We have two kidneys, mayflies lay packets of thousands of eggs, and duplicate genes abound in all living things. Gallifreyans even have two hearts. Redundancy is a universal fact of biology — but how does it play a role in HIV?
For HIV infection to grow within a person, it must spread from one T-cell to another. Typically, individual virus particles bud from an infected cell, diffuse through the lymph, and then enter a second T-cell. It just takes a single virus to infect that second cell — and so the many viruses exiting from one cell can infect many other cells. This mode is called cell-free transmission. But there is a second way that T-cell infection can occur: An infected T-cell can “link up” to an uninfected T-cell, forming a synapse between the two. Tens or hundreds of virus particles may then shuttle directly through the synapse, infecting the new cell. This second mode of transmission is called synaptic transmission or direct cell-to-cell transmission.
Why should doctors & patients care? It turns out that this mode of transmission may give the virus a way to avoid the effects of drug treatment. My back-of-the-envelope calculation suggests that synaptic transmission can halve or quarter a patient’s effective drug dose!
But why the virus evolved to spread in this manner is a bit of a puzzle — if only a single particle is needed to infect a cell, why waste hundreds all at once? Does the virus benefit from this alternate lifecycle, or is this redundant mode of transmission a quirky side-effect (a “spandrel”) of T-cell physiology that is just wasteful, from the virus’ perspective?
In PLoS ONE last month, Komarova, Levy, and Wodarz investigate exactly this question by modeling how the virus can evolve to maximize growth of the infection. Using a model of immune response, they conclude that there is room for two opposing viral strategies: a “stealth” strategy where just one or a few viruses infect each cell, and a “saturation” strategy where many viruses infect each cell, ganging up to overwhelm the person’s immune response. Synaptic transmission may have evolved to let HIV “gang up” on the intracellular immune response.
This saturation strategy is analogous to the commonly observed adaptation known as predator satiation — a strategy used by prey species (such as the mayfly laying thousands of eggs!) to flood their immediate environment with many more offspring than the cohabiting predator species can possibly consume. KLW’s model shows a way in which viral saturation can likewise be adaptive.
While they do not say so explicitly, an extension of KLW’s model might also explain the coexistence of cell-free and synaptic modes of transmission, as a way for the virus to “bet hedge” — using the cell-free mode to spread quickly in permissive immune environments, and using the synaptic mode to ensure safe passage from cell to cell when confronted with a stronger immune response. Even with the uncertainty surrounding exactly how to model the immune response, KLW’s theory should provide a helpful framework for virologists who want to think about the evolutionary causes and pathological effects of synaptic transmission.