Host B cells escape CAR-T immunotherapy by reversible downregulation of CD19

Anti-CD19-CAR-T cells are a successful clinical immunotherapy for B cell lymphomas, although some lymphomas can escape attack by downregulating surface CD19 levels. An undesirable consequence of this therapy is that it can also eliminate healthy B cells expressing CD19. Therefore, understanding the dynamics of CD19 expression in B cells under CAR-T cell immunotherapy can help mitigate both escape and adverse outcomes. Previous studies suggested that mechanisms responsible for the loss of CD19 expression in lymphomas usually involves genomic deletion or epigenetic modification which permanently removes CD19 as a therapeutic target in these cells. We examined if healthy B cells can use similar processes to lose CD19 expression and escape CAR-T attack. In the presence of CAR-T cells, untransformed B cells both when cultured in vitro or in vivo in non-tumor bearing animals downregulate expression of CD19. We then used adoptive transfer strategies to remove CD19-low B cells from αCD19-CAR-T pressure in vivo. Intriguingly, these B cells systematically recovered surface expression of CD19 comparable to wild-type levels. These data suggest that unlike many cases of lymphomas, healthy B cells downregulate CD19 in a reversible fashion. Taken together, these data suggest a dynamic regulatory process of CD19 surface expression on healthy B cells that could be exploited to modulate the expression of CD19 on cancer cells to improve immunotherapy or minimize the depletion of endogenous B cell compartment during treatment.