Antigen stimulation of any T cell initiates a complex series of events, culminating in T cell activation and an immune response. At the heart of this response is a cascade of signalling pathways, primarily — but not exclusively — involving mitogen-activated protein kinases (MAPKs). ZAP-70, a Syk-related tyrosine kinase, is also essential for an immune response in activated T cells, and although this kinase regulates MAPK activation, its precise targets have remained elusive. In the latest issue of Nature Immunology (Nature Immunol. 4, 46–50 (2003)) Alonso et al. identify at last a potential target of ZAP-70 in the regulation of T-cell activation.

Common sense dictates that the activation of any immune response must be highly regulated in both intensity and duration. Regulating the activity of kinase signalling pathways, such as those initiated by MAPK, through protein phosphatases (PPases) is crucial after any T cell–antigen interaction. These PPases curtail the kinase response in a temporal manner, depending on the type of T cell activated or the antigen presented to the cells. VHR (Vaccinia virus VH-1-related dual-specific protein phosphatase) is one such PPase that has an essential role in regulating T-cell activation through inhibition of the extracellular signal-regulated kinases (Erk/MAPK) or Jun kinase (JNK) signal cascades. In this context, it was unclear whether ZAP-70 is involved in VHR-mediated regulation.

Through expression studies in P116 cells lacking functional ZAP-70, Alonso et al. indicate that VHR activity is probably regulated by ZAP-70-mediated phosphorylation of VHR-Tyr 138. Rescue experiments in cells co-expressing either wild-type VHR or VHRY138F (in which the putative tyrosine phosphorylation site is mutated to phenylalanine) with recombinant ZAP-70 indicate that phosphorylation of this particular residue by ZAP-70 is essential for activation of VHR. Phosphorylation of VHR-Tyr 138 is also critical for inhibition of the Erk pathway in activated T cells. The localization of phosphorylated VHR to the immune synapse after antigen presentation to cells (see Figure) provides further correlative evidence to suggest that phosphorylated VHR is important in regulating T-cell activation.

Figure 1: CD8-positive T cells incubated with antigen-presenting cells.
figure 1

Confocal microscope images showing localization of T-cell receptor (a) localization of VHR (b) and colocalization of TCR T-cell receptor and VHR to sites of the immune synapse (c).

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Therefore, VHR-mediated inhibition of the Erk pathway after T-cell activation may involve phosphorylation of ZAP-70, making it one of the few known targets of this enzyme. This would ensure that while both Erk and ZAP-70 would be activated after T cell activation, ZAP-70 would then curtail the Erk response through phosphorylation and activation of the VHR PPase. However, as the VHRY138F mutant potentiates Erk signalling, the role of VHR in augmenting T-cell activation is not straightforward. These results raise the interesting possibilities that phosphorylation of VHR might not affect the catalytic activity of the PPase, but may cause alterations in protein localization, protein conformation or substrate specificity, all of which could be equally important in controlling the temporal regulation of MAPKs in activated T cells.