Reappraisal of biosafety risks posed by PERVs in xenotransplantation

Biomineralization of bone tissue: calcium phosphate-based inorganics in collagen fibrillar organic matrices

BACKGROUND

Bone regeneration research is currently ongoing in the scientific community. Materials approved for clinical use, and applied to patients, have been developed and produced. However, rather than directly affecting bone regeneration, these materials support bone induction, which regenerates bone. Therefore, the research community is still researching bone tissue regeneration. In the papers published so far, it is hard to find an improvement in the theory of bone regeneration. This review discusses the relationship between the existing theories on hard tissue growth and regeneration and the biomaterials developed so far for this purpose and future research directions.

MAINBODY

Highly complex nucleation and crystallization in hard tissue involves the coordinated action of ions and/or molecules that can produce different organic and inorganic composite biomaterials. In addition, the healing of bone defects is also affected by the dynamic conditions of ions and nutrients in the bone regeneration process. Inorganics in the human body, especially calcium- and/or phosphorus-based materials, play an important role in hard tissues. Inorganic crystal growth is important for treating or remodeling the bone matrix. Biomaterials used in bone tissue regeneration require expertise in various fields of the scientific community. Chemical knowledge is indispensable for interpreting the relationship between biological factors and their formation. In addition, sources of energy for the nucleation and crystallization processes of such chemical bonds and minerals that make up the bone tissue must be considered. However, the exact mechanism for this process has not yet been elucidated. Therefore, a convergence of broader scientific fields such as chemistry, materials, and biology is urgently needed to induce a distinct bone tissue regeneration mechanism.

CONCLUSION

This review provides an overview of calcium- and/or phosphorus-based inorganic properties and processes combined with organics that can be regarded as matrices of these minerals, namely collagen molecules and collagen fibrils. Furthermore, we discuss how this strategy can be applied to future bone tissue regenerative medicine in combination with other academic perspectives.

Screening and Identification of the First Non-CRISPR/Cas9-Treated Chinese Miniature Pig With Defective Porcine Endogenous Retrovirus pol Genes

Pig to human xenotransplantation is considered to be a possible approach to alleviate the shortage of human allografts. Porcine endogenous retrovirus (PERV) is the most significant pathogen in xenotransplantation. We screened for pigs that consistently did not transmit human-tropic replication competent PERVs (HTRC PERVs), namely, non-transmitting pigs. Then, we conducted whole-genome resequencing and full-length transcriptome sequencing to further investigate the sequence characteristics of one non-transmitting pig. Using in vitro transmission assays, we found 5 (out of 105) pigs of the Chinese Wuzhishan minipig inbred line that did not transmit PERV to human cells, i.e., non-transmitting pigs. Whole-genome resequencing and full-length transcriptome sequencing of one non-transmitting pig showed that all of the pol genes were defective at both the genome and transcript levels. We speculate that the defective PERV pol genes in this pig might be attributable to the long-term inbreeding process. This discovery is promising for the development of a strain of highly homozygous and genetically stable pigs with defective PERV pol genes as a source animal species for xenotransplantation.

Analysis of Protein Expression in Human Cells Cocultured with Porcine Peripheral Blood Mononuclear Cells

OBJECTIVE

Porcine endogenous retroviruses (PERV) involved in pig to human xenotransplantation have raised great concerns because of their ubiquitous nature in pigs and their ability of infecting human cells in vitro. Although no significant cytopathic effect attributed to PERV was evident on PERV-infected human embryonic kidney 293 (HEK293) cells, we did proteomic analysis to investigate the differences of protein profile in order to further characterize the effect of PERV infection.

METHODS

HEK293 cells were cocultured with porcine peripheral blood mononuclear cells (PBMCs). Protein profiles of PERV-infected and -noninfected HEK293 cells were analyzed by two-dimensional gel electrophoresis (2-DE). Protein spots with at least 1.5-fold alteration were identified by high-definition mass spectrometry (HDMS) analysis. Then real-time RT-PCR and Western blotting were performed to validate the proteomic results.

RESULTS

Differential analysis of PERV-infected and -noninfected HEK293 cells by 2-DE revealed ten differentially regulated proteins. The proteins identified by HDMS were involved in various cellular pathways including signal transduction, cell apoptosis, and protein synthesis.

