Late-onset neutropenia following rituximab results from a hematopoietic lineage competition due to an excessive BAFF-induced B-cell recovery.

Rituximab is used in the treatment of lymphoma and autoimmune diseases, for which late-onset neutropenia (LON) were reported. LON-related mechanisms remain unclear. To obtain insights into the mechanisms, we assessed serum, peripheral blood and bone marrow (BM) samples of a patient with LON. Factors classically associated with neutropenia such as anti-neutrophil antibodies, T-LGL, soluble Fas Ligand were not detectable. We then evaluated the kinetics of various cytokines involved in B-cell and granulocyte homeostasis. We found that LON is related to a lack of granulopoiesis in the BM that coincides with a very high level of BAFF, a strong stimulator of B-cell recovery, and hypothesized a hematopoietic lineage competition due to an excessive B-cell recovery in the BM by promotion of B-cell lymphopoiesis over granulopoiesis within common developmental niches. Assessment of serum BAFF levels following rituximab could detect patients at risk of developing LON.