Date Published: September 16, 2016
Publisher: Public Library of Science
Author(s): Karsten Eichholz, Thierry Bru, Thi Thu Phuong Tran, Paulo Fernandes, Hugh Welles, Franck J. D. Mennechet, Nicolas Manel, Paula Alves, Matthieu Perreau, Eric J. Kremer, Chris A. Benedict.
Human adenoviruses (HAdVs) are nonenveloped proteinaceous particles containing a linear double-stranded DNA genome. HAdVs cause a spectrum of pathologies in all populations regardless of health standards. Following repeat exposure to multiple HAdV types, we develop robust and long-lived humoral and cellular immune responses that provide life-long protection from de novo infections and persistent HAdV. How HAdVs, anti-HAdV antibodies and antigen presenting cells (APCs) interact to influence infection is still incompletely understood. In our study, we used physical, pharmacological, biochemical, fluorescence and electron microscopy, molecular and cell biology approaches to dissect the impact of immune-complexed HAdV (IC-HAdV) on human monocyte-derived dendritic cells (MoDCs). We show that IC-HAdV generate stabilized complexes of ~200 nm that are efficiently internalized by, and aggregate in, MoDCs. By comparing IC-HAdV, IC-empty capsid, IC-Ad2ts1 (a HAdV-C2 impaired in endosomal escape due to a mutation that impacts protease encapsidation) and IC-AdL40Q (a HAdV-C5 impaired in endosomal escape due to a mutation in protein VI), we demonstrate that protein VI-dependent endosomal escape is required for the HAdV genome to engage the DNA pattern recognition receptor AIM2 (absent in melanoma 2). AIM2 engagement induces pyroptotic MoDC death via ASC (apoptosis-associated speck protein containing a caspase activation/recruitment domain) aggregation, inflammasome formation, caspase 1 activation, and IL-1β and gasdermin D (GSDMD) cleavage. Our study provides mechanistic insight into how humoral immunity initiates an innate immune response to HAdV-C5 in human professional APCs.
Adenoviruses (AdVs) have a 28–42 kilobase pair double-stranded DNA genome encapsidated in a nonenveloped proteinaceous icosahedral shell. In immune-competent individuals, human AdVs (HAdVs) (of which there are approximately 70 types) cause self-limiting respiratory, ocular and gastro-intestinal tract infections. After repeated encounters, we typically develop multifaceted long-lived memory immune responses [1–3] that efficiently blunt HAdV-induced disease. In spite of the robust cross-reacting cellular and humoral immune responses, HAdVs can establish subclinical persistent infections that last for years, if not decades [4,5]. Not surprisingly, HAdV type-specific humoral immunity before hematopoietic stem cell transplantation is predictive of escape of the same type during immune suppression .
The aim of our study was to understand the impact of immune-complexed HAdV-C5 on human APCs. We show that anti-HAdV-C5 hexon Abs aggregate and stabilized the capsid at mildly acidic pH. The anti-hexon antibodies also perturb the intracellular trafficking of HAdV-C5: instead of release from early endocytic vesicles, the IC-HAdV+-vesicles fuse with TLR9+ vesicles. Here, the capsid must be partially disassembled to allow detection of the genome by TLR9. TLR9 engagement induces the transcriptional activation of pro-inflammatory cytokines and inflammasomes components via the MyD88-NF-κB signaling pathways. Concomitant with genome release from the capsid, the membrane lytic activity of protein VI impacts IC processing by permitting endocytic vesicle cargo escape into the cytoplasm. Here, the 36 kb dsDNA HAdV genome is detected by AIM2, which initiates the nucleation of an inflammasome. Recruitment and cleavage of caspase 1, pro-IL-1β and GSDMD culminate in pyroptotic MoDC death (Fig 8).