Effect of Synaptic Transmission on Viral Fitness in HIV Infection

<div><p>HIV can spread through its target cell population either via cell-free transmission, or by cell-to-cell transmission, presumably through virological synapses. Synaptic transmission entails the transfer of tens to hundreds of viruses per synapse, a fraction of which successfully integrate into the target cell genome. It is currently not understood how synaptic transmission affects viral fitness. Using a mathematical model, we investigate how different synaptic transmission strategies, defined by the number of viruses passed per synapse, influence the basic reproductive ratio of the virus, R₀, and virus load. In the most basic scenario, the model suggests that R₀ is maximized if a single virus particle is transferred per synapse. R₀ decreases and the infection eventually cannot be maintained for larger numbers of transferred viruses, because multiple infection of the same cell wastes viruses that could otherwise enter uninfected cells. To explain the relatively large number of HIV copies transferred per synapse, we consider additional biological assumptions under which an intermediate number of viruses transferred per synapse could maximize R₀. These include an increased burst size in multiply infected cells, the saturation of anti-viral factors upon infection of cells, and rate limiting steps during the process of synapse formation.</p> </div

The evolutionary simulations.

<p>The time-dependent solution of the evolutionary virus dynamics simulation is presented (please note the log axes). The uninfected cell population is <i>x₀₀</i>, and the infected populations are presented by two lines, one showing the sum of all cells containing the <i>s₁</i> virus, , and the other containing the <i>s₂</i> virus,. The inset shows the infectivity of the two strains. We used the base-line model for this simulation. The parameters are <i>s₁ = 1, s₂ = 3, z = 0, Q = 0.1,, a = d = 1, N = 5.</i></p

Virus-mediated induction of intracellular defense factors can lead to peaks in <i>R₀</i> for two different viral strategies, <i>s</i>: a “stealth strategy” and a “saturation strategy”.

<p>Plotted is the basic reproductive ratio, <i>R₀</i>, as a function of strategy, <i>s</i>, for two different values of the infectivity parameter, <i>r₂</i>. The rest of the parameters are as follows: , <i>d = 0.1</i>, <i>a = 4</i>, <i>n₁ = 16</i>. The infectivity parameter <i>r</i> depends on the strategy. It is given by <i>r = r₂-s(r₂-r₂/10)/10</i> if <i>s<10</i>, and <i>r = r₂/10</i> if . These functions are shown in the inset for the two values of <i>r₂</i>.</p

Limited ability of synapse formation.

<p>Same as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0048361#pone-0048361-g003" target="_blank">figure 3</a>, except for the relationship . The inset in (b) plots the number of infected cells as a function of strategy, s, for a fixed value <i>r = 0.5</i>. Other parameters are: <i>N = 15</i>, λ<i> = 300</i>, <i>d = 5</i>, <i>a = 30</i>, <i>z = 5</i>, <i>k = 6</i>.</p

The effect of flooding the immune defense.

<p>Same as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0048361#pone-0048361-g003" target="_blank">figure 3</a>, except for the function . The inset in (b) plots the number of infected cells as a function of strategy, <i>s</i>, for a fixed value <i>r = 0.05</i>. Other parameters are: <i>N = 1</i>, λ = 66.7, <i>d = 1.67</i>, <i>a = 3.33</i>, <i>k = 1</i>, <i>n₁ = 5</i>, <i>r = 0.1r₂</i>.</p

A schematic explaining the structure of the model.

<p>Here, uninfected cells are represented by white circles, infected cell by shaded circles, and viruses by black dots. (a) The overall virus dynamics, including production and death of target cells, the death of infected cells, and infection. (b) The process of infection contains two modes of transmission, free-virus and synaptic transmission. (c) Kinetics of synaptic transmission. Synaptic transmission can be performed by means of different strategies that vary by <i>s</i>, the number of viruses transferred per synapse. If <i>s</i> is small, may synapses must be formed (sequentially in time). If <i>s</i> is large, the viral load is transmitted by means of few synapses.</p

The basic model of virus dynamics with synapses.

<p>The basic reproductive ratio <i>R₀</i> (a) and the total number if infected cells <i>y</i> (b), are plotted as functions of the infectivity <i>r</i>. The horizontal dashed line in (a) corresponds to the infection threshold, <i>R₀ = 1</i>. The strategies capable of establishing successful infection for the given parameters are plotted by thick lines in (a). The inset in (b) plots the number of infected cells as a function of strategy, <i>s</i>, for a fixed value <i>r = 0.6</i>. Other parameters are: <i>N = 15</i>, λ<i> = 200</i>, <i>d = 4</i>, <i>a = 10</i>.</p

Model parameters and their definitions.

<p>Model parameters and their definitions.</p

Human tonsil CD8 T_FR inhibit T_FH and GC B cell function.

<p>Tonsil cells were sorted to isolate CD8 T_FR, CD8 conv, CD3+CD8-CXCR5+ T_FH, and CD19+CD38+ GC B cells and X4- or R5-spinoculated. Cells were then co-cultured at indicated ratios for 2 days and analyzed. (A) IL-21 production by T_FH with increasing (left to right) number of CD8 T_FR (n = 4). (B) Representative examples from X4- and R5-spinoculations showing IL-21 production by T_FH alone, 1:1 with CD8 T_FR, 1:1 with CD8 T_FR and anti-Tim3 antibody (500 ng/μl; right panels), and 1:1 with CD8 T_FR and an isotype control antibody (500 ng/μl). (C) Results from a total of 6 tonsils (isotype n = 3) as described in B. (D) IgG production in X4-spinoculated cultures with 2.5 μg/mL CpG-B stimulation in CD8 T_FR, T_FH, and B cell co-cultures as measured by ELISA. All co-cultures are 1:1 (n = 7). Statistical significance was determined by non-parametric Wilcoxon matched-pairs tests (B) or one-way ANOVA (Friedman test, C) and is displayed as * = p<0.05, ** = p<0.01 and *** = p<0.001.</p

Most human tonsil follicular homing CD8 T cells are CD8 T_FR.

<p>Disaggregated tonsil cell cultures were mock-spinoculated or spinoculated with X4- or R5-tropic HIV and cultured for 2 days (n = 6). (A) Of the viable CD3+CD8+ population expressing the follicular phenotype CXCR5+CCR7-, the percent CD44^hi (CD8 T_FR) and all other CD3+CD8+ (CD8 conv) in mock- and HIV-spinoculated cultures is shown. (B) The percent CCR7 expression on CD8 T_FR (red) and CD8 conv (blue) compared to an FMO control (black). Graphs depict median and range. Statistical significance was determined by non-parametric one-way ANOVA (Friedman test) and is displayed as * = p<0.05.</p