P59(fyn) is upregulated in anergic CD8+ T cells

Role of the Fyn -93A>G polymorphism (rs706895) in acute rejection after liver transplantation

The tyrosine kinase Fyn phosphorylates tyrosine residues on key targets involved in early T-cell signal transduction. T-cell signal transduction is one essential step for acute transplant rejection. The aim of this study was to evaluate the association of Fyn -93A>G single nucleotide polymorphism (SNP) (rs706895) with the susceptibility to acute rejection episodes in liver transplantation. In total, 72 liver transplant recipients with one biopsy proven acute rejection (S-BPAR), 56 with multiple BPAR (M-BPAR), 105 without BPAR (No-BPAR), and 145 healthy controls were enrolled in this case-control study. The SNP was genotyped by polymerase chain reaction-allele specific restriction enzyme analysis (PCR-ASRA) and was analyzed for a recessive and a dominant model. The Fyn -93G allele exhibits in healthy controls a statistically significant lower frequency than in liver recipients (18% vs. 24%; p=0.046) or in liver recipients with BPAR (18% vs. 27%; p=0.017). However, the genotype and allele frequencies of the Fyn -93A>G SNP demonstrate no significant differences between recipients with acute rejection episodes (S-BPAR and M-BPAR) and No-BPAR recipients. Thus our results provide no evidence that the Fyn -93A>G SNP contributes to the susceptibility to acute liver transplant rejection in a Caucasian population.

Changes in functional but not structural avidity during differentiation of CD8+ effector cells in vivo after virus infection

By the peak of the CD8(+) T cell response, the effector cell pool consists of a heterogeneous population of cells that includes both those with an increased propensity to become long-lived memory cells (memory precursor effector cells; MPEC) and those that are terminally differentiated cells (short-lived effector cells; SLEC). Numerous studies have established the critical role that functional avidity plays in determining the in vivo efficacy of CD8(+) effector cells. Currently, how functional avidity differs in MPEC versus SLEC and the evolution of this property within these two populations during the expansion and contraction of the response are unknown. The data presented in this study show that at the peak of the effector response generated after poxvirus infection, SLEC were of higher functional avidity than their MPEC counterpart. Over time, however, SLEC exhibited a decrease in peptide sensitivity. This is in contrast to MPEC, which showed a modest increase in peptide sensitivity as the response reached equilibrium. The decrease in functional avidity in SLEC was independent of CD8 modulation or the amount of Ag receptor expressed by the T cell. Instead, the loss in sensitivity was correlated with decreased expression and activation of ZAP70 and Lck, critical components of TCR membrane proximal signaling. These results highlight the potential contribution of avidity in the differentiation and evolution of the T cell effector response after viral infection.

The limited contribution of Fyn and Gab2 to the high affinity IgE receptor signaling in mast cells

Several studies with mast cells from knock-out mice have suggested that the tyrosine kinase Fyn and its downstream substrate Gab2 may play a role in high affinity IgE receptor (FcepsilonRI)-mediated mast cell activation. To better understand the role of these two molecules and of Syk, we transiently transfected mast cells with small interference RNA (siRNA) targeted to Fyn, Gab2, or Syk to specifically decrease their expression. The siRNA suppression of Gab2 but not Fyn reduced activation of the phosphoinositide-3-kinase (PI3K) pathway as demonstrated by the change in phosphorylation of Akt; this indicates that Gab2 but not Fyn regulates this pathway. The decreased expression of Gab2 and Fyn had minor effects on degranulation. There were also some minor changes in activation of the NFAT or NFkappaB transcription factors in cells with reduced expression of Fyn or Gab2. Decreased Gab2 but not Fyn reduced the FcepsilonRI-induced activation of the Erk, Jnk, and p38 MAP kinases and the release of TNF-alpha. In contrast, decreased expression of Syk dramatically reduced FcepsilonRI-induced degranulation, activation of NFAT and NFkappaB. Therefore, the reduction in expression of these proteins in mast cells indicates that Syk is the major regulator of FcepsilonRI-mediated reactions, whereas Fyn has minor if any effects and Gab2 regulates primarily late events including MAP kinase activation and release of cytokines.

T-cell receptor proximal signaling via the Src-family kinases, Lck and Fyn, influences T-cell activation, differentiation, and tolerance

T-cell development in the thymus and activation of mature T cells in secondary lymphoid organs requires the ability of cells to respond appropriately to environmental signals at multiple stages of their development. The process of thymocyte selection insures a functional T-cell repertoire, while activation of naive peripheral T cells induces proliferation, gain of effector function, and, ultimately, long-lived T-cell memory. The T-cell immune response is initiated upon engagement of the T-cell receptor (TCR) and coreceptor, CD4 or CD8, by cognate antigen/major histocompatibility complexes presented by antigen-presenting cells. TCR/coreceptor engagement induces the activation of biochemical signaling pathways that, in combination with signals from costimulator molecules and cytokine receptors, direct the outcome of the response. Activation of the src-family kinases p56(lck) (Lck) and p59(fyn) (Fyn) is central to the initiation of TCR signaling pathways. This review focuses on our current understanding of the mechanisms by which these two proteins orchestrate T-cell function.

