Antilymphocytic antibodies and marrow transplantation. XIV. Antibody- induced suppression of graft-versus-host disease in C3-decomplemented mice differentiates two T-cell-depletion pathways

Remarkable differences in the suppression of graft-versus-host disease (GVHD) have been found for anti-Thy-1 antibodies to relate to (1) antigen density and antibody coating on the target cells, (2) antibody isotype, and (3) uptake of complement subcomponent C1q. Regarding (2) and (3) we now demonstrate that depletion of the third complement component C3 by cobra venom factor (CVF) differentiates two T-cell elimination pathways in mice: four rat IgG2c anti-Thy-1 monoclonal antibodies (MoAbs) with low uptake of mouse C1q lost most of their T- cell-depleting and consequently GVHD-preventing effect in C3-depleted H2 IA incompatible semiallogeneic (C57BL/6xCBA)F1 mice. In contrast, eight rat IgG2b, mouse IgG2a, and 2b anti-Thy-1 MoAbs with high affinity for C1q still remained strongly T-cell-depleting and prevented GVHD even in fully mismatched CBA mice depleted of C3. In conjunction with our observation that anti-Thy-1 MoAbs also suppress GVHD in C5- deficient AKR mice, we conclude that complete complement activation until T-cell lysis is not required for our antibodies to be effective in vivo. Activation, but only until deposition of C3b on target-cells for opsonisation via C3b receptors, is necessary with the less immunosuppressive anti-Thy-1 IgG2c isotype with low affinity for C1q. Mouse C1q uptake and C3/C4 deposition on target cells were measured with labeled antibodies and localized in T-cell areas. Interestingly, not even activation until C3b is necessary with the most immunosuppressive C1q-high-affine isotypes. As far as the latter is concerned, we discuss whether elimination of antibody-coated cells via Fc receptors is enhanced by binding to C1q-receptors and/or by intercalating C1q expressed on macrophages.