Immunoglobulin heavy/light chain analysis enhances the detection of residual disease and monitoring of multiple myeloma patients

Aim

To evaluate the clinical utility of incorporating a novel heavy/light chain immunoassay (HLC) into the existing methods for the assessment of multiple myeloma (MM) patients.

Methods

Convenience sera samples from 90 previously treated IgG and IgA MM patients in different disease stages were analyzed. The study was conducted in Clinical Hospital Center Zagreb between 2011 and 2013. The collected sera were analyzed by standard laboratory techniques (serum protein electrophoresis, quantification of total immunoglobulins, serum immunofixation, serum free light chain [FLC] assay) and HLC assay.

Results

HLC ratios outside the normal range were found in 58 of 90 patients, including 28 out of 61 patients with total immunoglobulin measurements within the normal range and 5 out of 23 patients in complete response. Both elevated HLC isotype level and abnormal HLC ratio correlated with the parameters of tumor burden, including percentage of plasma cells in the bone marrow (P < 0.001 and P = 0.002, respectively) and an abnormal serum FLC ratio (for both P < 0.001). In addition, abnormal HLC isotype level correlated with serum beta-2-microglobulin level (P = 0.038). In terms of prognosis, abnormal HLC isotype level and abnormal HLC ratio were significantly associated with shorter overall survival (P < 0.001 and P = 0.002, respectively). Interestingly, suppression of the uninvolved (polyclonal) isotype pair, but not other non-myeloma immunoglobulin isotypes, was also associated with a shorter overall survival (P = 0.021). In a multivariate analysis, an abnormal HLC ratio and β₂-microglobulin level >3.5mg/L were independent risk factors for survival.

Conclusion

The new HLC assay has greater sensitivity in detecting monoclonal protein, correlates with tumor burden markers, and affects patients'' outcome.Hematological malignancy multiple myeloma is characterized by the proliferation of monoclonal plasma cells in the bone marrow, which usually leads to the secretion of monoclonal intact immunoglobulins (Ig), free light chains (FLC), or both, into the serum. Consequently, the measurement of monoclonal proteins is integral to diagnosing multiple myeloma, monitoring of response to treatment, and detecting relapse (1-3). Conventional methods rely principally on serum electrophoresis together with either urine electrophoresis for the detection of FLC M-proteins or nephelometric quantification of total Igs for intact Ig paraprotein measurement. However, these standard techniques often do not have the required sensitivity or do not correlate with the patient’s clinical status (4-7).The ability to detect monoclonal serum FLC has been substantially increased by the introduction of the serum FLC assay, improving the diagnosis, monitoring, and prognosis of a number of plasma cell dyscrasias (8,9). Still, a number of problems associated with the detection of monoclonal intact Igs persist. Serum Ig concentrations can change for ≥50% with fluctuations in blood volume and/or hematocrit (10). In the case of monoclonal IgGs, the serum concentration can be also influenced by altered metabolism due to recycling of IgG by the neonatal Fc receptors (11-13). Furthermore, monoclonal proteins, in particular IgA paraproteins, may not be accurately quantified by serum protein electrophoresis (SPE) due to co-migration with other serum proteins (14,15). Finally, one of the major limitations of measuring total Igs is the inability to distinguish between the monoclonal component and polyclonal Ig background.These problems can be addressed by a recently developed heavy/light chain (HLC) immunoassay. Using specific antibodies targeted to junctional epitopes of the heavy and light chains of Ig molecules, HLC immunoassay separately quantifies different light chain types of each Ig class (IgGκ, IgGλ, IgAκ, IgAλ, etc), thus providing accurate quantification of both the involved and uninvolved components of the patient''s affected Ig isotype. The ratio of the monoclonal and polyclonal Igs of the same isotype (IgGκ/IgGλ, IgAκ/IgAλ) provides a sensitive measure of monoclonality, while minimizing the influence of changes in blood volume/hematocrit and the effect of IgG recycling (16,17).Previous studies using this novel assay in specific, controlled patient cohorts found the HLC ratio to be useful for screening, monitoring, and risk stratification of patients with multiple myeloma, but also with other monoclonal gammopathies, such as monoclonal gammopathy of undetermined significance (MGUS) and amyloid light-chain (AL) amyloidosis (18-20). The aim of this study was to evaluate the ability of the new assay – HLC isotype measurements – to detect monoclonal protein (ie, residual disease) in previously treated multiple myeloma patients and to determine whether these patients can benefit from adding the new assay into a routine assessment.