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Requirement for complement in antibody responses is not explained by the classic pathway activator IgM
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Biochemistry and Microbiology.
Immune Disease Institute and Program in Cellular and Molecular Medicine, Children’s Hospital Boston, and Department of Pediatrics, Harvard Medical School, Boston, USA.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Biochemistry and Microbiology.
Immune Disease Institute and Program in Cellular and Molecular Medicine, Children’s Hospital Boston, and Department of Pediatrics, Harvard Medical School, Boston, USA.
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2011 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 108, no 43, 17589-17590 p.Article in journal (Refereed) Published
Abstract [en]

Animals lacking complement factors C1q, C2, C3, or C4 have severely impaired Ab responses, suggesting a major role for the classic pathway. The classic pathway is primarily initiated by antigen-Ab complexes. Therefore, its role for primary Ab responses seems paradoxical because only low amounts of specific Abs are present in naive animals. A possible explanation could be that the classic pathway is initiated by IgM from naive mice, binding with sufficient avidity to the antigen. To test this hypothesis, a knock-in mouse strain, Cμ13, with a point mutation in the gene encoding the third constant domain of the μ-heavy chain was constructed. These mice produce IgM in which proline in position 436 is substituted with serine, a mutation previously shown to abrogate the ability of mouse IgM to activate complement. Unexpectedly, the Ab response to sheep erythrocytes and keyhole limpet hemocyanin in Cμ13 mice was similar to that in WT mice. Thus, although secreted IgM and the classic pathway activation are both required for the normal primary Ab response, this does not require that IgM activate C. This led us to test Ab responses in animals lacking one of three other endogenous activators of the classic pathway: specific intracellular adhesion molecule-grabbing nonintegrin R1, serum amyloid P component, and C-reactive protein. Ab responses were also normal in these animals.

Place, publisher, year, edition, pages
2011. Vol. 108, no 43, 17589-17590 p.
National Category
Basic Medicine Immunology in the medical area Immunology
Research subject
Immunology
Identifiers
URN: urn:nbn:se:uu:diva-160601DOI: 10.1073/pnas.1109831108ISI: 000296378100011PubMedID: 21987785OAI: oai:DiVA.org:uu-160601DiVA: diva2:451731
Note
Author summaryAvailable from: 2011-10-26 Created: 2011-10-26 Last updated: 2017-12-08Bibliographically approved
In thesis
1. The Role of IgM and Complement in Antibody Responses
Open this publication in new window or tab >>The Role of IgM and Complement in Antibody Responses
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

