A role for complement in feedback enhancement of antibody responses by IgG3

Abstract

IgG1, IgG2a, and IgG2b, passively administered with soluble Ags, enhance specific Ab responses. The effect of IgG3 in this type of feedback regulation has not been studied previously. We immunized mice with trinitrophenyl (TNP)-coupled carrier proteins (bovine serum albumin [BSA] or ovalbumin [OVA]) alone or complexed to monoclonal TNP-specific IgG3. The carrier-specific Ab responses were enhanced by several hundred-fold by IgG3. Enhancement was significantly impaired in mice depleted of complement factor C3 and in mice lacking complement receptors 1 and 2 (Cr2-/-). In contrast, mice lacking the common Fc-receptor gamma chain (FcR gamma -/-), resulting in reduced expression of Fc gamma RI and lack of Fc gamma RIII, and mice lacking Fc gamma RIIB (Fc gamma RIIB-/-), responded equally well to immunization with IgG3-complexed Ag as wild-type controls. These findings demonstrate that IgG3 can induce feedback enhancement and that IgG3, in analogy with IgM, uses the complement system for this function.

Figure 1.

IgG3-, but not IgE-, mediated enhancement is impaired in CVF-treated mice. Groups of five FcRγ^+/+ mice, treated with CVF or sham-treated with PBS, were immunized intravenously with 20 μg BSA-TNP and 20 μg OVA alone or in combination with 50 μg TNP-specific IgG3 (A and B) or IgE (C and D). On the indicated days, mice were bled and the IgG anti-BSA and anti-OVA titers were measured by ELISA. OVA-specific responses were not enhanced by IgG3 or IgE (unpublished data). Asterisks indicate the statistical differences between experimental and control groups within the same strain; asterisks within parentheses indicate the statistical differences between corresponding not-C3-depleted and C3-depleted groups. *, P < 0.05; **, P < 0.01; ***, P < 0.001; NS, not significant.

Figure 2.

IgG3-mediated enhancement is impaired in Cr2^−/− mice. Groups of five Cr2^+/+ (A) and Cr2^−/− (B) mice were immunized intravenously with 20 μg OVA-TNP and 20 μg KLH alone or in combination with 50 μg TNP-specific IgG3. On the indicated days, mice were bled and the IgG anti-OVA and anti-KLH titers were measured by ELISA. KLH-specific responses were not enhanced by IgG3 (unpublished data). Asterisks indicate the statistical differences between experimental and control groups within the same strain; asterisks within parentheses indicate the statistical differences between the corresponding Cr2^+/+ and Cr2^−/− groups. *, P < 0.05; **, P < 0.01; ***, P < 0.001; NS, not significant.

Figure 3.

IgG2a-mediated enhancement is normal in Cr2^−/− mice. Groups of five Cr2^+/+ (A) and Cr2^−/− (B) mice were immunized intravenously with 20 μg OVA-TNP and 20 μg KLH alone or in combination with 50 μg TNP-specific IgG3 or IgG2a. Mice were bled 14 d after immunization and the IgG anti-OVA and anti-KLH titers were measured by ELISA. KLH-specific responses were not enhanced by IgG3. Asterisks indicate the statistical differences between experimental and control groups. ***, P < 0.001.

Figure 4.

IgG3-mediated enhancement is normal in FcRγ^−/− mice. Groups of four FcRγ^+/+ and FcRγ^−/− mice were immunized intravenously with 20 μg OVA-TNP and 20 μg KLH alone or in combination with 50 μg TNP-specific IgG3. On the indicated days mice were bled and the IgG anti-OVA and anti-KLH titers were measured by ELISA. Anti-KLH responses were not enhanced by IgG3 (unpublished data). Asterisks indicate the statistical differences between experimental and control groups within the same strain; asterisks within parentheses indicate the statistical differences between the corresponding FcRγ^+/+ and FcRγ^−/− groups. * P < 0.05; **, P < 0.01; ***, P < 0.001; NS, not significant.

Figure 5.

IgG3-mediated enhancement is normal in FcγRIIB^−/− mice. Groups of five FcγRIIB^+/+ and FcγRIIB^−/− mice were immunized intravenously with 20 μg BSA-TNP and 20 μg OVA alone or in combination with 50 μg TNP-specific IgG3. On the indicated days mice were bled and the IgG anti-BSA and anti-OVA titers were measured by ELISA. OVA-specific responses were not enhanced (unpublished data). Asterisks indicate the statistical differences between experimental and control groups within the same strain; asterisks within parentheses indicate the statistical differences between the corresponding FcγRIIB^+/+ and FcγRIIB^−/− groups. *, P < 0.05; **, P < 0.01; ***, P < 0.001; NS, not significant.