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Immune Enhancement Mechanisms by the Complement Protein C3d

Toapanta, Franklin Ramiro (2006) Immune Enhancement Mechanisms by the Complement Protein C3d. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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The final degradation product of the third (C3) component of complement, C3d, is a natural adjuvant. The adjuvant properties of this protein have been studied using several antigens fused to C3d. To determine the ability of this protein to enhance the humoral immune response in mice with different genetic backgrounds, inbred and outbred mice were immunized with C3d conjugated antigens. C3d induced enhanced immune responses that were comparable in all mouse strains. Analysis of the isotype class switch suggested that C3d favored the development of humoral immune responses. The classic mechanism of enhancement of the immune response involves interaction of C3d with its natural ligand, complement receptor 2 (CR2). This molecule is expressed mainly by follicular dendritic cells and B-cells. Simultaneous interaction of the antigen and C3d with the surface immunoglobulin M (sIgM) and CR2 on the surface of B-cells triggers two signaling pathways that cross-talk and synergize in cell activation. Most previous studies stressed the importance of the C3d-CR2 interaction for the adjuvant effect. However, the data supporting this hypothesis was derived from in vitro studies. To determine the importance of the C3d-CR2 interaction for enhancement of the immune response in vivo, mice deficient in CR2 (CR2-/-) were immunized with antigens fused to C3d. Contrary to the predicted, CR2-/- mice immunized with antigens fused to C3d, developed almost similar humoral immune responses than wild-type mice. These results suggested that C3d enhances the immune responses by CR2-dependent and CR2-independent mechanisms. Finally, the ability of C3d to emulate the antigenic redundancy of T-cell independent antigens and thus induce class switch in the absence of CD4+ T-cells was explored in a MHC type II knock-out mouse model (MHC II-/-). A soluble form of hemagglutinin (sHA) fused to C3d inefficiently induced IgG class switch in MHC II-/- mice. This demonstrated that C3d requires CD4+ T-cells to optimally enhance the immune responses. However, despite of the mild secondary immune responses, MHC II-/- sHA-C3d3-vaccinated mice had reduced morbidity and enhanced survival following a lethal virus challenge, suggesting that besides B-cells, C3d activates other immune cells and that the final enhancement effect is the accumulation of various mechanisms.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Toapanta, Franklin, frt2@pitt.eduFRT2
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairRoss,
Committee MemberNorris, Karenkan1@pitt.eduKAN1
Committee MemberStefano-Cole, Kellystefcole@pitt.eduSTEFCOLE
Committee MemberMontelaro, Ronaldrmont@pitt.eduRMONT
Committee MemberSalter, Russellrds@pitt.eduRDS
Date: 4 August 2006
Date Type: Completion
Defense Date: 21 July 2006
Approval Date: 4 August 2006
Submission Date: 31 July 2006
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Institution: University of Pittsburgh
Schools and Programs: School of Medicine > Molecular Virology and Microbiology
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: adjuvant; c3d; complement; CR2; vaccine
Other ID:, etd-07312006-135430
Date Deposited: 10 Nov 2011 19:55
Last Modified: 15 Nov 2016 13:47


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