The measles virus replication cycle

Measles occurs throughout the world. Interruption of indigenous transmission of measles was declared in the United States in the year and in other parts of the Western Hemisphere in However, outbreaks with sustained measles virus transmission have recently occurred in Venezuela and Brazil, leading to re-establishment of endemic transmission in these countries and loss of measles elimination in the Americas.

Measles is a human disease. There is no known animal reservoir, and an asymptomatic carrier state has not been documented. Measles transmission occurs person-to-person via large respiratory droplets and via airborne transmission of aerosolized droplet nuclei in closed areas e.

Measles is considered transmissible from 4 days before through 4 days after rash onset. Before , approximately , cases and measles deaths were reported annually, with epidemic cycles every 2 to 3 years. However, the actual number of cases was estimated at 3 to 4 million annually.

The occurrence of measles among previously vaccinated children i. Before , the highest number of measles cases following elimination in the United States occurred in when cases were reported. Increasing incidence of measles globally contributes to increased opportunities for measles importation into the United States.

Fortunately, public health measures and a long-standing vaccination program has prevented outbreaks form imported cases. Among children born during —, The inactivated vaccine was withdrawn in because it did not protect well against measles.

The original Edmonston B vaccine was withdrawn in because of a relatively high frequency of fever and rash in recipients. A live, further attenuated Schwarz strain vaccine was first introduced in , but also is no longer used in the United States. Another live, further attenuated strain Edmonston-Enders strain vaccine was licensed in These further attenuated vaccines caused fewer reactions than the original Edmonston B vaccine.

In , measles vaccine was licensed as a combined measles, mumps, and rubella MMR vaccine. Single-antigen measles vaccine is not available in the United States.

MMR and MMRV vaccines are supplied as a lyophilized freeze-dried powder and are reconstituted with sterile, preservative-free water and vaccine contains gelatin. It contains no adjuvant or preservative. MMR vaccine or MMRV vaccine can be used to implement the vaccination recommendations for prevention of measles, mumps, and rubella. MMR vaccine is licensed for use in persons age 12 months or older.

MMRV vaccine is licensed for use in persons age 12 months through 12 years; MMRV vaccine should not be administered to persons age 13 years or older. Two doses of MMR vaccine, separated by at least 4 weeks, are routinely recommended for children age 12 months or older. Dose 1 of MMR vaccine should be given at age 12 through 15 months. A second dose of MMR vaccine is recommended based on previous observations of the failure of some to generate an immune response to measles following dose 1.

Dose 2 is routinely given at age 4 through 6 years, before a child enters kindergarten or first grade. All students entering school should receive 2 doses of MMR vaccine with the first dose administered at age 12 months or older before enrollment.

Dose 2 of MMR vaccine may be administered as soon as 4 weeks after dose 1. The minimum interval between doses of MMRV vaccine is 3 months, although when dose 2 is administered 4 weeks following dose 1, it can be considered valid. Providers who are considering administering MMRV should discuss the benefits and risks of both vaccination options with the parents.

For the second dose of measles, mumps, rubella, and varicella vaccines at any age and for the first dose at age 48 months or older, the use of MMRV generally is preferred over separate injections of its equivalent component vaccines i. Adults born in or later should receive at least 1 dose of MMR vaccine unless they have documentation of vaccination with at least 1 dose of measles, mumps, and rubella-containing vaccine or other acceptable presumptive evidence of immunity to these three diseases.

Except for health care personnel who should have documented immunity, birth before generally can be considered acceptable evidence of immunity to measles, mumps, and rubella.

Colleges and other post-high-school educational institutions are potential high-risk areas for measles, mumps, and rubella transmission because of large concentrations of persons. Prematriculation vaccination requirements for measles immunity have been shown to significantly decrease the risk of measles outbreaks on college campuses where such requirements are implemented and enforced. All students entering colleges, universities, technical and vocational schools, and other institutions for post-high-school education should receive 2 doses of MMR vaccine or have other acceptable evidence of measles, mumps, and rubella immunity before entry.

