Abstract
Complement (C) plays a critical role in the immune response by opsonizing immune complexes (IC) and thymus-independent type 2 antigens with C3 breakdown product C3dg. We investigated the in vivo fate and handling in mice of anti-CR1/CR2 mAb 7G6. We used this rat IgG mAb as a surrogate for C3dg-opsonized IC; mAb 7G6 binds to CR1/CR2 with high affinity, blocks C3dg binding and saturates mouse B cell CR2 at inputs of only 2 ug. RIA, flow cytometry, and fluorescence immunohistochemistry were used to examine the disposition of 0.5–2 ug quantities of mAb 7G6 infused i.v. in mice. The mAb binds to circulating B cells and in the spleen binds preferentially to marginal zone (MZ) B cells. However, within 24 h MZ B cells relocate and transfer the mAb to regions rich in follicular dendritic cells (FDC). Localization of intact antigen to FDC should induce a substantial immune response, and therefore we immunized mice and monkeys i.v. with low doses (1–20 ug/kg) of prototype antigens constructed with anti-CR1/2 mAb 7G6 or anti-CR2 mAb HB135, respectively. We observed a strong immune response characterized by early development of IgG antibodies and long-lasting immunity extending out to at least one year. We applied our immunization paradigm to mouse IgG idiotypes, based on i.v. infusion of mouse IgG2a mAbs which were cross-linked with mAb 7G6. The purpose of these experiments was to determine if tolerance can be broken in order to develop a more powerful vaccine strategy to induce a cytotoxic humoral immune response to malignant B cells based on targeting the idiotype of immunoglobulin molecules expressed on their surfaces. I.V. immunization with the constructs indeed generated a mouse IgG1 immune response to two different mouse IgG2a mAbs, as demonstrated by ELISA. The immune response was idiotype specific, but some anti-isotype antibodies were also detected. Moreover, sera from immunized mice immunoprecipitated the specific radiolabeled mouse mAbs in the presence of 7.5% polyethylene glycol. This humoral immune response was also demonstrable in flow cytometry assays in which IgG1 in sera of immunized mice bound to erythrocytes opsonized with bispecific mAb constructs consisting of the IgG2a mAb crosslinked with an anti-CR1 mAb. The present approach, based on coupling the targeted immunoglobulin to an anti-CR2 mAb for delivery to FDC, may lead to a more effective immunotherapeutic vaccine compared to methods currently in clinical trials which require use of glutaraldehyde to effect crosslinking of the targeted immunoglobulin to KLH.
Babesia bovis RON2 contains conserved B-cell epitopes that induce an invasion-blocking humoral immune response in immunized cattle
