Rapid and efficient clearance of blood-borne virus by liver sinusoidal endothelium

Abstract

The liver removes quickly the great bulk of virus circulating in blood, leaving only a small fraction to infect the host, in a manner characteristic of each virus. The scavenger cells of the liver sinusoids are implicated, but the mechanism is entirely unknown. Here we show, borrowing a mouse model of adenovirus clearance, that nearly all infused adenovirus is cleared by the liver sinusoidal endothelial cell (LSEC). Using refined immunofluorescence microscopy techniques for distinguishing macrophages and endothelial cells in fixed liver, and identifying virus by two distinct physicochemical methods, we localized adenovirus 1 minute after infusion mainly to the LSEC (∼90%), finding ∼10% with Kupffer cells (KC) and none with hepatocytes. Electron microscopy confirmed our results. In contrast with much prior work claiming the main scavenger to be the KC, our results locate the clearance mechanism to the LSEC and identify this cell as a key site of antiviral activity.

Figure 1. Clearance of rAd5 from mouse blood circulation.

We infused by tail vein 3 different doses (1.6×10¹¹, 1.6×10¹⁰ and 1.6×10⁹ DRP) of rAd5, and then measured virus clearance from retroorbital sinus blood by qPCR assays, estimating the viral blood concentration at zero time as the dose divided by the blood volume. Panel A: Time course of blood virus concentration. The curve describes the mean percent of dose ± SD cleared over time using data averaged from all 3 administered doses as they were not statistically different. Since the blood concentrations of virus after all three doses decreased in a biphasic fashion, a biexponential decay model was employed to fit the viral concentration-time curves. The insert graph displays the log of blood virus concentration vs time for each of the 3 administered doses. Each data point represents mean ± SD of three mice. Panel B: Time course of blood virus concentration including “early time points” after the dose of 1.6×10¹⁰ DRP. Mice of a separate cohort were used to characterize precisely the viral clearance during 0–5 min period. In order to generate a complete concentration-time profile of viral clearance up to 30 min, the concentrations at later time points (10, 20, and 30 min), previously obtained (Panel A insert, 1.6×10¹⁰), were normalized to the mean concentration at 5 min using the concentration-decay ratio among time points. The complete curve also followed a biexponential decay model. Each data point represents mean ± SD of three mice.

Figure 2. Three-color immunofluorescence images showing LSEC and KC markers in mouse liver.

The top row of 4 panels shows LSEC markers, with mab 2.4G2 (green) identifying RIIb in the first column and anti-MR (red) in the second column. The middle row shows KC markers with anti-CD68 (green) in the first column and anti-F4/80 (red) in the second column. The bottom row illustrates the relative proportion of LSEC (labeled with anti MR, first column) and KC (labeled with anti-F4/80, second column). Column 3 shows merged images of the first two columns. Column 4 shows the merged color images plus differential interference contrast image (DIC) and DAPI staining of nuclei (blue). The bars in the panels of column 4 indicate 10 µm. Movies of 3D reconstructions of similar images, illustrating the distribution and abundance of LSEC and KC, are shown in Supplementary Videos S2 and S3, respectively.

Figure 3. rAd5 labeled with antibody or Cy3 localize predominantly to LSEC.

Two sets of 5 images are shown, each set from a separate 4-color fluorescence study in which the identifying markers were varied. Livers from mice infused 1 minute earlier with either 1.6×10¹¹ rAd5 particles (left set) or 10¹¹ Cy3-labeled rAd5 particles (right set) were fixed, sectioned (5 um), and examined by 4-color fluorescence microscopy using mab anti-F4/80 to label KC; rabbit anti-Ad5 or Cy3 to identify virus, anti-RIIb mab 2.4G2 or anti-MR to identify LSEC; and DAPI to label nuclei. Images were collected with an Olympus FluoView 1000 Laser Scanning Confocal microscope equipped with a spectral detection system (FV 1000 spectra). Representative ∼700 nm optical sections showing typical labeling patterns are presented here. A. Magenta color delineates the KC. B. Red puncta identify rAd5 particles. C. Green mab 2.4G2 and rabbit IgG anti-MR mark LSEC. D. Merged images of A, B, and C. E. Merged panels A, B, C with DAPI showing cell nuclei (blue) plus DIC defining tissue structure including sinusoidal lumens. The bars in panels E signify 10 µm. Videos of 3D projections from similar images are shown in Supplementary Videos S1–S8.

Figure 4. Cy3-labeled rAd5 localize predominantly to LSEC.

Livers from mice infused 1 minute earlier with 10¹¹ Cy3-rAd5 particles were fixed, sectioned (5 µm), and examined by 4-color fluorescence microscopy using mab anti-F4/80 to label KC; anti-MR to identify LSEC; and DAPI to label nuclei. Cy3 fluorescence identified virus. Images were collected and presented as described in legend to Fig. 3. A. Magenta color delineates the KC. B. Red puncta identify Cy3-rAd5 particles. C. Rabbit IgG anti-MR mark LSEC. D. Merged images of A, B, and C. E. Merged panels A, B, C with DAPI showing cell nuclei (blue) plus DIC defining tissue structure including sinusoidal lumens. The bars in panels D signify 10 µm. Videos of 3D projections from similar images are shown in Supplementary Videos S1–S8.

Figure 5. Quantification of rAd5 association with LSEC and KC.

We assessed the whereabouts of rAd5 in 4-color fluorescence images like the representative ones shown in Fig 3, Fig 4 and Supplementary Figs S1 and S2, employing 4 combinations of complementary markers to identify cells and virus. By counting total virus color (pixel area x mean intensity) associated with both LSEC and KC, we found that 83–96% of virus was associated with LSEC and 4–17% with KC and none with hepatocytes, as shown in the bar graphs as mean ± SD for each of 4 different experiments. Data are graphed for both strategies for labeling virus, both Cy3-labeling (Cy3) and labeling with anti-Ad5 antibody (Ab). Two different markers identified both LSEC and KC. The table below the Fig illustrates that the 4 experiments were comparable by several parameters including relative fluorescence intensity scored RFI, numbers of mice used (mice), numbers of KC and sinusoid lumens scored (lumens), number of images examined, and the area of tissue examined microscopically (µm²).