Guanylate Binding Protein (GBP) 5 Is an Interferon-Inducible Inhibitor of HIV-1 Infectivity

Guanylate binding proteins (GBPs) are an interferon (IFN)-inducible subfamily of guanosine triphosphatases (GTPases) with well-established activity against intracellular bacteria and parasites. Here we show that GBP5 potently restricts HIV-1 and other retroviruses. GBP5 is expressed in the primary target cells of HIV-1, where it impairs viral infectivity by interfering with the processing and virion incorporation of the viral envelope glycoprotein (Env). GBP5 levels in macrophages determine and inversely correlate with infectious HIV-1 yield over several orders of magnitude, which may explain the high donor variability in macrophage susceptibility to HIV. Antiviral activity requires Golgi localization of GBP5, but not its GTPase activity. Start codon mutations in the accessory vpu gene from macrophage-tropic HIV-1 strains conferred partial resistance to GBP5 inhibition by increasing Env expression. Our results identify GBP5 as an antiviral effector of the IFN response and may explain the increased frequency of defective vpu genes in primary HIV-1 strains.

Frequencies of lymphoid T-follicular helper cells obtained longitudinally by lymph node fine-needle aspiration correlate significantly with viral load in SIV-infected rhesus monkeys

BACKGROUND

T-follicular helper (T(FH)) cells are an important population in lymph nodes (LNs) contributing to the generation of highly specific B cells. For SIV studies in rhesus macaques (RM), analysis of LN is necessary, but restricted due to invasive sampling. We applied the minimally invasive LN fine-needle aspiration (LN-FNA) and examined dynamics of T(FH) cells during SIV infection.

MATERIALS AND METHODS

LN-FNA and LN resection were carried out on uninfected RM. Lymphocytes were analyzed by flow cytometry. Additionally, cells obtained by LN-FNA over time from SIV-infected RM were analyzed.

RESULTS

Percentages of lymphocyte subsets were similar in LN aspirates and whole LNs. Analysis of LN aspirates from SIV-infected RM demonstrated a decrease of CD4(+) T cells, while T(FH) cell frequencies increased over time and correlated significantly with plasma viral load.

CONCLUSIONS

By applying LN-FNA, we showed that T(FH) cell expansion in chronic SIV infection is associated with viral load.

Identification of potential HIV restriction factors by combining evolutionary genomic signatures with functional analyses

Background

Known antiretroviral restriction factors are encoded by genes that are under positive selection pressure, induced during HIV-1 infection, up-regulated by interferons, and/or interact with viral proteins. To identify potential novel restriction factors, we performed genome-wide scans for human genes sharing molecular and evolutionary signatures of known restriction factors and tested the anti-HIV-1 activity of the most promising candidates.

Results

Our analyses identified 30 human genes that share characteristics of known restriction factors. Functional analyses of 27 of these candidates showed that over-expression of a strikingly high proportion of them significantly inhibited HIV-1 without causing cytotoxic effects. Five factors (APOL1, APOL6, CD164, TNFRSF10A, TNFRSF10D) suppressed infectious HIV-1 production in transfected 293T cells by >90% and six additional candidates (FCGR3A, CD3E, OAS1, GBP5, SPN, IFI16) achieved this when the virus was lacking intact accessory vpr, vpu and nef genes. Unexpectedly, over-expression of two factors (IL1A, SP110) significantly increased infectious HIV-1 production. Mechanistic studies suggest that the newly identified potential restriction factors act at different steps of the viral replication cycle, including proviral transcription and production of viral proteins. Finally, we confirmed that mRNA expression of most of these candidate restriction factors in primary CD4+ T cells is significantly increased by type I interferons.

Conclusions

A limited number of human genes share multiple characteristics of genes encoding for known restriction factors. Most of them display anti-retroviral activity in transient transfection assays and are expressed in primary CD4+ T cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-015-0165-5) contains supplementary material, which is available to authorized users.

CD4+ T cell-derived IL-21 and deprivation of CD40 signaling favor the in vivo development of granzyme B-expressing regulatory B cells in HIV patients

IL-21 can induce both plasma cells and regulatory B cells. In this article, we demonstrate that untreated HIV patients display CD4(+) T cells with enhanced IL-21 expression and high in vivo frequencies of regulatory B cells overexpressing the serine protease granzyme B. Granzyme B-expressing regulatory B cells (GraB cells) cells from HIV patients exhibit increased expression of CD5, CD43, CD86, and CD147 but do not produce IL-10. The main functional characteristic of their regulatory activity is direct granzyme B-dependent degradation of the TCR-ζ-chain, resulting in significantly decreased proliferative T cell responses. Although Th cells from HIV patients secrete IL-21 in a Nef-dependent manner, they barely express CD40L. When culturing such IL-21(+)CD40L(-) Th cells with B cells, the former directly induce B cell differentiation into GraB cells. In contrast, the addition of soluble CD40L multimers to T cell/B cell cultures redirects B cell differentiation toward plasma cells, indicating that CD40L determines the direction of IL-21-dependent B cell differentiation. As proof of principle, we confirmed this mechanism in a patient lacking intact CD40 signaling due to a NEMO mutation. The majority of peripheral B cells from this patient were GraB cells and strongly suppressed T cell proliferation. In conclusion, GraB cells represent potent regulatory B cells in humans that are phenotypically and functionally distinct from B10 cells and occur in early HIV infection. GraB cells may contribute significantly to immune dysfunction in HIV patients, and may also explain ineffective Ab responses after vaccination. The use of soluble CD40L multimers may help to improve vaccination responses in HIV patients.

