PU.1

PU.1 is an Ets-family transcription factor that plays a broad range of roles in hematopoiesis. A direct regulator of myeloid, dendritic-cell, and B cell functional programs, and a well-known antagonist of terminal erythroid cell differentiation.

Introduction of PU.1

PU.1/proviral integration site-1 (Spi1) is an Ets-family transcription factor. Spi1 was first isolated by Moreau-Gachelin and coworkers as the product of the gene targeted by recurrent insertions of the spleen focus forming virus in friend’s erythroleukemia. Klemsz, McKercher, Celada, Van Beveren, and Maki (1990) then isolated PU.1 as a factor binding to a purine-rich motif in the MHC class II gene promoter. The subsequent knock-out of PU.1 established a major role for this protein as a master regulator of hematopoiesis, as PU.1 null mice die at birth and lack all myeloid and lymphoid lineage cells. PU.1 is also the factor that occupies a major fraction of all open regulatory sites in the genomes of pre-commitment (DN1 and DN2a) cells. This is a signature factor of myeloid cells, dendritic cells, and B cells, which is potent in reprogramming various other cell types into myeloid cells. Yet it is also initially well-expressed in virtually all DN1 and DN2a T-lineage progenitors and is silenced only during commitment. Either gain or loss of PU.1 in experimental contexts can appear to push pro-T cells “backward” or “forwards”, respectively, relative to the normal developmental program, implicating this factor in the control of developmental progression, while the further elevation of PU.1 expression in early T cell progenitors activates genes important for the myeloid program.

Fig.1 Structure of the PU.1 protein. (Kastner, 2008)

Expression of PU.1 in γδ T cells

The development of γδ T cells is regulated by extracellular signals and the expression of cell-intrinsic factors that regulate development and expansion. In a recent report, PU.1 was identified as a transcription factor that regulates numbers of γδ T cells in secondary lymphoid organs and at mucosal sites. This represents a distinct function of PU.1 between αβ and γδ T cells since αβ T cell development and numbers in various lymphoid organs are unaltered by PU.1-deficiency. PU.1 is expressed in γδ T cells from several organs. Splenic γδ T cells have higher expression of Spi1 than naive CD4⁺ T cells, although Spi1 is expressed in lower amounts by thymic and intra-epithelial intestinal γδ T cells, and not expressed in dermal γδ T cells. Since dermal γδ T cells have a fetal thymic origin, it is possible that there may be a difference in PU.1 expression between γδ T cells that derive during embryonic development and mature mice. PU.1 expression was observed in γδ T cells derived from fetal thymic organ culture suggesting that a fetal thymus does not result in the absence of PU.1 expression in γδ T cells. Further studies will be required to determine precisely when Spi1 is regulated during γδ T cell development.

Fig.2 Framework for experimental perturbation studies to define functions of PU.1 in early thymic development. (Rothenberg, 2019)

PU.1 Regulates γδ T Cell Expansion

PU.1-deficiency increases in vivo proliferation of γδ T cells, and in vitro TCR-induced proliferation of γδ T cells. In several cell types, PU.1 can function as a tumor suppressor. The development of myeloid leukemia in mice with reduced PU.1 expression is dependent on Jun. Therefore, Jun may help to promote the expansion of T cells in the absence of PU.1. In various cell types, PU.1 has also been shown to induce expression of TRAIL and Ink4b, which respectively induce apoptosis and inhibit cell proliferation. It may ultimately be a combination of effects on multiple genes that allows PU.1 to regulate γδ T cell expansion.

We have learned that PU.1 and γδ T cells can regulate each other. However, the transcription factor network that promotes the development and homeostasis of γδ T cells is not well-defined. It is also necessary to further study the functions and application of the interaction of PU.1 and γδ T cells, which may play an important role in hematology in the future.

References

1. Kastner, P.; Chan, S. PU.1: A crucial and versatile player in hematopoiesis and leukemia. Int J Biochem Cell Biol. 2008, 40(1): 22-7.
2. Rothenberg, E. V.; et al. Mechanisms of action of hematopoietic transcription factor PU.1 in initiation of T-cell development. Front Immunol. 2019, 10: 228.