Abstract: The human immunodeficiency virus type 1 (HIV-1) evades the immune responses of natural killer (NK) cells through mechanisms that have been partially deciphered. Here we show that in HIV-1-infected T lymphocytes, the early viral Nef protein downmodulates PVR (CD155, Necl-5), a ligand for the activating receptor DNAM-1 (CD226) expressed by all NK cells, CD8+ T cells, and other cell types. This novel Nef activity is conserved by Nef proteins of laboratory HIV-1 strains (NL4-3, SF2) and of a patient-derived virus, but it is not maintained by HIV-2. Nef uses the same motifs to downregulate PVR and HLA-I molecules, likely by the same mechanisms. Indeed, as previously demonstrated for HLA-I, Nef reduces the total amounts of cell-associated PVR. Optimal downregulation of cell surface PVR by Nef also requires the presence of the late viral factor Vpu. In line with PVR reduction, the NK cell-mediated lysis of T cells infected by a wild-type but not Nef-deficient virus is virtually abrogated upon blocking of both DNAM-1 and another activating receptor, NKG2D, previously shown to mediate killing of HIV-infected cells. Together, these data demonstrate that the PVR downmodulation by Nef and Vpu is a strategy evolved by HIV-1 to prevent NK cell-mediated lysis of infected cells. The PVR downregulation reported here has the potential to affect the immune responses of other DNAM-1-positive cells besides NK cells and to alter multiple PVR-mediated cellular processes, such as adhesion and migration, and may thus greatly influence HIV-1 pathogenesis.

Abstract: The poliovirus receptor CD155, is essential for poliovirus to infect and induce death in neural cells. Recently, CD155 has been shown to be selectively expressed on certain types of tumor cells originating from the neural crest, including malignant glioma and neuroblastoma. However, the expression pattern of CD155 in soft tissue sarcoma has not been examined. Therefore, we first examined CD155 expression in sarcoma cell lines, and found the expression of both CD155 mRNA and protein in 12 soft and bone tissue sarcoma cell lines. Furthermore, we examined the effect of live attenuated poliovirus (LAPV) on 6 bone and soft tissue sarcoma cell lines in vitro, and found that LAPV induced apoptosis by activating caspases 7 and 3 in all of these cell lines. Furthermore, in BALB/c nu/nu mice xenotransplanted with HT1080 fibrosarcoma cells, administration of live attenuated poliovirus caused growth suppression of the tumors. These results suggest that oncolytic therapy using a LAPV may represent a new option for the treatment of bone and soft tissue sarcomas.

Abstract: The Natural Killer (NK) cell activating receptor DNAM-1 (CD226) is stimulated through recognition of CD112 (nectin-2) and CD155 (nectin-like molecule 5; PVR) on target cells. HCMV UL141 elicits protection from NK-cells by down-regulating CD155 from the cell surface and sequestering it in the ER (Tomasec, 2005). Here, HCMV UL141 was shown to be involved in the down-regulation of CD112. Interestingly, UL141 appeared necessary but not sufficient to modulate CD112 expression. This thesis therefore focused on a hypothesis whereby UL141 was acting with one or more additional HCMV genes to target CD112 for degradation. This project was the first to utilise an entire recombinant adenovirus (RAd) library expressing individual HCMV ORFs (RAd-HCMV-ORF library) to screen for function. The RAd-HCMV-ORF library clearly provided an extremely powerful tool for the screening of HCMV gene function as results were highly repeatable and robust. The co-infection of RAd-UL141 and RAd-US2 resulted in a single, clear, positive hit in the final screening process. This hit was further verified by immunoblot where CD112 appeared to be down-regulated in cells infected with both RAd-UL141 and RAd-US2, compared to controls. While a Hela-US2 cell line which stably expressed US2 also down-regulated CD112 when infected with RAd-UL141. A RCMVΔUS1-11 virus was constructed, which failed to down-regulate CD112 from the cell surface of RCMVΔSU1-11 infected cells. The addition of proteasome inhibitors was able to partially restore CD112 expression in HCMV infected cells (Prod'homme et al., 2010). It therefore appeared that US2 and UL141 act to degrade CD112 via the proteasome during HCMV infection. CD112 downregulation may have the potential to prevent DNAM-1:CD112 interaction between HCMV infected targets and effector cells of the immune system, providing another facet to HCMV’s ability to avoid the human immune response.

Abstract: The nectin and nectin-like molecule (Necl) family includes important cell adhesion molecules (CAMs) characterized by their Ig-like nature. Such CAMs regulate a broad spectrum of cell–cell interactions, including the interaction between NK cells and cytotoxic T lymphocytes (CTLs) and their target cells. CAM members nectin-2 (CD112) and Necl-5 (CD155) are believed to form homodimers (for nectin-2) or heterodimers in their functions for cell adhesion, as well as to interact with immune costimulatory receptor DNAX accessory molecule 1 (DNAM-1) (CD226) to regulate functions of both NK and CTL cells. However, the structural basis of the interactive mode of DNAM-1 with nectin-2 or Necl-5 is not yet understood. In this study, a soluble nectin-2 Ig-like V-set domain (nectin-2v) was successfully prepared and demonstrated to bind to both soluble ectodomain and cell surface-expressed full-length DNAM-1. The 1.85-A crystal structure of nectin-2v displays a perpendicular homodimer arrangement, revealing the homodimer characteristics of the nectin and Necls. Further mutational analysis indicated that disruption of the homodimeric interface of nectin-2v led to a failure of the homodimer formation, as confirmed by crystal structure and biochemical properties of the mutant protein of nectin-2v. Interestingly, the monomer mutant also loses DNAM-1 binding, as evidenced by cell staining with tetramers and surface plasmon resonance assays. The data indicate that interaction with DNAM-1 requires either the homodimerization or engagement of the homodimeric interface of nectin-2v. These results have implications for immune intervention of tumors or autoimmune diseases in the DNAM-1/nectin-2–dependent pathway.

Abstract: In humans, paralytic poliomyelitis results from the invasion of the central nervous system by circulating poliovirus (PV) via the blood-brain barrier (BBB). After the virus enters the central nervous system (CNS), it replicates in neurons, especially in motor neurons (MNs), inducing the cell death that causes paralytic poliomyelitis. Along with this route of dissemination, neural pathway has been reported in humans, monkeys, and PV-sensitive human PV receptor (hPVR/CD155)-transgenic (Tg) mice. We demonstrated that a fast retrograde axonal transport process is required for PV dissemination through the sciatic nerve of hPVR-Tg mice and that intramuscularly inoculated PV causes paralysis in a hPVR-dependent manner. We also showed that hPVR-independent axonal transport of PV exists in hPVR-Tg and non-Tg mice, indicating that several different pathways for PV axonal transport exist in these mice. Circulating PV after intravenous inoculation in mice cross the BBB at a high rate in a hPVR-independent manner. Recently, we identified transferrin receptor 1 (TfR1) of mouse brain capillary endothelial cells as a binding protein to PV, implicating that TfR1 is a possible receptor for PV to permeate the BBB.