The fate of B cells in HIV infection - shifting the balance from plasma cell towards regulatory B cell (Breg) differentiation (LYM6P.776)

Certain regulatory lymphocyte populations suppress T cell expansion in a granzyme B (GrB)-dependent manner. Recently, we found that human B cells also contribute to immune regulation by interleukin (IL-)21-dependent expression of GrB. GrB-expressing Bregs are induced by incompletely activated CD4+ T cells, which express high levels of IL-21, but only low CD40 ligand. In contrast, fully activated CD4+ T cells strongly induce plasma cell differentiation. Here we demonstrate that up to 60% of B cells from HIV-infected patients with high viremia exhibit a regulatory (Breg) phenotype with expression of GrB and potent suppression of T cell proliferation by GrB-dependent degradation of the T cell receptor zeta-chain (TCR-zeta). These results are supported by additional findings showing that T cells in HIV-infected individuals produce high levels of IL-21, while expressing only low levels of CD40L and TCR-zeta. Importantly, soluble CD40L multimers can restore the differentiation of Bregs derived from HIV-infected individuals into plasma cells. Our results suggest that incompletely activated CD4+ T cells in HIV patients lead to a misrouting of B cell differentiation into Bregs at the cost of fully functional plasma cells. Apart from a disturbed humoral immune response these B cells may further aggravate the immune status of HIV patients. The use of CD40L multimers may disrupt the defective B cell-T cell interactions and may prove beneficial for future HIV vaccination approaches.

HIV triggers interleukin 21-mediated induction of granzyme B-secreting B cells with T cell regulatory potential. (P1441)

Certain regulatory lymphocyte subpopulations like regulatory T cells can suppress T cell expansion by granzyme B (GrB) secretion. Recently, we found that B cells can also produce GrB and acquire regulatory potential in response to interleukin (IL-)21. Since HIV is associated with elevated serum IL-21 levels, we hypothesized that GrB-expressing B cells may be induced during HIV infection. Here, we show that infection of CD4+ T cells with HIV 1 (NL4-3) induces strong expression of IL-21 without upregulation of CD40 ligand. We demonstrate that such IL-21+CD40Llow T cells secrete IL-21, thereby inducing GrB in cocultured B cells rather than supporting plasma cell differentiation. In line with these results, serum levels of both IL-21 and GrB are significantly higher in HIV-infected patients before HAART as compared to healthy controls. Up to 60% of freshly isolated B cells from HIV-infected patients, but not healthy control B cells, express GrB. Of note, coculture of HIV-infected CD4+ T cells with GrB+ B cells results in GrB transfer, and strongly suppresses both T cell proliferation and viral replication as indicated by significantly reduced p24 levels. The observed effects are enhanced by IL-21, and reduced by GrB inhibition. In summary, GrB induction in HIV-specific B cells may play a role in decelerating expansion of HIV-infected CD4+ T cells, while interfering with their terminal differentiation into plasma cells.

Incoming RNA virus nucleocapsids containing a 5'-triphosphorylated genome activate RIG-I and antiviral signaling

Host defense to RNA viruses depends on rapid intracellular recognition of viral RNA by two cytoplasmic RNA helicases: RIG-I and MDA5. RNA transfection experiments indicate that RIG-I responds to naked double-stranded RNAs (dsRNAs) with a triphosphorylated 5' (5'ppp) terminus. However, the identity of the RIG-I stimulating viral structures in an authentic infection context remains unresolved. We show that incoming viral nucleocapsids containing a 5'ppp dsRNA "panhandle" structure trigger antiviral signaling that commences with RIG-I, is mediated through the adaptor protein MAVS, and terminates with transcription factor IRF-3. Independent of mammalian cofactors or viral polymerase activity, RIG-I bound to viral nucleocapsids, underwent a conformational switch, and homo-oligomerized. Enzymatic probing and superresolution microscopy suggest that RIG-I interacts with the panhandle structure of the viral nucleocapsids. These results define cytoplasmic entry of nucleocapsids as the proximal RIG-I-sensitive step during infection and establish viral nucleocapsids with a 5'ppp dsRNA panhandle as a RIG-I activator.

HIV triggers interleukin 21-mediated induction of granzyme B-secreting B cells with antiviral properties (105.18)

Certain lymphocyte subsets including plasmacytoid dendritic cells and regulatory T cells can secrete granzyme B (GrB), thereby suppressing T cell expansion. Recently, we found that B cells can also produce GrB in response to interleukin (IL) 21. Since HIV has been shown to be associated with elevated serum IL-21 levels, we hypothesized that GrB-expressing B cells may be induced during HIV infection. Here, we demonstrate for the first time, that infection of CD4+ T cells with HIV 1 (NL4-3), but not mock infection, induces strong expression of IL-21. We further demonstrate that such T cells induce GrB in co-cultured B cells in an IL-21-dependent fashion. In support of these data, serum levels of both IL-21 and GrB are significantly higher in HIV-infected patients before HAART as compared to healthy controls. Up to 60% of B cells (36.2 ± 12.9%) from patients infected with HIV, but not normal B cells, express GrB. Importantly, co-culture of HIV-infected CD4+ T cells with GrB+ B cells resulted in GrB transfer, and strongly suppressed both, proliferation of T cells and virus replication as indicated by significantly reduced p24 levels. The observed effects were enhanced by IL-21, and reduced by GrB inhibition. In summary, we demonstrate that HIV induces IL-21 in CD4+ T cells, thereby indirectly triggering the development of GrB-secreting B cells with antiretroviral properties. GrB-secreting B cells may play a so far unappreciated role in decelerating the expansion of HIV.