Correlation between B-cell mitogenicity and immunosuppressor effects of a protein released by porcine monocytes infected with African swine fever virus.

Virus-free supernatants of cultured swine monocytes infected by African swine fever virus (ASFV) suppressed in vitro proliferation of porcine and human blood mononuclear cells in response to phytohemagglutinin and the in vivo primary immune response of C57BL/6 mice against sheep RBC. The supernatants were fractionated by discontinuous ion-exchange chromatography and subfractionated by double-step preparative isoelectric focusing. The pool of the most purified active subfractions (F5'EP-ASFV) is made up of heat-unstable material, can be stained by silver nitrate, and has an isoelectric point of 3.88, a maximal optical density at 280 nm, and a mass of 36,000 daltons. In vivo kinetic studies in nonimmunized C57BL/6 mice were performed on days 1, 2, 3, 5, and 7 after injection with 50 micrograms of F5'EP-ASFV protein. Compared with the untreated mice, the treated mice had a noticeable increase in nonspecific immunoglobulin-secreting splenic plaque-forming cells (PFC) with the following isotype profile: IgG2a greater than IgG2b greater than IgG3 greater than IgG1 congruent to IgM. Three days after treatment with the active material, specific IgM PFC against sheep RBC increased up to 23-fold. In C57BL/6 mice immunized against sheep RBC 2 days after treatment with F5'EP-ASFV, the increase in nonspecific PFC was followed by a suppression of specific PFC response in the respective isotype. When C57BL/6 mice were treated after priming with sheep RBC, however, there was little or no suppression of specific PFC and the increase in nonspecific PFC was considerably lower than that in the other F5'EP-ASFV-treated mice. In this case, kinetic curves of specific vs nonspecific PFC of each isotype were mirror images. Mice treated with 200 micrograms of F5'EP-ASFV protein died with hemorrhagic diastasis.