IL-7 receptor signaling drives human B-cell progenitor differentiation and expansion

Although absence of interleukin-7 (IL-7) signaling completely abrogates T and B lymphopoiesis in mice, patients with severe combined immunodeficiency caused by mutations in the IL-7 receptor α chain (IL-7Rα) still generate peripheral blood B cells. Consequently, human B lymphopoiesis has been thought to be independent of IL-7 signaling. Using flow cytometric analysis and single-cell RNA sequencing of bone marrow samples from healthy controls and patients who are IL-7Rα deficient, in combination with in vitro modeling of human B-cell differentiation, we demonstrate that IL-7R signaling plays a crucial role in human B lymphopoiesis. IL-7 drives proliferation and expansion of early B-cell progenitors but not of pre-BII large cells and has a limited role in the prevention of cell death. Furthermore, IL-7 guides cell fate decisions by enhancing the expression of BACH2, EBF1, and PAX5, which jointly orchestrate the specification and commitment of early B-cell progenitors. In line with this observation, early B-cell progenitors of patients with IL-7Rα deficiency still expressed myeloid-specific genes. Collectively, our results unveil a previously unknown role for IL-7 signaling in promoting the B-lymphoid fate and expanding early human B-cell progenitors while defining important differences between mice and humans. Our results have implications for hematopoietic stem cell transplantation strategies in patients with T- B+ severe combined immunodeficiency and provide insights into the role of IL-7R signaling in leukemogenesis.

Human and mouse early B cell development: So similar but so different

Early B cell development in the bone marrow ensures the replenishment of the peripheral B cell pool. Immature B cells continuously develop from hematopoietic stem cells, in a process guided by an intricate network of transcription factors as well as chemokine and cytokine signals. Humans and mice possess somewhat similar regulatory mechanisms of B lymphopoiesis. The continuous discovery of monogenetic defects that impact early B cell development in humans substantiates the similarities and differences with B cell development in mice. These differences become relevant when targeted therapeutic approaches are used in patients; therefore, predicting potential immunological adverse events is crucial. In this review, we have provided a phenotypical classification of human and murine early progenitors and B cell stages, based on surface and intracellular protein expression. Further, we have critically compared the role of key transcription factors (Ikaros, E2A, EBF1, PAX5, and Aiolos) and chemo- or cytokine signals (FLT3, c-kit, IL-7R, and CXCR4) during homeostatic and aberrant B lymphopoiesis in both humans and mice.

POS0400 MODULATION OF HUMAN EARLY B CELL DEVELOPMENT THROUGH TARGETED DEGRADATION OF IKAROS AND AIOLOS WITH IBERDOMIDE

Background

B differentiation in the bone marrow (BM) is impaired in patients carrying mutation in the IKFZ1 gene, coding for Ikaros a zinc-finger transcription factor. High Ikaros expression is on the contrary associated with systemic lupus erythematosus[1] and multiple myeloma[2]. Targeted treatment with iberdomide, a cereblon modulator which enhances degradation of Ikaros and Aiolos, is under clinical investigation in multiple myeloma patients and systemic lupus erythematosus. However, consequences of the treatment on human early B cell development remain elusive. Immature B cells develop in the BM from hematopoietic stem cells. An intricate network of transcription factors regulates the maturation process. Ikaros and Aiolos regulate gene expression during B cell development. As reported in mice, Ikaros is essential for the commitment to the lymphoid lineage and later, together with Aiolos, ensures the transition from pre-BII large to pre-BII small cells.

Objectives

Investigate the effect of iberdomide (CC-220) on human early B cell development simulated in vitro.

Methods

We tested the impact of iberdomide on short term culture of BM-derived lymphocytes and in a unique in vitro modeling of early B cell development starting from cord blood (CB)- CD34+ progenitors [3, 4]. We used multi-dimensional spectra flow cytometry (17-color pan-el) to dissect early B cell subpopulations.

Results

Iberdomide treatment led to enhanced degradation of Ikaros and Aiolos in both BM- and CB-derived cultures. Addition of iberdomide early (day 7) to the CB-derived culture impaired the specification to the lymphoid lineage and later also the commitment to the B cell lineage. These observations were confirmed by reduced E2A and PAX5 gene expression, respectively. Treatment with iberdomide on B cell precursors (pro- and pre-B cells, day 28 of culture) on one side it enhanced the proliferation of early progenitors resulting in increased amount of CD10+CD38+ lymphoid-committed cells. On the other side, it resulted in a accumulation of pre-B cells and inefficient development of immature B cells.

Conclusion

Iberdomide impairs the commitment to the lymphoid lineage by enhancing Ikaros’ degrada-tion. When targeting already committed B cells, iberdomide treatment undermines the transition of pre-BII large to pre-BII small cells due to increased Aiolos’ degradation, conse-quently impairing the development of immature B cells. Our data can instruct immunologi-cal monitoring of patients treated with iberdomide, and provide insights in the mechanisms of therapeutic efficacy.

References

[1]Rivellese, F., et al., Effects of targeting the transcription factors Ikaros and Aiolos on B cell activation and differentiation in systemic lupus erythematosus. Lupus Sci Med, 2021. 8(1).

[2]Thakurta, A., et al., Developing next generation immunomodulatory drugs and their combinations in multiple myeloma. Oncotarget, 2021. 12(15): p. 1555-1563.

[3]Kraus, H., et al., A Feeder-Free Differentiation System Identifies Autonomously Proliferating B Cell Precursors in Human Bone Marrow. The Journal of Immunology, 2014. 192(3): p. 1044-1054.

[4]Troilo, A., et al., Nonpermissive bone marrow environment impairs early B-cell development in common variable immunodeficiency. Blood, 2020. 135(17): p. 1452-1457.

Disclosure of Interests

Iga Janowska: None declared, Jakov Korzhenevich: None declared, Julian Staniek: None declared, Raquel Lorenzetti: None declared, Lukas Konstantinidis: None declared, Miriam Erlacher: None declared, Peter Schafer Employee of: BMS, Reinhard Voll: None declared, Jens Thiel Grant/research support from: BMS (former Cellgene), Nils Venhoff: None declared, Marta Rizzi Grant/research support from: BMS (former Cellgene)