CONCLUSION

The results of this study revealed differentially expressed proteins in HEK293 cells cocultured with porcine PBMCs and implied that these changes were probably induced by PERV infection. These results provide clues and potential links to understanding the molecular effect of the infection by human-tropic PERV.

The Collagen Suprafamily: From Biosynthesis to Advanced Biomaterial Development

Collagen is the oldest and most abundant extracellular matrix protein that has found many applications in food, cosmetic, pharmaceutical, and biomedical industries. First, an overview of the family of collagens and their respective structures, conformation, and biosynthesis is provided. The advances and shortfalls of various collagen preparations (e.g., mammalian/marine extracted collagen, cell-produced collagens, recombinant collagens, and collagen-like peptides) and crosslinking technologies (e.g., chemical, physical, and biological) are then critically discussed. Subsequently, an array of structural, thermal, mechanical, biochemical, and biological assays is examined, which are developed to analyze and characterize collagenous structures. Lastly, a comprehensive review is provided on how advances in engineering, chemistry, and biology have enabled the development of bioactive, 3D structures (e.g., tissue grafts, biomaterials, cell-assembled tissue equivalents) that closely imitate native supramolecular assemblies and have the capacity to deliver in a localized and sustained manner viable cell populations and/or bioactive/therapeutic molecules. Clearly, collagens have a long history in both evolution and biotechnology and continue to offer both challenges and exciting opportunities in regenerative medicine as nature's biomaterial of choice.

Cellular therapy and bioartificial approaches to liver replacement

PURPOSE OF REVIEW

The success of liver transplantation has increased over the past 20 years due to improved immunosuppressive medications, surgical technique and donor-recipient selection. To date, the number of patients waiting for a liver transplant exceeds the number of transplants performed yearly by over a 2 : 1 ratio. Despite efforts to expand the donor pool, mortality of patients waiting for a liver remains high due to the shortage of donor organs. Herein, we discuss options for liver replacement that are currently under development.

RECENT FINDINGS

Extracorporeal bioactive liver perfusion devices were investigated in the late 1990s and preliminarily demonstrated safety but failed to show clinical efficacy. Current research is ongoing, but the focus has shifted to xenotransplantation of whole organs, organ engineering and cell transplantation. These new modalities are limited to small and large animal studies and each present unique advantages and limitations.

SUMMARY

Discovery of new sources of organs or cells to replace a damaged liver may be the only long-term solution to provide definitive therapy to all patients who require transplantation. The past 2 years have seen notable achievements in xenotransplantation, tissue engineering and cell transplantation. Though challenges remain, now identified, they may be readily solved.

Identification and promoter analysis of PERV LTR subtypes in NIH-miniature pig

Porcine endogenous retroviruses (PERVs) are integrated into the genomes of all pigs. Since some PERVs can also infect human cells, they represent a potential risk for xenotransplantation involving pig cells or organs. The long terminal repeat (LTR) elements of PERVs show promoter activity that can affect human functional genes; therefore, we examined these elements in this study. We detected several expressed LTRs in the NIH-miniature pig liver, among which we identified 9 different subtypes. When these LTRs were compared, distinct structures that contained several insertion and deletion (INDEL) events and tandem repeats were identified in the U3 region. The transcriptional activity of the 9 LTR subtypes was analyzed in the PK15 porcine cell line and in the HepG2 and Hep3B human liver cell lines, and transcriptional regulation was found to be different in the 3 cell lines. The D LTR subtype was found to have stronger promoter activity than all other types in 4 different human cell lines (HepG2, Hep3B, U251, and 293). Using computational approaches, the D type was shown to contain 4 transcription factor-binding sites distinct from those in the U3 regions of the other subtypes. Therefore, deletion mutants were constructed and examined by a transient transfection luciferase assay. The results of this analysis indicated that the binding site for the Hand1:E47 transcription factor might play a positive role in the transcriptional regulation of PERV LTR subtype D in human liver cell lines.

