<p>Induction of collagen-induced arthritis (CIA), an animal model for human rheumatoid arthritis, is dependent on anti-collagen type II (CII) antibodies. The effector mechanism by which autoantibodies contribute to inflammatory reactions in autoimmune diseases is not well understood. In this thesis I have studied the effector pathways used by IgG anti-CII antibodies to initiate arthritis, namely the IgG Fc receptors (FcγRs) and the complement system. We have found that FcγRIII is crucial for development of CIA, as CII-immunized mice lacking this receptor do not develop arthritis and IgG1 and IgG2b anti-CII antibodies require FcγRIII to trigger arthritis when transferred to naïve mice. The antibody-mediated arthritis was further enhanced in mice deficient in the inhibitory FcγRIIB, indicating that FcγRIIB regulates the activation of FcγRIII. Furthermore, we demonstrate that FcγRIII exist as three distinct haplotypes in mice, FcγRIII:H, FcγRIII:V and FcγRIII:T. Mice expressing the FcγRIII:H haplotype are more susceptible to CIA than mice expressing the FcγRIII:V haplotype, indicating that certain FcγRIII haplotype predisposes for CIA. We also show that the most likely FcγRIII-expressing effector cell in CIA is the macrophage, since FcγRIII-expressing macrophages exclusively can induce arthritis in FcγRIII-deficient mice challenged for CIA.</p><p>The complement system was also investigated in development of CIA. We found that this effector pathway is also necessary for onset of arthritis, as CIA was inhibited by treatment with anti-complement factor 5 (C5) antibodies. C5-deficient mice could neither develop CIA unless provided with C5-containing sera. </p><p>Taken together, the work presented in this thesis indicates that FcγRs and the complement system are crucial for the induction of experimental arthritis. These findings are important for understanding the mechanisms behind rheumatoid arthritis and blocking of these effector pathways may in the future be used as treatment of rheumatoid arthritis. </p>

Induction of collagen-induced arthritis (CIA), an animal model for human rheumatoid arthritis, is dependent on anti-collagen type II (CII) antibodies. The effector mechanism by which autoantibodies contribute to inflammatory reactions in autoimmune diseases is not well understood. In this thesis I have studied the effector pathways used by IgG anti-CII antibodies to initiate arthritis, namely the IgG Fc receptors (FcγRs) and the complement system. We have found that FcγRIII is crucial for development of CIA, as CII-immunized mice lacking this receptor do not develop arthritis and IgG1 and IgG2b anti-CII antibodies require FcγRIII to trigger arthritis when transferred to naïve mice. The antibody-mediated arthritis was further enhanced in mice deficient in the inhibitory FcγRIIB, indicating that FcγRIIB regulates the activation of FcγRIII. Furthermore, we demonstrate that FcγRIII exist as three distinct haplotypes in mice, FcγRIII:H, FcγRIII:V and FcγRIII:T. Mice expressing the FcγRIII:H haplotype are more susceptible to CIA than mice expressing the FcγRIII:V haplotype, indicating that certain FcγRIII haplotype predisposes for CIA. We also show that the most likely FcγRIII-expressing effector cell in CIA is the macrophage, since FcγRIII-expressing macrophages exclusively can induce arthritis in FcγRIII-deficient mice challenged for CIA. The complement system was also investigated in development of CIA. We found that this effector pathway is also necessary for onset of arthritis, as CIA was inhibited by treatment with anti-complement factor 5 (C5) antibodies. C5-deficient mice could neither develop CIA unless provided with C5-containing sera. Taken together, the work presented in this thesis indicates that FcγRs and the complement system are crucial for the induction of experimental arthritis. These findings are important for understanding the mechanisms behind rheumatoid arthritis and blocking of these effector pathways may in the future be used as treatment of rheumatoid arthritis.

<p>Antibodies have the ability to influence the antibody response against the very antigen they are specific for, in a process called antibody feedback regulation. Depending on the nature of the antigen, the antibody response can be either enhanced or almost completely inhibited. This thesis focuses on the underlying mechanisms of antibody feedback regulation <i>in vivo</i>. </p><p>Antigen-specific IgG can inhibit the antibody response to a particulate antigen. Based on its ability to inhibit B cell activation, the inhibitory FcγRIIB (low affinity receptor for IgG) has been suggested to be involved. Here we show that although FcγRIIB is required for efficient suppression<i> in vitro, </i>it is not required <i>in vivo</i>. Therefore, even though FcγRIIB can inhibit antibody responses, other mechanisms (such as epitope masking and enhanced antigen clearance) play a more dominant role<i> in vivo</i>.</p><p>The antibody response to soluble antigen is greatly enhanced when it is introduced to the immune system in complex with antigen-specific IgG or IgE. We found that FcγRIIB attenuates the magnitude of IgG-mediated enhancement. In mice lacking FcγRIIB, IgG enhanced the antibody response much more efficiently than in normal mice.</p><p>Since B cells require CD4⁺ T cell help in order to become antibody-producing cells, we examined the CD4⁺ T cell response to immune complexes <i>in vivo</i>. Using an adoptive transfer strategy with transgenic ovalbumin (OVA)-specific CD4⁺T cells, we could show that the enhanced OVA-specific IgG response to IgG2a/OVA and IgE/OVA complexes was preceded by a potent OVA-specific CD4⁺ T cell response. IgG2a-mediated enhancement was dependent on activating Fcγ receptors, whereas IgE-mediated enhancement was dependent on CD23, the low affinity receptor for IgE. We identified CD23⁺ B cells as the responsible effector cells for IgE-mediated enhancement<i> in vivo</i>. Taken together, these results show that Fc receptor-mediated antigen presentation is a major mechanism underlying antibody feedback enhancement. </p>

Antibodies have the ability to influence the antibody response against the very antigen they are specific for, in a process called antibody feedback regulation. Depending on the nature of the antigen, the antibody response can be either enhanced or almost completely inhibited. This thesis focuses on the underlying mechanisms of antibody feedback regulation in vivo. Antigen-specific IgG can inhibit the antibody response to a particulate antigen. Based on its ability to inhibit B cell activation, the inhibitory FcγRIIB (low affinity receptor for IgG) has been suggested to be involved. Here we show that although FcγRIIB is required for efficient suppression in vitro, it is not required in vivo. Therefore, even though FcγRIIB can inhibit antibody responses, other mechanisms (such as epitope masking and enhanced antigen clearance) play a more dominant role in vivo. The antibody response to soluble antigen is greatly enhanced when it is introduced to the immune system in complex with antigen-specific IgG or IgE. We found that FcγRIIB attenuates the magnitude of IgG-mediated enhancement. In mice lacking FcγRIIB, IgG enhanced the antibody response much more efficiently than in normal mice. Since B cells require CD4+ T cell help in order to become antibody-producing cells, we examined the CD4+ T cell response to immune complexes in vivo. Using an adoptive transfer strategy with transgenic ovalbumin (OVA)-specific CD4+T cells, we could show that the enhanced OVA-specific IgG response to IgG2a/OVA and IgE/OVA complexes was preceded by a potent OVA-specific CD4+ T cell response. IgG2a-mediated enhancement was dependent on activating Fcγ receptors, whereas IgE-mediated enhancement was dependent on CD23, the low affinity receptor for IgE. We identified CD23+ B cells as the responsible effector cells for IgE-mediated enhancement in vivo. Taken together, these results show that Fc receptor-mediated antigen presentation is a major mechanism underlying antibody feedback enhancement.