Herpes Simplex Virus type 1 Inflammasome Activation in Human Macrophages is Dependent on NLRP3, ASC, and Caspase-1

Abstract

The pro-inflammatory cytokines interleukin (IL)-1{beta} and IL-18 are products of activation of the inflammasome, an innate sensing system, and important in the pathogenesis of herpes simplex type 1 (HSV-1). The release of IL-18 and IL-1{beta} from monocytes/macrophages is critical for protection from HSV-1 based on animal models of encephalitis and genital infection, yet if and how HSV-1 activates inflammasomes in human macrophages is unknown. To investigate this, we utilized both primary human monocyte derived macrophages and human monocytic cell lines (THP-1 cells) with various inflammasome components knocked-out. We found that HSV-1 activates inflammasome signaling in pro-inflammatory primary human macrophages. Additionally, HSV-1 inflammasome activation is dependent on nucleotide-binding domain and leucine-rich repeat-containing receptor 3 (NLRP3), apoptosis-associated speck-like molecule containing a caspase recruitment domain (ASC), and caspase-1, but not on absent in melanoma 2 (AIM2), or gamma interferon-inducible protein 16 (IFI16). Ultraviolet irradiation of HSV-1 enhanced inflammasome activation, demonstrating that viral replication suppresses inflammasome activation. These results confirm that HSV-1 is capable of activating the inflammasome in human macrophages through an NLRP3 dependent process and that the virus has an NLRP3 specific mechanism to inhibit inflammasome activation in monocytes and macrophages.\n\nAuthor SummaryThe inflammasome is a multi-protein complex that forms in response to pathogens and cellular damage. Active inflammasomes recruit pro-caspase-1 via ASC and cleave the cytokine precursors pro-IL-1{beta} and pro-IL-18 into mature IL-1{beta} and IL-18. These cytokines serve to activate other immune cells that either repair the damage or attempt to clear the invading pathogen. Upon activation, the inflammasome also promotes an inflammatory form of cell death called pyroptosis. Herpes simplex virus type 1 (HSV-1) is a common human pathogen that can cause cold sores, genital ulcers, encephalitis, and blindness. HSV-1 infection leads to induction of IL-1{beta} and IL-18, but whether it is capable of activating inflammasomes in macrophages, which play a role in severe forms of HSV-1 infection, was unclear. Here, we infected both primary human macrophages and a macrophage/monocytic cell line, THP-1 cells, with HSV-1. We found that HSV-1 does activate inflammasome signaling in macrophages in a process dependent on NLRP3, ASC, and caspase-1. This is important because it illustrates the mechanism by which HSV-1 infection leads to inflammasome activation in macrophages, known to be crucial for protection from severe disease in mouse models.