Studies on heterologous protection between Japanese type 1 and type 2 porcine reproductive and respiratory syndrome virus isolates

Abstract

The objective of the present study was to evaluate the cross-protective immunity between type 1 and type 2 porcine reproductive and respiratory syndrome virus (PRRSV) isolates in growing pigs. Japanese type 1 PRRSV, first isolated from a pig with respiratory disorders in a farm in 2009, exhibits unique genetic characteristics. The pathogenicity of a Japanese standard strain of type 2 PRRSV, EDRD1, in pigs immunized by the type 1 PRRSV isolate, Jpn EU 4-37 was determined by evaluating clinical signs, viremia, antibody response, and pathological lesions. Similarly, we evaluated the pathogenicity of Jpn EU 4-37 in pigs immunized by EDRD1 and compared the cross-protective immunity between these isolates. The EDRD1 challenge after Jpn EU 4-37 inoculation reduced viral clearance and shedding in pigs, compared to those treated with the EDRD1 single infection. On the other hand, the pathogenicity of Jpn EU 4-37 after EDRD1 infection did not differ significantly compared to non-immunized pigs treated with Jpn EU 4-37. Therefore, exposure to Jpn EU 4-37 could not induce enough immunity to reduce the viremia against subsequent infection by type 2 PRRSV. However, the immunity induced by Jpn EU 4-37 infection may play a role in reducing viremia caused by type 2 PRRSV. Moreover, the immunity induced by the EDRD1 and other genetically related viruses, which are broadly distributed in Japan, may not contribute to cross-protection against Jpn EU 4-37 as an emerging virus.

Keywords: Immunity; pig; porcine reproductive and respiratory syndrome virus.

Fig. 1.

Changes in the rectal temperature of pigs following challenge with virus. Mean rectal temperatures of pigs in the type1 (■), type2/type1 (●), type2 (□), type1/type2 (○), and non-infectious control (▲) groups. Statistically significant differences (P<0.05) are indicated between a) type2/type1 and control groups, b) type1/type2 and control groups, and c) type2 and control groups. Bars represent standard error of the mean.

Fig. 2.

Mean values of the quantity of type 1 (A) and type 2 (B) porcine reproductive and respiratory syndrome virus (PRRSV) RNA in the serum of pigs from type1 (■), type2/type1 (●), type2 (□), and type1/type2 (○) groups. Viral RNA (mean ± SE) content was measured by one-step RT-PCR. † indicates significant difference between groups (P<0.05). Error bars represent standard errors.

Fig. 3.

Mean S/P ratio of ELISA in the serum from type1 (■), type2/type1 (●), type2 (□), type1/type2 (○), and non-infectious control (▲) groups. Error bars represent standard errors.

Fig. 4.

Quantitative detection of type 1 porcine reproductive and respiratory syndrome virus (PRRSV) RNAs in lungs (A) and tonsils (B) collected from type2/type1 and type1 groups at 21 dpi of Jpn EU 4-37. Viral RNA (mean ± SE) content was measured by one-step RT-PCR. There were no significant differences between both organs of the 2 groups.

Fig. 5.

Quantitative detection of type 2 porcine reproductive and respiratory syndrome virus (PRRSV) RNAs in lungs (A) and tonsils (B) collected from type1/type2 and type2 groups at 21 dpi of EDRD1. Viral RNA (mean ± SE) content was measured by one-step RT-PCR. (A) No significant difference in the levels of RNA of EDRD1 in the lungs of both groups was detected. However, viral RNAs of the type1/type2 group tended to be lower than that of the type2 group. (B) A significant difference was observed when comparing the tonsils of both groups (†, P<0.05).

Fig. 6.

Histopathological findings in the lungs collected from type2/type1 (A), type1/type2 (B), type1 (C), and type2 (D) groups. Pathologically, no obvious gross lesions were detected although slight septal inflammatory reactions were observed in the type1/type2 group (B). Discoloration in the cranial and middle lobes and mild to moderate interstitial pneumonia were observed in the type1/type2 (A), type1 (C), and type2 (D) groups.