Revisiting Hypogammaglobulinemia in Chronic Lymphocytic Leukemia: A Combined Clinicobiological Approach

Chronic lymphocytic leukemia (CLL) is characterized by progressive hypogammaglobulinemia that can affect one or more immunoglobulin subclasses. Although many underlying mechanisms have been suggested, the pathogenesis of this phenomenon remains to be elucidated. In the present study, we revisit hypogammaglobulinemia in CLL through a combined clinicobiological approach aiming at identifying associations with particular disease profiles that would offer pathogenetic insight and guidance for further research. The study group included 412 CLL patients with available information about serum immunoglobulins either at diagnosis (n=380) or before treatment initiation (n=32). Patient characteristics were as follows: median age: 65 years; males/females: 266/146; Binet stage A: 272/335, unmutated IGHV genes (U-CLL): 140/412 cases (34%); CD38 expression: 59/330 cases (18%); clonotypic IG of the MD or G isotype: 250 and 43 cases, respectively; isolated del(13q): 64/136 (47%); trisomy 12: 18/183 (10%); del(11q): 18/186 (10%); del(17p): 11/189 (6%); NOTCH1 del7544-45/p.P2514Rfs*4: 8/219 (4%). With a median follow up of 5 years, 152/329 cases (46%) received treatment. Decreased immunoglobulin serum levels in at least one subclass were identified in 220/412 patients (53%), as follows: (i) decreased IgM, 172/412 cases (41%); (ii) decreased IgG, 78/412 cases (19%); (iii) decreased IgA, 100/412 cases (24%). In 36/412 cases (9%), a decrease in all serum immunoglobulin subclasses was noted. No statistically significant differences were identified between patients with normal serum immunoglobulin levels versus those with hypogammaglobulinemia regarding age, gender, disease burden at diagnosis, IGHV gene mutational status, CD38 expression, cytogenetic aberrations, NOTCH1 mutations and the incidence of a second malignancy. However patients with hypogammaglobulinemia exhibited increased need for treatment compared to patients with normal serum immunoglobulins (91/175 vs 61/154 respectively, p=0.025). Among cases with hypogammaglobulinemia, 90 (41%) and 26 (12%) exhibited isolated IgM and IgA subclass deficiency, respectively; isolated IgG decrease, was relatively rare (10/220 cases, 4%). Interestingly, when comparing isolated IgA versus other subclass deficiencies, statistically significant associations were identified with (i) advanced clinical stage (Binet B/C, Rai III/IV) (p=0.002); (ii) female gender (p=0.041); and, (iii) NOTCH1 mutations (p=0.004). A propos of the latter, it is noteworthy that in 5/8 (63%) mutant NOTCH1 cases with hypogammaglobulinemia, the affected subclass was IgA. Within our cohort, we identified cases belonging to one of three different, well characterized subsets with stereotyped B-cell receptor immunoglobulin (BcR IG), namely: (1) subset #1 (clan I IGHV genes/IGKV1(D)-39): U-CLL, clinically aggressive, n=12; (2) subset #2 (IGHV3-21/IGLV3-21), mixed IGHV mutational status, noted clinical aggressiveness, n=5; and, (3) subset #4, mutated IGHV4-34/IGKV2-30 BcR IG, clinically indolent, n=12. Notably, all subset #2 cases showed low levels of at least one serum subclass, while in 4/5 and 3/5 cases, two or all three immunoglobulin subclasses were affected. Although numbers are small, the incidence of hypogammaglobulinemia in subset #2 was significantly (p<0.05) higher compared to either subset #1 or subset #4). Univariate analysis revealed clinical stage, CD38 expression and IGHV mutational status as statistically important parameters (p<0.05) for both time-to-first–treatment (TTFT) and overall survival (OS); in contrast, hypogammaglobulinemia had no impact either on on TTFT or OS. In multivariate analysis, clinical stage and IGHV gene mutational status retained independent significance. In conclusion, abnormalities of serum immunoglobulins are detected in CLL patients with heterogeneous clinicobiological profiles, including different disease burden (clinical stage), cytogenetic aberrations and IGHV gene mutational status. However, certain observations reported herein, in particular the high incidence of hypogammaglobulinemia in subset #2 and the association of NOTCH1 mutations with IgA subclass deficiency, are noteworthy and indicate the need for research towards unraveling causal mechanisms among the observed interwined events.