SAMHD1 is a host restriction factor for human immunodeficiency virus 1 (HIV‐1) in cultured human cells. SAMHD1 mutations cause autoimmune Aicardi‐Goutières syndrome and are found in cancers including chronic lymphocytic leukaemia. SAMHD1 is a triphosphohydrolase that depletes the cellular pool of deoxynucleoside triphosphates, thereby preventing reverse transcription of retroviral genomes. However, in vivo evidence for SAMHD1's antiviral activity has been lacking. We generated Samhd1 null mice that do not develop autoimmune disease despite displaying a type I interferon signature in spleen, macrophages and fibroblasts. Samhd1−/− cells have elevated deoxynucleoside triphosphate (dNTP) levels but, surprisingly, SAMHD1 deficiency did not lead to increased infection with VSV‐G‐pseudotyped HIV‐1 vectors. The lack of restriction is likely attributable to the fact that dNTP concentrations in SAMHD1‐sufficient mouse cells are higher than the KM of HIV‐1 reverse transcriptase (RT). Consistent with this notion, an HIV‐1 vector mutant bearing an RT with lower affinity for dNTPs was sensitive to SAMHD1‐dependent restriction in cultured cells and in mice. This shows that SAMHD1 can restrict lentiviruses in vivo and that nucleotide starvation is an evolutionarily conserved antiviral mechanism.

Tetherin (Bst-2 CD317) is a cell-surface protein whose expression is induced by IFNα. Although tetherin expression causes the retention of retrovirus particles on the surface of infected cells, it is not known whether tetherin inhibits retroviral replication or pathogenesis in vivo. Mutation of tetherin antagonists often has little effect on retroviral replication in vitro, and, although tetherin can reduce the yield of extracellular viral particles, some studies suggest that tetherin actually enhances direct cell-to-cell viral transmission. We generated tetherin-deficient mice to determine the effect of this protein on murine retrovirus replication and pathogenesis. We find that tetherin markedly inhibits the replication of Moloney murine leukemia virus (Mo-MLV) and is required for the antiretroviral activity of IFNα to be fully manifested in vitro. Surprisingly, Mo-MLV replication and disease progression was not significantly different in WT and tetherin-deficient mice, but this finding was explained by the fact that Mo-MLV infection did not induce detectable tetherin expression on candidate target cells in vivo. Indeed, IFNα induction was required to reveal the antiMo-MLV activity of tetherin in vivo. Moreover, LP-BM5, an MLV strain that has been demonstrated to induce immune activation and IFNα expression, achieved higher levels of viremia and induced exaggerated pathology in tetherin-deficient mice. These data indicate that tetherin is a bona fide antiviral protein and can reduce retroviral replication and disease in vivo.

In yeast (Saccharomyces cerevisiae), transcriptional activators, such as Gal4 and Gal4-VP16, work ordinarily from sites located in the upstream activating sequence (UAS) positioned about 250 base pairs upstream of the transcription start site. In contrast to their behaviour in mammalian cells, however, such activators fail to work when positioned at distances greater than approximately 600-700 base pairs upstream, or anywhere downstream of the gene. Here we show that, in yeast, a gene bearing an enhancer positioned 1-2 kilobases downstream of the gene is activated if the reporter is linked to a telomere, but not if it is positioned at an internal chromosomal locus. These observations are explained by the finding that yeast telomeres form back-folding, or looped, structures. Because yeast telomeric regions resemble the heterochromatin found in higher eukaryotes, these findings might also explain why transcription of some higher eukaryotic genes depends on their location in heterochromatin.

Yeast telomeres reversibly repress the transcription of adjacent genes, a phenomenon called telomere position effect (TPE). TPE is thought to result from Rap1 and Sir protein-mediated spreading of heterochromatin-like structures from the telomeric DNA inwards. Because Rap1p is associated with subtelomeric chromatin as well as with telomeric DNA, yeast telomeres are proposed to form fold-back or looped structures. TPE can be eliminated in trans by deleting SIR genes or in cis by transcribing through the C 1-3 A/TG 1-3 tract of a telomere. We show that the promoter of a telomere-linked URA3 gene was inaccessible to restriction enzymes and that accessibility increased both in a sir3 strain and upon telomere transcription. We also show that subtelomeric chromatin was hypoacetylated at histone H3 and at each of the four acetylatable lysines in histone H4 and that histone acetylation increased both in a sir3 strain and when the telomere was transcribed. When transcription through the telomeric tract occurred in G 1 -arrested cells, TPE was lost, demonstrating that activation of a silenced telomeric gene can occur in the absence of DNA replication. The loss of TPE that accompanied telomere transcription resulted in the rapid and efficient loss of subtelomeric Rap1p. We propose that telomere transcription disrupts core heterochromatin by eliminating Rap1p-mediated telomere looping. This interpretation suggests that telomere looping is critical for maintaining TPE.

