 2550;5(1):17-28. Address request for reprints: Wasin Charerntantanakul, Department of Biology, Faculty of Science, Maejo University, Chiang Mai 50290, Thailand; E-mail:wasin@mju.ac.th Article received date: November 9 th 2006. Original article EVIDENCE FOR IMMUNOSUPPRESSIVE PROPERTIES OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS Wasin Charerntantanakul Department of Biology, Faculty of Science, Maejo University, Chiang Mai, Thailand Abstract This study reviews evidence for immunosuppressive properties of porcine reproductive and respiratory syndrome virus (PRRSV). PRRSV induces poor innate and adaptive immune responses after infection. Innate immune response to PRRSV was reduced as determined by phagocytosis, production of reactive oxygen species, bacteri- cidal activity, and production of innate cytokines by pulmonary alveolar macrophages and pulmonary intravascular macrophages. Humoral immune response to PRRSV was weak and delayed as compared to humoral immune response to other swine pathogens. The humoral immune response of PRRSV-infected pigs to other swine pathogens was sig- nificantly reduced during acute PRRSV infection. Cell-mediated immune response to PRRSV was weak and much delayed. The mechanisms of PRRSV affecting immune responses are not clearly understood. Chiang Mai Veterinary Journal 2007;5(1):17-28. Keywords: Immunosuppression, porcine reproductive and respiratory syndrome virus, pig Porcine reproductive and respiratory syndrome virus (PRRSV) is a small (approxi- mately 0.5 μM in size), positive-sense, single- stranded, enveloped RNA virus of the genus Arterivirus, family Arteriviridae. (1) The viral genome is approximately 15 kilobase in size and composed of 9 open-reading frames (ORF), designated ORF1a, 1b, 2a, 2b, and 3-7. (1) The virus infects and replicates in myelomonocytic cell lineages, including monocytes, macroph- ages, and dendritic cells. (2-5) The virus causes diseases in pigs of all ages. Neonatal and nursery pigs may experience high fever, anorexia, chemosis, conjunctivitis, and failure- to-thrive. (6-8) Grow to finish pigs may experi- ence respiratory disease that can vary from subclinical to fatal pneumonia. (6-9) Breeding pigs may demonstrate late-term abortions, increased numbers of stillborn pigs, mummified fetuses, weak live-born piglets, and delayed return to estrus. (10-14)