The transcription factor IFN regulatory factor (IRF)4 was shown to play a crucial role in the protective CD8+ T cell response; however, regulation of IRF4 expression in CD8+ T cells remains unclear. In this article, we report a critical role for Nr4a1 in regulating the expansion, differentiation, and function of CD8+ T cells through direct transcriptional repression of Irf4. Without Nr4a1, the regulation of IRF4 is lost, driving an increase in Irf4 expression and, in turn, resulting in a faster rate of CD8 T cell proliferation and expansion. Nr4a1-deficient mice show increases in CD8 T cell effector responses with improved clearance of Listeria monocytogenes. Our data support a novel and critical role for Nr4a1 in the regulation of CD8+ T cell expansion and effector function through transcriptional repression of Irf4.

The NR4A nuclear receptor family member Nr4a1 is strongly induced in thymocytes undergoing selection, and has been shown to control the development of Treg cells; however the role of Nr4a1 in CD8+ T cells remains undefined. Here we report a novel role for Nr4a1 in regulating the development and frequency of CD8+ T cells through direct transcriptional control of Runx3. We discovered that Nr4a1 recruits the corepressor, CoREST to suppress Runx3 expression in CD8+ T cells. Loss of Nr4a1 results in increased Runx3 expression in thymocytes which consequently causes a 2-fold increase in the frequency and total number of intrathymic and peripheral CD8+ T cells. Our findings establish Nr4a1 as a novel and critical player in the regulation of CD8 T cell development through the direct suppression of Runx3.

TRIM is a recently identified transmembrane adaptor protein which is exclusively expressed in T cells and natural killer (NK) cells. In peripheral blood T cells TRIM has been reported to coprecipitate, comodulate, and cocap with the T‐cell receptor (TCR), suggesting that it is an integral component of the TCR/CD3/ζ complex. Here we investigate the expression of TRIM mRNAs and proteins in developing thymocytes. Two splicing isoforms with open reading frames are observed, namely a full length (TRIM) and a truncated version (ΔTM‐TRIM). The latter lacks the extracellular and transmembrane domains as well as the first 10 cytoplasmic aminoacids and is significantly expressed only as mRNA in early fetal thymocytes. TRIM mRNA is detected in all mainstream thymocyte subsets in adult mice. TRIM protein, in contrast, first appears in the DN2 (CD44+ CD25+) subset of adult double negative (DN) cells. In fetal thymocyte development, TRIM mRNA is seen from dg 14.5 onwards whereas TRIM protein appears first on dg 16.5. In contrast to the adult, the TRIM protein was seen in a subset of fetal DN1 cells. In fetal and adult thymocytes, TRIM protein expression was highest in DN2, DN3 (CD44−25+) and in DP cells, compatible with a functional role at or around phases of thymic selection.

Mice reconstituted with human immune systems are instrumental in the investigation of HIV-1 pathogenesis and therapeutics. Natural killer (NK) cells have long been recognized as a key mediator of innate anti-HIV responses. However, established humanized mouse models do not support robust human NK cell development from engrafted human hematopoietic stem cells (HSCs). A major obstacle to human NK cell reconstitution is the lack of human interleukin-15 (IL-15) signaling, as murine IL-15 is a poor stimulator of the human IL-15 receptor. Here, we demonstrate that immunodeficient NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG) mice expressing a transgene encoding human IL-15 (NSG-Tg(IL-15)) have physiological levels of human IL-15 and support long-term engraftment of human NK cells when transplanted with human umbilical-cord-blood-derived HSCs. These Hu-NSG-Tg(IL-15) mice demonstrate robust and long-term reconstitution with human immune cells, but do not develop graft-versus-host disease (GVHD), allowing for long-term studies of human NK cells. Finally, we show that these HSC engrafted mice can sustain HIV-1 infection, resulting in human NK cell responses in HIV-infected mice. We conclude that Hu-NSG-Tg(IL-15) mice are a robust novel model to study NK cell responses to HIV-1.