Isolation and characterization of three subpopulations of IgG in the common cat (Felis catus)

Although the cat is an important model for a number of human diseases little is known of its basic humoral immunity. In this study we have isolated three subpopulations of IgG from cat serum by DEAE anion-exchange chromatography and protein A affinity chromatography. Subpopulation 3 did not bind to DEAE equilibrated in 2 mM phosphate buffer pH 8.0 while subpopulations 1 and 2 were eluted, as a mixed population, with a salt gradient between 3 mM and 25 mM. When this fraction was loaded onto protein A-Sepharose two distinct peaks were always generated in a pH gradient (pH 8.0-2.1). The first of these, subpopulation 2, was eluted on average at pH 4.3 while the second, subpopulation 1, was eluted on average at pH 3.4. Subpopulations 1 and 2 had similar immunoelectrophoretic mobilities and isoelectric points while subpopulation 3 was more cathodic in both tests. Antisera produced in both mice and rabbits were rendered specific, by absorption of cross-reacting antibodies, for determinants on the heavy chain of each subpopulation as shown by Western blotting. The data suggest, but are not conclusive, that each of these subpopulations are distinct subclasses of IgG.