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VIRAL IMMUNOLOGY

 

Viruses are strongly immunogenic and induces 2 types of immune responses; humoral and cellular. The repertoire of specificities of T and B cells are formed by rearrangements and somatic mutations. T and B cells do not generally recognize the same epitopes present on the same virus. B cells see the free unaltered proteins in their native 3-D conformation whereas T cells usually see the Ag in a denatured form in conjunction with MHC molecules. The characteristics of the immune reaction to the same virus may differ in different individuals depending on their genetic constitutions.

Humoral response is responsible for blocking the infectivity of the virus (neutralization). Those of the IgM and IgG class are especially relevant for defense against viral infections accompanied by viraemia, whereas those of the IgA class are important in infections acquired through a mucosa. (the nose, the intestine) In contrast, the cellular response kills the virus-infected cells expressing viral proteins on their surfaces, such as the glycoproteins of enveloped viruses and sometimes core proteins of these viruses.  

Humoral Response

Abs are elicited by the surface components of intact virions as well by the internal components of disrupted virions. Also they are elicited by viral products built into the surface of infected cells or released by the cells. Antibodies provide the key to protection against many viral infections. Sometimes, they are also pathogenic e.g. immune complexes are thought to be responsible for causing the rash in rubella. Interactions of virions with Abs to different components of their coats have different consequences.

Neutralization

virus neutralization consists of a decrease in the infectious titre of a viral preparation following its exposure to Abs. The loss of infectivity is bought about by interference by the bound Ab with any one o the steps leading to the release of the viral genome into the host cells. the consequences of the virion-Ab interaction therefore depends on many factors;-

  1. The structure of the virions
  2. The target of the Ab e.g. Abs against the HA but not the NA of influenza virus are neutralizing.
  3. Mutations affecting surface molecules that may alter the susceptibility to certain Abs
  4. The type of Ab, especially its affinity for the components of the virions
  5. The number of Ab molecules attached to the virions

Reversible neutralization - The neutralization process can be reversed by diluting the Ab-Ag mixture within a short time of the formation of the Ag-Ab complexes (30 mins). It is thought that reversible neutralization is due to the interference with attachment of virions to the cellular receptors. The process requires the saturation of the surface of the virus with Abs.

Stable neutralization - with time, Ag-Ab complexes usually become more stable (several hours) and the process cannot be reversed by dilution. Neither the virions nor the Abs are permanently changed in stable neutralization, for the unchanged components can be recovered. The neutralized virus can be reactivated by proteolytic cleavage. Intact Abs can be recovered by dissociating the Ab- Ag complexes at acid or alkaline pH.

Stable neutralization has a different mechanism to that of reversible neutralization. It had been shown that neutralized virus can attach and that already attached virions can be neutralized. The number of Ab molecules required for stable neutralization is considerably smaller than that of reversible neutralization, Kinetic evidence shows that even a single Ab molecule can neutralize a virion. Such neutralization is generally produced by Ab molecules that establish contact with 2 antigenic sites on different monomers of a virion, greatly increasing the stability of the complexes.

Virion sites for neutralization - only epitopes on molecules involved in the release of the viral genome into the cells are targets of neutralization. In influenza viruses, only the HA and not the NA are targets for neutralization. In polioviruses, all antigenic sites recognizable on the capsid are targets for neutralization, because the capsid is a unit for releasing the nucleic acid. For adenoviruses, the main targets are the hexons rather than the pentons, as the hexons are strongly interconnected and work together for the release of the viral DNA. Occasionally, Abs bound to non-neutralizing epitopes can be detected by neutralization in the presence of complement, whereby the viral enveloped is attacked by the complement cascade.

Protective role of neutralizing antibodies - the neutralizing power of a serum usually reflects the degree of protection in an infected animal. The correlation, however, is not always perfect. Discrepancies may be generated by differences in the neutralizability of a virus in the cells used for assay in vitro compared to those that the virus infects in vivo. e.g. the sera of mice protected from yellow fever did not neutralize the virus in vero cells but did so in a mouse neuroblastoma cell line. Another possible reason for discrepancy is that an Ab that does not neutralize in cultures may act in vivo by activating host responses against the virus or virus-infected cells. e.g. complement or macrophages. In addition, neutralizing Abs may fail to protect because rapid viral multiplication overcomes the neutralizing power. In the early period of immunization, low affinity Abs act predominantly by activating complement and have low neutralizing power in cultures. The degree of neutralization in cultures is probably best estimated by carrying out neutralization in the presence of complement.  

 

Evolution of viral antigens

Viral evolution must tend to select for mutations that change the antigenic determinants involved in neutralization. In contrast, other antigenic sites would tend to remain unchanged because mutations affecting them would not be selected for and could even be detrimental. A virus would thus evolve from an original type to a variety of types, different in neutralization (and sometimes in HI) tests, but retaining some of the original mosaic of antigenic determinants recognizable by CFTs.

