Virus-like Particles: Measures and Biological Functions

Abstract

Virus-like particles resemble infectious virus particles in size, shape, and molecular composition; however, they fail to productively infect host cells. Historically, the presence of virus-like particles has been inferred from total particle counts by microscopy, and infectious particle counts or plaque-forming-units (PFUs) by plaque assay; the resulting ratio of particles-to-PFUs is often greater than one, easily 10 or 100, indicating that most particles are non-infectious. Despite their inability to hijack cells for their reproduction, virus-like particles and the defective genomes they carry can exhibit a broad range of behaviors: interference with normal virus growth during co-infections, cell killing, and activation or inhibition of innate immune signaling. In addition, some virus-like particles become productive as their multiplicities of infection increase, a sign of cooperation between particles. Here, we review established and emerging methods to count virus-like particles and characterize their biological functions. We take a critical look at evidence for defective interfering virus genomes in natural and clinical isolates, and we review their potential as antiviral therapeutics. In short, we highlight an urgent need to better understand how virus-like genomes and particles interact with intact functional viruses during co-infection of their hosts, and their impacts on the transmission, severity, and persistence of virus-associated diseases.

Keywords: cell killing particles; clonogenic assay; co-infection; coulter counting; defective interfering particles; defective viral genomes; flow virometry; multiplicity of infection; plaque forming unit; resistive pulse sensing; semi-infectious particles; transmission electron microscopy; virus-like particles.

Figure 1

Transmission electron microscopy images of vesicular stomatitis virus defective interfering (DI-T) and standard (VSV) particles. DI-T particles are truncated relative to full-length VSV particles. Image adapted from [31].

Figure 2

Counting of virus-like particles by flow virometry. The apparatus employs a flow-focusing stream to align particles as they move single file through the beam of a powerful laser, followed by enhanced wide-angle FSC detection. The standard FSC signal used in conventional cell counting is blocked, and the wide-angle FSC signal is enhanced by setting a higher threshold for detection, reducing noise, and thereby enabling sensitive detection of nano-scale particles. Image adapted from [45].

Figure 3

Defective interfering particles (DIPs): emergence and biology. DIPs arise from normal virus infections (left), and they amplify during cell co-infections with fully infectious particles (middle). DIPs alone are unable to productively infect cells (right).

Figure 4

Production of (a) standard virus and (b) DI particles by co-infected cells is a complex function of DI particle inputs. All cells were co-infected with standard virus (MOI 20) and different input levels of DI particles. Levels of standard virus and DI particles were determined by plaque assay and plaque-reduction assay; adapted from [104].

Figure 5

Three potential fates of semi-infectious particles. (a) A particle infects its host cell but at least one essential gene fails to be expressed, so the cell makes no progeny, (b) a particle infects its host cell, all essential genes are expressed; the cell make progeny, and (c) two or more particles co-infect a host cell, gene or functional deficiencies are overcome by complementation, and the cell makes virus progeny.

Figure 6

A non-infectious cell killing particle can trigger apoptosis in a cell.