For unvaccinated health care personnel born before who lack laboratory evidence of measles, mumps, or rubella immunity or laboratory confirmation of disease, health care facilities should have policies that offer 2 doses of MMR vaccine at the appropriate interval for measles and mumps and 1 dose of MMR vaccine for rubella, respectively. Health care facilities should also have policies for such personnel that recommend 2 doses of MMR vaccine during an outbreak of measles or mumps and 1 dose during an outbreak of rubella.

Adequate vaccination for health care personnel born during or after consists of 2 appropriately spaced MMR doses for measles and mumps, and at least 1 dose of MMR for rubella. Persons who travel outside the United States are at increased risk of exposure to measles.

Measles is endemic or epidemic in many countries throughout the world. Although proof of immunization is not required for entry into the United States or any other country, persons traveling or living abroad should have evidence of measles immunity.

Adequate vaccination of persons who travel outside the United States is 1 dose of MMR vaccine for children age 6 through 11 months and 2 doses of an age-appropriate measles-, mumps-, and rubella-containing vaccine for children age 12 months and older and adults. Revaccination is recommended for certain persons. The following groups should be considered unvaccinated and should receive at least 1 dose of measles vaccine: 1 persons vaccinated before their first birthday, 2 persons vaccinated with killed measles vaccine, 3 persons vaccinated from through with an unknown type of vaccine, 4 persons who received immune globulin IG in addition to a further attenuated strain or vaccine of unknown type, and 5 persons with perinatal human immunodeficiency virus HIV infection who were vaccinated before establishment of effective antiretroviral therapy ART and who do not have evidence of current severe immunosuppression.

Measles-, mumps-, or rubella- virus-containing vaccine administered prior to age 12 months e. Children vaccinated before age 12 months should be revaccinated with 2 doses of appropriately spaced MMR or MMRV vaccine, the first dose administered when the child is age 12 through 15 months 12 months if the child remains in an area where disease risk is high and the second dose at least 4 weeks later.

Persons who experienced perinatal HIV-infection who may have received MMR vaccine prior to the establishment of effective combined antiretroviral therapy cART should be revaccinated with 2 appropriately spaced doses of MMR i. MMR series should be administered once effective cART has been established for at least 6 months and there is no evidence of severe immunosuppression.

Generally, persons can be considered immune to measles if they were born before , have serologic evidence of measles immunity equivocal test results should be considered negative , or laboratory confirmation of disease, or have documentation of adequate vaccination for measles.

MMR vaccine failure can occur because of passive antibody in the vaccine recipient, immaturity of the immune system, damaged vaccine, or other reasons. Most persons who fail to respond to the first dose will respond to a second dose. Although the titer of vaccine-induced antibodies is lower than that following natural disease, both serologic and epidemiologic evidence indicate that vaccine-induced immunity appears to be long-term and probably lifelong in most persons.

Most vaccinated persons who appear to lose antibody show an anamnestic immune response upon revaccination, indicating that they are probably still immune. Although revaccination can increase antibody titer in some persons, available data indicate that the increased titer may not be sustained. Some studies indicate that waning immunity may occur after successful vaccination, but this appears to occur rarely and to play only a minor role in measles transmission and outbreaks.

As with other vaccines, a history of a severe allergic reaction anaphylaxis to a vaccine component or following a prior dose is a contraindication to further doses.

Moderate or severe acute illness with or without fever in a patient is considered a precaution to vaccination, although persons with minor illness may be vaccinated. Persons with alpha-gal allergy may wish to consult their physician before receiving a vaccine that contains gelatin.

Severe immunocompromise e. Patients who have not received chemotherapy for at least 3 months, whose disease remains in remission, and who have restored immunocompetence, may receive MMR or MMRV vaccine. Healthy, susceptible close contacts of severely immunocompromised persons should be vaccinated. Persons receiving systemic high-dose corticosteroid therapy 2 milligrams per kilogram of body weight or more per day or 20 milligrams or more per day of prednisone for 14 days or more should not receive MMR or MMRV vaccine because of concern about vaccine safety.

