Fig 1.

HERV-K (HML-2) RNA, proteins, and VLPs are found in the plasma of HIV-1 and breast cancer patients. Plasma samples from one HIV-1 patient with a low HIV-1 RNA load (A) and another patient with a high HIV-1 RNA load (B) were fractionated by density in iodixanol gradients. The density of each gradient is given on the x axis. The HIV-1 p24 antigen levels were measured by enzyme-linked immunosorbent assay using the HIV-1 p24 Antigen Capture kit (Science Applications International Corporation, NCI-Frederick, Frederick, MD) in each fraction, and the values are represented by purple bars given on the y axis on the right side in pg/ml. The HERV-K (HML-2) gag and env and the HIV-1 gag RNA titers (lines) were assessed by real-time RT-PCR (viral load is indicated on the y axis on the left side). HERV-K (HML-2) viral proteins were detected by Western blotting of protein extracts from each fraction. A protein size ladder is included at the left of each blot. (C) Transmission electron micrograph of a viral particle visualized in a plasma fraction at a density of 1.16 g/ml obtained from the patient in panel A. Arrows indicate viral structures, such as the viral membrane, the capsid, and glycoprotein spikes. (D to Q) Viral particles found in the plasma fraction at a density of 1.16 g/ml were visualized by EM. (D to M) Immuno-EM images of particles labeled with an antibody specific to HERV-K Env (D to I) or Gag (J to M) and a secondary antibody linked to 5-nm (D) or 10-nm (E to M) gold particles. All viral particles detected have a size of ∼100 nm, as expected for HERV-K. Some particles (E and I) show condensed cores and spikes symmetrically distributed around the viral membrane, suggesting mature viruses. Immunolabeled HERV-K VLPs were detected in preparations from both breast cancer (D and E) and HIV-1-infected (F to M) patients. Some condensed cores, presumably unenveloped viruses resulting from the preparation procedure, were immunolabeled with an antibody specific for Gag and had a size of ∼70 nm (J to M). (N to Q) No clustering of gold particles was observed in the negative controls in which the same preparations were labeled with purified mouse IgG antibody and detected with a 5-nm gold particle-labeled secondary antibody.