Genetic prevalence of porcine endogenous retrovirus in chinese experimental miniature pigs

Pig-to-human xenotransplantation poses the potential risk of interspecies transmission of porcine endogenous retrovirus (PERV). The Chinese experimental miniature pig may be used as a pig-to-human xenograft donor. However, data for the distribution of PERV provirus in genomic DNA and PERV expression at the RNA level for the Chinese experimental miniature pig population are lacking. In this study, PERV was investigated in this regard using polymerase chain reaction (PCR), real-time quantitative PCR, and real-time quantitative reverse transcription PCR. The results showed that the genotype distribution was PERV-A subtype 100%, PERV-B subtype 100%, and PERV-C subtype 30% among 20 pig genomic DNA samples. Both PERV copy number in genomic DNA and PERV expression at the RNA level varied significantly among individuals, ranging from 3.95 ± 0.14 to 95.52 ± 2.20 and 3.66 ± 0.13 to 43.03 ± 2.50, respectively. For some individuals, the PERV copy number (eg, 3.95 ± 0.14) in genomic DNA and PERV expression (eg, 3.66 ± 0.13) at the RNA level were low. These results suggested that the Chinese experimental miniature pig is a possible donor for xenotransplantation. Our results provide reference information for selective breeding, which will benefit the application of these animals for the study of xenotransplantation.

Characterization of porcine endogenous retrovirus clones from the NIH miniature pig BAC library

Pigs have been considered as donors for xenotransplantation in the replacement of human organs and tissues. However, porcine endogenous retroviruses (PERVs) might transmit new infectious disease to humans during xenotransplantation. To investigate PERV integration sites, 45 PERV-positive BAC clones, including 12 PERV-A, 16 PERV-B, and 17 PERV-C clones, were identified from the NIH miniature pig BAC library. The analysis of 12 selected full-length sequences of PERVs, including the long terminal repeat (LTR) region, identified the expected of open reading frame length, an indicative of active PERV, in all five PERV-C clones and one of the four PERV-B clones. Premature stop codons were observed in only three PERV-A clones. Also, eleven PERV integration sites were mapped using a 5000-rad IMpRH panel. The map locations of PERV-C clones have not been reported before, thus they are novel PERV clones identified in this study. The results could provide basic information for the elimination of site-specific PERVs in selection of pigs for xenotransplantation.

Non-human viruses developed as therapeutic agent for use in humans

Viruses usually infect a restricted set of host species, and only in rare cases does productive infection occur outside the natural host range. Infection of a new host species can manifest as a distinct disease. In this respect, the use of non‐human viruses in clinical therapy may be a cause for concern. It could provide the opportunity for the viruses to adapt to the new host and be transferred to the recipient's relatives or medical caretakers, or even to the normal host species. Such environmental impact is evidently undesirable. To forecast future clinical use of non‐human viruses, a literature study was performed to identify the viruses that are being considered for application as therapeutic agents for use in humans. Twenty‐seven non‐human virus species were identified that are in (pre)clinical development, mainly as oncolytic agents. For risk management, it is essential that the potential environmental consequences are assessed before initiating clinical use, even if the virus is not formally classified as a genetically modified organism. To aid such assessment, each of these viruses was classified in one of five relative environmental risk categories, ranging from “Negligible” to “Very High”. Canary pox virus and the Autographa californica baculovirus were assigned a “Negligible” classification, and Seneca Valley virus, murine leukemia virus, and Maraba virus to the “High” category. A complicating factor in the classification is the scarcity of publicly available information on key aspects of virus biology in some species. In such cases the relative environmental risk score was increased as a precaution. Copyright © 2011 John Wiley & Sons, Ltd.

Evidence and controversies on the role of XMRV in prostate cancer and chronic fatigue syndrome

The recent discovery of xenotropic murine leukaemia virus-related virus (XMRV) in prostate cancer tissues and in the blood of individuals suffering from chronic fatigue syndrome has attracted considerable interest. However, the relevance and significance of XMRV to human disease remain unclear, since the association has not been confirmed in other studies. XMRV is the first gammaretrovirus to be found in humans. XMRV and murine leukaemia viruses share similar structures and genomic organisation. Human restriction factors such as APOBEC3 or tetherin inhibit XMRV replication. Although XMRV induces low rates of transformation in cell culture, it might be able to induce cancer by low-frequency insertional activation of oncogenes or through the generation of highly active transforming viruses. A preference for regulatory regions of transcriptional active genes has been observed after a genomic-wide analysis of XMRV integration sites. Genes related to carcinogenesis and androgen signalling have been identified in the vicinity of integration sites. The XMRV genome contains a glucocorticoid responsive element, and androgens could modulate viral replication in the prostate. Evidence supporting the involvement of XMRV in chronic fatigue syndrome is still very weak, and needs further confirmation and validation. Currently approved anti-retroviral drugs such as zidovudine, tenofovir and raltegravir are efficient inhibitors of XMRV replication in vitro. These drugs might be useful to treat XMRV infection in humans. The identification of XMRV has potentially serious health implications for the implementation of novel techniques including gene therapy or xenotransplantation, while raising concerns on the need for screening donated blood to prevent transmission through transfusion.

