Immunological implications of the use of plant lectins for drug and vaccine targeting to the gastrointestinal tract.

Plant lectins are under consideration as targeting agents to enhance the efficacy of orally administered drugs and vaccines. A significant issue that must be considered is the immunogenicity of these molecules since an immune response to the targeting agent may interfere with its ability to interact with the epithelium. In contrast, the ability of certain lectins to activate the immune system may be exploited in the delivery of vaccines. We previously demonstrated that plant lectins vary widely in their immunogenicity and in particular that mistletoe lectins (ML) I, II and II (MLI, MLII, MLIII) are potent immunogens when administered nasotracheally. Here, we measured immune responses following oral delivery of the MLs and assessed their ability to enhance responses to a co-administered antigen to determine if the molecules possess adjuvant activity. Oral administration of the lectins induced potent lectin-specific systemic and mucosal antibody responses. In addition, each of the three lectins possessed adjuvant activity when delivered orally together with ovalbumin (OVA). The lectins enhanced both serum and mucosal antibody responses to the co-delivered antigen. This shows for the first time that MLI, MLII and MLIII possess adjuvant activity when administered orally and may provide a platform for the generation of effective mucosal adjuvants.

Mistletoe lectins enhance immune responses to intranasally co-administered herpes simplex virus glycoprotein D2.

The mucosal adjuvant properties of the three type 2 ribosome-inactivating proteins (RIPs) from the European mistletoe, Viscum album L., were investigated. Mistletoe lectins were compared with cholera toxin (CT) as adjuvants when delivered nasotracheally together with herpes simplex virus glycoprotein D2 (gD2). All three mistletoe lectins (MLI, MLII, MLIII) were potent mucosal adjuvants. Co-administration of MLI, MLII or MLIII with gD2 led to significantly higher levels of gD2-specific mucosal immunoglobulin A (IgA) and systemic immunoglobulin G (IgG) antibody than when the antigen was delivered alone. The levels of antibodies induced were similar to those generated in mice immunized with gD2 and the potent mucosal adjuvant CT. Administration of ML1 with gD2 enhanced the antigen-specific splenic T-cell proliferative response. Interleukin-5 (IL-5), but not interferon-gamma (IFN-gamma), was detected in supernatants from splenocytes stimulated in vitro with gD2. This indicates that MLI enhanced type 2 T-helper cell (Th2) responses to the bystander antigen, gD2. Analysis of the gD2- and lectin-specific IgG subclass titres in mice immunized with gD2 and MLI, MLII or MLIII revealed a high ratio of IgG1 : IgG2a, which is compatible with the selective induction of Th2-type immune responses.

The identification of plant lectins with mucosal adjuvant activity.

To date, the most potent mucosal vaccine adjuvants to be identified have been bacterial toxins. The present data demonstrate that the type 2 ribosome-inactivating protein (type 2 RIP), mistletoe lectin I (ML-I) is a strong mucosal 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. Co-administration of ML-I or CT with OVA stimulated high titres of OVA-specific serum immunoglobulin G (IgG) in addition to OVA-specific IgA in mucosal secretions. CT and ML-I were also strongly immunogenic, inducing high 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 elicited a lectin-specific response but did not stimulate an enhanced response to OVA compared with the antigen alone. Intranasal delivery of tomato lectin (LEA) elicited a strong lectin-specific systemic and mucosal antibody response but only weakly potentiated the response to co-delivered OVA. In contrast, administration of wheatgerm agglutinin (WGA) or Ulex europaeus lectin 1 (UEA-I) with OVA stimulated a serum IgG response to OVA while the lectin-specific responses (particularly for WGA) were relatively low. Thus, there was not a direct correlation between immunogenicity and adjuvanticity although the strongest adjuvants (CT, ML-I) were also highly immunogenic.

Biodegradable lamellar particles of poly(lactide) induce sustained immune responses to a single dose of adsorbed protein.

The adjuvanticity of lamellar particles of poly(L-lactide) (PLA) towards adsorbed ovalbumin (OVA) was investigated. The aim of vaccine formulation was to maximise the amount of antigen retained on the particles and the time of retention during incubation of the formulations in PBS at 37 degrees C. Unmodified PLA lamellae were capable of adsorbing large quantities of OVA (up to 12.5% w/w) but major and rapid desorption occurred in PBS at 37 degrees C (80% released in 24 h). Retention of OVA on PLA lamellae was improved (25% released in 24 h) by precipitating the particles using aqueous sodium deoxycholate solution (DOC-modified PLA lamellae and lyophilising the lamellae-protein preparation after adsorption. Sustained immune responses were elicited in mice to a single sub-cutaneous injection of OVA adsorbed onto DOC-modified PLA lamellae. The level of antibodies induced and the pattern of response was similar to that induced by an alum-adsorbed OVA formulation. Normally boosting is required to obtain high levels of antibody when OVA is adsorbed on poly(DL-lactide co-glycolide) (PLG) microspheres. The lamellar forms of PLA may function as an efficient immunomodulator by effectively retaining adsorbed antigen and by activating immune cells due to their irregular shape. PLA lamellae have potential to stimulate enhanced immune responses to a variety of adsorbed antigens.