Human complement receptor type 2 (CR2/CD21) transgenic mice provide an in vivo model to study immunoregulatory effects of receptor antagonists.

We found that transgenic (tg) mice stably expressing a bacterial artificial chromosome (BAC)-derived human complement receptor type 2 (CR2/CD21) gene demonstrate B cell specific hCR2 protein expression, normal B cell development and no changes in B cell subpopulations. To determine whether this BAC-encoded human CR2 (hCR2) can replace mouse CR2/CR1 in Cr2(-/-) mice and restore humoral immune responses to model foreign antigens (Ags), we generated hCR2(+/-)Cr2(-/-) tg mice and immunized them with sheep red blood cells (SRBC). We found that hCR2(+/-)Cr2(-/-) mice demonstrated anti-SRBC antibody (Ab) levels that were initially comparable to Cr2(-/-) mice after a single injection of the Ag, but then showed marked increases in anti-SRBC IgM and IgG1 levels after a second immunization. Identical results were found with a second model Ag, NP-Ficoll. To further confirm that this improvement in Ag-specific Ab production over Cr2(-/-) mice was indeed due to hCR2 expression, as well as to examine the effects of treating hCR2(+/-)Cr2(-/-) mice with an inhibitory anti-hCR2 monoclonal Ab (mAb) in vivo, we used mAb 171, an anti-hCR2 mAb that we have shown directly recognizes the C3d ligand binding site on hCR2. We first found that mAb 171 completely blocked hCR2-dependent co-activation of hCR2-tg B cells by anti-BCR/C3d complexes as measured in vitro by intracellular calcium influx. The i.p. injection of 1mg of mAb 171 was then found to induce for at least three weeks only partial loss of hCR2 surface expression, without modifying B and T cell numbers or the apparent activation status of the cells. Treatment of hCR2(+/-)Cr2(-/-) mice with mAb 171 also substantially suppressed the development of anti-SRBC and anti-NP Abs following immunization with Ags. The development of this model system should allow the study of the effects of manipulating hCR2 function in vivo with potential therapeutic compounds.