Differential cytokine mRNA expression induced by binding of virulent and avirulent molecularly cloned equine infectious anemia viruses to equine macrophages

Equine infectious anemia virus (EIAV) causes rapid development of acute disease followed by recurring episodes of fever, thrombocytopenia and viremia, termed chronic EIA. Most infected horses control the virus by immune mechanisms and become inapparent carriers. To further our understanding of the equine immune response to EIAV, a multi-probe ribonuclease protection assay (RPA) was developed to quantitate equine-specific cytokine mRNAs. Eleven template plasmids specific to ten equine cytokine genes and the ?-actin gene were generated, from which radiolabeled anti-sense RNA probes were produced. The RPA simultaneously quantitated mRNA levels of interleukin (IL)-1?, IL-1?, IL-6, IL-8, IL-10, IL-12 p35, IL-12 p40, interferon (IFN)-?, transforming growth factor (TGF)-1? and tumor necrosis factor (TNF)-? in equine peripheral blood mononuclear cells and equine monocyte-derived macrophages (EMDM). The assay detected as few as 5?105 RNA molecules and displayed coefficients of variation of 0.03-0.08 when normalized to ?-actin expression. Using this RPA, cytokine expression in EMDM infected with 2 molecularly cloned viruses (EIAV17 and EIAV19) was determined. EIAV17 varies from EIAV19 only in env, rev and LTR and causes fatal disease in Shetland ponies. When added to EMDM cultures, virulent EIAV17 stimulated expression of IL-1?, IL-1?, IL-6, IL-10 and TNF-?. These cytokine mRNAs were significantly elevated by 0.5 to 1 hr post infection (hpi) and returned to basal levels by 12 to 24 hpi, indicating modulation by early event(s), such as receptor binding. In contrast to EIAV17, EIAV19 is avirulent in vivo and failed to induce any of the tested cytokines in EMDM. These data show a direct correlation between the virulence of the EIAV clone and the induction of cytokines. The cytokines stimulated by EIAV17 may contribute to EIA-associated symptoms, enhance viral replication in the host, and regulate the host immune response. To determine whether cytokine induction requires EIAV17 replication, EMDM cultures were exposed to UV-inactivated EIAV17 and cytokine induction was monitored. UV-inactivation did not block cytokine induction by EIAV17, suggesting dispensability of viral replication. Given that EIAV17 induces cytokines in a rapid and replication-independent manner, the activation of cytokine expression is likely mediated by binding of EIAV17 to equine macrophage receptor(s).