Correlation between serum levels of free light chain and phenotype of plasma cells in bone marrow in primary AL amyloidosis

Abstract

To investigate whether there is a correlation between subtypes of plasma cells in the bone marrow and the production of M-protein, flow cytometry and serum free light chain (FLC) analyses were carried out in 17 patients with primary systemic AL amyloidosis (mean age, 59.9 ± 8.8 years) and controls with M-protein (MGUS controls, n = 6) and without it (negative controls, n = 9). The patients showed a significantly higher value in the serum predominant FLC:serum creatinine ratio (43.8 ± 63.2) and CD38 + + CD19-CD56 + subpopulation (monoclonal plasma cells) (2.57 ± 5.35%) than either the negative (p < 0.0005 and p < 0.001, respectively) or MGUS controls (p < 0.05). With respect to maturation of plasma cells in the bone marrow, the intermediate (MPC-1 + CD45-CD49e-) and mature (MPC-1 + CD45 + CD49e-) subtypes were significantly higher (49.2 ± 23.2%, p < 0.005) and lower (27.6 ± 21.3%, p < 0.005) in the patients than in the negative controls, respectively. The serum predominant FLC:serum creatinine ratio was elevated in parallel with an increase in CD38 + + CD19-CD56 + and MPC-1 + CD45-CD49e- cells and a decrease in mature subtypes (MPC-1 + CD45 + CD49e- and MPC-1 + CD45 + CD49e + cells), There was a significantly positive correlation between the serum predominant FLC:serum creatinine ratio and either CD38 + + CD19-CD56 + (r = 0.510, p < 0.05) or MPC-1 + CD45-CD49e- cells (r = 0.481, p < 0.05). In primary AL amyloidosis M-protein is probably produced by increased monoclonal plasma cells in the bone marrow, particularly by the intermediate subpopulation with a phenotype of MPC-1 + CD45-CD49e-.