Cyclophilin A Regulates TCR Signal Strength in CD4+ T Cells via a Proline-Directed Conformational Switch in Itk

Abstract

Cyclophilin A (CypA/Ppia) is a peptidyl-prolyl isomerase (PPIase) that binds the immunosuppressive drug cyclosporine. The resulting complex blocks T cell function by inhibiting the calcium-dependent phosphatase calcineurin. To identify the native function of CypA, long suspected of regulating signal transduction, we generated mice lacking the Ppia gene. These animals develop allergic disease, with elevated IgE and tissue infiltration by mast cells and eosinophils, that is driven by CD4+ T helper type II (Th2) cytokines. Ppia(-/-) Th2 cells were hypersensitive to TCR stimulation, a phenotype consistent with increased activity of Itk, a Tec family tyrosine kinase crucial for Th2 responses. CypA bound Itk via the PPIase active site. Mutation of a conformationally heterogeneous proline in the SH2 domain of Itk disrupted interaction with CypA and specifically increased Th2 cytokine production from wild-type CD4+ T cells. Thus, CypA inhibits CD4+ T cell signal transduction in the absence of cyclosporine via a regulatory proline residue in Itk.

Tec Kinase Itk Forms Membrane Clusters Specifically in the Vicinity of Recruiting Receptors

“…Thus, membrane recruitment is a critical component of their activation, and Itk can be found at the plasma membrane of cells, although other events are required for its full activity (17)(18)(19). It is, however, not clear whether this is general membrane localization of the protein in anticipation of activation or a specific localization in certain regions of the plasma membrane.The structure of Itk in cells is not known, although it has been suggested to form dimers in its inactive state, with a resultant monomerization upon activation (20,21). We have examined these issues using a split YFP system (22) and find that Itk forms dimers or higher order clusters; however, it does so only at the plasma membrane and only in the vicinity of a receptor that can recruit PI3 kinase.…”

“…3a). In addition to potential interactions with the PRR of the TH domain, the SH3 domain of Itk has been suggested to interact with a proline residue in the SH2 domain of Itk in an intermolecular fashion (20,21). The SH2 domain can interact with other proteins upon their tyrosine phosphorylation.…”

“…The structure of Itk in cells is not known, although it has been suggested to form dimers in its inactive state, with a resultant monomerization upon activation (20,21). We have examined these issues using a split YFP system (22) and find that Itk forms dimers or higher order clusters; however, it does so only at the plasma membrane and only in the vicinity of a receptor that can recruit PI3 kinase.…”

127 Proteomic analysis of filaggrin deficiency identifies molecular signatures characteristic of atopic eczema

“…38 PPIA knockout mice experience spontaneous T H 2-polarized allergic disease and an immune profile akin to AE, although no dermatitis is reported. 39 No concurrent reduction in PPIA mRNA expression was observed in shFLG LSEs (Fig 5, C); the reduction in intracellular protein expression might instead reflect an increase in PPIA secretion known to be triggered after inflammatory/stress stimuli in epithelia. 35 Extracellular PPIA functions as both a proinflammatory stimuli (autocrine/paracrine) and potent chemoattractant of CD147 1 immune cells.…”

Cyclophilin function in Cancer; lessons from virus replication

“…It is intriguing that normal cells harbour relatively high cyclophilin levels [5], yet, knockout studies in mice indicate that CypA is not essential for mammalian cell viability [176]. In contrast, depletion of CypA causes cytokinesis defects and reduces the proliferation of tumour cells, supporting the view that CypA provides a growth advantage to tumour cells.…”