Costimulatory function of CD28 in avian gamma delta T cells is evolutionarily conserved

CD28 costimulatory signals are required for T-cell proliferation and lympho kine production. In this work, the functional conservation of CD28 was stud ied in avian gamma delta T cells. The avian CD28 molecule is expressed on a ll alpha beta T cells and is capable of giving a costimulatory signal. Most peripheral gamma delta T cells are CD28 negative; however, we identified a CD28-positive gamma delta T-cell subset from peripheral blood comprising a bout 12% of gamma delta T cells. The peripheral CD28(+) gamma delta T-cell subset included all CD8(+) gamma delta T cells known to be a responding sub set during activation. After polyclonal activation, the frequency of CD28() gamma delta T cells was increased and the activation also up-regulated CD 5, CD25 and major histocompatibility complex (MHC) class II molecules. Thes e changes were detected after both polyclonal and antigen-specific T-cell a ctivation. In addition, we also showed that CD28 can give a costimulatory s ignal to gamma delta T cells and that this signal leads to up-regulation of IL-2 and bcl-x transcripts. These results indicate that the function of CD 28 is evolutionarily conserved and can already be detected in avian gamma d elta T cells.