THU0013 Normal CD19-Negative Plasma Cells Are Biologically Distinct from Other Normal Plasma Cells and Likely To Be Involved in Lack of Response To B-Cell Depletion for Autoimmune Disorders
Authors: G. Arumugakani, R.M. de Tute, D.J. Newton, G.J. Morgan, J.A. Child, R. Owen, P. Emery, A.S. Jack, D. McGonagle, R. Tooze, A.C. Rawstron
Journal: Annals of the Rheumatic Diseases (2016)

<h3>Background</h3> 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<sup>Neg</sup> PCs constitute approximately 15–35% of total PCs in people aged over 50 years. Here we further characterize normal CD19<sup>Neg</sup> PC populations in the context of normal and therapeutically perturbed bone marrow. <h3>Objectives</h3> To establish the phenotypic and functional differences between normal CD19<sup>Pos</sup> and CD19<sup>Neg</sup> human PCs <h3>Methods</h3> 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<sup>Pos</sup> and CD19<sup>Neg</sup> human PCs were sequenced as described previously [4] to identify phylogenetic relationships. <h3>Results</h3> CD19<sup>Neg</sup> PCs are phenotypically distinct from normal CD19<sup>Pos</sup> plasma cells with weaker CD95 (p&lt;0.0001) and stronger (p&lt;0.0001) BCL2 expression. Normal CD19<sup>Neg</sup> PCs share some characteristics with neoplastic PCs including variable expression of CD28, CD56, and CD45, but neoplastic PCs are reliably distinguished from normal CD19<sup>Neg</sup> PCs by differential expression of CD27, CD81 and CD117 as well as CD56 in some cases. After effective myeloablation for myeloma CD19<sup>Pos</sup> PCs recovered in 74 out of 80 samples, while CD19<sup>Neg</sup> 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<sup>Pos</sup> PCs (p&lt;0.0001) whereas CD19<sup>Neg</sup> 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<sup>Pos</sup> and CD19<sup>Neg</sup> fractions, suggestive of a largely discrete immunoglobulin repertoire in the two fractions at steady state. <h3>Conclusions</h3> CD19<sup>Pos</sup> and CD19<sup>Neg</sup> PCs are biologically distinct with different immunoglobulin repertoires and production kinetics consistent with CD19<sup>Neg</sup> PCs having greater potential lifespan. Identifying the developmental pathway of normal CD19<sup>Neg</sup> PC is likely to allow selectively targeting PC compartments for autoimmune disorders. <h3>References</h3> Mei HE, et al. Blood 2015, 125(11):1739–1748. Rawstron AC, et al. Journal of clinical oncology 2013, 31(20):2540–2547. Rawstron AC, et al. Blood 2004, 103(6):2027–2031. Cocco M, et al. Journal of Immunology 2012, 189(12):5773–5785. <h3>Disclosure of Interest</h3> None declared