Rationale of anti-CD19 immunotherapy: an option to target autoreactive plasma cells in autoimmunity

Abstract

Anti-CD20 therapy using rituximab directly targeting B cells has been approved for treatment of non-Hodgkin lymphoma, rheumatoid arthritis and anti-neutrophil cytoplasmic antibody-associated vasculitides and has led to reappreciation of B-lineage cells for anti-rheumatic treatment strategies. Moreover, blocking B-cell activating factor with belimumab, a drug that is licensed for treatment of active, seropositive systemic lupus erythematosus (SLE), represents an alternative, indirect anti-B-cell approach interfering with proper B-cell development. While these approaches apparently have no substantial impact on antibody-secreting plasma cells, challenges to improve the treatment of difficult-to-treat patients with SLE remain. In this context, anti-CD19 antibodies have the promise to directly target autoantibody-secreting plasmablasts and plasma cells as well as early B-cell differentiation stages not covered by anti-CD20 therapy. Currently known distinct expression profiles of CD19 by human plasma cell subsets, experiences with anti-CD19 therapies in malignant conditions as well as the rationale of targeting autoreactive plasma cells in patients with SLE are discussed in this review.

Figure 1

Expression of CD19 and CD20 during human B-cell development. B cells emerge from hematopoietic stem cells (HSC) and acquire maturity in the bone marrow (BM). During this process, CD19 expression starts at the stage of late pro-B cells and precedes that of CD20, starting in immature B cells. CD19 and CD20 expression is maintained during B-cell differentiation and activation in the periphery, while activation and differentiation into memory B cells may fine-tune expression levels. During activation of B cells in lymphoid organs - such as the spleen and tonsil, resulting in their differentiation into plasmablasts and finally plasma cells - CD20 expression is downregulated and subsequently lost. At the same time, CD19 expression is partially downregulated but not extinguished. Being released into circulation, only few plasmablasts express low amounts of CD20, while most have lost CD20 expression but express CD19. After successful immigration into deposits in the BM or the lamina propria (LP), all plasma cells lack CD20, while CD19 expression is maintained by one subset of tissue-resident plasma cells, while another subset acquires a CD19^-/CD20^- phenotype. Other surface molecules such as CD27, CD38 and CD138 do not qualify as therapeutic targets for global B-cell depletion approaches, as these are not continuously expressed throughout B-cell differentiation and their expression is shared with, for example, T lymphocytes and endothelial cells.

Figure 2

CD19 expressed by peripheral B-cell subsets from healthy donors and systemic lupus erythematosus patients. Peripheral blood mononuclear cells from 33 systemic lupus erythematosus (SLE) patients and 10 controls were stained for CD3, CD14, CD20, CD19 (clone SJ25C1) and CD27. (a) Lymphocytes including large cells were gated based on forward scatter (FSC) and side scatter (SSC) properties; cell aggregates were excluded in a dotplot depicting forward-scatter height (H) versus area (A) signals. Dead cells, T cells and monocytes were excluded from the analysis, and B cells were identified by gating on the DAPI^-CD3^-CD14^-CD19⁺ population. (b) Geometric mean fluorescence intensity (MFI) values reflecting CD19 expression were compared among B-cell subsets in SLE patients vs. healthy controls (HD); that is, CD27^-CD20⁺ naive and CD27⁺CD20⁺ memory B cells and CD27^highCD20^-/low plasmablasts. (c) SLE patients were subdivided into three equally sized groups according to ascending expression levels of CD19 by CD27^- B cells, CD27⁺ B cells or plasmablasts (indicated by the triangles below the graph) and analyzed for disease activity (Systemic Lupus Erythematosus Disease Activity Index (SLEDAI)). (d) SLE patients were divided into two groups according to disease activity (SLEDAI <6 vs. SLEDAI ≥6) and analyzed for B-cell CD19 expression levels. Horizontal lines represent median values. P < 0.05 was considered to reflect a statistically significant difference according to the Wilcoxon or Mann-Whitney test used for intra-donor or inter-group comparisons, respectively. (e) CD19 expression levels by CD27^- and CD27⁺ B cells and plasmablasts were analyzed between the individual B-cell subsets using Spearman's rank correlation, demonstrating close relations between their CD19 surface expression levels. *P < 0.05, **P < 0.01 and ***P < 0.001 classify significant values. DAPI, 4',6-diamidino-2-phenylindole.

Figure 3

CD19⁺ and CD19^- plasma cells detectable in bone marrow cells from autoimmune patients and controls. Bone marrow (BM) cell suspensions were obtained from iliac crest or femoral head samples from autoimmune or nonautoimmune control individuals and stained for CD38 and CD19 (clone SJ25C1), and in some cases additionally for CD138, CD3 and CD14. Dead cells were excluded by adding DAPI and gating on DAPI^- cells. Plasma cells (PCs) were identified based on light scatter properties and high expression of CD38. CD38^high PCs showed co-expression of CD138 and lacked that of CD3 and CD14. (a), (b) Nonautoimmune individuals. (c), (d) Patients as indicated. (a) Red histogram depicts PCs stained for CD19, black histogram shows isotype control-stained PCs, blue histogram shows lymphoid cells including CD19⁺ B cells for comparison. Note that control-stained and CD19-negative PCs exhibit an increased fluorescent background signal as compared with other lymphoid cells. (b) to (d) Black histograms represent PCs; red histograms depicting lymphoid cells including CD19⁺ B cells are shown for comparison. Markers define CD19⁺ and CD19^- PCs, respectively. Frequencies represent proportions of CD19⁺ cells among PCs. AAV, ANCA-associated vasculitis; DAPI, 4',6-diamidino-2-phenylindole; SLE, systemic lupus erythematosus.