Downregulation of robust acute type I interferon responses distinguishes nonpathogenic simian immunodeficiency virus (SIV) infection of natural hosts from pathogenic SIV infection of rhesus macaques

Abstract

The mechanisms underlying the AIDS resistance of natural hosts for simian immunodeficiency virus (SIV) remain unknown. Recently, it was proposed that natural SIV hosts avoid disease because their plasmacytoid dendritic cells (pDCs) are intrinsically unable to produce alpha interferon (IFN-alpha) in response to SIV RNA stimulation. However, here we show that (i) acute SIV infections of natural hosts are associated with a rapid and robust type I IFN response in vivo, (ii) pDCs are the principal in vivo producers of IFN-alpha/beta at peak acute infection in lymphatic tissues, and (iii) natural SIV hosts downregulate these responses in early chronic infection. In contrast, persistently high type I IFN responses are observed during pathogenic SIV infection of rhesus macaques.

FIG. 1.

In vivo IFN-α/β protein expression in pDCs during acute SIV infection of RMs, SMs, and AGMs. (A) Immunohistochemical staining (1,000×) of IFN-α (top panels) and IFN-β (bottom panels) in lymph node sections from animals 14 dpi. Arrows identify examples of IFN-α expression in cells with a clear plasmacytoid morphology, putative pDCs. Tissues were stained with antibodies to IFN-α (clones MMHA-2 and MMHA-3) or IFN-β (MMHB-16) (all from PBL InterferonSource) using the biotin-free MACH-3 polymer-ALP system (Biocare Medical), developed with Vulcan Fast Red (Biocare Medical) and counterstained with hematoxylin. (B) Immunofluorescent staining (400×) shows that CD123⁺ pDCs (green) in SMs at 14 dpi do in fact express IFN-α protein (red), as the merged images clearly demonstrate colocalization (yellow).

FIG. 2.

Rapid and robust LN type I IFN responses in SIV-infected RMs, SMs, and AGMs that are promptly resolved in SMs and AGMs but not RMs. (A) Representative high-magnification (400×) images from the T-cell zone immunohistochemically stained for MxA, a specific type I IFN-responsive gene in RMs, AGMs, and SMs prior to and during the acute and chronic stage of SIV infection. (B and C) Quantitative image analysis of high-resolution whole-lymph-node scans immunohistochemically stained for MxA (left panels) and ISG15 (right panels) from SIV-infected RMs (black symbols), SMs (blue symbols), and AGMs (red symbols). Archived tissue sections were from three previous acute infection studies, in which we had collected lymph node biopsy specimens that spanned from preinfection time points through 30 days postinfection and into the chronic stage longitudinally (Table 1) (5, 9, 11, 15). In addition, we used cross-sectional tissue samples collected at various times during the chronic stage and from SIV-negative animals to add to our original data set (Table 1). Quantitative image analysis was performed on immunohistochemically stained, high-resolution whole-lymph-node tissue scans obtained from an Aperio ScanScope CS (Aperio Technologies, Inc.) to measure the protein expression levels of both MxA (anti-MxA antibody M143 generously provided by Georg Kochs, Universitätsklinikum Freiburg) and ISG15 (Sigma Prestige antibodies powered by Atlas antibodies) and shown kinetically for each nonhuman primate species (B) or at each time point showing comparative expression levels for all the species (C). Each circle represents a quantified lymph node section. For some animals there were multiple lymph nodes collected and measured and these are thus represented as separate independent data points. Additional lymph nodes from individual animals are indicated by unfilled plot symbols. Mann-Whitney nonparametric U tests were performed using Prism 4.0 software (Prism, San Diego, CA) with resulting P values shown.