The identification of plant lectins with mucosal adjuvant activity

To date, the most potent mucosal vaccine adjuvants to be identified have be en bacterial toxins. The present data demonstrate that the type 2 ribosome- inactivating protein (type 2 RIP), mistletoe lectin I (ML-I) is a strong mu cosal adjuvant of plant origin. A number of plant lectins were investigated as intranasal (i.n.) coadjuvants for a bystander protein, ovalbumin (OVA). As a positive control, a potent mucosal adjuvant, cholera toxin (CT), was used. Go-administration of ML-I or CT with OVA stimulated high titres of OV A-specific serum immunoglobulin G (IgG) in addition to OVA specific IgA in mucosal secretions. CT and ML-I were also strongly immunogenic, inducing hi gh titres of specific serum IgG and specific IgA at mucosal sites. None of the other plant lectins investigated significantly boosted the response to co-administered OVA. Immunization with phytohaemagglutinin (PHA) plus OVA e licited a lectin-specific response but did not stimulate an enhanced respon se to OVA compared with the antigen alone. Intranasal delivery of tomato le ctin (LEA) elicited a strong lectin-specific systemic and mucosal antibody response but only weakly potentiated the response to co-delivered OVA. In c ontrast, administration of wheatgerm agglutinin (WGA) or Ulex europaeus lec tin 1 (UEA-I) with OVA stimulated a serum IgG response to OVA while the lec tin-specific responses (particularly for WGA) were relatively low. Thus, th ere was not a direct correlation between immunogenicity and adjuvanticity a lthough the strongest adjuvants (CT. ML-I) were also highly immunogenic.