We found that FCRL5 surface protein itself was induced temporarily upon BCR stimulation of human, naive B cells, indicating precise control over timing of FCRL5 engagement. Cross-linking of FCRL5 on cells induced to express FCRL5 enhanced B cell proliferation significantly.
This enhancement required costimulation of the BCR and TLR9, two signals required for optimal proliferation of naive B cells, whereas T cell help in the form of anti-CD40 and IL-2 was dispensable.
In addition, we found that FCRL5 stimulation generated a high proportion of cells displaying surface IgG and IgA. Optimal development of cells expressing switched isotypes required T cell help, in addition to stimuli found necessary for enhanced proliferation. Surprisingly, cells that developed upon FCRL5 stimulation simultaneously displayed surface IgM, IgG, and IgA. Cells expressing multiple Ig isotypes were described in hairy cell leukemia, a disease in which FCRL5 is overexpressed.
Enhanced proliferation and downstream isotype expression upon FCRL5 stimulation could reflect a physiological role for FCRL5 in the expansion and development of antigen-primed B cells.
In addition, FCRL5 may promote growth of malignant cells in hairy cell leukemia and other FCRL5-expressing tumors.