Engagement of the CD4 Receptor Affects the Redistribution of Lck to the Immunological Synapse in Primary T Cells: Implications for T-Cell Activation during Human Immunodeficiency Virus Type 1 Infection

Abstract

Human immunodeficiency virus (HIV) infection is characterized by impaired cell-mediated immune responses, which are primarily manifested by a progressive decline in the number and function of CD4 ϩ T cells. Of note, the qualitative loss in the CD4 ϩ T-cell function begins well in advance of its quantitative decline during HIV infection (31, 59, 68), although there is a selective loss of memory/effector CD4 ϩ CCR5ϩ cells occurring early after HIV/simian immunodeficiency virus infection, particularly in the mucosal areas (reviewed in references 8, 52, and 55). This implies that the impairment in immune response could be due to mechanisms other than direct viral destruction. HIV-1 binds through its envelope glycoprotein, gp120, to the CD4 receptor molecule (13). The CD4 receptor plays a critical role during antigenic stimulation by major histocompatibility complex (MHC)-bearing cells. It is important in subsequent signal transduction through activation of the CD4-associated tyrosine kinase, p56Lck (Lck) (22), which in turn modulates T-cell differentiation as well as T-cell receptor (TCR)-induced signaling (67). This phenomenon could involve an up-or a downregulation of signaling effects depending on the timing of CD4 engagement relative to TCR-induced stimulation (26). Indeed, in vitro studies have demonstrated that CD4-induced stimulation, in the absence of or prior to TCR engagement, leads to apoptosis of the cell (1, 5) or inhibition of subsequent TCR-induced T-cell activation (6), which underscores the possible role of CD4 engagement in HIV pathogenesis.Lck is a member of the Src family of non-receptor tyrosine kinases expressed primarily in thymocytes and lymphocytes and predominantly in T cells. In MHC II-restricted T cells, approximately 75 to 95% of cellular Lck is found associated with the cytoplasmic portion of CD4, involving about 85 to 95% of CD4 molecules (7). Lck interacts specifically with CD4 molecule through a dicysteine motif which binds to a corresponding motif in the cytoplasmic domain of the CD4 molecule (62). TCR-induced signaling involves the activation of lck, which, in turn, phosphorylates the immuno-receptor tyrosine activation motifs (ITAMs) within the TCR complex, as well as the tyrosine kinase ZAP-70 that docks onto the phosphorylated ITAMs. In T lymphocytes, following stimulation, Lck redistributes into lipid rafts and accumulates at the stable region between T cells and antigen-presenting cells, the immunological synapse (IS) (20,43,60). Together with cytoskeletal reorganization (15,27,34,66), the membrane lipid raft domains are thought to orchestrate protein interactions in space and time by regulating raft coalescence and/or to control the recruitment of proteins to these domains. The temporal and

The Orientation of HIV-1 gp120 Binding to the CD4 Receptor Differentially Modulates CD4+ T Cell Activation

“…In line with this, we demonstrate that cross-linking of CD4 receptors by ICs not just at the site of TCR engagement hinders the accumulation of signaling units beyond the engaged TCRs, reflected by impaired MTOC orientation toward the site of TCR stimulation and hence compromised T cell activation. These results attribute gp120 a role in interfering with CD4 + T cell activation at the level of IS formation, which is in line with a previous report that describes defective recruitment of p56 lck to the IS upon CD4 receptor engagement prior to TCR engagement (39).…”

“…HIV-1 replication is one major driving force of progressive immunodeficiency (3). One constituent of HIV-1, namely gp120, has the capacity to directly modulate CD4 + T cell function, as shown previously (30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47) and in the present study. This effect has mostly been shown in vitro using relatively high concentrations of gp120, which leads to the question whether these effects might similarly operate in an in vivo scenario.…”

“…CD4 receptors play a crucial role in enhancing sensitivity of TCR-triggered T cell activation by interacting with MHC class II (MHC II) molecules on APCs (25,26) and by their noncovalent interaction with the src family tyrosine kinase p56 lck whose activation initiates TCR signaling progression (26)(27)(28)(29). In line with this, a plethora of in vitro studies provide experimental proof that gp120 binding to the CD4 receptor interferes with TCR-induced CD4 + T cell activation (30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47). However, the effect of noninfectious gp120-CD4 receptor interaction on CD4 + T cell activation is still a contradictory issue in the literature, and the gp120 levels measured in plasma of HIV + patients may be below those that have functional effects on cells in vitro (48).…”

“…Such direct interference with the function of central regulators of the immune system is unique among human persistent viruses and represents a crucial factor in undermining HIV-specific and heterologous immune control. CD4 receptor binding by the HIV-1 envelope mediates not only viral infection (11,12), but it can also amplify TCR-induced signaling by recruiting p56 lck to the engaged TCRs (26-29, 89, 90), attributing the noninfectious interaction between gp120 and the CD4 receptor a potential role in modulating CD4 + T cell responses [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. Several studies demonstrated the presence of soluble gp120 in the plasma or lymphoid tissue of HIV-1 patients (20)(21)(22)(23)(24), but its effect on TCR-induced CD4 + T cell activation is still a contentious issue in the literature.…”

“…On the other hand, gp120 binding to CD4 + T cells was shown to reduce their activation mediated through TCR stimulation (30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43). Mechanisms to explain gp120-induced negative effects on CD4 + T cell activation are manifold: gp120-mediated cross-linking of CD4 receptors was suggested to induce CD4 receptor endocytosis (33,35,36,41,46), to prevent peptide-MHC II-CD4 receptor interaction (37,50) through interference of gp120 with the MHC II binding site on the CD4 receptor (51), to abolish proximal TCR signaling (32,34,38,40,52), or to redistribute p56 lck away from TCRs or from the immunological synapse (IS) (39,53).…”