Summary: Cytotoxic CD4⁺ T cells have been found in patients with chronic lymphocytic leukaemia (CLL) and seem to be involved in the regulation of malignant B cells. The CD4⁺ T regulatory cells (Tregs) can regulate various immune cells, including B cells, by inducing their apoptosis. Hence, different subgroups of CD4⁺ T cells may be involved in the regulation of malignant B cells. In this study, the cytotoxic phenotype and function of various CD4⁺ T‐cell subgroups were investigated in patients with B‐cell malignancies. Peripheral blood was collected from patients with CLL, various B‐cell lymphomas, healthy adult donors, children with precursor B‐cell acute lymphoblastic leukaemia (pre‐B ALL) and from healthy children. CD4⁺ T cells (CD3⁺ CD4⁺ FoxP3⁻), Tregs (CD3⁺ CD4⁺ CD127^low FoxP3⁺) and CD127^high FoxP3⁺ T cells (CD3⁺ CD4⁺ CD127^high FoxP3⁺) were analysed for their expression of the cytolytic markers CD107a and Fas ligand. Patients with CLL had increased CD107a expression on all tested T‐cell subgroups compared with healthy donors. Similar results were found in patients with B‐cell lymphomas whereas the CD107a expression in children with pre‐B ALL was no different from that in healthy controls. Fas ligand expression was similar between patient cells and cells of healthy donors. CD4⁺ T cells and Tregs from patients with CLL and healthy donors were subsequently purified and cultured in vitro with autologous B cells. Both subgroups lysed B cells and killing was confirmed by granzyme ELISAs. In conclusion, cytotoxic populations of CD4⁺ T cells, including Tregs, are present in patients with B‐cell malignancy and may be an important factor in immune‐related disease control.

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