T-cell responses: The choice between anergy or activity

signaling update home

SOURCE:

Two isoforms of otubain 1 regulate T cell anergy via GRAIL.

GRAIL (gene related to anergy in lymphocytes), as its name indicates, is involved in the induction of anergy in CD4⁺ T cells. However, until now, the factors that regulate the activity of this RING-type E3 ligase (which attaches ubiquitin to proteins that are destined for degradation) were unknown. Soares et al. now report in Nature Immunology that two isoforms of the ubiquitin-specific protease otubain-1 control the expression and function of GRAIL in the induction of anergy.

Full activation of CD4⁺ T cells requires two signals, and anergy results when a signal is received through the T-cell receptor (TCR) in the absence of a co-stimulatory signal. It is thought that GRAIL might induce anergy by ubiquitylation of membrane-associated targets that are required for T-cell activation. To investigate this system further, the authors used a yeast two-hybrid system to identify GRAIL-binding partners. Two isoforms of a gene containing a OTU domain, otubain-1 and alternative reading frame otubain-1 (otubain-1 ARF1), were shown to bind with high affinity to GRAIL and to co-precipitate with GRAIL in further assays.

What effect do these otubain isoforms have on GRAIL function in T cells? Co-expression of otubain-1 and GRAIL by mouse T-cell hybridomas resulted in the degradation of GRAIL and decreased GRAIL-mediated inhibition of interleukin-2 (IL-2) production, whereas co-expression of otubain-1 ARF1 caused an increase in GRAIL expression and enhanced inhibition of IL-2 production. Further experiments showed that the otubain-1 protein, rather than directly affecting the ubiquitylation of GRAIL, binds a deubiquitylating enzyme USP8 in a trimolecular complex with GRAIL, which regulates GRAIL ubiquitylation and degradation. Otubain-1 ARF1 binds GRAIL, not USP8, allowing USP8 to deubiquitylate polyubiquitylated GRAIL and so stabilize GRAIL function.

Next, the authors investigated the effects of overexpression of otubain-1/otubain-1 ARF1 on GRAIL function and the induction of anergy. Lethally irradiated mice were reconstituted with bone marrow from TCR-transgenic mice that had been retrovirally transduced to express one of these otubain isoforms. CD4⁺ T cells that constitutively expressed otubain-1 showed increased IL-2 production and enhanced proliferation in response to antigen in comparison to control cells. By contrast, cells expressing otubain-1 ARF1 were functionally anergic and they responded poorly to antigen in terms of IL-2 production and proliferation.

This study shows that two isoforms of otubain-1, in conjunction with the deubiquitylating enzyme USP8, have opposing effects on the expression and function of GRAIL in the induction of anergy.

Jenny Buckland

References

Soares, L. et al. Two isoforms of otubain 1 regulate T cell anergy via GRAIL. Nature Immunol. 5, 45–54 (2004) | PubMed |