Identification of a Novel Proliferating Cell Fraction in Chronic Lymphocytic Leukaemia with High Expression of IgM and Chemokine Receptors

Chronic lymphocytic leukaemia (CLL) is characterised by the accumulation of malignant CD5+ B cells in the peripheral blood (PB), secondary lymphoid tissues and bone marrow. Currently considered an incurable disease, B cell receptor (BCR) signalling plays a key role in the disease aetiology as evidenced by the therapeutic success of BCR signalling inhibitors such as ibrutinib. Previous studies using incorporation of ²H-labelling of DNA in vivo demonstrated sub-clonal heterogeneity in PB CLL cell fractions sorted based on reciprocal densities of chemokine C-X-C motif receptor 4 (CXCR4) and CD5. The CXCR4 ^loCD5 ^hi fraction was shown to be enriched in recently born proliferating cells while the CXCR4 ^hiCD5 ^lo fraction consists of resting, quiescent cells thought to reflect their migratory and BCR signalling histories in tissue. Whilst these proliferating/resting fractions have since been more closely examined, the remaining bulk PB CLL population has been left relatively unexplored leaving other therapeutically relevant cell fractions undetected.

Here, we have comprehensively analysed the phenotype of subpopulations of PB cells from 11 CLL patients using flow cytometry to identify activated and proliferating cell fractions. CD19 ⁺CD5 ⁺cells were divided into 9 fractions based on CXCR4/CD5 densities and to permit comparisons between fractions, each cell fraction was defined as containing 1-2% of the total clonal CD19 ⁺CD5 ⁺ population. Surprisingly, we detected enrichment for Ki67+ proliferating cells and high expression of AID in the cell fraction with highest expression levels of both CXCR4 and CD5 (CXCR4 ^hiCD5 ^hi), demonstrating that CXCR4 ^loCD5 ^hi cells are not the only proliferating fraction in the blood. Moreover, we could detect mitotic cells in the CXCR4 ^hiCD5 ^hi fraction using imaging flow cytometry of a nuclear stain. This CXCR4 ^hiCD5 ^hi fraction showed the highest surface expression levels of IgM, CD86, CCR7, CXCR3 and CXCR5 of all the fractions assessed (p<0.05), indicating they are highly activated and primed for migration to lymph nodes (LNs) for further activation and proliferation.

Proliferation of CLL cells is highest in secondary lymphoid tissues, however the phenotype of proliferating cells in tissue is unknown. To examine the phenotype of proliferating CLL cells in LNs, we analysed a fine-needle aspirate obtained from an enlarged cervical node using flow cytometry and compared this to a matched PB sample. Flow cytometric gates set on the PB sample were used to define and quantify LN cell fractions. Expression levels of both Ki67 and surface IgM were highest in the CXCR4 ^hiCD5 ^hi fraction which was expanded to 20% of the CD19 ⁺CD5 ⁺ population in the LN whilst CXCR4 ^loCD5 ^hi cells (accounting for 2% of the bulk LN population) expressed very low surface IgM and Ki67 levels, suggesting CXCR4 ^hiCD5 ^hi cells may be the most proliferative cells in CLL.

The CXCR4 ^loCD5 ^hi cell fraction has been shown to be a key target of ibrutinib, however the impact of ibrutinib on the CXCR4 ^hiCD5 ^hi fraction is unknown. Administration of ibrutinib to PB CLL cells for 48hr in vitro resulted in selective targeted depletion of the CXCR4 ^loCD5 ^hi fraction, as evidenced by induction of apoptotic markers in this compartment; conversely, persistent cells after 48hr ibrutinib administration in vitro were exclusively of the CXCR4 ^hi phenotype.

In conclusion, we have identified a potentially dangerous fraction of proliferating cells in the PB of CLL patients with high expression of CXCR4, CD5, IgM, CCR7, CXCR3 and CXCR5 open for both migration to tissue and reception of BCR signals. Furthermore, CXCR4 ^hiCD5 ^hi cells in the periphery may closely mirror tissue-resident activated cell phenotypes and may represent critical targets for therapeutic intervention, particularly in high-risk CLL patients refractory to BCR inhibitor therapies.