Characteristics and dynamics of the residual tumor clone determined by multicolor flow cytometry in Waldenstrom’s macroglobulinemia

Background. Minimal residual disease (MRD), as determined by immunophenotyping, is an indicator of therapy response and a marker of relapse in many hematological diseases. There are some foreign publications devoted to the assessment of the residual tumor population using multicolor flow cytometry in Waldenstrom’s macroglobulinemia (WM). A characteristic feature of WM is the infiltration of bone marrow by two tumor populations from one tumor clone: clonal B-lymphocytes and clonal plasma cells. The study of these two aberrant populations was only possible using flow cytometry. In 90 % of WM cases, the L265P mutation of the MYD88 gene is detected: it is a diagnostic marker and is much less common in other lymphomas.

Aim. To investigate the residual tumor clone characteristics in WM patients, their correlation with progression, assess the relationship between the monoclonal immunoglobulin M dynamics and MRD status, as well as between the presence of the MYD88 mutation at onset and the rate of tumor clone reduction.

Materials and methods. Patients underwent immunophenotypic analysis of bone marrow cells at disease onset and at follow-up time points post-induction using multicolor flow cytometry. The following antigens were investigated for B-cells: surface CD19, CD22, CD20, CD45, and CD27, as well as cytoplasmic λ and κ immunoglobulin M antigens. For plasma cells, the following were assessed: surface CD38, CD138, CD27, CD45, CD81, and CD19, along with cytoplasmic λ and κ immunoglobulin M antigens. Standard methods of descriptive statistics and graphical visualization were used to analyze the results. To evaluate the dynamics of aberrant cell populations, multivariate statistical methods were employed, accounting for repeated measures within the same participants. The same methods were applied to study the association of the MYD88 gene mutation with the dynamics of these parameters.

Results. Heterogeneity of the residual tumor clone was revealed, which could be represented by three variants of tumor populations: only aberrant B cells; only aberrant plasma cells; and populations of B- and plasma cells. Different reduction rates of residual aberrant B- and plasma cells were observed after the end of induction therapy: in the 1^st month after treatment, the number of aberrant B cells decreased 1.4 times faster than plasma cells. The long-term persistence phenomenon of trace immunoglobulin M secretion after induction therapy even in MRD-negative patients was revealed (70 % cases). A relationship was noted between the presence of L265P mutation of the MYD88 gene at the disease onset and the number of residual B cells. In patients without a MYD88 gene mutation at the disease onset, MRD-negative B-cell status was achieved by the first month after the end of induction. No relationship was found between the presence / absence of the MYD88 gene mutation and the residual tumor plasma cell population. Patients with MRD-positive status in any combination (only aberrant B cells; only aberrant plasma cells; and populations of B- and plasma cells) demonstrated a higher probability of disease progression compared to the MRD-negative group.

Conclusion. The use of multicolor flow cytometry to detect residual neoplastic B-cell and plasma cell populations provides additional insights into the depth of remission, the rate of tumor cell reduction, and the heterogeneity of the residual malignant clone.