Tetherin/BST-2 is a host restriction factor that could directly inhibit retroviral particle release by tethering nascent virions to the plasma membrane. However, the immunological impact of Tetherin during retrovirus infection remains unknown. We now show that Tetherin influences antiretroviral cell-mediated immune responses. In contrast to the direct antiviral effects of Tetherin, which are dependent on cell surface expression, the immunomodulatory effects are linked to the endocytosis of the molecule. Mice encoding endocytosis-competent C57BL/6 Tetherin exhibited lower viremia and pathology at 7 days post-infection with Friend retrovirus (FV) compared to mice encoding endocytosis-defective NZW/LacJ Tetherin. Notably, antiretroviral protection correlated with stronger NK cell responses. In addition, FV infection levels were significantly lower in wild-type C57BL/6 mice than in Tetherin knock-out mice at 2 weeks post-infection, and antiretroviral protection correlated with stronger NK cell and virus-specific CD8+ T cell responses. The results demonstrate that Tetherin acts as a modulator of the cell-mediated immune response against retrovirus infection in vivo.

Highlights d VSV-displayed HIV-1 envelope trimers identified five HIV-1 bNAbs d BCR repertoires identified two bNAb lineages class-switched to both IgG and IgA d The V3 crown-targeting bNAb M4008_N1 conferred tier 2 virus neutralization d Cryo-EM structure of bNAb M1214_N1 with CH505 SOSIP defined a V2V5 corridor epitope

Yeast telomeres reversibly repress the transcription of adjacent genes, a phenomenon called telomere position effect (TPE). TPE is thought to result from Rap1 and Sir protein-mediated spreading of heterochromatin-like structures from the telomeric DNA inwards. Because Rap1p is associated with subtelomeric chromatin as well as with telomeric DNA, yeast telomeres are proposed to form fold-back or looped structures. TPE can be eliminated in trans by deleting SIR genes or in cis by transcribing through the C 1-3 A/TG 1-3 tract of a telomere. We show that the promoter of a telomere-linked URA3 gene was inaccessible to restriction enzymes and that accessibility increased both in a sir3 strain and upon telomere transcription. We also show that subtelomeric chromatin was hypoacetylated at histone H3 and at each of the four acetylatable lysines in histone H4 and that histone acetylation increased both in a sir3 strain and when the telomere was transcribed. When transcription through the telomeric tract occurred in G 1 -arrested cells, TPE was lost, demonstrating that activation of a silenced telomeric gene can occur in the absence of DNA replication. The loss of TPE that accompanied telomere transcription resulted in the rapid and efficient loss of subtelomeric Rap1p. We propose that telomere transcription disrupts core heterochromatin by eliminating Rap1p-mediated telomere looping. This interpretation suggests that telomere looping is critical for maintaining TPE.

A role for c-Abl in B cell development and signaling has been suggested by previous work showing that c-Abl-deficient mice have defects in bone marrow B cell development and that c-Abl-deficient B cells are hypoproliferative in response to antigen receptor stimulation. Here we show that in addition to defects in early B cell development, c-Abl-deficient mice have defects in peripheral B cell development, including reduced percentages of peritoneal B-1 cells as well as transitional and marginal zone B cells in the spleen. It has been shown that c-Abl kinase activity increases upon B cell receptor (BCR) stimulation and that one of the targets of tyrosine phosphorylation by c-Abl is CD19. However, the consequences of c-Abl activity on B cell activation and CD19 signaling remain unknown. Here, we show that c-Abl-deficient splenic B cells exhibit reduced calcium flux in response to CD19 cross-linking, consistent with a role for c-Abl in CD19-dependent signaling. Additionally, we show that c-Abl-deficient B cells are defective in their ability to be activated in response to antigen receptor engagement, suggesting a functional role for c-Abl in BCR-dependent activation signaling pathways.