CAPÍTULO IV: MARCO PROPOSITIVO
4.2. Desarrollo de la propuesta
4.2.9. Registro y asignación de costos
Ful 1 length copies of each segment of the virus genome are synthesized in
equimolar amounts. The synthesis is not regulated and represents 5% of
total viral RNA transcription (Hay et a l . . 1977). Like mRNA synthesis, the
synthesis of template RNA occurs in the cell nucleus (Herz et a l .. 1961;
Shapiro et al.. 1967) and uses the same viral enzyme complex as described
above (Beaton & Krug. 1984) . Unlike mRNA synthesis there is no requirement
7 4 that other proteins are involved which modify the activity of the
transcriptase complex (Beaton & Krug, 1906). The inhibition of secondary
transcription by cycloheximide confirms that other viral proteins are
necessary (Taylor et a l .. 1977; Barrett et a l .. 1979).
4^4,3, Synthesis of progeny vRNA.
Progeny vRNA is synthesised from the full length template RNA transcripts.
Polypeptide PA is involved and the event is nuclear (Shapiro et a l .. 1907).
The characteristics of influenza virus replication can vary depending on
the host cell (Stanth & Hay, 1902) and virus strain (Lamb & Choppin, 1977;
Hay et a l .. 1977). Sub-genomic RNAs which interfere with subsequent virus
replication are sometimes synthesised and are termed defective-interfering
(DI) RNAs (reviewed by Nayak et a l .. 1905). The generation of DI RNAs is
favored by high multiplicities of infection and the host cell type (Choppin
& Pons. 1970; Janda et a l .. 1979).
4 4-4- Synthesis of influenza virus proteins and assembly of progeny virus.
Influenza virus utilises the normal host cell processes to synthesis viral
proteins. Approximately 1 hour after infection NS1 and NP proteins are
detected in the host cell and this is followed by a gradual reduction in
host cell protein synthesis which is not completely inhibited (Skehel 1972;
1973). Later in infection HA, NA and Ml proteins are produced, although the
host cell type can influence viral protein synthesis (Minor et a l .. 1979).
Viral glycoproteins (HA and NA) are synthesised on membrane-bound
polyribosomes and are contranslationally inserted into the rough
7 5 modified as they progress from the RE2? through the Golgi apparatus to
smooth vesicles. These vesicles migrate to and fuse with the apical plasma
membrane of the infected cells. Other viral proteins are synthesised on
free ribosomes in the cytoplasm and migrate to different sites of the cell
by diffusion (Sfcith et al.. 1967). The P proteins become tightly associated
with the cell nucleus, though PA is also found in the cytoplasm. The NP
protein is initially found associated with the nucleus, later it is found
in the cytoplasm before it migrates to the plasma membrane prior to progeny
virus release.
Progeny virus particles are produced by budding from the infected cell
membranes. The HA and NA glycoproteins concentrate in the apical plasma
membrane and displace host cell proteins. The Ml (and M2) proteins migrate
to the plasma membrane where they may interact with the viral
glycoproteins, although this is not proven (see section 1.). The RNPs fora
in the cytoplasm before also migrating to the plasma membrane. The
mechanism of how the eight RNPs are incorporated into each virus particle,
76
5
. He O w n gas of neutralization of virusesThe binding of antibodies, both neutralizing and non-neutralizing, can
inactivate virus infectivity in conjunction with accessory factors e.g.
complement (reviewed by Lachmann, 1965; Davies & Metzger. 1963; Beebe et
a l .. 1963; Math lessen et a l .. 1968; Vasantha et a l . ■ 1968). rheumatoid
factor (Coutelier & van Shick. 1968) or antibody-dependent cytotoxic cells
(section 3.1.1.). In this section I will focus on neutralization resulting
from the binding of antibody alone. This type of neutralization depends on
the interaction of three components; the virus, the antibody and the cell
(reviewed by Dismock. 1967).
Neutralization (reviewed by D inmock. 1964; 1967; Mandel, 1965) results from
the binding of one or more antibody molecules to a particular protein on
the virus particle leading to a loss of infectivity. For influenza virus,
antibodies directed against the HA glycoprotein are neutralizing whereas
those directed against the NA glycoprotein are non-neutralizing, although
they do limit infection (Seto & Rott. 1966; Webster & Laver. 1967). In
order to neutralize influenza virus, antibodies have to bind not only to
the HA but to specific epitopes (critical sites) on the molecule (section
1.5.). Antibodies that bind to the HA at epitopes outside these antigen
regions (non-critical sites) fail to neutralize the virus (Breschkin si
a l .. 1961).
Neutralization can be mediated by all three major immunoglobulin classes.
IgG. IgM and IgA (Ogra et a l .. 1975; reviewed by Mandel, 1964). The class of
antibody is a major factor in determining the mechanism of influenza virus
7 7 neutralization process for other viruses has involved the use of the IgG
and unless stated otherwise, it is this class of antibody to which I am
referring. The virus occupies a central position when considering
neutralization. The diversity of virus structures means that the findings
pertaining to one type of virus (e.g. the non-enveloped picomavirus,
polio) cannot be automatically interpreted as applying to others (e.g. the
enveloped orthomyxovirus, influenza). Considering the diversity of
different virus structures it is not surprising there is no single
mechanism of neutralization for all viruses. The target cell type has
received comparatively little attention, although some studies, using
influenza and other virus systems, have demonstrated a clear and important
role for the target cell in the neutralization process and will be
discussed below.
The mechanisms by which the binding of antibody can result in a loss of
virus infectivity can divided into four categories:
(i) Aggregation
(ii) Inhibition of virus attachment to cells
(iii) Inhibition of virus penetration into cells
(iv) Inhibition of subsequent intracellular stages of infection
Neutralization may not necessarily result from the inhibition of just one
of these but be due to a combination of these events.