Marina Cibati

Clonal B cells of HCV-associated mixed cryoglobulinemia patients contain exhausted marginal zone-like and CD21 low cells overexpressing Stra13

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.

Telomere-dependent replicative senescence of B and T cells from patients with type 1a common variable immunodeficiency

A subset of patients with common variable immunodeficiency (CVID), group 1a of the Freiburg classification, is characterized by increased B cells expressing low levels of CD21 (CD21low), lymphoproliferation and autoimmunity. The CD21low B cells have been shown to be profoundly anergic, and defects of BCR‐mediated calcium signaling and of T cells have been described in CVID 1a. We found that also the classical naïve B cells from CVID 1a patients, but not from CVID non‐1a patients, proliferated poorly. The B cells of CVID 1a patients had a reduced capacity to divide reminiscent of the proliferative arrest associated with replicative senescence. Thus, we investigated whether lymphocyte dysfunction in CVID 1a was related to telomere‐dependent replicative senescence, and found that both the B and the T cells from CVID 1a patients had significantly shorter telomeres compared with B and T cells from CVID non‐1a patients. Telomere lengths in B and T cells were significantly correlated, indicating that the rate of telomere attrition in lymphocytes is an individual characteristic of CVID patients. Our findings suggest that telomere‐dependent replicative senescence contributes to the immune dysfunction of CVID 1a patients, and may provide an important clue for a better understanding of the pathogenesis of CVID.

Measurement of telomere length using PNA probe by cytometry.

Peptide nucleic acid (PNA) probes hybridize to denatured telomeric sequences in cells permeabilized in hot formamide. In reported protocols, the hybridization was conducted in solutions with high formamide concentrations to avoid the DNA renaturation that can hamper binding of the oligo-PNA probe to specific sequences. We postulated that telomeric DNA, confined in the nuclear microvolume, is not able to properly renature after hot formamide denaturation. Therefore, to improve hybridization conditions between the probe and the target sequences, it might be possible to add probe to sample after the complete removal of formamide.