Howard C.H. Yim

Role of HIV-1 Tat in AIDS pathogenesis: its effects on cytokine dysregulation and contributions to the pathogenesis of opportunistic infection.

HIV is the primary etiological agent of AIDS. Effective treatment and prevention of HIV remain a top priority for governments and international health authorities worldwide. Nowadays, although the use of antiretroviral drugs remains an effective treatment to control AIDS progression, new strains of HIV subtypes continue to evolve. They are demonstrating drug resistance due to their effective and continuous mutations in vivo. Similarly, failure of HIV vaccine development is due to the unique mechanisms of HIV in evading the host immune response. Thus, it is imperative to further investigate the molecular basis of HIV pathogenesis and identify new targets for therapeutic intervention.

A role for c-Myc in regulating anti-mycobacterial responses.

c-Myc (Myc) is a well known transcription factor that regulates many essential cellular processes; however, its role in modulating immunity is not known. Here, we showed different species of mycobacteria can induce Myc expression via ERK1/2 and JNK activation. Unexpectedly, the induced Myc is localized in the cytoplasm but not in the nucleus. This induced Myc expression is associated with the induction of TNF-α and IL-6 and with the suppression of intracellular mycobacterial growth. To delineate the underlying mechanisms, we demonstrated that Myc enhances IRAK1 degradation, leading to specific activations of ERK1/2 and p38 MAPK but not Akt, and reduces IκBα protein recovery upon degradation. Hence, our findings may provide insights into a potential role for Myc in regulating the antimicrobial responses.

Role for Nonstructural Protein 1 of Severe Acute Respiratory Syndrome Coronavirus in Chemokine Dysregulation

ABSTRACT Severe acute respiratory syndrome (SARS) is an emerging infectious disease caused by a novel coronavirus. Since its associated morbidity and mortality have been postulated to be due to immune dysregulation, we investigated which of the viral proteins is responsible for chemokine overexpression. To delineate the viral and cellular factor interactions, the role of four SARS coronavirus proteins, including nonstructural protein 1 (nsp-1), nsp-5, envelope, and membrane, were examined in terms of cytokine induction. Our results showed that the SARS coronavirus nsp-1 plays an important role in CCL5, CXCL10, and CCL3 expression in human lung epithelial cells via the activation of NF-κB.

BTB-ZF transcriptional regulator PLZF modifies chromatin to restrain inflammatory signaling programs

Significance Maintaining physiological balance is vital in the primary response to infectious and other stress stimuli to avert damaging inflammation. Delineation of the cell regulatory processes that control inflammatory processes better enable the development of informed strategies to treat associated pathologies. Toward this end, we identify that the promyelocytic leukemia zinc finger (PLZF) transcription factor limits pathogen-induced inflammation. PLZF stabilizes a repressor complex that encompasses histone deacetylase activity, which modifies the state of chromatin. This activity maintains homeostasis by decreasing the scale of induction of select immune response genes. In the absence of PLZF, the chromatin structure is altered, enabling active transcriptional complexes to immediately assemble on gene promoters, resulting in inordinate production of inflammatory cytokines. Inflammation is critical for host defense, but without appropriate control, it can cause chronic disease or even provoke fatal responses. Here we identify a mechanism that limits the inflammatory response. Probing the responses of macrophages to the key sensory Toll-like receptors, we identify that the Broad-complex, Tramtrack and Bric-a-brac/poxvirus and zinc finger (BTB/POZ), transcriptional regulator promyelocytic leukemia zinc finger (PLZF) limits the expression of inflammatory gene products. In accord with this finding, PLZF-deficient animals express higher levels of potent inflammatory cytokines and mount exaggerated inflammatory responses to infectious stimuli. Temporal quantitation of inflammatory gene transcripts shows increased gene induction in the absence of PLZF. Genome-wide analysis of histone modifications distinguish that PLZF establishes basal activity states of early response genes to maintain immune homeostasis and limit damaging inflammation. We show that PLZF stabilizes a corepressor complex that encompasses histone deacetylase activity to control chromatin. Together with our previous demonstration that PLZF promotes the antiviral response, these results suggest a strategy that could realize one of the major goals of immune therapy to retain immune resistance to pathogens while curbing damaging inflammation.

Protein kinase R and the inflammasome

Protein kinase R (PKR) was first identified as a mediator of the double-stranded RNA (dsRNA)-mediated inhibition of protein synthesis in extracts from interferon-treated cells. In a physiological context, viral replication results in production of dsRNA, activation of PKR by autophosphorylation, and phosphorylation of the protein synthesis initiation factor eIF2α. Subsequent biochemical, structural, and genetic analyses have identified the dsRNA and kinase domain structure of PKR, and shown that its deletion from the germline of mice results in impaired resistance to infection by many different viruses. These studies have also opened up roles for PKR in different signaling pathways, the most recent being regulation of the inflammasome. Here we review evidence for this newly ascribed function for PKR and discuss roles in inflammasome regulation and associated diseases.

HIV-1 Tat dysregulation of lipopolysaccharide-induced cytokine responses: microbial interactions in HIV infection.

