No abstract

Dendritic cells (DCs) are bone marrow (BM)-derived APCs central to both innate and adaptive immunity. DCs are a heterogeneous cell population composed of multiple subsets with diverse functions. The mechanism governing the generation of multiple DC subsets is, however, poorly understood. In this study we investigated the roles of closely related transcription factors, IFN regulatory factor (IRF)-4 and IRF-8, in DC development by analyzing IRF-4−/−, IRF-8−/−, and IRF-4−/−IRF-8−/− (double-knockout) mice. We found that IRF-4 is required for the generation of CD4+ DCs, whereas IRF-8 is, as reported previously, essential for CD8α+ DCs. Both IRFs support the development of CD4−CD8α− DCs. IRF-8 and, to a lesser degree, IRF-4 contribute to plasmacytoid DC (PDC) development. Thus, the two IRFs together regulate the development of all conventional DCs as well as PDCs. Consistent with these findings, IRF-4, but not IRF-8, was expressed in CD4+ DCs, whereas only IRF-8 was expressed in CD8α+ DCs. CD4−CD8α− DCs and PDCs expressed both IRFs. We also demonstrate in vitro that GM-CSF-mediated DC differentiation depends on IRF-4, whereas Fms-like tyrosine kinase 3 ligand-mediated differentiation depends mainly on IRF-8. Gene transfer experiments with double-knockout BM cells showed that both IRFs have an overlapping activity and stimulate a common process of DC development. Nonetheless, each IRF also possesses a distinct activity to stimulate subset-specific gene expression, leading to the generation of functionally divergent DCs. Together, IRF-4 and IRF-8 serve as a backbone of the molecular program regulating DC subset development and their functional diversity.

Cytokines that use the common gamma chain ␥c are critical for lymphoid development and function. Mutations of the IL-7 receptor, ␥c, or its associated kinase, Jak3, are the major cause of human severe combined immunodeficiency. Although activated by IL-7, Stat5a͞b (Stat, signal transducer and activator of transcription) have been thought to play limited roles in lymphoid development. However, we now show that mice completely deficient in Stat5a͞b have severely impaired lymphoid development and differentiation. Absence of Stat5 also abrogates T cell receptor ␥ rearrangement and survival of peripheral CD8 ؉ T cells. Thus, deficiency of Stat5 results in severe combined immunodeficiency, similar in many respects to deficiency of IL-7R, ␥c, and Jak3.cytokine ͉ jak ͉ lymphocyte ͉ severe combined immunodeficiency ͉ interleukin T he development and homeostasis of lymphoid cells are tightly regulated by cytokines such as interleukin (IL)-7 (1). Its receptor comprises a ligand-specific subunit (IL-7R) associated with a shared receptor subunit designated the cytokine common gamma chain (␥c), which binds the Janus kinase 3 (Jak3). Importantly, mutations of IL-7R, ␥c, or Jak3 underlie the majority of cases of human severe combined immunodeficiency and mouse models in which these genes are deleted also have severe combined immunodeficiency phenotypes (2-8).Activated Jaks phosphorylate cytokine receptors, providing docking sites that recruit Signal transducers and activators of transcription (Stats), which are also phosphorylated. Stats then dimerize, bind DNA, and regulate gene transcription (9, 10). The predominant Stat activated by IL-7 and other ␥c cytokines is Stat5 (11-13). Encoded by two separate genes, the two isoforms of this transcription factor, Stat5a and Stat5b, have distinct physiological functions (14). Deficiency of Stat5a results in impaired prolactindependent mammary cell differentiation (15), whereas deficiency of Stat5b results in impaired growth (16).With respect to T and B cell development, deficiency of Stat5a or Stat5b individually does not have severe consequences (13,(17)(18)(19). Furthermore, analysis of mice in which both genes were targeted also led to the conclusion that Stat5 was not essential for T or B cell development (20,21). Peripheral B cells and bone marrow precursors were reduced but not eliminated and it was suggested that Stat5 is differentially required for T and B cell development (22-26). More recently, a 5-to 10-fold reduction in thymocytes was demonstrated in this model during fetal development, but after birth, the number of thymocytes normalized (21,27,28).Thus, the differences in phenotypes between Stat5a͞b knockout mice and mice lacking IL-7R, Jak3, or ␥c were striking, suggesting that ␥c cytokines like IL-7 must employ Stat5-independent mechanisms to direct lymphocyte development. However, the gene targeting strategy used in the original Stat5 knockout mice encodes a N-terminally truncated and partially functional Stat5 protein (Stat5 ⌬N ) (ref. 29; see also Fig. 7, which is p...