Immunophenotype of normal and myelomatous plasma-cell subsets

Abstract

Plasma cells (PCs) are essentially characterized by the co-expression of CD138 and CD38, which allows their identification in flow cytometry in bone marrow (BM), peripheral blood, or cell suspensions from tissues. These terminally differentiated B-cells may lose the expression of surface CD19 and that of CD20 while retaining CD27. When malignant, they can gain a number of other markers such as CD28, CD33, CD56, or CD117 and lose CD27. Moreover, since each PC is only able to produce a single type of immunoglobulins (Igs), they display isotypic restriction and clonal malignant PCs can be further characterized by their homogeneous expression of either kappa or lambda light chains. In multiple myeloma (MM), such PC clones produce the Ig identified in plasma as an abnormal peak. In the BM where they essentially accumulate, these PCs may however display various immunophenotypes. The latter were explored in a two-way approach. Firstly, the various subsets delineated by the selective or common expression of CD19 together with combined CD56/CD28 were explored in normal and MM BM. Then, other aberrant markers' expression was investigated, i.e., CD20, CD27, CD33, CD56, CD117. These data were compared to literature information. They underline the vast heterogeneity of MM PCs possibly accounting for the various answers to therapy of MM patients.

Keywords: bone marrow; flow cytometry; immunophenotype; multiple myeloma; plasma cells.

Figure 1

Partition of PC percentages among the four immunophenotypic subgroups defined by the expression or not of CD19 and the combination CD56/CD28 in MM patients (left, open circles) and controls (right, black circles). In diagnosis samples from MM patients, most of the abnormal plasma cells belong to the CD19− CD56+/28+ subgroup 3 while most but not all normal PC in controls retain expression of CD19 in the absence of CD56 and CD28 (subgroup 1).

Figure 2

Example of FCM scattergrams of a control BM sample. The left panel shows the partition in four subgroups of CD38+/CD138+ cells (initial gate, not shown) according to the expression or not of CD19 and the combination of CD56/CD28. Most of normal PC are in subgroup 1 but cells can also be seen in the other compartments. Polyclonality is confirmed by the investigation of cytoplasmic light chains in the four subsequent scattergrams gated on groups 1–4 from left to right.

Figure 3

Examples of abnormal PCs in MM BM samples at diagnosis. The disposition is the same as in Figure 2, light-chain expression being shown in the four subsets (1–4). (A) Patient with κ-restricted abnormal PC presents mostly in subgroups 1 and 2 but also 3 and 4. (B) Patient with κ-restricted abnormal PC presents mostly in subgroups 2 and 3, with polyclonal residual normal PC in subgroup 1. (C) “Classical” MM patient with the vast majority of κ-restricted abnormal PC present in subgroup 3, i.e., lacking CD19 and expressing either CD56 or CD28 (or both). (D) Atypical MM patient with abnormal PC most of which lack all three surface antigens yet clearly display λ-restriction in subsets 4 and 3. Note the small population of residual polyclonal normal PC in subgroup 1.

Figure 4

Expression patterns of CD56 and CD28 on MM PCs when tested separately. This figure shows that 90% of MM patients express at least one of these two aberrant markers.

Figure 5

Immunophenotypic features of MM PC when investigating other aberrant surface markers. Data are expressed in percentages for the series presented here.