In experiments to test antigen presentation to mouse or human cells, COS cells are often transfected with an antigen plus a mouse or human MHC molecule. My question is why the mouse and human cells aren't alloreactive towards the endogenous COS cell MHCs?
Alloreactivity is by definition within a species (e.g. B6 against Balb/c) and these tend to be very strong responses. Xenoreactivity (between species, eg mouse against monkey) are in general much weaker, especially at short term. In case you perform the experiment with a T cell line (i.e. a monoclonal TCR population), the likelyhood that your TCR has reactivity against the endogenous MHC molecules is very small.
Alloreactivity is by definition within a species (e.g. B6 against Balb/c) and these tend to be very strong responses. Xenoreactivity (between species, eg mouse against monkey) are in general much weaker, especially at short term. In case you perform the experiment with a T cell line (i.e. a monoclonal TCR population), the likelyhood that your TCR has reactivity against the endogenous MHC molecules is very small.
I'm not sure and I'm going from memory here, but I don't think that cell line expresses class II histocompatibility antigens. I might be wrong, but if I'm not T cells will only respond to transfected class I/class II antigen complexes.
It is likely that COS cells do not have a large endogenous pool of antigens to form targets for allorecogniton. For a good allorecognition, a large numbers of T cells must react to their targets. From your question, it sounds as if you are transfecting with one MHC and one antigen. That is not sufficient for detection of few reactive T cells that might be specific for that combination of pMHC.
Very interesting question and very good answers from which we can learn a lot!
I would like to keep track of this discussion and maybe give a look at this issue!
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