One of the best things about being a research scientist is that there is always good news to be had from somewhere in my own field or one close by. It always cheers me up to be reminded that the knowledge base is growing every day.
Closest look yet at HIV surface: a Florida State University team has used electron tomography to put together the most detailed images ever seen of the surface of HIV and SIV (S = “simian”) particles. The pictures on the right show engineered SIV expressing high levels of the surface protein gp120 (indicated by arrows), from above (right) and from the side (far right). The propeller-like shape of the gp120 trimer can be clearly seen in the top-down view and again in the close-up below (also from above). The final picture shows a wild type HIV particle, showing the much lower density of gp120 (which came as a surprise; most researchers thought it would look like the SIV pictures). The white bar in the HIV picture is 100 nm, about 1/500th the thickness of a human hair; the same scale applies to the SIV pictures.
This is neat, but it isn’t new: FRET (fluorescence resonance energy transfer) has been around for a while. The basic idea is to take two fluorescent molecules whose excitation and emission spectra overlap in such a way as to make it possible to excite one using the emission of the other. If you put energy in to the system at the lower excitation frequency and get back light at the higher emission frequency, it proves that the two molecules are in close contact. If you’ve attached one to protein A and one to protein B, wherever you see the higher frequency emission proteins A and B must be very close together (interacting); you can do this in a living cell to see when and where A and B interact. What’s new in this study is that the two fluorophores have been attached to opposite ends of a single molecule which happens to fold differently in its active and inactive states. That’s neat, because it allows you to monitor the activation state of the protein by monitoring the emission from the fluorophores. It’s true that this is “the first probe of its kind that allows us to actually see in a living system where, when and how proteins are activated”, but only because that’s very narrowly defined, and the authors do not claim (as the article does) that it’s an entirely new kind of probe. Bad reporter, no vodka.
Classic science; first, some background. Listeria monocytogenes is a most unpleasant organism, one of the most common causes of bacterial meningitis. Most bacteria, when taken up by specialized immune cells — professional eaters-of-foreign-bodies called macrophages — find themselves trapped in a small bubble of membrane called a phagosome, in which they are rapidly killed by acidification and digestive enzymes. Listeria, though, has found a way to break out of the phagosome, using a phospholipase called listeriolysin O. Macrophages are also professional antigen-presenting cells, meaning that they present foreign proteins to other cells of the immune system, enabling those cells to mount a specific response. Macrophages typically present proteins taken up from outside themselves (like bacterial proteins) to T-helper cells, which act to drive humoral (antibody-based) immunity. When what you want is to drive cellular immunity, macrophages can do that too, but usually the protein of interest has to be synthesized within the macrophage (like a viral protein). One of the long-standing problems in vaccinology has been how to get a foreign protein which is taken up from outside the cell (your vaccine) to be presented like a viral protein, as though it came from within the cell, so as to get a cellular immune response to back up the antibody response, which is often insufficient on its own.
So, what Lee’s team has done is to make a vaccine formulation containing their vaccine target protein together with listeriolysin O. When liposomes containing both proteins are taken up by cells, the listeriolysin acts to release the vaccine protein into the cytosol of the cell, at which point it can enter the presentation pathway normally reserved for proteins made within the cell. In other words, Lee and co. have taken the method that Listeria uses to get safely out of the phagosome, and used it to get a vaccine protein out of the phagosome and into a cellular pathway that has until now been very difficult to access. Beautiful, elegant work. They showed, using a mouse viral meningitis model, that liposomes containing listeriolysin plus viral protein elicited a stronger cellular immune response than liposomes containing viral protein alone without antagonising the antibody response, and that this dual response was sufficient to provide sterile immunity against a challenge that killed half of the viral-protein-only group and 100% of unvaccinated controls. Inclusion of listeriolysin in vaccine formulations may provide a way to boost the levels of protection that can be obtained against agents that attack from within a cell, notably viral infections (SARS, HIV, Ebola, ‘flu…) and cancer.
(You can read the whole paper for this one; because the study was reported in the first issue of a new journal (Molecular Pharmaceuticals) it’s available online as a free sample).