TCR ligand discrimination is enforced by competing ERK positive and SHP-1 negative feedback pathways

Abstract

Functional discrimination between structurally similar self and foreign antigens is a main attribute of adaptive immunity. Here we describe two feedback mechanisms in T lymphocytes that together sharpen and amplify initial signaling differences related to the quality of T cell receptor (TCR) engagement. Weakly binding ligands predominantly trigger a negative feedback loop leading to rapid recruitment of the tyrosine phosphatase SHP-1, followed by receptor desensitization through inactivation of Lck kinase. In contrast, strongly binding ligands efficiently activate a positive feedback circuit involving Lck modification by ERK, preventing SHP-1 recruitment and allowing the long-lasting signaling necessary for gene activation. The characteristics of these pathways suggest that they constitute an important part of the mechanism allowing T cells to discriminate between self and foreign ligands.

Decreased CD4 expression by polarized T helper 2 cells contributes to suboptimal TCR-induced phosphorylation and reduced Ca2+ signaling

“…Whether the previously reported loss of lipid domain association of CD4 in Th2 cells might thus represent a secondary effect of this quantitative change and the alteration in signaling leads to this, will require additional investigation. Our description of very early acting SHP-1-dependent negative feedback in controlling the TCR signal quality and duration [19], along with the evidence from this and other laboratories that the early phosphorylation of TCR-associated components precedes synapse formation [22], is consistent with the view that these previously reported aspects of CD4 behavior may be a downstream consequence rather than a primary cause of the Th2 pattern of signaling.…”

“…Control 5C.C7 Th1 cells showed substantial levels of the p23 phosphorylated form of TCR f chain, whereas those exposed to antigen in the presence of anti-CD4 showed little p23 TCR f generation and predominant p21 production, consistent with our earlier study. Our previous studies have shown that in addition to CD4-associated Lck, there is a separate pool of Lck associated with the TCR [19], providing a source of kinase for generation of the antigen-induced p21f seen in this experiment even when CD4-Lck is not involved. …”

“…In some experiments, anti-CD4 (clone H129. 19) was added to the cell mixture. After incubation, cells were lysed with 1% Nonidet-P40, 140 mM NaCl, 10 mM Tris-HCl (pH 7.2), 2 mM EDTA, 5 mM iodoacetamide, 1 mM Na 3 VO 4 , and proteinase inhibitor mixture (Boehringer Mannheim, Indianapolis, IN) on ice for 30 min.…”

“…Horseradish peroxidase activity was detected with ECL (Amersham Pharmacia Biotech). Previous studies have shown that these solubilization and precipitation conditions are appropriate for the detection of phosphoproteins associated with the proximal events in TCR-dependent signaling [14,15,[17][18][19]. Controls for sample loading consisted of re-probing of blots for total ZAP-70.…”

Epi-allelic Erk1 and Erk2 knockdown series for quantitative analysis of T cell Erk regulation and IL-2 production

“…Multiple mechanisms have been postulated to generate these differences in Erk dynamics. Recently, a positive feedback loop in which Erk phosphorylates Lck in a regulatory manner to prevent SHP-1 deactivation of Lck was shown to sustain TCR signaling (7). A mathematical modeling effort has demonstrated how the balance between this novel Erk positive feedback and negative feedback via phosphatase-mediated deactivation of molecules proximal to the TCR allows for sensitivity in ligand discrimination (8).…”