Background Plasma cells (PCs) are heterogeneous in terms of function and lifespan. Long-lived PCs are not depleted by B-cell depletion therapy for autoimmune disorders and therefore likely to be responsible for lack of clinical response in a significant proportion of patients. Until recently [1], the phenotype of long-lived PCs was not known. Understanding the biological features of long-lived PCs could greatly facilitate identification of therapeutic targets. Long-lived PCs contribute to humoral memory, and may represent the cell of origin for myeloma while neoplastic PCs are generally distinguished by lack of CD19 expression. However, polyclonal CD19^Neg PCs constitute approximately 15–35% of total PCs in people aged over 50 years. Here we further characterize normal CD19^Neg PC populations in the context of normal and therapeutically perturbed bone marrow.

Objectives To establish the phenotypic and functional differences between normal CD19^Pos and CD19^Neg human PCs

Methods Extensive phenotyping of surface and intracellular proteins of normal and myeloma PCs were performed by flow cytometry. The regeneration of normal PCs was monitored following bone marrow ablation for myeloma [2] and B-cell depletion with alemtuzumab [3] in a series of clinical trials. The immunoglobulin heavy chain variable (IGHV) region of normal CD19^Pos and CD19^Neg human PCs were sequenced as described previously [4] to identify phylogenetic relationships.

Results CD19^Neg PCs are phenotypically distinct from normal CD19^Pos plasma cells with weaker CD95 (p<0.0001) and stronger (p<0.0001) BCL2 expression. Normal CD19^Neg PCs share some characteristics with neoplastic PCs including variable expression of CD28, CD56, and CD45, but neoplastic PCs are reliably distinguished from normal CD19^Neg PCs by differential expression of CD27, CD81 and CD117 as well as CD56 in some cases. After effective myeloablation for myeloma CD19^Pos PCs recovered in 74 out of 80 samples, while CD19^Neg PCs remained at less than 1% of total PCs in 30 out of 80 samples. Therapeutic B-cell depletion with alemtuzumab for chronic lymphocytic leukaemia results in loss of CD19^Pos PCs (p<0.0001) whereas CD19^Neg PCs persist. There was little overlap between the IGHV repertoire with only 2.1 to 3.7% of unique IGHV@ rearrangements shared between the CD19^Pos and CD19^Neg fractions, suggestive of a largely discrete immunoglobulin repertoire in the two fractions at steady state.

Conclusions CD19^Pos and CD19^Neg PCs are biologically distinct with different immunoglobulin repertoires and production kinetics consistent with CD19^Neg PCs having greater potential lifespan. Identifying the developmental pathway of normal CD19^Neg PC is likely to allow selectively targeting PC compartments for autoimmune disorders.

1. Mei HE, et al. Blood 2015, 125(11):1739–1748.

2. Rawstron AC, et al. Journal of clinical oncology 2013, 31(20):2540–2547.

3. Rawstron AC, et al. Blood 2004, 103(6):2027–2031.

4. Cocco M, et al. Journal of Immunology 2012, 189(12):5773–5785.

Disclosure of Interest None declared