Objective:To examine whether the HIV-1 Tat protein impairs the lipopolysaccharide (LPS)-induced cytokine responses. Design:Concurrent infections with pathogens including bacteria and viruses are common in AIDS patients. However, cytokine and interferon responses during infection with or translocation from the gut of these pathogens in HIV-infected patients are not well studied. As HIV-1 Tat contributes partly to the HIV-induced immune dysregulation, we investigated whether the protein may play a role in perturbing the LPS-induced cytokine responses. Methods:Expression levels of cytokines in human primary blood monocytes/macrophages were determined by quantitative polymerase chain reaction and enzyme-linked immunosorbent assay. Expression level of the cell surface Toll-like receptor 4 was examined by flow cytometry. Activations of signaling molecules were assayed by western blot and immunofluorescence. Results:We demonstrated that HIV-1 Tat downregulated the LPS-induction of IFN-β and concomitantly upregulated IL-6 expression in primary blood monocytes/macrophages, whereas the viral protein had no significant effects on TNF-α expression. To delineate the underlying mechanism, we showed that Tat inhibited the LPS-activation of ERK1/2 but not the p38 mitogen-activated protein kinases. The viral protein suppressed the LPS-induced activation of NFκB p65 via its induction of IκBα expression, which resulted in retention of NFκB p65 in the cytosol. Conclusion:These findings suggest that Tat may play a role in modulating the immune responses triggered by other coinfecting pathogens and thus providing a permissive environment for both HIV and other opportunistic microbes.

The kinase activity of PKR represses inflammasome activity.

The protein kinase R (PKR) functions in the antiviral response by controlling protein translation and inflammatory cell signaling pathways. We generated a transgenic, knock-in mouse in which the endogenous PKR is expressed with a point mutation that ablates its kinase activity. This novel animal allows us to probe the kinase-dependent and -independent functions of PKR. We used this animal together with a previously generated transgenic mouse that is ablated for PKR expression to determine the role of PKR in regulating the activity of the cryopyrin inflammasome. Our data demonstrate that, in contradiction to earlier reports, PKR represses cryopyrin inflammasome activity. We demonstrate that this control is mediated through the established function of PKR to inhibit protein translation of constituents of the inflammasome to prevent initial priming during innate immune signaling. These findings identify an important role for PKR to dampen inflammation during the innate immune response and caution against the previously proposed therapeutic strategy to inhibit PKR to treat inflammation.

Interleukin-17A differentially modulates BCG induction of cytokine production in human blood macrophages

The pathogenesis of Mtb depends in part on cytokine cross‐regulation between macrophages and T cells in host immunity. Th17 cells produce IL‐17A to induce granuloma formation and to restrict mycobacterial dissemination. IL‐17A also mediates cytokine responses induced by proinflammatory cytokines such as TNF‐α. Our previous results showed that BCG induces IL‐6, IL‐10, and TNF‐α via activity of protein kinases, including dsRNA‐activated serine/threonine protein kinase and glycogen synthase kinase‐3 in primary human monocytes. Therefore, we investigated whether IL‐17A, upon its induction by BCG, plays an additional role to aid the production of downstream proinflammatory cytokines in macrophages. Here, we showed that IL‐17A enhanced IL‐6 mRNA and protein levels inducible by BCG in a time‐ and dose‐dependent manner, whereas it had no effect on IL‐10 and TNF‐α production. We also demonstrated that IL‐17A activated the phosphorylation of ERK1/2 triggered by BCG. With the use of a specific chemical inhibitor of a MAPK/ERK‐activating kinase (MEK1/2), we confirmed the correlation between the enhanced ERK1/2 activation and augmented IL‐6 production. Additionally, we revealed that IL‐17A acts in concert with BCG‐induced TNF‐α to enhance the level of IL‐6 synthesis. Taken together, our results suggest a significant role of IL‐17A to serve as a modulator of cytokine expression in innate immune response during mycobacterial infection.

Integrin-linked kinase expression in myeloid cells promotes inflammatory signaling during experimental colitis

The pathology of inflammatory bowel diseases is driven by the inflammatory signaling pathways associated with mucosal epithelial damage. Myeloid cells are known to play an essential role in mediating epithelial inflammatory responses during injury. However, the precise role of these cells in stimulating intestinal inflammation and the subsequent tissue damage is unclear. In this article, we show that expression of integrin-linked kinase (ILK) in myeloid cells is critical for the epithelial inflammatory signaling during colitis induced by dextran sodium sulfate. Myeloid ILK (M-ILK) deficiency significantly ameliorates the pathology of experimental colitis. In response to dextran sodium sulfate, colonic infiltration of neutrophils and inflammatory cytokine production are impaired in M-ILK–deficient mice, and activation of epithelial NF-κB and PI3K signaling pathways are restricted by the M-ILK deficiency. In contrast, reduced epithelial damage in M-ILK–deficient mice is correlated with elevated levels of epithelial Stat3 activation and proliferation. Moreover, M-ILK–dependent inflammatory signaling in the mucosal epithelium can be therapeutically targeted by the pharmacological inhibition of ILK during experimental colitis. Collectively, these findings identify M-ILK as a critical regulator of epithelial inflammatory signaling pathways during colitis and, as a consequence, targeting M-ILK could provide therapeutic benefit.

The innate immune receptor MDA5 limits rotavirus infection but promotes cell death and pancreatic inflammation

Melanoma differentiation‐associated protein 5 (MDA5) mediates the innate immune response to viral infection. Polymorphisms in IFIH1, the gene coding for MDA5, correlate with the risk of developing type 1 diabetes (T1D). Here, we demonstrate that MDA5 is crucial for the immune response to enteric rotavirus infection, a proposed etiological agent for T1D. MDA5 variants encoded by minor IFIH1 alleles associated with lower T1D risk exhibit reduced activity against rotavirus infection. We find that MDA5 activity limits rotavirus infection not only through the induction of antiviral interferons and pro‐inflammatory cytokines, but also by promoting cell death. Importantly, this MDA5‐dependent antiviral response is specific to the pancreas of rotavirus‐infected mice, similar to the autoimmunity associated with T1D. These findings imply that MDA5‐induced cell death and inflammation in the pancreas facilitate progression to autoimmune destruction of pancreatic β‐cells.