Correspondence: Dr SS Watowich, Department of Immunology, UT MD Anderson Cancer Center, Houston, TX 77030-1904, USA. E-mail: swatowic@mail.mdanderson.org

Understanding the molecular control of blood and immune cell development is a major interest in biomedicine because of the potential to design strategies to treat conditions such as leukemia, autoimmunity and chronic inflammation. Indeed, therapeutics, such as erythropoietin, granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor, that modulate blood and immune cell production or function are among the most successful drugs to emerge from biomedical research to date. In the recent issue of Cell, Boris Reizis, Babacar Cisse and colleagues provided a new advance in this field through their discovery of an essential molecular regulator of the plasmacytoid dendritic cell (PDC) lineage, the transcription factor E2-2.¹ Their research also revealed the basis for persistent respiratory infection in patients with the devastating Pitt–Hopkins syndrome (PHS).

Plasmacytoid dendritic cells were discovered only recently² and are integral to the host defense against viruses. A major function of PDCs is their ability to rapidly produce high amounts of type I interferon,² which elicits a cascade of antiviral countermeasures in the infected host. Conversely, deregulation of PDCs seems to contribute to autoimmune diseases such as psoriasis and systemic lupus erythematosus.^{3, 4} However, little is understood about the molecular control of PDC development and the events that specify the distinction between PDCs and other immune lineage cells.

Reizis, Cisse and colleagues addressed this question by first studying the pattern of basic helix-loop-helix (E protein) transcription factor expression within PDCs and determining how it differed from other immune cells such as 'conventional' dendritic cells (cDCs) and B and T lymphocytes. Transcription factors specify immune and blood cell development, with certain combinations being essential for the correct amounts and ratios of individual cell lineages.⁵ Cisse et al. identified E2-2 as being expressed to high amounts in PDCs relative to other immune cells. Because of this pattern and its similarity with other helix-loop-helix lineage transcription factors in the immune system, E2-2 appeared to be a strong candidate for regulating PDC development. Indeed, a recent report described E2-2 as an important transcriptional regulator of human PDC development.⁶

Cisse et al. performed a series of molecular genetic studies to test the function of E2-2 in PDCs, including analysis of constitutive and conditional E2-2 'knockout' mice. E2-2^-/- bone marrow chimeric mice showed a severe deficiency in PDC development but not any other blood lineages, including the closely related cDCs, which share a common progenitor with PDCs in the bone marrow^{7, 8} (Figure 1). In fact, cells that have characteristics of PDCs (that is, CD11c⁺ CD11b^- Ly-6C⁺ Bst2^lo/-) are found in E2-2-deficient bone marrow, but these cells lack the full compliment of PDC markers. Therefore, E2-2 function is critically required to pass beyond a PDC-restricted developmental stage, downstream of the common DC progenitor (Figure 1).

Figure 1.

E2-2 function in the plasmacytoid dendritic cell (PDC) developmental pathway. DC development proceeds from hematopoietic stem cells (HSCs) through a common DC progenitor (CDP). Hypothetical precursors for PDCs (pre-PDC) may be specified by the differential expression or activation of transcription factors within CDPs. In the PDC lineage, E2-2 regulates terminal development and interferon production by controlling PDC-related genes.

Full figure and legend (45K)

Furthermore, conditional E2-2 deletion resulted in a significant and rapid decline in PDC amounts in bone marrow and spleen, major sites of PDC accumulation in adult mice. These results underscore the necessity for E2-2 in PDC development and confirm that PDCs undergo rapid turnover in vivo, requiring constant replenishment from bone marrow progenitors. Significantly, bone marrow and blood samples from E2-2-deficient sources failed to respond to the viral mimetic CpG by upregulating type I interferon production. This was important evidence of a true PDC deficiency as it was formally possible that E2-2 controlled the expression of cell surface markers used to define PDCs, but did not affect terminal maturation. Both copies of the E2-2 gene are essential for PDC development and for efficient production of type I interferon, as shown by defects in PDCs from E2-2 heterozygous (E2-2^+/-) mice.

One of the most exciting findings by Reizis, Cisse and colleagues was the confirmation of PDC deficiency in patients with PHS. This is a multifaceted genetic disease, recently linked with loss-of-function mutations in E2-2,^{9, 10} that affects neural and craniofacial development, causing mental retardation and motor dysfunction. Notably, two of the three patients examined suffer from recurrent respiratory infections, indicating a deficiency in their immune response. Cisse et al. showed that PDC amounts appear lower in PHS patients. Significantly, the PDCs have an abnormal surface phenotype and fail to respond to CpG treatment by secreting type I interferon, similar to murine E2-2^+/- PDCs. This indicates that E2-2 function is conserved between mice and humans, providing rationale for future studies with murine models that mimic human E2-2 deficiency.

The molecular basis of E2-2's action appears to be the control of genes that are essential for PDC terminal maturation and function (Figure 1). For example, E2-2 directly regulates the synthesis of other transcription factors that are important for PDC development (SpiB, IRF8), specific markers for the PDC lineage (BDCA-2, ILT7, PTCRA), and genes that control the production of interferon (IRF7).¹ The regulation of E protein target genes in PDCs by E2-2 explains the apparent 're-activation' of lymphoid genes within this lineage, which diverges from lymphocytes early in the developmental pathway.

Collectively, Cisse et al. showed that E2-2 has many characteristics of a blood cell lineage determination factor, as its correct dosage is essential for PDC development and it directly controls genes critical for PDC maturation and function. The mechanisms that dictate specification to the PDC lineage from the common DC progenitor remain, however, obscure. Similarly, the pathways that regulate cDC lineage commitment are unclear (Figure 1). For PDCs, lineage specification is most likely to involve E2-2 but it does not appear to be absolutely dependent upon E2-2 function, as apparent PDC precursors are seen in E2-2-deficient bone marrow. E2-2 is most likely to work in concert with other PDC-related transcription factors to regulate PDC specification and lineage development. It will be important to determine how E2-2 and other PDC transcription factors are controlled, and how they cooperate to instruct the specification and development of PDCs.