Cutting edge: impaired MHC class I expression in mice deficient for Nlrc5/class I transactivator

MHC class I and class II are crucial for the adaptive immune system. Although regulation of MHC class II expression by CIITA has long been recognized, the mechanism of MHC class I transactivation has been largely unknown until the recent discovery of NLRC5/class I transactivator. In this study, we show using Nlrc5-deficient mice that NLRC5 is required for both constitutive and inducible MHC class I expression. Loss of Nlrc5 resulted in severe reduction in the expression of MHC class I and related genes such as β(2)-microglobulin, Tap1, or Lmp2, but did not affect MHC class II levels. IFN-γ stimulation could not overcome the impaired MHC class I expression in Nlrc5-deficient cells. Upon infection with Listeria monocyogenes, Nlrc5-deficient mice displayed impaired CD8(+) T cell activation, accompanied with increased bacterial loads. These findings illustrate critical roles of NLRC5/class I transactivator in MHC class I gene regulation and host defense by CD8(+) T cell responses.     

Expression regulation and function of NLRC5

The NOD like receptors (NLRs), a class of intracellular receptors that respond to pathogen attack or cellular stress, have gained increasing attention. NLRC5, the largest member of the NLR protein family, has recently been identified as a critical regulator of immune responses. While NLRC5 is constitutively and widely expressed, it can be dramatically induced by interferons during pathogen infections. Both in vitro and in vivo studies have demonstrated that NLRC5 is a specifi c and master regulator of major mistocompatibility complex (MHC) class I genes as well as related genes involved in MHC class I antigen presentation. The expression of MHC class I genes is regulated by NLRC5 in coordination with the RFX components through an enhanceosome-dependent manner. And the involvement of NLRC5 in MHC class I mediated CD8⁺ T cell activation, proliferation and cytotoxicity is proved to be critical for host defense against intracellular bacterial infections. Nevertheless, the role of NLRC5 in innate immunity remains to be further explored. Here, we review the research advances on the structure, expression regulation and function of NLRC5.     

NLRC5 regulates MHC class I antigen presentation in host defense against intracellular pathogens

NOD-like receptors (NLRs) are a family of intracellular proteins that play critical roles in innate immunity against microbial infection. NLRC5, the largest member of the NLR family, has recently attracted much attention. However, in vitro studies have reported inconsistent results about the roles of NLRC5 in host defense and in regulating immune signaling pathways. The in vivo function of NLRC5 remains unknown. Here, we report that NLRC5 is a critical regulator of host defense against intracellular pathogens in vivo. NLRC5 was specifically required for the expression of genes involved in MHC class I antigen presentation. NLRC5-deficient mice showed a profound defect in the expression of MHC class I genes and a concomitant failure to activate L. monocytogenes-specific CD8⁺ T cell responses, including activation, proliferation and cytotoxicity, and the mutant mice were more susceptible to the pathogen infection. NLRP3-mediated inflammasome activation was also partially impaired in NLRC5-deficient mice. However, NLRC5 was dispensable for pathogen-induced expression of NF-κB-dependent pro-inflammatory genes as well as type I interferon genes. Thus, NLRC5 critically regulates MHC class I antigen presentation to control intracellular pathogen infection.     

A Role for the Human Nucleotide-binding Domain,Leucine-rich Repeat-containing Family Member NLRC5 in Antiviral Responses

Proteins of the nucleotide-binding domain, leucine-rich repeat (NLR)-containing family recently gained attention as important components of the innate immune system. Although over 20 of these proteins are present in humans, only a few members including the cytosolic pattern recognition receptors NOD1, NOD2, and NLRP3 have been analyzed extensively. These NLRs were shown to be pivotal for mounting innate immune response toward microbial invasion. Here we report on the characterization of human NLRC5 and provide evidence that this NLR has a function in innate immune responses. We found that NLRC5 is a cytosolic protein expressed predominantly in hematopoetic cells. NLRC5 mRNA and protein expression was inducible by the double-stranded RNA analog poly(I·C) and Sendai virus. Overexpression of NLRC5 failed to trigger inflammatory responses such as the NF-κB or interferon pathways in HEK293T cells. However, knockdown of endogenous NLRC5 reduced Sendai virus- and poly(I·C)-mediated type I interferon pathway-dependent responses in THP-1 cells and human primary dermal fibroblasts. Taken together, this defines a function for NLRC5 in anti-viral innate immune responses.     

Regulation of MHC class II gene expression by the class II transactivator

MHC class II molecules are pivotal for the adaptive immune system, because they guide the development and activation of CD4+ T helper cells. Fulfilling these functions requires that the genes encoding MHC class II molecules are transcribed according to a strict cell-type-specific and quantitatively modulated pattern. This complex gene-expression profile is controlled almost exclusively by a single master regulatory factor, which is known as the class II transactivator. As we discuss here, differential activation of the three independent promoters that drive expression of the gene encoding the class II transactivator ultimately determines the exquisitely regulated pattern of MHC class II gene expression.     

Modulation of gene expression by the MHC class II transactivator

The class II transactivator (CIITA) is a master regulator of MHC class II expression. CIITA also modulates the expression of MHC class I genes, suggesting that it may have a more global role in gene expression. To determine whether CIITA regulates genes other than the MHC class II and I family, DNA microarray analysis was used to compare the expression profiles of the CIITA expressing B cell line Raji and its CIITA-negative counterpart RJ2.2.5. The comparison identified a wide variety of genes whose expression was modulated by CIITA. Real time RT-PCR from Raji, RJ2.2.5, an RJ2.2.5 cell line complemented with CIITA, was performed to confirm the results and to further identify CIITA-regulated genes. CIITA-regulated genes were found to have diverse functions, which could impact Ag processing, signaling, and proliferation. Of note was the identification of a set of genes localized to chromosome 1p34-35. The global modulation of genes in a local region suggests that this region may share some regulatory control with the MHC.