Real-time quantitative polymerase chain reaction with SYBR green i detection for estimating copy numbers of porcine endogenous retrovirus from Chinese miniature pigs

Porcine endogenous retrovirus (PERV) in the pig genome represents a potential infectious risk in xenotransplantation. Chinese miniature pigs have been considered to be potential organ donors in China. However, an adequate level of information on PERV from Chinese miniature pigs has not been available. We established an SYBR Green I-based real-time quantitative polymerase chain reaction (PCR) assay for estimating copy numbers of PERV integrated in the host genome. The assay was 100-fold more sensitive compared with conventional PCR. We also evaluated the specificity and reproducibility of the assay. We statistically analyzed the difference in PERV copy numbers integrated into the genomes of Wuzhishan pigs versus Bama minipigs. This approach will be useful to screen donor pigs as well as to examine clinical samples from human subjects treated with porcine xenotransplantation products for evidence of PERV transmission.

Pig liver xenotransplantation as a bridge to allotransplantation: which patients might benefit?

Acute liver failure is a potentially devastating clinical syndrome that, without liver transplantation (Tx), is associated with high mortality. Rapid deterioration in clinical status and a shortage of deceased human organs prohibits liver Tx in many patients. Bridging to liver Tx has been attempted by various approaches, for example, bioartificial liver support, extracorporeal pig liver perfusion, and hepatocyte Tx, but none of these approaches has convincingly improved patient survival. The orthotopic Tx of a genetically engineered pig liver could theoretically provide successful bridging. Immediate availability, perfect metabolic condition, adequate size-match and hepatocyte mass, and freedom from potentially pathogenic microorganisms could be assured. The advantages and disadvantages of bridging by pig liver Tx compared with other approaches are discussed. The selection of patients for an initial clinical trial of pig liver Tx would be similar to that of various prior trials in patients experiencing rapid and severe deterioration in liver function. The ability to give truly informed consent for a pig bridging procedure at the time of listing for liver Tx renders the patient with acute-on-chronic liver failure or primary allograft failure is a preferable candidate for this procedure than a patient who is admitted urgently with acute (fulminant) liver failure in whom consent may not be possible. Although several barriers to successful pig organ xenoTx remain, for example, coagulation dysfunction between pig and primate, if these can be resolved by further genetic engineering of the organ-source pigs, a pig liver may prove life saving to patients dying rapidly of liver failure.

Porcine endogenous retrovirus released by a bioartificial liver infects primary human cells

BACKGROUND

Porcine endogenous retrovirus (PERV) remains a safety risk in pig-to-human xenotransplantation. There is no evidence of in vivo productive infection in humans because PERV is inactivated by human serum. However, PERV can infect human cell lines and human primary cells in vitro and inhibit human immune functions.

AIMS

We investigated the potential of primary porcine liver cells to transmit PERV to primary human cells in a bioreactor-based bioartificial liver (BAL).

METHODS

Primary human hepatocytes, endothelial cells and the human cell line HEK 293 were exposed to supernatants from BAL or from the porcine cell line PK-15. PERV polymerase-specific reverse-transcriptase polymerase chain reaction (RT-PCR) and PCR were used to investigate PERV transmission to human cells. An assay of RT activity was used to detect the presence of retrovirus in the supernatants of BAL, primary human hepatocytes and endothelial cells.

RESULTS

Primary human hepatocytes (hHep), endothelial cells and HEK 293 cells were reproducibly infected by PERV, originating from primary porcine liver cells within the BAL and from PK-15 cells. Infected cells were positive for PERV-specific DNA and RNA after 8-10 days on an average, and RT activity was detectable in the supernatants of infected hHep and HEK 293 cells.

CONCLUSION

A risk of PERV infection in human cells is documented in this study, indicating that short-term contact of primary porcine liver cell supernatants with primary human cells could result in PERV transmission.