These evolutionary arguments are consistent with the observation that the clearest differentiation of types within a family is present in viruses of rather complex architecture, in which the Ags involved in the interaction with the cell vary more than other proteins. Thus enveloped viruses have a strain-specific envelope but a cross-reactive internal capsid; adenoviruses have type-specific fibers and family-specific (and also type-specific) capsomers. Moreover, the C Ag of polioviruses, which appears only after heating, reveals antigenic sites that are normally hidden and hence are not affected by selective pressure. The extent of antigenic variation differs widely among viruses and is most extensive with lentiviruses and influenza viruses.  

Types of virus-specific antibodies

Different types of viral preparations elicit the formation of different Abs;-

  1. Killed virus preparations elicit Abs predominantly directed against the surface of the virions. These Abs have neutralizing and HI activities against the virions as well as CF and precipitating activities against the Ags of the viral coat.
  2. Live virus preparations elicit antibodies against all the viral antigens, including both external and internal antigens.
  3. Immunization with internal components of the virions produces CF and precipitating Abs active only toward the Ags of these components.
  4. Immunization with peptides reproducing segments of virion proteins elicit Abs, the properties of which depend both on the protein and the specific sequences reproduced.    

Specificity of test methods

The Abs that react in the different tests may overlap though they may not be altogether identical. Neutralization is primarily caused by Ab molecules specific for the sites of the virion that are involved in the release of viral nucleic acid into the cell. CF usually involves additional surface or internal Ags. Neutralization probably requires molecules with a higher affinity for virions than do HI and CF. After viral infection, the titres of Abs to different components rise and fall with quite different time courses.

Because of their high specificity, immunological methods can differentiate not only between viruses of different families but also between closely related viruses of the same family or subfamily. By these means, family Ags may be identified. Usually, antibodies detected by neutralization tend to be less cross-reactive and thus are useful in defining the immunological type. Whereas those detected by CF tend to be more cross-reactive and the useful in defining the family. By proper procedures, however, such as immunization with purified Ags, highly specific CF Abs can be prepared.

The resolving power of Abs is maximized by the use of monoclonal Abs. Whereas all the methods for measuring viral antigens are needed for classifying a new isolate, the method of choice for diagnostic purposes is ELISA, for its high sensitivity and low cost.    

 

Cell-Mediated Immunity 

Cytotoxic T lymphocytes

CMI is very important in localizing viral infections, in recovery, and in the pathogenesis of viral diseases. In experimental animals, primary CTLs reach maximal abundance about 6 days after a viral infection and then disappears as infection subsides. However, memory T cells persists and can be recognized by culturing spleen cells with virus-infected cells where within a few days, secondary CTLs appear in culture with much greater activity than in the initial response.

Formation of CTLs is elicited by cell-associated Ags present at the cell surface, not only for enveloped viruses, but also for other viruses whose core or nonvirion proteins reach the cell surface. As in humoral immunity, type specific and group specific responses can be seen. Even noninfectious or inactivated viruses can elicit a cellular response because their envelopes fuse with the cell plasma membrane in the initial stage of viral penetration. Moreover, the virions themselves may also be able to elicit the response after absorbing to the macrophages. Both internal virion proteins and nonvirion proteins are often recognized by CTLs. An example is the nucleocapsid proteins of enveloped viruses, fragments of which reach the cell surface by an unknown route and are recognized very efficiently, giving rise mainly to cross-reactive CTLs. Often, Abs to viral surface proteins do not block their interaction with CTLs, because the humoral and cellular responses recognize different epitopes.

 

Antibody-dependent cell-mediated cytotoxicity

The K cells are the effector cells in ADCC. In vitro, these cells kill virus-infected cells sensitized by IgG from immune donors but not unsensitized targets. ADCC is very efficient in vitro against HSV or VZV infected cells, preventing the usual spread of the virus from infected to neighboring uninfected cells. Therefore, it may play a role in the defense against human infection with these viruses. K cells had been shown to mediate immunity to vaccinia infection rather than Tc cells.  

Natural Killer (NK) cells

In man, the principal NK cell is the large granular lymphocyte (LGL) which comprise 2-5% of peripheral blood lymphocytes. However, not all lytic cells are LGLs and not all LGLs are NK cells. There is overlap of the NK population with K cells. The Fc receptor of the NK cell is however, not involved in the lytic process. There are also mechanistic differences and K cell activity is less consistently augmented by interferon and other immune modulators. NK activity is subject to both positive and negative regulation in vivo and in vitro. Interferon gamma and IL-2 are potent inducers. Besides producing lysis, NK cells can produce alpha-interferon.

The target molecules recognized but NK cells have not been defined but it appears that some determinants are ubiquitous whilst others have a more restricted distribution. An alternative suggestion is that NK cell susceptibility depends on the absence of normal cell surface antigens such as MHC molecules. The importance of NK cells in viral infection is partially understood. It had been shown that mice depleted of NK cells by treatment with Ab against asialo GM1 show an increased susceptibility to CMV.

 

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