The interferon response inhibits HIV particle production by induction of TRIM22

Abstract

Treatment of human cells with Type 1 interferons restricts HIV replication. Here we report that the tripartite motif protein TRIM22 is a key mediator. We used transcriptional profiling to identify cellular genes that were induced by interferon treatment and identified TRIM22 as one of the most strongly up-regulated genes. We confirmed, as in previous studies, that TRIM22 over-expression inhibited HIV replication. To assess the role of TRIM22 expressed under natural inducing conditions, we compared the effects of interferon in cells depleted for TRIM22 using RNAi and found that HIV particle release was significantly increased in the knockdown, implying that TRIM22 acts as a natural antiviral effector. Further studies showed that TRIM22 inhibited budding of virus-like particles containing Gag only, indicating that Gag was the target of TRIM22. TRIM22 did not block the release of MLV or EIAV Gag particles. Inhibition was associated with diffuse cytoplasmic staining of HIV Gag rather than accumulation at the plasma membrane, suggesting TRIM22 disrupts proper trafficking. Mutational analyses of TRIM22 showed that the catalytic amino acids Cys15 and Cys18 of the RING domain are required for TRIM22 antiviral activity. These data disclose a pathway by which Type 1 interferons obstruct HIV replication.

Figure 1. Expression of TRIM22 in HOS-CD4/CXCR4 cells blocks late steps in HIV replication.

A) Induction of TRIM22 RNA levels by IFNβ treatment of HOS-CD4/CXCR4 cells. Cells were pre-treated with 1000 units/ml of IFNβ, RNA isolated and subjected to Northern blot analysis. The TRIM22 3′UTR was labeled and used as a probe. B) Stable expression of HA-tagged TRIM22 in HOS-CD4/CXCR4 cells verified by Western blotting. C) Inhibition of HIV replication in cells stably expressing TRIM22. Cells were infected with different concentrations of replication-competent HIV-1 R9. After 72 hours, virus released into the supernatant was pelleted and assayed for p24CA levels by Western blot. The numbers below the Western blots correspond to the amounts of virus added to each well, quantified according to the amount of reverse transcriptase activity. D) TRIM22 does not inhibit early HIV replication steps, subsequent to entry. The TRIM22 expressing cells were infected with different amounts of HIV-1/VSVG pseudotyped virus and analyzed by FACS for GFP expression. Data obtained from a second independently generated cell line expressing TRIM22 (#2) are also shown. Data shown are representative of two independent infections for each performed in quadruplicate (+/−StdDev).

Figure 2. TRIM22 inhibits accumulation of viral particles in cell supernatants.

HOS, U2OS, 143B and HeLa cells were studied (labeled to the left of the gels). Cells were co-transfected with pR9 (encoding full-length HIV-1) and pTRIM22 or the control empty expression vector pcDNA3.1. After 48 hours, Western blots were performed on the supernatants (left panels) and cell pellets (right panels) using p24CA antibodies.

Figure 3. Knockdown of TRIM22 by RNA interference abrogates interferon-induced inhibition of HIV replication.

A) HOS-CD4/CXCR4 cells were transfected with the empty vector control plasmid pLKO.1 or pLKO.1/TRIM22 shRNA_#1 followed by treatment with 0, 50, 500 or 1000 units/ml of IFNβ (represented by triangles at the top of the Northern blots). Total RNA was extracted and subjected to Northern blot analysis to detect TRIM22 RNA transcripts. TRIM34 (variant 3) RNA transcript levels were also detected for a comparison (middle row). Input RNA was normalized using Northern blots probed with an actin fragment (bottom row). B) Quantitation of the Northern blots in A, indicating the fold induction of TRIM22 RNA in response to the IFNβ treatment. C) Cells were transfected with the empty vector pLKO.1 or pLKO.1/TRIM22_shRNA#1 and treated with 1000 units/ml of IFNβ. Cells were then transfected with pR9 and Western blot analysis of HIV capsid antigen in cells and supernatants was analyzed using an anti-p24 antibody. D) Cells were transfected with the control plasmids pLKO.1, pLKO.1/eGFP_shRNA or pLKO.1/scrambled_shRNA or TRIM22 shRNA plasmids pLKO.1/TRIM22_shRNA#1 and pLKO.1/TRIM22_shRNA#2 and treated with 1000 units/ml of IFNβ. Following transfection with pR9, HIV capsid antigen released into the supernatant was detected by Western blot and quantified densitometrically. Data represents the averages (+/−StdDev) of at least 3 experiments performed in triplicate. E) A comparison of the average fold inhibition in particle release of the pooled control shRNAs and the pooled TRIM22 shRNAs (+/−StdDev). *, P = 0.0002 (Mann-Whitney).

Figure 4. TRIM22 expression blocks release of HIV-1 Gag-only particles.

A) Cells were transfected with pGag and pTRIM22 or pcDNA. Cells and Gag-only particles released into the supernatant were isolated 24 hours after transfection and analyzed by Western blotting using an anti-p24CA or anti-actin antibody. B) Cells were pre-treated for 16 hours with 1000 units/ml of IFNβ followed by transfection with pGag. Cells and Gag-only particles released into the supernatant were isolated after 24 hours and analyzed by Western blotting using an anti-p24CA antibody. C) Cells were co-transfected with pR9 or a plasmid expressing MLV GagPol (pMLV) or an EIAV packaging vector (pEIAV) with or without TRIM22 for 24 hours. Cells and Gag-containing particles were isolated and analyzed by Western blotting using an anti-capsid antibody.

Figure 5. TRIM22 alters the sub-cellular localization of Gag protein.

A) Analysis of Gag localization by fluorescence microscopy. HOS-CD4/CXCR4 cells were co-transfected with pTRIM22 and pGag/GFP for 3 hours, washed and incubated with 100 µM cycloheximide for 3 hours. IFNβ pre-treatment was done for 16 hours prior to transfection with pGag/GFP. Cells were washed and incubated in fresh media for a further 48 hours. Gag-GFP localization was assessed using fluorescence microscopy. High magnification images of the white squares are shown to the right. B) Quantifications of images as in A from three independent experiments. GFP-positive cells were classified as showing either punctate fluorescence at the cell periphery or diffuse cytoplasmic fluorescence. The number of cells counted was: pcDNA, n = 332; TRIM22, n = 386; −IFNβ = 497; +IFNβ = 398. C) HOS-CD4/CXCR4 cells were co-transfected with pGag in the absence or presence of pTRIM22 and pulse-chase analysis of intracellular Gag protein was done. D) Detection of ³H-myristic acid-labeled Gag. Cells were co-transfected with pGag in the absence or presence of pTRIM22 for 24 hours and then labeled with ³H-myristic acid overnight. Gag was immunoprecipitated and resolved on a SDS-PAGE gel before exposure to film (“Labeled Gag”) or Western blot analysis using anti-p24CA (“Total Gag”).

Figure 6. E3 ligase catalytic site is required for HIV particle release.

pR9 was transfected with or without pcDNA, pTRIM22 or pT22-C15A/C18A for 24 hours. Western blots were performed on virus released into the supernatants (top panel) using anti-p24CA and on the cell pellets (lower panels) using anti-p24CA, anti-HA or anti-actin respectively.

Figure 7. TRIM22 interacts with Gag.

HOS-CD4/CXCR4 cells were co-transfected with pGag-GFP and pFLAG-TRIM22 (or empty vector control pFLAG) and immunoprecipitated with anti-GFP or anti-FLAG. Precipitated Gag and TRIM22 were detected by Western blotting using p24CA or FLAG antibodies.