Gain-of-function <i>IKZF1</i> variants in humans cause immune dysregulation associated with abnormal T/B cell late differentiation

Abstract

IKZF1/IKAROS is a key transcription factor of lymphocyte development expressed throughout hematopoiesis. Heterozygous germline <i>IKZF1</i> haploinsufficient (<i>IKZF1</i>^HI) and dominant-negative (<i>IKZF1</i>^DN) variants in humans cause B cell immune deficiency and combined immunodeficiency. Here, we identified previously unidentified heterozygous <i>IKZF1</i> variants (R183C/H) located in the DNA binding domain in eight individuals with inflammatory, autoimmune, allergic symptoms, and abnormal plasma cell (PC) proliferation. Leukocytes of patients exhibited specific defects including impaired IL-2 production by T cells, T helper (T_H) skewing toward T_H2, low numbers of regulatory T cells (T_reg), eosinophilia, and abnormal PC proliferation. In contrast to IKZF1^HI and IKZF1^DN, IKZF1^R183H/C proteins showed increased DNA binding associated with increased gene expression of T_H2 and PC differentiation, thus demonstrating that <i>IKZF1</i>^R183H/C behave as gain-of-function (GOF) alleles. In vitro treatment with lenalidomide, known to degrade IKZF1, corrected T_H2 and PC abnormalities caused by IKZF1^R183H/C. These data extend the spectrum of pathological mechanisms associated with <i>IKZF1</i> deficiencies and highlight the role of IKZF1 in late lymphoid differentiation stages.