登革病毒的血清型

    Dengue Fever (DF登革热) is due to mild infection caused by one of the four serotypes of Dengue virus that may progress to a severe Dengue Hemorrhagic Fever (DHF) and subsequently to a fatal Dengue Shock Syndrome (DSS), if treatment not initiated in right time (WHO 1999). Dengue viruses belong to the family Flaviviridae, genus Flavivirus. Dengue virus (DENV) an enveloped RNA virus, is an arthropod-borne human viral pathogen that causes dengue fever epidemics in tropical and subtropical areas every year. DENV is categorized into four distinct antigenic serotypes (DENV1–4)(血清型1-4), which are further divided into different genotypes（基因型）. Within each DENV antigenic type, genetic and antigenic diversity exists which results in multiple distinct genotypes.

    Primary infection with one DENV serotype has been shown to provide life-long immunity against a similar serotype and shortterm immunity against other serotypes. A longitudinal survey of DENV clinical strains over a 20-year time span from Bangkok, where all four DENV serotypes have co-circulated suggests the extinction of one clade and the replacement with another. These shifts have been shown to be linked to the epidemic cycle in the region.

   Dengue epidemics in Philippines and in Thailand in the 50′s of the last century raised attention to the virus occurring in 4 serologic types. A successful cure of the infection caused by one of the serotypes does not mean immunity against the other serologic types. Dengue can thus affect one individual repeatedly and the reinfections are caused by different serotypes. Moreover, the course of the infection is

predetermined by the sequence of the individual serotypes, which induce the re-infection in one individual. Both older children and adults are involved.

    Anti-envelope human antibodies are primarily directed against two glycoproteins on the surface of the viral envelope, the precursor membrane (pre-M) and envelope (E) proteins. Genetic diversity of pre-M and E proteins can significantly modulate antibody neutralization activity against DENV2 strains. Antigenic variation in the E protein has also been shown to impact the neutralization efficiency of monoclonal antibodies, antisera from DENV-infected patients, and immune sera from DENV-vaccinated individuals. Additionally, DENV serotypes contain antigenic heterogeneity. Mapping of E protein amino acid differences from different DENV antigenic serotypes revealed four distinct clusters whereby each serotype was clustered closely together on the antigenic map. However, cross-reactivity of antisera with different serotypes can lead to discrepancies in the DENV antigenic cluster and certain DENV strains have shown more antigenic variance to inter-typic viruses than intra-typic viruses.

    Antigenic characterization of DENV would greatly aid in DENV vaccine development. In order to promptly identify DENV antigenic variants, an in-silico model for DENV has been devised based on possible antigenicity-dominant positions of the E protein providing a convenient way to calculate the difference in viral antigenicity. 

    Although the four dengue serotypes are antigenically distinct, there is evidence that serologic subcomplexes may exist within the group. For example, a close genetic relationship has been demonstrated between DENV-1 and DENV-3 and between DENV-2 and DENV-4. The sizes of the genomic open reading frames of DENV-1, DENV-2, DENV-3, and DENV-4 are 3392, 3391, 3390, and 3387 amino acids, respectively, the shortest among the mosquito-borne flaviviruses. An amino acid sequence positional homology of 63%–68% is observed among the DENV serotypes compared to 44%–51% between DENVs and other flaviviruses such as yellow fever and West Nile.

  Serotypes within the dengue virus complex are most accurately and easily identified with an indirect immunofluorescent antibody (IFA) assay using serotype-specific monoclonal antibodies which react with epitopes on the structural proteins, or by a polymerase chain reaction (PCR) using serotype specific primers.

    Both antigenic and biological variation among dengue viruses has been documented. As mentioned above, DENV-3 viruses isolated in the Caribbean and the South Pacific in the 1960s were found to be antigenically distinct from the prototype and Asian strains of DENV-3 using PRNT. They were also biologically unique in that they did not grow as well in baby mice and mosquitoes as did the Asian strains. DENV-4 viruses isolated in the Caribbean after the introduction of this serotype into that region in 1981 were antigenically distinct from DENV-4 viruses from Asia.

    Factors responsible for the emergence and spread of the severe form of the disease, DHF/DSS, are not fully understood. The changing disease pattern described above provides support for the principal hypotheses regarding the pathogenesis of DHF/DSS, a secondary infection, and an increased virulence of the virus. Thus, increased transmission in urban areas and the development of hyperendemicity increases the probability of secondary infections and of genetic changes in the virus which may result in more severe disease due to antibody dependent enhancement (ADE) or to an epidemic virus strain with greater virulence. And increased spread of viruses between population centers via modern transportation increases the probability of introducing a virus strain with increased epidemic potential and virulence into new geographic areas.

    Increased transmission of multiple dengue serotypes thus increases the probability that severe disease will occur, regardless of whether the underlying cause is due to increased virulence, ADE, or, more likely, a combination of both.

    冠状病毒（Coronavirus）   

    In some species, the Spike（S，刺突蛋白） protein is cleaved into two subunits, the N-terminal S1 fragment being slightly smaller than the C-terminal S2 sequence. The S protein is anchored in the envelope by a transmembrane region near the C-terminus of S2. The functional S protein is highly glycosylated and exists as a trimer. The bulbous outer part of the mature S protein is formed largely by S1 while the stalk is formed largely by S2, having a coiled-coil structure.  S1 is the most variable part of the S protein; some serotypes of the avian coronavirus, infectious bronchitis virus (IBV) differ from one another by 40% of S1 amino acids. S1 is the major inducer of protective immune responses. Variation in the S1 protein enables one strain of virus to avoid immunity induced by another strain of the same species. 

 名词解释：

Serotype

  An isolate or group of isolates that are distinguished from biologically related isolates by reaction (or lack of reaction) with key serological reagents such as defined polyclonal antisera or monoclonal antibodies. 

  血清型的定义：A serotype is defined as either exhibiting no cross-reaction with others or showing a homologous/heterologous titer ratio of greater than 16 (in both directions).

Hyperendemicity

The co-circulation of multiple dengue virus serotypes in the same population.

Viral Isolates, Strains, and Serotypes

    The term virus isolate refers to any particular virus culture that is being studied and it is thus simply an instance of a given virus.

    A viral strain is a biological variant of a virus that is recognizable because it possesses some unique phenotypic properties that remain stable under natural conditions. Characteristics that allow strains to be recognized include (1) biological properties such as a particular disease symptom or a particular host, (2) chemical or antigenic properties, and (3) the genome sequence when it is known to be correlated with a unique phenotypic character. If the only difference between a ‘wild type’ virus taken as reference and a particular variant is a small difference in genome sequence, such a variant or mutant is not given the status of a separate strain in the absence of a distinct phenotypic characteristic.

    Strains that possess unique, stable antigenic properties are called serotypes. Serotypes necessarily also possess unique structural, chemical, and genome sequence properties that are related to the differences in antigenicity. Serotypes constitute stable replicating lineages which allow them to remain distinct over time. The infectivity of individual serotypes of animal viruses can be neutralized only by their own specific antibodies and not by antibodies directed to other serotypes. This inablility of serotype-specific antibodies to cross-neutralize other serotypes is important in the case of animal viruses that are submitted to the immunological pressure of their hosts.

参考资料：

 《Encyclopedia of Virology》by Dennis Bamford and Mark Zuckerman.