“AbroGATAed” human NK cell development

In this issue of Blood, Mace et al from the Orange laboratory define a role for the GATA2 transcription factor in human natural killer (NK) cell development and function, highlighting how the study of human disease leads to new insights into normal immunobiology.¹ 

GATA2 is a master transcription factor that is critical for hematopoiesis and regulates the hematopoietic stem cell compartment. GATA2^−/− mice have a fatal embryonic failure of hematopoiesis caused by hematopoietic stem cell (HSC) defects,²  making the mechanistic study of this transcription factor difficult in specific blood lineages. Spontaneous or autosomal dominant mutations in GATA2 have recently been described, unifying a number of syndromes with varying clinical presentations that are characterized immunologically by alterations in dendritic cells, B cells, and NK cells.^3-6  These patients have recurrent opportunistic infections by Mycobacterium species and certain viruses, especially human papilloma virus and herpesviruses. In addition, these patients have an increased risk of developing myelodysplasia and myeloid malignancies early in life, as well as defects in lymphatic vascular development. Thus, for the practicing hematologist encountering patients with these clinical presentations, mutations in GATA2 should be considered. Currently, it is not apparent why mutation of GATA2 results in heterogeneous phenotypes in affected patients, including the immunologic abnormalities.

While clearly affecting upstream HSCs, a specific role for GATA2 has not been defined in maturing peripheral human NK cells—innate immune lymphocytes important for host defense against viruses that also respond to malignant tumor cells.⁷  Human NK cells are divided into 2 developmentally related, but functionally distinct, subsets in the peripheral blood based on the CD56 expression.^8,9  CD56^bright NK cells (also referred to as stage 4 NK cells) are the immediate precursors to CD56^dim NK cells, reside primarily in secondary lymphoid tissues, produce cytokines when activated, and may play an immunoregulatory role. CD56^dim NK cells (stage 5) are the major NK-cell subset in peripheral blood, express KIR and CD16 (FcγRIIIa), and are normally competent for cytotoxicity when triggered by the appropriate virus-infected or tumor target cell. Mace and colleagues carefully investigate the NK cell compartment in a cohort of 8 patients with GATA2 mutations,¹  which includes the first patient reported with an NK cell deficiency.¹⁰  This study confirms reduced overall NK cell (CD56⁺CD3⁻) numbers in the majority of GATA2-mutated patients, as well as decreased natural killing and antibody-dependent cellular cytotoxicity in their peripheral blood—defects in classical NK cell functions. Moreover, after accounting for reduced NK cell numbers, a functional defect in NK cell killing on a per cell basis was also identified. The immunophenotype of residual NK cells, including maturation and functional markers, appeared normal but revealed that the only NK cells present were from the CD56^dim subset. Thus, the authors made an unexpected discovery: the CD56^bright NK-cell subset was selectively absent from the peripheral blood in GATA2-mutated patients (see figure). GATA2 protein was found to be expressed 10-fold higher in CD56^bright compared with CD56^dim NK cells in normal donors, suggestive of an important role for this transcription factor in regulating the CD56^bright NK-cell subset, including its differentiation. Supporting an intrinsic role for GATA2 in this phenotype, CD34⁺ HSCs from a GATA2-mutated patient developed fewer NK cells compared with normal donor controls using an established in vitro differentiation system, and those NK cells that did develop were CD56^dim NK cells, recapitulating the findings in the patients' peripheral blood. Moreover, 2 patients treated with systemic interferon therapy exhibited improvements in NK cell number and/or a partial rescue of the functional NK defect, but no CD56^bright NK cells were detected. Finally, 2 patients with relatively intact NK cell numbers also selectively lacked CD56^bright NK cells. Thus, GATA2 deficiency results in reduced overall NK cells, compromised NK cell cytotoxicity, and lack of CD56^bright NK cells, likely contributing to the recurrent viral (and potentially other) infections in the majority of these patients.

For human NK cell biology, this study raises intriguing questions about the role of GATA2 at several levels. First, it appears that CD56^bright NK cells depend on normal GATA2 expression for development and/or survival. It will be interesting to investigate secondary lymphoid tissues and bone marrow from GATA2-mutated patients (and normal HSCs with altered GATA2 expression) to define the impact on numbers of CD56^bright NK cells and upstream NK cell precursors and progenitors present. Similarly, what is the role of GATA2 in other related innate lymphoid cells (ILCs), such as thymic NK cells and ILC-22s? Second, CD56^dim NK cells were present at reduced numbers and their cytotoxic capacity was reduced on a per-cell basis. Is this defect a result of altered development or education through the CD56^bright stage, or does it represent a novel role for GATA2 in mature NK cell killing? Further, is this a global functional alteration that extends to interferon-γ production and proliferation or specific to cytotoxicity? Third, although the preponderance of data supports a linear development of CD56^dim NK cells from CD56^bright NK cells,^7,9  this study raises the possibility that alternative precursors exist for the CD56^dim NK cell subset. Finally, why do a small number of GATA2-mutated patients have near normal NK cell numbers? Possibilities include responses to active infection, the type of GATA2 mutation present, and the potential for alterations in this immune cell compartment over time. Thus, while emphasizing the importance of NK cells for host defense to viral infections in humans afflicted with GATA2 mutations, this study generates a number of novel hypotheses regarding the contribution of GATA2 as a regulator of the NK cell molecular program.

In summary, this report is an outstanding example of how studying the immunologic defects in patients with rare diseases can inform our understanding of normal immune cell development and function, and leads to a new avenue of research regarding the role of GATA2 in human NK cell biology.