T Cells from Patients with Chronic Lymphocytic Leukaemia Prevent Apoptosis of Autologous CLL Cells in a Dose- and Cell:Cell-Contact Dependent Fashion; Rationale for New Therapeutic Possibilities.

Background: Multiple T cell abnormalities in Chronic Lymphocytic Leukemia (CLL) patients are well established. Compared to healthy individuals, total T lymphocyte count, CD8⁺ cells and CD4⁺ cells in CLL patients are increased and follow disease activity, i.e. increase at CLL progression. Major alterations in T cell functions and TCR repertoire have also been reported as well as T cells specifically recognizing the leukemic CLL cells. It may be hypothesized that aberrantly functioning T cells contribute to a microenvironment which may allow CLL cells to avoid apoptosis and/or support the proliferative capacity. However, the underlying mechanisms are largely unknown.

Materials and Methods: T cells and CLL cells were highly purified from PBMC of 20 patients with indolent CLL as well as normal blood B- and T-cells from healthy control donors (n=7). Autologous T cells and B cells were co-cultured at different T:B ratios for 72 h in ordinary and transwell plates. Apoptosis of B cells were assayed using Annexin-V and supernatants were analysed by Luminex for the following cytokines: RANTES, IL-10, IL-8, TNF-α, IFN-γ, IL-4, MCP-1, CD40L and GM-CSF.

Results: Apoptosis of CLL cells was significantly (p<0.01) prevented by autologous T cells in a cell-dose-dependent fashion; the highest rate of apoptosis was observed when CLL cells were cultured alone in the absence of T cells. When the experiments were repeated in transwell plates the apoptosis-inhibiting effect of the T cells was completely ameliorated. No corresponding effects were observed when T cells were co-cultured with normal B cells of healthy donors. Luminex analysis revealed that the levels of IL-4, RANTES, MCP-1, TNF-α and IFN-γ were significantly higher in CLL co-culture experiments (especially at high T:B ratios) as compared to CLL cells alone.

Conclusions: Our data provide proof-of-principle that autologous T cells prevent apoptosis of CLL cells in a dose-dependent fashion and the effect was critically dependent on cell:cell contact. A number of cytokine candidates were identified for further studies. The results provide a basis for exploring therapeutic approaches aiming at selective inhibition/depletion of T cells in patients with CLL.