The Basis of the Oral and Parenteral Adjuvant Properties of Immune Stimulating Complexes (ISCOMs)

Maloy, Kevin Joseph (1996) The Basis of the Oral and Parenteral Adjuvant Properties of Immune Stimulating Complexes (ISCOMs). PhD thesis, University of Glasgow.

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There is currently a great deal of interest in the development of vaccines using purified recombinant protein antigens. For practical and scientific reasons, it would be advantageous if these vaccines could be administered orally. This is esssential for stimulating widespread immunity at the mucosal surfaces where most pathogens are encountered and, as the success of the oral polio vaccine shows, this can also protect against systemic infection. Furthermore, oral immunisation is one of the few effective means allowing passive transfer of immunity from mother to infant via milk. The poor immunogenicity of purified soluble proteins, their inability to induce class I MHC-restricted CTL responses and the systemic tolerance induced by their oral administration are major obstacles to the development of synthetic oral vaccines. Thus, there is a need for well characterised vaccine vectors which will circumvent these difficulties and allow soluble protein antigens to be immunogenic when administered orally. Immune stimulating complexes (ISCOMS) are lipophilic, cage-like particles composed of cholesterol, phospholipid and the saponin adjuvant Quil A. Parenteral immunisation with ISCOMS is known to induce humoral and cell-mediated immune responses as well as protective immunity against a number of infections. However, their efficacy by mucosal routes and their use with non-hydrophobic proteins had received little attention. My studies exploited a recently established method for incorporating purified OVA into ISCOMS and aimed to extend preliminary work in the laboratory showing that ISCOMS-OVA were immunogenic by oral and parenteral routes. I therefore explored the full range of responses induced and investigated the immunological basis of the adjuvant properties of ISCOMS when used by the oral and parenteral routes. A single parenteral immunisation with as little as 5-10mug ISCOMS-OVA primed potent OVA-specific systemic DTH and IgG responses and high levels of class I MHC-restricted OVA-specific CTL activity were detected in the spleen. These findings indicate that ISCOMS allow presentation of incorporated protein to the class II MHC-restricted T cells required for antibody and DTH responses, as well as permitting efficient entry of the protein into the endogenous antigen processing pathway required for stimulation of class 1 MHC-restricted CTL responses. A multiple dose oral immunisation schedule induced the same range of systemic immune responses as parenteral immunisation and also stimulated the production of secretory IgA antibodies in the intestine and local CTL precursors in MLN. These findings indicate that ISCOMS may allow protein antigen to subvert the regulatory mechanisms which normally cause protein antigens to induce tolerance at mucosal surfaces. One of the most novel properties of ISCOMS was their ability to stimulate potent class I MHC-restricted CTL responses to exogenous protein antigens in vivo. As this would be important for a vaccine capable of stimulating protection against many viral infections, I characterised the CTL responses primed by ISCOMS- OVA and investigated the mechanisms involved in their induction. My results showed that the CTL primed by ISCOMS-OVA were CD8+ T cells which recognised the immunodominant epitope contained within the endogenously processed peptide OVA 258-276, containing the minimal motif for binding the H-2Kb class I MHC molecule. This is the epitope recognised by OVA-specific CTL in H-2b mice following physiological processing of OVA. OVA-specific precursor CTL appeared rapidly in the draining lymph node after subcutaneous immunisation with ISCOMS-OVA and these pCTL differentiated into effector cells when cultured merely with IL-2, indicating the efficiency of priming in vivo. Phagocytic accessory cells (M?) were required for the priming of a large proportion of the OVA-specific CTL, as paralysis or depletion of these cells produced a marked decrease in the priming of OVA-specific CTL in vivo. However, detectable CTL activity remained in the absence of M(?), suggesting that there may be more than one pathway of CTL induction utilised by ISCOMS in vivo. The CTL responses were entirely dependent on helper T cells, as depletion of CD4+ T cells completely abolished priming of CTL responses by ISCOMS-OVA. These findings suggest that the induction of CTL by ISCOMS is a complicated process, involving both phagocytic accessory cells and CD4+T helper cells. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Allan Mowat
Keywords: Immunology, Pharmacology
Date of Award: 1996
Depositing User: Enlighten Team
Unique ID: glathesis:1996-74948
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 27 Sep 2019 15:02
Last Modified: 27 Sep 2019 15:02

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