When Neutrophils attack

Today in class the dental students learned about blood, and the cells contained within. I promised to post a cool video of a neutrophil "chasing" a bacteria, so here it is:

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Poor bacteria never stood a chance.

There are a lot of interesting processes going on here. The first, obviously, is that the neutrophil appears to "see" the bacteria. Since we all got a good look at cell structure last week, we know that cells don't have eyes! So just how does this neutrophil know to attack?

The neutrophil is most likely sensing chemical signals coming from the bacteria. So-called "quorum sensing signals" are basically messages to other bacteria to come and join the infection party. It is likely that our neutrophil has picked up on these quorum sensing molecules and is using them to sniff out the bacteria. In addition, we know that neutrophils, like other phagocytes, express what are called "Toll-like receptors",or TLRs, on their cell surface. These receptors, important for innate immunity, are designed to recognize bacteria-specific sugars and DNA sequences.

So the neutrophil has identified the target, the target is escaping - how does the neutrophil move? This involves a whole lot of specialized cell signaling. First, the neutrophil is exhibiting "chemotaxis", or movement along a chemical gradient. It wants to go where the chemical signal from the bacteria is most concentrated. So, receptors on the cell surface bind the bacterial proteins, then trigger changes in the cell.

One thing that's happening is that our neutrophil has developed polarity -now it has a front end and a back end. We're not quite sure how this happens, but it seems likely that the cell is probably sending out random pseudopodia in all directions. Whichever psuedopod gets positive reinforcement (e.g. lots of binding to bacterial protein receptors) gets to stay and grow, while the other pseudopodia which don't encounter their target are pulled back into the cell.

To move, the neutrophil needs to rearrange some cellular structures. This involves polymerization of actin and myosin, and probably some growth of microtubules as well.

Once the neutrophil catches up to the bacteria, the process of phagocytosis begins. The bacteria is "grabbed" by specific receptors and pulled into an endosome. That endosome will eventually fuse with a lysosome, and the resulting phagosome is where the bacteria ultimately meets its end.