Background/Purpose:

The interplay between effector and regulatory T cells (Tregs) is a key element among peripheral tolerance mechanisms in Systemic Lupus Erythematosus (SLE). Resistance to suppression has been recently acknowledged as part of the defects shown by T cells from SLE patients. The E3 ligase Cbl-b has been shown to modulate T cell unresponsiveness in SLE. However its potential role in the regulation of peripheral Tregs tolerance has not been fully addressed. The aim of this study was to assess the expression of Cbl-b and its relationship to the resistance to suppression phenotype in SLE patients.

Methods: We included 25 patients with SLE (10 in remission and 15 with active untreated disease) according to the classification criteria of the American College of Rheumatology and 25 age and gender-matched healthy controls. PBMCs were isolated by density gradient and effector  (CD4⁺CD25^–) and Tregs (CD4⁺CD25⁺CD127^–) were purified by magnetic selection. The expression of Cbl-b and p27^kip1 was analyzed by Western blotting. Interaction between Cbl-b and p27^kip1 was addressed by immunoprecipitation (IP). Proliferative responses were assessed in allogeneic and autologous cocultures by CFSE. Differences were assessed by t Student test. p<0.05 was considered as statistically significant. In all cases, an informed consent was obtained, and the ethics committee approved this study. 

Results: We found diminished Cbl-b expression in Tregs from SLE patients in comparison to healthy controls (1.3±1.0 vs 2.8±1.8, p=0.002), which was associated with resistance to suppression in proliferation assays (r=0.553, p=0.041). Moreover, this phenomenon was related to deficient expression of the cell cycle regulator p27^kip1 in Tregs from SLE patients when compared to healthy controls. We also found by IP assays, that p27^kip1 interacts with Cbl-b in Tregs. We found no significant differences regarding to disease activity.

Conclusion: Our data suggest that the ligase Cbl-b is able to regulate the interplay between effector and Tregs, particularly, the resistance to suppression via ubiquitination of p27^kip1 in SLE patients. To our knowledge, this is the first study to demonstrate that p27^kip1 is able to interact with Cbl-b, which might constitute another mechanism by which this ubiquitin ligase is able to modulate the T cell receptor activation threshold.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-e3-ligase-casitas-b-lineage-lymphoma-b-cbl-b-modulates-peripheral-regulatory-t-cell-function-via-p27kip1-in-patients-with-systemic-lupus-erythematosus/