Rituximab, an antibody directed against the CD20 antigen expressed on the surface of B lymphocytes, has for several years constituted a first-line strategy in B-cell-mediated autoimmune diseases. By depleting these cells, it aims to suppress the production of the autoantibodies responsible for the disease. However, a substantial proportion of patients fail to respond to treatment or relapse upon reconstitution of the B-cell compartment. The cellular basis of these relapses within secondary lymphoid organs has remained poorly understood in humans, owing to the lack of access to these tissues. The authors took advantage of a particular clinical situation: therapeutic splenectomy performed in patients with relapsed immune thrombocytopenia, which provides access to the spleen, a major lymphoid organ.
By analyzing the immunoglobulin gene repertoire of B lymphocytes and plasma cells, both in bulk and at the single-cell level for antigen-specific cells, the team demonstrates that relapses rely on two responses coexisting within the splenic germinal centers. The first involves pre-existing, already mutated memory B cells that survived rituximab treatment; the second involves naïve B cells newly engaged during reconstitution of the B-cell compartment. Clones specific for the glycoprotein GPIIbIIIa, the characteristic autoimmune target of immune thrombocytopenia, were found in both populations, confirming their pathogenic nature.
To characterize the memory B cells that escaped depletion, the researchers studied refractory patients who were non-responders at the time of depletion. Single-cell RNA sequencing revealed a population of quiescent splenic memory B cells displaying a distinctive phenotype, shaped by rituximab but reversible: reduced expression of B-cell-specific factors and expression of genes promoting cell survival. These rituximab-resistant autoreactive cells reactivate once the molecule is cleared, giving rise to plasma cells and fueling new germinal center reactions.
A key point is that these resistant memory B cells retain surface expression of the CD19 antigen. The authors show that they can therefore be eliminated in vitro by anti-CD19 CAR-T cells, making them accessible targets for currently available anti-CD19 therapies. This study thus identifies a cellular contributor to the persistence of autoimmune diseases that may be reached by existing therapeutic agents.