MMR or MMRV should not be administered for at least 1 month after cessation of systemic high-dose corticosteroid therapy. Although persons receiving high doses of systemic corticosteroids daily or on alternate days for less than 14 days generally can receive MMR or MMRV immediately after cessation of treatment, some experts prefer waiting until 2 weeks after completion of therapy.

Moreover, the structural flexibility of the disordered N tail is important for interactions between the N tail and multiple cellular proteins, including the 70 KDa heat shock protein Hsp72 , eukaryotic translation initiation factor 3 eIF3-p40 and interferon regulatory factor 3 IRF-3 [ 25 , 26 , 27 ].

The second gene codes for three proteins—P, V, and C—via an RNA editing process and an alternative reading frame [ 28 ]. In addition, the P protein tethers the polymerase onto and progresses along the N-RNA template by binding to the NC [ 31 , 32 , 33 , 34 ]. The main function of the V and C proteins is to suppress the host innate immune response by interfering with IFN signaling pathways [ 1 ].

These proteins also function as virulence factors in that they are indispensable for virus infection in vivo [ 35 ]. For the related Sendai virus, the C protein even enhances the release of the M protein in a manner dependent on the endosomal sorting complexes required for transport ESCRT pathway [ 36 ].

The third gene codes for the M protein, which is a hydrophobic protein. Although M is not a membrane protein, it associates with membranes, probably through its hydrophobic surface [ 37 ]. It also binds RNPs, associates with the cytoplasmic tails of F and H proteins and modulates cell fusion [ 24 , 38 ]. In addition, it acts as an inhibitor of viral polymerase activity, affecting both mRNA transcription and genome replication [ 24 , 39 ].

Thus, the M protein plays a crucial role in many stages of the viral lifecycle. The last gene codes for the RNA-dependent RNA polymerase RdRP , which is believed to possess all catalytic functions required for RNA synthesis, including ribonucleotide polymerization, capping and methylation, and polyadenylation [ 40 , 41 ].

The initial binding of MeV to the cell surface is mediated by the tetrameric H protein via interaction with cell surface receptors, which triggers the conformational change of the trimeric F protein and then, the membrane fusion and the delivery of the viral RNP core into the cytoplasm [ 44 , 45 ].

Similar to the H proteins of the Morbillivirus, the H protein of MeV cannot bind sialic acid and lacks neuraminidase activity; thus, it is named H not HN [ 1 , 46 , 47 ]. MeV shares the gene order and transcription strategy that are fundamental characteristics of all other paramyxoviruses [ 22 , 53 ]. Following cell entry, the genomic RNPs are released into the cytosol and the encapsidated viral RNA serves as a template of the RdRP complex for both transcription and replication [ 5 ].

Newly synthesized viral mRNAs are translated to viral proteins by using the host translation machinery. The negative-strand genome is also used to synthesize a positive-strand anti-genome, which is a complementary copy of the entire genome that produces more genomes via the same viral RNA polymerase.

During replication, the newly synthesized genomic RNA is tightly wrapped with the N protein to provide a helical template for viral transcription and replication [ 32 , 54 ].

Although the mechanism of the switch from transcription to replication remains unclear, evidence suggests that the accumulation of N proteins is critical for it [ 42 ]. The assembly of the M protein, the RNP complex, and the glycoproteins at selected sites on the plasma membranes of infected cells lead to the formation of fully infectious MeV particles, which is a result of coordinated interactions between viral components as well as between viral and cellular factors [ 45 , 55 ].

The C -terminal domain of the N protein has been proved to be essential for the interaction with the M protein by yeast two-hybrid binding assay and co-immunoprecipitation in mammalian cells [ 24 ], and mutations or deletions in the M gene block the transport of RNP complex to the plasma membrane during infection, further supporting the crucial role of M protein in incorporating the RNP complex into virions [ 56 , 57 ]. In addition, the M protein can assemble to form higher structure, and binds cellular membranes and cellular factors as well [ 45 , 55 , 58 , 59 ].

