Expression patterns in feline blood and tissues of α-acid glycoprotein (AGP) and of an AGP-related protein (AGPrP).

α-Acid glycoprotein (AGP) is an acute-phase protein (APP) that modulates immune responses, probably - at least in humans - owing to the modification of its glycosylation pattern. On this perspective, feline AGP can be a useful comparative model, as it has different concentrations in cats susceptible or resistant to some disease. As a preliminary approach to the study of feline AGP (fAGP) we have purified this protein from feline serum by HPLC using human AGP (hAGP) as a model. Immunoblotting with a polyclonal antibody against fAGP and with a monoclonal antibody against hAGP was performed on serum from healthy cats, from cats exposed to feline coronavirus (FCoV) infection and from cats with purulent inflammations, such as feline infectious peritonitis (FIP), feline immunodeficiency virus (FIV) and feline leukemia virus (FeLV). Immunohistochemistry on tissues from healthy cats and from cats with different diseases (FIP, FIV, FeLV, locally extensive inflammation) was also performed with the same antibodies. Both hAGP and fAGP have been purified to homogenity as determined by SDS-PAGE. fAGP did not react with the anti-hAGP antibody which, in contrast, detected in feline serum a low MW protein that we called fAGP-related protein (fAGPrP). This protein was underexpressed in cats with FeLV and FIP. Both fAGP and fAGPrP were immunohistochemically detected in plasma and hepatocytes with a stronger intensity in cats with FIP and some inflammatory conditions. Moreover, fAGPrP was detected in the cytoplasm of tissue cells, most likely identifiable with plasma cells. These cells were rarely detectable in cats with FIV and FeLV, and numerous in cats with FIP and with locally extensive inflammation. In conclusion, purified fAGP has physicochemical characteristics similar to those of hAGP, but does not cross-react with anti-hAGP antibodies. In contrast, the anti-hAGP detected an AGP-related protein whose blood concentration and tissue distribution was not related to that of fAGP. Moreover, both fAGP and fAGPrP were differently expressed in cats with pathologic conditions compared to controls. Further study of these proteins by analysing their structural characteristics is required.