59Some of the interactions between the porcine reproductive and respiratory syndrome virus (PRRSV) and the porcine interferon-a (IFN-a) system were studied. In a first experiment, it was shown that pretreatment of primary porcine alveolar macrophages (AMs) with recombinant porcine (rPo) IFN-a I resulted in significant reductions ofPRRSVyield and numbers of antigen expressing cells. In a second experiment, sensitivity of PRRSV to IFN-a was confirmed in vivo. In pigs inoculated with porcine respiratory coronavirus (PRCV)-a potent inducer of endogenous IFN-a in the lungs of pigs-followed 2 days later by PRRSV-Iung PRRSV titers were 1.7 to 2.9 loglo TCID so reduced compared to those in singly PRRSV inoculated pigs. It was concluded therefore that PRRSV has a fairly good sensitivity to the antiviral effects of IFN-a. A third experiment documented that in vivo PRRSV infection generally does not affect PRCV-induced IFN-a production in the lungs of pigs. In addition, it was shown that the IFN-inducing capacity ofPRRSV is at least 159 times lower than that of PRCV. This finding suggests that cells other than AMs may be responsible for IFN production in the lungs of pigs.

In the present study, the outcome of an inoculation of equine peripheral blood mononuclear cells (PBMC) with equine herpesvirus type 1 (EHV-1) was studied in vitro. Cytoplasmic and plasma membrane expression of viral antigens, intra-and extracellular virus titres, and plaque formation in co-culture were determined. EHV-1 replicated in monocytes, although in a highly restricted way. Viral antigens were found at maximum levels (8n7 % of the monocytes) at 12 h post-infection. The infection was productive in 0n16 % of the monocytes. The virus yield was 10 0n7 TCID 50 per productive cell. In a population of resting lymphocytes, 0n9 % of cells were infected and less than 0n05 % produced infectious virus. After prestimulation with different mitogens, the number of infected lymphocytes increased four to twelve times. The susceptible lymphocytes were T-lymphocytes. In mitogenstimulated lymphocytes, clear expression of viral antigens was found on the plasma membrane.Equine herpesvirus type 1 (EHV-1), a member of the Alphaherpesvirinae, is a major pathogen of horses, responsible for respiratory disorders, abortion, neonatal foal disease and neurological disorders. Starting from 4-6 days after EHV-1 infection, an extensive cell-associated viraemia is detected, which lasts until 9-14 days after infection (Gibson et al., 1992). T-lymphocytes seem to be the most susceptible of the peripheral blood mononuclear cells (PBMC) (Scott et al., 1983). Viraemia is associated with, for example, T-cell lymphopenia and appearance of blastic cells (McCulloch et al., 1993) and may occur in the presence of virus-neutralizing antibodies (Doll & Bryans, 1963 ;Mumford et al., 1987). Carried by infected leukocytes, EHV-1 spreads to internal organs.Information on the interaction between EHV-1 and leuko- et al. (1983). In vitro-infected, non-stimulated PBMC gave a higher number of plaques than mitogen-stimulated PBMC, in contrast to the results obtained in vivo.The main purpose of this study is to obtain more detailed information about the replication of EHV-1 in freshly isolated, equine PBMC and to investigate the effect of mitogen stimulation on the replication kinetics of EHV-1 in lymphocytes.PBMC were isolated by density centrifugation of heparinized blood from adult, infection-immune horses on Ficoll-Paque and afterwards separated into two subpopulations by plasma-mediated adhesion as described by Nauwynck & Pensaert (1994). Adherent cells consisted predominantly of monocytes and non-adherent cells consisted predominantly of lymphocytes. Remaining monocytes in the lymphocyte-enriched population were removed by plastic adhesion during 1 h at 37 mC. The composition of the PBMC and subpopulations was determined by flow cytometry (FACSCalibur ; Becton Dickinson) using MAbs HB88A and DH59B (VMRD) to stain T-lymphocytes and monocytes, respectively (Tumas et al., 1994), and a hyperimmune goat serum against horse IgM (Kirkegaard and Perry Laboratories) to stain B-lymphocytes. In the population of PBMC, the percentages of T-lymphocytes, B-lymphocytes...