The expression of many putative antiviral genes is upregulated when cells encounter type I interferon (IFN), but the actual mechanisms by which many IFN-induced gene products inhibit virus replication are poorly understood. A recently identified IFN-induced antiretroviral protein, termed tetherin (previously known as BST-2 or CD317), blocks the release of nascent human immunodeficiency virus type 1 (HIV-1) particles from infected cells, and an HIV-1 accessory protein, Vpu, acts as a viral antagonist of tetherin. Here, we show that tetherin is capable of blocking not only the release of HIV-1 particles but also the release of particles assembled using the major structural proteins of a variety of prototype retroviruses, including members of the alpharetrovirus, betaretrovirus, deltaretrovirus, lentivirus, and spumaretrovirus families. Moreover, we show that the release of particles assembled using filovirus matrix proteins from Marburg virus and Ebola virus is also sensitive to inhibition by tetherin. These findings indicate that tetherin is a broadly specific inhibitor of enveloped particle release, and therefore, inhibition is unlikely to require specific interactions with viral proteins. Nonetheless, tetherin colocalized with nascent virus-like particles generated by several retroviral and filoviral structural proteins, indicating that it is present at, or recruited to, sites of particle assembly. Overall, tetherin is potentially active against many enveloped viruses and likely to be an important component of the antiviral innate immune defense.

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.

The signaling cascade initiated by IL-4 is classically divisible into two major pathways: one mediated by STAT6, and the other by insulin receptor substrates-1 and -2 via activation of PI3K. In murine splenic B cells, the suppressor of cytokine signaling (SOCS)3 is inducible by IL-4 via a mechanism independent of STAT6 and PI3K. SOCS3 expression increases 9-fold within 5 h of IL-4 treatment. This induction occurs normally in B cells deficient in STAT6 and is unaffected by pretreatment with the PI3K inhibitor wortmannin, or with the ERK pathway inhibitor, PD98059. However, the IL-4 induction of SOCS3 is blocked by inhibitors of either the JNK or p38 MAPK pathways (SP600125 and SB203580, respectively). Direct examination of these pathways reveals rapid, IL-4-directed activation of p38 MAPK, uncovering a previously unappreciated pathway mediating IL-4 signal transduction.

Effector CD4+ T cells rapidly activate high-level cytokine expression following TCR stimulation. Consistent with accelerated protein production in these cells, global mRNA profiles revealed that, after cytokines, the most impressive cluster of activated genes encode rRNA-maturation factors. Activation of these genes was ERK-MAPK dependent, accompanied by increased rRNA transcription and faster maturation kinetics, and much greater in effector CD4+ T cells than in naive cells. Ribosomal protein subunit (RPS) synthesis was also ERK-MAPK dependent and increased to match rRNA production, but without evident increase in RPS mRNA. Instead, stimulation promoted polysome loading of RPS mRNA via cis-acting, 5'-terminal oligopyrimidines. These results demonstrate how, in response to extracellular signals, effector CD4+ T cells coordinately increase multiple ribosomal components to accommodate burgeoning cytokine production.