Consequently, it is generally considered the key driver of paramyxovirus particle assembly and budding. For many paramyxoviruses, including MeV, M protein expressed in the absence of other viral proteins is sufficient to form virus-like particles [ 37 , 56 ]. Taken together, the aforementioned findings suggest that, for MeV, the F and H proteins assemble intracellularly prior to receptor binding and are co-transported to the plasma membrane.

The M protein associates with the RNP complex in the cytoplasm and then carries it to the plasma membrane, where the assembly with F and H proteins occurs. As an obligate intracellular parasite, MeV interacts with numerous cellular molecules to manipulate cellular processes and to subvert anti-viral responses for its replication.

At the entry level, at least three cellular receptors have been identified for wild type strains and laboratory-adapted strains. In addition to the two aforementioned receptors, the laboratory-adapted and vaccine strains use CD46, which is expressed on nearly all nucleated cells [ 65 , 66 ]. Consequently, using CD46 as an additional receptor results in a tropism alteration of MeV.

Despite the lymphocyte and epithelia cell tropism of wild type MeV strains, MeV may also infect other types of cells. Thus far, the mechanism accounting for MeV neuronal infection and transport is unclear.

Many putative host factors involved in MeV RNA synthesis have been identified via yeast two-hybrid, co-immunoprecipitation and some proteomic approaches. By competing with P protein for binding to these domains, Hsp72 would loosen the binding between RdRP and the ribonucleocapsid so that RdRP can move to the next N tail, which sustains the RdRP processivity, resulting in increased genome transcription, replication, and virulence [ 25 , 71 ]. In addition, another host factor, SHC binding and spindle associated 1 SHCBP1 , was found to interact with both the C and the P proteins, and they did not compete for the binding [ 72 ].

Besides just exploiting the regulation on itself, MeV inhibits the translation of cellular mRNA via the interaction between the p40 subunit of eIF3 and the N protein [ 26 ]. Other host factors that interact with MeV proteins include several kinases, such as casein kinase II that phosphorylates P protein and some unidentified kinases that phosphorylate N and P proteins [ 73 , 74 ]. Phosphorylation of these two proteins has totally different functions.

Phosphorylation of P protein at S86 and S downregulates viral transcriptional activity, whereas MeV RNA synthesis is increased upon the phosphorylation of serine residues and in N protein, as confirmed [ 75 ] in a mini-genome expression system [ 76 , 77 ].

Many viruses take advantage of the cellular trafficking system during their replication. Cytoskeletal tubulin has been shown to be an essential component for the transcription and replication of Sendai virus and vesicular stomatitis virus [ 78 , 79 ]. Moreover, the MeV RNP complex is transported in a microtubule-dependent manner associated with recycling endosomes containing Ras-related protein RabA [ 81 ].

Together with microtubules, another cytoskeleton component, actin, is essential for the reproduction of MeV as well, especially for budding. Experimental data revealed that the accumulation of RNP and defects in the maturation and release of infectious MeV particles were connected with the disruption of actin filaments and that actin filaments were packaged within the virions, suggesting a close association between actin filaments and MeV assembly and budding [ 75 , 79 , 82 ].

These host factors interacting with MeV during its lifecycle are summarized in Figure 1. Schematic of host factors involved in the measles virus MeV lifecycle. The attachment and entry of MeV is mediated by H and F proteins, associated with cellular receptors CD, nectin-4 for wild-type strains and CD46 for attenuated strains, respectively. The N protein can also bind to eukaryotic translation initiation factor 3 eIF3-p40 to inhibit the translation of cellular mRNA.

As for assembly and budding, the RNP complex is transported to the plasma membrane driven by the M protein, and the process is dependent on actin filaments and microtubules associated with Ras-related protein Rab11A.

The host has innate immunity, in particular the IFN system, to sense and protect it from MeV infection. However, MeV has also evolved multifaceted strategies to antagonize the immune attack. MeV has evolved multifaceted strategies to counteract viral RNA sensing. The data revealed that overexpression of SK1 promoted the replication of MeV.

