Mechanism Cellular Recognition and Entry by Porcine Circovirus - Report Example

Mechanism Cellular Recognition and Entry by Poreine Circovirus Introduction Viruses are infectious agents that are made of particles known as virions, which are composed of three parts: a protein coat to protect the genes, lipid envelope, and genetic material (either RNA or DNA). Their shapes vary from simple helical or icosahedral to complex structures (Morilla, Kyoung-Jin, and Jeffrey 283). In terms of genomic sizes, the ranges are from 1.80 kb to 1259.2 kb, which confirms that most of the viruses are tiny to be observed by using an optical microscope. When viewed under sophisticated electron microscopes, their surface contents appear to be either enveloped or non-enveloped (Murphy 359). Porcine circovirus disease (PCVD) is a viral disease, which attack pigs and has recently been a problem in US and many parts of China. The viral exists in two types 1, and 2, however, the Porcine Circovirus Type (PCV2) is the common one, which shows clinical signs when it infects swine. Evidently, PCV2 is spread all over the world and causes a lot of problems in domestic swine production. The disease exhibits so many syndromes, which are collectively known as porcine circovirus diseases (Murphy 400). The most relevant ones are Postweaning multisystemic wasting syndrome (PMWS), reproductive disorders, and porcine nephropathy and dermatitis syndromes. Nonetheless, only PMWS has proved to be the most severe and highly affect swine production and is common in pigs at nursery or those that are fattening. Among the clinical signs, wasting is the most prominent (Morilla, Kyoung-Jin, and Jeffrey 284-286). Fig 1: shows a pig with Circovirus infection Structure of the PCV Virus This is observed under a cryo-electron microscope (cryo-EM) to reconstruct a full length of the virus particles at a resolution of 9.6-Å resolution. The image reveals the features such as strands, binding sites, and capsid epitopes visualization (Mettenleiter, Thomas, and Francisco 123). Fig 2: shows the Structure of the PCV virus Taxonomy PCV1 and PCV2 are found in the Circoviridae family, which is composed of viruses with single strand DNA genome that are circular and to the generic members called Circovirus. However, other known generic names include, pigeon circovirus, caranary circovirus, goose circovirus, and feather, and beak disease virus. Several species result to lymphoid depletion within the infected hosts, although, some cause subclinical infection. Again, the family members are small and non-envelope and have a capsid that is icosahedral (Cann 302). Strains of PVC There are strains type 1 (PCV1) and type 2 (PCV2); however, PCAD is caused by PCV2. PCV-1 was discovered in 1974, though it has not caused any notable disease in pigs while PCV-2 was isolated in 1997 are is known to cause problems of PMWS, which depletes significant counts of lymphocytes in swine resulting to enlarged lymph nodes and affects the lung tissues. However, PCV-2 infection only causes mild diseases but when other co-factors like other infections and immune-stimulation initiates developments of severe stages of the disease (Cann 327). Pathogenesity of the two strains has never been clearly distinct; nevertheless, they exhibit a high degree of identity in sequence and similarity in genomic organization. Fig 3: the nature of the Strains of PVC Moreover, PCV2 is divided into two genotypes named PCV2-group 1 and group 2 but, have no difference in their pathogenesis. However, the groups differ in size as PCV2-group 1 is made up of 1767 nucleotides (nt) while PCV2-group 2 has 1768 nt. PCV-group 1 is again divided into three clades group 2 into 5 clades. Chronologically, only isolates of PCV-group 2 were recorded in United States; however in 2005, a variety of outbreaks more than normalcy were found in Kansas, Iowa, and North Carolina. The outbreaks were linked to isolates of PCV-group 1. PCV Genome The genome of PCV is very simple as related to other viruses because it uses only a single capsid protein (ORF2) together with two replicase proteins (ORF1) to allow its replication and production of a functional virus. Due to its simplicity, PCV depends much on the machinery of the host in order to replicate. Therefore, PCV undergoes replication in the infected cells’ nucleus through the help of host polymerase to amplify their genome. Origin of replication of the virus is found on an octanucleotide stem-loop, which is sandwiched by palindromic repeats that are synchronized by ORF’S located head to head on either sides of the Ori. Specifically, ORF 1 is found on the clockwise while ORF 2 on the anticlockwise side of Ori (Cann 338). Early stages of the virus have two replicase enzymes which are created by ORF1 called Rep and Rep’ and conserved between the two PCV types. In distinction, Rep is a complete transcript of 312 protein building blocks (amino acids) whereas Rep’ shows a truncation of ORF1 due to splicing and has a length of only 168 amino acids. Again, a promoter for Rep (Prep) has an interferon-Stimulated Response Element (ISRE) that means that cytokine involvement regulates Rep and Rep’, and this forms the mechanism through which the virus counters the host’s immune defense to infection (Murphy 401-405). The two replicases form a dimer, which links two hexameric regions that are above the stem-loop