Date Published: October 31, 2013
Publisher: Public Library of Science
Author(s): Qiliang Cai, Shuvomoy Banerjee, Amanda Cervini, Jie Lu, Andrew D. Hislop, Richard Dzeng, Erle S. Robertson, Shou-Jiang Gao.
Peptides presentation to T cells by MHC class II molecules is of importance in initiation of immune response to a pathogen. The level of MHC II expression directly influences T lymphocyte activation and is often targeted by various viruses. Kaposi’s sarcoma-associated herpesvirus (KSHV) encoded LANA is known to evade MHC class I peptide processing, however, the effect of LANA on MHC class II remains unclear. Here, we report that LANA down-regulates MHC II expression and presentation by inhibiting the transcription of MHC II transactivator (CIITA) promoter pIII and pIV in a dose-dependent manner. Strikingly, although LANA knockdown efficiently disrupts the inhibition of CIITA transcripts from its pIII and pIV promoter region, the expression of HLA-DQβ but no other MHC II molecules was significantly restored. Moreover, we revealed that the presentation of HLA-DQβ enhanced by LANA knockdown did not help LANA-specific CD4+ T cell recognition of PEL cells, and the inhibition of CIITA by LANA is independent of IL-4 or IFN-γ signaling but dependent on the direct interaction of LANA with IRF-4 (an activator of both the pIII and pIV CIITA promoters). This interaction dramatically blocked the DNA-binding ability of IRF-4 on both pIII and pIV promoters. Thus, our data implies that LANA can evade MHC II presentation and suppress CIITA transcription to provide a unique strategy of KSHV escape from immune surveillance by cytotoxic T cells.
Major histocompatibility complex (MHC) class II is known to play critical roles in the induction and regulation of adaptive immune responses to pathogenic agents . In human, there are at least six major MHC II molecules: HLA-DRα, HLA-DRβ, HLA-DPα, HLA-DPβ, HLA-DQα and HLA-DQβ. During the initiation of the immune response, MHC II molecules expressed from antigen presenting cells (APC) are responsible for binding and presenting peptides to CD4+ T lymphocytes . This process triggers the activation and proliferation of the T cells and so elicits an immune response directed against the antigen derived from MHC II-bound peptides. All mature B cells constitutively express MHC class II molecules on their cell surfaces and the Class II transactivator CIITA is the master regulator of MHC class II and its downstream gene expression activities. Previous reports showed that genetic mutations of CIITA are tightly associated with pathogenesis linked to Hodgkin lymphoma and primary mediastinal B cell lymphoma . Transfection of CIITA into cell lines and primary cells which normally lack MHC II expression has been shown to be sufficient to induce MHC II expression . Consistent with these studies, MHC II mRNA was barely detectable, and the cell surface expression of MHC II was undetectable in CIITA-deficient cells , . In humans, the transcription of CIITA is controlled by a multi-promoter region which harbors 4 independent promoter units . Among these, promoter pI is constitutively activated in dendritic cells, while pIII promoter is designated as the main regulator of CIITA expression in many hematopoietic lineages including B lymphocytes, dendritic cells, monocytes, and activated T cells . Of particular interest to our studies, promoter pIV is predominantly involved in IFN-γ–inducible CIITA expression in APCs as well as other cell types . However, the function of the pII promoter is still poorly understood. For CIITA-mediated MHC II expression by cytokines like IL-4 and IFNγ, it was shown that IFN-γ activates CIITA through the promotion of STAT1 binding to the GAS site, IRF-1/2 to the IRF-E box, and USF-1 to the E-box within the pIV promoter . In contrast, it remains unclear if IL-4 targets the CIITA promoter, although it was previously shown that IL-4 induces MHC II expression .
Latency is a phase of the viral life cycle where expressions of encoded proteins susceptible to immune recognition are significantly reduced. During KSHV latency, it has been widely demonstrated that LANA is one of the key latent antigens responsible for KSHV episome maintenance and pathogenesis . In the present study, we further characterize another function that LANA evolves an elaborate mechanism to modulate host immune response through targeting IRF-4-mediated transcription of CIITA. The interaction of LANA with IRF-4 blocks the ability of IRF-4 bound to CIITA promoters, which results in the shut-off of all of CIITA-mediated MHC II molecule expression (HLA-DR, DP and DQ). In contrast, suppression of LANA only led to a specific MHC II namely HLA-DQβ expression and presentation instead of HLA-DQα, DR or DP, as well as the evidence of vIRF3 inhibition further enhanced the HLA-DQβ expression caused by LANA suppression, indicating that LANA is directly involved with the deregulation of CD4+ T cells immune response against viral infection through cooperation with other viral antigen like vIRF3. In addition, the inhibition of CBP which results in blocking of LANA-mediated suppression of HLA-DQβ expression indicated that LANA can play a global role on deregulation of MHCII in a CIITA dependent and independent manner (Figure 8).