The GB virus C (GBV-C) NS3 serine protease inhibits HIV-1 replication in a CD4+ T lymphocyte cell line without decreasing HIV receptor expression

Abstract

Introduction: Persistent infection with GBV-C (GB Virus C), a non-pathogenic virus related to hepatitis C virus (HCV), prolongs survival in HIV infection. Two GBV-C proteins, NS5A and E2, have been shown previously to inhibit HIV replication in vitro. We investigated whether the GBV-C NS3 serine protease affects HIV replication.

Results: GBV-C NS3 protease expressed in a human CD4+ T lymphocyte cell line significantly inhibited HIV replication. Addition of NS4A or NS4A/4B coding sequence to GBV-C NS3 increased the effect on HIV replication. Inhibition of HIV replication was dose-dependent and was not mediated by increased cell toxicity. Mutation of the NS3 catalytic serine to alanine resulted in loss of both HIV inhibition and protease activity. GBV-C NS3 expression did not measurably decrease CD4 or CXCR4 expression.

Conclusion: GBV-C NS3 serine protease significantly inhibited HIV replication without decreasing HIV receptor expression. The requirement for an intact catalytic serine at the active site indicates that inhibition was mediated by proteolytic cleavage of an unidentified target(s).

Figure 1. Schematic of the GBV-C genome and protein expression constructs.

(A) The GBV-C genome, including the 5′ and 3′ non-translated regions (NTR), is shown. The structural genes include the envelope proteins E1 and E2, and the non-structural genes include NS2, NS3, NS4A, NS4B, NS5A, and NS5B. The regions coding for NS3, NS3/4A, and NS3/4A/4B that were cloned are shown. The relative position of the NS3 serine protease catalytic serine residue (S137) is shown. (B) Comparison of the HCV and GBV-C serine protease catalytic triad. Partial protease sequences of HCV1b (CON1 sequence), HCV1a (H77 sequence), HCV2a (JFH-1 sequence), are compared with the GBV-C AY196904 sequence. The catalytic triad consists of histidine 56, aspartic acid 79, and serine 137 (blue letters, underlined and in bold). Histidine 56 and serine 137 are expressed in similar peptide contexts in both HCV and GBV-C (blue letters). The GBV-C residues are numbered from the experimentally determined amino terminus of GBV-C NS3. (C) Comparison of the known cleavage recognition sequences of the HCV and GBV-C NS3 serine proteases. *Adapted from . **Adapted from , .

Figure 2. GBV-C NS3/4A/4B expression and protease activity in Jurkat cells.

(A) Relative primary structures of the GBV-C proteins employed are shown. HA indicates the location of the HA epitope tag. The location of serine 137 is indicated along with the relative sizes of NS3 and NS4B. (B) CD4+ Jurkat cells were transfected with equal amounts of HA-NS3/4A/4B, NS3/4A/4B-HA, or S137A-HA expression plasmids and cell lysates were collected one day post transfection. Cell lysates were analyzed by western blot developed with an anti-HA monoclonal antibody. Molecular weight markers are shown at right, and the mobility of NS3 (73kDa), NS4B (30kDa), or the complete polypeptide S137A (102kDa) is shown at left. (C) Jurkat cells were transfected with NS3/4A/4B-HA and equal numbers of viable cells determined by trypan blue staining were lysed on days 1, 4, and 6 post transfection. Protein from lysates was analyzed by western blots developed with an anti-HA monoclonal antibody. Expression of GBV-C NS3/4A/4B-HA on days 1, 4, and 6 post transfection is shown compared with cell lysates from cells transfected with NS3/4A/4B and a highly concentrated NS3/4A/4B-HA cell lysate. Molecular weight markers are indicated at the left, and the mobility of NS4B-HA (30kDa) is indicated at the right. A western blot of actin from the same cell lysates run in parallel is shown below as a loading control.

Figure 3. Transfection of Jurkat cells with GBV-C NS3/4A/4B or NS5A expression plasmids does not cause cellular toxicity.

Jurkat cells were transfected with equal amounts of NS3/4A/4B (red line), NS5A (solid black line), or control protein (CAT, luciferase) expression plasmids in pCDNA3.1/Zeo+ or empty vector (dotted black lines). Cellular viability (A) and proliferation (B) were measured for 10 days. Standard errors for each measurement on each day are shown.

Figure 4. Expression of GBV-C NS3, NS3/4A, or NS3/4A/4B in Jurkat cells inhibits HIV replication for up to 9 days.

Jurkat cells were transfected with equal amounts of plasmids expressing NS3 (solid black line), NS3/4A (blue line), NS3/4A/4B (red line), GST, CAT, or empty vector (dotted black lines) plus HIV pNL4-3 plasmid. Mean and standard error p24 values of supernatants collected on days 1, 3, 6, and 9 are shown.

Figure 5. GBV-C NS3/4A inhibition of HIV replication in Jurkat cells is dose-dependent.

(A) Jurkat cells were transfected with either 3 µg HIV pNL4-3 (HIV), 3 µg of HIV pNL4-3+3 µg pCDNA3.1/Zeo+ (HIV+vector), or 3 µg HIV pNL4-3+3 µg NS3/4A in pCDNA3.1/Zeo+ (HIV+NS3/4A) (blue line). Mean and standard errors of p24 values in supernatants collected on days 1, 3, and 7 are shown. (B) Jurkat cells were transfected with HIV pNL4-3 and either 6 or 9 µg of NS3/4A expression plasmid (blue lines) or equivalent amounts of CAT expression plasmid; mean and standard errors of p24 values in supernatants collected on days 1, 3, and 6 are shown.

Figure 6. Mutating the protease catalytic serine of GBV-C NS3/4A/4B to alanine results in loss of HIV inhibition.

Jurkat cells were transfected with HIV pNL4-3 and equal amounts of NS3/4A (blue line), NS3/4A/4B (red line), NS5A (solid black line), S137A (dotted red line), CAT, or luciferase expression plasmids or empty vector (dotted black lines). Supernatants were collected and p24 measured on days 1, 3, 5, and 7. Mean and standard error of p24 values are shown.

Figure 7. GBV-C NS3/4A/4B expression does not decrease CD4 or CXCR4 expression.

Jurkat cells were transfected with 1 µg GFP in pCDNA3.1/Zeo+ and either 6 µg NS3/4A/4B expression plasmid (red line) or CAT expression plasmid (blue line) and scrambled anti-CD4 siRNA. Equal numbers of viable cells were labeled with anti-CD4 PE-Cy7 and anti-CXCR4-APC and analyzed by FACS on days 3 (A and B), 4 (C and D), and 5 (E and F) post transfection. Cells were gated for GFP expression and then analyzed for CD4 and CXCR4 expression. As a control to demonstrate reduction in receptor expression, cells were transfected with 1 µg GFP in pCDNA3.1/Zeo+ and 6 µg CAT expression plasmid plus anti-CD4 siRNA (A,C,E: gray shaded area, B,D,F: black line).