However, the C protein of vaccine strains cannot localize into the nucleus, suggesting that altered C protein intracellular localization of vaccine strains contributes to its attenuation [ ]. However, a direct interaction between IRF-3 and N could not be confirmed experimentally by using purified recombinant proteins and yeast two hybrid assay: this finding led to the hypothesis of an indirect binding requiring a specific cellular context [ 27 ].

MeV is capable of interfering with both IFN synthesis and the signal transduction pathway mediated by the release of IFN, thus allowing the successful escape of MeV from the innate immune system. A detailed overview of host factors involved in MeV-stimulated interferon IFN induction and signaling. Furthermore, V protein inhibits the activation and function of interferon regulatory transcription factor 3 IRF-3 and IRF-7 via interaction with them.

In addition to the synthesis of IFN and the subsequent signaling cascades, another response of host cells to infection of several paramyxoviruses, including MeV, is the formation of stress granules SGs , which are cytoplasmic aggregates containing various translationally stalled mRNAs, 40S ribosomes, and RNA-binding proteins [ , , ]. Of particular note, SG cannot form in cells infected by wild-type MeV; however, a mutant lacking C protein expression is a robust inducer of SG formation, which is consistent with the suppression of the activation of PKR by the C protein [ ].

Autophagy is a highly conserved mechanism that mediates the dysfunctional cytoplasmic components for lysosomal degradation and recycling, whose hallmark is the formation of the double-membraned autophagosomes that sequently fuse with lysosomes to form autolysosomes for degradation.

Autophagy not only maintains cellular and tissue homeostasis but also regulates innate immune responses against intracellular virus invasion [ ].

Consequently, viruses have developed various mechanisms to escape or even hijack the autophagy machinery for their own benefit. For MeV, the infection can induce successive proviral autophagy signaling via different pathways, which ultimately promotes the formation of infectious viral particles [ ].

Both virulent and attenuated strains induce a late and sustained autophagy wave, which is initiated after viral replication and relies on the expression of C protein of MeV. The late autophagy signaling can be sustained overtime as a result of the formation of syncytia [ ]. Moreover, the extensive syncytia formation mediated by the expression of both H and F proteins in cells expressing one of the cellular receptors is sufficient to induce autophagy. Therefore, the expression of C protein is not necessary to induce autophagy in syncytia [ ].

As for attenuated strains being able to bind CD46, the infection can induce two waves of autophagy, including an early but transient autophagy wave via the engagement of CDCyt-1 in a Golgi-associated PDZ and coiled-coil motif-containing protein GOPC -dependent pathway and the late autophagy wave [ ].

Research findings have suggested that the involvement of CD46 in the autophagic digestion of MeV peptides and the subsequent presentation by MHC-II may explain the acquisition of protective immunity via vaccine strains of MeV infection [ ].

Of note, MeV induces de novo formation of autophagosomes, and such autophagosomes mature into autolysosomes, whereas some paramyxoviruses make use of autophagy machinery to replicate but inhibit autophagosome maturation by blocking the fusion of autophagosomes with lysosomes [ , ].

Apart from bulk autophagy, the Edmonston-MeV strain also exploits the selective mitophagy in non-small cell lung cancer NSCLC cells, which enhances its replication as well [ ].

By clearing damaged mitochondria before they release cytochrome c, mitophagy prevents the beclin 1-mediated activation of Bid or the degradation of active caspase-8 and then inhibits apoptosis, resulting in migitated cell death and the consequent oncolysis in NSCLC induced by Edmonston-MeV [ ]. An overview of host factors involved in MeV inducible stress granule formation and autophagy is depicted in Figure 3.

A diagram of host factors involved in MeV inducible stress granule formation and autophagy. The expression of H and F proteins in cells expressing one of the cellular receptors is also sufficient to induce autophagy.

