## Archive for April, 2018

### Macronectes giganteus

April 15, 2018

Southern Giant Petrel

This blog is named after the Storm Petrel, Hydrobates pelagicus. It is a small bird, looking superficially like a swallow, and with a wingspan of less than 20 centimetres and a weight of about 30 grams. Looking back to my recent trip to South America, I see that the bird which made the biggest impression on me was a relative of the title species, the Southern Giant Petrel, Macronectes giganteus. It is on quite a different scale, with a wingspan of about two metres and a weight of about 5 kilograms. Thus it approaches the size of one of the smaller species of albatross. In form it looks a bit like a giant version of the Fulmar. The first ones I saw were in the harbour of Ushuaia. I then saw many more in flight during the cruise on the Beagle Channel. Before the trip I was not informed about how to distinguish Macronectes giganteus from the very similar Northern Giant Petrel, Macronectes halli. Fortunately for me, Eva was very active with her camera and took a photograph (see above) of an individual in Ushuaia which shows what I later learned to be a characteristic feature of M. giganteus, namely the fact that the tip of the bill is green. There does exist a light morph which is mainly white but we did not see any of those.

When reading immunology textbooks I had the feeling that one important point was not explained. The T cell receptor is almost entirely outside the cell and so when it encounters its antigen it cannot transmit this information into the cytosol the way a transmembrane receptor does. But since the activation of the cell involves the phosphorylation of the cytoplasmic tails of proteins associated to the receptor (CD3 and the $\zeta$-chains) the information must get through somehow. So how does this work? This process, which precedes the events relevant to the models for T cell activation I discussed here, is referred to as T cell triggering. I had an idea about how this process could work. If the T cell receptor and the coreceptor CD8 both bind to a peptide-MHC complex they are brought into proximity. As a consequence CD3 and the $\zeta$-chains are then close to CD8. On the other hand the kinase Lck is associated to CD8. Thus Lck is brought into proximity with the proteins of the T cell receptor complex and can phosphorylate them. I had never seen this described in detail in the literature. Now I found a review article by van der Merwe and Dushek (Nature Reviews in Immunology 11, 47) which explains this mechanism (and gives it a name, co-receptor heterodimerization) together with a number of other alternatives. It is mentioned that this mechanism alone does not suffice to explain T cell triggering since there are experiments where T cells lacking CD4 and CD8 were triggered. The authors of this paper do not commit themselves to one mechanism but instead suggest that a combination of mechanisms may be necessary.