Utility of Multiparameter Flow Cytometry Immunophenotypic Studies In Patients with Systemic Light Chain (AL) Amyloidosis

Abstract 4051

Multiparameter flow cytometry (MFC) immunophenotyping has shown to be of value for differential diagnosis and minimal residual disease assessment in multiple myeloma. However, the clinical value of MFC immunophenotyping in other plasma cell disorders (PCD) remains largely unexplored. Systemic light chain (AL) amyloidosis is a rare PCD characterized by the accumulation of monoclonal light chain fragments leading to end-organ damage and short survival. Bone marrow (BM) plasma cell (PC) infiltration in AL is usually low and thus the identification of clonal PC can be often difficult by immunohistochemistry and/or immunofluorescence.

In the present study we focused on 34 BM samples sent to our institution with a suspected diagnosis of AL. MFC immunophenotypic studies were performed using the following 4-color combinations of MoAbs (FITC/PE/PerCP-Cy5.5/APC): CD38/CD56/CD19/CD45 (n=34); in addition cy-Kappa/cy-Lambda/CD19/CD38 staining was add to confirm the clonal or polyclonal nature of BMPC in equivocal cases. Ploidy and cell cycle analysis were additionally performed in a subset of cases (n=12/34).

From the total 34 cases included in the present study, 28 had a confirmed diagnosis of AL. The remaining 6 cases were finally diagnosed with localized - amyloidoma - (n=2) and familial (n=1) forms of amyloidosis, multiple myeloma-associated amyloid (n=2) and congestive pericarditis (n=1). Interestingly, the presence of clonal PC was detected by MFC in 27 of the 28 (96%) patients with AL; in turn, clonal PC were undetectable in the BM of all cases with localized and familial forms of amyloidosis.

The median overall level of PC (M-PC plus N-PC) seen in MFC immunophenotypic analyses of BM samples of the 28 patients with AL was 1.9% (range: 0.1% - 15%), with a significant positive correlation between PC enumerated by MFC and conventional morphology (r=0.5; p=.01). Within the BMPC compartment, the median proportion of clonal PC was of 94% (mean 81% ± 29%); in 6 cases all BMPC were clonal while in the remaining 22 patients residual normal PC persisted (median of normal PC/BMPC 13% ± 31%). The most common aberrant phenotypes were down-regulation of CD19 (92%) and CD45 (83%), followed by overexpression of CD56 (56%) and infra-expression of CD38 (42%). Aneuploidy was only found in 18% of cases, all of them hyperdiploid. Cell cycle analysis showed a median % of S-phase and G2-Mitosis PC of 0.7% and 3.5%, respectively.

Concerning patients' outcome, cases with undetectable normal PC (6/28, 21%) had a significantly decreased overall survival (OS) compared to patients with persistent BM normal PC at diagnosis (22/28, 79%) with 3-year OS rates of 0% vs. 59%, respectively (p=.001).

In summary, these preliminary data suggests that MFC immunophenotyping investigations may be clinically relevant in patients with suspected amyloidosis for i) differential diagnosis between AL and other forms of amyloidosis and, ii) prognostication of patients with AL according to the presence or absence of baseline persistent normal PC.