Regulatory issues in xenotransplantation: recent developments

PURPOSE OF REVIEW

This article reviews recent literature on regulatory issues of xenotransplantation, focusing primarily on current progress with transplantation of pancreatic islets for diabetic patients. The need for a global system that connects the national and international regulatory levels within a coherent framework is emphasized.

RECENT FINDINGS

Recent progress in preclinical xenotransplantation of pancreatic islets has enhanced the hopes of providing patients with safe and effective treatments from animal cells and tissues in the near future. At the same time the increasing number of clinical trials proposed and sometimes authorized calls for attention from regulatory authorities.

SUMMARY

Although the main regulatory aspects of xenotransplantation have been identified since the early 1990s, regulatory problems connected to the promising scientific data published in the field of pancreatic islets still require attention. In fact most normative issues, such as criteria for patients' enrollment in clinical trials and the management of safety measures have been primarily explored in relation to solid organ transplantation. Also the issue of animal rights has become more compelling in relation to the treatment of nonhuman primates in preclinical trials. In these rapidly evolving conditions the establishment of agreed guidelines at both the national and international levels remains the most urgent goal to be achieved.

No in vivo infection of triple immunosuppressed non-human primates after inoculation with high titers of porcine endogenous retroviruses

Porcine endogenous retroviruses (PERVs) released from pig tissue can infect selected human cells in vitro and therefore represent a safety risk for xenotransplantation using pig cells, tissues, or organs. Although PERVs infect cells of numerous species in vitro, attempts to establish reliable animal models failed until now. Absence of PERV transmission has been shown in first experimental and clinical xenotransplantations; however, these trials suffered from the absence of long-term exposure (transplant survival) and profound immunosuppression.

METHODS

We conducted infectivity studies in rhesus monkeys, pig-tailed monkeys, and baboons under chronic immunosuppression with cyclosporine A, methylprednisolone, and the rapamycin derivative. These species were selected because they are close to the human species and PERVs can be transmitted in vitro to cells of these species. In addition, the animals received twice, a C1 esterase inhibitor to block complement activation before inoculation of PERV. In order to overcome the complications of microchimerism, animals were inoculated with high titers of cell-free PERV. In addition, to enable transmission via cell-cell contact, some animals also received virus-producing cells. For inoculation the primate cell-adapted strain PERV/5 degrees was used which is characterized by a high infectious titer. Produced on human cells, this virus does not express alpha 1,3 Gal epitopes, does not contain porcine antigens on the viral surface and is therefore less immunogenic in non-human primates compared with pig cell-derived virus. Finally, we present evidence that PERV/5 degrees productively infects cells from baboons and rhesus monkeys.

RESULTS

In a follow-up period of 11 months, no antibody production against PERV and no integration of proviral DNA in blood cells was observed. Furthermore, no PERV sequences were detected in the DNA of different organs taken after necropsy.

CONCLUSION

These results indicate that in a primate model, in the presence of chronic immunosuppression, neither the inoculation of cell-free nor cell-associated PERV using a virus already adapted to primate cells results in an infection; this is despite the fact that peripheral blood mononuclear cells of the same animals are infectible in vitro.

Update: cardiac xenotransplantation

PURPOSE OF REVIEW

To review the latest development in cardiac xenotransplantation in small and large animal models and related in-vitro studies.

RECENT FINDINGS

With the recent introduction of alpha1,3-galactosyltransferase gene-knockout (GT-KO) pig organs for xenotransplantation, improved cardiac graft survival has been obtained. However, this experience has demonstrated the importance of pig antigens other than Galalpha1,3Gal (Gal) antigens (so-called nonGal antigens) as targets for primate anti-pig antibodies. Several in-vitro studies have confirmed that, although the incidence and levels of anti-nonGal antibodies in nonhuman primates and humans are significantly less when compared with total anti-pig antibodies (i.e., anti-Gal + anti-nonGal), they can result in complement-mediated lysis of GT-KO pig cells. More recently, it has been demonstrated that regulatory T cells suppress the cellular xenogeneic response, thus potentially preventing or reducing T-cell-mediated rejection. The importance of thrombotic microangiopathy as a feature of the immune/inflammatory response and incompatibilities between the coagulation-anticoagulation systems of pig and primate are receiving increasing attention. Development of GT-KO pigs transgenic for one or more 'antithrombotic' genes, for example, CD39 or tissue factor pathway inhibitor, may contribute to overcoming these problems.

