The question of CD4 and CD8 expression on APCs is an interesting one.
I don't know much about the subject but can offer some conceptual ideas.
Firstly, the state of activation of the APC (dendritic cell) may be
important in whether or not CD4 is expressed and how much of it is
expressed. Is more CD4 expressed during an active immune response, i.e.,
after the initial activation of T cells, or humoral immunity?
Secondly, if only a subset of human dendritic cells or macrophages
express CD4, has this been reproducibly proven at the level of Northern blot
expression?
Thirdly, it is possible that APCs can bind soluble CD4 and CD8 on
their surface, thus giving some FACS +ive staining.
Fourthly, if CD4 is expressed on APCs as a generalized phenomenon,
then I offer this idea:
During an immune response soluble receptors such as CD4, CD8 and both
class I and class II MHC can be shed from the surface of activated cells. In
fact, a number of studies have demonstrated this in humans and mice. One
study suggests that activated CD8 cells can bind soluble class II and
peptide and may mediate suppressor (veto-like activity) on CD4 cells in
response to Mls antigens (Nature or Science paper by Sprent's group a few
years ago).
Thus, is it possible that during inflammation APCs, via CD4, can
bind and internalize soluble/shed class II MHC + peptide complexes? This
may serve as an additional mechanism of antigen processing and presentation
during an immune response. The same may hold true for APCs expressing CD8,
except class I + peptide would be involved.
Laszlo Radvanyi
Ontario Cancer Institute, Princess Margaret Hospital
Toronto, Ontario M4X 1K9
tel: 416-924-0671 ext. 4994
fax: 416-926-6529
e-mail: radvanyi at oci.utoronto.ca
