Temporary CD8+ T-cell depletion in pigs does not exacerbate infection with porcine reproductive and respiratory syndrome virus (PRRSV).

Several studies have demonstrated a consistent increase in the CD8+ T-cell subset of pigs following infection with porcine reproductive and respiratory virus (PRRSV). Consequently, it has been suggested that CD8+ T-cells may play an important role in protection against this infection. In order to test this hypothesis, we examined five 5-week-old pigs, which had been depleted for CD8+ T-cells by treatment with anti-CD8 mAb injections, starting 2 days before inoculation with PRRSV. Virus-inoculated and sham-inoculated age-matched pigs served as controls. Blood samples were collected continuously, together with organ material at necropsy, to study kinetics of leukocyte subpopulations, antibody production and virus persistence in individual pigs. Significant lower CD8+ T-cell counts on day 0, that is, before virus challenge, in the anti-CD8 mAb treated pigs compared to the control pigs, confirmed the depletion effect of specific mAb therapy. Almost complete depletion of cell subsets expressing the CD8+ antigen was obtained on day 2 and 5 post infection (PI) with nadir less than 1% of peripheral blood mononuclear cells (PBMC). One week PI, an increase in T-cell subsets was observed for both anti-CD8 mAb treated pigs and virus-inoculated control pigs. T-memory cells and cytotoxic T-cells reached levels comparative with the virus-inoculated control pigs on days 8 and 12 PI, respectively, whereas NKcells remained suppressed for the rest of the experimental period. An extraordinary increase of T-helper cells in the anti-CD8 mAb treated pigs with a significantly higher level than in the virus-inoculated and sham-inoculated control pigs, was observed from day 12 PI. In conclusion, it was established that CD8+ T-cell depletion in the early phase of PRRSV infection neither caused increased disease nor influenced the ability to clear virus in the treated pigs.