Fas (CD95) expression and death-mediating function are induced by CD4 cross-linking on CD4+ T cells.

Abstract

The CD4 receptor contributes to T-cell activation by coligating major histocompatibility complex class II on antigen presenting cells with the T-cell receptor (TCR)/ CD3 complex, and triggering a cascade of signaling events including tyrosine phosphorylation of intracellular proteins. The CD4 coreceptor and the T-cell receptor(TCR)/CD3 complex on T cells bind peptide-major histocompatibility complex class II complexes on antigen presenting cells during antigen specific immune responses (1). CD4 contributes to T-cell activation by signaling via its associated src-family tyrosine kinase, p56lck (lck), which phosphorylates the CD3 'y, s, and ; chains (2). CD4 coengagement with the TCR facilitates T-cell activation. In contrast, CD4 cross-linking prior to activation via the TCR results in apoptotic cell death (3). CD4 cross-linking by HIV glycoprotein (gp) 120 also results in apoptosis (4). Furthermore, in vivo CD4 ligation by injection of anti-CD4 antibodies triggered loss of surface CD4 expression, and DNA fragmentation and death (5). In vivo CD4-induced apoptosis appeared to be Fas (CD95)-dependent, since it did not occur in MRL-lpr/lpr mice, which have a mutation in the Fas gene resulting in a nonfunctional protein.Fas, a member of the tumor necrosis factor receptor family, is a mediator of cell death (6). Fas-Fas ligand (FasL) interactions mediate activation-induced cell death, the apoptotic death which follows TCR/CD3-induced T-cell activation (7-9). In vivo, Fas/FasL are thought to be responsible for controlling clone size after expansion in responses to peptide antigens and superantigens, and thus are critical in the regulation of immune homeostasis (10, 11). Fas knockout mice and mice with natural mutations in the Fas or FasL genes, lpr (lymphoproliferative) and gld (generalized lymphoproliferative defect), respectively, develop massive lymphadenopathy that becomes worse with age (12)(13)(14).In this report we demonstrate that CD4 cross-linking induces a small but rapid increase in cell surface Fas expression on isolated CD4+ T lymphocytes. CD4 crosslinking also results in Fas-dependent cell death in normal and gld, but nor lpr, strains of mice. Finally, we show that CD4 cross-linking triggers a subset of CD4+ cells to become sensitive to Fasinduced cell death, by a mechanism separate from control of Fas surface expression. MATERIALS AND METHODSMice. Female mice 6-10 weeks of age were obtained from the The Jackson Laboratories. Mice of the inbred strains C3H, C3H-lpr/lpr, and C3H-gld/gld were used. CD4+ Cell Isolation. Single cell suspensions were prepared from murine spleens and depleted of erythrocytes with Gey's solution. CD4+ cell isolation was accomplished by negative selection on commercial CD4 isolation columns (Biotex Laboratories, Edmonton, Canada). In normal mice, yields were 90% or more CD4+ cells as determined by flow cytometry. In lpr andgld mutant mice, a larger percentage of CD4-cells were present due to the accumulation of CD3+4-8-cells (15), which could not be removed by the negativ...

A novel mechanism for HIV1-mediated bystander CD4+ T-cell death: neighboring dying cells drive the capacity of HIV1 to kill noncycling primary CD4+ T cells

“…[20][21][22][24][25][26] While our results confirm previous findings indicating that antibody-mediated engagement of CD4 or CXCR4 can indeed induce CD4 þ T-cell death, [27][28][29][30] they also indicate that the interactions of the HIV1 Env protein with CD4 and/or CXCR4 are not responsible for the killing by HIV1 particles of unstimulated, noncycling primary CD4 þ T cells. Rather, in the presence of cellular factors released by dying cells, CD4 þ T-cell killing required the presence of the Env protein on the viral particle simply because of a requirement for HIV1 postbinding fusion or entry into the CD4 þ T cells.…”

“…38 Both treatments prevented the capacity of HIV1 to induce CD4 þ T-cell death (Figure 1a), indicating that an interaction of the HIV1 Env with the CD4 cell-surface receptors was one of the events required for the induction of CD4 þ T-cell death. While the Leu3a antibody added in solution did not by itself affect CD4 þ T-cell survival (Figure 1a), CD4 receptor engagement by plateimmobilized Leu3a induced death in the same proportion of CD4 þ T cells as HIV1 (Figure 1a), consistent with previous reports, [27][28][29][30] and with the suggestion that HIV1 Env-mediated engagement of its CD4 cell-surface receptor may by itself cause CD4 þ T-cell death. [20][21][22]24 However, because CD4-specific antibodies may induce different cell signaling than HIV1 viral particles, we investigated whether HIV1-mediated CD4 þ T-cell death might rather be due to post-CD4-binding events.…”

“…3 Because most of the viral particles produced during HIV1 infection are replication-defective, 6 but can still bind to-, and enter into-CD4 þ T cells, viral proteins present at the surface or inside the viral particles have been proposed as candidates for the killing of noncycling CD4 þ T cells in the absence of productive infection of these cells. In vitro findings have suggested that among such viral proteins, the HIV1 surface envelope (Env) constitutes a major potential candidate for the HIV1-mediated killing of CD4 þ T cells in the absence of productive infection, 16,[20][21][22][23][24][25][26] through death signaling caused by engagement of either its cell-surface receptor, CD4 [27][28][29] or, for the Env of X4-tropic HIV1 strains, of its CXCR4 coreceptor. 25,26,30,31 Incubation of noncycling, primary CD4 þ T cells with X4-tropic HIV1 viral particles has been previously reported to cause CD4 þ T-cell death, after a delay of 3-4 days.…”

Caspase-dependent and -independent T-cell death pathways in pathogenic simian immunodeficiency virus infection: relationship to disease progression

“…The interaction of the HIV envelope protein with the CD4 molecule may prime CD4 þ and CD8 þ T cells for CD95-mediated apoptosis. [64][65][66][67] We recently demonstrated that the incubation of resting CD4 þ T cells from healthy donors, even in the presence of an inhibitor of the viral replication, was sufficient to prime CD4 þ T cells for apoptosis in response to CD95 ligation. 68,69 This priming did not require further T-cell activation, suggesting that interactions between the virus and T cells are sufficient.…”

Apoptosis in SIV infection

“…The gp120 is present on the surface of infected T cells, on viral particles, or as a soluble protein 129,130 and can bind to and crosslink CD4. The interaction of the gp 120 with the CD4 molecule can prime CD4 þ and CD8 þ T cells for apoptosis, 95,[131][132][133][134][135] and can promote cell-to-cell fusion leading to syncytia formation that can undergo apoptosis. This apoptosis is characterized by the translocation of Bax from the cytosol to mitochondria leading to the mitochondrial membrane permeabilization with loss of the DC m ), release of apoptogenic intermembrane proteins, in particular apoptosis-inducing factor and cytochrome c, caspase activation, and nuclear chromatin condensation.…”