known as H1 and H2 required for replication. When the dimer attaches to this region, Rep and Rep’ cleave the loop-region of the stem loop and then remain covalently bonded to both H1 and H2 DNA regions, which finally forms the 5’ end part of the DNA. Newly formed 3’OH end produces a primer with the aid of host RNA polymerase (Cann 332). Consequently, DNA polymerase of the host initiates transcription process for the viral DNA through rolling circle replication. After the creation of the contemporary DNA strand, the stem-region of the stem-loop produces a loose a quadruplet DNA structure that is non-hydrogen bonded. The loose structure form short spanned DNA trimmers, which creates two templates to retain nucleic integrity of the stem-loop and to allow replication. Unfortunately, the termination of replication has never been clearly understood; however, evidence shows that Rep also represses the promoter of its own, Prep (Mettenleiter, Thomas, and Francisco 156). On the other hand, ORF2 encoding differs slightly between the two PCV strains (1 and 2) and this difference explains why PCV-1 is considered non-pathogenic contrasted to the pathogenic PCV-2. Fig 4: shows the replication phases of the virus The Life Cycle of the Virus The mechanism of cell recognition, entry, and attachment of PCV 2 are still being researched and probably not well understood. However, evidence shows that PCV2 uses a common cell receptor because PCV2 antigens and viral replication have been isolated in a number of different cell types. PCV2 binds to Chondrioitin Sulphate and Heparin sulfate, which are known to be Glycosaminoglycans (GAGs0). Also, PCV2 uses another co-receptor for its cellular entry (Maclachlan, Edward, and Frank 253-258). Histiocytic replacement occurs together with lymphoid depletion and cause hallmark lesion at the clinical stages of PCV2 infection. In the affected macrophages, dendritic cells, and lymph organs, result to replacement of lymphocytes, which contain many PCV2 viruses. The viruses do not degrade in these cells; also, the high mobility of dendritic cells allows viral dissemination to other unaffected tissues. Continuous degradation of the lymphocytes leads to a reduction of the counts of the cells. The action of the PCV2 virus on lymphocytes are not well understood, however, plausible hypotheses have been preliminarily supported in many studies that were initiated due to increased outbreaks of the cases (Straw 301). First, a hypothesis referred to as Apoptosis explains that the virus inhibits lymphocyte production that takes place in the bone marrow. Further, other hypotheses purport that the reduction of the proliferation of lymphocytes within the secondary lymphoid tissues results to the lymphoid depletions and the consequential enlargement of the lymph glands. Fig 5: show an oedematous kidney that has white spots seen on a cut surface Because PCV2 only encodes for two proteins, it’s believed that the virus depends on the host cell for both replication and protein expression. The virus needs an actively dividing cell, particularly during the S-phase, for replication of DNA catalyzed by DNA polymerase. PCV2 replication is postulated to follow a mechanism involving a rolling circle. A study of cells of porcine kidney (PK-15) illustrated that the Cap protein led to post-infection and was localized at the perinuclear portion of the cell. After 36 hours of inoculation, it was noted that the viral titer was stabilized, which was an indicator that replication had been completed (Mettenleiter, Thomas, and Francisco 143). Unfortunately, there are no sufficient information to illustrate how and why the virus capsid is made, assembling of the virus and how the virus is released out of the infected cells. PCV Entry PCV infects a large number of cell types, which include macrophages, cardiomyocytes, hepatocytes. However, until recently, there was no clear information on how the PCV2 attaches or enters into the cellular medium. Through thorough and consistent researches, results have indicated that PCV uses a clathrin-mediated endocytosis mechanism to necessitate enter the cell, however, it is believed that there are other factors that modulate the entry but have not been identified. Although PCV cells that are targeted are well characterized, the mechanistic details during the early phases of PCV2 infection, which involve the attachment of the virus particles known as virions to the surface of the cell, are poorly understood. But the virions are believed to first bind to cellular receptors then enter to the targeted cells, which are characterized as the first phase of PCV2 infection. The virus then interacts with the cell surface receptors to create endocytic pathways, which facilitates the direct entry of PCV2 into the cell via the plasma membrane. Examples of the possible endocytic pathways are caveolae-independent endocytosis, macropinocytosis, and clathrin- and caveolae-mediated endocytosis. After the virus is endosytosed, the lysosome and the endosome formations create an acidic shift. The condition then causes ATP-driven removal of the coat of the virus; hence PCV2 can escape from the lysosome and the endosome. Thereafter, the virus travels to the cell nucleus by using unknown means (Maclachlan, Edward, and Frank 248-249). Fig 6: Illustrates the pathways of the viral cellular entry. Actin takes part in all the