A distinct inflammatory gene expression profile in patients with psoriatic arthritis

Psoriatic arthritis (PsA) is a systemic inflammatory condition featuring polyarthritis associated with psoriasis. Apart from clinical indicators, few biomarkers exist to aid in the diagnosis and management of PsA. We hypothesized that whole blood gene expression profiling would provide new diagnostic markers and/or insights into pathogenesis of the disease. We compared whole blood gene expression profiles in PsA patients and in age-matched controls. We identified 310 differentially expressed genes, the majority of which are upregulated in PsA patients. The PsA expression profile does not significantly overlap with profiles derived from patients with rheumatoid arthritis or systemic lupus erythematosus. Logistic regression identified two lymphocyte-specific genes (zinc-finger protein 395 and phosphoinositide-3-kinase 2B) that discriminate PsA patients from normal controls. In addition, a highly coregulated cluster of overexpressed genes implicated in protein kinase A regulation strongly correlates with erythrocyte sedimentation rate. Other clusters of coregulated, yet suppressed genes in PsA patient blood include molecules involved in T-cell signaling. Finally, differentially expressed genes in PsA fall into diverse functional categories, but many downregulated genes belong to a CD40 signaling pathway. Together, the data suggest that gene expression profiles of PsA patient blood contain candidate novel disease markers and clues to pathogenesis.

Protein Tyrosine Phosphatase α Regulates Fyn Activity and Cbp/PAG Phosphorylation in Thymocyte Lipid Rafts

A role for the receptor protein tyrosine phosphatase alpha (PTPalpha) in immune cell function and regulation of Src family kinases was investigated using thymocytes from PTPalpha-deficient mice. PTPalpha-null thymocytes develop normally, but unstimulated PTPalpha-/- cells exhibit increased tyrosine phosphorylation of specific proteins, increased Fyn activity, and hyperphosphorylation of Cbp/PAG that promotes its association with C-terminal Src kinase. Elevated Fyn activity in the absence of PTPalpha is due to enhanced phosphorylation of Fyn tyrosines 528 and 417. Some PTPalpha is localized in lipid rafts of thymocytes, and raft-associated Fyn is specifically activated in PTPalpha-/- cells. PTPalpha is not a Cbp/PAG phosphatase, because it is not required for Cbp/PAG dephosphorylation in unstimulated or anti-CD3-stimulated thymocytes. Together, our results indicate that PTPalpha, likely located in lipid rafts, regulates the activity of raft Fyn. In the absence of PTPalpha this population of Fyn is activated and phosphorylates Cbp/PAG to enhance association with C-terminal Src kinase. Although TCR-mediated tyrosine phosphorylation was apparently unaffected by the absence of PTPalpha, the long-term proliferative response of PTPalpha-/- thymocytes was reduced. These findings indicate that PTPalpha is a component of the complex Src family tyrosine kinase regulatory network in thymocytes and is required to suppress Fyn activity in unstimulated cells in a manner that is not compensated for by the major T cell PTP and SFK regulator, CD45.

Apoptosis of antigen-specific T lymphocytes upon the engagement of CD8 by soluble HLA class I molecules is Fas ligand/Fas mediated: evidence for the involvement of p56lck, calcium calmodulin kinase II, and Calcium-independent protein kinase C signaling pathways and for NF-kappaB and NF-AT nuclear translocation

The binding of soluble HLA class I (sHLA-I) molecules to CD8 on EBV-specific CTL induced up-regulation of Fas ligand (FasL) mRNA and consequent sFasL protein secretion. This, in turn, triggered CTL apoptosis by FasL/Fas interaction. Molecular analysis of the biochemical pathways responsible for FasL up-regulation showed that sHLA-I/CD8 interaction firstly induced the recruitment of src-like p56(lck) and syk-like Zap-70 protein tyrosine kinases (PTK). Interestingly, p59(fyn) was activated upon the engagement of CD3/TCR complex but not upon the interaction of sHLA-I with CD8. In addition, sHLA-I/CD8 interaction, which is different from signaling through the CD3/TCR complex, did not induce nuclear translocation of AP-1 protein complex. These findings suggest that CD8- and CD3/TCR-mediated activating stimuli can recruit different PTK and transcription factors. Indeed, the engagement of CD8 by sHLA-I led to the activation of Ca2+ calmodulin kinase II pathway, which eventually was responsible for the NF-AT nuclear translocation. In addition, we found that the ligation of sHLA-I to CD8 recruited protein kinase C, leading to NF-kappaB activation. Both NF-AT and NF-kappaB were responsible for the induction of FasL mRNA and consequent CTL apoptosis. Moreover, FasL up-regulation and CTL apoptotic death were down-regulated by pharmacological specific inhibitors of Ca2+/calmodulin/calcineurin and Ca2+-independent protein kinase C signaling pathways. These findings clarify the intracellular signaling pathways triggering FasL up-regulation and apoptosis in CTL upon sHLA-I/CD8 ligation and suggest that sHLA-I molecules can be proposed as therapeutic tools to modulate immune responses.