CD4⁺ T Cells Sustain Aggressive Chronic Lymphocytic Leukemia through a CD40L-Independent Mechanism

In Chronic Lymphocytic Leukemia (CLL), mature CD5⁺ B cells accumulate in lymphoid organs such as bone marrow and lymph nodes where they proliferate and expand within localized proliferation centers. In vitro and in vivo data suggest that survival and proliferation of CLL cells within proliferation centers may be also dependent on microenvironmental interactions originating from the surrounding cellular elements (e.g. monocyte-derived nurse-like cells, mesenchymal stromal cells, or CD4⁺ T lymphocytes), that deliver both membrane-bound and soluble signals to CLL cells. In particular, the role of CD4⁺ T cells in vivo is less defined and data in animal models are conflicting as they appear to sustain CLL clone expansion and survival through CD40L-CD40 interactions, though in approximately 40% of patients with CLL, aggressive leukemic clones appear to be independent of CD40 stimulation.

We aimed at clarifying the role of CD4⁺ T lymphocytes taking advantage of the Eμ-TCL1 mouse model, which develops a disease that mimics aggressive, human CLL. To this aim, we generated genetically modified Eμ-TCL1 mice lacking either CD4⁺ T cells (TCL1^+/+AB0), CD40 (TCL1^+/+CD40^-/-), or CD8⁺ T cells (TCL1^+/+TAP^-/-). In these mice, disease appearance and progression were monitored in lymphoid organs and blood by flow cytometry and immunohistochemistry analyses. Findings were confirmed by adoptive transfer of leukemic clones into mice either lacking CD4⁺ T cells, or CD40L, or treated with monoclonal antibodies depleting selected T cell populations, or blocking CD40L-CD40 interactions.

Interestingly, we observed that CLL clones did not expand in mice either lacking or depleted of CD4⁺ T cells, thus confirming that CD4⁺ T cells are essential for CLL development in Eμ-TCL1 mice. On the contrary, in TCL1^+/+TAP^-/- mice, lacking CD8⁺ T cells, disease progression was accelerated, suggesting an anti-tumor activity exerted by this subset of T cells. Specificity of CD4⁺ T cells was marginal for CLL development, as leukemic clones developed regularly in transgenic mice whose CD4⁺ T cells had TCR with CLL-unrelated specificities. Similarly, TCL1^+/+CD40^-/- mice developed frank CLL with no differences compared to controls, as well as leukemic clones expanded when transferred into wild type mice treated with monoclonal antibodies blocking CD40, or into CD40L^-/- mice, suggesting a dispensable role for CD40/CD40L stimulation in the development of murine CLL. Analysis of peritoneal fluid, spleen, lymph nodes and bone marrow showed similar CLL development in Eμ-TCL1 and TCL1^+/+CD40^-/- mice.

In conclusion, our data demonstrates that CD8⁺ and CD4⁺ T cells exert opposite roles in CLL: CD8⁺ T cells restrain CLL progression, whereas CD4⁺ T cells support the expansion of CLL clones in Eμ-TCL1 mice through CD40-indipendent, and apparently non-cognate mechanisms. Further studies are warranted to dissect the nature of the molecules, either soluble or membrane-bound, responsible for the interactions occurring between CD4⁺ T cells and CLL B cells fueling the onset and expansion of CLL cells.