High Prevalence of Multiple B Cell Clones in Patients with Chronic Lymphocytic Leukemia

INTRODUCTION: Studies suggest that the first clonal events in chronic lymphocytic leukemia (CLL) may be acquired at hematopoietic stem cell stage. Consistently, it has been reported that CD34+cells from CLL patients engrafted in immunodeficient mice give rise to clonal B cell lymphoproliferation while CD34+ cells from healthy donors did not (Kikushige, Cancer Cell 2011). Furthermore, in rare patients with early CLL or monoclonal B cell lymphocytosis two clonal populations can be demonstrated. We hypothesized that immunofixation electrophoresis could serve as a sensitive tool to assess the prevalence of a second, clonally distinct, lymphoproliferative event in CLL patients.

METHODS: We studied 262 treatment-naïve patients with a diagnosis of chronic lymphocytic leukemia (CLL) enrolled in our natural history study (NCT00923507). All patients had diagnostic peripheral blood flow cytometry testing (Laboratory of Pathology, NCI) and serum immunofixation electrophoresis (Department of Laboratory Medicine, Clinical Center, NIH) done.

RESULTS: Of the 262 patients, 148 (56.49%) were male and 114 (43.51%) were female. The median age of the study population was 65 years. Amongst the 262 patients, 243 (92.74%) were Caucasians, 8 (3.05%) were African Americans, 3 (1.14%) were Asians and 3 (1.14%) were multiracial. The race was undisclosed for 5 patients. Two of the 262 CLL patients had another B-cell malignancy on diagnostic workup, Waldenstrom's macroglobulinemia (Lymphoplasmacytic Lymphoma) and Hairy Cell Leukemia. Nine (3.44%) patients had two distinct clones of CLL by flow cytometry. In 4 both kappa and lambda clones were present. In 5, the two clones expressed the same light chains but had a distinct immunophenotype resulting in bi-phenotypic clones. By serum IFE, 62 (23.66%) of 262 patients had monoclonal gammopathy; in 44 (16.8%) patients one monoclonal band was detected, while 18 (6.9%) patients had 2 or more bands. Among patients with one monoclonal band 25 were kappa and 19 lambda light chain. In 19 of 25 patients with kappa clonal CLL by flow cytometry, the monoclonal band on IFE was kappa (concordant flow and IFE): IgM Kappa (in 8), IgG Kappa (10), and IgA Kappa (1); and in 6 patients with kappa clonal CLL the monoclonal band was lambda (discordant flow and IFE): IgM Lambda (in 2), and IgG Lambda (4). In 15 of 19 patients with lambda clonal CLL, flow cytometry and IFE were concordant: IgM Lambda (in 6), IgG Lambda (6), IgA Lambda (1), free lambda light chain (2), and discordant in 4: IgM Kappa (in 2), IgG Kappa (2). Amongst the 18 patients with 2 or more bands on IFE, only 5 patients had concordant flow cytometry and IFE, while 13 had discordant findings. In summary, using IFE we detected a second clonal event in 23 (8.8%) of 262 CLL patients. Considering the 11 patients with two distinct clones by flow cytometry, 34 (13%) of 262 treatment-naive CLL patients had at least one additional clonal B cell process. In addition, 18 patients with concordant flow and IFE findings had IgG or IgA type gammopathy, consistent with a concurrent plasma cell dyscrasia of the same light chain restriction as the CLL clone.

CONCLUSION: In 13% of CLL patients we found evidence for a second B cell lymphoproliferative process. The prevalence of biclonal CLL by flow cytometry at 3.4% is comparable to findings by others, reporting between 0.7% and 3.4%. In contrast, the prevalence of monoclonal gammopathy (23.7%) in our patients (median age 65) was much higher than reported in the general population: 3.2 % at age 50 and above, and 5.3% at age 70 and above (Kyle, NEJM 2006). Importantly, in 8.8% of all patients the monoclonal gammopathy was discordant with the CLL clone. These data support the existence of a genetic event in an early progenitor cell that predisposes to the development of clonal B cell expansions.

This research was supported by the Intramural Research Program of NHLBI and NCI.