In Vitro Co-Culture of CLL-B Cells Reveals Long-Term Survival, Proliferation, and Maintenance of Telomere Length

Telomere length is a prognostic factor in Chronic Lymphocytic Leukemia (CLL) with short telomere length a powerful predictor of early time to first treatment and reduced overall survival. However, little is known about telomere dynamics through the course of an individual patient's disease. Our recent longitudinal analysis of CLL B-cell telomere length revealed very little dynamic change within individual patients with a mean erosion rate of -52bp/year (p=0.05). In marked contrast, T-cells derived from the same patients showed a significantly higher mean erosion rate of -119bp/year (p=0.02) with a median follow up time of 69 months. Here we present data derived from long-term in-vitro co-culture of peripheral blood from CLL patients coupled with temporal analysis of their telomere length dynamics.

We utilized a multi-cellular co-culture system, comprised of autologous T-cells and CD40L-expressing mouse fibroblasts, to maintain CLL cells in long-term culture. Patient-derived peripheral blood mononuclear cells (n=16) were maintained for a median of 70 days (range 54-154); samples were analyzed every two weeks for tumor cell telomere length and evidence of proliferation. We used fluorescence-activated cell sorting (FACS) to sort populations of CD19⁺CD5⁺ CLL B-cells and CD3⁺ T-cells from each of the cultures. We then performed high-resolution single telomere length analysis (STELA) on these sorted subsets of cells and analyzed their telomere dynamics over this extended time course. Analysis of CLL B-cells from these cultures revealed significantly increased Ki-67⁺ at day 14 when compared to day 0 (p<0.001) and this was evident for the duration of the cultures. Despite sustained tumor cell proliferation, we observed no significant difference in the CLL B-cell telomere length with a mean TL at the start of 4.5kb vs 4.3kb at the end (p=0.14).

The presence of T-cells was shown to be critical for the maintenance of the long-term cultures in two ways. Firstly, cultures that were treated with 4μM fludarabine showed a catastrophic reduction in T-cells (p=0.01), which was associated with a significantly shorter duration of survival of CLL B-cells when compared to untreated controls (median 17.5 days (range 7-70); p<0.001). Secondly, it proved impossible to maintain T-cell depleted, purified CLL B-cells, in long-term culture. T-cells isolated from the long-term cultures showed evidence of proliferation with Ki-67⁺ again being increased at day 14 in comparison to baseline (p=0.003). Furthermore, T-cells derived from these cultures showed a significant alteration in subset composition over time with a decrease in the numbers of naive CD4⁺ (p=0.05) and CD8⁺ (p=0.02) T-cells and a corresponding increase in effector memory (p=0.2) and terminally differentiated effector memory (EMRA) subsets (p=0.07).

In conclusion, this study demonstrates that we have developed a robust, long-term culture method for the maintenance of CLL cells. Despite evidence of sustained CLL proliferation, CLL B-cells showed little telomere length erosion during long-term co-culture and this is compatible with our recent ex-vivo analysis, which showed that the telomere length of CLL B-cells are remarkably stable with a mean erosion rate of only -52bp/year. In both ex-vivo and in-vitro analysis, telomere erosion correlated with starting telomere length (r²=0.14, p=0.04 and r²=0.3 p=0.03 respectively). Taken together, our in-vitro and ex-vivo data imply that the radically short telomeres observed in some CLL patients are not the result of increased proliferation of the malignant B-cell, but rather the mutagenic event occurs in a B-cell which already has short telomeres. Furthermore, our novel long-term culture model has reinforced the vital role of T-cells in sustaining CLL B-cells viability and proliferation in-vitro. Given the consistent skewing of the T-cell pool towards a memory phenotype it seems unlikely that this is driven in-vitro by cognate TCR antigen recognition but rather a cytokine-mediated response.