Background Epratuzumab, a humanized monoclonal antibody targeting CD22, has demonstrated therapeutic activity in clinical trials of patients with SLE, yet an understanding of its mechanism of action (MOA) is still emerging. Because epratuzumab reduces on average 35% of circulating B cells in patients, its therapeutic efficacy involves MOA beyond B-cell depletion. Modulation of CD22 and other surface molecules that regulate B-cell antigen receptor (BCR) signaling may alter B-cell functions and ultimately mitigate symptoms of the underlying disease.
Objectives Using an experimental ex-vivo analysis, we identified trogocytosis as a novel MOA for epratuzumab, which may be important for therapy of SLE and other autoimmune disorders. Clinical specimens of SLE patients were evaluated for evidence of trogocytosis induced by epratuzumab in vivo.
Methods PBMCs from either healthy donors or SLE patients were incubated with epratuzumab, and the relative surface levels of CD22, CD19, CD21, and CD79b on B cells were analyzed by flow cytometry (FCM). Trogocytosis was studied with FCM and fluorescence microscopy using B-cell NHL cell lines (Daudi and Raji) mixed with PBMCs, T cells, monocytes, or granulocytes. We further measured the relative levels of CD22, CD19, CD21, and CD79b on B cells from five SLE patients who were receiving epratuzumab, four treatment-naïve SLE patients, and two receiving belimumab.
Results Epratuzumab promptly induced a marked decrease of surface CD22 (>80%), CD19 (>50%), CD21 (>50%), and CD79b (>30%) on B cells in PBMCs obtained from normal donors or treatment-naïve SLE patients. CD27- B cells were more responsive than CD27+ cells. Within a few hrs, B-cell surface proteins were reduced to a similar level over a broad concentration range (0.01 – 100 µg/mL) of epratuzumab. Although some Fc-independent loss of CD22 is expected from internalization, the concurrent and prominent reduction of CD19, CD21, and CD79b is Fc-dependent and results from trogocytosis of epratuzumab-bound B cells to FcgR-expressing effector cells, including monocytes, NK cells, and granulocytes. Reduced staining of surface antigens on B cells coincided with positive CD19 and CD22 staining of the effector cells. Epratuzumab-induced transfer of membrane components from Daudi cells to monocytes was also evident by fluorescence microscopy. Unlike rituximab, which reduced the B-cell count by 50% in the ex-vivo assay, epratuzumab did not cause significant B-cell depletion. Analysis of SLE patient samples suggests that similar epratuzumab-mediated trogocytosis, as observed ex-vivo, also occurs clinically. As expected, CD22 was significantly (P<.0001) lower (>80%) on the B cells of epratuzumab-treated patients. Notably, CD19, CD21 and CD79b were each also significantly (P<.02) lower for the epratuzumab group.
Conclusions This study revealed a previously unknown, and potentially important, MOA of epratuzumab. The findings of reduced levels of CD19 are of particular relevance for the efficacy of epratuzumab in SLE, because elevated CD19 has been correlated with susceptibility to SLE in animal models as well as in patients, and down-regulation of CD19 should attenuate activation of B cells by raising the BCR signaling threshold.
Disclosure of Interest D. Goldenberg Shareholder of: Immunomedics, Inc., Employee of: Immunomedics, Inc., E. Rossi Employee of: Immunomedics, Inc., R. Michel Employee of: Immunomedics, Inc., D. Wallace: None Declared, C.-H. Chang: None Declared