Background Plasma cells (PC) are the unique source of protective and autoreactive immunoglobulin. Mature PC are resting and have downregulated CD20, rendering them refractory to almost all conventional and modern antirheumatic therapies including rituximab. Consequently, the failure to eridacte autoantibody production by plasma cells appears as a therapeutical hurdle in patients with autoimmune diseases. We have identified two distinct subsets of plasma cells based on differing expression of CD19 in the human BM which are comparatively characterised here against the background of the development of anti-CD19 directed immunotherapies.
Objectives To characterise CD19+ vs. CD19-negative PC subsets in normal human bone marrow.
Methods BM samples from iliac crest, femural heads and sternum as well as blood, spleen and tonsil specimen were analyzed for CD38hi plasma cells by FACS and the expression of CD19, CD138, Ki-67, HLA-DR, CD95, CD28 and CD56 wa studied. Transcriptional analyses of sort-purified plasma cell subsets were performed on an Affymetrix platform. The antibodies expressed by individual plasma cells were characterized using Elispot, cytoplasmic Ig staining and single cell RT-PCR.
Results CD19+ and CD19-negative plasma cells were readily detectable in all human BM samples irrespective of the anatomic site analyzed and co-expressed CD138. At average, 60-70% of the plasma cells expressed CD19 in the BM, while 85% were CD19+ in spleen and tonsils and 95-100% in the blood, respectively. Transcriptional and phenotypical analyses of CD19-negative BMPC suggest their advanced maturity, inasmuch they express CD56 and CD28 while HLA-DR, CD95 and Ki-67 are downregulated compared to CD19+ BMPC. CD19-negative BMPC were enriched for IgG-secreting cells which showed suprisingly low mutational load within their VH gene rearrangements. Treatment of RA patients with rituximab significantly reduced numbers of CD19+ BMPC but not CD19-negative BMPC.
Conclusions Our data indicate an unexpected heterogeneity within the mature plasma cell compartment in the human bone marrow. Detailled characterisation according to CD19 expression suggests that CD19-negative plasma cells aquired a more mature differentiation state and represents a rather static population as compared to CD19+ BMPC. A potential anti-CD19 directed B cell depletion approach for therapy of autoimmunity will likely target many but not all plasma cells, while the relationship betwen CD19 expression by plasma cells and autoantibody production remains to be analysed in order to estimate targeting of autoantibody production. Our data provides new insight into the homeostasis of normal mature plasma cells which opens new perspectives for studies on plasma cells as sources of auto-antibodies in patients with autoimmune diseases.
Disclosure of Interest None Declared
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