Viral-sized particles

The passage of viral-sized particles through ostensibly intact NR latex films has been directly observed in the laboratory (Carey et ah, 1992 Kiernan, 1996), and indirectly observed in the field (The New York Times, 1994). Evidence for such flaws in NR comes from microscopic observations (Arnold et al., 1988), as well as water absorption measurements, wherein the initial rapid uptake suggests the existence of capillary channels (Gazeley et al., 1988). 

Mlcrofiltra.tlon, Various membrane filters have been used to remove viral agents from fluids. In some cases, membranes which have pores larger than the viral particle can be used if the filtration is conducted under conditions which allow for the adsorption of the viral particle to the membrane matrix. These are typically single-pass systems having pore sizes of 0.10—0.22 lm. Under situations which allow optimum adsorption, between 10—10 particles of poHovims (28—30 nm) were removed (34—36). The formation of a cake layer enhanced removal (35). The titer reduction when using 0.10—0.22 p.m membrane filters declined under conditions which minimized adsorption. By removal standards, these filters remove vimses at a rate on the low end of the desired titer reduction and the removal efficiency varies with differences in fluid chemistry and surface chemistry of viral agents (26). 

Smaller pore size microfilters in single-pass systems which have pore sizes small enough to remove some vimses by size exclusion have been examined (26,37,38). Minimum levels of vims removal can be estabhshed for these systems if fluid and process conditions are employed which minimize removal of viral particles by mechanisms other than size selection. 

The size of this viral particle is of course larger than that of a virus with only 60 subunits. The diameter of tomato bushy stunt virus is 330 A compared with 180 A for satellite tobacco necrosis virus. The increase in volume of the capsid means that a roughly four times larger RNA molecule can be accommodated. 

The bacterial RNA viruses are all of quite small size, about 26 nm in size, and they are all icosahedral, with 180 copies of coat protein per virus particle. The complete nucleotide sequence of several RNA phages are known. In the RNA phage MS2, which infects Escherichia coli, the viral RNA is 3,569 nucleotides long. The virus RNA, although single stranded, has extensive regions of secondary and tertiary structure. The RNA strand in the virion has the plus (+) sense, acting directly as mRNA upon entry into the cell. 

Removal of viruses from the product stream can be achieved in a number of ways. The physicochemical properties of viral particles differ greatly from most proteins, ensuring that effective fractionation is automatically achieved by most chromatographic techniques. Gel-filtration chromatography, for example, effectively separates viral particles from most proteins on the basis of differences in size. 

Reverse transcriptase is an enzyme that makes a DNA copy of the virus RNA (Step 3). Once made, the DNA enters the cell nucleus and rephcates many times. Another enzyme, the protease, is required to cut the HIV proteins into proper sizes and assemble into the viral particles (Step 7). 

Immunopotentiating reconstituted influenza virosomes (IRTV) are spherical 150-nm sized particles consisting of a phospholipid bilayer in which influenza virus A/Singapore strain-derived hemagglutinin (HA) and neuraminidase (NA) are intercalated. As such, they resemble and mimic the influenza virus envelope. The difference from conventional liposome formulations lies in the inclusion of the viral envelope proteins HA and NA as well as viral phospholipids. Especially, the inclusion of influenza virus HA provides IRIV with delivery and immimogenic capacities. IRTV are licensed for human use as adjuvant in hepatitis A vaccination and as influenza subunit vaccine (1). 

Unlike in bacteria and fungi, viruses do not have a protective coat that separates essential proteins and nucleic acids from the environment. The majority of viruses consist of nucleic acid polymers (DNA or RNA) enclosed within a protein coat (capsid). Sometimes, viruses pick up a lipid membrane (envelope) from the host cell that surroimds the capsid. The average size of viral particles is in the range 10-300 nm. The most common... 

HBsAg was the first viral antigen to be produced in transgenic plants. The protein self-assembles into subviral mammalian particles of 22 nm, and is virtually indistinguishable from serum-derived and yeast-derived HBsAg in both infected sera as well as commercial vaccines with respect to size, density sedimentation, and immunogenicity. 

TABLE 24-1 The Sizes of DNA and Viral Particles for Some Bacterial Viruses (Bacteriophages)... 

Virus Size of viral DNA (bp) Length of viral DNA (nm) Long dimension of viral particle (nm)... 

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