types of endosytosis by facilitating the uptake and delivery into the destructive parts of ligands internalizes through clathrin-mediated endocytosis. Furthermore, polymerization of Actin is needed to form membrane protrusions at the internalization sites during macropinocytosis. Also, the filaments of actin organize and confine caveolae close to the surface of the cell. It is also worth stating that the polymerization of the actin occurs at the sites of endocytosis during caveolae-mediated endocytosis (Mettenleiter, Thomas, and Francisco 147). Fig 7: Shows clathrin-mediated endosytosis mechanism Viral Transmission PCV2 is transmitted in a number of ways. The main route of entry is through oro-nasal contact. The contact can be with infected urine, feces, or direct contact with infected pigs. The virus is also shed in the oral secretions, respiratory secretions, and urine of feces excretions both from pigs that show clinical sign or not but infected (Straw 290). However, pigs with clinical signs shed higher amounts of the virus. Transmission can also occur vertically, when piglets contract the disease from the sow via the placenta, which results persistently infected piglets; however, this method appears to be rare. PCV2 are also shed in colostrums, non-sperm fraction, and seminal fluids. The PCV2 from the semen infects when injected intraperitoneally to pigs. Again, infected products such as skeletal muscle, gastric mucosa, lymphoid tissue, and bone marrow can cause infection when fed forabout three days to naïve piglets. Fig 8: shows Haemorrhages when PCV2 enters gastric mucosa The incubation period of infected pigs ranges between two to four weeks. Once in the host’s body, the virus infects the lymph nodes in the head and tonsils then begins to replicate. Also, PCV2 infects B-cell that possibly leads to dissemination of the virus all over the body through the lymphatic system (Maclachlan, Edward, and Frank 247). The virus has been detected in many lymph nodes, the spleen, lung, kidney, bone marrow, Peyer’s patches, and many body organs. Replication of PCV2 then starts at the T-cells and the mononuclear peripheral blood cells. In pigs, viremia is detected from seven to fourteen days of post inoculation. Nonetheless, peculiar instances of prolonged infections have been recorded of up to 125 days of post inoculation when experimental infections were conducted (Mettenleiter, Thomas, and Francisco 145). Host Immunity Many breeding sows are seropositive to PCV2; thus many piglets acquire antibodies against it from the mother. Antibody levels wane in low level at 4-6 weeks, moderate levels at 6-10 weeks, and high at 8.5-13.5 weeks (Straw 291-294). However, the mean antibody half-life is about 19 days. The maternal antibodies make piglet not to exhibit any clinical sign for up to 4 weeks but, this is dependent on the levels of the antibodies. After 2-3 months, pigs develop their own defense mechanism, but not fully protected as the swine still can develop viremia (Maclachlan, Edward, and Frank 240). Pathogenesis is dependent on immune-modulatory factors of the virus. The infection decreases B-cell growth (IL-4) and T-cell and cytokine (IL2), which causes lymphoid depletion, interferon antiviral response, and increases pro-inflammatory cytokines (IL-8 and IL-1B). Factors Modulating Diseases Caused by PCV2 Viral factors-the virus genotype affect expression of the syndromes, for example, when amino acid mutations occurs in the PCV2 virus, histopathological lesions in pigs are altered. Host factors-both pure and crossbreeds of pigs are susceptible to infections; however, there is variation in adaptive immune response, resulting to different patterns of PCV2 replications, especially in conventionally crossbred pigs (Straw 285). Co-infections is achieved by co-factors such as several bacterial, Mycoplasma hyopneumoniae, porcine parvovirus (PPV) or viral pose a high risk and cause a full spectrum of clinical signs. Immunomodulation- Immunomodulation prior to infections, for example by keyhole limpet hemocyanin results to severe histopathological Lesions. Again swine treated by dexamethasone develop Granulomatous lymphadenitis (Morilla, Kyoung-Jin, and Jeffrey 282). Conclusion Diagnosis and treatment via diagnosis of PCVAD, which is pegged on demonstration and clinical signs of PCV2 antigen in organ systems such as lungs, intestine, lung, kidney, spleen, blood vessels, small muscles, connective tissues, smooth muscles and so on. To prevent prevalence, vaccination, quarantine, disinfection, monitoring, cleanliness are necessary, however when the clinical signs appear, veterinary intervention is mandatory to avoid proliferations and severe pathogenesis.   References Cann, Alan. Principles of Molecular Virology. San Diego: Academic Press, 2001. Print. Maclachlan, Nigel J, Edward J. Dubovi, and Frank Fenner. Fenners Veterinary Virology. Amsterdam: Elsevier Academic Press, 2011. Print. Mettenleiter, Thomas C, and Francisco Sobrino. Animal Viruses: Molecular Biology. Norfolk, UK: Caister Academic Press, 2008. Print. Morilla, González A, Kyoung-Jin Yoon, and Jeffrey J. Zimmerman. Trends in Emerging Viral Infections of Swine. Ames, Iowa: Iowa State Press, 2002. Print. Murphy, Frederick A. Veterinary Virology. San Diego: Academic Press, 1999. Print. Straw, Barbara E. Diseases of Swine. Ames, Iowa: Blackwell Pub, 2006. Print. Read More