Thymocytes Tempted by TEC Wnt Ligands See article p.5261

Hunting HSV See article p.5285

Illuminating the History of IL-17 See article p.5440

Signaling a Switch See article p.5461

A recent study showed that CXCR3 expression on T cells was necessary for control of vaccinia virus in mice, although the exact search strategy used to locate virally infected cells—directed chemotaxis or random chemokinesis—was not elucidated. Ariotti et al. (p. 5285) performed intravital imaging of the epidermis and computational modeling following infection with type 1 herpesvirus (HSV-1), which requires CD8+ T cells for its control. Recombinant viruses expressing td-tomato fluorescent protein (HSVTOM) alone or with the OTI CD8+ T cell OVA257–264 epitope (HSVTOM-OVA) allowed for visualization of infected cells and the study of Ag-specific and bystander CD8+ T cells. Twenty-four hours postinfection, the migration of CD8+ T cells appeared erratic, but more detailed analyses revealed a slight preference for movement toward HSV lesions, regardless of the Ag specificity of the T cell. OVA257–264-specific GFP-OTI or GFP-OTI-CXCR3−/− (CXCR3−/−) cells were transferred into wild-type mice, which were then immunized with OVA257–264 peptide prior to HSVTOM infection. CXCR3−/− cells were observed at the site of infection, suggesting that CXCR3 is not critical for T cell tissue extravasation; however, although wild-type OTI cells demonstrated directed migration at long distances from the infection, CXCR3−/− cells did not. Computational modeling based on experimental speeds and angles produced long-lasting tracks that matched experimental observations. When random angles were used, only about a third of cells were calculated to reach the site of infection, and in twice the amount of time as cells whose simulated migration was directed, of which almost all were calculated to reach the site of infection. Simulations of random movement of HSV-specific T cells could not account for T cell accumulation at the site of infection, indicating that directional migration was responsible. Taken together, these results suggest that CD8+ T cells directionally target virus-infected cells in a CXCR3-dependent manner.

The generation of class-switched Ab isotypes with distinct effector functions is accomplished by class switch recombination (CSR), which is dependent on activation-induced deaminase (AID) initiation of DNA double strand breaks. The upstream signaling events that lead to the careful molecular orchestration of CSR are not fully elucidated, as previous CSR studies have been complicated by the fact that some genetic knockout approaches affect B cell development. Chen et al. (p. 5461) examined the roles two enzymes that control the regulation of PI(3,4,5)P3, PI3K and the phosphatase PTEN, play in CSR induction in vitro. Relative to cells from wild-type mice, splenic B cells from mice that lacked Pten only in mature B cells were deficient in IgG1 and IgE expression following anti-CD40 stimulation in the presence of IL-4. These deficiencies were noted even though the Pten-deficient cells exhibited no defects in proliferation or the expression of AKT, a kinase previously suggested to downregulate AID expression. Accordingly, genetic overexpression of Pten in mature B cells led to relatively greater IgG1 and IgE switching. Mature B cells that expressed constitutively active p110α, a subunit of PI3K, also had reduced IgG1 and IgE expression. Pharmacological inhibition of endogenous p110α or p110δ enhanced CSR, possibly through decreased phosphorylation of AKT. AID expression was reduced in Pten-deficient B cells and those with constitutively active p110α, and inhibition of AKT led to dramatic increases in CSR in these cells. The authors concluded that PI3K and PTEN work antagonistically to regulate AKT activity, which in turn modulates AID expression to repress or induce CSR.

Altering Wnt signaling in the thymus is known to affect thymic epithelial cell (TEC) integrity; however, the primary source of Wnt ligands in the thymus and their role in shaping the development of the thymocyte compartment are still unclear. In this issue, Brunk et al. (p. 5261) sorted thymic cell populations by flow cytometry and used real-time PCR to identify TECs as the major source of Wnt ligands in the thymus. To determine the contributions of Wnt signaling to thymic stromal and T cell development without genetically altering thymic T cell populations, the authors used a mouse harboring a TEC conditional knockout of a receptor required for Wnt ligand secretion (FoxN1-Gpr177). They found that these mice exhibited hypotrophic thymii and increased TEC apoptosis, suggesting that TECs were a nonredundant source of Wnt ligands. FoxN1-Gpr177 mice also had a 2-fold reduction in the number of thymocytes, although subset populations appeared to be present at normal frequencies. Peripheral T cell populations in these mice were also slightly reduced and had a greater tendency to display a memory-like phenotype, but their capacity for homeostatic proliferation was intact. Together, these data suggest T cell development is not directly dependent on Wnt but do show that TECs provide a nonredundant primary source of Wnt ligands essential for normal thymus development.

The mammalian IL-17 cytokine gene family has six members (IL-17A–F) and five receptors (IL-17RA–E) which play important roles in autoimmunity and defense against infection. Orthologs of these genes have been described in multiple fish species, and Han et al. (p. 5440) characterized IL-17 family members and receptors in lamprey, a useful organism in the study of immune evolution. Lampreys are jawless vertebrates that recognize Ag with variable lymphocyte receptors (VLRs); cells that express VLRA or VLRC are T cell–like, and cells that express VLRB are B cell–like. Previous work demonstrated that VLRB+ lymphocytes expressed IL-17R transcripts and VLRA+ cells expressed an IL-17 ortholog, suggesting cross-talk between the lymphocyte subsets. Examination of available genome data for multiple lamprey species identified orthologs of four IL-17 genes (but not IL-17A or IL-17F) and six IL-17R genes, which were confirmed to be expressed in various sea lamprey tissues by real-time PCR. All lamprey lymphocyte subsets expressed IL-17B, IL-17C, and IL-17D.2. VLRA+ and VLRC+ lymphocytes preferentially expressed IL-17D.1, whereas VLR triple-negative and VLRB+ cells expressed IL-17E. IL-17RA.1 expression was confirmed in VLRB+ lymphocytes as well as monocytes, and IL-17RA.3 was predominately found in VLRA+ and VLRC+ lymphocytes. Lamprey IL-17D.1 was cloned by nested PCR, and an mAb was generated to recognize recombinant IL-17D.1, which facilitated detection of IL-17D.1 in the intestine, skin, and gills. Flow cytometry analysis using the recombinant IL-17D.1 demonstrated IL-17D.1 binding was only observed in VLRB+ lymphocytes and monocytes, which was concordant with transfection experiments with different receptors demonstrating that IL-17D.1 only associated with IL-17RA.1. These studies provide valuable insight into the evolution of this ancient cytokine family.