Transcriptional regulation of human NK cells: TAL1 as a modulator

Background - Natural killer (NK) cells are major cytotoxic lymphocytes subset with potent activity against hematopoietic malignancies and are strong candidates for immunotherapy. However, their clinical application is limited by our incomplete understanding of the transcriptional programs that control NK cell development and function. Human NK cells development from hematopoietic stem cells to mature NK cells is orchestrated by a network of lineage-committing transcription factors (TF). TAL1-a class II basic helix-loop-helix TF-is essential for early hematopoiesis and is maintained in several mature myeloid lineages, but it is absent from mature B- and T-cells. However, we recently discovered that mature human NK cells express TAL1, making them a rare lymphoid population retaining TAL1 expression. We also identified three TAL1 isoforms, suggesting isoform-specific roles in NK cell maturation and function. This suggests that TAL1 may shape NK cell identity and function. We hypothesize that TAL1 is a previously unrecognized regulator of human NK-cell development and cytotoxic activity. To test this, we used (a) an inducible TAL1 knockout mouse model to identify TAL1-dependent checkpoints in NK cell development and (b) TAL1 overexpression in NK-92 cells to characterize TAL1 binding partners. Method - For development and differentiation of NK cells, we generated an inducible TAL1 fl/fl Mx1 Cr e knockout (KO) mouse model, in which poly(I:C) was used to activate Cre. Bone marrow and spleen tissues were then analyzed by flow cytometry. We used CRISPR/Cas9 to generate HEB, ID2, and E2A KO NK-92 cells and created a TAL1-overexpressing NK-92 line. Protein expression and interactions were assessed by Western blot and co-immunoprecipitation. Results - TAL1 KO in mouse showed a significantly increased percentage of NK progenitor cells (CD3ε - CD122 + NK1.1 - NCR1 - ; 91.3% in KO vs 33.0% in WT) and a reduction of immature and mature NK cells (CD3ε - CD122 + NK1.1 + and CD3ε - CD122 + NCR1 + ) in the bone marrow compared to control. Co-immunoprecipitation of TAL1 in NK92 cells revealed protein interactions with methylase SETD1A, and E-proteins HEB, and ID2. Conclusion - TAL1 KO mice show NK cell maturation arrested at the CD122 + NK1.1 + progenitor stage, suggesting a central role for TAL1 in NK cell development. Co-immunoprecipitation in NK-92 cells revealed TAL1 interactions with SETD1A, HEB, and ID2. We propose that TAL1 forms a heterodimer with HEB that recruits the demethylase LSD1 to repress genes required for NK cell maturation. When ID2 joins the complex, it blocks LSD1 recruitment and instead brings in the methylase SETD1A to activate maturation-associated genes. Together, these findings raise the possibility that TAL1 recruits specific partners that influence NK cell maturation. Understanding the role of TAL1 in human NK cell development may enable the therapeutic modulation of NK cell function for clinical applications.