Alix serves as an adaptor that allows human parainfluenza virus type 1 to interact with the host cell ESCRT system

The cellular ESCRT (endosomal sorting complex required for transport) system functions in cargo-sorting, in the formation of intraluminal vesicles that comprise multivesicular bodies (MVB), and in cytokinesis, and this system can be hijacked by a number of enveloped viruses to promote budding. The respiratory pathogen human parainfluenza virus type I (HPIV1) encodes a nested set of accessory C proteins that play important roles in down-regulating viral transcription and replication, in suppressing the type I interferon (IFN) response, and in suppressing apoptosis. Deletion or mutation of the C proteins attenuates HPIV1 in vivo, and such mutants are being evaluated preclinically and clinically as vaccines. We show here that the C proteins interact and co-localize with the cellular protein Alix, which is a member of the class E vacuolar protein sorting (Vps) proteins that assemble at endosomal membranes into ESCRT complexes. The HPIV1 C proteins interact with the Bro1 domain of Alix at a site that is also required for the interaction between Alix and Chmp4b, a subunit of ESCRT-III. The C proteins are ubiquitinated and subjected to proteasome-mediated degradation, but the interaction with Alix_Bro1 protects the C proteins from degradation. Neither over-expression nor knock-down of Alix expression had an effect on HPIV1 replication, although this might be due to the large redundancy of Alix-like proteins. In contrast, knocking down the expression of Chmp4 led to an approximately 100-fold reduction in viral titer during infection with wild-type (WT) HPIV1. This level of reduction was similar to that observed for the viral mutant, P(C-) HPIV1, in which expression of the C proteins were knocked out. Chmp4 is capable of out-competing the HPIV1 C proteins for binding Alix. Together, this suggests a possible model in which Chmp4, through Alix, recruits the C proteins to a common site on intracellular membranes and facilitates budding.

Pathogenesis of acute respiratory illness caused by human parainfluenza viruses

Human parainfluenza viruses (HPIVs) are a common cause of acute respiratory illness throughout life. Infants, children, and the immunocompromised are the most likely to develop severe disease. HPIV1 and HPIV2 are best known to cause croup while HPIV3 is a common cause of bronchiolitis and pneumonia. HPIVs replicate productively in respiratory epithelial cells and do not spread systemically unless the host is severely immunocompromised. Molecular studies have delineated how HPIVs evade and block cellular innate immune responses to permit efficient replication, local spread, and host-to-host transmission. Studies using ex vivo human airway epithelium have focused on virus tropism, cellular pathology and the epithelial inflammatory response, elucidating how events early in infection shape the adaptive immune response and disease outcome.

Progress in the development of human parainfluenza virus vaccines

In children under 5 years of age, human parainfluenza viruses (HPIVs) as a group are the second most common etiology of acute respiratory illness leading to hospitalization, surpassed only by respiratory syncytial virus but ahead of influenza viruses. Using reverse genetics systems for HPIV serotypes 1, 2 and 3 (HPIV1, 2 and 3), several live-attenuated HPIVs have been generated and evaluated as intranasal vaccines in adults and in children. Two vaccines against HPIV3 were found to be well tolerated, infectious and immunogenic in Phase I trials in HPIV3-seronegative infants and children and should progress to proof-of-concept trials. Vaccines against HPIV1 and HPIV2 are less advanced and have just entered pediatric trials.

The C proteins of human parainfluenza virus type 1 limit double-stranded RNA accumulation that would otherwise trigger activation of MDA5 and protein kinase R

Human parainfluenza virus type 1 (HPIV1) is an important respiratory pathogen in young children, the immunocompromised, and the elderly. We found that infection with wild-type (WT) HPIV1 suppressed the innate immune response in human airway epithelial cells by preventing not only phosphorylation of interferon regulatory factor 3 (IRF3) but also degradation of IκBβ, thereby inhibiting IRF3 and NF-κB activation, respectively. Both of these effects were ablated by a F170S substitution in the HPIV1 C proteins (F170S) or by silencing the C open reading frame [P(C-)], resulting in a potent beta interferon (IFN-β) response. Using murine knockout cells, we found that IFN-β induction following infection with either mutant relied mainly on melanoma-associated differentiation gene 5 (MDA5) rather than retinoic acid-inducible gene I (RIG-I). Infection with either mutant, but not WT HPIV1, induced a significant accumulation of intracellular double-stranded RNA (dsRNA). These mutant viruses directed a marked increase in the accumulation of viral genome, antigenome, and mRNA that was coincident with the accumulation of dsRNA. In addition, the amount of viral proteins was reduced compared to that of WT HPIV1. Thus, the accumulation of dsRNA might be a result of an imbalance in the N protein/genomic RNA ratio leading to incomplete encapsidation. Protein kinase R (PKR) activation and IFN-β induction followed the kinetics of dsRNA accumulation. Interestingly, the C proteins did not appear to directly inhibit intracellular signaling involved in IFN-β induction; instead, their role in preventing IFN-β induction appeared to be in suppressing the formation of dsRNA. PKR activation contributed to IFN-β induction and also was associated with the reduction in the amount of viral proteins. Thus, the HPIV1 C proteins normally limit the accumulation of dsRNA and thereby limit activation of IRF3, NF-κB, and PKR. If C protein function is compromised, as in the case of F170S HPIV1, the resulting PKR activation and reduction in viral protein levels enable the host to further reduce C protein levels and to mount a potent antiviral type I IFN response.

In silico identification of a putative new paramyxovirus related to the Henipavirus genus

A database search for genes encoding paramyxoviral proteins revealed sequences that were designated as human but presented strong evidence of being of viral origin. The two cDNA-derived sequences designated AngRem104 and AngRem52 were originally described as human gene products that were upregulated by angiotensin II in primary mesangial kidney cells. However, their high degree of sequence relatedness to known viral proteins suggests that they represent the P/C/V, M, and F genes of a putative new member of family Paramyxoviridae. Comparison of deduced amino acid sequences and nucleotide motifs suggests that this putative virus is a divergent relative of the Hendra and Nipah viruses; hence, we suggest henipa-like virus or HNLV as a provisional name. Compared to Nipah virus, the percentage of identical (similar) amino acids varied from 19% (42%) for the C protein to 51% (75%) for the M protein. The presence and conservation of presumptive viral transcription start and stop signals and an apparent P editing motif also indicate a relationship of this putative virus to the henipaviruses. Given the highly pathogenic nature of the henipaviruses, the origin of these sequences is enigmatic, and attempts to identify and isolate HNLV are warranted.