SUMMARY

Although GT-KO pigs have provided an advance over wild-type pigs as a source of organs for transplantation into primates, further genetic modification of GT-KO pigs is required to overcome the remaining immune barriers before a clinical trial of cardiac xenotransplantation can be contemplated.

Xenotransplantation of solid organs in the pig-to-primate model

Xenotransplantation using pig organs could solve the significant increasing shortage of donor organs for allotransplantation. In the last two decades, major progress has been made in understanding the xenoimmunobiology of pig-to-nonhuman primate transplantation, and today we are close to clinical trials. The ability to genetically engineer pigs, such as human decay-accelerating factor (hDAF), CD46 (membrane cofactor protein), or alpha1,3-galactosyltransferase gene-knockout (GT-KO), has been a significant step toward the clinical application of xenotransplantation. Using GT-KO pigs and novel immunosuppressant agents, 2 to 6 months' survival of heterotopic heart xenotransplants has been achieved. In life-supporting kidney xenotransplantation, promising survival of close to 3 months has been achieved. However, liver and lung xenotransplantations do not have such encouraging survival as kidney and heart xenotransplantation. Although the introduction of hDAF and GT-KO pigs largely overcame hyperacute rejection, acute humoral xenograft rejection (AHXR) remains a challenge to be overcome if survival is to be increased. In several studies, when classical AHXR was prevented, thrombotic microangiopathy and coagulation dysregulation became more obvious, which make them another hurdle to be overcome. The initiating cause of failure of pig cardiac and renal xenografts may be antibody-mediated injury to the endothelium, leading to the development of microvascular thrombosis. Potential contributing factors toward the development of the thrombotic microangiopathy include: 1) the presence of preformed anti-non-Gal antibodies, 2) the development of very low levels of elicited antibodies to non-Gal antigens, 3) natural killer cell or macrophage activity, and 4) inherent coagulation dysregulation between pigs and primates. The breeding of pigs transgenic for an 'anticoagulant' or 'anti-thrombotic' gene, such as human tissue factor pathway inhibitor, hirudin, or CD39, or lacking the gene for the prothrombinase, fibrinogen-like protein-2, is anticipated to inhibit the change in the endothelium to a procoagulant state that takes place in the pig organ after transplantation. A further limitation for organ xenotransplantation is the potential for cross-species infection. As far as exogenous viruses are concerned, porcine cytomegalovirus has been detected in the tissues of recipient non-human primates, although no invasive disease was reported. Until today, no formal evidence has been presented from in vivo studies in non-human primates or from humans exposed to pig organs, tissues, or cells that porcine endogenous retroviruses infect primate cells. Xenotransplantation is a potential answer to the current organ shortage. Its future depends on; 1) further genetic modification of pigs, 2) the introduction of novel immunosuppressive agents that target the innate immune system and plasma cells, and 3) the development of clinically-applicable methods to induce donor-specific tolerance.

Functional hierarchy of two L domains in porcine endogenous retrovirus (PERV) that influence release and infectivity

The porcine endogenous retrovirus (PERV) Gag protein contains two late (L) domain motifs, PPPY and P(F/S)AP. Using viral release assays we demonstrate that PPPY is the dominant L domain involved in PERV release. PFAP represents a novel retroviral L domain variant and is defined by abnormal viral assembly phenotypes visualized by electron microscopy and attenuation of early PERV release as measured by viral genomes. PSAP is functionally dominant over PFAP in early PERV release. PSAP virions are 3.5-fold more infectious in vitro by TCID(50) and in vivo results in more RNA positive tissues and higher levels of proviral DNA using our human PERV-A receptor (HuPAR-2) transgenic mouse model [Martina, Y., Marcucci, K.T., Cherqui, S., Szabo, A., Drysdale, T., Srinivisan, U., Wilson, C.A., Patience, C., Salomon, D.R., 2006. Mice transgenic for a human porcine endogenous retrovirus receptor are susceptible to productive viral infection. J. Virol. 80 (7), 3135-3146]. The functional hierarchies displayed by PERV L domains, demonstrates that L domain selection in viral evolution exists to promote efficient viral assembly, release and infectivity in the virus-host context.