Induction of oral tolerance in carp (Cyprinus carpio L.) after feeding protein antigens

Induction of oral tolerance against ferritin, recombinant surface glycoprotein of viral haemorrhagic septicemia virus (KLG18) and ovalbumin (OVA) was studied in carp. Feeding of ferritin or KLG18 resulted in lower Ab titres compared to unprimed controls when animals were intramuscularly (i.m.) injected with protein 10 weeks later and sampled 21 days after this injection. After administration of OVA by different routes (oral, anal, i.m.) and i.m. injection with OVA + Freund's incomplete adjuvant 2 months later, only a few fish responded to OVA as measured by serum Ab titres. Responsiveness to OVA appeared to be carp strain dependent. When an isogenic carp strain was selected for an optimal response to i.m. injection with OVA, this carp strain did not develop oral tolerance after feeding. In contrast, 6 x feeding high doses of OVA on subsequent days, resulted in immunological memory formation. Oral tolerance can be induced in carp, but differences in tolerance induction may depend on the protein used. A possible role of genetic factors in the induction of oral tolerance in fish is discussed.