Regulation of Homeostatic and Malignant B Cell Development By the Tetraspanin CD53

Acute lymphoblastic leukemia (ALL) is the most frequent pediatric malignancy, most commonly originating from the transformation of progenitor cells of the B cell lineage (B cell precursor-ALL; BCP-ALL). Treatment of patients with high-risk or relapsed disease is difficult and prognosis remains poor in pediatric patients, with an even worse survival rate for adult BCP-ALL. Previous studies have shown an association of enhanced CD53 expression with many B cell malignancies, suggesting upregulation of CD53 may be implicated in carcinogenesis or maintenance of malignant cells. CD53 is a member of the tetraspanin family of transmembrane proteins, classically involved in cell adhesion, proliferation, and survival, and expressed exclusively on hematopoietic cells. While several studies have implicated a role for CD53 in regulating mature B cell proliferation, its role in early B cell development is not yet known. To elucidate the contribution of CD53 to normal and malignant B cell development, we have generated a CD53 knockout mouse.

In our CD53^-/- mouse, we observe no differences in total white blood cell counts, yet the fraction of peripheral blood B cells is significantly reduced by 31% compared to wild-type (WT) controls (28.3% vs. 19.5%; p<0.005). During homeostatic B lymphopoiesis, CD53 increases through development, beginning at the pre-pro-B cell stage and reaching highest expression on mature B cells. Further investigation into the loss of B cells revealed that immature pre-B cells in the bone marrow and mature B cells in the spleen and lymph nodes are significantly diminished upon loss of CD53, resulting from increased apoptosis in CD53^-/- mice. B cell differentiation of CD53^-/- hematopoietic stem cells (HSCs) in vitro corroborates the dependence on CD53 for normal differentiation, as CD53^-/- cultures have 26% fewer B cells than controls (p=0.033). Investigation into the signaling differences between WT and CD53^-/- B cell progenitors by mass cytometry (CyTOF) suggests that decreased PI3K/Akt and MAPK signaling could be driving this loss.

With the observed loss of both B cell progenitors and mature B cells in CD53-deficient mice, CD53^-/- mice were recently crossed to Eμ-Myc transgenic mice, a model of B-lineage leukemia/lymphoma, to generate WT, CD53^-/-, Eμ-Myc⁺;CD53^+/+, and Eμ-Myc⁺;CD53^-/- groups to assess whether loss of CD53 alters the pathology or survival of these mice. As observed in human patients, moribund Eμ-Myc⁺ mice significantly upregulate CD53 on malignant cells, suggesting a potential role for CD53 during pathogenesis. Ongoing experiments are aimed at elucidating the mechanism by which CD53 promotes homeostatic B cell development and determining the potential of CD53 as a therapeutic target for B lineage malignancies.