Clonal B cells of HCV-associated mixed cryoglobulinemia patients contain exhausted marginal zone-like and CD21low cells overexpressing Stra13


Correspondence: Dr. Massimo Fiorilli, Department of Internal Medicine and Clinical Immunology, Sapienza University of Rome, Viale dell'Università 37, 00185 Rome, Italy

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A clonal population of B cells expressing a VH1-69-encoded idiotype accumulates in hepatitis C virus (HCV) associated mixed cryoglobulinemia (MC). These cells are phenotypically heterogeneous, resembling either typical marginal zone (MZ) B cells (IgM+IgD+CD27+CD21+) or the exhausted CD21low B cells that accumulate in HIV infection or in common variable immunodeficiency. We show that both the MZ-like and the CD21low VH1-69+ B cells of MC patients are functionally exhausted, since they fail to respond to TLR and BCR ligands. The proliferative defect of VH1-69+ B cells can be overcome by co-stimulation of TLR9 and BCR in the presence of interleukin(IL)-2 and IL-10. The MZ-like VH1-69+ B cells do not express the inhibitory receptors distinctive of CD21low B cells, but display constitutive activation of extracellular signal regulated kinase (ERK) and attenuated BCR/ERK signaling. These cells also express abundant transcripts of Stra13 (DEC1, Bhlhb2, Sharp2, Clast5), a basic helix-loop-helix transcription factor that acts as a powerful negative regulator of B-cell proliferation and homeostasis. Our findings suggest that MZ B cells activated by HCV undergo functional exhaustion associated with BCR signaling defects and overexpression of a key antiproliferative gene, and may subsequently become terminally spent CD21low B cells. Premature exhaustion may serve to prevent the outgrowth of chronically stimulated MZ B cells.