Reduced Expression of the CD94/NKG2C NK Cell Receptor in Chronic Lymphocytic Leukemia (CLL) and CLL-like Monoclonal B-Cell Lymphocytosis (MBL)

Background. Dysregulated NK-cell responses have been reported in chronic lymphocytic leukemia (CLL) patients, but little is known about the NK cell compartment in CLL-like monoclonal B cell lymphocytosis (MBL). Human cytomegalovirus (HCMV) infection induces an adaptive reconfiguration of the NK cell compartment characterized by the differentiation and persistent expansion of a subset displaying the CD94/NKG2C NK receptor (NKR). Moreover, a deletion of the NKG2C (KLRC2) gene has been reported to modulate the magnitude of the NK cell repertoire redistribution. Little is known about the expression of NKG2C in CLL and MBL patients.

Aims. To analyse the distribution of NKR, with special attention to NKG2C, in MBL and CLL patients, assessing the relation of the NK cell immunophenotype with clinical features.

Methods. The study cohort included 61 patients, 24 were diagnosed with clinical MBL and 37 were treatment-naïve CLL (32/37 Binet A). The expression of NKG2C, NKG2A, ILT2 (LIR1, LILRB1), CD161, CD57 and KIRs (identified with a cocktail of monoclonal antibodies) was assessed by flow cytometry in peripheral blood NK cells. The NKG2C (KLRC2) genotype was analysed in a larger representative MBL/CLL cohort (n=135).

Results. The proportions of NK cells were reduced in CLL patients compared to MBL (median 5.5% vs. 10%; P=0.003), whereas their absolute numbers were increased (median 0.85x10⁹/L vs. 0.57x10⁹/L; P=0.002). No significant differences between MBL and CLL were detected regarding the distribution of the different NKR: NKG2C (median: 2.7 vs. 5.9%, respectively), NKG2A (31.4 vs. 30.8%), ILT2 (18.0 vs.15.8%), KIRs (54.4 vs. 52.7%), CD161 (16.1 vs. 16.4%) and CD57 (40.4 vs. 38.9%). Though a reduced NKG2C expression was noticed in both entities, it was specially marked in patients with a greater (>30x10⁹ cells/L) lymphocytosis (1.4 vs. 7.7%, P=0.016). The proportions of NKG2C+ NK cells in HCMV⁺ patients (85%, 47/55) as compared to HCMV- individuals were not significantly different (6.3% vs. 2.9%, respectively). HCMV⁺ patients showed a significantly lower NKG2C expression when compared with two independent age-matched cohorts of HCMV⁺ non-CLL/-MBL individuals, including 43 non-metastatic breast cancer patients (4.2% vs. 15.3% , P<0.001); and 30 renal transplantation donors (4.2% vs.12.4% in P=0.028). The frequencies of NKG2C^+/+ (56%), NKG2C^+/del (37%) and NKG2C^del/del (7%) genotypes were comparable to those previously defined in healthy blood donors. Moreover, cases with very low (<2%) or undetectable NKG2C expression were found in NKG2C^del/^del patients (100%, 6/6), but also among NKG2C⁺/^- (45%, 10/22) and NKG2C⁺/⁺ (12%, 3/26) genotypes.

Conclusions. 1. MBL and CLL exhibited low proportions of NKG2C⁺ NK cells. This immunophenotype was particularly evident in CLL patients with increased lymphocytosis and could not be explained by differences in HCMV seropositivity, NKG2C zygosity nor age. 2. Additional studies are required to define the mechanism(s) and putative implications of the reduced NKG2C expression in these lymphoproliferative disorders.

Acknowledgements. PI11/1621; PI15/437; 2017/SGR437; Fundació La Caixa; Fundación Española de Hematología y Hemoterapia (FEHH).