Primary CLL Xenograft: A Model System to Study the Role of T-Cells in CLL Biology and Therapeutic Response

The interaction between chronic lymphocytic leukaemia (CLL) cells and T-cells is an important aspect of CLL biology. CLL cells require T-cell support for their proliferation and in addition induce proliferation of regulatory and cytotoxic (CD8⁺) T-cells. T-cell number and repertoire are both markedly affected by CLL therapy and there is considerable interest in how current treatments modulate the interaction between T-cells and the tumour clone. In this study we investigated whether this relationship was maintained in a xenotransplantation model.

CLL engraftment in NOG mice was facilitated by humanisation of the murine microenvironment by allogeneic CD34⁺ umbilical cord cells or CD14⁺ monocytes. Accelerated engraftment of both CLL and T-cell compartments was observed in xenografts derived from patients with progressive CLL, suggesting that the biological properties of both subsets are maintained in the murine model. Furthermore, the distribution of helper (CD4⁺), cytotoxic (CD8⁺) and regulatory (CD4⁺CD25⁺FoxP3⁺) T-cells was maintained within the xenografts, including retention of the CD4:CD8 ratio. Interestingly, the anergic PD-1⁺CD160⁺CD244⁺TIM3⁺ T-cell phenotype reported in CLL patients was also evident in T-cells expanded in xenograft models. Consistent with an anergic T-cell phenotype, T-cells from CLL xenografts lacked anti-tumour activity in vitro. Importantly, such anergic cells were observed when T-cells were reconstituted from allogeneic cord blood cells as well as autologous cells, suggesting that CLL cells have the ability to shape T-cell populations of different origin in diverse microenvironments. Finally, to investigate the interaction between specific T-cell subsets and engrafted CLL cells, CD4⁺, CD8⁺, and CD25⁺ T-cells were depleted prior to generation of xenografts. CD8⁺ T-cell depletion significantly prolonged CLL engraftment (p≤0.01) whereas neither depletion of CD4⁺ nor CD25⁺cells had a significant impact.

In summary, our results demonstrate that the relationship between CLL tumour cells and reactive T-cells is accurately maintained in a murine xenograft model. Such models will be of great value for investigation of aspects of T-cell function in CLL biology.