Session Type: Abstract Submissions (ACR)
Epratuzumab is a humanized monoclonal antibody that targets the B cell-specific protein CD22 currently in Phase 3 clinical trials in patients (pts) with systemic lupus erythematosus (SLE). Epratuzumab does not deplete B cells but does cause long-term changes in B cell numbers (~50–60% reduction after 9–12 months) in SLE pts. The mechanism of action of epratuzumab appears to involve immunomodulation of B cells e.g. by inhibiting activation through the B cell receptor (BCR).1 It has also been shown to modulate B cell adhesion molecule expression and responsiveness to chemokines.2 This study aimed to understand the effect of epratuzumab on B cells in vivo using human CD22 knock-in (Huki) mice in which B cells express the human, instead of murine, CD22 gene.3
Huki mice (n=4–8) received a single intravenous injection of epratuzumab (0.5mg) or phosphate-buffered saline (PBS) and at time points up to 12 weeks (wks) B cell sub-populations (immature, transitional, mature, germinal center, marginal zone) in peripheral blood, bone marrow, spleen and lymph nodes were measured along with B cell activation markers (CD69, MHCII) and the CD62L homing marker. Ex vivo functional assays were also performed: Ca2+ flux after anti-IgM BCR stimulation, apoptosis (based on numbers of sub-G1 phase cells) and CD22 internalization on B cells were measured (flow cytometry) and the proliferation of B cell subsets assessed after 7 days in vivo administration of BrdU.
In Huki mice, a single dose of epratuzumab did not appear to affect absolute numbers or proportions of B cell subsets in peripheral blood or lymphoid organs at Wks 3, 5 or 12 (comparable to PBS-treated Huki mice). Similarly, there were no consistent changes in activation markers or CD62L. However Huki mice receiving epratuzumab showed human CD22 internalization in B cells from blood and all other organs. Internalization was detected at 24 hours and maintained for >8 wks; long after antibody clearance. Splenic B cells purified 10 days after receiving epratuzumab demonstrated an increased rate of apoptosis when cultured ex vivo relative to PBS-treated mice and decreased BCR-activated Ca2+ flux was demonstrated in Huki mouse splenic B cells after epratuzumab treatment in vitro. BrdU incorporation in several B cell subsets was unchanged 7 days after administration of epratuzumab, suggesting there was no increase in proliferation.
Epratuzumab administration to Huki mice induced functional effects on B cells assessed ex vivo in keeping with in vitro data using human B cells. Specifically, epratuzumab decreased CD22 expression for a long time period, increased B cell apoptosis and reduced Ca2+ flux upon BCR activation. Single dose epratuzumab did not seem to strongly influence B cell development or B cell populations in blood and various organs. These data have implications for understanding the effects of epratuzumab treatment on B cell function in SLE pts particularly in relation to how BCR inhibition leads to long-term changes in the survival and physiology of B cells.
1. Sieger N. Arthritis Rheum 2013;65:770
2. Daridon C. Arthritis Res Ther 2010;12:R204
3. Wöhner M. Eur J Immunol 2012;42:3009
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ACR Meeting Abstracts - https://acrabstracts.org/abstract/in-vivo-effects-of-epratuzumab-a-monoclonal-antibody-targeting-human-cd22-on-b-cell-function-in-human-cd22-knock-in-huki-mice/