Abstract
The present study characterized the homologous and heterologous immune response in type-I porcine reproductive and respiratory syndrome virus (PRRSV) infection. Two experiments were conducted: in experiment 1, eight pigs were inoculated with PRRSV strain 3262 and 84 days post-inoculation (dpi) they were challenged with either strain 3262 or strain 3267 and followed for the next 14 days (98 dpi). In experiment 2, eight pigs were inoculated with strain 3267 and challenged at 84 dpi as above. Clinical course, viremia, humoral response (neutralizing and non-neutralizing antibodies, NA) and virus-specific IFN-γ responses (ELISPOT) were evaluated all throughout the study. Serum levels of IL-1, IL-6, IL-8, TNF-α and TGF-β were determined (ELISA) after the second challenge. In experiment 1 primo-inoculation with strain 3262 induced viremia of ≤ 28 days, low titres of homologous NA but strong IFN-γ responses. In contrast, strain 3267 induced longer viremias (up to 56 days), higher NA titres (≤ 6 log2) and lower IFN-γ responses. Inoculation with 3267 produced higher serum IL-8 levels. After the re-challenge at 84 dpi, pigs in experiment 1 developed mostly a one week viremia regardless of the strain used. In experiment 2, neither the homologous nor the heterologous challenge resulted in detectable viremia although PRRSV was present in tonsils of some animals. Homologous re-inoculation with 3267 produced elevated TGF-β levels in serum for 7–14 days but this did not occur with the heterologous re-inoculation. In conclusion, inoculation with different PRRSV strains result in different virological and immunological outcomes and in different degrees of homologous and heterologous protection.
Highlights
One of the main obstacles for the development of new vaccines of greater efficacy against porcine reproductive and respiratory syndrome virus (PRRSV) is the limited understanding of the mechanisms involved in protection [1,2,3,4]
The present study shows that two different PRRSV isolates of genotype I subtype I can produce two different models of infection based on the clinical, virological and immunological parameters examined
Both models were absent of very overt clinical signs, in one of them fever and the decrease in weekly weight gains were very mild compared to the other
Summary
One of the main obstacles for the development of new vaccines of greater efficacy against porcine reproductive and respiratory syndrome virus (PRRSV) is the limited understanding of the mechanisms involved in protection [1,2,3,4]. Most studies have focused in the development of neutralizing antibodies (NA) and to virus-specific interferon-γ secreting cells (IFN-γ-SC) as the main correlates of protection [5,6,7,8,9,10] the precise role. It is worth to note that after a careful review of the available scientific literature, there are very few characterized models of homologous/heterologous challenge considering simultaneously potential correlates of protection (NA and IFN-γ-SC), the development of clinical signs and the virological outcome of the challenge model. We characterized the clinical and virological course and the evolution of neutralizing antibodies and interferon responses after inoculation with two PRRSV strains previously reported to be different [15,16]. We evaluated the neutralizing and IFN-γ-SC responses against a heterologous strain after immunization and we tested the immunological responses after the homologous and heterologous challenges of previously immunized pigs
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