Selective induction of apoptosis in antigen-presenting cells in mice by Parapoxvirus ovis

Abstract

Viruses have evolved numerous mechanisms to avoid host immune reactions. Here we report a mechanism by which Parapoxvirus ovis (PPVO) interferes with antigen presentation. PPVO (orf virus) causes orf, an acute skin disease of sheep and goats worldwide. Importantly, PPVO can repeatedly infect its host in spite of a vigorous inflammatory and host immune response to the infection. We demonstrate in a mouse system that PPVO induces apoptosis in a significant number of antigen-presenting cells after intraperitoneal injection using the CD95 pathway, thus preventing a primary T-cell response. We also show that PPVO induces a compensatory activation of the immune system. Our results may help to explain the phenomenon that natural PPVO infections in sheep occur repeatedly even after short intervals. They also suggest that the combination of immunosuppressive and immunostimulatory mechanisms is an effective survival strategy that might be used in other viruses as well.

FIG. 1

PPVO leads to disappearance of monocytes/macrophages in peritoneal lavages 6 h after intraperitoneal injection (A). The number of cells relative to the placebo (100%) is indicated. In contrast to placebo-treated (B, panel 1) or mock-treated (B, panel 2) animals, monocytes/macrophages (R3) disappear from the application site in PPVO-treated animals (B, panels 3 [active PPVO] and 4 [inactivated PPVO]). A significant amount of granulocytes (R2) accumulates 6 h after application at the site of application of PPVO and in CFA- (B, panel 5) but not in mock- or PBS-treated animals. No changes are observed in lymphocytes (R1) at this time. The data that are shown are typical examples for six animals per group and time point. ∗, P < 0.05; ∗∗, P < 0.001.

FIG. 2

The induction of apoptosis does not depend on infection or viral replication. Administration of either active or inactivated PPVO leads to a four- to sixfold increase of the number of apoptotic cells 6 h after administration at the site of injection (six mice per group and time point). The total number of annexin V-positive monocytes/macrophages in mock-treated animals was set at 1. ∗, P < 0.05.

FIG. 3

Priming of naive T cells is absent in PPVO-treated mice. BALB/cJ mice (eight per group) were injected subcutaneously with PBS, PPVO, or CFA, each mixed with 10 μg of OVA/20 μl at the base of the tail to provoke hypersensitivity. Seven days later, OVA was injected intradermally into the right hind foot. After another 48 h, the specific foot swelling was measured. To correct for the different individual sizes of feet, the diameter from the unchallenged left hind foot was taken and subtracted from the right measurement. n.s., not statistically significant.

FIG. 4

CD95 mRNA expression is induced in peritoneal lavage cells of PPVO-treated (P < 0.01) and CFA-treated (P < 0.01) animals and marginally in placebo-treated animals 6 h after administration. In contrast, after 12 h, CD95 mRNA expression is detectable only in animals treated with PPVO (P < 0.0001) (A). CD95L mRNA expression is induced in peritoneal cells in all treatment groups. However, mRNA expression declines to nearly undetectable levels after 24 h in placebo-treated animals but remains high in PPVO-treated animals and detectable in CFA-treated animals (B).

FIG. 5

The CD95/CD95L pathway plays the dominant role in the PPVO-mediated apoptosis of monocytes/macrophages. The mice (six/group) were treated with placebo, mock supernatant, or PPVO and were analyzed 6 h later. In MRL/MpJ-Fas^lpr mice, we found a reduction in the population of monocytes/macrophages of about 25% after PPVO treatment, whereas the number of monocytes/macrophages was reduced 68.8% in wild-type mice in comparison to the placebo control (P < 0.05).

FIG. 6

The local suppression of the immune response is systemically compensated. BALB/cJ mice were treated with PPVO or placebo. After 16 h, anti-CD95L antibodies were administered and the mice were infected with a lethal dose of herpesvirus. Eighty percent of the placebo-pretreated mice died after 12 days. In contrast, PPVO pretreatment led to significantly prolonged survival (n = 10; P < 0.05), indicating a systemic stimulatory effect that occurs about 1 day after PPVO exposure. Blocking the CD95/CD95L pathway of apoptosis did modulate the course of herpes and led to a higher survival rate in these mice (not statistically significant). However, when pretreated with PPVO, 80% of the animals survived (P < 0.05).