Expression of Interleukin-10 and Interleukin-12 in Piglets Experimentally Infected with Porcine Reproductive and Respiratory Syndrome Virus (PRRSV)

Abstract

The expression of mRNA encoding interleukin-10 (IL-10) and IL-12 was studied, by the reverse transcription-polymerase chain reaction and by in-situ hybridization with a non-radioactive digoxigenin-labelled cDNA probe, in formalin-fixed, paraffin wax-embedded lung tissue from piglets inoculated intranasally with a Korean isolate (North American genotype) of porcine reproductive and respiratory syndrome virus (PRRSV). IL-12p35-positive cells were detected in the lung at 1 day post-inoculation (dpi), their number increasing at 5 dpi, and rapidly decreasing thereafter. In contrast, IL-10- and IL-12p40-positive cells were detected in the lung at 1 dpi, their number increasing at 3 dpi, and rapidly decreasing thereafter. Hybridization signals for IL-10, IL-12p35 and IL-12p40 were always associated with inflammation. Expression of these cytokines was minimal in non-lesional lung of PRRSV-infected piglets and in normal lung from control piglets. In-situ hybridization in serial sections of lung tissues indicated close co-localization of PRRSV and these cytokines in interstitial pneumonia. The results suggest that the expression of IL-10 and IL-12 plays a role in pulmonary defence mechanisms against PRRSV infection.

Introduction

Porcine reproductive and respiratory syndrome (PRRS), caused by PRRS virus (PRRSV), is characterized by reproductive failure due to abortion, stillbirth and fetal mummification and by severe respiratory disease in newborn and nursing pigs (Cheon et al., 1997a, Cheon and Chae, 1999, Cheon and Chae, 2000b, Cheon and Chae, 2001a). PRRSV, a member of the genus Arterivirus (family Arteriviridae, order Nidovirales), has a positive single-stranded polyadenylated RNA molecule approximately 15 kD in length that contains eight open reading frames (ORFs) (Conzelmann et al., 1993, Meulenberg et al., 1993, Cavanagh, 1997). There are two distinct genotypes of PRRSV: the North American and European genotypes (Meng et al., 1995). All Korean PRRSV isolates were identified as North American genotypes (Cheon and Chae, 2000a, Chung et al., 2002). PRRSV is endemic in Korea (Cheon et al., 1997b).

Interleukin-10 (IL-10) was originally identified as a cytokine synthesis inhibitory factor generated by T helper 2 (Th2) cells acting upon Th1 cells (Moore et al., 1993). IL-12 is a heterodimeric 70 kD (p70) cytokine composed of two covalently linked glycosylated 40 kD (p40) and 35 kD (p35) chains (Gubler et al., 1991) which are produced by monocytes, macrophages, B lymphocytes and other antigen-presenting cells in response to microbial pathogens (Trinchieri, 1995, Stern et al., 1996, Gately et al., 1998). IL-12 appears to be a requisite for generating optimal Th1 responses and to play a important role in promoting cell-mediated immunity against microbial pathogens (Trinchieri, 1995, Stern et al., 1996, Gately et al., 1998). Thus, IL-10 and IL-12 regulate the balance between Th1 and Th2 cells (Moore and O'Garra, 1993, Trinchieri, 1995, Stern et al., 1996, Gately et al., 1998).

Macrophages play a central role in resistance to, and recovery from, many viral infections (Heise and Virgin, 1995). PRRSV primarily infects macrophages (Cheon et al., 1997a, Cheon and Chae, 1999). Production of proinflammatory cytokines by virus-infected macrophages is a prelude to activation of the effector cells of innate resistance. IL-12 plays a major role in the pathogenesis of viral diseases (Heise and Virgin, 1995, Malmgaard et al., 2000), and it induces the production of large amounts of interferon (IFN)-γ from resting and activated T and natural killer cells (Gazzinelli et al., 1993, Trinchieri, 1995, Stern et al., 1996, Gately et al., 1998). The secretion of IFN-γ by these cells contributes to the control of acute infections caused by viruses in general (Smith et al., 1994, Cantin et al., 1995).

The promotion of both Th1 maturation and enhancement of cytotoxic T lymphocytes and natural killer cell activity by IL-12 is likely to be important for its ability to protect against viral infection (Rogerson et al., 1996). The ability, however, of IL-10 to inhibit IL-12 production in monocytes/macrophages has particular interest in the light of the opposite effects of these two cytokines in Th1–Th2 cell differentiation (Aste-Amegaza et al., 1998). Expression of IFN-γ in PRRSV-infected lung tissues (Choi et al., 2002) suggests that IL-12 plays a role in PRRSV infection. However, little is known regarding IL-10 and IL-12 expression in PRRSV infection. Therefore, the objective of this study was to determine, by reverse transcription-polymerase chain reaction (RT-PCR) and in-situ hybridization, the expression of IL-10 and IL-12 mRNA in the lungs of pigs experimentally infected with PRRSV.