidyl-prolyl isomerases, to regulate Crk proteins in human T lymphocytes.

Development of unique antibodies directed against each of the six different phosphotyrosine residues within the T cell receptor CD3ζ chain.

Signal transduction from the T cell antigen receptor (TCR)/CD3 complex involves six different immunoreceptor tyrosine-based activation motifs (ITAM) located within the cytoplasmic tails of the CD3 chains. Each ITAM possesses two conserved tyrosine residues that can undergo phosphorylation upon TCR/CD3 crosslinking and become a docking site for SH2-containing effector molecules. Specificity of the SH2 domains is determined by their ability to bind a phosphorylated tyrosine in the context of a longer peptide motif within the target protein.