T cell anergy: An NFATic response

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The prevention of autoimmunity by T cell anergy is controlled by a complex multistep negative signaling pathway, requiring the coordinated action of several E3 ubiquitin ligases.

The process of T cell anergy, which renders cells unresponsive to antigens, is important for the prevention of autoimmunity. Central to this process is signaling through Ca²⁺ and calcineurin to the transcription factor NFAT, resulting in the upregulation of a set of anergy-associated genes. These genes are known to encode both proteases and E3 ubiquitin ligases, which Heissmeyer et al. now show are important for the degradation of key signaling proteins.

Full activation of T cells requires engagement of both the T cell receptor (TCR) and costimulatory molecules, which form an immunological synapse between the T cell and the antigen-presenting cell (APC). NFAT then combines with another transcription factor, AP-1, leading to expression of interleukin 2. In the absence of a costimulatory signal, only NFAT becomes activated, resulting in the induction of anergy. Upon subsequent activation of both the TCR and costimulatory molecules, these anergic cells remain unresponsive. A state of anergy can also be produced artificially through the addition of ionomycin, which inhibits Ca²⁺ mobilization.

Heissemeyer et al. observed decreases in the levels of phospholipase C- gamma 1 (PLC- gamma 1) and protein kinase C- theta (PKC- theta ) – key signaling factors in the T cell response – following the induction of anergy, and demonstrated the importance of calcineurin in this process. However, these decreases were dependent on homotypic cell adhesion (as well as restimulation), ruling out a direct effect from Ca²⁺-calcineurin signaling.

Several E3 ubiquitin ligases were upregulated during ionomycin-induced anergy, including Itch and Cbl-b. Both Itch^-/- and Cblb^-/- mice failed to show a reduction in PLC- gamma 1 or PKC- theta levels following induction of anergy. Moreover, degradation of PLC- gamma 1 and PKC- theta was preceded by translocation of Itch and Nedd4 to the plasma membrane following restimulation. However, treatment with proteasome inhibitors failed to prevent degradation; instead, accumulation of a mono-ubiquitinated form of PKC- theta was observed, suggesting that degradation proceeds via the lysosomal pathway. Supporting this, greater levels of Tsg101 mRNA—which encodes a ubiquitin receptor associated with endosomal sorting—were observed in anergic T cells.

The mature immunological synapse contains a core TCR-major histocompatibility complex (MHC)-peptide contact region, and a peripheral lymphocyte function-associated antigen-1 and intercellular adhesion molecule 1 (LFA-1-ICAM-1) ring. Partial, but sometimes complete, breakdown of the LFA-1 ring, was observed in anergic T cells and also following the addition of phospholipase inhibitors.

This work supports a multistep model for the induction of T cell anergy, whereby sustained calcineurin signaling upregulates several E3 ubiquitin ligases and Tsg101. Next, degradation of signaling proteins occurs following a second step of T cell-APC contact, during which the E3 ligases translocate to the plasma membrane. Degradation of PLC- gamma 1 and PKC- theta impairs TCR and LFA-1 signaling, and disrupts the mature immunological synapse, further reducing the antigenic response of anergic T cells.

Jon Reynolds, Copy Editor
Nature Cell Biology

References

Heissmeyer, Vigo Macian, Fernando Im, Sin-Hyeog Varma, Rajat Feske, Stefan Venuprasad, K Gu, Hua Liu, Yun-Cai Dustin, Michael L Roa, Anjana Calcineurin imposes T cell unresponsiveness through targeted proteolysis of signaling proteins. Nature Immunology 5, 255–265 (March 2004)
Davis, Matti Ben-Neriah, Yinon Behind the scenes of anergy: a tale of three E3s. Nature Immunology 5, 238–240 (March 2004)