Endogenous CD4 T cells that recognize ALK and the NPM1::ALK fusion protein can be expanded from human peripheral blood.

Anaplastic lymphoma kinase (ALK)-fusion proteins resulting from chromosomal rearrangements are promising targets for cancer immunotherapy. While ALK-specific CD8+ T cells and epitopes presented on MHC class I have been identified in patients with ALK-positive malignancies, little is known about ALK-specific CD4+ T cells. We screened peripheral blood of ten ALK-positive anaplastic large cell lymphoma (ALK+ALCL) patients in remission and six healthy donors for CD4+ T-cell responses to the whole ALK-fusion protein, nucleophosmin (NPM1)::ALK. ALK-specific CD4+ T cells were detected in 15 individuals after stimulation with autologous dendritic cells pulsed with long-overlapping ALK peptide pools. CD4+ T-cell epitopes were predominantly located within three specific regions (p102-188, p257-356, p593-680) in the ALK portion of the fusion protein. We detected CD4+ T cells in one patient that recognized the NPM1::ALK fusion neoepitope and identified a corresponding T-cell receptor (TCR) by TCR single-cell sequencing. The NPM1::ALK fusion-specific TCR was HLA-DR13 restricted and conferred antigen specificity when expressed in a TCR- reporter cell line (58--). Together, our data provide evidence of ALK-specific CD4+ T cells in human peripheral blood, describe target epitopes in patients and support the consideration of CD4+ T cells in the development of ALK-specific immunotherapies.