The killer cell lectin-like receptor G1 (KLRG1) is a natural killer cell receptor expressed by T cells that exhibit impaired proliferative capacity. Here, we determined the KLRG1 expression by virus-specific T cells. We found that repetitive and persistent antigen stimulation leads to an increase in KLRG1 expression of virus-specific CD8 ؉ T cells in mice and that virus-specific CD8 ؉ T cells are mostly KLRG1 ؉ in chronic human viral infections (human immunodeficiency virus, cytomegalovirus, and Epstein-Barr virus) but not in resolved infection (influenza virus). Thus, by using KLRG1 as a T-cell marker, our results suggest that the differentiation status and function of virus-specific CD8 ؉ T cells are directly influenced by persistent antigen stimulation.Inhibitory NK cell receptors are expressed in variable frequency by memory-phenotype T cells (11,16,18,30). These receptors include the killer cell immunoglobulin-like receptors (KIRs) and the leukocyte immunoglobulin-like receptors (LIRs) in humans and the Ly49 molecules in mice. In humans and mice, CD94/NKG2A and the killer cell lectin-like receptor G1 (KLRG1) represent additional inhibitory NK cell receptors expressed by T cells (33). Classical and nonclassical major histocompatibility complex (MHC) molecules serve as ligands for KIRs, LIRs, CD94/NKG2A, and Ly49 molecules, while the ligand of KLRG1 is unknown.In humans and mice, KLRG1 is expressed by CD4 and CD8 T cells that exhibit a memory cell phenotype and by a large proportion of NK cells but not by monocytes, granulocytes, or mast cells (6,7,14,24,31,32). Human KLRG1 is also found in a substantial subset of ␥/␦ T cells (12) and, surprisingly, in a large proportion of naive-phenotype CD4 and CD8 T cells in umbilical cord blood (15). After birth, naive-phenotype KLRG1 ϩ T cells disappear rapidly from peripheral blood. Experiments with P14 T-cell receptor (TCR) transgenic mice specific for lymphocytic choriomeningitis virus (LCMV) further showed that P14 memory cells expressing KLRG1 were impaired in their proliferation potential but not in their capacity to perform immediate effector cell functions (31). Similar to the data in mice, human T cells expressing KLRG1 also exhibit a poor proliferation potential, while their ability to secrete gamma interferon is preserved (32).Stimulation of T cells in vitro fails to induce KLRG1 expression, while infection of mice with viruses dramatically and transiently increases the frequency of KLRG1-expressing CD8 T cells (6,7,17,24,32). However, information about the expression of KLRG1 by virus-specific CD8 ϩ T cells during repetitive or persistent antigen stimulation in mice and humans is limited (22). To address this important question, we examined KLRG1 expression by antigen-experienced CD8 T cells in different viral infections using antibodies specific for human and mouse KLRG1 and MHC class I tetramers loaded with viral epitopes to identify virus-specific CD8 T cells in both species.To determine the kinetics of KLRG1 expression by antigenspecific CD8 T cells duri...

Tumor-specific CD8 T cell responses to MCA102 fibrosarcoma cells expressing the cytotoxic T cell epitope gp33 from lymphocytic choriomeningitis virus were studied. MCA102gp33 tumors grew progressively in C57BL/6 mice, despite induction of peripheral gp33-tetramer+ T cells that were capable of mediating antiviral protection, specific cell rejection, and concomitant tumor immunity. MCA102gp33 tumors were infiltrated with a high number (∼20%) of CD11b+CD11c− macrophage-phenotype cells that were able to cross-present the gp33 epitope to T cells. Tumor-infiltrating CD8 T cells exhibited a highly activated phenotype but lacked effector cell function. Strikingly, a significant portion of tumor-infiltrating lymphocytes expressed TCRs specific for gp33 but bound MHC tetramers only after cell purification and a 24-h resting period in vitro. The phenomenon of “tetramer-negative T cells” was not restricted to tumor-infiltrating lymphocytes from MCA102gp33 tumors, but was also observed when Ag-specific T cells derived from an environment with high Ag load were analyzed ex vivo. Thus, using a novel tumor model, allowing us to trace tumor-specific T cells at the single cell level in vivo, we demonstrate that the tumor microenvironment is able to alter the functional activity of T cells infiltrating the tumor mass.

To study liver cell damage by CTL, CD8 T cells from P14 TCR transgenic (tg) mice specific for the gp33 epitope of lymphocytic choriomeningitis virus with either deficiency in IFN-γ (P14.IFN-γ°), functional Fas ligand (P14.gld), or perforin (P14.PKO) were transferred into H8 tg mice ubiquitously expressing gp33 Ag. Treatment of H8 recipient mice with agonistic anti-CD40 Abs induced vigorous expansion of the transferred P14 T cells and led to liver cell destruction determined by increase of glutamate dehydrogenase serum levels and induction of caspase-3 in hepatocytes. Liver injury was mediated by the Fas/Fas ligand (FasL) pathway and by perforin, because P14.gld and P14.PKO T cells failed to induce increased glutamate dehydrogenase levels despite strong in vivo proliferation. In addition, H8 tg mice lacking Fas were resistant to the pathogenic effect of P14 T cells. Besides FasL and perforin, IFN-γ was also required for liver cell damage, because P14.IFN-γ° T cells adoptively transferred into H8 mice failed to induce disease. Moreover, Fas expression on hepatocytes from H8 recipient mice was increased after transfer of wild-type compared with P14.IFN-γ° T cells, and wild-type P14 T cells expressed higher levels of FasL than P14 T cells lacking IFN-γ. Thus, our data suggest that IFN-γ released by activated CD8 T cells upon Ag contact facilitates liver cell destruction.