Immunophenotypic Analysis of Myeloma Precursors: Antigens for Therapeutic Targeting

Abstract SCI-5

Multiple myeloma (MM) is a malignant disorder characterized by the (mono)clonal expansion of terminally-differentiated plasma cells (M-PC) in the bone marrow (BM) that produce and secrete a monoclonal immunoglobulin (Ig), detectable in the serum and/or urine. Infiltration by the expanded M-PC is easily identified in the involved tissues, through conventional morphology and immunophenotyping. However, the possibility exists that rather than M-PC, a less differentiated B-cell that represents a minor fraction of all tumor cells and retains self-renewal properties, is responsible for the outgrowth of the more differentiated M-PC compartment. In B-cell disorders, the idiotypic Ig produced by tumor cells and defined by its CDR3 sequence, acts as a genetic fingerprint for clonally-related B-cells. In MM, the specificity of the idiotype is further enhanced through modification of V genes by somatic hypermutation (SHM) at the germinal center (GC), since M-PC display extensively mutated VH genes which are stable throughout the disease. This suggests that in MM, malignant transformation could occur in a post-GC B-cell. Based on the CDR3 sequences of the Ig genes of M-PC, preliminary studies have identified tumor-associated circulating peripheral blood (PB) CD19+ B-cells, whose malignant/clonogenic potential remained to be demonstrated. More recently, the Matsui group has reported that while in MM cell lines both CD138− and CD138+ cells retain clonogenic capacity after in vitro serial plating clonogenic assays, in primary MM samples, such (ex vivo and in vivo) ability would be restricted to the CD34−/CD138− compartment. Of note, the clonogenic growth of these later cells significantly decreased after depletion by CD19, CD22, CD20 and CD45 antibodies, and it was associated with an in vitro multidrug-resistant functional phenotype (restricted to CD19+/CD27+, CD138− cells but not CD138+ cells) and the Hedgehog (Hh) stem cell-associated signaling pathway. These results point out the potential existence of a CD19+, CD20+, CD138− pre-PC compartment responsible for the expansion of M-PC in MM. In turn, evidence also exists in both the SCID-Hu model and in Rituximab treated MM patients, which suggests that plasmablasts/PC -but not pre-plasmablasts-, could act as MM “stem” cells, the precise characteristics of such cells remaining to be precisely defined. Alternatively, it could also be possible that both cell cellular components coexist and are relevant to MM progression through appropriate interaction with the BM stroma. Independently of all the above, trafficking of such cells through PB to BM niches could also play a key role in the spread of the tumor and its malignant behavior. In this regard, we recently confirmed that a relatively high percentage of MM patients (and a substantial fraction of all MGUS cases) show circulating PB PC with i) tumor-related clonal VH gene rearrangements and ii) an aberrant immunophenotypic profile which largely overlaps with that of BM M-PC from the same subjects; the only minor differences consisted of a significantly lower expression of CD38 and CD138, smaller size and internal complexity, features that indicate a slightly more immature plasmablastic/PC profile. Noteworthy, this was the only PB B-cell compartment for which clonally-related B-cells were detected with a sensitivity of <1 cell/50μ L in all cases investigated. In summary, accumulating evidence suggest the existence of a clonal hierarchy in MM but uncertainties remain as regards the precise immunophenotypic features of those cells responsible for tumor growth in primary patient samples, that could be of help in developing new targeted therapies.