The arrows indicate the activation of innate immune responses while the T-ended arrows indicate the repression by MeV. In this review, we provide a summary of current achievements in MeV research and review the experimental efforts aimed at elucidating the molecular mechanisms of MeV replication and host—pathogen interactions.

We focus on recent insights related to the MeV genome, the function of viral proteins, the replication cycle, and the involvement of host cell factors during the MeV lifecycle, with an emphasis on cellular factors mediating MeV-stimulated innate immune responses that play key roles in the MeV replication cycle. The elucidation of the mechanisms of MeV infection has provided valuable information on viral replication and countermeasures to mitigate cellular innate immune responses, which will ultimately provide new targets for antiviral therapy against MeV.

However, many details of the mechanisms remain unclear; therefore, further investigation is needed to provide a clearer and more comprehensive understanding of the aforementioned aspects. This work was supported by a grant from the China Natural Science Foundation grant , and Yanliang Jiang drafted the manuscript.

All authors corrected, edited and approved the text. National Center for Biotechnology Information , U. Journal List Viruses v. Published online Nov Richard Plemper, Academic Editor.

Author information Article notes Copyright and License information Disclaimer. Received Aug 31; Accepted Nov 7. This article has been cited by other articles in PMC. Abstract The measles virus MeV is a contagious pathogenic RNA virus of the family Paramyxoviridae , genus Morbillivirus , that can cause serious symptoms and even fetal complications.

Keywords: measles virus, paramyxoviruses, viral replication, host factors. Introduction Measles, also known as rubeola or morbilli, is a contagious infection caused by measles virus MeV , an RNA virus of the genus Morbillivirus within the family Paramyxoviridae [ 1 ]. Genome and Lifecycle 2. Genome Currently, 24 MeV genotypes compiled in eight clades A—H have been recognized by sequencing nucleotides nt that code for the C -terminal amino acids of the N gene [ 16 , 17 ].

Cell Entry The initial binding of MeV to the cell surface is mediated by the tetrameric H protein via interaction with cell surface receptors, which triggers the conformational change of the trimeric F protein and then, the membrane fusion and the delivery of the viral RNP core into the cytoplasm [ 44 , 45 ]. Transcription and Replication MeV shares the gene order and transcription strategy that are fundamental characteristics of all other paramyxoviruses [ 22 , 53 ].

Assembly and Egress The assembly of the M protein, the RNP complex, and the glycoproteins at selected sites on the plasma membranes of infected cells lead to the formation of fully infectious MeV particles, which is a result of coordinated interactions between viral components as well as between viral and cellular factors [ 45 , 55 ]. Interaction between MeV and Cellular Factors 3. Host Factors Involved in MeV Entry As an obligate intracellular parasite, MeV interacts with numerous cellular molecules to manipulate cellular processes and to subvert anti-viral responses for its replication.

Open in a separate window. Figure 1. Figure 2. Host Factors Involved in Stress Granule Formation In addition to the synthesis of IFN and the subsequent signaling cascades, another response of host cells to infection of several paramyxoviruses, including MeV, is the formation of stress granules SGs , which are cytoplasmic aggregates containing various translationally stalled mRNAs, 40S ribosomes, and RNA-binding proteins [ , , ].

Host Factors Involved in Autophagy Autophagy is a highly conserved mechanism that mediates the dysfunctional cytoplasmic components for lysosomal degradation and recycling, whose hallmark is the formation of the double-membraned autophagosomes that sequently fuse with lysosomes to form autolysosomes for degradation.

Figure 3. Conclusions In this review, we provide a summary of current achievements in MeV research and review the experimental efforts aimed at elucidating the molecular mechanisms of MeV replication and host—pathogen interactions.

Acknowledgments This work was supported by a grant from the China Natural Science Foundation grant , and Author Contributions Yanliang Jiang drafted the manuscript.

Conflicts of Interest The authors declare no conflict of interest. References 1. Chesney R. Encyclopedia of Virology. Measles Virus; pp. King A. A given virus may exhibit extraordinary diversity in genomic content and particle morphology, so candidate therapeutics must be pan-protective against a heterogeneous viral population.

