Background/Purpose:

The CR2/CD19 complex on B cells serves as a co-receptor for membrane IgM and amplifies B cell responses through physical associations of the bound antigen with the complement C3 activation fragment designated C3d.  Prior studies have used Cr2 -/- mice which lack CR2 expression; however, because the mouse CR2 protein is derived through an alternately spliced mRNA from the same gene as the complement receptor 1 (CR1) protein, a C3b/C4b receptor, one cannot discern whether the primary immune effect is due to deficiency of one or the other receptor, or whether compensatory mechanisms play important roles in the gene-targeted mice.  To better understand the unique role of CR2 in autoimmune disease, while minimizing manipulation of CR1, we have generated a novel mouse monoclonal antibody (mAb), designated 4B2, to mouse CR2 that uniquely blocks the CR2/C3d interaction.

Methods:

mAb 4B2 (IgG1) was generated in Cr2-/- mice by immunizing with recombinant mouse CR2 spanning the ligand-binding site within short consensus repeat domains 1-4 and fusion with the 653 myeloma cell line.  A mouse model of collagen-induced arthritis (CIA) was used, and more recently a mouse model of lupus-like disease is being used to study the effects of blocking the CR2/C3d ligand-receptor interaction with mAb 4B2. 

Mice were immunized twice with sheep red blood cells (SRBC) to evoke a primary and secondary antibody response in the presence or absence of a single pre-injection of mAb 4B2.  Using the CIA model, the durability of the inhibitory effect of 4B2 was measured, and B cell levels were recorded. Immune responses to antigen were measured in CIA mice pretreated with 4B2, or control isotype mAb.  In CIA, mice pre-treated with 4B2 or control mAb were injected with bovine type II collagen, and IgG2a antibodies to mouse and bovine type II collagen (mCII and bCII, respectively) were measured. Peripheral and splenic B and CD4+ T cells were measured as well as B cell subpopulations from spleen. Additionally, a clinical scoring system was used to evaluate arthritis. 

Results:

Mice pre-injected with 4B2 antibody demonstrated reduced levels of anti-SRBC IgG1 levels. Mice with CIA treated with 4B2 demonstrated a 6 week blockade of CR2 and significantly decreased anti-mCII and anti-bCII IgG2a.  No anti-idiotype antibodies were generated to 4B2. Splenic B cell follicular, transitional, immature and marginal zone subpopulations were not changed.  B and T cell hematopoietic changes were not seen. Clinical disease activity was substantially decreased in mice treated with mAb 4B2.  Additionally, 4B2 is being used in studies of (NZBxNZW)F1 mice, which spontaneously develop lupus-like disease. 

Conclusion:

We have shown that the mouse monoclonal antibody to CR2, 4B2, is able to block CR2/C3d interaction and decrease immunological responses to model antigens as well as the development of CIA.  Studies using 4B2 in (NZBxNZW)F1 mice  are being undertaken to determine whether chronic blockade of the CR2/C3d interaction can be used as a therapeutic approach in lupus.  Chronic inhibition of the CR2/C3d interaction can be further studied with this new tool as a potential therapeutic approach in other mouse models of autoimmune and inflammatory disease.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/blocking-the-complement-receptor-2-cr2-ligand-receptor-interaction-with-a-novel-mouse-anti-mouse-cr2-monoclonal-antibody-decreases-antigen-specific-humoral-immune-responses-and-the-evolution-of-coll/