Neonatal colonization of rats induces immunological tolerance to bacterial antigens

We wanted to investigate the immunological events occurring in rats intestinally colonized from birth (neonatally) or at adult age with an ovalbumin (OVA)-producing Escherichia coli O6K13 strain, carrying type 1 pili. The neonatally colonized animals responded with lower delayed type hypersensitivity (DTH) against OVA and lower levels of IgG antibodies against OVA, O6 lipopolysaccharide (LPS) and type 1 pili compared to age-matched controls. The IgG antibody response against the bystander antigen, human serum albumin (HSA), was lower in the neonatally colonized animals than in the controls co-immunized with HSA and E. coli, indicating a release of suppressive factors induced by the bacterial antigens. The adult colonized animals showed an increased DTH and antibody response against OVA after immunization. They also had high pre-immunization levels of IgG anti-O6 LPS antibodies compared to controls. However, the relative increase in IgG anti-O6 LPS antibody levels after the immunization with dead E. coli was much lower in the adult colonized animals. The present results suggest that neonatal animals develop tolerance against antigen on bacterial colonizers of the intestine. In addition, this tolerance contains components of suppression.