Consideration should be given to the cell type and virion morphology as well as the diversity in entry and egress processes.

Many antiviral therapies identified to date are entry inhibitors, yet we have seen how diverse mechanisms of entry and egress enable rapid dissemination through tissues. The use of TNTs for delivery of viral genomes to a neighbouring cell bypasses receptor-mediated entry altogether. Therefore, entry inhibitors may prove ineffective against such infections.

TNTs may also facilitate viral escape from antibody neutralization, thus weakening the effectiveness of vaccination. Such infections may progress faster than a classical infection would, as rapid dissemination to cells hundreds of microns apart can readily occur , Post-entry therapeutics or combination therapies with nanotube inhibitors might prove to be more effective against such viral diseases. A diverse set of viruses are cloaked in host microvesicles. Cloaked picornaviruses have higher infectivity than traditionally described non-enveloped particles and may disseminate more efficiently.

Cloaked particles may have other functions in addition to spreading virus particles; in the case of influenza virus, they may complement semi-infectious virus particles lacking one or more gene segments. Given that picornaviruses reportedly package multiple viral particles per microvesicle to achieve dissemination of many virions en masse , one could envision that microvesicles promote co-infection.

Interestingly, microvesicle cloaking does not increase genetic complementation or population diversity during co-infection with multiple CVB3 variants This finding suggests that packaging of CVB3 into host microvesicles is highly selective. Considering that genetic reassortment is commonly reported at high multiplicity of infection during influenza virus co-infection , further investigation will be necessary to determine whether the selectivity observed in CVB3 microvesicles applies to other viruses.

The mechanism of such high-fidelity packaging in microvesicles is nonetheless an exciting new area of research. Pleomorphic particles have been reported for decades, but the effect of heterogeneous assembly mechanisms on viral pathogenesis remains understudied. An intriguing function in mucus layer penetration and clearance has been proposed for long-filament IAV particles, which are defective for genomic packaging Such a role for low-fidelity assembly of IAV particles would have clear implications for pathogenesis and may be a viable target in the development of future therapeutics.

Alternatively, the production of pleomorphic virions in a host can result in viruses with different HA to NA ratios see Fig. Altered HA to NA levels on a virion could impact antibody binding and may, therefore, influence vaccine efficacy.

Although outside the scope of this Review, the contribution of diverse polymicrobial communities to the morbidity and mortality of heterogeneous viruses is a public health dilemma in need of deeper investigation.

Active tuberculosis in HIVinfected patients is associated with elevated HIV-1 infection of macrophages found in the lungs and pleural effusions. Active tuberculosis skews human monocyte differentiation towards an anti-inflammatory M2 macrophage pathway and blood monocytes treated with M. This particular phenotypic diversity likely contributes to M.

Further study of bacterial and viral co-infection will undoubtedly reveal additional layers of phenotypic heterogeneity in these complex systems. As new technologies emerge, our appreciation of phenotypic heterogeneity in the viral replication cycle grows.

From alternate egress and entry pathways to variations in genome assembly and virion morphology, these diverse replication mechanisms challenge how we classify viruses and what is considered an infectious particle. Here, we have highlighted the use of nanotubular bridges in cell-to-cell spread, exploitation of the microvesicle secretory pathway to cloak virions and promote infection, and how low-fidelity replication can drive viral heterogeneity.

By redefining the canonical viral replication cycle, we will be better equipped to develop antiviral therapies against the many nuances of viral replication and address adaptive evasion strategies.

Future studies examining the relationship between viral heterogeneity and disease severity are particularly needed to help refine specific antiviral targets and biosensors. Investigation of heterogeneity in infections with emerging pathogens such as SARS-CoV-2 are particularly essential for combating emerging viral threats. Studies already indicate heterogeneity in SARS-CoV-2 entry mechanisms in different cell types, which ultimately determines susceptibility to antivirals like chloroquine and its derivatives The gravity of the COVID pandemic underscores the critical role of such studies in guiding global health policies.

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