An antibody response against structural proteins of porcine reproductive and respiratory syndrome (PRRS) virus after the experimental infection

PRRS is one of the most economically important viral diseases in swine industry. The genome of the causative virus encodes seven structural proteins, namely, GP2a, GP2b, GP3, GP4, GP5, M and N. Although the antibody response against N protein after the infection has been well investigated using an available kit, the antibody response against other structural proteins is yet to be evaluated. Here, except for GP2b, we evaluated by an immunofluorescence assay-based method the antibody response against each structural protein after an experimental infection.

The genes encoding ORF2-ORF7 in the field-isolated PRRSV (INP-002; American type belongs to Cluster Ⅲ) were cloned into an expression vector and transfected into HEK293 cells. Eight piglets at 35 days of age were nasally infected with the above PRRSV. The serum sample was collected at week 0, 2, 3 and 4 post infection (p.i.). The serum samples were serially diluted from 20 to 1,280-fold and incubated with the HEK293 cells expressing each structural protein. The antibody reacting to the structural protein was labeled with FITC-conjugated secondary antibody and detected by a flow cytometer. The copy number of virus RNA in the serum was determined by real-time PCR.

A strong antibody response (≥1280-fold dilution of the serum) against N protein was detected in six of eight piglets from week-2 p.i. and it was detected from week-3 p.i. in the remaining piglets. The responses against other structural proteins were detected in most piglets from week-3 p.i. except for the one against GP4. The antibody against GP4 was detected only at week-4 p.i. in seven piglets but the antibody titre in five of them was very low (≤40- fold dilution). The antibody response against GP2a was observed only in three piglets. In two piglets, the response against GP5 was not observed.

Nonetheless, the viral RNA was decreased in these two piglets in the same manner as with the other piglets. Half of the piglets showed a strong response against M protein from week-3 p.i., while the remaining half at week-4 p.i.. The copy number of viral RNA at week-4 p.i. was markedly smaller in the piglets showing an earlier response against M protein compared with the others showing a later response.

Antibodies against GP2a and GP4 seem to be not always produced upon PRRSV infection. The antibody against GP5 is one of the major antibodies induced by infection but it is seemingly not a unique antibody possessing a neutralizing activity. It is of interest that piglets quickly producing the antibody against M protein showed a quicker decrease of viral RNA in serum. The antibody against M protein may play a role in the clearance of PRRSV.

Disclosure of Interest: None Declared.