Activation of T cells usually requires two signals. Signal 1 is mediated vi
a a peptide-MHC on the APC; signal 2 is mediated via a costimulatory molecu
le on the APC surface. We demonstrate here that naive CD4(+) T cells actual
ly acquire the costimulatory molecule CD80 (B7-1) from syngeneic APCs after
activation. This phenomenon was demonstrated showing acquisition of CD80 b
y T cells from CD80/CD86 (B7-2) knockout mice, and by treating T cells with
cyclohexamide to further rule out endogenous expression of CD80 by T cells
. Moreover, no CD80 mRNA could be detected in T cells that had acquired CD8
0, The amount of acquisition of CD80 by T cells was shown to be directly re
lated to both the strength of signal 1 and the amount of CD80 on the APC, S
pecificity of this acquisition was also shown by the lack of acquisition by
T cells from CD28 knockout mice (implicating CD28 in this process), the la
ck of acquisition of CD40 (another molecule on the APC surface) by T cells,
and confocal microscopy studies. We demonstrate for the first time that 1)
naive T cells, following acquisition of CD80 from APCs, were themselves sh
own to be capable of acting as APCs; and 2) memory T cells that have acquir
ed CD80 from APCs undergo apoptosis in the presence of increased levels of
signal 1. Thus we demonstrate both immunostimulatory and immunoregulatory f
unctions as a result of CD80 acquisition by different T cell populations. T
he Journal of Immunology, 2001, 166: 2505-2513.