Plasticity of NK-cell differentiation

In this issue of Blood, Grzywacz and colleagues provide in vitro experimental evidence that human natural killer (NK) lymphocytes may derive from myeloid precursors and that microenvironmental factors such as stromal cells and hydrocortisone (HDC) may play a key role in favoring such differentiation.¹ 

In the past years the model of hematopoiesis has been revised. The statement that an early dicothomy between myeloid cell fate and lymphoid cell fate occurs during hematopoiesis, has been disclaimed both in mice and humans.^2,3  Current models of hematopoiesis suggest that hematopoietic stem cells (HSCs) may commit early to the erythroid/platelet lineages or to leukocyte lineages. However, once committed to the leukocyte lineage, hematopoietic precursors retain a high degree of plasticity. It has been conclusively shown that human multilymphoid progenitors (MLPs) retain myeloid differentiation potential.³  The choice to terminally differentiate toward myeloid or lymphoid lineages would then depend on the role of lineage-specific transcription factors and on a permissive microenvironment.^2,3 

NK cells are lymphoid cells belonging to natural immunity. They play a dual role as effector cells and regulatory cells. NK cells are able to interact both with cells of the innate and of the acquired immune response.⁴  They share common precursors with dendritic cells (DCs) and T lymphocytes,⁵  and it has been proposed that they share precursors also with monocytes.^3,6 

Grzywacz et al confirm previous reports,^5-7  showing that stromal cells or HDCs, used alone or in combination, may greatly increase the in vitro NK-cell differentiation from CD34⁺CD38^−/+ cells in the presence of appropriate cytokines. More importantly, they demonstrate, at single-cell level, that common myeloid precursors (CMPs) and granulocyte-monocyte precursors (GMPs) retain the ability to differentiate into functional mature NK cells. They show that NK lineage maintenance is conserved until late steps of monocyte development, and that only the combined use of stromal cells and HDCs can efficiently recruit CMP and GMP cells to drive their differentiation toward NK cells.¹  Only in cases of bright expression of myeloid surface markers (CD13) or high concentrations of macrophage colony-stimulating factor (M-CSF) growth factor, myeloid precursors irreversibly differentiate toward monocytes.¹  These results imply that the microenvironment may play a key role. The presence of soluble factors (cytokines and/or hormones) and of peculiar stromal cells expressing NOTCH ligands⁸  or membrane-bound interleukin-15⁷  would make the difference in CD34⁺ precursors differentiation. Notably, these factors/cells have been shown to play a crucial role also in NK-cell differentiation from NK-committed CD34⁺ precursors present in human secondary lymphoid organs and human decidua.^5,9 

Grzywacz and colleagues suggest that the positive effect of the combined use of HDCs and stroma may induce up-regulation of the WNT pathway leading to the activation of the TCF/LEF transcription pathway, known to be involved in lymphocyte development.¹  However, previous reports suggested that corticosteroids could induce NK-cell differentiation from CD33⁺ (CD14^+/−) myeloid cells even in the absence of stromal cells.^6,10  In particular, pharmacologic concentrations of Methylprednisolone induced differentiation of CD33⁺ cell precursors toward NK cells.¹⁰  These results suggest that corticosteroids may support NK-cell differentiation from myeloid precursors, possibly exerting a direct effect on these cells. In this context, it would be interesting to evaluate whether corticosteroids could be involved in the up-regulation of the expression of transcription factors such as E4BP4, shown to be involved NK-cell development.¹¹ 

NK cells develop in a step-by-step process, characterized by phenotypically identified stages. They first acquire CD161, 2B4, CD56, and CD94/NKG2A surface molecules. The acquisition of LFA-1, NKp46, NKp30, NKG2D, and DNAM-1 activating receptors correlates with the gradual acquisition of cytotoxicity. The expression of CD16 and KIR occurs at later stages and is hardly observed in vitro.⁵ 

Grzywacz et al show that NK cells derived from M-CSFR⁺ myeloid progenitors express higher percentages of CD16⁺KIR⁺ cells and display higher cytolytic activity against B-Epstein-Barr virus (B-EBV) cell lines than NK cells derived from more immature M-CSFR⁻ progenitors. These results suggest that NK cells derived from M-CSFR⁺ progenitors may differentiate more rapidly or follow an alternative differentiation pathway compared with NK cells derived from M-CFR⁻ progenitors. In this context, it may be relevant to analyze the 2B4 receptor function. 2B4 acts as inhibitory rather activating receptor in immature NK-cell precursors, because of the lack of the cytoplasmic adaptor protein called signaling lymphocytic activation molecule–associated protein (SAP) that is not expressed at early stages of differentiation.¹²  The inhibitory activity of 2B4 impairs the NK cell–mediated lysis of B-EBV cell lines. Thus, cytolytic NK cells derived from M-CFR⁺ progenitors could display an activating 2B4 in agreement with their more advanced stage of differentiation. Conversely, M-CSFR⁻ progenitor-derived NK cells could still express an inhibitory 2B4 receptor because of the lack of SAP.

The possibility that myeloid precursors, in the presence of stromal cells and HDCs, could generate more rapidly functional NK cells also offers important clues to several clinical observations. It is conceivable that NK cells, present in high percentages in peripheral blood of patients at the earliest time intervals after HSC transplantation may derive, at least in part, from myeloid precursors. NK cells have been shown to exert an important role in preventing leukemic relapses after transplantation,¹³  and an increasing number of clinical trials are assessing the efficacy of NK cell–mediated immunotherapy. In this context, the study by Grzywacz and colleagues offers new insights in the physiology of the NK-cell development and underlines the need for further investigation on the effect exerted by conditioning regimens and corticosteroids on the myeloid/NK-cell differentiation pathways after HSC transplantation.