An intact complement system including the complement receptors 1 and 2 (CR1/2) is crucial for the generation of a normal antibody response in animals and humans. Moreover, activation of the classical pathway is thought to be important since deficiency in complement components C1q, C2, C4 or C3 lead to impaired antibody responses. The classical pathway is mainly initiated by antibodies bound to their antigen. It is unclear how classical pathway activation can be crucial for primary antibody responses since the levels of specific antibodies are very low in naïve animals. It has been hypothesized that natural IgM, with high enough affinity, can initiate the classical pathway after immunization. To test this, we generated the knock-in mouse strain Cμ13, producing IgM unable to activate complement. Surprisingly, the antibody response against SRBC and KLH in Cµ13 mice was normal. Thus, the importance of classical pathway activation and natural IgM in antibody responses is not dependent on the ability of IgM to activate complement. SIGN-R1, SAP and CRP are other known activators of the classical pathway, but mice lacking these also had normal antibody responses. Complement activation leads to the generation of C3 split products which are ligands for CR1/2. In mice, CR1/2 are expressed on B cells and follicular dendritic cells (FDC), but it is unclear on which cell-type expression of CR1/2 is needed for the generation of a normal antibody response. Some reports argue that increased antigen retention by CR1/2+ FDC would increase the effective antigen concentration, giving more effective B-cell stimulation. In contrast, several mechanisms involving CR1/2 on B cells are suggested. First, marginal zone B cells could transport complement-coated antigen or IC via CR1/2 into the follicle. Second, different ways of co-crosslinking the B-cell receptor with CR1/2, lowering the threshold for B-cell activation, have been proposed. Finally, CR1/2 on B cells are shown in vitro to facilitate endocytosis and thereby presentation of antigen to T cells. We show that abrogated antibody responses in mice lacking CR1/2 are not due to lack of CR1/2-mediated antigen presentation to T cells. Chimeric mice with CR1/2 expression on both B cells and FDC, on neither B cells nor FDC, or on either B cells or FDC, were generated. The antibody response against SRBC was completely dependent of CR1/2-expression on FDC. However, when this requirement was fulfilled, B cells without expression of CR1/2 were equally efficient antibody producers as wildtype B cells. Antigen-specific IgM together with its antigen can enhance the antibody response to that antigen and CR1/2-expression is crucial for the enhancement. We show that the response to IgM in complex with SRBC is dependent on CR1/2 expression on both B cells and FDC.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 63 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 720
Keyword
Complement, antibody response, mutated IgM, natural IgM, complement receptors, SIGN-R1, SAP, CRP, B cells
National Category
Basic Medicine Immunology in the medical area Immunology
Research subject
Medical Science
Identifiers
urn:nbn:se:uu:diva-160610 (URN)978-91-554-8210-7 (ISBN)
Public defence
2011-12-08, C10:301, BMC, Husargatan 3, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2011-11-17 Created: 2011-10-27 Last updated: 2011-11-23Bibliographically approved
2. IgM and Complement in Regulation of Antibody Responses
Open this publication in new window or tab >>IgM and Complement in Regulation of Antibody Responses
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Animals deficient in complement components C1q, C4, C3, and CR1/2 have severely impaired antibody responses. C1q is primarily activated by antibody-antigen complexes. Antigen-specific IgM in complex with an antigen is able to enhance the antibody response against that antigen. This is dependent on the ability of IgM to activate complement. Naïve mice have very low amounts of specific antibodies and therefore it is surprising that classical pathway activation plays a role for primary antibody responses. It was hypothesized that natural IgM, present in naïve mice, would bind an antigen with enough affinity to activate C1q. To test this, a knock-in mouse strain, Cm13, with a point mutation in m heavy chain, making its IgM unable to activate complement was constructed. Surprisingly, the antibody responses in Cm13 were normal. Puzzled by the finding that the ability of IgM to activate complement was required only for some effects, the immunization protocol was changed to mimic an infectious scenario. With this regime, Cm13 mice had an impaired antibody response compared to wildtype (WT) mice. The antibody response in WT mice to these repeated low-dose immunizations was also enhanced. These observations suggest that IgM-mediated enhancement indeed plays a physiological role in initiation of early antibody responses. IgM-mediated enhancement cannot however compensate for the dependecy of T-cell help. Although IgM from WT mice enhanced the antibody response, the T-cell response was not enhanced. The connection between classical pathway activation and CR1/2 is thought to be generation of ligands for CR1/2. In mice, CR1/2 are expressed on B cells and follicular dendritic cells (FDC). Although CR1/2 are crucial for a normal antibody response, the molecular mechanism(s) are not understood. To investigate whether CR1/2 must be expressed on B-cells or FDC to generate a normal antibody response, chimeric mice between WT and CR1/2-deficient mice were constructed. The results show that CR1/2+ FDC were crucial for the generation of antibody responses. In the presence of CR1/2+ FDC, both CR1/2+ and CR1/2- B cells were equally good antibody producers. However, for an optimally enhanced antibody response against IgM-antigen complexes, both B cells and FDC needed to express CR1/2.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 62 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 1140
Keyword
IgG, GC responses, feedback regulation, T-cell responses, antigen presentation, complement receptors 1 and 2
National Category
Immunology in the medical area
Research subject
Immunology
Identifiers
urn:nbn:se:uu:diva-263472 (URN)978-91-554-9357-8 (ISBN)
Public defence
2015-11-19, C4:305, BMC, BMC, Husargatan 3, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2015-10-28 Created: 2015-09-30 Last updated: 2015-11-10

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Rutemark, ChristianBergman, AnnaHeyman, Birgitta

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