MO257BELIMUMAB ADD-ON THERAPY MOBILIZES MEMORY B CELLS INTO CIRCULATION OF SLE PATIENTS
Authors: Arends E J , Zlei Mihaela , Tipton Christopher M , Osmani Zgjim , Kamerling S W A , Rabelink Ton , Sanz Ignacio , Van Dongen J J M , Van Kooten Cees , Teng Y K O 
Published: May 2021

Abstract

Background and Aims
Belimumab (BLM), a recombinant human IgG-1λ monoclonal antibody directed against B-cell activating factor (BAFF), is the first approved biological agent for patients with active severe systemic lupus erythematosus (SLE) and lupus nephritis (LN). There is clinical evidence that combining belimumab with B cell depleting therapy can ameliorate disease activity in severe, refractory SLE patients. Although BLM is a B cell directed therapy and has been shown to significantly decrease total B cells, flow cytometry observations suggest a rapid increase of circulating memory B cells. The present study investigated the dynamics of B cell subsets in patients treated with or without BLM, with a special focus on the memory compartment in order to assess the characteristics of these memory B cells.

Method
We first performed extensive B cell subset phenotyping by high sensitivity flowcytometry according to the Euroflow protocol on whole blood from active lupus nephritis or SLE patients with other major organ involvement treated with standard of care (SOC) consisting of high dose steroids and mycophenolate mofetil combined with or without the addition of BLM. Next we characterized memory B cell gene expression profiles with single-cell RNA and V(D)J sequencing (ScRNA-SEQ).

Results
By employing high sensitivity flowcytometry, we established that the absolute increase in circulating memory B cells in SLE and LN patients was significant for patients who initiated BLM but not for patients treated with standard of care (Figure 1). The increase was observed in a broad range of memory B cell subsets (Unswitched, IgG1+, IgG2+, IgA1+, IgA2+) at 2 and 4 weeks following initiation of BLM treatment. This rise in memory B cells could hypothetically be attributed to either proliferation or homeostatic modulation of tissue-resident memory B cells causing a release into the circulation. ScRNA-SEQ cell cycle gene-expression was performed and established in both groups a non-proliferating phenotype [in approximately ∼94%] of memory B cells post-treatment, including absence of MKI67 as active proliferation marker. Additionally, BLM treatment did not result in an increased, and even in a severe reduction of the largest memory B cells clones after two weeks. In contrast, no change in clonality was seen after treatment with SOC. Together these data indicate that proliferation is not likely to be responsible for the observed increase in memory B cells by BLM. Furthermore, a clear difference was found in gene-expression levels between both treatment groups: BLM was responsible for the upregulation of 72 vs 10 genes in SOC, likewise 162 vs 32 genes were downregulated. Most importantly, a significant downregulation of the migration genes SELL (CD62L), CCR7, ITGB1, RAC2 and ICAM2, were specifically seen in BLM treated patients, possibly reflecting disrupted lymphocyte trafficking preventing memory B cells to either transmigrate into tissue or be retained at the tissue level.

Conclusion
The addition of BLM compared to standard of care in SLE patients leads to an increase of memory B cells in the circulation which appeared independent of proliferation. The process was accompanied with a strong modulation of gene-expression levels including a reduced expression of migration markers pointing towards disrupted lymphocyte trafficking. Altogether, these data could have important implications to further improve treatment strategies in severe SLE or lupus nephritis patients, for instance by establishing a deeper depletion of (autoreactive) memory B cells by the addition of rituximab after the initiation of belimumab.