Characterization of Residual CLL Following Long-Term Bruton Tyrosine Kinase Inhibitor Therapy

Introduction:

The development of Bruton tyrosine kinase inhibitors (BTKi) and their introduction into clinical practice represent a major advance in the treatment of chronic lymphocytic leukemia (CLL). Ibrutinib and other second generation BTKi as monotherapies generally do not produce minimal residual disease negative (MRD-) complete remissions even with extended therapy. The reason for lack of continued elimination of CLL to a MRD- status over time is unknown, and we hypothesized that biological differences in the CLL cells or immune microenvironment might make them resistant to elimination.

Methods:

Samples were obtained from patients on continuous ibrutinib who hadn't relapsed at time points of 3 years on treatment and 5 years on treatment; and pre-ibrutinib. Isolated CLL cells were subject to B-cell receptor (BCR) sequencing using NEBNext Immune Sequencing Kit by New England Biolabs (NEB, Inc., USA). In a separate cohort, 10X VDJ+5'-sequencing was performed on peripheral blood mononuclear cells. Flow cytometry and ELISA were used to identify alterations in immune cell subtype and identify immune profiles associated with MRD positive (MRD+) status.

Results:

To identify the clonal pattern in MRD+, we performed deep sequencing of the BCR repertoire on samples from 13 patients with 3 time points each. We found that dominant clones tended to remain constant, but new clones appeared in later time points (Figure 1). MiXCR (v3.0.5) was used with default parameters to identify preprocessed reads containing CDR3 regions from B-cell heavy, kappa, and lambda chains, generating a list of unique productive and nonproductive CDR3 sequences associated with their relative abundances and specific V(D)J gene usage. Two out of three patients (patients 1 and 3) showed significant change in the clone over time. In patients 1 and 2, we saw that heavy chain clones emerge at later time points. In patient 3 alone, we observed that at 5 years there are two dominant clones. Our findings suggest that each patient shows a diverse repertoire of CLL clones and that the dominant clone does not change significantly across time points.

To identify cell populations based on gene expression patterns, we performed 10X VDJ+5'-seq. Based on the expression of known markers, we identified CLL cells and other immune cell subtypes. We identified differentially expressed genes (DEGs) for CLL cells in each time points. Over time, we observed upregulation of CD79a, LTB, TAGLN2, and LGALS, genes typically associated with leukemic cell survival. Suggesting differential expression of pro-survival genes contribute to continued presence of MRD over time. T cells are known to be dysfunctional in CLL and have not previously been extensively studied in the setting of long term BTKi. We performed flow cytometry to determine the repertoire and function of T cells at 3 and 5 years of ibrutinib therapy. We found that the percentage of CD3+ T cells increases at later time points in all the 8 patients (p<0.05). Although T cell numbers increase, we do see skewing of these cells towards a terminally differentiated phenotype (p<0.05). We also observed significant increases in NK cells across time points (p<0.05), albeit non-functional due to high expression of inhibitory receptor KLRG1 in 7 out of 8 patients (p<0.05). Although overall the number of immune cells increase in long time ibrutinib therapy, they exhibit exhausted or non-functional phenotypes.

Conclusion:

Extended ibrutinib treatment yields a subset of patients who become MRD- whereas a large majority remain MRD+. Our findings suggest that BCR repertoire in CLL MRD might change in long term ibrutinib therapy and induce necessary genes for its survival in the microenvironment. Although T cells and NK cells are non-functional at later time points, better understanding of these subtypes may lead to new strategies and to improve antitumor function of these cells. Differentiating the biology of why certain patients attain MRD- status on BTK inhibitor is of high interest as it could provide rationale for therapy discontinuation or add on approaches.

Figure 1

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Figure 1

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