In article <antibody-1801952038560001 at seven.bii.ch> antibody at bii.ch (antibody) writes:
>Some people out there might like to discuss this topic
>as a recent paper in this weeks Nature proves (?) that there
>appears to be hypermutation in GC T cells specific for a
>given antigen.
>>This is a bit to a surprise as it was generally (?) agreed
>that T cells do not show such a mechanism; in contrary to
>B cells were it is used to generate high affinity Abs.
>>As many labs in past times might have looked for hypermutated
>T cells it is a bit confusing that this mechanism was not seen
>before. E.g. some T cell clones were established from hyper-
>immunized mice but their sequence of TCRa or TCRb was entirely
>germline (of course not the junctions where non templated N-
>nucleotide additions took place).
Remember that germinal centers are mostly B cells. Only a miniscule percentage
of T cells responsive to a particular antigen are likely to be found there.
It could be a frequency issue, and the likely reason that Kelsoe saw this
is that he went in and manually picked out the T cells from the germinal
centers. I would imagine if he did as others had and used bulk splenic T
cells, the frequency of T cells _from germinal centers_ would be a very small
percentage of the _total_ number of antigen reactive T cells and he probably
would have had a much harder time of finding mutations. Furthermore, while
B cell memory is generated in germinal centers, this isn't thought to be true
of germinal center T cells so there's no reason to expect those T cells to
be enriched through hyperimmunization protocols.
Basically, Garnett Kelsoe's lab has been hunting for needles in the
proverbial haystack, but in his case it's been worthwhile (my guess is
that if he lost a needle in a haystack, he'd just go buy a new needle :)
>>What do you think about this? Maybe I missed many studies
>showing TCRas of non germline encoded sequences? How were
>they explained at those old days?
>
Sorry, I'm not old enough to know (and hey, the genes for the TCR weren't
cloned THAT long ago! Sheesh! Kids... :)
>BTW: It is not discussed what influence these mutatins have
>on the specificity of these cells.
>
Actually, it was. Some cells likely lost their specificity. As for the
potential for autoreactivity of new specificities, while B cell "memory"
appears to be generated in germinal centers, this isn't thought to be
so for T cells. Also, since there are very small numbers of T cells in a
germinal center, it's possible that the frequency of autoreactive T cells
that could be generated wouldn't result in a final frequency high enough to
respond to a new antigen anyway (I don't think anyone believes that NO
autoreactive T cells make it through negative selection, so there must be
other controls either at the level of precursor frequency in the periphery
or extrathymic deletion/anergy).
>Although discussed (and I did not understand it) why there are
>so many non-productive sequences amplified? If the mouse was
>immunized one should have expanded cells with the antigenic
>specificity and therefore with productive rearrangements.
No. There is no selection for the ability to express protein at the level
of a PCR reaction. Remember that there are also unproductive rearrangements
and that V alpha genes appear not to undergo allelic exclusion. Yes, there
is a productive rearrangement in each cell, but nothing will stop the
PCR primers from amplifying the non-productive one too, as long as the
primer will anneal. PCR doesn't care about incorrect reading frame.
Also, the primers would amplify related genes to which the primers
could anneal, so you wouldn't expect all the rearrangements to be
specific, productive or not (not all the T cells in the GC were antigen
specific to begin with).
To me, the main issue is WHY T cells undergo somatic mutation. I find it
interesting that a recent issue of Cell contained an article showing that
HIV mutates at high frequencies at germinal centers (I forget the authors
and I'm not overly familiar with HIV mutation, but the paper came out
within the last couple of months). To me, this suggests a potential trigger
in the microenvironment of the germinal center that may be MEANT for B cells,
but may affect other cells that happen to be there too. While it isn't
beyond the realm of possibility that the TCR and Ig loci share enough
similarity that they would both be affected by the "mutator", why HIV?
DNA accessibility? I don't get it, but I'd love to know. I keep forgetting
to ask Garnett what he thinks, but I'll try to remember the next time
I talk to him (we're collaborating on a germinal center B cell project),
or if someone on this newsgroup is at UMAB, ask him yourself and let us
know :)
>>--
>******************************************************************
>>Joerg Kirberg EMAIL: kirberg at bii.ch>Basel Institute for Immunology FAX: 41-61-605 13 72
>Grenzacherstr. 487 PHONE: 41-61-605 12 77 (lab)
>Postfach
>CH-4005 Basel
>>Switzerland
Stacy
