In vitro immunosuppressive effect of serum from orally immunized mice

Mice were orally immunized by 4 daily intragastric (IG) intubations of 4 X 10(9) SHEEP RED BLOOD CELLS (SRBC). The serum of such mice (OS), taken two weeks after the first intubation, was compared to normal mouse serum for its suppressive effect on in vitro primary responses of normal spleen cells to SRBC. OS regularly depressed the in vitro response to less than 10% of the normal response. The suppressive effect was dose-dependent, antigen-specific and removable by absorption with SRBC. When cultures were stimulated with SRBC pre-exposed to OS, the response was also depressed. The suppression was less when the addition of OS was delayed. OS from BALB/c mice efficiently depressed in vitro responses of spleen cells from other strains. Spleen cells, taken 2 weeks after IG immunization, responded normally to in vitro challenge with SRBC. The suppressive factor had the molecular size of an IgG and could be removed by anti-Ig and anti-IgG1 but not by anti-alpha or anti-mu immunosorbents. Purified IgG1 was more suppressive than IgG2a and IgG2b despite a similar anti-SRBC antibody content.